Kyushu University Academic Staff Educational and Research Activities Database
List of Papers
Michitaka Ohtaki Last modified date:2022.04.05

Professor / Molecular and Materials Physical Chemistry / Department of Advanced Materials Science and Engineering / Faculty of Engineering Sciences


Papers
1. Bui Duc LONG, Le Hong THANG, Nguyen Hong HAI, Koichiro SUEKUNI, Katsuaki HASHIKUNI, TRAN Quang Minh Nhat, Wojciech KLICH, Michitaka OHTAKI, Thermoelectric Quaternary Sulfide Cu2+xZn1−xSnS4 (x = 0–0.3): Effects of Cu substitution for Zn, Materials Science and Engineering B, 10.1016/j.mseb.2021.115353, 272, 115353, 2021.07.
2. Yuta SHIMIZU, Koichiro SUEKUNI, Hikaru SAITO, Pierric LEMOINE, Emmanuel GUILMEAU, Bernard RAVEAU, Raju CHETTY, Michihiro OHTA, Toshiro TAKABATAKE, Michitaka OHTAKI, Synergistic Effect of Chemical Substitution and Insertion on the Thermoelectric Performance of Cu26V2Ge6S32 Colusite, Inorganic Chemistry, 10.1021/acs.inorgchem.1c01321, accepted for publication on 6 July 2021., 2021.07.
3. Rauf KHAN, Michitaka OHTAKI, Satoshi HATA, Koji MIYAZAKI, Reiji HATTORI, Thermal Conductivity of Nano-Crystallized Indium-Gallium-Zinc Oxide Thin Films Determined by Differential Three-Omega Method, Nanomaterials, 10.3390/nano11061547, 11, 6, 1547 (10 pages), 2021.06.
4. Ahrong JEONG, Koichiro SUEKUNI, Michitaka OHTAKI, Byung Koog JANG, Thermoelectric Properties of In- and Ga-doped Spark Plasma Sintered ZnO Ceramics, Ceramics International, 10.1016/j.ceramint.2021.05.101, in press, available online 19 May 2021., 2021.05.
5. Nhat Quang Minh TRAN, Michitaka OHTAKI, Koichiro SUEKUNI, Rapid Synthesis of W18O49 via Reactive Spark Plasma Sintering with Controlled Anisotropic Thermoelectric Properties, Evergreen, 8, 2, accepted for publication on 16 May 2021., 2021.05.
6. Takashi HAGIWARA, Koichiro SUEKUNI, Pierric LEMOINE, Andrew R. SUPKA, Raju CHETTY, Emmanuel GUILMEAU, Bernard RAVEAU, Marco FORNARI, Michihiro OHTA, Rabih Al Rahal Al ORABI, Hikaru SAITO, Katsuaki HASHIKUNI, Michitaka OHTAKI, Key Role of d0 and d10 Cations for the Design of Semiconducting Colusites: Large Thermoelectric ZT in Cu26Ti2Sb6S32 Compounds, Chemistry of Materials, 10.1021/acs.chemmater.1c00872, 33, 9, 3449-3456, 2021.04.
7. Katsuaki HASHIKUNI, Koichiro SUEKUNI, Hidetomo USUI, Raju CHETTY, Michihiro OHTA, Toshiro TAKABATAKE, Michitaka OHTAKI, A Comparative Study of Thermoelectric Cu2TrTi3S8 (Tr = Co and Sc) Thiospinels: Enhanced Seebeck Coefficient via Electronic Structure Modification, Journal of Alloys and Compounds, 10.1016/j.jallcom.2021.159548, 871, 159548 (9 pages), 2021.03.
8. Wojciech KLICH, Michitaka OHTAKI, Thermoelectric Properties of Mo-doped Bulk In2O3 and Prediction of its Maximum ZT, Ceramics International, 10.1016/j.ceramint.2021.03.129, 47, 13, 18116-18121, 2021.03.
9. Sampad GHOSH, Sivasankaran HARISH, Michitaka OHTAKI, Bidyut Baran SAHA, Thermoelectric Figure of Merit Enhancement in Cement Composites with Graphene and Transition Metal Oxides, Materials Today Energy, 10.1016/j.mtener.2020.100492, 18, 100492 (6 pages), 2020.08.
10. Shinji HIRATA, Michitaka OHTAKI, Kosuke WATANABE, Highly Improved Thermoelectric Performance of Nb-doped SrTiO3 due to Significant Suppression of Phonon Thermal Conduction by Synergetic Effects of Pores and Metallic Nanoparticles, Ceramics International, 10.1016/j.ceramint.2020.07.085, 46, 16A, 25964-25969, 2020.07.
11. Anh Tuan Thanh Pham, Tuyen Anh Luu, Ngoc Kim Pham, Hanh Kieu Thi Ta, Truong Huu Nguyen, Dung Van Hoang, Hoa Thi Lai, Vinh Cao Tran, Jong Ho Park, Jae Ki Lee, Sungkyun Park, Ohtaki Michitaka, Su Dong Park, Hung Quang Nguyen, Thang Bach Phan, Multi-scale defects in ZnO thermoelectric ceramic materials co-doped with In and Ga, Ceramics International, 10.1016/j.ceramint.2020.01.084, 46, 8, 10748-10758, 2020.06, In this work, several X-ray and nuclear analysis techniques were used to examine ZnO materials co-doped with In and Ga, or IGZO materials. X-ray diffraction analysis, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy revealed multi-scale defects in the materials. A nanoscale secondary Ga2Zn9O12 spinel phase, mesoscale grain boundaries, and atomic-scale lattice defects were detected. The lattice defects included oxygen vacancies, zinc vacancies, and complex defects. Positron annihilation spectroscopy and Doppler broadening spectroscopy provided evidence of interactions between charge carriers and defects sites, which explained the low thermal conductivities of the IGZO materials (κtotal ≈ 3.9 W/mK) at 773 K. This combination of X-ray and nuclear analytical techniques can be viewed as a novel approach for investigating the thermoelectric properties of materials with complex crystal structures that contain atomic-scale voids, nanoscale secondary phases, and mesoscale grain boundaries..
12. Dung Van HOANG, Anh Tuan Thanh PHAM, Hanh Kieu Thi TA, Truong Huu NGUYEN, Ngoc Kim PHAM, Lai Thi HOA, Vinh Cao TRAN, Michitaka OHTAKI, Quang Minh Nhat TRAN, Jong-Ho PARK, Jae-Ki LEE, Su-Dong PARK, Tae-Seong JU, Hongjun PARK, Sungkyun PARK, Thang Bach PHAN, Effects of Multi-scale Defects on the Thermoelectric Properties of Delafossite CuCr1-xMgxO2 Materials, Journal of Alloys and Compounds, 10.1016/j.jallcom.2020.156119, 844, 156119, 2020.06.
13. Takuya Tanimoto, Koichiro Suekuni, Taiki Tanishita, Hidetomo Usui, Terumasa Tadano, Taiga Kamei, Hikaru Saito, Hirotaka Nishiate, Chul Ho Lee, Kazuhiko Kuroki, Michitaka Ohtaki, Enargite Cu3PS4: A Cu–S-Based Thermoelectric Material with a Wurtzite-Derivative Structure, Advanced Functional Materials, 10.1002/adfm.202000973, 30, 22, 2000973, 2020.05, Compound semiconductors derived from ZnS (zincblende and wurtzite) with tetrahedral framework structures have functions for various applications. Examples of such materials include Cu–S-based materials with zincblende-derivative structures, which have attracted attention as thermoelectric (TE) materials over the past decade. This study illuminates superior TE performance in polycrystalline samples of enargite Cu3P1−xGexS4 with a wurtzite-derivative structure. The substitution of Ge for P dopes holes into the top of the valence band composed of Cu-3d and S-3p, whereby its multiband characteristic leads to a high TE power factor. Furthermore, a reduction in the grain size to 50–300 nm can effectively decrease phonon mean free paths, leading to low thermal conductivity. These features result in a dimensionless TE figure of merit ZT of 0.5 at 673 K for the x = 0.2 sample. Environmentally benign and low-cost characteristics of the constituent elements of Cu3PS4, as well as its high-performance thermoelectricity, make it a promising candidate for large-scale TE applications. Furthermore, this finding extends the development field of Cu–S-based TE materials to those with wurtzite-derivative structures..
14. Sampad Ghosh, Sivasankaran Harish, Michitaka Ohtaki, Bidyut Baran Saha, Enhanced figure of merit of cement composites with graphene and ZnO nanoinclusions for efficient energy harvesting in buildings, Energy, 10.1016/j.energy.2020.117396, 198, 2020.05, In this work, we have reported the thermoelectric performance of graphene-zinc oxide hybrid cement composites for the first time. Nanostructured graphene and zinc oxide were incorporated into cement to increase the thermoelectric properties of cement composites. The as-received materials were blended, compressed, and subsequently cured at room temperature to make it bulk. On bulk samples, electrical conductivity, Seebeck coefficient, and thermal conductivity were measured. As a consequence of high electrical conductivity and the Seebeck coefficient, the composites exhibited a high value of thermoelectric efficiency. The maximum figure of merit (ZT) of 0.01 is obtained when the contents of graphene and ZnO are 10 wt%, respectively, by mass of cement. This ZT is the highest reported to date for the thermoelectric cement composites, a nearly 70% increase over the previous record with carbon based inclusions. High figure of merit is crucial for energy harvesting applications in future buildings..
15. Shinji Hirata, Michitaka Ohtaki, Simultaneous enhancement in the electrical conductivity and reduction in the lattice thermal conductivity leading to enhanced thermoelectric zt realized by incorporation of metallic nanoparticles into oxide matrix, Evergreen, 10.5109/2740934, 7, 1, 1-6, 2020.03, Nanoporous Fe3O4 composites containing metallic Cu nanoparticles were synthesized by reducing-leaching treatment of CuFe2O4 precursor. Thermoelectric properties of the composites sintered at 1270 ºC greatly improved compared with Fe3O4 without Cu, presumably due to a nanoporous structure and the Cu nanoparticles remaining in the Fe3O4 matrix after acid leaching. The electrical and thermal conductivity indicate that Cu in the matrix promotes the electron conduction and suppresses the phonon conduction, improving the dimensionless figure-of-merit, ZT..
16. Taiki Tanishita, Koichiro Suekuni, Hirotaka Nishiate, Chul Ho Lee, Michitaka Ohtaki, A strategy for boosting the thermoelectric performance of famatinite Cu3SbS4, Physical Chemistry Chemical Physics, 10.1039/c9cp06233e, 22, 4, 2081-2086, 2020.01, Famatinite Cu3SbS4 has attracted attention for its potential application in thermoelectric (TE) contexts. In this work, we report the impacts of co-substituting Ge and P for Sb on TE properties. Melting and heat treatment methods were adopted to synthesize samples of Cu3Sb1-x-yGexPyS4 (x ≤ 0.4, y ≤ 0.3). In this system, Ge functioned as an acceptor for doping a hole to the valence band, which led to enhancement of the TE power factor. Contrastingly, P barely altered the electronic structure. Furthermore, both Ge and P acted as point defects, which effectively decreased lattice thermal conductivity. The combined effects of the co-substitution gave rise to an enhanced dimensionless figure of merit, ZT, of 0.67 at 673 K..
17. Sampad Ghosh, Sivasankaran Harish, Kaiser Ahmed Rocky, Michitaka Ohtaki, Bidyut Baran Saha, Graphene enhanced thermoelectric properties of cement based composites for building energy harvesting, Energy and Buildings, 10.1016/j.enbuild.2019.109419, 202, 2019.11, Cement based thermoelectric materials can convert the available ambient heat absorbed by the building surfaces in summer into electrical energy. These materials can have an impact on the improvement of indoor climate of buildings, reduction of energy consumption and energy harvesting applications especially in the urban areas. In this paper, the thermoelectric properties of cement based composites with graphene nanoplatelets (GNP) inclusions were reported for the first time. GNP-cement mixtures were prepared by planetary ball milling and compressed subsequently to form bulk composites. We report the maximum electrical conductivity of 16.2 Scm−1 and Seebeck coefficient of +34.0 µVK−1 in this work. Hall measurement was performed to determine the material type and carrier concentration. It was found that all specimen exhibit p-type semiconductor behavior. Thermal diffusivity measurements were carried out using laser flash measurement technique. The highest figure of merit 0.44 × 10−3 was achieved at about 70 °C. Enhanced thermoelectric properties of graphene nanoplatelets cement based composites have a promising prospect in the urban heat island effect alleviation, thus saving electricity consumption and energy harvesting..
18. Koichiro Suekuni, Hidetomo Usui, Siying Qiao, Katsuaki Hashikuni, Tatsuya Hirano, Hirotaka Nishiate, Chul Ho Lee, Kazuhiko Kuroki, Kosuke Watanabe, Michitaka Ohtaki, Electronic structure and thermoelectric properties of Sn1.2-xNbxTi0.8S3 with a quasi-one-dimensional structure, Journal of Applied Physics, 10.1063/1.5093183, 125, 17, 2019.05, We report the electronic structure and thermoelectric properties of a tin titanium trisulfide, Sn1.2Ti0.8S3. The crystal structure is composed of infinite "ribbons" of double edge-sharing (Sn4+/Ti4+)S6 octahedra capped by Sn2+. First-principles calculations predict a nearly unidirectional transport of electrons along the ribbon axis for a single crystal and the existence of lone-pair electrons on Sn2+. Experiments on polycrystalline pressed samples demonstrate that Sn1.2Ti0.8S3 exhibits semiconducting temperature dependence of electrical resistivity and a large negative Seebeck coefficient at room temperature. Substitution of Nb5+
for Sn4+ at the octahedral sites increases the electron carrier concentration, leading to an enhancement of the thermoelectric power factor. Anisotropy in the electronic properties is weak because of a weak orientation of the ribbon axis of crystallites in the pressed sample. The lattice thermal conductivity is less than 1 W K-1m-1 for the pristine and substituted samples, which is attributed to weak bonding between the ribbons via the lone-pair electrons of Sn2+ and to random occupation of S4+, Ti4+, and Nb5+ at the octahedral sites..
19. Koichiro Suekuni, Yuta Shimizu, Eiji Nishibori, Hidetaka Kasai, Hikaru Saito, Daichi Yoshimoto, Katsuaki Hashikuni, Yohan Bouyrie, Raju Chetty, Michihiro Ohta, Emmanuel Guilmeau, Toshiro Takabatake, Kosuke Watanabe, Michitaka Ohtaki, Atomic-scale phonon scatterers in thermoelectric colusites with a tetrahedral framework structure, Journal of Materials Chemistry A, 10.1039/c8ta08248k, 7, 1, 228-235, 2019.01, Copper-based chalcogenides with tetrahedral framework structures have been attracting increasing attention as environmentally friendly thermoelectric materials. A representative group of such thermoelectric chalcogenides is the Cu 26 A 2 M 6 S 32 (A = V, Nb, Ta; M = Ge, Sn) family of colusites, which exhibit low electrical resistivity, a large Seebeck coefficient, and low thermal conductivity; these properties are necessary for efficient thermal-to-electronic energy conversion. Here, we show the impact of crystal structure on the lattice thermal conductivity of colusite with A = Nb, M = Sn. The crystal structure can be modified by controlling the cationic compositions and the deficiency in the sulfur content as Cu 26-x Nb 2 Sn 6+x S 32-δ . The Cu/Sn ratio is found to be the key parameter for exsolution into distinct phases with ordered and disordered arrangements of cations. For the ordered-structure phase, sulfur sublimation induces atomic-scale defects/disordered states including interstitial defects, anti-site defects, and site splitting, which function as strong phonon scatterers, and the lowest lattice thermal conductivity of ∼0.5 W K -1 m -1 is achieved for the modified ordered structure. This finding provides a simple approach to modifying the crystal structure of thermoelectric chalcogenides via the loss of anions to reduce their lattice thermal conductivity..
20. Katsuaki Hashikuni, Koichiro Suekuni, Hidetomo Usui, Raju Chetty, Michihiro Ohta, Kazuhiko Kuroki, Toshiro Takabatake, Kosuke Watanabe, Michitaka Ohtaki, Thermoelectric Properties and Electronic Structures of CuTi2S4 Thiospinel and Its Derivatives: Structural Design for Spinel-Related Thermoelectric Materials, Inorganic Chemistry, 10.1021/acs.inorgchem.8b02955, 58, 2, 1425-1432, 2019.01, We report the preparations, thermoelectric and magnetic properties, and electronic structures of Cu-Ti-S systems, namely, cubic thiospinel c-Cu1-xTi2S4 (x ≤ 0.375), a derivative cubic and Ti-rich phase c-Cu1-xTi2.25S4 (x = 0.5, 0.625), and a rhombohedral phase r-CuTi2S4. All samples have the target compositions except for r-CuTi2S4, whose actual composition is Cu1.14Ti1.80S4. All of the phases have n-type metallic character and exhibit Pauli paramagnetism, as proven by experiments and first-principles calculations. The Cu and Ti deficiencies in c-Cu1-xTi2S4 and r-CuTi2S4, respectively, decrease the electron-carrier concentration, whereas the "excess" of Ti ions in c-Cu1-xTi2.25S4 largely increases it. For r-CuTi2S4, the reduced carrier concentration increases the electrical resistivity and Seebeck coefficient, leading to the highest thermoelectric power factor of 0.5 mW K-2m-1 at 670 K. For all of the Cu-Ti-S phases, the thermal conductivity at 670 K is 3.5-5 W K-1m-1, where the lattice part of the conductivity is as low as 1 W K-1m-1at 670 K. As a result, r-CuTi2S4 shows the highest dimensionless thermoelectric figure of merit ZT of 0.2. The present systematic study on the Cu-Ti-S systems provides insights into the structural design of thermoelectric materials based on Cu-M-S (M = transition-metal elements)..
21. Yulia Eka Putri, Suhana Mohd Said, Refinel Refinel, Michitaka Ohtaki, Syskri Syukri, Low Thermal Conductivity of RE-Doped SrO(SrTiO3)1 Ruddlesden Popper Phase Bulk Materials Prepared by Molten Salt Method, Electronic Materials Letters, 10.1007/s13391-018-0062-x, 1-7, 2018.04.
22. Katsuaki Hashikuni, Koichiro Suekuni, Kosuke Watanabe, Yohan Bouyrie, Michihiro Ohta, Michitaka Ohtaki, ToshiroTakabatake, Carrier Concentration Tuning in Thermoelectric Thiospinel Cu2CoTi3S8 by Oxidative Extraction of Copper, Journal of Solid State Chemistry, 10.1016/j.jssc.2017.12.031, 259, 5-10, 2018.03.
23. Shinji Hirata and Michitaka Ohtaki, Synthesis and Thermoelectric Properties of Nanoporous Oxide Composites Containing Metal Nanoparticles, Proceedings of the 18th Cross Straits Symposium on Energy and Environmental Science and Technology, 18, 35-36, 2016.12.
24. Takuya Kagawa and Michitaka Ohtaki, Synthesis and Thermoelectric Properties of Layered Oxyselenides and Oxyphosphides, Proceedings of the 18th Cross Straits Symposium on Energy and Environmental Science and Technology, 18, 269-270, 2016.12.
25. Kohei Mizuta, Michitaka Ohtaki, Crystal Structure and Thermoelectric Properties of β-Pyrochlore-Type Alkali Iron Tungsten Oxides with Cage-Like Structure, Journal of Electronic Materials, 10.1007/s11664-015-4179-5, 45, 3, 1695-1699, 2016.03.
26. Toshifumi Ogawa and Michitaka Ohtaki, Effects of microstructure on thermoelectric properties of dually doped ZnO, Proceedings of the 17th Cross Straits Symposium on Energy and Environmental Science and Technology, 17, 97-98, 2015.12.
27. Kei Watanabe and Michitaka Ohtaki, Switching the Thermal Conductivity by Phase Transitions in Metal Oxides, Proceedings of the 17th Cross Straits Symposium on Energy and Environmental Science and Technology, 17, 105-106, 2015.12.
28. Shinji Hirata and Michitaka Ohtaki, Synthesis and Thermoelectrics Properties of Nanoporous Oxide Formed by Selective Leaching of Metal Nanoparticles, Proceedings of the 17th Cross Straits Symposium on Energy and Environmental Science and Technology, 17, 111-112, 2015.12.
29. Fumihiro Ishibashi and Michitaka Ohtaki, Synthesis of Low-dimensional Nanomaterials in Mesoporous Silica, Proceedings of the 17th Cross Straits Symposium on Energy and Environmental Science and Technology, 17, 113-114, 2015.12.
30. Yosuke Araki and Michitaka Ohtaki, Synthesis of NaFeO2-xPx by P-doping of Layered Oxide NaFeO2, Proceedings of the 17th Cross Straits Symposium on Energy and Environmental Science and Technology, 17, 115-116, 2015.12.
31. Shin-ichi Hayano and Michitaka Ohtaki, Synthesis of Nanosized Layered Compounds with Molecular Assembly Template and Characteristics Changes with Post-treatment, Proceedings of the 17th Cross Straits Symposium on Energy and Environmental Science and Technology, 17, 117-118, 2015.12.
32. Yoshinao Kai and Michitaka Ohtaki, Thermoelectric Properties of Ferroelectric Oxides Reduced by Hydrogen, Proceedings of the 17th Cross Straits Symposium on Energy and Environmental Science and Technology, 17, 121-122, 2015.12.
33. Michitaka Ohtaki, Nanostructured oxide thermoelectric materials with enhanced phonon scattering, Oxide Thin Films, Multilayers, and Nanocomposites, 10.1007/978-3-319-14478-8_7, 107-122, 2015.01, Rapid progress in thermoelectric performance of oxide materials has been conducted virtually exclusively in Japan, resulting in more than ten times increase in the ZT values of oxides within the last two decades. This has caused a revolutionary change in the guiding principles of thermoelectric materials research, in which oxide materials had been disregarded as a potential candidate until early 1990s. Promising oxide thermoelectric materials having been discovered include CaMnO 3 -based perovskites, Al-doped ZnO, layered cobalt oxides represented by NaCo2O4 and Ca3Co4O9, and SrTiO 3 -related phases. This chapter reviews the current aspects of bulk oxide thermoelectric materials, and some strategies for selective reduction of the lattice thermal conductivity (selective phonon scattering) in bulk oxides are also discussed..
34. Michitaka Ohtaki, Oxide Thermoelectric Materials for Power Generation from Mid-to-High Temperature Unused Heat Energy, Proceedings of the 16th Cross Straits Symposium on Energy and Environmental Science and Technology, 29-32, 2014.11.
35. Kohei Mizuta, Michitaka Ohtaki, Cage-like Structure Oxides AAl0.33Te1.67O6 with Unconventional Phonon Scattering, Proceedings of the 16th Cross Straits Symposium on Energy and Environmental Science and Technology, 175-176, 2014.11.
36. Taiga Kuragaki, Michitaka Ohtaki, Performance Stability of ZnO-based Oxide Thermoelectric Materials at High Temperature, Proceedings of the 16th Cross Straits Symposium on Energy and Environmental Science and Technology, 181-182, 2014.11.
37. Li Han, Ngo Van Nong, Wei Zhang, Le Thanh Hung, Tim Holgate, Kazunari Tashiro, Michitaka Ohtaki, Nini Pryds, Søren Linderoth, Effects of morphology on the thermoelectric properties of Al-doped ZnO, RSC Advances, 10.1039/c3ra47617k, 4, 24, 12353-12361, 2014.03, The nanoparticles of Al-doped ZnO were successfully grown into rod-like and platelet-like morphologies by soft chemical routes. These powders were consolidated using spark plasma sintering (SPS) technique. The samples consolidated from rods and platelets exhibited characteristic structures with preferential orientation while the sample consolidated from nanoparticles did not show any sign of preferential orientation. The measured ZT values along the preferred orientation directions were found to be 0.16 and 0.25 at 1223 K for the samples consolidated from rods and platelets, respectively. The sample consolidated from nanoparticles exhibited fine grains and highly distributed nanoprecipitates, resulting in a ZT value of 0.3 at 1223 K due to the lower thermal conductivity resulting from nanostructuring. Using the simple parabolic band model and Debye-Callaway thermal transport model, the anisotropic properties of the nanostructured samples were elucidated and the influence of the grain size and nanoprecipitates on the electron and phonon transport was analyzed and discussed in detail..
38. Li Han, Ngo Van Nong, Wei Zhan, Le Thanh Hung, Tim Holgate, Kazunari Tashiro, Michitaka Ohtaki, Nini Pryds, Søren Linderoth, Effects of morphology on the thermoelectric properties of Al-doped ZnO, RSC Advances, 4, 24, 12353-12361, 2014.02.
39. Shigeru Katsuyama, Koji Akai, Hiroaki Anno, Kazuhiro Hasezaki, Kaoru Kimura, Michitaka Ohtaki, Masatoshi Takeda, Preface, Materials Transactions, 10.2320/matertrans.MPR2014904, 55, 8, 2014.01.
40. Tohru Sugahara, Michitaka Ohtaki, Katsuaki Suganuma, La doped effects on structure and thermoelectric properties of Sr2MnMoO6 double-perovskite oxides, Journal of Asian Ceramic Societies, 1, 3, 282-288, 2013.09.
41. Tohru Sugahara, Michitaka Ohtaki, Katsuaki Suganuma, La doped effects on structure and thermoelectric properties of Sr 2 MnMoO 6 double-perovskite oxides, Journal of Asian Ceramic Societies, 10.1016/j.jascer.2013.06.006, 1, 3, 282-288, 2013.09, The thermoelectric properties of the double-perovskite-type oxide Sr 2 MnMoO 6 were investigated in terms of La doping at the A-site of the oxide. The Bragg reflection of Sr 2-x La x MnMoO 6 was indexed with a monoclinic unit cell and the structure was determined in the monoclinic space group P2 1 /n, due to the good agreement between the observed and calculated patterns after Rietveld refinement. The electrical conductivity of the oxides, σ, showed a semiconductor characteristic, increasing monotonically from 0.1 to 0.5S/cm at room temperature to 5-50S/cm at 1250K. The Seebeck coefficient, S, indicated the oxides to be of n-type. The absolute values of S increased with increasing temperature. These absolute values increased at just x=0.1, and then drastically decreased with increasing x ranging from 0.3 to 1.0. The power factor of the oxides, S 2 σ, increased with increasing temperature; Sr 1.9 La 0.1 MnMoO 6 showed approximately 0.12mW/mK 2 at 1250K, which is the largest value among all the samples examined in this study. The thermal conductivities of the oxides showed a temperature independent behavior, ranging approximately 0.6-1.0W/mK, and the values were significantly low. The dimensionless figure of merit, ZT=S 2 σT/κ, increased with the increasing temperature. The highest ZT value of 0.13 was observed for Sr 1.9 La 0.1 MnMoO 6 at 1100K..
42. Nguyen Viet Long, Cao Minh Thi, Yang Yong, Masayuki Nogami, Michitaka Ohtaki, Platinum and palladium nano-structured catalysts for polymer electrolyte fuel cells and direct methanol fuel cells, Journal of nanoscience and nanotechnology, 10.1166/jnn.2013.7570, 13, 7, 4799-4824, 2013.07, In this review, we present the synthesis and characterization of Pt, Pd, Pt based bimetallic and multi-metallic nanoparticles with mixture, alloy and core-shell structure for nano-catalysis, energy conversion, and fuel cells. Here, Pt and Pd nanoparticles with modified nanostructures can be controllably synthesized via chemistry and physics for their uses as electro-catalysts. The cheap base metal catalysts can be studied in the relationship of crystal structure, size, morphology, shape, and composition for new catalysts with low cost. Thus, Pt based alloy and core-shell catalysts can be prepared with the thin Pt and Pt-Pd shell, which are proposed in low and high temperature proton exchange membrane fuel cells (PEMFCs), and direct methanol fuel cells (DMFCs). We also present the survey of the preparation of Pt and Pd based catalysts for the better catalytic activity, high durability, and stability. The structural transformations, quantum-size effects, and characterization of Pt and Pd based catalysts in the size ranges of 30 nm (1-30 nm) are presented in electro-catalysis. In the size range of 10 nm (1-10 nm), the pure Pt catalyst shows very large surface area for electro-catalysis. To achieve homogeneous size distribution, the shaped synthesis of the polyhedral Pt nanoparticles is presented. The new concept of shaping specific shapes and morphologies in the entire nano-scale from nano to micro, such as polyhedral, cube, octahedra, tetrahedra, bar, rod, and others of the nanoparticles is proposed, especially for noble and cheap metals. The uniform Pt based nanosystems of surface structure, internal structure, shape, and morphology in the nanosized ranges are very crucial to next fuel cells. Finally, the modifications of Pt and Pd based catalysts of alloy, core-shell, and mixture structures lead to find high catalytic activity, durability, and stability for nano-catalysis, energy conversion, fuel cells, especially the next large-scale commercialization of next PEMFCs, and DMFCs..
43. Michitaka Ohtaki, So Miyaishi, Extremely Low Thermal Conductivity in Oxides with Cage-like Crystal Structure
, Journal of Electronic Materials, 10.1007/s11664-012-2382-1, 42, 2013.06.
44. Li Han, Ngo Van Nong, Le Thanh Hung, Nini Pryds, Michitaka Ohtaki, Søren Linderoth, The Influence of α- and γ-Al2O3 Phases on the Thermoelectric Properties of Al-doped ZnO
, Journal of Alloys and Compounds, 555, 291-296, 2013.04.
45. Nguyen Viet Long, Michitaka Ohtaki, Masayoshi Yuasa, Satoshi Yoshida, Taiga Kuragaki, Gao Minh Thi, Masayuki Nogami, Synthesis and Self-Assembly of Gold Nanoparticles by Chemically Modified Polyol Methods under Experimental Control, Journal of Nanomaterials, 2013, 793125, 2013.04.
46. Nguyen Viet Long, Michitaka Ohtaki, Masayoshi Yuasa, Satoshi Yoshida, Taiga Kuragaki, Cao Minh Thi, Masayuki Nogami, Synthesis and self-assembly of gold nanoparticles by chemically modified polyol methods under experimental control, Journal of Nanomaterials, 10.1155/2013/793125, 2013, 2013.04, In our present research, bottom-up self-assembly of gold (Au) nanoparticles on a flat copper (Cu) substrate is performed by a facile method. The very interesting evidence of self-assembly of Au nanoparticles on the top of the thin assembled layer was observed by scanning electron microscopy (SEM). We had discovered one of the most general and simple methods for the self-assembly of metal nanoparticles. The general physical and chemical mechanisms of the evaporation process of the solvents can be used for self-assembly of the as-prepared nanoparticles. The important roles of molecules of the used solvents are very critical to self-assembly of the as-prepared Au nanoparticles in the case without using any polymers for those processes. It is clear that self-assembly of such one nanosystem of the uniform Au nanoparticles is fully examined. Finally, an exciting surface plasmon resonance (SPR) phenomenon of the pure Au nanoparticles in the solvent was fully discovered in their exciting changes of the narrow and large SPR bands according to synthesis time. The SPR was considered as the collective oscillation of valence electrons of the surfaces of the pure Au nanoparticles in the solvent by incident ultraviolet-visible light. Then, the frequency of light photons matches the frequency of the oscillation of surface electrons of the Au nanoparticles that are excited..
47. Li Han, Ngo Van Nong, Le Thanh Hung, Tim Holgate, Nini Pryds, Michitaka Ohtaki, Søren Linderoth, The influence of α- And γ-Al2O3 phases on the thermoelectric properties of Al-doped ZnO, Journal of Alloys and Compounds, 10.1016/j.jallcom.2012.12.091, 555, 291-296, 2013.04, A systematic investigation on the microstructure and thermoelectric properties of Al-doped ZnO using α- and γ-Al2O 3 as dopants was conducted in order to understand the doping effect and its mechanism. The samples were prepared by the spark plasma sintering technique from precursors calcined at various temperatures. Clear differences in microstructure and thermoelectric properties were observed between the samples doped with α- and γ-Al2O3. At any given calcination temperature, γ-Al2O3 resulted in the formation of a larger amount of the ZnAl2O4 phase in the Al-doped ZnO samples. The average grain size was found to be smaller for the γ-Al2O3-doped samples than that for the α-Al2O3-doped ones under the same sintering condition. It is proposed that the ZnAl2O4 phase is the reason for the observed suppression of grain growth and also for the slightly reduced lattice thermal conductivity exhibited by these samples. The γ-Al2O3 promoted the substitution for donor impurities in ZnO, thus resulting in shrinkage of the unit cell volume and an increase in the electrical conductivity compared with the α-Al 2O3-doped ZnO. At a calcination temperature of 1173 K, the γ-Al2O3-doped sample showed a ZT value of 0.17 at 1173 K, which is 27% higher than that of the α-Al2O 3-doped sample..
48. Nguyen Viet Long, Tomokatsu Hayakawa, Takashi Matsubara, Nguyen Duc Chien, Michitaka Ohtaki, Masayuki Nogami, Erratum to Controlled synthesis and properties of palladium nanoparticles ((426-439), 10.1080/17458080.2010.543988), Journal of Experimental Nanoscience, 10.1080/17458080.2012.755868, 7, 6, 711-712, 2012.12.
49. Nguyen Viet Long, Masayuki Nogami, Cao Minh Thi, Michitaka Ohtaki, Experimental Evidences of Crystal Nucleation and Growth of Platinum Nanoparticles with Most Characteristic Roughness Heteromorphologies and Nanostructures from Homogeneous Solution, Journal of Advanced Microscopy Research, 7, 1-20, 2012.09.
50. Nguyen Viet Long, Tomokatsu Hayakawa, Takashi Matsubara, Nguyen Duc Chien, Michitaka Ohtaki, Masayuki Nogami, Controlled synthesis and properties of palladium nanoparticles, Journal of Experimental Nanoscience, 10.1080/17458080.2010.543988, 7, 4, 426-439, 2012.07, In our research, the preparation processes of Pd nanoparticles (Pd NPs) with different methods by adding a certain amount of silver nitrate, iron chloride or sodium iodide are presented in order to control their size and morphology. Various sizes and shapes of Pd NPs are observed by alcohol reduction. The results show the size and morphology control of Pd NPs with chemical reductions of Pd salts by ethanol and ethylene glycol (EG) that have very different reduction processes. The modified polyol method with the gradual addition of Pd and polyvinylpyrollidone precursors in EG at 160°C has led to control their size and morphology in the 10 nm range using 0.02 M AgNO3. It is observed that the Pd nanorod is also formed. The main factors that will control the shapes of Pd NPs have been presented to explain their growth and formation mechanisms in a control process..
51. Nguyen Viet Long, Cao Minh Thi, Masayuki Nogami, Michitaka Ohtaki, Pt and Pd based catalysts with novel alloy and core-shell nanostructures for practical applications in next fuel cells
Patents and highlights, Recent Patents on Materials Science, 5, 3, 175-190, 2012.07, In this review, we have investigated research results of recent patents of various kinds of Pt or Pd based nanoparticles for catalysis. It includes aspects of practical applications of metal, bimetal or multi-metal based nanoparticles in catalysis and fuel cells. The synthetic methods and catalysts engineering are comprehensively presented in their excellent applications for fuel cells. The aims of this review are to provide achievements and highlights of patents of recent applications of Pt or Pd based material catalysts for various fuel cells. In particular, the Pt or Pd based nanoparticles of certain size, shape, structure, composition show great and promising applications in fuel cells and energy issues. The new and modified catalysts associated with the Pt or Pd based nanoparticles can improve future fuel cells with very high and robust performance. Our ideas and proposals of using a very low weight of Pt metal in novel robust and efficiently designed catalysts are one of the best ways for the large-scale commercialization of fuel cells technology. In addition, the characterization and controlled synthesis of metal, bimetal, multi-metal, and multi-component nanoparticles are discussed in potential applications for fuel cells. Finally, we think that greatly novel discoveries in science and technology through succesful synthesis of novel alloy or core-shell nanoparticles and their excellent applications in catalysis, medicine and biology can be clearly predicted..
52. Tohru Sugahara, Teppei Araki, Michitaka Ohtaki, Katsuaki Suganuma, Structure and Thermoelectric Properties of Double-perovskite Oxides: Sr2-xKxFeMoO6, Journal of the Ceramic Society of Japan, 120, 6, 211-216, 2012.06.
53. Nguyen Viet Long, Cao Minh Thi, Masayuki Nogami, Michitaka Ohtaki, Novel Issues of Morphology, Size, and Structure of Pt Nanoparticles in Chemical Engineering: Surface Attachment, Aggregation or Agglomeration, Assembly, and Structural Changes, New Journal of Chemistry, 10.1039/C2NJ40027H, 2012.06.
54. Nguyen Viet Long, Michitaka Ohtaki, Takashi Matsubara, Cao Minh Thi, Masayuki Nogami, New Experimental Evidences of Pt-Pd Bimetallic Nanoparticles with Core-Shell Configuration and Highly Fine-ordered Nanostructures by High-Resolution Electron Transmission Microscopy, The Journal of Physical Chemistry C, 116, 22, 12265-12274, 2012.06, In our facile synthesis method, poly(vinylpyrrolidone) protected Pt and Pt-Pd bimetallic nanoparticles with controllable polyhedral core-shell morphologies are precisely synthesized by the reduction of Pt and Pd precursors at a certain temperature in ethylene glycol and silver nitrate as structure-controlling agent. The Pt nanoparticles exhibited well-shaped polyhedral morphology with highly fine and specific nanostructures in the nanosized range of 20 nm. Important evidences of core-shell configurations of the Pt-Pd core-shell nanoparticles were clearly characterized by high-resolution transmission electron microscopy (HRTEM) measurements. The results of HRTEM images showed that the core-shell Pt-Pd nanoparticles in the nanosized range of 25 nm with polyhedral morphology were synthesized with the thin Pd shells of the nanosized range of about 3 nm in thickness as the atomic Pd layers grown on the as-prepared Pt cores in the nanosized range. The very interesting characterization of surface structure of Pt nanostructures and Pt-Pd core-shell nanostructures with surface defects were observed. The high-resolution TEM images of Pt-Pd bimetallic nanoparticles showed that the Frank-van-der Merwe and Stranski-Krastanov growth modes coexist in the nucleation and growth of the Pd shells on the as-prepared Pt cores. The lattice-fringe spacing was estimated as around 0.240 nm, which is assigned exactly to the lattice-fringe spacing of the {111} planes of the Pd metal shell observed. It is predicted that the FM growth becomes a more favorable growth mode compared to the SK growth in the formation of the very thin Pd shells of Pt-Pd core-shell nanoparticles. The experimental evidences of the deformations of lattice fringes and lattice-fringe patterns were found in polyhedral Pt nanoparticles as well as polyhedral Pt-Pd core-shell nanoparticles. The interesting re-nucleation and re-crystallization at their connections or their attachments of the as-prepared nanoparticles are found in a good lattice match. In addition, our novel ideas of the largest surface-area superlattices and suitable utilization of such large surface areas are proposed for next generations of various fuel cells with low cost. Finally, the products of as-prepared Pt-Pd core-shell nanoparticles can potentially utilized as highly efficient catalysts in the realization of polymer electrolyte membrane cell (PEMFC) and direct methanol fuel cell (DMFC) using the very low Pt loading with better cost effective design..
55. Nguyen Viet Long, Masayuki Nogami, Cao Minh Thi, Michitaka Ohtaki, Experimental evidences of crystal nucleation and growth of platinum nanoparticles with most characteristic roughness heteromorphologies and nanostructures from homogeneous solution, Journal of Advanced Microscopy Research, 10.1166/jamr.2012.1103, 7, 2, 98-117, 2012.06, In our research, Pt nanoparticles by polyol method using AgNO3 were studied by UV-vis-IR spectroscopy, X-ray diffraction analysis, transmission electron microscopy (TEM) and high-resolution (HR) TEM methods. Their specific surfaces were investigated in the controlled size and morphology in the range of 10 nm without AgNO3, and in the range of 20 nm with AgNO 3. Here, AgNO3 is crucial to appearance of polyhedral Pt nanoparticles but their convex or concave roughness characterizations are confirmed in the range of 20 nm without AgNO3. The high crystallization degree of the as-prepared Pt nanoparticles was discovered in the HRTEM evidences. In addition, the interesting complexity of surface roughness showed important evidences of bonding and chemical changes. Re-nucleations and re-crystallizations at their connections by surface attachment were discovered. The results show importance of controlling size and morphology, and big challenge for HRTEM method is predicted, which is how to measure many nanoparticles with high resolution with good quality of fine lattice-fringe systems around 10 nm..
56. Viet Long Nguyen, Michitaka Ohtaki, Takashi Matsubara, Minh Thi Cao, Masayuki Nogami, New experimental evidences of Pt-Pd bimetallic nanoparticles with core-shell configuration and highly fine-ordered nanostructures by high-resolution electron transmission microscopy, Journal of Physical Chemistry C, 10.1021/jp303117y, 116, 22, 12265-12274, 2012.06, In our facile synthesis method, poly(vinylpyrrolidone)-protected Pt and Pt-Pd bimetallic nanoparticles with controllable polyhedral core-shell morphologies are precisely synthesized by the reduction of Pt and Pd precursors at a certain temperature in ethylene glycol with silver nitrate as structure-controlling agent. The Pt nanoparticles exhibited well-shaped polyhedral morphology with highly fine and specific nanostructures in the size range of 20 nm. Important evidences of core-shell configurations of the Pt-Pd core-shell nanoparticles were clearly characterized by high-resolution transmission electron microscopy (HRTEM) measurements. The results of HRTEM images showed that the core-shell Pt-Pd nanoparticles in the size range of 25 nm with polyhedral morphology were synthesized with the thin Pd shells of ∼3 nm in thickness as the atomic Pd layers grown on the Pt cores. Very interesting characteristics of surface structure of Pt nanostructures and Pt-Pd core-shell nanostructures with surface defects were observed. The high-resolution TEM images of Pt-Pd bimetallic nanoparticles showed that the Frank-van der Merwe and Stranski-Krastanov growth modes coexist in the nucleation and growth of the Pd shells on the as-prepared Pt cores. It is predicted that the FM growth becomes the main favorable growth compared with the SK growth in the formation of the thin Pd shells of Pt-Pd core-shell nanoparticles. The experimental evidence of the deformations of lattice fringes and lattice-fringe patterns was found in Pt and Pt-Pd core-shell nanoparticles. The interesting renucleation and recrystallization at the attachments between the nanoparticles are revealed to form a good lattice match. In addition, our novel ideas of the largest surface-area superlattices and promising utilization of them are proposed for next generations of various fuel cells with low cost. Finally, the products of Pt-Pd core-shell nanoparticles can be potentially utilized as highly efficient catalysts in the realization of polymer electrolyte membrane fuel cell and direct methanol fuel cell using the very low Pt loading with better cost-effective design..
57. Nguyen Viet Long, Cao Minh Thi, Masayuki Nogami, Michitaka Ohtaki, Novel issues of morphology, size, and structure of Pt nanoparticles in chemical engineering
Surface attachment, aggregation or agglomeration, assembly, and structural changes, New Journal of Chemistry, 10.1039/c2nj40027h, 36, 6, 1320-1334, 2012.06, In this paper, the polyhedral Pt nanoparticles were prepared by a modified polyol method using AgNO3 as a nanostructure-shaping agent. TEM and HRTEM images of Pt nanoparticles show the particle size in the 10 nm range for the well-controlled case. In contrast, Pt nanoparticles have the particle size in the 50 nm range for the uncontrolled case. To understand the important issues of morphology, size, surface and structure, the as-prepared Pt nanoparticles were investigated through UV-vis spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), and high-resolution (HR)TEM measurements. In the two cases, the as-prepared Pt nanoparticles with and without the control procedures exhibit surface attachment, aggregation or agglomeration and assembly. The mechanisms can lead to the formation of the ultra-porous mesostructure of the as-prepared Pt nanoparticles by using various sophisticated control methods. Therefore, the experimental findings and observations showed the formations of the porous Pt nanostructures as the new Pt textures from self-aggregation or self-agglomeration and self-assembly of extreme importance in designing great superlattices under experimentally chemical and physical methods. This also proved the important role of PVP polymer in the protection of the as-prepared nanoparticles. In particular, a new phenomenon was found in the randomly natural collapse and self-breaking in the Pt nanostructures originating from the naked Pt nanoparticles without protective polymer agents. As a result, a porous meso-nanostructure was formed by the structural changes of Pt nanoparticles without stabilization of the PVP polymer. Finally, the discoveries of surface structure changes of polyhedral Pt shapes and morphologies in future are very important in further catalysis investigation..
58. Tohru Sugahara, Ngo Van Nong, Michitaka Ohtaki, Structure and Thermoelectric Properties of Ca2-xSrxFeMoO6 (0 ≤ x ≤ 0.3) Double-perovskite Oxides, Materials Chemistry and Physics, 133, 2-3, 630-634, 2012.04.
59. Tohru Sugahara, Ngo Van Nong, Michitaka Ohtaki, Structure and thermoelectric properties of Ca 2-xSr xFeMoO 6 (0 ≤ x ≤ 0.3) double-perovskite oxides, Materials Chemistry and Physics, 10.1016/j.matchemphys.2012.01.032, 133, 2-3, 630-634, 2012.04, The thermoelectric properties of double perovskite-type oxides Ca 2FeMoO 6 are investigated in terms of Sr substitution at the A site of the oxides. The electrical conductivity, σ, of Ca 2-xSr xFeMoO 6 (0 ≤ x ≤ 0.3) showed a metallic behavior, decreasing monotonically from ca. 10 3 S cm -1 at room temperature to ca. 10 2 S cm -1 at 1250 K. At room temperature, although the σ values of the oxides increased with increasing substitution level, x, the σ values maintained almost the same values at high temperature range of 1000-1250 K. The absolute values of the Seebeck coefficient, S, for the samples at x < 0.3 abruptly increase at around 1000 K. The Rietveld refinement of the XRD patterns of the oxides indicated that the anti-site defects in the oxides decreased with increasing Sr concentration. The power factor, S 2σ, of the oxides largely increased with increasing temperature; the S 2σ value of Ca 2FeMoO 6 was ca. 0.35 mW mK -2 at 1200 K, the largest value of all the samples in this study. Although the thermal conductivity, κ, of the oxides generally decreased from ca. 3.5 to 4.5 W mK -1 at room temperature to ca. 2-3 W mK -1 at 1050 K, the sample at x = 0.3 showed the lowest and most T-independent κ values, implying that the relative increase in the temperature is independent from the phonon-impurity scattering caused by the A-site substitution. The power factor of the oxides increased above 900 K; thereby, the dimensionless figure of merit, ZT = (S 2σ/κ)T, significantly increased at the same temperature range. The largest ZT value of 0.15 was observed for Ca 2FeMoO 6, Ca 1.9Sr 0.1FeMoO 6, and Ca 1.8Sr 0.2FeMoO 6 at 1250 K..
60. Nguyen Viet Long, Nguyen Duc Chien, Tomokatsu Hayakawa, Takashi Matsubara, Michitaka Ohtaki, Masayuki Nogami, Sharp Cubic and Octahedral Morphologies of Poly(vinylpyrrolidone)-stabilised Platinum Nanoparticles by Polyol Method in Ethylene Glycol: Their Nucleation, Growth and Formation Mechanisms, Journal of Experimental Nanoscience, 7, 2, 133-149, 2012.03.
61. Nguyen Viet Long, Nguyen Duc Chien, Tomokatsu Hayakawa, Takashi Matsubara, Michitaka Ohtaki, Masayuki Nogami, Sharp cubic and octahedral morphologies of poly(vinylpyrrolidone)-stabilised platinum nanoparticles by polyol method in ethylene glycol
Their nucleation, growth and formation mechanisms, Journal of Experimental Nanoscience, 10.1080/17458080.2010.513017, 7, 2, 133-149, 2012.03, Pt nanoparticles (NPs) were synthesised by a modified polyol method with the addition of silver nitrate. The results showed that the specific shapes of Pt NPs were influenced by the relevant factors, which are the contents of silver nitrate, synthetic time and temperature. A small content of silver nitrate has played an important role in determining their final shapes of platinum NPs. We observed that Pt NPs in the forms of very sharp shapes such as Pt cubes, octahedrons, cuboctahedrons and tetrahedrons have been obtained. In addition, the shape growth mechanisms and formation of Pt NPs have been studied. They exist in both cubic and octahedral shapes. Importantly, Pt nanocrystals can grow into main cubic and octahedral shapes for a short time less than 15 min. Moreover, Pt nanocrystals can also grow into different shapes from cubic and octahedral into spherical ones for several hours. Especially, they exhibited interesting shapes of multiple-branched Pt nanostructures because of their overgrowth and aggregations. Clearly, large cubic and octahedral Pt NPs of 160 nm diameter were observed. The growth and formation of large cubic and octahedral Pt NPs were due to the aggregation of Pt clusters or initial Pt seeds, even small Pt nanocrystals..
62. Nguyen Viet Long, Michitaka Ohtaki, Van Nong Ngo, Minh Thi Cao, Masayuki Nogami, Structure and morphology of platinum nanoparticles with critical new issues of low- and high-index facets, Advances in Natural Sciences: Nanoscience and Nanotechnology, 10.1088/2043-6262/3/2/025005, 3, 2, 2012.01, In this paper, Pt nanoparticles were successfully prepared by modified polyol method silver nitrate as an effective structure-modifying agent. The characterization of Pt nanoparticles was investigated by using UV-Vis-NIR spectroscopy, transmission electron microscopy (TEM) and high resolution (HR) TEM, and x-ray diffraction (XRD). The method of selected area electron diffraction (SEAD) was used to study the structure of Pt nanoparticles. The results showed that the as-prepared Pt nanoparticles exhibiting the complexity of surface structure and morphology could be used as efficient catalysts for polymer electrolyte membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs)..
63. Tohru Sugahara, Teppei Araki, Michitaka Ohtaki, Katsuaki Suganuma, Structure and thermoelectric properties of double-perovskite oxides
Sr2-xKxFeMoO6, Journal of the Ceramic Society of Japan, 10.2109/jcersj2.120.211, 120, 1402, 211-216, 2012.01, The thermoelectric properties of the double perovskite-type oxides Sr 2-xKxFeMoO6 were investigated in terms of K-doping at the A site of the oxides. The electrical conductivity, of the oxides showed a metallic behavior, decreasing monotonically from ca. 103 S/cm at room temperature to ca. 102 S/cm at 1250 K. The Seebeck coefficient, S, showed that the oxides are of n-type over the measured temperature range. The absolute value of S increased with increasing temperature. The absolute value of S at 300K increased up to x = 0.2 and then decreased dramatically for x = 0.304. The Rietveld refinement of the XRD patterns of the oxides suggested that the increase in orbital degeneracy due to the eased distortion of FeO 6 and MoO6 octahedra at x = 0.2 was responsible for the S maximum at this composition. The power factor, S2, of the oxides increased sharply with increasing K-doping level; the value for Sr 1.6K0.4FeMoO6 was ca. 4.2 × 10 -4W/mK2, which is the highest among all the samples in this study. The thermal conductivity, of the oxides generally decreased from ca. 35W/ mK at room temperature to ca. 24W/ mK at 1100 K. Because the power factor of the oxides increased above 900 K, the dimensionless figure of merit, ZT = S2T/, increased dramatically above 900 K. The largest ZT value of 0.24 was observed for Sr1.6K0.4FeMoO6 at 1250 K..
64. Michitaka Ohtaki, Kazuhiko Araki, Thermoelectric Properties and Thermopower Enhancement of Al-doped ZnO with Nanosized Pore Structure, Journal of the Ceramic Society of Japan, 119, 11, 813-816, 2011.11.
65. Michitaka Ohtaki, Recent aspects of oxide thermoelectric materials for power generation from mid-to-high temperature heat source, Journal of the Ceramic Society of Japan, 10.2109/jcersj2.119.770, 119, 1395, 770-775, 2011.11, Rapid progress in thermoelectric performance of oxide materials has been conducted virtually exclusively in Japan, resulting in more than 10 times increase in the ZT values of oxides within the last two decades. This has caused a revolutionary change in the guiding principles of thermoelectric materials research, in which oxide materials had been disregarded as a potential candidate until early 1990s. Promising oxide thermoelectric materials having been discovered include CaMnO3-based perovskites, Al-doped ZnO, layered cobalt oxides represented by NaCo2O4 and Ca 3Co4O9, and SrTiO3-related phases. This paper reviews the current aspects of oxide thermoelectric materials, and some strategies of nanostructure control for selective reduction of the lattice thermal conductivity (selective phonon scattering) in bulk oxide ceramics will also be discussed..
66. Michitaka Ohtaki, Kazuhiko Araki, Thermoelectric properties and thermopower enhancement of Al-doped ZnO with nanosized pore structure, Journal of the Ceramic Society of Japan, 10.2109/jcersj2.119.813, 119, 1395, 813-816, 2011.11, Thermoelectric properties of Al-doped ZnO (Zn0.98Al 0.02O) with nanosized pore (nanovoid) structure were investigated. Nanovoids were formed by using monodisperse polymethylmethacrylate (PMMA) particles of 150, 425, and 1800nm in average diameter as a void forming agent (VFA). Whereas the thermal conductivity of the samples sintered with the 150nm PMMA particles of 510 wt% was efficiently suppressed, the magnitude of the suppression was almost the same as that of the electrical conductivity, revealing that the selective phonon scattering was failed in these samples. However, being helped by the contribution from an enhancement in the Seebeck coefficient, the samples sintered with 510 wt% of 150nm VFA successfully attained the ZT values of 0.55-0.57 at 1000°C. Furthermore, the reduction of the thermal conductivity with increasing porosity was much steeper than ordinary porous materials, suggesting significantly stronger phonon scattering by the nanosized pore structure..
67. Nguyen Viet Long, Michitaka Ohtaki, Tong Duy Hien, Jalem Randy, Masayuki Nogami, A Comparative Study of Pt and Pt-Pd Core-shell Nanocatalysts, Electrochimica Acta, 56, 25, 9133-9143, 2011.10.
68. Nguyen Viet Long, Michitaka Ohtaki, Tong Duy Hien, Jalem Randy, Masayuki Nogami, Synthesis and Characterization of Polyhedral and Quasi-sphere Non-polyhedral Pt Nanoparticles: Effects of Their Various Surface Morphologies and Sizes on Electrocatalytic Activity for Fuel Cell Applications, Journal of Nanoparticle Research, 13, 10, 5177-5191, 2011.10.
69. Nguyen Viet Long, Michitaka Ohtaki, Tong Duy Hien, Jalem Randy, Masayuki Nogami, A comparative study of Pt and Pt-Pd core-shell nanocatalysts, Electrochimica Acta, 10.1016/j.electacta.2011.07.090, 56, 25, 9133-9143, 2011.10, This comparative study characterizes two types of metallic and core-shell bimetallic nanoparticles prepared with our modified polyol method. These nanoparticles consist of Pt and Pt-Pd core-shell nanocatalysts exhibiting polyhedral morphologies. The controlled syntheses of Pt metallic nanoparticles in the 10-nm regime (4-8 nm) and Pt-Pd bimetallic core-shell nanoparticles in the 30-nm regime (15-25 nm) are presented. To realize our ultimate research goals for proton exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs), we thoroughly investigate the dependence of the electrocatalytic properties of the nanoparticles on the structure, size and morphology. Significant differences in the electrocatalysis are also explained in experimental evidences of both Pt and Pt-Pd nanocatalysts. We suggested that the core-shell controlled morphologies and nanostructures of the Pd nanoshell as the Pd atomic monolayers will not only play an important role in producing inexpensive, novel Pt- and Pd-based nanocatalysts but also in designing more efficient Pt- and Pd-based nanocatalysts for practical use in DMFC technology. Our comparative results show that Pt-Pd nanocatalysts with Pd nanoshells exhibited much better electrocatalytic activity and stabilization compared to Pt nanocatalysts. Interestingly, we found that the size effect is not as strong as the nanostructuring effect on the catalytic properties of the researched nanoparticles. A nanostructure effect of the core-shell bimetallic nanoparticles was identified..
70. Nguyen Viet Long, Michitaka Ohtaki, Tong Duy Hien, Randy Jalem, Masayuki Nogami, Synthesis and characterization of polyhedral and quasi-sphere non-polyhedral Pt nanoparticles
Effects of their various surface morphologies and sizes on electrocatalytic activity for fuel cell applications, Journal of Nanoparticle Research, 10.1007/s11051-011-0503-z, 13, 10, 5177-5191, 2011.10, In this article, polyhedral and non-polyhedral Pt nanoparticles were prepared by modified polyol method using AgNO3 as a good structure-modifying agent. Their TEM and HRTEM images showed the particle size in the range of 8-16 nm for both the above cases. The structures and properties of the surfaces of Pt nanoparticles were investigated through cyclic voltammetry in dilute perchloric acid (HClO4) electrolyte solution. A comparison of the electrocatalytic property in methanol electrooxidation was made. Here, the effects of polyhedral and non-polyhedral morphologies on their catalytic properties were studied. The results revealed that the special catalytic activity of quasi-sphere non-polyhedral Pt nanoparticles is higher than that of polyhedral Pt nanoparticles. In addition, Pt nanoparticles of un-sharp and quasi-sphere morphologies exhibit the tolerance to poisoning species better than that of Pt nanoparticles of sharp and polyhedral morphologies due to the various morphologies of the catalyst surfaces in the chronoamperometric plots. Therefore, these experimental evidences showed the morphology-dependent catalytic property according to the various morphologies and complexity of their catalyst surfaces..
71. Tohru Sugahara, Michitaka Ohtaki, Structural and Semiconductor-to-metal Transitions of Double-perovskite Cobalt Oxide Sr2-xLaxCoTiO6-d with Enhanced Thermoelectric Capability, Applied Physics Letters, 99, 6, 062107, 2011.08.
72. Nguyen Viet Long, Michitaka Ohtaki, Masayuki Nogami, Tong Duy Hien, Effects of Heat Treatment and Poly(vinylpyrrolidone) (PVP) Polymer on Electrocatalytic Activity of Polyhedral Pt Nanoparticles towards their Methanol Oxidation, Colloid and Polymer Science, 289, 12, 1373-1386, 2011.08.
73. Nguyen Viet Long, Michitaka Ohtaki, Masayuki Nogami, Tong Duy Hien, Effects of heat treatment and poly(vinylpyrrolidone) (PVP) polymer on electrocatalytic activity of polyhedral Pt nanoparticles towards their methanol oxidation, Colloid and Polymer Science, 10.1007/s00396-011-2460-6, 289, 12, 1373-1386, 2011.08, In this paper, the polyhedral Pt nanoparticles under control were prepared by polyol method using AgNO3 and poly(vinylpyrrolidone) (PVP) in the reduction of H2PtCl6 with ethylene glycol (EG). Transmission electron microscopy (TEM) and high resolution (HR) TEM measurements were used to investigate their characterization. In the case of the previous removal of PVP by washing and heating at 300°C, the specific morphologies of polyhedral Pt nanoparticles were still observed. However, the removal of PVP only by heat treatment at 300°C without washing causes the significant variation of their morphology. The large Pt particles were observed in the self-aggregation and assembly of the as-prepared polyhedral Pt nanoparticles. The pure Pt nanoparticles by washing and heat treatment showed the electrocatalytic property better than PVP-Pt nanoparticles by heat treatment due to the incomplete removal of PVP and by-products from AgNO3. Therefore, the removal modes of PVP without changing their characterization are required to obtain the good catalytic performance..
74. Tohru Sugahara, Michitaka Ohtaki, Structural and semiconductor-to-metal transitions of double-perovskite cobalt oxide Sr 2-xLa xCoTiO 6-δ with enhanced thermoelectric capability, Applied Physics Letters, 10.1063/1.3623476, 99, 6, 2011.08, The thermoelectric properties of double-perovskite oxide Sr 2-xLa xCoTiO 6-δ were revealed to vary anomalously with the La concentration, plausibly due to a structural transition found in this study. Although the temperature dependence of the resistivity and thermopower of the present oxide showed a semiconductor-to-metal transition similar to those observed for other perovskite-related Co oxides such as Sr 1-xY xCoO 3-δ, the transition temperature was more than 350 K higher, implying considerable stabilization of the low-spin state of Co ions in the double-perovskite oxide. Consequently, the operating temperature range of the oxide for potential thermoelectric applications was significantly expanded toward higher temperatures..
75. Nguyen Viet Long, Tong Dui Hien, Toru Asaka, Michitaka Ohtaki, Masayuki Nogami, Synthesis and Characterization of Pt-Pd Nanoparticles with Core-shell morphology: Nucleation and Overgrowth of the Pd Shells on the as-prepared and Defined Pt Seeds, Journal of Alloys and Compounds, 509, 29, 7702-7709, 2011.07.
76. Nguyen Viet Long, Masaya Uchida, Jalem Randy, Hirohito Hirata, Nguyen Duc Chien, Michitaka Ohtaki, Masayuki Nogami, Synthesis and Characterization of Pt-Pd Alloy and Core-shell Bimetallic Nanoparticles for Direct Methanol Fuel Cells (DMFCs): Enhanced Electrocatalytic Properties of Well-shaped Core-shell morphologies and Nanostructures, International Journal of Hydrogen Energy, 36, 14, 8478-8491, 2011.07.
77. Nguyen Viet Long, Michitaka Ohtaki, Masaya Uchida, Randy Jalem, Hirohito Hirata, Nguyen Duc Chien, Masayuki Nogami, Synthesis and Characterization of Polyhedral Pt Nanoparticles: Their Catalytic Property, Surface Attachments, Self-aggregation and Assembly, Journal of Colloid and Interface Science, 359, 2, 339-350, 2011.07.
78. Nguyen Viet Long, Tong Duy Hien, Toru Asaka, Michitaka Ohtaki, Masayuki Nogami, Synthesis and characterization of Pt-Pd nanoparticles with core-shell morphology
Nucleation and overgrowth of the Pd shells on the as-prepared and defined Pt seeds, Journal of Alloys and Compounds, 10.1016/j.jallcom.2011.04.031, 509, 29, 7702-7709, 2011.07, In the present research, Pt-Pd core-shell nanoparticles based on the as-prepared and defined Pt-seed cores with well-controlled size and morphology were synthesized. Their characterizations were investigated by using UV-vis spectroscopy, transmission electron microscopy (TEM), and high resolution (HR)TEM measurements. The high resolution elemental mappings were performed in the operation of high angle annular dark field (HAADF) in conjunction with scanning (S)TEM mode and X-ray energy dispersive spectroscopy (XEDS). It is found that not only the Pt-Pd core-shell nanoparticles were formed, but also the nucleation, growth, and the separate formation of single Pd nanoparticles as well. Interestingly, the as-prepared Pt cores without the morphological changes were protected by the overgrowths of the Pd shells during the successive reduction of sodium tetrachloropalladate (II) hydrate. There were the co-existence of the Frank-van der Merwe (FM) layer-by-layer and Stranski-Krastanov (SK) island-on-wetting-layer growth modes of the Pd shells on the as-prepared Pt cores. It is predicted that the SK growth became the favorable growth mode in the formation of the Pd shells in the formation Pt-Pd core-shell nanoparticles..
79. Nguyen Viet Long, Michitaka Ohtaki, Masaya Uchida, Randy Jalem, Hirohito Hirata, Nguyen Duc Chien, Masayuki Nogami, Synthesis and characterization of polyhedral Pt nanoparticles
Their catalytic property, surface attachment, self-aggregation and assembly, Journal of Colloid And Interface Science, 10.1016/j.jcis.2011.03.029, 359, 2, 339-350, 2011.07, In this paper, we presented the preparation procedure of Pt nanoparticles with the well-controlled polyhedral morphology and size by a modified polyol method using AgNO3 in accordance with the reduction of H2PtCl6 in EG at high temperature around 160°C. The methods of UV-vis spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), and high resolution (HR) TEM measurements were used to characterize their surface morphology, size, and crystal structure. We have observed that the polyhedral Pt nanoparticles of sharp edges and corners were produced in the preferential homogenous growth as well as the formation of porous and large Pt particles by self-aggregation and assembly originating from as-prepared polyhedral Pt nanoparticles. It is most impressive to find that the arrangement of Pt nanoparticles was observed in their surface attachments, self-aggregation, random and directed surface self-assembly by the bottom-up approach. Their high electrocatalytic activity for methanol oxidation was predicted. The findings and results showed that the polyhedral Pt nanoparticle-based catalysts exhibited the high electrocatalytic activity for their potential applications in developing the efficient Pt-based catalysts for direct methanol fuel cells..
80. Nguyen Viet Long, Tong Duy Hien, Toru Asaka, Michitaka Ohtaki, Masayuki Nogami, Synthesis and characterization of Pt-Pd alloy and core-shell bimetallic nanoparticles for direct methanol fuel cells (DMFCs)
Enhanced electrocatalytic properties of well-shaped core-shell morphologies and nanostructures, International Journal of Hydrogen Energy, 10.1016/j.ijhydene.2011.03.140, 36, 14, 8478-8491, 2011.07, In our research, we present the controlled synthesis of poly(vinylpyrrolidone) (PVP) protected Pt-Pd nanoparticles of various alloy and core-shell morphologies by modified polyol method with the assistance of AgNO3. The Pt-Pd alloy and core-shell nanoparticles were characterized by transmission electron microscopy (TEM), high-resolution TEM, and electrochemical measurements. The comparison of electrocatalytic properties of Pd-Pt bimetallic nanoparticles was described to confirm their highest catalytic performance. Importantly, the catalytic activity of Pt-Pd alloy and core-shell nanoparticles was investigated to develop novel electrocatalysts in direct methanol fuel cells (DMFCs). The results showed that the core-shell nanoparticles with the thin nanoshells as monolayers exhibit as great nanocatalysts. The correlation among structure, size and morphology was presented in their catalytic characterization..
81. Ngo Van Nong, Nini Pryds, Soren Linderoth, Michitaka Ohtaki, Enhancement of the Thermoelectric Performance of p-Type Layered Oxide Ca3Co4O9+d Through Heavy Doping and Metallic Nanoinclusions, Advanced Materials, 23, 21, 2484-2490, 2011.06.
82. Ngo Van Nong, Nini Pryds, Søren Linderoth, Michitaka Ohtaki, Enhancement of the thermoelectric performance of p-type layered oxide Ca3Co4O9+δ through heavy doping and metallic nanoinclusions, Advanced Materials, 10.1002/adma.201004782, 23, 21, 2484-2490, 2011.06, An effective way to improve the thermoelectric performance (ZT) of layered structured oxide materials by carefully choosing heavy ion doping and introducing metallic nanoinclusions is proposed. A p-type oxide material with remarkable highly improved ZT is successfully fabricated using this approach. Long-term durability at high temperature testing confirms this material is a very promising p-type material for high temperature power generation..
83. Ngo Van Nong, Syun-ichi. Yanagiya, Sonne Monica, Nini Pryds, Michitaka Ohtaki, High-Temperature Thermoelectric and Microstructural Characteristics of Cobalt-Based Oxides with Ga Substituted on the Co-Site, Journal of Electronic Materials, 40, 5, 716-722, 2011.05.
84. N. V. Nong, S. Yanagiya, S. Monica, N. Pryds, M. Ohtaki, High-temperature thermoelectric and microstructural characteristics of cobalt-based oxides with ga substituted on the Co-site, Journal of Electronic Materials, 10.1007/s11664-011-1524-1, 40, 5, 716-722, 2011.05, The effects of Ga substitution on the Co-site on the high-temperature thermoelectric properties and microstructure are investigated for the misfitlayered Ca 3Co 4O 9 and the complex perovskite-related Sr 3RECo 4O 10.5 (RE = rare earth) cobalt-based oxides. For both systems, substitution of Ga for Co results in a simultaneous increase in the Seebeck coefficient (S) and the electrical conductivity (σ), and the influence is more significant in the high temperature region. The power factor (S 2 σ) is thereby remarkably improved by Ga substitution, particularly at high temperatures. Texture factor calculations using x-ray diffraction pattern data for pressed and powder samples reveal that the Ga-doped samples are highly textured. Microstructure observed by scanning electron microscopy shows very well-crystallized grains for the samples with Ga substitution for Co. Among the Ga-doped samples, Ca 3Co 3.95Ga 0.05O 9 shows the best ZT value of 0.45 at 1200 K, which is about 87.5% higher than the nondoped one, a considerable improvement..
85. Nguyen Viet Long, Nguyen Duc Chien, Hirohito Hirata, Takashi Matsubara, Michitaka Ohtaki, Masayuki Nogami, Highly monodisperse cubic and octahedral rhodium nanocrystals
Their evolutions from sharp polyhedrons into branched nanostructures and surface-enhanced Raman scattering, Journal of Crystal Growth, 10.1016/j.jcrysgro.2011.02.006, 320, 1, 78-89, 2011.04, We report the preparation of poly(vinylpyrrolidone) (PVP)-protected Rh nanoparticles by modified polyol method with the reduction of RhCl3 in ethylene glycol at 160 °C using AgNO3. This method is used to synthesize highly cubic and octahedral Rh nanoparticles with their well-controlled size ∼714 nm characterized by transmission electron microscopy (TEM). At a low AgNO3 concentration, the appearance of the sharply good morphology of Rh nanoparticles due to their controlled growth of {1 0 0} and {1 1 1} selective surfaces for ∼1015 min is observed. Their morphology between cubic and octahedral shape exhibits their competitive growth along the [1 1 1] and [1 0 0] directions. Interestingly, sharply and highly branched Rh nanoparticles with their shapes of very sharp corners or sharp-tip arms growing in the [1 1 1] directions are synthesized for ∼30 min at the same concentration of RhCl3 and AgNO3 precursors. However, at a higher concentration of AgNO3, the assemblies of Rh multipods leading to large porous Rh nanoparticles are observed. The problems of reaction time and temperature are discussed to clarify the nucleation and growth of Rh nanoparticles. In addition, monolayers of Rh nanoparticles on SERS substrates using a probe as malachite green (MG) molecule exhibit the high sensitivity for their potential applications in chemical and biochemical sensing..
86. Nguyen V. Long, Michitaka Ohtaki, Masayuki Nogami, Control of Morphology of Pt Nanoparticles and Pt-Pd Core-Shell Nanoparticles, Journal of Novel Carbon Resource Sciences, 3, 40-44, 2011.02.
87. N. V. Nong, C.-J. Liu, M. Ohtaki, High Temperature Thermoelectric Properties of Late Rare Earth-doped Ca3Co4-xGaxO9+d
, J. Alloy Compd., 509, 977-981, 2011.01.
88. N. V. Nong, Chia Jyi Liu, M. Ohtaki, High-temperature thermoelectric properties of late rare earth-doped Ca 3Co4O9+δ, Journal of Alloys and Compounds, 10.1016/j.jallcom.2010.09.150, 509, 3, 977-981, 2011.01, Misfit-layered oxides Ca3-xLnxCo4O 9+δ with Ln = Dy, Er, Ho, and Lu were synthesized using solid state reactions. The resulting samples were hot-pressed (HP) at 1123 K in air for 2 h under a uniaxial pressure of 60 MPa. Thermoelectric properties of Ca3-xLnxCo4O9+δ were investigated up to 1200 K. Both the Seebeck coefficient and electrical resistivity increase upon Ln substitution for Ca. Among the Ln-doped samples, the magnitude of Seebeck coefficient tends to increase with decreasing ionic radius of Ln3+. The Ln-doped samples exhibit a lower thermal conductivity than the non-doped one due to a decrease of their lattice thermal conductivity. The dimensionless figure of merit, ZT, reaches 0.36 at 1073 K for the Ca2.8Lu0.2Co4O9+δ sample, which is about 1.6 times larger than that for the non-doped counterpart..
89. Oxide thermoelectric materials for heat-to-electricity direct energy conversion.
90. N. V. Nong, Chia Jyi Liu, Michitaka Ohtaki, Improvement on the high temperature thermoelectric performance of Ga-doped misfit-layered Ca3Co4-xGaxO9+δ (x = 0, 0.05, 0.1, and 0.2), Journal of Alloys and Compounds, 10.1016/j.jallcom.2009.11.009, 491, 1-2, 53-56, 2010.02, Highly densified Ca3Co4-xGaxO9+δ (0 ≤ x ≤ 0.2) misfit-layered thermoelectric oxides are prepared by solid state reaction methods followed by hot-pressing. Thermoelectric properties of the samples are measured from room temperature to 1200 K. The results show that partial Ga substitution leads to a simultaneous increase of the electrical conductivity and thermopower. The Ga-doped samples have lower thermal conductivity than that of the non-doped sample in the high temperature region (>600 K). The x = 0.05 sample shows a higher figure of merit (Z = 3.37 × 10-4 K-1) than that of the non-doped sample (Z = 1.98 × 10-4 K-1) at 1073 K, indicating significant improvement of the thermoelectric performance of Ca3Co4O9+δ by partial Ga substitution for Co..
91. N. V. Nong, C.-J. Liu, M. Ohtaki, Improvement on the High Temperature Thermoelectric Performance of Ga-doped Misfit-layered Ca3Co4-xGaxO9+d (x = 0, 0.05, 0.1, and 0.2), J. Alloy Compd., 491, 1-2, 53-56, 2010.01.
92. Nguyen Viet Long, Duc Chien Nguyen, Hirohito Hirata, Michitaka Ohtaki, Tomokatsu Hayakawa, Masayuki Nogami, Chemical synthesis and characterization of palladium nanoparticles, Advances in Natural Sciences: Nanoscience and Nanotechnology, 10.1088/2043-6262/1/3/035012, 1, 3, 2010.01, This work presents the results of the successful preparation of Pd nanoparticles by the polyol method and the proposed techniques of controlling their size and shape. Polyvinylpyrrolidone (PVP) stabilized Pd nanoparticles of various shapes with the largest sizes in the forms of octahedrons (24 nm), tetrahedrons (22 nm) and cubes (20 nm) have been obtained by alcohol reduction in ethanol with the addition of a hydrochloric acid catalyst. Moreover, PVP-Pd nanoparticles of well-controlled spherical shapes have also been prepared by a modified polyol method. PVP-Pd nanoparticles of cubic, octahedral, tetrahedral and spherical shapes with well-controlled size achieved by using ethylene glycol (EG) as reductant and various inorganic species were also fabricated. In particular, Pd nanorods with sizes of 47 nm and 16 nm formed due to the anisotropic growth mechanism of Pd nanoparticles were found. At the same time, tetrahedral particles of sharp shapes of 120 nm and 70 nm sizes have been observed. A high concentration of inorganic species was used to control the size and shape of the Pd nanoparticles, leading to the appearance of various irregular sizes and shapes. There was evidence of the very sharp corners and edges of tetrahedral and octahedral Pd nanoparticles or others that were formed in the clustering and combination of the seeds of smaller particles..
93. S. Teraoka, T. Sugahara, M. Ohtaki, Anomalous Reduction in the Thermal Conductivity of Perovskite-type Oxide on a Structural Phase Transition between the Perovskite and Brownmillerite Structures, Proc. 11th Cross Straits Symp. Mater. Ener. Environ. Sci., 91-92, 2009.11.
94. M. Mori, M. Ohtaki, Molecular Assembly-assisted Synthesis and Photocatalytic Activity of Lamellar Sodium Tantalate, Proc. 11th Cross Straits Symp. Mater. Ener. Environ. Sci., 87-88, 2009.11.
95. K. Yamamoto, M. Ohtaki, Thermoelectric Properties of co-doped ZnO Oxides for Heat-to-Electricity Direct Conversion, Proc. 11th Cross Straits Symp. Mater. Ener. Environ. Sci., 83-84, 2009.11.
96. G. Yamamoto, M. Ohtaki, Synthesis and Thermoelectric Properties of Nanoporous Al-doped ZnO, Proc. 11th Cross Straits Symp. Mater. Ener. Environ. Sci., 79-80, 2009.11.
97. Michitaka Ohtaki, Kazuhiko Araki, Kiyoshi Yamamoto, High thermoelectric performance of dually doped ZnO ceramics, Journal of Electronic Materials, 10.1007/s11664-009-0816-1, 38, 7, 1234-1238, 2009.07, A marked improvement in the thermoelectric performance of dense ZnO ceramics is achieved by employing a third element as a co-dopant with Al. Dual doping of ZnO with Al and Ga results in a drastic decrease in the thermal conductivity of the oxide, while the decrease in the electrical conductivity is relatively small. With the aid of a significant enhancement in the thermopower, the dually doped oxide shows thermoelectric figure of merit values, ZT, values of 0.47 at 1000 K and 0.65 at 1247 K at the composition Zn 0.96Al 0.02Ga 0.02O. These results appear to be the highest ZT values so far reported for bulk n-type oxides. Microscopic observation of the samples reveals a granular texture in the densely sintered oxide matrix, suggesting that considerable reduction of the thermal conductivity while maintaining high electrical conductivity could be achieved by such a bulk nanocomposite structure in the samples..
98. Kaoru Kimura, Ken Ichi Takagi, Takeshi Koyanagi, Michitaka Ohtaki, Hidero Unuma, Shigeru Katsuyama, Kouji Akai, Masatoshi Takeda, Special issue on thermoelectric conversion materials V
Preface, Materials Transactions, 50, 7, 1581, 2009.07.
99. M. Ohtaki, K. Araki, K. Yamamoto, High Thermoelectric Performance of Dually Doped ZnO Ceramics, Journal of Electronic Materials, 38, 1234-1238, 2009.06.
100. T. Sugahara, M. Ohtaki, T. Souma, Thermoelectric Properties of Double-perovskite Oxide Sr2-xMxFeMoO6 (M = Ba, La), J. Ceram. Soc. Jpn., 116 (12), 1278-1282, 2008.12.
101. Tohra Sugahara, Michitaka Ohtaki, Takeshi Souma, Thermoelectric properties of double-perovskite oxide Sr2-xM xFeMoO6 [M = Ba, La), Journal of the Ceramic Society of Japan, 10.2109/jcersj2.116.1278, 116, 1360, 1278-1282, 2008.12, Doping effects for the A-site in double-perovskite type oxide Sr 2-xMxFeMoO6 (M = Ba, La) are investigated on the thermoelectric properties of the oxides. The Seebeck coefficient, S, shows that all the samples of both Ba- and La-doped Sr2FeMoO6 systems are n-type over the measured temperature range. In particular, the power factor, S2 σ, of the Sr2FeMoO6 system largely increased with increasing Ba doping level up to x = 0.3. As a consequence, S2 σ of Sr1.7 Bao 0.3FeMoO6 reached ca. 0.8 × 10-4 W/mK2, the largest value of all the samples in this study. The thermal conductivity, κ, shows a general trend in which the value decreases from ca. 1 W/mK at room temperature to ca. 0.7 W/mK at 1100 K. The dimensionless figure of merit, ZT, of the Badoped samples increased with increasing Ba doping level. Moreover, the ZT value drastically increased above 700 K due to the S2 σ value increasing above 700 K and the κ value decreasing monotonically. As a result, the ZT value of Sr 1.7Ba0.3FeMoO6 reached 0.31 at 1100 K..
102. Kaoru Kimura, Ken Ichi Takagi, Takeshi Koyanagi, Michitaka Ohtaki, Hidero Unuma, Shigeru Katsuyama, Kouji Akai, Masatoshi Takeda, Special issue on thermoelectric conversion materials IV
Preface, Materials Transactions, 49, 8, 2008.08.
103. T. Souma, D. Isobe, M. Ohtaki, Synthesis and Rietveld Analysis of New Thermoelectric Oxides F-doped Na1.6Co2O4, Trans. Mater. Res. Soc. Jpn., 33 (4), 897-901, 2008.06.
104. Thermoelectric Properties of Double-perovskite Oxide A2 B'MoO6 with A-site Substitution.
105. Nanostructure Control and Thermoelectric Properties of Metal Oxide Ceramics.
106. M. Ohtaki, R. Hayashi, K. Araki, Thermoelectric Properties of Sintered ZnO Incorporating Nanovid Structure: Influence of the Size and Number Density of Nanovoids, Proc. 26th Int. Conf. Thermoelectrics, pp. 112-116, 2008.04.
107. T. Souma, M. Ohtaki, K. Ohnishi, M. Shigeno , Y. Ohba, N. Nakamura, T. Shimozaki, Power Generation Characteristics of Oxide Thermoelectric Modules Incorporating Nanostructured ZnO Sintered Materials, Proc. 26th Int. Conf. Thermoelectrics, pp. 38-41, 2008.04.
108. Takeshi Souma, Michitaka Ohtaki, Kyohei Ohnishi, Masayuki Shigeno, Yasuhiro Ohba, Toshitada Shimozaki, Power generation characteristics of oxide thermoelectric modules incorporating nanostructured ZnO sintered materials, ICT'07 - 26th International Conference on Thermoelectrics Proceedings ICT'07 - 26th International Conference on Thermoelectrics, 10.1109/ICT.2007.4569418, 38-41, 2007.12, A practical oxide thermoelectric module consisting of 12 pairs of p-NaCo2O4 and nanostructured n-ZnO sintered materials was successfully fabricated by using a diffusion welding technique. By applying a nanostructure using a void forming agency (VFA), the electrical conductivity of the n-ZnO sintered materials were drastically enhanced, leading that the power factor of the materials were dramatically improved. It is revealed that the oxide module can be operated up to 930 K on the hot side temperature. A maximum power output of 52.5 mW was achieved at the temperature condition of T H/TL = 934/479 K, where TH and TL are temperatures at the hot and cold sides of the module, respectively. The details of the power generation characteristics of the present oxide module will be presented and the performance will be discussed with those of our previous and other oxide modules..
109. Michitaka Ohtaki, Ryosuke Hayashi, Kazuhiko Araki, Thermoelectric properties of sintered ZnO incorporating nanovid structure
Influence of the size and number density of nanovoids, ICT'07 - 26th International Conference on Thermoelectrics Proceedings ICT'07 - 26th International Conference on Thermoelectrics, 10.1109/ICT.2007.4569436, 112-116, 2007.12, Investigated in this paper is influence of the size and amount of combustible nanosized particles added as a void forming agent (VFA) for generating isolated closed pores (nanovoids) within a densely sintered Al-doped ZnO matrix on the thermoelectric properties of the oxide. With increasing amount of VFA, the electrical conductivity (σ) and the thermal conductivity (κ) of the samples decrease, while the Seebeck coefficient (S) is relatively insensitive to the amount of VFA; these results appear to be a natural consequence concerning increasing porosity. However, conventional κ-porosity relation is revealed to be unapplicable to the present system, showing a much steeper decrease in κ with increasing porosity. These results suggest that the nanovoid structure is capable of scattering phonons more effectively compared to conventional porous structures with similar porosity. Moreover, the S values significantly increase with decreasing average diameter of VFA from 1800 nm down to 150 nm. Consequently, the samples prepared with 5-10wt% of 150 nm VFA show ZT = 0.54 - 0.59 at 1000°C. Detailed investigation of the transport properties of the samples reveals that the &sigma/κ ratios can be larger for the nanoviod samples than that of the dense matrix without VFA, implying a possible selective phonon scattering..
110. K. Ohnishi, M. Ohtaki, Development of Oxide Thermoelectric Modules with a High Density Packing by Using Ceramic Honeycomb, Proc. 9th Cross Straits Symp. Mater. Ener. Environ. Sci., pp. 63-64, 2007.11.
111. H. Ikeda, M. Ohtaki, Molecular Templating Electrochemical Synthesis of Nano-Superlattice Thin Films of Layered Oxides and Their Electromagnetic Properties, Proc. 9th Cross Straits Symp. Mater. Ener. Environ. Sci., pp. 95-96, 2007.11.
112. T. Masuda, M. Ohtaki, Suppression of Thermal Conductivity In Metal Oxides Due to Order-Disorder Transition of Oxide Ion Sublattice, Proc. 9th Cross Straits Symp. Mater. Ener. Environ. Sci., pp. 115-116, 2007.11.
113. T. Sugahara, M. Ohtaki, T. Souma, Thermoelectric Properties of Double-Perovskite Oxide A2B'B"O6 with A-Site Substitution, Proc. 9th Cross Straits Symp. Mater. Ener. Environ. Sci., pp. 137-138, 2007.11.
114. Kaoru Kimura, Ken Ichi Takagi, Takeshi Koyanagi, Michitaka Ohtaki, Hidero Unuma, Shigeru Katsuyama, Kouji Akai, Masatoshi Takeda, Special issue on thermoelectric conversion materials III, Materials Transactions, 48, 8, 2072, 2007.08.
115. T. Souma, M. Ohtaki, M. Shigeno, Y. Ohba, N. Nakamura, T. Shimozaki, Jointing Technique and Power Generation Characteristics of p-NaCo2O4/n-ZnO Oxide Thermoelectric Modules, Trans. Mater. Res. Soc. Jpn., 32(3), 701-704, 2007.06.
116. Michitaka Ohtaki, Ryosuke Hayashi, Enhanced thermoelectric performance of nanostructured ZnO
A possibility of selective phonon scattering and carrier energy filtering by nanovoid structure, ICT'06 - 25th International Conference on Thermoelectrics Proceedings ICT'06 - 25th International Conference on Thermoelectrics, 10.1109/ICT.2006.331368, 276-279, 2006.12, Highly dispersed nanosized closed pores (nanoviods) are revealed to be effective to substantially enhance the thermoelectric performance of bulk sintered body of n-type Al-doped ZnO oxide, resulting in a dimensionless figure-of-merit of ZT - 0.65 at 1250 K. The nanovoid structure is built in a densely sintered Al-doped ZnO matrix by using combustible nanosized polymer particles as a void forming agent (VFA), the uniformity of the WA distribution in the sintering mixture being greatly improved by employing planetary-type ball milling with high pulverizing capability. A combination of shortened mixing period and liquid mixing media enables us to prevent formation of oxygen-related defects in ZnO, and sintered samples thus obtained show the electrical conductivity (σ) higher than that of those prepared with conventional ball milling. The sintered samples obtained in the present study also show the Seebeck coefficient (S). considerably larger than that of the control sample over the whole temperature range from 300 K to 1273 K, implying an enhancement of the thermopower possibly due to a carrier energy filtering effect by low-energy nanosized defects. Although a decrease in the thermal conductivity (κ) is only of the same magnitude to that of the σ values, the marked increase in both σ and S gives rise to a significant enhancement of the power factor. With fairly suppressed κ values, the nanovoid ZnO samples successfully attain a largest ZT value so far observed for n-type bulk oxide materials..
117. Takeshi Souma, Michitaka Ohtaki, Masayuki Shigeno, Yasuhiro Ohba, Norikazu Nakamura, Toshitada Shimozaki, Fabrication and power generation characteristics of p-NaCo 2O4/n-ZnO oxide thermoelectric modules, ICT'06 - 25th International Conference on Thermoelectrics Proceedings ICT'06 - 25th International Conference on Thermoelectrics, 10.1109/ICT.2006.331386, 603-606, 2006.12, A prototype of thermoelectric module using oxide materials has been successfully fabricated, and the power generation characteristics have been examined in the high temperature region around 773 K (500 °C). Twelve couples of sintered p-NaCo2O4 and n-ZnO materials were connected via silver conducting strips with a planer arrangement jointed by using a diffusion welding technique under 16 MPa at 1023 K in Ar. A maximum power output of 58 mW was achieved at a temperature condition of T H/TL= 839/377 K. The details of the power generation characteristics of the oxide module will be presented and the performance will be discussed with other oxide modules..
118. Takeshi Souma, Michitaka Ohtaki, Optimization of synthesis conditions for CoSb3 compounds prepared by Sb self-flux method, ICT'06 - 25th International Conference on Thermoelectrics Proceedings ICT'06 - 25th International Conference on Thermoelectrics, 10.1109/ICT.2006.331385, 598-602, 2006.12, CoSb3 bulk materials were successfully prepared by Sb self-flux technique, and the synthesis conditions were widely investigated to obtain the high purity compounds utilizing a quantitative powder XRD study with the Rietveld analysis applying a multi-phase-fitting condition. The reaction temperature range for the Sb self-flux technique was extended up to 1123 K to promote the reaction in the Co-Sb system. The relation between the reaction temperature and the chemical composition is discussed to optimize the preparation conditions in the Sb self-flux technique..
119. Takeshi Souma, Michitaka Ohtaki, Relation between Zn content and thermoelectric properties of Zn 4+xSb3 (-0.12≤x≤0.12), ICT'06 - 25th International Conference on Thermoelectrics Proceedings ICT'06 - 25th International Conference on Thermoelectrics, 10.1109/ICT.2006.331384, 594-597, 2006.12, A series of seven bulk crystals of Zn4+xSb3 (-0.12 ≤ x ≤ 0.12) are successfully prepared, and the relation between the Zn content and the thermoelectric properties is investigated. The sample with the nominal composition of Zn3.96Sb3 (x = -0.04) contain the maximum mass fraction of Zn4Sb3. The shapes of the S(T) and ρ(T) curves are drastically changed with varying x in the temperature range from 300 K to 700 K. Whereas the lower x leads to lower ρ, the magnitude of S unchanges, resulting in the fact that the power factor of the Zn-poor compositions in the Zn4+xSb3 system is lower than that of the Zn-rich compositions. The details of the influence of the Zn content and the scope of the improvement of the durability are discussed..
120. N. V. Nong, Michitaka Ohtaki, Thermoelectric properties and local electronic structure of rare earth-doped Ca3Co2O6, ICT'06 - 25th International Conference on Thermoelectrics Proceedings ICT'06 - 25th International Conference on Thermoelectrics, 10.1109/ICT.2006.331270, 62-65, 2006.12, Thermoelectric properties of a series of rare earth metaldoped polycrystalline samples of (Ca1-xRx)3Co 2O6 with R = Gd, Tb, Dy and Ho (x = 0 - 0.1) were investigated in the temperature range from 300 K to 1300 K. In a high temperature region above 900 K, a partial rare earth substitution with R 3+ for Ca2+ resulted in appreciable increase in the Seebeck coefficient (S). However, the S value decreased abruptly at low temperatures, and turned to negative values for the Gd- and Tb-doped samples at temperatures below 400 K. With decreasing ionic radii of rare earth elements (Gd3+ > Tb3+ > Dy3+ > Ho 3+), the S values increased, while the thermal conductivity (Κ) decreased particularly at temperatures above 700 K. Contrastingly, the influence of rare earth metal substitution on the electrical resistiivity (p) was small; in high temperature region the p values increased only slightly with decreasing ionic radii of rare earth metals. High-temperature thermoelectric figure-of-merit (Z) of the samples was thereby improved by the late rare-earth metal substitution for Ca2+, particularly for those with Ho 3+. A maximum Z value of the Ho-doped sample for x = 0.03 was 1.83×10-4 K-1 at 1100 K as compared with 0.37×10-4 K-1 for non-doped sample. The electronic structure of the samples was also investigated by x-ray photoemission spectroscopy (XPS) technique. The charge-transfer satellite structure of Co 2p core-level spectra was observed for the Gdand Tb-doped samples, while the satellite is negligible for the other samples..
121. N. V. Nong, M. Ohtaki, Thermoelectric Properties and Local Electronic Structure of Rare Earth-doped Ca3Co2O6, Proc. 25th Int. Conf. Thermoelectrics, pp.62-65., 2006.11.
122. M. Ohtaki, R. Hayashi, Enhanced Thermoelectric Performance of Nanostructured ZnO: A possibility of Selective Phonon Scattering and Carrier Energy Filtering by Nanovoid Structure, Proc. 25th Int. Conf. Thermoelectrics, pp.276-279., 2006.11.
123. T. Souma, M. Ohtaki, Relation between Zn Content and Thermoelectric Properties of Zn4+xSb3 (-0.12≦x≦0.12), Proc. 25th Int. Conf. Thermoelectrics, pp.594-597., 2006.11.
124. T. Souma, M. Ohtaki, Optimization of Synthesis Conditions for CoSb3 Compounds Prepared by Sb Self-flux Method, Proc. 25th Int. Conf. Thermoelectrics, pp.598-602., 2006.11.
125. T. Souma, M. Ohtaki, M. Shigeno, Y. Ohba, N. Nakamura, T. Shimozaki, Fabrication and Power Generation Characteristics of p-NaCo2O4/n-ZnO Oxide Thermoelectric Modules, Proc. 25th Int. Conf. Thermoelectrics, pp.603-606., 2006.11.
126. N. V. Nong, M. Ohtaki, Structural Transition and Magnetic Field Induced Strain in Ni50Mn26.5Ga23.5 Alloy, 九州大学大学院総合理工学報告, 28(2), 237-239., 2006.10.
127. Ngo Van Nong, Michitaka Ohtaki, Power factors of late rare earth-doped Ca3Co2O6 oxides, Solid State Communications, 10.1016/j.ssc.2006.05.033, 139, 5, 232-234, 2006.08, Polycrystalline samples of (Ca1-xRx)3Co2O6 with R = Gd, Tb, Dy and Ho at x = 0 - 0.1 were synthesized and the effects of rare earth substitution on their thermoelectric properties were investigated. In the high-temperature region, the rare earth substitution resulted in an increase in the Seebeck coefficients (S), and the S values increased with decreasing ionic radius of rare earth elements in the order Gd3+>Tb3+>Dy3+>Ho3+ . In contrast, the influence of rare earth substitution on the electrical resistivity was small. The high-temperature power factor was thereby improved by the late rare earth substitutions, particularly those with Ho3+ for Ca2+. For the Ho-doped samples (x ≤ 0.05), the power factor was significantly improved by increasing Ho concentration..
128. N. V. Nong, M. Ohtaki, High-temperature Thermoelectric properties of Late Rare Earth-doped Ca3Co2O6, Trans. Mater. Res. Soc. Jpn., 31(2), 399-402, 2006.06.
129. H. Hirobe, M. Ohtaki, Enhanced Phonon Scattering by Oxygen Defects in Metal Oxides, Trans. Mater. Res. Soc. Jpn., 31(2), 403-406., 2006.06.
130. Kaoru Kimura, Ken Ichi Takagi, Takeshi Koyanagi, Michitaka Ohtaki, Hidero Unuma, Shigeru Katsuyama, Koji Akai, Masatoshi Takeda, Special issue on thermoelectric conversion materials II, Materials Transactions, 47, 6, 1421, 2006.06.
131. Takeshi Souma, Michitaka Ohtaki, Synthesis and Rietveld analysis of Zn4 - XCdxSb 3 bulk crystals in the Zn-rich region, Journal of Alloys and Compounds, 10.1016/j.jallcom.2005.06.062, 413, 1-2, 289-297, 2006.03, A series of nine bulk crystals of the Zn4 - xCd xSb3 compounds (x = 0-2.00 with 0.25 step) has been successfully synthesized and the detailed crystal structure has been refined by powder XRD study using the Rietveld method including the quantitative analysis. The substitution limit of the Cd atoms for the Zn4 - xCd xSb3 system has been extended to x = 2.00 by employing the vacuum casting method without annealing. The purities and densities of the bulk crystals exceeded approximately 96 mass% and 95% of XRD density, respectively. The lattice parameters of the a and c-axes in the system increase linearly with increasing Cd content x with obeying the Vegard law..
132. Takeshi Souma, Michitaka Ohtaki, Comparison of structural parameters for Zn4-xCd xSb3 compounds analyzed by the Rietveld method using two crystallographic models, ICT'05: 24th International Conference on Thermoelectrics Proceedings - ICT'05 24th International Conference on Thermoelectrics, 10.1109/ICT.2005.1519973, 410-413, 2005.12, A comparison of detailed structural parameters between the 3IS and Mayer models for the Zn4-xCdxSb3 system in the Zn-rich region has been successfully performed by the powder XRD study with the Rietveld method using 9 bulk crystals synthesized by vacuum casting method without annealing. The 3IS model gives a better fitting on the Rietveld analysis not only for the undoped Zn4Sb3 but also for the whole compositional range of the Zn-rich region of the Zn4-xCd xSb3 compounds. The purities and densities of the bulk crystals exceeded approximately 96 mass% and 95 % of XRD density, respectively..
133. Takeshi Souma, Michitaka Ohtaki, Evaluation of durability of thermoelectric properties for Zn 4Sb3 compounds, ICT'05: 24th International Conference on Thermoelectrics Proceedings - ICT'05 24th International Conference on Thermoelectrics, 10.1109/ICT.2005.1519974, 414-417, 2005.12, Durability of the Zn4Sb3 compounds was evaluated in term of their thermoelectric properties by applying repeated heat cycles to the vacuum-casted and hot-pressed samples. The power factor of both samples decreased by applying 20 cycle heat load. The major reason of the deterioration was formation of the ZnCd compounds due to evaporating Zn from the Zn 4Sb3 compounds. The degree of the deterioration of the thermoelectric performance and a solution to enhanced durability of the compounds are discussed..
134. Michitaka Ohtaki, Kimihiro Shouji, Strong influence of CO2 partial pressure on inhomogeneous Na distributions and the thermoelectric performance of polycrystalline NaCo 2C4, ICT'05: 24th International Conference on Thermoelectrics Proceedings - ICT'05 24th International Conference on Thermoelectrics, 10.1109/ICT.2005.1519988, 2005, 472-475, 2005.12, Influence of gas-solid equilibrium and partial pressures of gaseous species was investigated on solid state reactions of Na2CO 3 and Co3O4 for synthesis of bulk sintered bodies of layered thermoelectric oxide NaCo2O4. It was revealed that the presence of CO2 in the reaction atmosphere strongly inhibits the formation reaction of NaCo2O4. However, precise control of the CO2 partial pressure in the oxygen atmosphere enabled us to avoid otherwise significant Na loss during sintering at temperature as high as 900°C, thereby allowing high reaction temperature required for incorporation of Na into the crystal lattice. Selective quantification of the Na species in order to distinguish those within the crystallites and at the grain boundaries revealed that the samples synthesized in the presence of dilute CO2 contain more Na in the crystallites with less Na segregation at the boundaries. Improved interconnection between the grains containing larger amounts of Na was suggested as a reason of higher electrical conductivity and larger Seebeck coefficient of the samples thus obtained, compared to the samples containing the same amount of total Na but synthesized in air..
135. Takeshi Souma, Michitaka Ohtaki, Synthesis and Rietveld analysis for CoSb3 compounds prepared by Sb self-flux method, ICT'05: 24th International Conference on Thermoelectrics Proceedings - ICT'05 24th International Conference on Thermoelectrics, 10.1109/ICT.2005.1519903, 121-124, 2005.12, High purity CoSb3 bulk crystals have been successfully synthesized by Sb self-flux method and a relation between reaction conditions and chemical composition on the method has been systematically analyzed by powder XRD study using the Rietveld analysis. Sb self flux method at 923 K using 100 mesh Co elements can directly provide a single phase CoSb3 bulk crystal in a brief time of 10 h. Advantages of Sb self-flux methods will be discussed compared with other preparation methods..
136. N. V. Nong, N. P. Thuy, Y. D. Yao, Michitaka Ohtaki, N. T. Lam, The influence of composition change on structural and magnetic properties of non-stoichiometric Ni-Mn-Ga alloys, INTERMAG ASIA 2005: Digests of the IEEE International Magnetics Conference INTERMAG ASIA 2005 Digests of the IEEE International Magnetics Conference, 2005.12.
137. Kaoru Kimura, Ken Ichi Takagi, Takeshi Koyanagi, Michitaka Ohtaki, Materials Transations
Preface, Materials Transactions, 46, 7, 2005.07.
138. Site-Selective Substitution by Transition Metal Cations for NaCo_2O_4 via Ion-Exchange.
139. Yoshihiro Nojiri, Michitaka Ohtaki, Site-selective substitution by transition metal cations for NaCo 2O4 via ion-exchange, Journal of the Ceramic Society of Japan, 10.2109/jcersj.113.400, 113, 1318, 400-404, 2005.01, Incorporation of Ni ions into two different sites of the Na and Co sites of NaCo2O4 was carried out by the processes of conventional elemental substitution for Co ion and solid state ion exchange of mobile Na + ion. The changes of the lattice parameters of " substituted" and "exchanged" samples were different, suggesting that the Ni ions occupy different sites. The Ni-substituted sample for Co showed a significant decrease of Seebeck coefficient keeping the electrical conductivity almost intact, while Ni-exchanged samples for Na showed a lower electrical conductivity than that of the non-substituted sample with keeping the Seebeck coefficient unchanged. These results suggest that Ni cations introduced to the Co site decouples the Co-Co spin exchange, whereas those at the Na site compensates positive holes by providing conduction electrons..
140. M. Ohtaki, K. Shouji, Sintering process and nonstoichiometry of NaCo2O4 layered thermoelectric oxide, 22nd International Conference on Thermoelectrics, ICT 2003 Proceedings ICT 2003 - 22nd International Conference on Thermoelectrics, 10.1109/ICT.2003.1287490, 227-230, 2003.01, Sintering process of layered oxide NaCo2O4, which has been the best p-type thermoelectric oxide candidate to date since its debut in the world of thermoelectrics in 1997, was investigated in terms of sodium nonstoichiometry occurring during solid state reaction between Na2CO3 and Co3O4 for the synthesis of the oxide. Thermal gravimetry (TG) and differential thermal analysis (DTA) measurements revealed that solid-gas equilibrium plays an important role on phase formation process and on controlling chemical composition of the oxide, Partial pressure of oxygen and carbon dioxide in the reaction atmosphere was revealed to be crucial for controlling vaporization of Na, as well as for promotion of phase formation reaction of NaCo2O4, In particular, whereas the CO2 atmosphere effectively suppressed abrupt dissociation reaction of Na2CO3, low oxygen partial pressure accelerated vaporization of Na at lower temperature. Interestingly, high partial pressure Of CO2 appeared to hinder phase formation reaction of NaCo2O4 even at the same oxygen partial pressure as that under the aerobic conditions..
141. Michitaka Ohtaki, S. Maehara, S. Shige, Thermoelectric properties of Al-doped ZnO sintered with nanosized void forming agents, 22nd International Conference on Thermoelectrics, ICT 2003 Proceedings ICT 2003 - 22nd International Conference on Thermoelectrics, 10.1109/ICT.2003.1287476, 171-174, 2003.01, Reported are the electrical and thermal transport properties of Al-doped zinc oxide sintered in the presence of void forming agents (VFA) such as small carbon or organic polymer particles of several hundreds nanometers in size. Nanosized cavities (nanovoids) thus formed within dense oxide matrices lead to a marked enhancement in the thermopower, showing a large negative maxima at the temperature range around 400-600°C. On the other hand, the formation of the nanovoids scarcely influences the electrical conductivity, if the size of VFA is sufficiently small. Although the reduction of the thermal conductivity is also fairly small, the enhanced thermopower coupled with the virtually unchanged electrical conductivity results in a considerable improvement in the power factor with the maximum value of 40×10-4 W/mK2, being twice as large as that of the completely dense sample without addition of VFA. The ZT value thereby exceeds 0.4, and the operation temperature range markedly expands toward the lower temperatures. Different behavior of the scattering cross sections for electrons and phonons will be discussed..
142. The State of the Art of Research and Development in Electroceramics.
143. Yoshihiro Nojiri, Michitaka Ohtaki, Site-selective doping of transition metal cations into NaCo2O4, 21st International Conference on Thermoelectrics, ICT 2002 Proceedings ICT 2002: 21st International Conference on Thermoelectrics, 10.1109/ICT.2002.1190308, 2002-January, 234-237, 2002.12, We have investigated site-selective doping of Ni in order to experimentally explore the roles of the Na and Co sites of NaCO2O4. The selective doping is carried out by changing preparation procedures of the oxide. Cation "substitution" is intended by incorporating Ni(NO3)2 into a Co-deficient starting mixture of Na2CO3 and Co3O4 prior to the first heat treatment. Cation "exchange" is, on the other hand, carried out by heating a mixture of Ni(NO3), and a single phase sample ofNaCo2O4, which has been prepared beforehand from a stoichiometric starting mixture. Comparison with control samples without addition of Ni(NO3)2 reveals that the "substituted" sample shows a significant decrease in the thermo power with keeping the electrical conductivity almost intact, while the "exchanged" sample shows the lower electrical conductivity with leaving the thermo power unchanged. These results strongly suggest that Ni cations occupy different sites in the Ni-doped samples; disturbing Co-Co spin exchange at the Co site, and compensating holes at the Na site..
144. Research Aspects of Oxide Thermoelectric Materials.
145. Synthesis and Optical Properties of Low-Dimensional Quantum-Confined Oxide Semiconductors by Liquid Crystal Templating-Hydrothermal Technique.
146. H. Fujii, K. Inata, Michitaka Ohtaki, K. Eguchi, H. Arai, Synthesis of TiO2/CdS nanocomposite via TiO2 coating on CdS nanoparticles by compartmentalized hydrolysis of Ti alkoxide, Journal of Materials Science, 10.1023/A:1004857419242, 36, 2, 527-532, 2001.01, Hydrolysis of Ti alkoxide in the presence of inverse micelles containing CdS nanoparticles in their hydrophilic interior results in formation of TiO2/CdS nanocomposites in which the CdS nanoparticles are embedded in a TiO2 matrix with a thickness of ≤10 nm at the surface of the particles. The primary hydroxyl groups introduced by 2-mercaptoethanol as a capping agent used for preparation of the CdS nanoparticles are considered to play an important role for successful adhesion and growth of the TiO2 layer on the CdS surfaces. TEM observation strongly supports formation of semiconductor-in-semiconductor heteronanostructure by compartmentalized hydrolysis of Ti alkoxide within the inverse micelles in which the surface-capped CdS nanoparticles coexist..
147. Mingxin Qiu, Toru Mizunami, Teruo Shimomura, Michitaka Ohtaki, Threshold conditions for bulk second-order nonlinearity and near-surface second-order nonlinearity in thermally poled infrasil silica, Optical Review, 10.1007/s10043-001-0159-5, 8, 3, 159-162, 2001.01, Generation of bulk second-order nonlinearity in silica glass requires higher poling temperature or longer poling time than that of near-surface second-order nonlinearity. The threshold conditions for initiating the bulk second-order nonlinearity are studied on Infrasil fused silica glass. The threshold poling time is strongly dependent on the poling temperature. The near-surface second-order nonlinearity is also studied, especially the dependence of thickness of the nonlinear layer on the poling temperature, poling voltage and poling time. Secondary-ion mass-spectroscopy measurement showed depletion of Na+ ions at the anodic surface. We assume there is an ionic wave during poling traveling from the anodic surface to generate the dipolar electric field that induces the near-surface second-order nonlinearity..
148. Microstructures and Thermoelectric Properties of NaCo_2O_4 Prepared by Double-step Sintering.
149. Michitaka Ohtaki, Eishi Maeda, Microstructures and thermoelectric properties of NaCo2O4 prepared by double-step sintering, Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy, 10.2497/jjspm.47.1159, 47, 11, 1159-1164, 2000.11, The thermoelectric properties of sintered bodies of NaCo2O4, which is so far the most promising p-type candidate for oxide thermoelectric material, are investigated in terms of their sintering conditions. Double-step sintering, for which single-phase NaCo2O4 obtained by calcination and sintering with addition of 10% excess of Na each time is re-ground and sintered again, is revealed to attain a marked improvement in the thermoelectric performance of the oxide. The improvement is due to increase in both Seebeck coefficient and electrical conductivity, and is attributed to a distinct enhancement of the two-dimensionality in the crystallites constituting the sintered body. The oxide sintered body consequently attains the figure-of-merit of 0.88×10-3 K-1 at 600 °C, and the maximum ZT of 0.78 at 800 °C..
150. Michitaka Ohtaki, Hirofumi Sato, Hiroyuki Fujii, Koichi Eguchi, Intramolecularly selective decomposition of surfactant molecules on photocatalytic oxidative degradation over TiO2 photocatalyst, Journal of Molecular Catalysis A: Chemical, 10.1016/S1381-1169(99)00325-8, 155, 1-2, 121-129, 2000.04, Investigation on oxidative mineralization of various surfactants over TiO2 photocatalyst reveals an anomalously retarded degradation of the trimethylammonium moiety in cationic surfactants. Model reactions using tetraalkylammonium chlorides as substrates confirm that the stepwise behavior of the mineralization time courses is ascribed to intramolecularly selective decomposition caused by a considerably slow degradation rate of the methyl groups directly bound to the quaternary nitrogen. A strong inhibition effect of bromide or iodide anions preventing the complete mineralization is also revealed..
151. Michitaka Ohtaki, Thermoelectric properties and crystal chemistry of promising oxide candidate NaxCoO2, 18th International Conference on Thermoelectrics (ICT'99) International Conference on Thermoelectrics, ICT, Proceedings, 565-568, 1999.12, Promising p-type thermoelectric properties of a layered transition metal oxide NaxCoO2 are discussed in terms of crystal chemistry and the conduction mechanism. Highly anisotropic spatial distribution of 3d orbitals coupled with an unusually short Co-Co interatomic distance in the ab plane, along which edge-sharing CoO6 octahedra connecting each other to form the CoO2 sheets, strongly suggest that Co3d dε-dε (t2g-t2g) direct overlapping is plausible for establishing hole conduction in the valence band with a fairly covalent character. An extremely low thermal conductivity (≤1 W/mK) of the oxide is also revealed, implying contribution of the randomly distributed Na vacancies and an inherent advantage of materials with low-dimensional crystal structure. In spite of insufficient densification, sintered bodies fired under atmospheric pressure shows ZT≈0.5. Possible influence of the ionic conduction of Na+ on the thermoelectric properties of the oxide is also examined..
152. 坪田 敏樹, 大瀧 倫卓, 江口 浩一, Thermoelectric Properties of Chevrel-Type Sulfides AMo_6S_8 (A=Fe, Ni, Ag, Zn, Sn, Pb, Cu), 日本セラミックス協会学術論文誌 : Nippon Seramikkusu Kyokai gakujutsu ronbunshi, 10.2109/jcersj.107.697, 107, 1248, 697-701, 1999.08, Thermoelectric properties of Chevrel-type sulfides AMo_6S_8 (A=Fe, Ni, Ag, Zn, Sn, Pb, Cu) were investigated with respect to many advantageous characteristics of the Chevrel compounds for thermoelectric applications. The electrical conductivity, σ, was 10^2.5S・cm^<-1><10^3S・cm^<-1> for most of the samples from room temporature to 1000℃; AgMo_6S_8 gave the largest σ up to 250℃. The Seebeck coefficient, S, was positive for all the samples, indicating the p-type conduction. All the S values were 10μV・K^<-1>and AgMo_6S_8 showed smallest S, probably owing to the higher hole concentration for A=Ag compared to other samples. The maximum S^2σ value was attained by SnMo_6S_8, reaching ca. 1.7×10^<-4> W・m^<-1.>K^<-2> at 800℃. All the k values were 1 W・m^<-1.>K^<-1><κ<3W・m^<-1.>K^<-1> from room temperature to 1000℃. The maximum Z value was ca. 0.07×10^<-3>K^<-1> attained by SnMo_6S_8 at 800℃..
153. Toshiki Tsubota, Michitaka Ohtaki, Koichi Eguchi, Thermoelectric properties of chevrel-type sulfides AMo6S8 (A=Fe, Ni, Ag, Zn, Sn, Pb, Cu), Journal of the Ceramic Society of Japan, 10.2109/jcersj.107.697, 107, 8, 697-701, 1999.01, Thermoelectric properties of Chevrel-type sulfides AMo6S8 (A=Fe, Ni, Ag, Zn, Sn, Pb, Cu) were investigated with respect to many advantageous characteristics of the Chevrel compounds for thermoelectric applications. The electrical conductivity, σ, was 102.5 S·cm-1<σ<103 S·cm-1 for most of the samples from room temperature to 1000°C; AgMo6S8 gave the largest σ up to 250°C. The Seebeck coefficient, S, was positive for all the samples, indicating the p-type conduction. All the S values were 10 μV·K-1-1, and AgMo6S8 showed the smallest S, probably owing to the higher hole concentration for A=Ag compared to other samples. The maximum S2σ value was attained by SnMo6S8, reaching ca. 1.7 × 10-4 W·m-1· K-2 at 800°C. All the κ values were 1 W·m-1·K-1<κ<3 W·m-1·K-1 from room temperature to 1000°C. The maximum Z value was ca. 0.07 × 10-3 K-1 attained by SnMo6S8 at 800°C..
154. H. Fujii, Michitaka Ohtaki, K. Eguchi, Synthesis and photocatalytic activity of lamellar titanium oxide formed by surfactant bilayer templating, Journal of the American Chemical Society, 10.1021/ja980430d, 120, 27, 6832-6833, 1998.07.
155. Hiroyuki Fujii, Michitaka Ohtaki, Koichi Eguchi, Hiromichi Arai, Preparation and photocatalytic activities of a semiconductor composite of CdS embedded in a TiO2 gel as a stable oxide semiconducting matrix, Journal of Molecular Catalysis A: Chemical, 10.1016/S1381-1169(97)00132-5, 129, 1, 61-68, 1998.03, A semiconductor composite of CdS particles embedded in a TiO2 gel after heat treatment at 250°C in air is revealed to show catalytic H2 evolution from an EDTA aqueous solution with light of λ ≤ 450 nm, in contrast to bare CdS which is stoichiometrically photocorroded. A significant improvement in the activity even with considerable loses of CdS in the gel during the heat treatment suggests that the TiO2 gel crystallized into anatase can prevent photocorrosion of CdS embedded therein, and serve as a semiconducting matrix to facilitate charge transfer from the embedded CdS to Pt at the surfaces of the gel..
156. Y. Mishima, H. Mitsuyasu, M. Ohtaki, K. Eguchi, Solid oxide fuel cell with composite electrolyte consisting of samaria-doped ceria and yttria-stabilized zirconia, Journal of the Electrochemical Society, 10.1149/1.1838378, 145, 3, 1004-1007, 1998.03, A composite oxide containing samaria-doped eeria [SDC, (CeO2)0.8(SmO1.5)0.2] and yttria-stabilized zirconia [YSZ, (ZrO2)0.92(Y2O3)0.08] was developed as an electrolyte for a solid oxide fuel cell (SOFC). A SOFC with a YSZ electrolyte was characterized by a high open-circuit voltage (OCV) and a steep decrease in voltage, V, with increasing current, I, whereas that with a SDC electrolyte exhibited a low OCV and a gradual decrease of its V-I curve. A biphasic electrolyte microstructure with SDC and YSZ was proposed in which SDC grains are dispersed in a YSZ matrix. In this composite structure, the electronic conduction caused by reduction of the SDC was effectively blocked by the YSZ matrix. An SOFC with this composite electrolyte offered the advantages of YSZ with its high OCV and SDC with its gradual decrease of the V-I curve. The SDC content and the particle size significantly affected the power-generation characteristics of a cell with this composite electrolyte. The electrode polarization was significantly lowered by using the SDC or the composite electrolyte..
157. Michitaka Ohtaki, Misako Ozaki, Koichi Eguchi, PO2-dependent nonohmic current-voltage characteristics and current-induced redox behavior of junctioned SrTiO3, Journal of the Electrochemical Society, 10.1149/1.1838433, 145, 4, 1165-1173, 1998.01, Oxide semiconductor junctions formed by attaching p- and n-type SrTiO3 exhibits a markedly nonohmic current-voltage characteristics at the steady state, showing no typical rectifying characters. Although the voltages which appeared immediately after applying the currents, are completely linear vs. the current values, the probe voltages vary with time and finally result in a significantly nonlinear behavior strongly dependent on oxygen partial pressure, showing distinct asymmetry to the current direction. Separation of the voltage drop into those within the bulks and at the junction reveals that the highly resistive p-type SrTiO3 bulk near the junction is mainly responsible for the nonohmic character. For the p-type SrTiO3 single bulk, absorption of oxygen in the gas phase is observed on applying large bias currents, regardless of the current directions. However, the junctioned sample desorbs oxygen for the forward bias, while the reverse bias causes oxygen absorption. Conductivity changes of the highly resistive p-type SrTiO3 as a consequence of these current-induced redox behavior give a consistent explanation for the direction-dependent nonohmic character of the junctioned sample. Dissociation/association of oxygen due to a shift of the solid-gas equilibrium caused by the applied currents is discussed..
158. Hiroyuki Fujii, Hirofumi Sato, Michitaka Ohtaki, Koichi Eguchi, Hiromichi Arai, Retarded decomposition of the trimethylammonium moiety observed on photocatalytic mineralization of surfactants over TiO2 photocatalyst, Chemistry Letters, 10.1246/cl.1998.251, 3, 251-252, 1998.01, The CO2 evolution rate for photocatalytic mineralization of alkyltrimethylammonium chloride sharply decreased when the mineralization yield became nearly equal to the proportion of the long alkyl carbon to the total organic carbon, suggesting that the photodecomposition rate of the alkyl chains is much faster than that of the trimethylammonium group. Moreover, the presence of Br- significantly suppressed the initial CO2 evolution rate, and resulted in incompleted mineralization with the final mineralization yield virtually equal to the proportion of the carbon atoms not attaching to the quarternary nitrogen..
159. Michitaka Ohtaki, Kazumasa Inata, Koichi Eguchi, Selective Incorporation of Inorganic Precursors into the Channels of MCM-41 by Molecular Assembly Template as a Hydrophobic Carrier, Chemistry of Materials, 10.1021/cm9801744, 10, 10, 2582-2584, 1998.01.
160. Toshiki Tsubota, Michitaka Ohtaki, Koichi Eguchi, Hiromichi Arai, Transport properties and thermoelectric performance of (Zn1-yMgy)1-xAlxO, Journal of Materials Chemistry, 10.1039/a706213c, 8, 2, 409-412, 1998.01, Addition of MgO to Al-doped ZnO was successful in reduction of the phonon thermal conductivity, κph, in order to suppress the unfavorably high thermal conductivity, κ, of the material in terms of applications to thermoelectric conversion. The electrical conductivity, σ, of Zn0.98Al0.02O decreased with increasing amount of the added MgO, whereas the Seebeck coefficient was virtually unchanged up to (Zn0.9Mg0.1)0.98Al0.02O. Further Al2O3 doping was ineffective in improving σ. The carrier mobility decreased with increasing amount of the added MgO but was independent of the extent of the Al doping. In spite of the significant suppression of κ, the figure of merit was smaller for the MgO-added samples because of the marked decrease of σ..
161. Michitaka Ohtaki, Toshiki Tsubota, Koichi Eguchi, Thermoelectric properties of oxide solid solutions based on Al-doped ZnO, Proceedings of the 1998 17th International Conference on Thermoelectrics, ICT International Conference on Thermoelectrics, ICT, Proceedings, 610-613, 1998, Investigation on thermoelectric properties of oxide solid solutions based on Zn0.98Al0.02O revealed that the thermal conductivity of Al-doped ZnO was markedly suppressed via formation of solid solution, particularly for those with MgO. The decrease in the thermal conductivity was significant at the lower temperatures, and at room temperature, (Zn0.9Mg0.1)0.98Al0.02O attained a value less than a quarter of that for Zn0.98Al0.02O. However, the electrical conductivity also decreased, while the Seebeck coefficients almost unchanged. The Hall measurements revealed reduction in both the carrier concentration and the Hall mobility for the solid solution samples. In spite of the significant suppression of the thermal conductivity, improvement in the figure of merit was unsuccessful for the MgO-doped samples because of the unfavorable decrease in the electrical conductivity..
162. H. Fujii, Michitaka Ohtaki, K. Eguchi, H. Arai, Photocatalytic activities of CdS crystallites embedded in TiO2 gel as a stable semiconducting matrix, Journal of Materials Science Letters, 10.1023/A:1018599229805, 16, 13, 1086-1088, 1997.01.
163. Toshiki Tsubota, Michitaka Ohtaki, Koichi Eguchi, Hiromichi Arai, Thermoelectric properties of Al-doped ZnO as a promising oxide material for high-temperature thermoelectric conversion, Journal of Materials Chemistry, 10.1039/a602506d, 7, 1, 85-90, 1997.01, The thermoelectric properties of a mixed oxide (Zn1-xAlx)O (x = 0, 0.005, 0.01, 0.02, 0.05) are investigated in terms of materials for high-temperature thermoelectric conversion. The electrical conductivity, σ, of the oxide increases on Al-doping by more than three orders of magnitude up to ca. 103 S cm-1 at room temperature, showing metallic behaviour. The Seebeck coefficient, S, of (Zn1-xAlx)O (x > 0) shows a general trend in which the absolute value increases gradually from ca. -100 μV K-1 at room temperature to ca. -200 μV K-1 at 1000°C. As a consequence, the power factor, S2σ, reaches ca. 15 × 10-4 W m-1 K-2, the largest value of all reported oxide materials. The thermal conductivity, κ, of the oxide decreases with increasing temperature, owing to a decrease in the lattice thermal conductivity which is revealed to be dominant in the overall κ. In spite of the considerably large values of κ-, the figure of merit, Z = S2σ/κ, reaches 0.24 × 10-3 K-1 for (Zn0.98Al0.02)O at 1000°C. The extremely large power factor of (Zn1-xAlx)O compared to other metal oxides can be attributed to the high carrier mobility revealed by the Hall measurements, presumably resulting from a relatively covalent character of the Zn-O bond owing to a fairly small difference of the electronegativities of Zn and O. The dimensionless figure of merit, ZT, of 0.30 attained by (Zn0.98Al0.02)O at 1000°C demonstrates the potential usefulness of the oxide..
164. Michitaka Ohtaki, Toshiki Tsubota, Koichi Eguchi, Hiromichi Arai, High-temperature thermoelectric properties of (Zn1-xAlx)O, Journal of Applied Physics, 10.1063/1.360976, 79, 3, 1816-1818, 1996.02, A mixed oxide (Zn1-xAlx)O exhibits promising thermoelectric properties attaining a dimensionless figure of merit ZT of 0.30 at 1000 °C, which value is much superior to other oxides and quite comparable to conventional state-of-the-art thermoelectric materials. The addition of a small amount of Al2O3 to ZnO results in a large power factor of 10-15×10-4 W/mK2, showing a marked increase in the electrical conductivity while retaining moderate thermoelectric power. A large product of the carrier mobility and density of states would be responsible for the favorable electrical properties of the present oxide. A figure of merit Z=0.24×10-3 K-1 is attained by (Zn0.98Al0.02)O at 1000 °C, even with a high thermal conductivity. A predominant proportion of the phonon thermal conductivity promises a further improvement in the thermoelectric performance by selective enhancement of phonon scattering..
165. Emi Hifumi, Rie Kanmei, Masanori Ishimaru, Kosuke Shimizu, Michitaka Ohtaki, Taizo Uda, Immunochemical detection of methamphetamine using polymer-protected ultrafine platinum particles, Journal of Fermentation and Bioengineering, 10.1016/0922-338X(96)89164-1, 82, 4, 417-419, 1996.01, Ultrafine platinum particles protected by poly (methyl acrylate-co-N- vinyl-2-pyrrolidone) were applied to the immunological detection of methamphetamine (MA). The polymer-protected ultrafine particles could chemically bind anti-methamphetamine monoclonal antibody to their surfaces. The antibody-fixed particles behaved like an antibody in the immunoreaction. The antibody-bound particles competitively reacted against the immobilized and the free antigens in the ELISA system. MA could be detected down to a concentration of ca. 10 ng/ml. In the immuno-dot assay, 10-100 ng/ml of MA was detected within 30 min using the ultrafine particles..
166. Michitaka Ohtaki, Kunihiro Oda, Koichi Eguchi, Hiromichi Arai, Preparation of nanosized CdS particles using decomposition of P2S5 in a non-aqueous solvent, Chemical Communications, 10.1039/cc9960001209, 10, 1209-1210, 1996.01, Nanosized CdS particles of ≤6 nm average diameter are prepared at ambient conditions by use of controlled hydrolysis of P2S5 in absolute ethanol initiated by addition of an alcoholic Cd(NO3)2·4H2O solution, resulting in an excitonic absorption peak of the particles at 350 nm corresponding to a LUMO-HOMO gap energy of 3.1 eV..
167. Michitaka Ohtaki, Hisako Koga, Tsutomu Tokunaga, Koichi Eguchi, Hiromichi Arai, Electrical transport properties and high-temperature thermoelectric performance of (Ca0.9M0.1)MnO3 (M = Y, La, Ce, Sm, In, Sn, Sb, Pb, Bi), Journal of Solid State Chemistry, 10.1006/jssc.1995.1384, 120, 1, 105-111, 1995.11, The electrical transport properties of (Ca0.9M0.1)MnO3(M = Y, La, Ce, Sm, In, Sn, Sb, Pb, Bi) are investigated in terms of a new material for high-temperature thermoelectric conversion. The substitution at the Ca site causes a marked increase in the electrical conductivity σ, along with a moderate decrease in the absolute value of the Seebeck coefficient S. The plots of log σT vs 1/T show a negative linear relation, indicating that hopping conduction occurs in these oxides. The σ values at room temperature increase with increasing ionic radii of the cation substituents, implying an increase in the carrier mobility due to larger intersite distance for hopping. The sample of (Ca0.9Bi0.1)MnO3 attains the largest power factor, 2.8 × 10-4/W m-1K-2, at 800°C and leads to the figure of merit 0.7-0.75 × 10-4K-1 over the wide range of temperatures 600-900°C. The oxide shows a maximum ZT value of 0.085 at 900°C..
168. Koichi Eguchi, Shigeru Kawakami, Michitaka Ohtaki, Hiromichi Arai, Atomic force microscopic observation of molybdenum trioxide surface, Journal of the Ceramic Society of Japan. International ed., 103, 1, 73-77, 1995.01, Surface morphology of MoO3 was observed with an atomic force microscope. The array of (MoO6)6- octahedra was clearly observable for the oxidized surface of a thin crystal prepared by the evaporation-condensation process. Similar high resolution AFM image was also obtained for the powder sample which was coated and dried on the substrate using an ethanol suspension. The high resolution image of the (MoO6)6- array disappeared after heating the thin crystal in reducing atmosphere at 350°C, but restored after reoxidation in air at 500°C. Vacuum deposited MoO3 film on silica glass was amorphous and its surface was smooth and flat after heating at or below 300°C. The film crystallized on heating at 400°C accompanying a slight microstructural change at the film surface. This film exhibited sharp diffraction lines from (0k0) planes. The thin film morphology was destroyed and changed into randomly oriented particles after heating at 500°C..
169. Taizo Uda, Rie Kanmei, Shigeru Akasofu, Emi Hifumi, Michitaka Ohtaki, Application of polymer-protected ultrafine platinum particles to the immunological detection of human serum albumin, Analytical Biochemistry, 10.1006/abio.1994.1176, 218, 2, 259-264, 1994.05, The immunological properties of ultrafine platinum particles protected with poly(methyl acrylate)-co-N-vinyl-2-pyrrolidone, having a group chemically reactive with protein, were examined. Polymer-protected ultrafine particles were able to chemically bond multiple human serum albumin (HSA) molecules on their surface. The particles behaved as a multiepitope-type antigen for the immunoreaction. In an ELISA, the HSA-bound particles and HSA molecules competitively reacted against the anti-HSA monoclonal antibodies. The limit of detection of HSA was ca. 10 ng/ml in the assay. The particles were used in an immunodot analysis of anti-HSA polyclonal antibody. Within 30 min of the reaction, 110 μg/ml of HSA was detected. The protein fixed polymer-protected ultrafine particles are a useful new material in immunochemistry..
170. Michitaka Ohtaki, Daisuke Ogura, Koichi Eguchi, Hiromichi Arai, High-temperature thermoelectric properties of ln2O3-based mixed oxides and their applicability to thermoelectric power generation, Journal of Materials Chemistry, 10.1039/JM9940400653, 4, 5, 653-656, 1994.01, The thermoelectric properties of mixed oxides ln2O3-MOx(MOx=Cr2O3, Mn2O3, NiO, ZnO, Y2O3, Nb2O5, SnO2) are investigated in terms of the thermoelectric materials at high temperature. The Seebeck coefficients, S, of all the samples have negative values, and those of ln2O3•SnO2 and ln2O3•ZnO increase linearly with temperature, attaining values of -90 and -210 μV K-1 at 1000 °C, respectively. The electrical conductivities, σ, of these oxides are significantly high. The power factor S2σ of the oxides has constantly positive temperature coefficients up to over 1000 °C. Rather low thermal conductivities, k, of the sintered bodies of the oxides, ca. 1.7 W m-1 K-1 at room temperature, lead to the largest value of the thermoelectric figure of merit Z=0.4 × 10-4 K-1 for M = Sn at 1000 °C, and the Z value increases toward higher temperatures..
171. Michitaka Ohtaki, Jun Peng, Koichi Eguchi, Hiromichi Arai, Oxygen sensing properties of Ti-doped Nb2O5, Sensors and Actuators: B. Chemical, 10.1016/0925-4005(93)85062-F, 14, 1-3, 495-496, 1993.06.
172. Michitaka Ohtaki, K. Sumiyoshi, T. Tamai, K. Eguchi, H. Arai, Effect of barium substitution at the yttrium site in YBa2Cu3O7-y prepared from nitrate precursors, Journal of Materials Science Letters, 10.1007/BF00729402, 11, 5, 249-251, 1992.01.
173. Naoki Toshima, Michitaka Ohtaki, Toshiharu Teranishi, Substrate selectivity by the polymer support in hydrogenation over crosslinked polymer-immobilized metal catalysts, Reactive Polymers, 10.1016/0923-1137(91)90157-J, 15, C, 135-145, 1991.11, Metal clusters were immobilized on crosslinked polymers by two methods. One is the immobilization of polymer-protected platinum or rhodium clusters by covalent bond formation between the protective polymer and the crosslinked polymer support. The immobilization was attributed to the formation of amide bonds by the reaction of methyl acrylate residues of the protective polymer with amino groups of the aminoethylated polyacrylamide gel. In the other method, palladium clusters were immobilized on chelate resin-metal complexes by reduction of palladium ions supported on the resin complexes which are characterised by high porosity. In this case the chelate resin works as both the support and as the protective polymer of the clusters. Both the immobilized metal clusters preferably catalyzed the hydrogenation of C=C bonds in hydrophilic substrates, in particular the substrate containing a carboxyl group. This substrate selectivity can be explained by the acid-base interaction between the substrate and the polymer support..
174. Michitaka Ohtak, Makoto Komiyama, Hidefumi Hirai, Naoki Toshima, Effects of Polymer Support on the Substrate Selectivity of Covalently Immobilized Ultrafine Rhodium Particles as a Catalyst for Olefin Hydrogenation, Macromolecules, 10.1021/ma00020a014, 24, 20, 5567-5572, 1991.09, Ultrafine rhodium particles prepared by alcohol reduction in the presence of a protective polymer were covalently immobilized onto a cross-linked polymer support by forming the amide bond between the primary amino group contained in the support and the methyl acrylate residue in the protective polymer of the particles. Catalytic activity of the immobilized particles was investigated in catalytic hydrogenation of olefins. The activity strongly depended on the substrates used in the reactions. The immobilized ultrafine rhodium particles preferentially hydrogenated hydrophilic substrates. In particular, the substrates containing a carboxyl group were hydrogenated by the immobilized catalyst more easily than by the dispersed catalyst of the ultrafine rhodium particles before the immobilization. The difference in the hydrogenation rates can be correlated with the solubility parameter of the substrates. Electrostatic interaction between the substrates and the polymer support leads to increased reaction rates..