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Michitaka Ohtaki Last modified date:2021.07.27

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


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Homepage
https://kyushu-u.pure.elsevier.com/en/persons/michitaka-ohtaki
 Reseacher Profiling Tool Kyushu University Pure
http://www.asem.kyushu-u.ac.jp/~ohtaki/
Ohtaki-Suekuni Laboratory Official Website .
Phone
092-583-7947
Fax
092-583-7947
Academic Degree
Doctor of Engineering
Country of degree conferring institution (Overseas)
No
Field of Specialization
Applied Physical Chemistry, Inorganic Materials Chemistry
Total Priod of education and research career in the foreign country
00years00months
Research
Research Interests
  • Oxide thermoelectric materials for recuperating unused waste heat energy
    keyword : thermoelectric material, oxide semiconductor, energy conversion material, conductive ceramics, thermal conductivity, zinc oxide
    1991.07Development of oxide thermoelectric materials for waste heat recovery.
  • Development of oxide thermoelectric modules for mid-to-high temperature waste heat recovery
    keyword : zinc oxide, layered cobalt oxide, microstructure control, long-term stability assessment
    2002.11Development of oxide thermoelectric modules for mid- to high-temperature power generation applications.
  • Self-assembly synthesis and properties of inorganic nanomaterials with low-dimensional quantum confinement structures
    keyword : low-dimensional nanomaterial, molecular assembly, self-assembly, nano-superlattice, photocatalyst, magnetism, inorganic-organic hybrid
    1996.04Self-assembly synthesis of inorganic nanomaterials with low-dimensional quantum confinement structures.
  • Self-assembly low-dimensional nanomaterials showing quantum-confined properties
    keyword : quantum-confined nanomaterials, spatial regularity, nanospace, interlayer interaction
    1998.04.
Current and Past Project
  • "Development of Thermoelectric Power Generator Modules for 500C-Class Waste Heat Recovery (Ohtaki Project)" Project Director: Michitaka Ohtaki (Kyushu Univ.)
    "Science and Technology Incubation Program in Advanced Region" at Innovation Plaza Fukuoka, sponsored by Japan Science and Technology Agency (JST)
  • "Creation of Thermoelectric Oxide Materials with Layered Structures through Nano-Block Integration" Research Director: Prof. K. Koumoto (Nagoya Univ.)
    Leader of Kyushu University Group: Michitaka Ohtaki
    "Development of Advanced Nanostructured Materials for Energy Conversion and Storage" Research Area; Research Supervisor: Akira Fujishima
    Proposal-Oriented Type Research, Core Research for Evolutional Science and Technology(CREST-Type) of JST Basic Research, sponsored by Japan Science and Technology Agency (JST)
Academic Activities
Books
1. Michitaka Ohtaki, Oxide Thin Films, Multilayers and Nanocomposites, Springer, Chapter 7, Nanostructured Oxide Thermoelectric Materials with Enhanced Phonon Scattering, pp. 109-122., 2015.03.
2. Ed. D. M. Rowe, Thermoelectrics Handbook: macro to nano, CRC Press, ISBN: 0-8493-2264-2, pp.35-1〜35-15, 2006.01.
3. Ed. K. Koumoto, I. Terasaki, N. Murayama, Oxide Thermoelectrics, Research Signpost, ISBN 81-7736-100-7, pp. 159-180., 2002.12.
Reports
1. 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, 119 (11), 770-775, 2011.11.
Papers
1. 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.
2. 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.
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. 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.
5. 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.
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. 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.
8. 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.
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. 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.
12. 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..
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. 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.
24. 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..
25. 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.
26. 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.
27. 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.
28. 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.
29. 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.
30. 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.
31. 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.
32. 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.
33. 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..
34. 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.
35. 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.
36. 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.
37. 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.
38. 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.
39. 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.
40. 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.
41. 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.
42. 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.
43. 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.
44. 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.
45. 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.
46. 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.
47. 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.
48. M. Ohtaki, K. Araki, K. Yamamoto, High Thermoelectric Performance of Dually Doped ZnO Ceramics, Journal of Electronic Materials, 38, 1234-1238, 2009.06.
49. 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.
50. 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.
51. 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.
52. 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.
53. 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.
54. 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.
55. 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.
56. N. V. Nong, M. Ohtaki, Power Factors of Late Rare Earth-doped Ca3Co2O6 Oxides, Solid State Commun., 139(5), 232-234., 2006.08.
57. 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.
58. H. Hirobe, M. Ohtaki, Enhanced Phonon Scattering by Oxygen Defects in Metal Oxides, Trans. Mater. Res. Soc. Jpn., 31(2), 403-406., 2006.06.
59. T. Souma, M. Ohtaki, Synthesis and Rietveld Analysis of Zn4-xCdxSb3 Bulk Crystals in the Zn-rich Region, J. Alloy. Compd., 413, 289-297, 2006.03.
Presentations
1. Sasikan Maneeyom, Koichiro Suekuni, Michitaka Ohtaki, Synthesis and Thermoelectric Properties of β-pyrochlore Oxide CsW2-xRuxO6, The 13th Pacific Rim Conference of Ceramic Societies (PACRIM13)、2019年10月27〜31日、沖縄コンベンションセンター(沖縄県宜野湾市), 2019.10.
2. Nhat Quang Minh Tran, Koichiro Suekuni, Michitaka Ohtaki, Anisotropic Thermoelectric Properties of W18O49 Prepared by SPS, The 13th Pacific Rim Conference of Ceramic Societies (PACRIM13)、2019年10月27〜31日、沖縄コンベンションセンター(沖縄県宜野湾市), 2019.10.
3. Wojciech Klich, Koichiro Suekuni, Michitaka Ohtaki, Thermoelectric Performance of Mo-doped bulk In2O3, The 13th Pacific Rim Conference of Ceramic Societies (PACRIM13)、2019年10月27〜31日、沖縄コンベンションセンター(沖縄県宜野湾市), 2019.10.
4. Sasikan Maneeyom, Koichiro Suekuni, Michitaka Ohtaki, Thermoelectric properties of Ru-doped β-pyrochlore CsW2O6, The 38th International Conference on Thermoelectrics & the 4th Asian Conference on Thermoelectrics (ICT/ACT2019), 2019.06.
5. Nhat Q. M. Tran, Koichiro Suekuni, Michitaka Ohtaki, Nano-structured ReO3-type Building Blocks for Reduced Lattice Thermal Conductivity in High Temperature WO2.70 Penta-columnar Phase, The 38th International Conference on Thermoelectrics & the 4th Asian Conference on Thermoelectrics (ICT/ACT2019), 2019.06.
6. Takuya Tanimoto, Koichiro Suekuni, Hirotaka Nishiate, Chul-Ho Lee, Michitaka Ohtaki, Thermoelectric Properties of P- and Si-substituted Tetrahedrites, The 38th International Conference on Thermoelectrics & the 4th Asian Conference on Thermoelectrics (ICT/ACT2019), 2019.06.
7. Koichiro Suekuni, Hidetomo Usui, Siying Qiao, Katsuaki Hashikuni, Tatsuya Hirano, Hirotaka Nishiate, Chul-Ho Lee, Kazuhiko Kuroki, Kosuke Watanabe, Michitaka Ohtaki, Quasi-one-dimensional Electronic Structure and Low Thermal Conductivity in Sn1.2Ti0.8S3, The 38th International Conference on Thermoelectrics & the 4th Asian Conference on Thermoelectrics (ICT/ACT2019), 2019.06.
8. Katsuaki Hashikuni, Koichiro Suekuni, Hidetomo Usui, Raju Chetty, Michihiro Ohta, Kazuhiko Kuroki, Toshiro Takabatake, Kosuke Watanabe, Michitaka Ohtaki, Rhombohedral CuTi2S4: a thiospinel-derived thermoelectric material, The 38th International Conference on Thermoelectrics & the 4th Asian Conference on Thermoelectrics (ICT/ACT2019), 2019.06.
9. Tatsuya Hirano, Koichiro Suekuni, Katsuaki Hashikuni, Hirotaka Nishiate, Chul-Ho Lee, Toshiro Takabatake, Michitaka Ohtaki, Effects of Se Substitution on the Thermoelectric Properties of (SnS)1.2(TiS2)n (n = 1, 2), The 37th International and 16th European Conference on Thermoelectrics (ICT/ECT2018), 2018.07.
10. Michitaka Ohtaki, Nanostructure Engineering in Oxide Thermoelectric Materials, 3rd Global Congress & Expo on Materials Science & Engineering, 2018.06.
11. Michitaka Ohtaki, Development of Oxide Ceramics for Thermoelectric Power Generation Applications, 42nd International Conference and Expo on Advanced Ceramics and Composites (ICACC2018), 2018.01.
12. Michitaka Ohtaki, Ultra-low Thermal Conductivity in Oxides with Rattler-in-a-Cage Crystal Structure Promising for High-temperature Thermoelectric Conversion, TThe 6th International Symposium on Advanced Ceramics and Technology for Sustainable Energy Applications toward a Low Carbon Society (ACTSEA 2017), 2017.11.
13. Michitaka Ohtaki, Self-assembly Synthesis and Physicochemical Properties of Highly Regulated Layered Oxides by Two-dimensionally Confined Growth in Single-nanometer Thickness
, The 6th International Workshop on Nanotechnology and Application (IWNA 2017), 2017.11.
14. Michitaka Ohtaki, Nanostructure Engineering in Oxide Thermoelectric Materials for Higher Energy Efficiency, The 13th International Conference on Ecomaterials (ICEM13), 2017.11.
15. Michitaka Ohtaki, Rattling Phenomena in Oxides for Enhanced Phonon Scattering, Workshop on Advanced Materials and Principles to Develop Viable Thermoelectrics and Advanced Thermal Management, 2017.09.
16. Michitaka Ohtaki, Kohei Mizuta, Koichiro Suekuni, Ultra-low Thermal Conductivity in β-Pyrochlore-type Oxides, 36th International Conference on Thermoelectrics (ICT2017), 2017.08.
17. Katsuaki Hashikuni, Koichiro Suekuni, Kosuke Watanabe, Bouyrie Yohan, Michihiro Ohta, Michitaka Ohtaki, and Toshiro Takabatake, Carrier Density Tuning in Cu2CoTi3S8 by Oxidative Extraction of Cu, 36th International Conference on Thermoelectrics (ICT2017), 2017.08.
18. Michitaka Ohtaki, Nanostructure Engineering for High-performance Oxide Thermoelectric Materials, 3rd World Congress on Materials Science & Engineering (Materials Science & Engineering), 2017.08.
19. Michitaka Ohtaki, Nanostructure Engineering in Oxide Ceramics for High Temperature Thermoelectric Energy Conversion
, 15th Conference & Exhibition of the European Ceramic Society (ECerS2017), 2017.07.
20. Michitaka Ohtaki, Thermoelectric Power Generation ― from fundamentals to applications with a demo of how to generate electricity , The Third International Workshop on Nano Materials for Energy Conversion (NMEC-3), 2017.05.
21. Michitaka Ohtaki, Crystal and Nanostructure Engineering in Oxide Thermoelectric Materials for Waste Heat Recovery, The Third International Workshop on Nano Materials for Energy Conversion (NMEC-3), 2017.05.
22. Michitaka Ohtaki, Kohei Mizuta, and Koichiro Suekuni, Ultra-low Thermal Conductivity in β-Pyrochlore Oxides with a Rattler-in-a-Cage Crystal Structure, The 12th Pacific Rim Conference on Ceramic and Glass Technology (PacRim12), 2017.05.
23. Michitaka Ohtaki, Nanostructure and Phonon Engineering in Oxide Thermoelectric Materials, TMS 2017 146th Annual Meeting and Exhibition, 2017.02.
24. Michitaka Ohtaki, Thermal Conductivity Reduction in Oxides for Thermoelectric Energy Conversion, The 33rd International Korea-Japan Seminar on Ceramics (K-J Ceramics 33), 2016.11.
25. Michitaka Ohtaki, Oxide Thermoelectric Materials for Waste Heat Energy Recuperation, Global Congress & Expo on Materials Science & Nanoscience (Materials Science-2016), 2016.10.
26. Michitaka Ohtaki, ZnO-based Oxide Thermoelectric Materials with Microstructure Control for Selective Phonon Scattering, Materials Challenges in Alternative & Renewable Energy Conference (MCARE) 2016, 2016.04.
27. Michitaka Ohtaki and Kohei Mizuta, Enhanced Phonon Scattering in Cage-Like Structure Oxides with Rattling Atoms, 2016 MRS Spring Meeting and Exhibit, 2016.03.
28. Michitaka Ohtaki, Oxide Materials with Ultra-low Lattice Thermal Conductivity, The EMN Meeting on Thermoelectric Materials, 2016.02.
29. Kosuke Watanabe and Michitaka Ohtaki, Thermoelectric Properties of Ag/ZnO Sintered Bodies Prepared from Soft-chemical Precursors, 2015 MRS Fall Meeting & Exhibit, 2015.12.
30. Kohei Mizuta and Michitaka Ohtaki, The Crystal Structures and Thermoelectric Properties of Defect Pyrochlore Oxides with Anharmonic Vibration, 2015 MRS Fall Meeting & Exhibit, 2015.12.
31. Michitaka Ohtaki, Yuki Iwano, and Taiga Kuragaki, Thermoelectric Performance of Multinary-doped ZnO with Enhanced Phonon Scattering, The 3rd International Renewable and Sustainable Energy Conference (IRSEC'15), 2015.12.
32. Michitaka Ohtaki, Recent Progress in Oxide Thermoelectric Materials with Microstructure Control for Selective Phonon Scattering, 14th International Union of Materials Research Societies- International Conference on Advanced Materials (IUMRS-ICAM2015), 2015.10.
33. Michitaka Ohtaki, Taiga Kuragaki, Yuki Iwano, Kiyoshi Yamamoto , Selective Phonon Scattering in ZnO-based Bulk Thermoelectric Oxides


, Advanced Materials World Congress 2015 (AMWC2015), 2015.08.
34. Michitaka Ohtaki, ZnO-based Oxide Thermoelectric Materials: problems solved and unsolved yet, The 11th International Conference on Ceramic Materials and Components for Energy and Environmental Applications (CMCEE-11), 2015.06.
35. Michitaka Ohtaki and Taiga Kuragaki, Microstructure and Thermoelectric Properties of 
Ga/Cu-doped ZnO, 34th International Conference on Thermoelectrics/13th European Conference on Thermoelectrics (ICT/ECT2015), 2015.06.
36. Kohei Mizuta and Michitaka Ohtaki, Crystal structure and thermoelectric properties of β-pyrochlore-type alkali iron tungsten oxides with cage-like structure, 34th International Conference on Thermoelectrics/13th European Conference on Thermoelectrics (ICT/ECT2015), 2015.06.
37. Michitaka Ohtaki, Enhanced Phonon Scattering in Nanostructured Oxide Materials, XVI International Forum on Thermoelectricity, 2015.05.
38. Michitaka Ohtaki, Recent Aspects of Oxide Ceramic Materials for Thermoelectric Energy Conversion, BIT's 1st Annual World Congress of Smart Materials-2015 (WCSM-2015), 2015.03.
39. Michitaka Ohtaki, New Aspects of Oxide Ceramic Materials for Thermoelectric Energy Conversion, Energy, Materials, and Nanotechnology (EMN) Meeting on Ceramics, 2015.01.
40. Michitaka Ohtaki, Kohei Mizuta, Yuki Iwano, New Aspects in Oxide Thermoelectric Materials for Heat-to-Electricity Direct Energy Conversion, The 13th Eurasia Conference on Chemical Sciences (EuAsC2S-13), 2014.12.
41. Michitaka Ohtaki, Yuki Iwano, Enhanced Phonon Scattering and Carrier Compensation in Multinary-doped ZnO Thermoelectric Oxide, 2014 MRS Fall Meeting, 2014.12.
42. Michitaka Ohtaki, Self-assembly Synthesis of 2-D Semiconducting Oxide Nano-superlattice with Enhanced Photocatalytic Activity by Incorporation of Au Nanoparticles, The Second International Workshop on Nano Materials for Energy Conversion (NMEC-2), 2014.11, A highly regulated superlattice structure consisting of two-dimensional titanium oxide of ca. 1.5 nm in thickness with a d-spacing of 3 - 4 nm was synthesized in liquid phase using bilayer micelles of sodium alkylphosphate surfactant as an organic self-assembly template. A powder XRD study revealed that the product showed only (00l) diffractions in the low-2θ region, which are characteristic of a lamellar structure with a d001 spacing of ca. 4 nm. Moreover, hydrothermal (HT) post-treatment at 120 °C in distilled water markedly sharpened the (00l) diffractions, suggesting a substantial improvement in the spatial regularity of the layered structure. Precise temperature control of the solutions during the mixing and aging was revealed to be crucial to obtain highly regulated superlattice-like layered nanostructure after the HT treatment.
The optical absorption edge of the superlattice samples showed a large blue-shift of compared to that of bulk TiO2, which can be accounted for electron confinement effects within the titanium oxide layers.
Au ions were intercalated by using a phase-transfer catalyst into the hydrophobic interior of bilayer micelle template interleaving the TiO2 layers, and were subsequently reduced to obtain Au nanoparticles. The samples thus prepared showed an absorption peak in the visible light region and exhibited higher photocatalytic activity compared with layered TiO2 without Au as well as commercial TiO2 (P-25). It demonstrated that the Au nanoparticles enhanced the visible light photocatalytic activity of the layered TiO2 with the superlattice structure.
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43. Michitaka Ohtaki, Oxide Thermoelectric Materials for Power Generation From Mid-to-High Temperature Unused Heat Energy, The 16th Cross Straits Symposium on Energy and Environmental Science and Technology(CSS-EEST16), 2014.11.
44. Michitaka Ohtaki, Taiga Kuragaki, Taketoshi Tomida, High-temperature Durability of doped ZnO on Thermal Cycling under Aerobic Conditions, IUMRS International Conference in Asia (IUMRS-ICA) 2014, 2014.08, Oxide materials are promising candidates for thermoelectric power generation from waste heat at mid-to-high temperatures. The n-type thermoelectric properties of Al-doped ZnO, ZT ~ 0.3 at 1273 K, were first reported by the authors in 19961). Recently, we have obtained the ZT values of ~0.55 and ~0.65 at around 1273 K for nanostructured Al-doped ZnO and dually doped ZnO with Al and Ga, respectively2,3). These results strengthen the opportunity of using polycrystalline ZnO for low-cost practical waste heat harvesting. However, controversial results reported by several research groups on the thermoelectric performance of Al-doped ZnO have caused argument on the candidacy of ZnO-based thermoelectric materials. Therefore, it is noteworthy to investigate the influences of temperature and atmosphere on the thermoelectric properties of Al-doped ZnO systematically.

In the present paper, thermoelectric properties of ZnO doped with Al and/or other transition metals are investigated by thermally cycling the samples under various atmospheres, particularly focusing on the possible changes in the electrical conductivity and Seebeck coefficient. Repeated cycling from room temperature to 1273 K is carried out to examine the high-temperature stability of the oxide in the oxidative and inert atmospheres. The thermogravimetric analysis is also carried out up to 1573 K in order to investigate the non-stoichiometry of the oxide and to determine the desorbing species in various atmospheres at high temperature.

The results of the thermal cycling up to 1273 K in air clearly showed that the electrical conductivity and Seebeck coefficient of Al-doped ZnO sintered in N2 certainly changed during the first heating (H1), causing a large hysteresis in the first cycle (H1+C1). However, strikingly, the properties measured on the second heating (H2) were identical to those observed on the first cooling (C1), and hence the data corrected on the second cycle (H2+C2) reproduced those on C1, showing no hysteresis in the second cycle. These results demonstrate that the first heating in air up to 1273 K stabilizes the electrical properties of Al-doped ZnO sintered in N2. The details and the upper limit of the stable temperature range will be discussed..
45. Michitaka Ohtaki, Kohei Mizuta, Enhanced Phonon Scattering in Oxides with “Rattling” Atoms in Oversized Cage Structure, The 33rd International Conference on Thermoelectrics (ICT2014), 2014.07, Oxide thermoelectric materials, which are highly durable at high temperature in air, non-toxic, low cost with minimal environmental impact, are apparently promising for recuperation of decentralized waste heat energy at the temperature range of > 400 °C, where all the non-oxide candidate materials will eventually be oxidized under aerobic conditions. Although strongly ionic characters of oxide materials has been regarded as an inherent disadvantage leading to low carrier mobility and high lattice thermal conductivity, it has been revealed that such disadvantages are not always the case with all oxides. Recent reports on reduced ferroelectric oxides [1] and cage-like structure oxides [2] are convincing that the simple picture of ionic compounds no longer holds for these oxides.
In this paper, some new aspects in metal oxides that show unconventionally enhanced phonon scattering will be highlighted in terms of their transport properties and crystal structures. In particular, β-pyrochlore oxides ABB’O6 (A = K, Rb, Cs) show lower lattice thermal conductivity with decreasing the mass and size of the A-site alkali cations, clearly evidencing that the larger size mismatch between the A-site cations and the surrounding oversized cage framework enhances the “rattling” motion of the A-site cations and thereby efficiently shortens the phonon mean free path more for the smaller A-site cations. As a consequence, the thermal conductivity of the oxide with the smallest A cation, KBB’O6, was revealed to be virtually the same with its theoretical minimum, κmin [3].
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46. Michitaka Ohtaki, Kohei Mizuta, Enhanced Phonon Scattering in “Rattling” Oxides for Thermoelectric Energy Conversion, The 6th Forum on New Materials, CIMTEC 2014, 2014.06, Increasing energy demands in worldwide definitely require safer energy security options. Thermoelectric (TE) conversion is becoming more and more of vital importance for higher overall energy efficiency.
In terms of durability at high temperature in air, metal oxides are most attractive. Since the early 1990s, several oxide materials such as CaMnO3, ZnO, SrTiO3, NaCoO2, and Ca3Co4O9 have been reported to show a promising thermoelectric performance. However, strongly ionic characters and the light constituent element, oxygen, of metal oxides are apparently against the conventional guiding principles for higher ZT.
In this paper, we report the thermal and electrical properties of defect pyrochlore (β-pyrochlore) oxides AB2O6 with the “rattler-in-a-cage” structure, in which small A cations sit in an oversized cage-like crystal structure. Extremely shortened phonon mean free path with smaller A cations of in the oxides will be reported. Moreover, improvement in the electrical conduction in the B2O6 framework of the oxides will be discussed.
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47. Michitaka Ohtaki, So Miyaishi, Kohei Mizuta, New Aspects in Oxide Thermoelectric Materials with Unconventionally Enhanced Phonon Scattering, 11th European Conference on Thermoelectrics (ECT 2013), 2013.11.
48. Michitaka Ohtaki, Yuki Iwano, Oxide Thermoelectric Materials Promising for Power Generation from Mid-to-High Temperature Waste Heat, The 2nd International Symposium on Inorganic and Environmental Materials (ISIEM 2013), 2013.10.
49. Michitaka Ohtaki, Oxide Materials with Unconventional Transport Properties Promising for Thermoelectrics, The 15th Asian Chemical Congress (15ACC),, 2013. 8. 19-23. , 2013.08.
50. Michitaka Ohtaki, Nanostructured Oxide Ceramics for Efficient Thermoelectric Energy Conversion, 12th International Conference on Ceramic Processing Science (12-ICCPS), 2013.08.
51. 大瀧 倫卓, 富田 健稔, 倉垣 大河, High-temperature Stability of ZnO-based Thermoelectric Materials with Thermal Cycling under Aerobic Conditions, The 32nd International Conference on Thermoelectrics (ICT2013), 2013.07.
52. 大瀧 倫卓, Oxide Materials in Thermoelectrics: Beyond a Simple Picture of Ionic Compounds, The 10th Pacific Rim Conference on Ceramic and Glass Technology (PacRim10), 2013.06.
53. Michitaka Ohtaki, Recent Aspects of R&D for Next Generation Thermoelectric Materials, The 1st Symposium of Research and Education Center for Advanced Energy Materials, Devices, and Systems, 2013.03.
54. Michitaka Ohtaki, Oxide Materials for Mid-to-High Temperature 
Thermoelectric Energy Conversion, The 29th International Korea-Japan Seminar on Ceramics, 2012.11.
55. Michitaka Ohtaki, Oxide Thermoelectric Materials Promising for Heat-to-Electricity Direct Conversion from Mid-to-High Temperature Waste Heat, 2nd International Conference on Competitive Materials and Technology Processes (ic-cmtp2), 2012.10.
56. Michitaka Ohtaki, Yuki Iwano, So Miyaishi, Optimizing the Thermoelectric Properties of Oxide Materials: some case studies and strategies, IUMRS-International Conference on Electronic Materials (IUMRS-ICEM 2012), 2012.09.
57. Michitaka Ohtaki, Oxide Thermoelectric Materials with Bulk Nanocomposite Structures for Heat-to-Electricity Direct Conversion 
and Applicability of Self-assembly Low-dimensional Oxide Nanomaterials, First International Workshop on Nano Materials for Energy Conversion and Fuel Cell (NMEC-1), 2012.08.
58. Michitaka Ohtaki, So Miyaishi, Extremely Low Thermal Conductivity in Oxides with Cage-like Crystal Structure
, The 31st International Conference on Thermoelectrics 2012 (ICT-2012), 2012.07.
59. Michitaka Ohtaki, Yuki Iwano, Thermoelectric Properties and Impurity-related Heterogeneous Microstructure of Doped ZnO, 
2012 Spring Meeting of E-MRS, 2012.05.
60. Michitaka Ohtaki, Zinc Oxide as a Promising Material for Thermoelectric Power Generation Utilizing Wasted Heat Energy, The 12th International Confernce on Chemical Sciences (EuAsC2S-12), 2012.04, Increasing energy demands in worldwide definitely require safer energy security options. Moreover, the threat of the nuclear power plant disaster after the massive tsunami attack in Japan in March 2011 has obviously been making energy policies in Japan and many other countries shift more toward renewable energies. Thermoelectric power generation is one of the most promising energy options for better energy efficiency by recovering a huge amount of unused waste heat energy, which is currently unable to utilize and thus discarded.
Thermoelectric (TE) power generation directly converts heat (more strictly a temperature difference) to electricity by exploiting the thermoelectric effects in solid semiconductor materials. However, conventional TE materials such as Bi2Te3 and PbTe are unlikely to accomplish wide commercialization in our society because of their toxicity, poor heat resistance, and low abundance of the comprising elements. In terms of durability and stability at high temperature in air, oxide materials are obviously most attractive. Among them, ZnO [1,2] is one of the prospective n-type oxide thermoelectric materials showing a fairly high figure-of-merit, Z = S2σ/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, and κ is the thermal conductivity. In fact, the power factor, S2σ, is large enough for Al-doped ZnO to be competitive to conventional non-oxide materials such as PbTe and Si-Ge alloys. Nevertheless, the light constituting elements and strong interatomic bonding in ZnO results in a very high κ compared to conventional thermoelectric materials based on heavy elements, limiting the Z values relatively low [1,2].
In this paper, simultaneous doping of ZnO with two or more elements is investigated in order to reduce the lattice thermal conductivity, κph, a contribution of lattice vibration or phonons to the total thermal conductivity κ = κph + κel. Analyses of the carrier mobility and κph revealed that a selective reduction of κph with maintaining relatively high σ values (selective phonon scattering) would be operative in ZnO doped with Al and Ga [3]. Moreover, binary doping of Al and Cu resulted in further intense and anomalous reduction of κ. The phonon scattering will be discussed in terms of the phonon mean free path and nanostructure in the oxide with concerning the theoretical prediction of the lower limit of κ in ZnO.

References
[1] M. Ohtaki, T. Tsubota, K. Eguchi, H. Arai, J. Appl. Phys., 79, 1816-1818 (1996).
[2] T. Tsubota, M. Ohtaki, K. Eguchi, H. Arai, J. Mater. Chem., 7, 85-90 (1997).
[3] M, Ohtaki, K, Araki, K. Yamamoto, J. Electron. Mater., 38, 1234-1238 (2009)..
Membership in Academic Society
  • Chemical Society of Japan
  • The Ceramic Society of Japan
  • Thermoelectrics Society of Japan
  • The Electrochemical Society of Japan