||Michitaka Ohtaki, Nanostructure Engineering in Oxide Thermoelectric Materials, 3rd Global Congress & Expo on Materials Science & Engineering, 2018.06.
||Michitaka Ohtaki, Development of Oxide Ceramics for Thermoelectric Power Generation Applications, 42nd International Conference and Expo on Advanced Ceramics and Composites (ICACC2018), 2018.01.
||Michitaka Ohtaki, Nanostructure Engineering in Oxide Thermoelectric Materials for Higher Energy Efficiency, The 13th International Conference on Ecomaterials (ICEM13), 2017.11.
||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.
||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.
||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.
||Michitaka Ohtaki, Nanostructure Engineering for High-performance Oxide Thermoelectric Materials, 3rd World Congress on Materials Science & Engineering (Materials Science & Engineering), 2017.08.
||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.
||Michitaka Ohtaki, Kohei Mizuta, Koichiro Suekuni, Ultra-low Thermal Conductivity in β-Pyrochlore-type Oxides, 36th International Conference on Thermoelectrics (ICT2017), 2017.08.
||Michitaka Ohtaki, Nanostructure Engineering in Oxide Ceramics for High Temperature Thermoelectric Energy Conversion
, 15th Conference & Exhibition of the European Ceramic Society (ECerS2017), 2017.07.
||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.
||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.
||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.
||Michitaka Ohtaki, Nanostructure and Phonon Engineering in Oxide Thermoelectric Materials, TMS 2017 146th Annual Meeting and Exhibition, 2017.02.
||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.
||Michitaka Ohtaki, Oxide Thermoelectric Materials for Waste Heat Energy Recuperation, Global Congress & Expo on Materials Science & Nanoscience (Materials Science-2016), 2016.10.
||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.
||Michitaka Ohtaki and Kohei Mizuta, Enhanced Phonon Scattering in Cage-Like Structure Oxides with Rattling Atoms, 2016 MRS Spring Meeting and Exhibit, 2016.03.
||Michitaka Ohtaki, Oxide Materials with Ultra-low Lattice Thermal Conductivity, The EMN Meeting on Thermoelectric Materials, 2016.02.
||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.
||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.
||Kosuke Watanabe and Michitaka Ohtaki, Thermoelectric Properties of Ag/ZnO Sintered Bodies Prepared from Soft-chemical Precursors, 2015 MRS Fall Meeting & Exhibit, 2015.12.
||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.
||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.
||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.
||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.
||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.
||Michitaka Ohtaki, Enhanced Phonon Scattering in Nanostructured Oxide Materials, XVI International Forum on Thermoelectricity, 2015.05.
||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.
||Michitaka Ohtaki, New Aspects of Oxide Ceramic Materials for Thermoelectric Energy Conversion, Energy, Materials, and Nanotechnology (EMN) Meeting on Ceramics, 2015.01.
||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.
||Michitaka Ohtaki, Yuki Iwano, Enhanced Phonon Scattering and Carrier Compensation in Multinary-doped ZnO Thermoelectric Oxide, 2014 MRS Fall Meeting, 2014.12.
||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.
||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.
||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..
||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  and cage-like structure oxides  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 .
||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.
||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.
||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.
||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.
||Michitaka Ohtaki, Oxide Materials with Unconventional Transport Properties Promising for Thermoelectrics, The 15th Asian Chemical Congress (15ACC),, 2013. 8. 19-23. , 2013.08.
||Michitaka Ohtaki, Nanostructured Oxide Ceramics for Efficient Thermoelectric Energy Conversion, 12th International Conference on Ceramic Processing Science (12-ICCPS), 2013.08.
||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.
||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.
||Michitaka Ohtaki, Oxide Materials for Mid-to-High Temperature
Thermoelectric Energy Conversion, The 29th International Korea-Japan Seminar on Ceramics, 2012.11.
||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.
||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.
||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.
||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.
||Michitaka Ohtaki, Yuki Iwano, Thermoelectric Properties and Impurity-related Heterogeneous Microstructure of Doped ZnO,
2012 Spring Meeting of E-MRS, 2012.05.
||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 . 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.
 M. Ohtaki, T. Tsubota, K. Eguchi, H. Arai, J. Appl. Phys., 79, 1816-1818 (1996).
 T. Tsubota, M. Ohtaki, K. Eguchi, H. Arai, J. Mater. Chem., 7, 85-90 (1997).
 M, Ohtaki, K, Araki, K. Yamamoto, J. Electron. Mater., 38, 1234-1238 (2009)..