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Materials Science and Engineering
Yu Pan, Mengyu Yao, Xiaochen Hong, Yifan Zhu, Fengren Fan, Kazuki Imasato, Yangkun He, Christian Hess, Jörg Fink, Jiong Yang, Bernd Büchner, Chenguang Fu, G. Jeffrey Snyder, and Claudia Felser
Energy and Environmental Science, ISSN: 17545692, eISSN: 17545706, Pages: 1717-1724, Published: June 2020 Royal Society of Chemistry (RSC)
Ternary Mg3(Bi,Sb)2 single crystals showing high thermoelectric performance are for the first time grown by the Mg flux method.
Kazuki Imasato, Chenguang Fu, Yu Pan, Max Wood, Jimmy Jiahong Kuo, Claudia Felser, and G. Jeffrey Snyder
Advanced Materials, ISSN: 09359648, eISSN: 15214095, Published: 1 April 2020 Wiley
Mg3 (Sb,Bi)2 alloys have recently been discovered as a competitive alternative to the state-of-the-art n-type Bi2 (Te,Se)3 thermoelectric alloys. Previous theoretical studies predict that single crystals Mg3 (Sb,Bi)2 can exhibit higher thermoelectric performance near room temperature by eliminating grain boundary resistance. However, the intrinsic Mg defect chemistry makes it challenging to grow n-type Mg3 (Sb,Bi)2 single crystals. Here, the first thermoelectric properties of n-type Te-doped Mg3 Sb2 single crystals, synthesized by a combination of Sb-flux method and Mg-vapor annealing, is reported. The electrical conductivity and carrier mobility of single crystals exhibit a metallic behavior with a typical T-1.5 dependence, indicating that phonon scattering dominates the charge carrier transport. The absence of any evidence of ionized impurity scattering in Te-doped Mg3 Sb2 single crystals proves that the thermally activated mobility previously observed in polycrystalline materials is caused by grain boundary resistance. Eliminating this grain boundary resistance in the single crystals results in a large enhancement of the weighted mobility and figure of merit zT by more than 100% near room temperature. This work experimentally demonstrates the accurate understanding of charge-carrier scattering is crucial for developing high-performance thermoelectric materials and indicates that single-crystalline Mg3 (Sb,Bi)2 solid solutions can exhibit higher zT compared to polycrystalline samples.
Max Wood, Kazuki Imasato, Shashwat Anand, Jiong Yang, and G. Jeffrey Snyder
Journal of Materials Chemistry A, ISSN: 20507488, eISSN: 20507496, Pages: 2033-2038, Published: 2020 Royal Society of Chemistry (RSC)
Herein we study the effect alloying Yb onto the octahedral cite of Te doped Mg3Sb1.5Bi0.5 has and show that the reduction in mobility can be explained with an alloy scattering argument due to disrupting the Mgoctahedral–Mgtetrahedral interaction.
C. Fu, Mengyu Yao, X. Chen, L. Z. Maulana, X. Li, J. Yang, Kazuki Imasato, Fengfeng Zhu, G. Li, G. Auffermann, U. Burkhardt, W. Schnelle, Jianshi Zhou, Tiejun Zhu, Xinbing Zhao, M. Shi, M. Dressel, A. Pronin, G. Snyder and C. Felser
Advanced Science, eISSN: 21983844, Published: 1 January 2020
Abstract Accurate determination of the intrinsic electronic structure of thermoelectric materials is a prerequisite for utilizing an electronic band engineering strategy to improve their thermoelectric performance. Herein, with high‐resolution angle‐resolved photoemission spectroscopy (ARPES), the intrinsic electronic structure of the 3D half‐Heusler thermoelectric material ZrNiSn is revealed. An unexpectedly large intrinsic bandgap is directly observed by ARPES and is further confirmed by electrical and optical measurements and first‐principles calculations. Moreover, a large anisotropic conduction band with an anisotropic factor of 6 is identified by ARPES and attributed to be one of the most important reasons leading to the high thermoelectric performance of ZrNiSn. These successful findings rely on the grown high‐quality single crystals, which have fewer Ni interstitial defects and negligible in‐gap states on the electronic structure. This work demonstrates a realistic paradigm to investigate the electronic structure of 3D solid materials by using ARPES and provides new insights into the intrinsic electronic structure of the half‐Heusler system benefiting further optimization of thermoelectric performance.
Kazuki Imasato, Stephen Dongmin Kang, and G. Jeffrey Snyder
Energy and Environmental Science, ISSN: 17545692, eISSN: 17545706, Pages: 965-971, Published: March 2019 Royal Society of Chemistry (RSC)
An n-type material with intrinsically higher thermoelectric conversion efficiency than Bi2Te3 in the low-grade waste-heat range has finally been developed.
Mingyi Wang, Ramya Gurunathan, Kazuki Imasato, Nicholas R. Geisendorfer, Adam E. Jakus, Jun Peng, Ramille N. Shah, Matthew Grayson, and G. Jeffrey Snyder
Advanced Theory and Simulations, eISSN: 25130390, Published: 1 February 2019 Wiley
Maxwell Wood, Jimmy Jiahong Kuo, Kazuki Imasato, and Gerald Jeffrey Snyder
Advanced Materials, ISSN: 09359648, eISSN: 15214095, Published: 2019 Wiley
Materials with high zT over a wide temperature range are essential for thermoelectric applications. n-Type Mg3 Sb2 -based compounds have been shown to achieve high zT at 700 K, but their performance at low temperatures (<500 K) is compromised due to their highly resistive grain boundaries. Syntheses and optimization processes to mitigate this grain-boundary effect has been limited due to loss of Mg, which hinders a sample's n-type dopability. A Mg-vapor anneal processing step that grows a sample's grain size and preserves its n-type carrier concentration during annealing is demonstrated. The electrical conductivity and mobility of the samples with large grain size follows a phonon-scattering-dominated T-3/2 trend over a large temperature range, further supporting the conclusion that the temperature-activated mobility in Mg3 Sb2 -based materials is caused by resistive grain boundaries. The measured Hall mobility of electrons reaches 170 cm2 V-1 s-1 in annealed 800 °C sintered Mg3 + δ Sb1.49 Bi0.5 Te0.01 , the highest ever reported for Mg3 Sb2 -based thermoelectric materials. In particular, a sample with grain size >30 mm has a zT 0.8 at 300 K, which is comparable to commercial thermoelectric materials used at room temperature (n-type Bi2 Te3 ) while reaching zT 1.4 at 700 K, allowing applications over a wider temperature scale.
Matthias T. Agne, Kazuki Imasato, Shashwat Anand, Kathleen Lee, Sabah K. Bux, Alex Zevalkink, Alexander J.E. Rettie, Duck Young Chung, Mercouri G. Kanatzidis, and G. Jeffrey Snyder
Materials Today Physics, eISSN: 25425293, Pages: 83-88, Published: August 2018 Elsevier BV
Abstract The thermoelectric figure of merit reported for n-type Mg3(Sb,Bi)2 compounds has made these materials of great engineering significance, increasing the need for accurate evaluations of their thermal conductivity. Thermal conductivity is typically derived from measurements of thermal diffusivity and determination of the specific heat capacity. The uncertainty in this method (often 10% or more) is frequently attributed to measurement of heat capacity such that estimated values are often more accurate. Inconsistencies between reported thermal conductivity of Mg3(Sb,Bi)2 compounds may be attributed to the different values of heat capacity measured or used to calculate thermal conductivity. The high anharmonicity of these materials can lead to significant deviations at high temperatures from the Dulong-Petit heat capacity, which is often a reasonable substitute for measurements at high temperatures. Herein, a physics-based model is used to assess the magnitude of the heat capacity over the entire temperature range up to 800 K. The model agrees in magnitude with experimental low-temperature values and reproduces the linear slope observed in high-temperature data. Owing to the large scatter in experimental values of high-temperature heat capacity, the model is likely more accurate (within ±3%) than a measurement of a new sample even for doped or alloyed materials. It is found that heat capacity for the solid solution series can be simply described (for temperatures: 200 K ≤ T ≤ 800 K ) by the polynomial equation: c p [ Jg − 1 K − 1 ] = 3 N R M W ( 1 + 1.3 × 10 − 4 T − 4 × 10 3 T − 2 ) , where 3 N R = 124.71 J mol − 1 K − 1 , M W is the molecular weight [ g mol − 1 ] of the formula unit being considered, and T is temperature in K. This heat capacity is recommended to be a standard value for reporting and comparing the thermal conductivity of Mg3(Sb,Bi)2 including doped or alloyed derivatives. A general form of the equation is given which can be used for other material systems.
Biao Xu, Tianli Feng, Matthias T. Agne, Qing Tan, Zhe Li, Kazuki Imasato, Lin Zhou, Je-Hyeong Bahk, Xiulin Ruan, G. Jeffery Snyder, and Yue Wu
Angewandte Chemie - International Edition, ISSN: 14337851, eISSN: 15213773, Pages: 2413-2418, Published: 23 February 2018 Wiley
Reconstructing canonical binary compounds by inserting a third agent can significantly modify their electronic and phonon structures. Therefore, it has inspired the semiconductor communities in various fields. Introducing this paradigm will potentially revolutionize thermoelectrics as well. Using a solution synthesis, Bi2 S3 was rebuilt by adding disordered Bi and weakly bonded I. These new structural motifs and the altered crystal symmetry induce prominent changes in electrical and thermal transport, resulting in a great enhancement of the figure of merit. The as-obtained nanostructured Bi13 S18 I2 is the first non-toxic, cost-efficient, and solution-processable n-type material with z T=1.0.
Jimmy Jiahong Kuo, Stephen Dongmin Kang, Kazuki Imasato, Hiromasa Tamaki, Saneyuki Ohno, Tsutomu Kanno, and G. Jeffrey Snyder
Energy and Environmental Science, ISSN: 17545692, eISSN: 17545706, Pages: 429-434, Published: February 2018 Royal Society of Chemistry (RSC)
The influence of grain boundaries is modelled to show that there is much room for improvement in some thermoelectric materials.
Saneyuki Ohno, Kazuki Imasato, Shashwat Anand, Hiromasa Tamaki, Stephen Dongmin Kang, Prashun Gorai, Hiroki K. Sato, Eric S. Toberer, Tsutomu Kanno, and G. Jeffrey Snyder
Joule, eISSN: 25424351, Pages: 141-154, Published: 17 January 2018 Elsevier BV
Zintl compounds make excellent thermoelectrics with many opportunities for chemically tuning their electronic and thermal transport properties. However, the majority of Zintl compounds are persistently p-type even though computation predicts superior properties when n-type. Surprisingly, n-type Mg_3Sb_2-based thermoelectrics have been recently found with exceptionally high figure of merit. Excess Mg is required to make the material n-type, prompting the suspicion that interstitial Mg is responsible. Here we explore the defect chemistry of Mg_3Sb_2 both theoretically and experimentally to explain why there are two distinct thermodynamic states for Mg_3Sb_2 (Mg-excess and Sb-excess) and why only one can become n-type. This work emphasizes the importance of exploring all of the multiple thermodynamic states in a nominally single-phase semiconductor. This understanding of the existence of multiple inherently distinct different thermodynamic states of the same nominal compound will vastly multiply the number of new complex semiconductors to be discovered for high zT thermoelectrics or other applications.
Tsutomu Kanno, Hiromasa Tamaki, Hiroki K. Sato, Stephen Dongmin Kang, Saneyuki Ohno, Kazuki Imasato, Jimmy Jiahong Kuo, G. Jeffrey Snyder, and Yuzuru Miyazaki
Applied Physics Letters, ISSN: 00036951, Volume: 112, Published: 15 January 2018 AIP Publishing
Zintl compound n-type Mg3(Sb,Bi)2 was recently found to exhibit excellent thermoelectric figure of merit zT (∼1.5 at around 700 K). To improve the thermoelectric performance in the whole temperature range of operation from room temperature to 720 K, we investigated how the grain size of sintered samples influences electronic and thermal transport. By increasing the average grain size from 1.0 μm to 7.8 μm, the Hall mobility below 500 K was significantly improved, possibly due to suppression of grain boundary scattering. We also confirmed that the thermal conductivity did not change by increasing the grain size. Consequently, the sample with larger grains exhibited enhanced average zT. The calculated efficiency of thermoelectric power generation reaches 14.5% (ΔT = 420 K), which is quite high for a polycrystalline pristine material.
Kazuki Imasato, Max Wood, Jimmy Jiahong Kuo, and G. Jeffrey Snyder
Journal of Materials Chemistry A, ISSN: 20507488, eISSN: 20507496, Pages: 19941-19946, Published: 2018 Royal Society of Chemistry (RSC)
n-Type conduction in a Mg3Sb1.5Bi0.5 system is achieved with La-doping at cation sites with a peak zT > 1.
Kazuki Imasato, Saneyuki Ohno, Stephen Dongmin Kang, and G. Jeffrey Snyder
APL Materials, eISSN: 2166532X, Published: 1 January 2018 AIP Publishing
The thermoelectric performance of Mg_(3+x)Sb_(1.5)Bi_(0.49)Te_(0.01) was improved by reducing the amount of excess Mg (x = 0.01-0.2). A 20% reduction in effective lattice thermal conductivity at 600 K was observed by decreasing the nominal xfrom 0.2 to 0.01 in Mg_(3+x)Sb_(1.5)Bi_(0.49)Te_(0.01), leading to a 20% improvement in the figure-of-merit zT. Since materials with different amounts of Mg have similar electronic properties, the enhancement is attributed primarily to the reduction in thermal conductivity. It is known that excess Mg is required to make n-type Mg_(3+x)Sb_(1.5)Bi_(0.49)Te_(0.01); however, too much excess Mg in the material increases the thermal conductivity and is therefore detrimental for the overall thermoelectric performance of the material.
Kazuki Imasato, Stephen Dongmin Kang, Saneyuki Ohno, and G. Jeffrey Snyder
Materials Horizons, eISSN: 20516355, Pages: 59-64, Published: January 2018 Royal Society of Chemistry (RSC)
The optimum Mg3Sb2–Mg3Bi2 alloy composition for thermoelectrics is modeled by analyzing transport properties and understanding changes in the electronic structure.
Thaneer Malai Narayanan, Kazuki Imasato, Satoshi Takeya, Saman Alavi, and Ryo Ohmura
Journal of Physical Chemistry C, ISSN: 19327447, eISSN: 19327455, Volume: 119, Pages: 25738-25746, Published: 19 November 2015 American Chemical Society (ACS)
Binary structure II (sII) clathrate hydrates of tetrahydropyran (THP) with methane and carbon dioxide are synthesized and characterized with powder X-ray diffraction (PXRD) and molecular dynamics (MD) simulations. Analysis of PXRD results shows that 83% of the small 12-sided (dodecahedral, D) sII cages are occupied by CO2 in CO2 + THP hydrate, whereas 93% of the D cages are occupied by CH4 in CH4 + THP hydrate. The effects of the tighter fit of CO2 molecules in the D cages of the binary hydrate were observed in longer lattice constants and lower cage occupancies compared to binary THP hydrates with CH4 molecule in the D cage. MD simulations of 1,4-twist-boat and chair conformers of the THP in the sII clathrate hydrate phase with the CO2 and CH4 help gases are performed at temperatures between 153 to 250 K. Simulations suggest shape and size of D cage guest molecules does not affect the conformation of H cage guest molecule, but the steric repulsions between CO2 molecule and cage’s water molecules limits i...
Muhammad Aifaa, Kazuki Imasato, and Ryo Ohmura
Crystal Growth and Design, ISSN: 15287483, eISSN: 15287505, Pages: 2853-2858, Published: 3 June 2015 American Chemical Society (ACS)
The growth of clathrate hydrate crystals in a flow of water saturated with simulated natural gas was visually observed. The simulated natural gas was a mixture of methane, ethane, and propane in a molar ratio of 90:7:3 or 98.5:1.4:0.1. The morphology (i.e., the shape and size) of the hydrate crystals that grew in the water flow changed depending on the system subcooling, which denotes the difference between the hydrate-equilibrium and experimental temperatures. At lower subcooling conditions, polygonal flat-plate crystals were observed. When the subcooling temperature was larger than 11.5 K, polygonal crystals were completely replaced by dendritic crystals. Crystals formed in flowing liquid water grew for a longer period of time than those in the quiescent system without any further guest supply. In addition, the coexistence of structure I and II hydrates with the 98.5:1.4:0.1 gas mixture in the continuous supply system was visually confirmed. From these observations, we note that the crystal morphology a...
Kazuki Imasato, Kotaro Murayama, Satoshi Takeya, Saman Alavi, and Ryo Ohmura
Canadian Journal of Chemistry, ISSN: 00084042, Pages: 906-912, Published: 3 March 2015 Canadian Science Publishing
The effect of substituting nitrogen heteroatoms in the cyclohexane ring of methylcyclohexane on the structure and guest dynamics of structure H (sH) clathrate hydrates with methane help gases are studied through experimental synthesis, powder X-ray diffraction (PXRD) measurements, and classical molecular dynamics simulation of methylcyclohexane and 1-methylpiperidine. The PXRD measurements were performed for temperatures in the range of 138 to 183 K, and the a axis and c axis lattice constants were determined in this temperature range. The PXRD results show the different thermal expansivity of lattice constants in both sH hydrate cages. Simulations on methylcyclohexane and 1-methylpiperidine are performed, and the effects of methane cage occupancy on the lattice constants were studied by simulations with 100% and 80% of the small and medium cages occupied by methane. The sH phases do not expand isotropically, and the a and c lattice constants can vary over a range of 0.015 Å and 0.06 Å, respectively, depending on the large cage guest and methane occupancy. Hydrogen bonding between the 1-methylpiperidine nitrogen atom and cage water in the sH hydrate are not observed in the simulations, and differences in behavior of the two sH hydrates are likely related to the differences in geometry of the large guests and occupancies of methane in the small and medium sH cages.
Kazuki Imasato, Hiroki Tokutomi, and Ryo Ohmura
Crystal Growth and Design, ISSN: 15287483, eISSN: 15287505, Pages: 428-433, Published: 7 January 2015 American Chemical Society (ACS)
This paper reports the visual observation of the formation and growth of methane hydrate crystals in the methane + decane + water three-phase quiescent system. This three-phase system is a simplified model of the condition within an oil-producing pipeline. A hydrate crystal nucleated at a random point on the water droplet surface and then became a hydrate film that covered the whole surface of the water droplet. The morphology of individual hydrate crystals grown at the water droplet surface were triangular or polygonal. The hydrate crystal size and the hydrate-covered water droplet shape distinctly varied depending on the system subcooling (ΔTsub). The hydrate crystal size got smaller and more slender with the increase in subcooling, and the hydrate-covered water droplet shape collapsed even more at a lower subcooling. To estimate the effect of non-hydrate-forming liquid existence, we compared the observational results obtained in the systems with and without liquid hydrocarbon. Although the liquid hydro...