KAZUKI IMASATO
@northwestern.edu
Materials Science and Engineering
Northwestern University
Scopus Publications
Scopus Publications
Kazuki Imasato, Hidetoshi Miyazaki, Philipp Sauerschnig, Kishor Kumar Johari, Takao Ishida, Atsushi Yamamoto, and Michihiro Ohta
American Chemical Society (ACS)
In thermoelectric and other inorganic materials research, the significance of half-Heusler (HH) compositions following the 18-electron rule has drawn interest in developing and exploiting the potential of intermetallic compounds. For the fabrication of thermoelectric modules, in addition to high-performance materials, having both p- and n-type materials with compatible thermal expansion coefficients is a prerequisite for module development. In this work, the p-type to n-type transition of valence balanced/unbalanced HH composition of Mg1-xVxNiSb was demonstrated by changing the Mg:V chemical ratio. The Seebeck coefficient and power factor of Ti-doped Mg0.57V0.33Ti0.1NiSb are -130 μV K-1 and 0.4 mW m-1 K-2 at 400 K, respectively. In addition, the reduced lattice thermal conductivity (κL < 2.5 W m-1 K-1 at 300 K) of n-type compositions was reported to be much smaller than κL of conventional HH materials. As high thermal conductivity has long been an issue for HH materials, the synthesis of p- and n-type Mg1-xVxNiSb compositions with low lattice thermal conductivity is a promising strategy for producing high-performance HH compounds. Achieving both p- and n-type materials from similar parent composition enabled us to fabricate a thermoelectric module with maximum output power Pmax ∼ 63 mW with a temperature difference of 390 K. This finding supports the benefit of exploring the huge compositional space of valence balanced/unbalanced quaternary HH compositions for further development of thermoelectric devices.
Sylvain Le Tonquesse, Hugo Bouteiller, Yoshitaka Matsushita, Araseli Cortez, Sabah K. Bux, Kazuki Imasato, Michihiro Ohta, Jean-François Halet, Takao Mori, Franck Gascoin,et al.
American Chemical Society (ACS)
Hossein Sepehri-Amin, Kazuki Imasato, Maxwell Wood, Jimmy Jiahong Kuo, and G. Jeffrey Snyder
American Chemical Society (ACS)
n-type Mg3Sb2-Mg3Bi2 alloys have been investigated as one of the most promising thermoelectric materials. To achieve high performance, a detailed understanding of the microstructure is required. Although Mg3Sb2-Mg3Bi2 is usually considered to be a complete solid solution, nanosized compositional fluctuations were observed within a matrix and in the vicinity of the grain boundary. As an inhomogeneous microstructure can be beneficial or detrimental to thermoelectric performance, it is important to investigate the evolution of compositional variations for the engineering and long-term use of these materials. Using scanning transmission electron microscopy and atom probe tomography, a Bi-rich phase and compositional fluctuations are observed in sintered and annealed samples. After annealing, the broad intergranular phase was sharpened, resulting in a greater compositional change in the intergranular region. Annealing considerably reduces the fluctuations of Bi and Mg content within the grain as observed in atom probe tomography. Weighted mobility and lattice thermal conductivity were both increased as a result of the homogenized matrix phase. The combined microstructure features of intragrain and grain boundary effects resulted in an increased thermoelectric figure-of-merit zT of Mg3Sb0.6Bi1.4. These findings imply that the optimization of thermal and electrical properties can be realized through microstructure tuning.
Kazuki Imasato, Philipp Sauerschnig, Shashwat Anand, Takao Ishida, Atsushi Yamamoto, and Michihiro Ohta
Royal Society of Chemistry (RSC)
Triple half-Heusler Mg2VNi3Sb3 was successfully synthesized by following an unconventional valence balance strategy. A new strategy to explore the huge compositional space for extended tunability of intermetallic compounds was demonstrated.
Sankalp Kota, Matthias T. Agne, Kazuki Imasato, Tarek Aly El-Melegy, Jiayi Wang, Christine Opagiste, Yexiao Chen, Miladin Radovic, G. Jeffrey Snyder, and Michel W. Barsoum
Elsevier BV
Kazuki Imasato, Maxwell Wood, Shashwat Anand, Jimmy Jiahong Kuo, and G. Jeffrey Snyder
Wiley
n‐Type Mg3Sb2‐Mg3Bi2 alloys are some of the most promising thermoelectric materials in the low–mid temperature range. While discovered relatively recently, these materials have garnered intense attention, and numerous papers from the international thermoelectric community have been published in a relatively short period of time. As with all materials, detailed insights into the underlying mechanisms that contribute to these alloys’ distinguished thermoelectric properties are important for future researchers to push the performance of this material to new heights. Herein, experimental studies on the role defects, synthesis conditions, electronic band structure, and microstructure along with future prospects arecompiled to establish a guide for fully exploiting the potential of this material system. Considering the limited number of n‐type thermoelectric materials with this performance for low‐grade heat recovery and cooling technologies, further development of the Mg3Sb2‐Mg3Bi2 alloys is an important step toward commercial applications of thermoelectric materials, including cooling technologies and waste heat recovery applications.
Robert Freer, Dursun Ekren, Tanmoy Ghosh, Kanishka Biswas, Pengfei Qiu, Shun Wan, Lidong Chen, Shen Han, Chenguang Fu, Tiejun Zhu,et al.
IOP Publishing
Abstract This paper presents tables of key thermoelectric properties, which define thermoelectric conversion efficiency, for a wide range of inorganic materials. The twelve families of materials included in these tables are primarily selected on the basis of well established, internationally-recognized performance and promise for current and future applications: tellurides, skutterudites, half Heuslers, Zintls, Mg–Sb antimonides, clathrates, FeGa3-type materials, actinides and lanthanides, oxides, sulfides, selenides, silicides, borides and carbides. As thermoelectric properties vary with temperature, data are presented at room temperature to enable ready comparison, and also at a higher temperature appropriate to peak performance. An individual table of data and commentary are provided for each family of materials plus source references for all the data.
Kazuki Imasato, Shashwat Anand, Ramya Gurunathan, and G. Jeffrey Snyder
Royal Society of Chemistry (RSC)
The effect of Mg3As2 alloying on thermoelectric properties of Mg3(Sb, Bi)2 has been investigated. While the crystal structure of pure Mg3As2 is different from Mg3(Sb, Bi)2, at least 15% arsenic solubility on anion site is confirmed.
Ting Luo, Jimmy J. Kuo, Kent J. Griffith, Kazuki Imasato, Oana Cojocaru‐Mirédin, Matthias Wuttig, Baptiste Gault, Yuan Yu, and G. Jeffrey Snyder
Wiley
Tyler J. Slade, Shashwat Anand, Max Wood, James P. Male, Kazuki Imasato, Dean Cheikh, Muath M. Al Malki, Matthias T. Agne, Kent J. Griffith, Sabah K. Bux,et al.
Elsevier BV
Summary High phonon velocities, i.e., as measured by the speed of sound (vs) lead to high lattice thermal conductivity (κlat), which is detrimental to thermoelectric performance. Conventional wisdom associates vs exclusively with structural features such as average atomic mass but not the number of conducting electrons. Here, we demonstrate vs reduction from electronic doping in eight well-known thermoelectric semiconductors and establish carrier density nH as the main cause for the observed lattice softening by ruling out alternative factors such as changes in density, average atomic mass, and defect formation. In p-type SnTe and n-type La3–xTe4, we find respective decreases of 16% and ∼20% in vs when raising the nH from ∼1019 to 1021 cm–3, which is sufficient to decrease κlat by nearly 50%. Such giant softening effects can account for 25% of the optimized thermoelectric figure of merit (zTmax) in high-performing materials (zTmax > 1) by suppressing total thermal conductivity.
Yu Pan, Feng‐Ren Fan, Xiaochen Hong, Bin He, Congcong Le, Walter Schnelle, Yangkun He, Kazuki Imasato, Horst Borrmann, Christian Hess,et al.
Wiley
The emerging class of topological materials provides a platform to engineer exotic electronic structures for a variety of applications. As complex band structures and Fermi surfaces can directly benefit thermoelectric performance it is important to identify the role of featured topological bands in thermoelectrics particularly when there are coexisting classic regular bands. In this work, the contribution of Dirac bands to thermoelectric performance and their ability to concurrently achieve large thermopower and low resistivity in novel semimetals is investigated. By examining the YbMnSb2 nodal line semimetal as an example, the Dirac bands appear to provide a low resistivity along the direction in which they are highly dispersive. Moreover, because of the regular-band-provided density of states, a large Seebeck coefficient over 160 µV K-1 at 300 K is achieved in both directions, which is very high for a semimetal with high carrier concentration. The combined highly dispersive Dirac and regular bands lead to ten times increase in power factor, reaching a value of 2.1 mW m-1 K-2 at 300 K. The present work highlights the potential of such novel semimetals for unusual electronic transport properties and guides strategies towards high thermoelectric performance.
Yue Lin, Maxwell Wood, Kazuki Imasato, Jimmy Jiahong Kuo, David Lam, Anna N. Mortazavi, Tyler J. Slade, Stephen A. Hodge, Kai Xi, Mercouri G. Kanatzidis,et al.
Royal Society of Chemistry (RSC)
Expression of energy filtering to boost thermoelectric performance through grain boundary engineering utilising graphene.
Yu Pan, Mengyu Yao, Xiaochen Hong, Yifan Zhu, Fengren Fan, Kazuki Imasato, Yangkun He, Christian Hess, Jörg Fink, Jiong Yang,et al.
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
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
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.
Chenguang Fu, Mengyu Yao, Xi Chen, Lucky Zaehir Maulana, Xin Li, Jiong Yang, Kazuki Imasato, Fengfeng Zhu, Guowei Li, Gudrun Auffermann,et al.
Wiley
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.
Maxwell Wood, Jimmy Jiahong Kuo, Kazuki Imasato, and Gerald Jeffrey Snyder
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.
Kazuki Imasato, Stephen Dongmin Kang, and G. Jeffrey Snyder
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
Wiley
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
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,et al.
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
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
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.