KAZUKI IMASATO
@northwestern.edu
Materials Science and Engineering
Northwestern University
Scopus Publications
Scopus Publications
Soumei Baba, Kazuki Imasato, Atsushi Yamamoto, Takao Ishida, and Michihiro Ohta
Elsevier BV
Piyawat Piyasin, Supree Pinitsoontorn, Philipp Sauerschnig, Kazuki Imasato, and Michihiro Ohta
Royal Society of Chemistry (RSC)
The thermoelectric figure of merit zT in n-type Ni-doped NbCoSn and p-type Mn-doped NbFeSb half-Heusler (HH) compounds was successfully improved using the 18 valence electron count concept, demonstrating reliable power generation in HH-based modules.
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.
Kazuki Imasato, Philipp Sauerschnig, Masanobu Miyata, Takao Ishida, Atsushi Yamamoto, and Michihiro Ohta
Royal Society of Chemistry (RSC)
The adjustment of the Fe/Ni ratio in the double half-Heusler composition HfFexNi1−xSb leads to a p-type to n-type transition. The thermoelectric figures of merit zT = 0.36 and 0.22 at 950 K for n- and p-type, respectively, were demonstrated.
Premakumar Yanda, Leila Noohinejad, Ning Mao, Nikolai Peshcherenko, Kazuki Imasato, Abhay K. Srivastava, Yicheng Guan, Bimalesh Giri, Avdhesh Kumar Sharma, Kaustuv Manna,et al.
Wiley
AbstractColossal magnetoresistance (CMR) is an exotic phenomenon that allows for the efficient magnetic control of electrical resistivity and has attracted significant attention in condensed matter due to its potential for memory and spintronic applications. Heusler alloys are the subject of considerable interest in this context due to the electronic properties that result from the nontrivial band topology. Here, the observation of CMR near room temperature is reported in the shape memory Heusler alloy Ni2Mn1.4In0.6, which is attributed to the combined effects of magnetic field‐induced martensite twin variant reorientation (MFIR) and magnetic field‐induced structural phase transformation (MFIPT). This compound undergoes a structural phase transition from a cubic (austenite‐L21) ferromagnetic (FM) to a monoclinic (martensite) antiferromagnetic (AFM), which leads to an effective increase in the size of the Fermi surface and consequently in CMR. Additionally, it exhibits significant anomalous Hall conductivity in both antiferromagnetic and ferromagnetic phases. Furthermore, it demonstrates a giant topological Hall resistivity (THR) ≈6 µΩ.cm in the vicinity of martensite transition due to the enhanced spin chirality resulting from the formation of magnetic domains with Bloch‐type domain walls. The findings contribute to the understanding of the magnetotransport of Ni‐Mn‐In Heusler alloys, which are prospective candidates for room‐temperature spintronic applications.
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