Developing thermoelectric materials with superior performance means tailoring interrelated thermoelectric physical parameters – electrical conductivities, Seebeck coefficients, and thermal conductivities – for a crystalline system. High electrical conductivity, low thermal conductivity, and a high Seebeck coefficient are desirable for thermoelectric materials. Therefore, knowledge of the relation between electrical conductivity and thermal conductivity is essential to improve thermoelectric properties. In general, research in recent years has focused on developing thermoelectric structures and materials of high efficiency. The importance of this parameter is universally recognized; it is an established, ubiquitous, routinely used tool for material, device, equipment and process characterization both in the thermoelectric industry and in research. In this paper, basic knowledge of thermoelectric materials and an overview of parameters that affect the figure of merit ZT are provided. The prospects for the optimization of thermoelectric materials and their applications are also discussed.

A review on thermoelectric renewable energy: Principle parameters that affect their performance

Recent advances in thermoelectric materials

Recent progress of half-Heusler for moderate temperature thermoelectric applications

Thermoelectric (TE) materials facilitate direct heat-to-electricity conversion. The performance of a TE material is characterised by its figure of merit zT (=S2 σT/κ) that depends on both electronic transport properties, i.e. the Seebeck coefficient S and the electrical conductivity σ, and on thermal transport properties, i.e. the thermal conductivity κ of a material. The intrinsically counter-correlated behaviour between electronic and thermal transport properties makes the enhancement of zT a very challenging task. In the past 10 years, the zTs in bulk TE materials have been significantly enhanced due to intensive exploratory efforts, the discovery of new physical phenomena and effects, and applications of advanced synthesis methods. In this review, we summarise the recent progress in bulk TE materials. After the introduction of fundamental principles of thermoelectricity, the recently achieved enhancements in the TE performance encompassing the use of electronic band structure engineering, lattice phon...

Recent advances in high-performance bulk thermoelectric materials

Half-Heusler compounds are an impressive class of materials with a huge potential for different applications such as future energy applications and for spintronics. The semiconducting Heusler compounds can be identified by the number of valence electrons. The band gap can be tuned between 0 and 4 eV by the electronegativity difference of the constituents. Magnetism can be introduced in these compounds by using rare-earth elements, manganese or ‘electron’ doping. Thus, there is a great interest in the fields of thermoelectrics, solar cells and diluted magnetic semiconductors. The combination of different properties such as superconductivity and topological edge states leads to new multifunctional materials, which have the potential to revolutionize technological applications. Here, we review the structure, the origin of the band gap and the functionalities of semiconducting half-Heusler compounds.

https://iopscience.iop.org/article/10.1088/0268-1242/27/6/063001/pdf

Half-Heusler compounds: Novel materials for energy and spintronic applications

Thermoelectric properties of the TiX(Zr0.5Hf0.5)1 − XNiSn half-Heusler compounds

The structural and magnetic properties of (R,Zr)(Fe,Co)10(R=Nd,Sm) prepared by the rapid‐quenching method have been investigated. The presence of zirconium makes it possible to realize a TbCu7‐type crystal structure with a high c/a ratio of more than 0.87, while the same crystal structure has a c/a ratio of just 0.84–0.85 in the zirconium‐free case. The saturation magnetization of crystals having such a high c/a ratio as 0.87 exceeds 1.7 T at room temperature. These materials can be nitrogenated, and remarkable enhancement of magnetocrystalline anisotropy by nitrogenation can be seen in the case of Sm. The magnetic hardening of these nitrogenated materials can be achieved by annealing before nitrogenation. The highest value of (BH)max obtained so far for the nitrogenated materials is 144 kJ/m3 (18.0 MGOe). The iHc and Br values for this sample are 503 kA/m (6.3 kOe) and 1.08 T, respectively.

Structural and magnetic properties of rapidly quenched (R,Zr)(Fe,Co)10Nx (R=Nd,Sm)

In this study, we investigated intermediate-heat treatment (IHT) at temperatures between those of sintering and solution treatment, and evaluated the effects on the macrostructure, microstructure, and magnetic properties of Sm(Cobal.Fe0.35Cu0.06Zr0.018)7.8. We found that squareness was clearly improved by adopting the IHT, which promotes grain growth. These results indicate that reducing the fraction of grain boundaries by increasing the grain size affects the behavior of domain-wall motion. Magnetic properties of Mr 1.22 T, HcJ 1580 kA/m, and (BH)max 282 kJ/m3 (>35 MGOe) were obtained for Sm(Cobal.Fe0.35Cu0.06Zr0.018)7.8 subjected to IHT at 1453 K.

Influence of intermediate-heat treatment on the structure and magnetic properties of iron-rich Sm(CoFeCuZr)Z sintered magnets

The effects of solution-treatment temperature on the phase constitution, microstructure and magnetic properties of Sm(Cobal.Fe0.35Cu0.06Zr0.018)7.8 were investigated systematically. The formation of Sm2Co7 around the Sm2O3 was confirmed and the volume fraction of Sm2Co7 increases with decreasing solution-treatment temperature. An area without cellular structure was observed around the Sm2Co7 phase in aged specimens solution treated at 1403 K. Thus, it was found that precipitation of Sm2Co7 phase prevents the formation of cellular structure during aging treatment. The coercivity of those magnets was less than 600 kA/m. On the other hand, the obvious formation of cellular structure was confirmed in the case of solution treatment at 1423 K and these magnets showed high coercivity of more than 1000 kA/m. The magnetic properties of Mr = 1.22 T and HcJ = 1011 kA/m have been obtained for Sm(Cobal.Fe0.35Cu0.06Zr0.018)7.8 by solution treatment at 1423 K.

Effects of Solution Treated Temperature on the Structural and Magnetic Properties of Iron-Rich ${\rm Sm}({\rm CoFeCuZr})_{\rm Z}$ Sintered Magnet

In one embodiment, a permanent magnet includes: a composition expressed by R p Fe q M r Cu s CO 100-p-q-r-s (R is a rare-earth element, M is at least one element selected from Zr, Ti, and Hf, 10 ≤ p ≤ 13.5 at%, 25 ≤ q ≤ 40 at%, 1.35 ≤ r ≤ 1.75 at%, and 0.88 ≤ s ≤ 13.5 at%); and a metallic structure including Th 2 Zn 17 crystal phases each having a Fe concentration of 25 at% or more, and Cu-rich crystal phases each having a Cu concentration of from 25 at% to 70 at%. An average thickness of the Cu-rich crystal phases is 20 nm or less, and an average distance between the Cu-rich crystal phases is 200 nm or less.

Shinya Sakurada

Papers

Thermoelectric properties of the TiX(Zr0.5Hf0.5)1 − XNiSn half-Heusler compounds

Structural and magnetic properties of rapidly quenched (R,Zr)(Fe,Co)10Nx (R=Nd,Sm)

Influence of intermediate-heat treatment on the structure and magnetic properties of iron-rich Sm(CoFeCuZr)Z sintered magnets

Effects of Solution Treated Temperature on the Structural and Magnetic Properties of Iron-Rich ${\rm Sm}({\rm CoFeCuZr})_{\rm Z}$ Sintered Magnet

Permanent magnet, and motor and power generator using the same