https://dc.engconfintl.org/cgi/viewcontent.cgi?article=1032&context=superalloys_ii

A critical review of high entropy alloys and related concepts

The past two decades have witnessed the rapid growth of thermoelectric (TE) research. Novel concepts and paradigms are described here that have emerged, targeting superior TE materials and higher TE performance. These superior aspects include band convergence, "phonon-glass electron-crystal", multiscale phonon scattering, resonant states, anharmonicity, etc. Based on these concepts, some new TE materials with distinct features have been identified, including solids with high band degeneracy, with cages in which atoms rattle, with nanostructures at various length scales, etc. In addition, the performance of classical materials has been improved remarkably. However, the figure of merit zT of most TE materials is still lower than 2.0, generally around 1.0, due to interrelated TE properties. In order to realize an "overall zT > 2.0," it is imperative that the interrelated properties are decoupled more thoroughly, or new degrees of freedom are added to the overall optimization problem. The electrical and thermal transport must be synergistically optimized. Here, a detailed discussion about the commonly adopted strategies to optimize individual TE properties is presented. Then, four main compromises between the TE properties are elaborated from the point of view of the underlying mechanisms and decoupling strategies. Finally, some representative systems of synergistic optimization are also presented, which can serve as references for other TE materials. In conclusion, some of the newest ideas for the future are discussed.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/adma.201702816

Compromise and Synergy in High-Efficiency Thermoelectric Materials.

The long-standing popularity of thermoelectric materials has contributed to the creation of various thermoelectric devices and stimulated the development of strategies to improve their thermoelectric performance. In this review, we aim to comprehensively summarize the state-of-the-art strategies for the realization of high-performance thermoelectric materials and devices by establishing the links between synthesis, structural characteristics, properties, underlying chemistry and physics, including structural design (point defects, dislocations, interfaces, inclusions, and pores), multidimensional design (quantum dots/wires, nanoparticles, nanowires, nano- or microbelts, few-layered nanosheets, nano- or microplates, thin films, single crystals, and polycrystalline bulks), and advanced device design (thermoelectric modules, miniature generators and coolers, and flexible thermoelectric generators). The outline of each strategy starts with a concise presentation of their fundamentals and carefully selected examples. In the end, we point out the controversies, challenges, and outlooks toward the future development of thermoelectric materials and devices. Overall, this review will serve to help materials scientists, chemists, and physicists, particularly students and young researchers, in selecting suitable strategies for the improvement of thermoelectrics and potentially other relevant energy conversion technologies.

Advanced Thermoelectric Design: From Materials and Structures to Devices

Thermoelectric materials: Energy conversion between heat and electricity

Thermoelectric materials could be used to convert waste heat into useful electricity, but the ideal substance needs to both optimize the electrical power factor and suppress thermal conductivity. Here, the authors report a high figure of merit of 1.5 at 1,200 K in the p-type half-Heusler alloy FeNbSb.

/pdf/realizing-high-figure-of-merit-in-heavy-band-p-type-half-i1ey9au6d0.pdf

Realizing high figure of merit in heavy-band p-type half-Heusler thermoelectric materials.

High-performance half-Heusler thermoelectric materials Hf1−x ZrxNiSn1−ySby prepared by levitation melting and spark plasma sintering

It has been shown previously that the thermoelectric properties of the Zintl phase compound, YbZn2Sb2 can be finely tuned via substitution at the cationic Yb-site Here we report the results of the investigation of isoelectronic substitution of Zn by Mn in the anionic (Zn2Sb2)2− framework The p-type YbZn2−xMnxSb2 (00⩽x⩽04) samples have been synthesized via a solid-state reaction followed by suitable cooling, annealing, grounding, and hot-pressing densification processes In samples with x=00, 005, 010, 015, 02, 03, and 04, the electrical conductivity, Seebeck coefficient, and thermal conductivity measurements have been performed as a function of temperature from 300to750K It is found that the Mn substitution effectively lowers the thermal conductivity for all samples, while it significantly increases the power factor for x⩽015 As a result, a dimensionless figure of merit ZT of ∼061–065 has been attained at 726K for x=005–015 as compared to the ZT of ∼048 in the unsubstituted YbZn2Sb2

Improved thermoelectric performance in the Zintl phase compounds YbZn2−xMnxSb2 via isoelectronic substitution in the anionic framework

Half-Heusler thermoelectric materials Hf(/Zr)NiSn were prepared by levitation melting followed by melt-spinning to refine the boundary structures, and then they were consolidated by spark plasma sintering. X-ray diffraction analysis and scanning electron microscopy showed that single phased half-Heusler compounds without compositional segregation had been obtained. It was found that the thermoelectric properties, especially the thermal conductivity, depended strongly on the boundary structures. The melt-spinning samples with refined boundary structures had a lower thermal conductivity but a power factor comparable to that of the sample prepared by levitation melting, thus providing good thermoelectric properties.

Reduced Grain Size and Improved Thermoelectric Properties of Melt Spun (Hf,Zr)NiSn Half-Heusler Alloys

The (Y,Sb) codoped ( Hf 0.6 Zr 0.4 ) 1 − x Y x NiSn 0.98 Sb 0.02 ( x = 0 , 0.01, 0.015, 0.02, and 0.025) half-Heusler alloys were prepared by levitation melting and spark plasma sintering. The effects of Y doping on the electrical conductivity, the Seebeck coefficient, and the thermal conductivity have been investigated in the temperature range of 300–900 K. It was found that the Y doping decreased the carrier concentration and electrical conductivity due to the introduction of hole carriers. The thermal conductivity was also reduced upon Y doping, mainly due to the reduced carrier thermal conductivity. The Y-doping substantially increased the Seebeck coefficient because of the decrease in carrier concentration. Pisarenko plot showed that the measured room temperature Seebeck coefficients agrees well with the predicted single parabolic band behavior as a function of the carrier concentration, suggesting that no additional mechanisms cause the extra enhancement of Seebeck coefficient in the Y–Sb codoped half-Heusler alloys. The figure of merit Z T of 1% Y-doped sample was increased by a factor of about 25% than that of the undoped sample.

Effects of yttrium doping on the thermoelectric properties of Hf 0.6 Zr 0.4 NiSn 0.98 Sb 0.02 half-Heusler alloys

(Zr, Hf)NiSn-based half-Heusler alloys with refined grains were prepared by melt spinning and spark plasma sintering. The grain size of the melt-spun (MS) thin ribbons varied from ∼500 nm to ∼3 μm. X-ray diffraction analysis showed that single phased alloys were obtained. Nanoscale precipitates dispersed in the matrix could be observed in both the MS ribbons and sintered bulk samples, which increased the carrier concentration and electrical conductivity. The lattice thermal conductivity decreased by more than 20% below 100 K and 5–20% from 200 to 1000 K, compared with the levitation melted counterparts, due to the refined grain sizes. The maximum dimensionless figure of merit ZT value reached ∼0.9 for the MS Hf0.6Zr0.4NiSn0.98Sb0.02sample.

Cui Yu

Papers

High-performance half-Heusler thermoelectric materials Hf1−x ZrxNiSn1−ySby prepared by levitation melting and spark plasma sintering

Improved thermoelectric performance in the Zintl phase compounds YbZn2−xMnxSb2 via isoelectronic substitution in the anionic framework

Reduced Grain Size and Improved Thermoelectric Properties of Melt Spun (Hf,Zr)NiSn Half-Heusler Alloys

Effects of yttrium doping on the thermoelectric properties of Hf 0.6 Zr 0.4 NiSn 0.98 Sb 0.02 half-Heusler alloys

High performance half-Heusler thermoelectric materials with refined grains and nanoscale precipitates