High performance p-type half-Heusler compounds FeNb1−xTixSb are developed via a band engineering approach and a record zT of 1.1 is achieved.

Electron and phonon transport characteristics determines the potential of thermoelectric materials for power generation or refrigeration. This work shows that, different from most of high performance thermoelectric materials with dominant acoustic phonon scattering, the promising ZrNiSn based half‐Heusler thermoelectric solid solutions exhibit an alloy scattering dominated charge transport. A low deformation potential and a low alloy scattering potential are found for the solid solutions, which is beneficial to maintain a relatively high electron mobility despite of the large effective mass, and can be intrinsic favorable features contributing to the noticeably high power factors of ZrNiSn based alloys. A quantitive description of the different phonon scattering mechanisms suggests that the point defect scattering is the most important mechanism that determines the phonon transport process of the solid solutions. The present results indicate that alloying can be an effective approach for such materials systems to enhance thermoelectric figure of merit ZT by reducing phonon thermal conductivity, while minimizing the deterioration of charge mobility due to the low alloy scatteirng potential.

respectively the temperature of the hot side and cold side, Δ T = T H -T C and T avg are the temperature gradient and average between hot and cold sides. [ 4 ] The fi gure of merit zT is defi ned as zT = α 2 σT /( κ e + κ L ), where α , σ , T , κ e and κ L are respectively the Seebeck coeffi cient, the electrical conductivity, the absolute temperature, and the electrical and lattice components of total thermal conductivity κ . [ 2 ] High conversion effi ciency of 15%-20% is thought of as the "Holy Grail" for large scale application of TE technologies. [ 5 ] As depicted in Figure 1 , this conversion efficiency can be realized by using middle temperature (500-900 K) TE materials with very high zT or high temperature (>900 K) TE materials with compromised zT . Recently, much progress has been made in developing high effi ciency middle temperature TE materials: high zT 's of ≈1.5 at 750 K for n-type and ≈2.0 at for p-type have been achieved in PbTe-based alloys, [6][7][8][9] while n-type and p-type fi lled skutterudites have the maximum zT 's of ≈1.7 and ≈1.0, respectively, near 800 K. [ 10,11 ] Some other TE materials made of earth-abundant and non-toxic elements have also been identifi ed, such as single crystal SnSe ( zT ≈ 2.6 at 923 K), [ 12 ] silicides [ 13,14 ] and α -MgAgSb. [ 15 ] However, there remain tremendous challenges for most of these TE materials for large scale commercial application, mainly due to their relatively poor thermal stability and weak mechanical strength at high temperatures. In Half-Heusler (HH) compounds have gained ever-increasing popularity as promising high temperature thermoelectric materials. High fi gure of merit zT of ≈1.0 above 1000 K has recently been realized for both n-type and p-type HH compounds, demonstrating the realistic prospect of these high temperature compounds for high effi ciency power generation. Here, recent progress in advanced fabrication techniques and the intrinsic atomic disorders in HH compounds, which are linked to the understanding of the electrical transport, is discussed. Thermoelectric transport features of n-type ZrNiSn-based HH alloys are particularly emphasized, which is benefi cial to further improving thermoelectric performance and comprehensively understanding the underlying mechanisms in HH thermoelectric materials. The rational design and realization of new high performance p-type Fe(V,Nb)Sb-based HH compounds are also demonstrated. The outlook for future research directions of HH thermoelectric materials is also discussed.

The intrinsic structural disorder dramatically affects the thermal and electronic transport in semiconductors. Although normally considered an ordered compound, the half-Heusler ZrNiSn displays many transport characteristics of a disordered alloy. Similar to the (Zr,Hf)NiSn based solid solutions, the unsubstituted ZrNiSn compound also exhibits charge transport dominated by alloy scattering, as demonstrated in this work. The unexpected charge transport, even in ZrNiSn which is normally considered fully ordered, can be explained by the Ni partially filling interstitial sites in this half-Heusler system. The influence of the disordering and defects in crystal structure on the electron transport process has also been quantitatively analyzed in ZrNiSn1-xSbx with carrier concentration nH ranging from 5.0×1019 to 2.3×1021 cm−3 by changing Sb dopant content. The optimized carrier concentration nH ≈ 3–4×1020 cm−2 results in ZT ≈ 0.8 at 875K. This work suggests that MNiSn (M = Hf, Zr, Ti) and perhaps most other half-Heusler thermoelectric materials should be considered highly disordered especially when trying to understand the electronic and phonon structure and transport features.