Strategies for efficient thermoelectrics: Thermoelectric materials convert heat into electricity and can provide solid-state cooling for spot-sized refrigeration. One important barrier for adopting these materials beyond niche applications is their low efficiency. He and Tritt review the mechanisms and strategies for improving thermoelectric efficiency. They discuss how to report material performance and highlight the most promising materials. With new materials and strategies for performance enhancement, thermoelectrics are poised to alter the renewable energy landscape. Science , this issue p. eaak9997

Thermoelectric materials, which have applications in refrigeration and power generation, are experiencing a surge in research activity. Many different materials are investigated with the goal of maximizing electrical conductivity while minimizing thermal conductivity, which is required for good thermoelectric performance. Only recently was a 30-year deadlock in thermoelectric performance overcome. Predictions for the new materials suggest that important further improvements are on the horizon, with promising applications in the computer and other high-tech industries.

Half-Heusler (HH) alloys have attracted considerable interest as promising thermoelectric (TE) materials in the temperature range around 700 K and above, which is close to the temperature range of most industrial waste heat sources. The past few years have seen nanostructuing play an important role in significantly enhancing the TE performance of several HH alloys. In this article, we briefly review the recent progress and advances in these HH nanocomposites. We begin by presenting the structure of HH alloys and the different strategies that have been utilized for improving the TE properties of HH alloys. Next, we review the details of HH nanocomposites as obtained by different techniques. Finally, the review closes by highlighting several promising strategies for further research directions in these very promising TE materials.

The resistivity of single-crystal pentatellurides, ${\mathrm{HfTe}}_5$ and ${\mathrm{ZrTe}}_5,$ has been measured as a function of temperature and applied magnetic field. At zero magnetic field these materials exhibit a peak in their resistivity (at $T_P)$ as a function of temperature that corresponds to an, as yet, undetermined phase transition. The application of a transverse magnetic field (B ⊥ to the current I) has a profound effect on the resistive peak in these materials, shifting the peak to slightly higher temperatures and producing a large enhancement of the resistivity at the peak, up to a factor of 3 in ${\mathrm{ZrTe}}_5$ ${(T}_P=145\mathrm{}\mathrm{K})$ and 10 in ${\mathrm{HfTe}}_5$ ${(T}_P=80\mathrm{}\mathrm{K}).$ Larger magnetoresistance is observed at even lower temperatures, $T<\mathrm{}20\mathrm{}\mathrm{K}.$

The importance of the future development of materials for use in low-temperature thermoelectric refrigeration devices is discussed. Specifically, results are presented on an interesting class of materials called pentatellurides, ${\mathrm{HfTe}}_5$ and ${\mathrm{ZrTe}}_5,$ which have shown promising low-temperature thermoelectric properties $(100\mathrm{}\mathrm{K}<T<\mathrm{}250\mathrm{}\mathrm{K}).\mathrm{}$ Substitutional doping occurs both on the metal site as ${\mathrm{Hf}}_{1-x}{\mathrm{Zr}}_x{\mathrm{Te}}_5$ solid solutions, and on the Te site with Se. Proper doping leads to a decrease in resistivity and an enhancement of thermopower which results in a doubling of the power factor (electronic properties) which is then very competitive with the power factor of existing thermoelectric materials in these temperature regimes.