Half Heusler compounds: promising materials for mid-to-high temperature thermoelectric conversion

Half-Heusler (HH) compounds (space group Fm[ image ]m) have garnered increasing attention in recent years in the thermoelectric community. Three decades ago, refractory RNiSn HH compounds (R represents refractory metals such as Hf, Zr, Ti) were found to be narrow-gap semiconductors with large Seebeck coefficients in 100 s of micro-volt per Kelvin. Today, HH compounds have emerged as promising thermoelectric materials in the intermediate temperature range (400 °C-800 °C). HH materials are endowed with good thermal stability and scalability. Thermoelectric n-p modules based on HH materials demonstrate conversion efficiency near 10% and power density output near 9 W cm^-2. The objective of this article is to present a historical account of the research and development of thermoelectric HH compounds. Particularly, there have been notable achievements since 2012 thanks to the emergence of new approaches. As a result, ZT has risen from 1 to 1.5. The various advances made since the early 1990s to the present are recounted by categorizing HH materials into three generations (Gen): Gen-1 Gen-2, and Gen-3 HH materials.