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

Half-Heusler compounds (space group Fm3m) has garnered increasing attention in recent years in the thermoelectric community. Three decades ago, refractory RNiSn half-Heusler compounds (R represents refractory metals such as Hf, Zr, Ti) were found to be narrow-gap semiconductors with large Seebeck coefficients in 100s of micro-volt per Kelvin. Today, half-Heusler (HH) compounds have emerged as promising thermoelectric materials in the intermediate temperature range (400-800oC). 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/cm2. The objective of this article is to present a historical account of the research and development of thermoelectric half-Heusler 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 half-Heusler materials into three generations (Gen): Gen-1 Gen-2, and Gen-3 HH materials.