Enhanced Density of States Mass in Cu Containing XNiSn Half-Heusler Alloys – A New Route Towards High Thermoelectric Performance

Half-Heusler alloys are leading contenders for application in commercial thermoelectric generators. They combine strong performance with good engineering properties and thermal stability.¹ There has been great progress in p-type half-Heusler alloys in the past decade, with <i>zT</i> = 1.5 achieved in NbFeSb and ZrCoBi. There have been fewer developments in n-type Heusler alloys with the best performance in compositions based on ZrNiSn, with peak <i>zT</i> = 1.2-1.3.<br/><br/>We have investigated the use of interstitial Cu in the XNiSn half-Heusler alloys. This cheap base metal is an alternative to the usual n-type dopant Sb. In addition to being an efficient n-type donor, its use leads to a suppression of thermal conductivity and a more homogeneous microstructure.^2-4 Our latest work reveals an enhanced density of states effective mass, m_DoS* ~ 4.1 m_e, for Cu-doped XNiSn half-Heusler samples rich in X = Ti. This enhancement over typical values of 3 m_e occurs without substantial drop in electron mobility. This enables improved power factors and peak <i>zT</i> = 1 at 773 K in TiNiSn-based samples with interstitial Cu used as a mineraliser, dopant and to suppress thermal transport.⁵Bandstructure calculations show the emergence of a second band at the conduction band minimum. This brings the performance of TiNiSn-based compositions close to the leading n-types based on Sb-doped ZrNiSn and is a significant development.<br/><br/>This contribution will summarise our current understanding of the impact of interstitial Cu in the XNiSn Heuslers, which goes beyond carrier doping and affects underlying electronic properties and microstructure of these materials.<br/><br/><b>References</b><br/>[1] R. J. Quinn and J.W.G. Bos, <i>Materials Advances,</i> <b>2</b> 6246 (2021).<br/>[2] S. A. Barczak et al. <i>ACS Applied Materials and Interfaces</i> <b>10</b>, 4786 (2018)<br/>[3] S. A. Barczak et al. <i>Journal of Materials Chemistry A </i><b>7</b>, 27124 (2019).<br/>[4] J. E. Halpin et al. <i>ACS Applied Electronic Materials</i> <b>4</b>, 4446 (2022).<br/>[5] R. J. Quinn et al. <i>Submitted</i> (2023).