Interpreting the combustion process for high performance ZrNiSn thermoelectric materials.

ZrNiSn alloy, a member of the half-Heusler family of thermoelectric materials, shows great potential for mid-to-high temperature power generation applications due to its excellent thermoelectric properties, robust mechanical properties, and good thermal stability. The existing synthesis processes of half-Heusler alloys are, however, rather time and energy intensive. In this study, single-phase ZrNiSn bulk materials were prepared by self-propagating high-temperature synthesis (SHS) combined with spark plasma sintering (SPS) for the first time. The analysis of thermodynamic and kinetic processes shows that the SHS reaction in the ternary ZrNiSn alloy is different from the more usual binary systems. It consists of a series of SHS reactions and mass transfers triggered by the SHS fusion of the binary Ni-Sn system that eventually culminates in the formation of single-phase ternary ZrNiSn in a very short time, which reduced the synthesis period from few days to less than an hour. Moreover, the non-equilibrium feature induces Ni interstitials in the structure, which simultaneously enhances the Seebeck coefficient and decreases the thermal conductivity, favorable for thermoelectric. The maximum thermoelectric figure of merit ZT of the SHS+SPS-processed ZrNiSn1-xSbx alloy reached 0.7 at 870 K. This study opens a new avenue for the fast and low-cost fabrication of half-Heusler thermoelectric materials.