Optimization Study of (Ta_0.4Nb_0.4Ti_0.2)–Fe–Sb Half-Heusler Thin Films Using Combinatorial Synthesis, High-Throughput Power Factor Mapping and Thermoreflectance Thermal Property Measurements

Half-Heusler (hH) thermoelectric (TE) intermetallic alloys are promising candidates for commercial modules, but their conversion efficiency can be limited due to high thermal conductivity. One method to improve hH alloy performance is to decrease the lattice contribution to the thermal conductivity through solid-solution alloying. Several publications reported p-type NbFeSb hH alloys can accommodate off-stoichiometry on the Fe- and Sb-sites, which could positively impact the both the electrical properties and the thermal properties, similar to the lattice thermal conductivity decrease observed via Ta-alloying. Combinatorial approaches are an ideal method to explore hH alloy properties and phase stability with the advantage of rapid sample fabrication and characterization of a wide range of compositions. This approach can provide insights into materials systems that could be missed using conventional approaches but requires unique and custom synthesis and transport property measurement instrumentation. Both continuous-spread composition gradient and homogeneous discrete co-sputtered combinatorial thin film synthesis methodologies were leveraged to produce maps of the composition-structure-property relationships as a function of Fe- and Sb-content in (Ta_0.40Nb_0.40Ti_0.20)-Fe-Sb hH alloys for the first time. The Seebeck coefficient and electrical resistivity were measured using our custom-built high-throughput scanning probe. Thermal conductivity and heat capacity were measured on our discrete combinatorial hH thin films using a custom-built, automated Frequency Domain Thermoreflectance (FDTR) instrument. Maximum TE figure-of-merit <i>zT</i> values at room temperature are calculated to be ≈ 0.08 for compositions (Nb_0.41Ta_0.33Ti_0.26)_28.5Fe_40.3Sb_31.2 and (Nb_0.42Ta_0.33Ti_0.25)_35.0Fe_31.7Sb_33.3 having thermal conductivity values ≈ 2.25 ± 0.27 W m^-1 K^-1. Our custom suite of high-throughput combinatorial instruments can now measure all of the properties needed to calculate the <i>zT</i> of any promising and unoptimized TE material system.