COMBINATORIAL EXPLORATION OF HALF-HEUSLER (Ta0.4 Nb0.4 Ti0.2)–Fe–Sb THIN FILMS VIA HIGH-THROUGHPUT POWER FACTOR MAPPING AND FREQUENCY-DOMAIN THERMOREFLECTANCE (FDTR)

Abstract

Thermoelectrics (TEs) are a class of materials capabl¬¬e of converting heat into electricity in the solid state. Their widespread application is limited by the low efficiency (≈ 5 %) for commercial modules in applications such as waste heat recovery and refrigeration. Half-Heusler (hH) TE intermetallic alloys have good electrical properties that are easily tuned by doping but are limited in commercial deployment due to high thermal conductivity (TC). This limits the achievable thermal gradient across a TE module, reducing the efficiency. One method to improve hH alloy performance is to decrease the lattice contribution to the TC through solid-solution alloying. Combinatorial synthesis approaches have the advantage of rapid sample fabrication and characterization over a wide range of material compositions. This approach can provide insights into materials systems that could be missed using conventional synthesis approaches. Several publications reported p-type NbFeSb hH alloys can accommodate off-stoichiometry, which could positively impact the TE properties similar to TC decrease observed via Ta-alloying. Combinatorial thin film co-sputter synthesis of hH alloy (Ta0.40Nb0.40Ti0.20)-Fe-Sb composition spread libraries coupled with high throughput (HiTp) characterization is utilized to produce maps of the composition-structure-property relationships as a function of Fe- and Sb-content in this system for the first time. Continuous spread composition gradient and homogeneous discrete co-sputtered combinatorial thin film synthesis methodologies are leveraged to investigate the hH stability region and TE performance in (Ta0.40Nb0.40Ti0.20)-Fe-Sb. Combinatorial thin film characterization requires specialized custom or commercial instrumentation capable of scanning across samples. Established HiTp tools were utilized to characterize the crystal structure, electrical transport properties (Seebeck coefficient and electrical resistivity), and chemical composition of the films. A scanning thin film TC measurement instrument was not available prior to this dissertation. Without this ability, the dimensionless TE figure-of-merit zT cannot be calculated. To address this need, a custom, automated Frequency Domain Thermoreflectance (FDTR) instrument was designed and constructed. FDTR TC measurements are presented on single-phase F¯4 3m off stoichiometric discrete combinatorial hH (Ta0.40Nb0.40Ti0.20)-Fe-Sb for the first time. Maximum zT values at 296 K are calculated to be 0.076 for compositions (Nb0.412Ta0.327Ti0.261)28.5Fe40.3Sb31.2 and (Nb0.418Ta0.328Ti0.254)35.0Fe31.7Sb33.3 having TC values around 2.25 ± 0.27 W m-1 K-1.