Dylan Kirsch1,2,Joshua Martin1,Nathan Johnson3,4,Rohit Pant2,Ichiro Takeuchi2,Apurva Mehta4
National Institute of Standards and Technology1,University of Maryland2,Stanford University3,SLAC National Accelerator Laboratory4
Dylan Kirsch1,2,Joshua Martin1,Nathan Johnson3,4,Rohit Pant2,Ichiro Takeuchi2,Apurva Mehta4
National Institute of Standards and Technology1,University of Maryland2,Stanford University3,SLAC National Accelerator Laboratory4
The NbFeSb ternary system is one of the higher performing p-type Half Heusler (HH) thermoelectrics but has a relatively high thermal conductivity. Several theoretical and experimental reports suggest that forming a solid solution mixture between Nb and Ta could lower lattice thermal conductivity without negatively affecting the electrical transport. Experimental investigations have studied (Nb,Ta)FeSb solid solution mixtures up to (Nb<sub>0.6</sub>Ta<sub>0.4</sub>)FeSb, which is the Nb/Ta ratio that was found to exhibit the highest thermoelectric figure of merit. However, the effects of an excess or a deficit of Fe or Sb on both the electrical and thermal transport properties have been largely unexplored. There could be additional alloy stoichiometries with more optimized properties. Combinatorial thin film deposition provides an efficient route to synthesize a wide range of stoichiometries on a single substrate, enabling rapid screening of composition/structure/transport property relationships. Here we used combinatorial sputter deposition to investigate thin film mixtures of Sb, Fe, and a Ta<sub>0.4</sub>Nb<sub>0.4</sub>Ti<sub>0.2</sub> alloy target. Room temperature electrical and thermal transport properties were mapped as a function of atomic composition (micro-X-ray Fluorescence) using our custom-built suite of high-throughput measurement instrumentation, which includes a scanning Seebeck coefficient and 4-probe resistivity probe, and a scanning frequency domain thermoreflectance (FDTR) instrument. The structural properties were examined using a high throughput synchrotron X-ray diffraction (XRD) at Stanford Synchrotron Radiation Lightsource. Rietveld refinement of the single phase XRD patterns provide key insights into the role of excess Fe or Sb on the site occupancy and atomic ordering of the HH phase and the resulting electrical and thermal transport properties. A region of off-stoichiometric HH phase stability was identified that exhibits electrical and thermal transport properties which agree with bulk values reported in literature.