Composite design of half-Heusler thermoelectrics: Selective doping of grain boundary phases in NbFeSb by InSb
Ruben Bueno Villoro, Raana Hatami Naderloo, Dominique Alexander Mattlat, Chanwon Jung, Kornelius Nielsch, Christina Scheu, Ran He, Siyuan Zhang
Abstract
Thermoelectric materials require complex microstructures to optimize the power conversion efficiency. Grain boundaries (GBs) reduce the thermal conductivity in nanocrystalline materials but often also reduce the electrical conductivity. We have recently shown that elemental segregation at GBs can make them electrically conductive or non resistive and thereby improving the thermoelectric properties. Nevertheless, the doping elements participate in the chemistry within the grain matrix, which can limit the design space. In this work, we present an independent control of doping to the GBs. Specifically, we add InSb to Nb0.95Ti0.05FeSb to selectively modify the chemistry of the GBs without increasing the carrier concentration of the matrix. Using high resolution transmission electron microscopy and atom probe tomography, we understand the role of InSb in the selective modification of the chemistry of the GBs and we establish structure-property relationships between the chemistry of the GBs and their electrical behaviour. InSb successfully increases power factor and zT, thus, we prove that the negative impact of GBs in the power factor can be overcame by GB engineering.