Effect of Doping in Defective Half-Heusler Nb0.8CoSb

Abstract

A thermoelectric (TE) device converts heat energy into electrical energy and can be used to tap the waste heat and turn it into clean and useful electrical energy that can reduce our dependence on fossil fuels whose excess use has led to climate change and global warming. The efficiency of a TE device depends on the figure of merit (zT) of the n-type and p-type materials used in its fabrication. The higher the zT, the greater the efficiency. The zT of the TE material is given by the formula: zT = S2σ/k where S is the Seebeck coefficient, σ is the electrical conductivity and k is the thermal conductivity at an absolute temperature T. Some of the most commonly used TE materials include PbTe, Bi2Te3, etc. They exhibit a reasonably high zT (>1) near room-temperature; however, their applicability in the mid-to-high temperature range is severely limited by their low-melting temperatures. In this regard, the alloys of the Heusler family, and half-Heusler (hH) XYZ with valence electron could (VEC) 18 as they are stable in particular, have attracted a great deal of attention. They have high melting temperatures and their closed-shell electronic structure ensures good semiconducting properties but the main disadvantage of it is they have high thermal conductivity. The NbCoSb system, despite having a 19-valence electron count (VEC), stabilizes itself in Nb0.8CoSb by introducing vacancies at the Nb site. These vacancies contribute to a reduction in thermal conductivity, resulting in a maximum thermoelectric figure of merit (zT) of 0.9 at 1123K in Nb0.83CoSb. In my work, I attempted to substitute Si and Al at the Sb site, but the Al doping was incomplete and the enhancement in Si doping was less than the highest reported value. Additionally, I attempted to achieve resonant doping in Nb0.8CoSb using Mo and W doping at the Nb site, but resonant doping did not occur. However, there was an enhancement in transport properties, and a zT of 0.88 was achieved at approximately 973K with 0.5% Mo doping. The cause of this enhancement is still being investigated.