Study of Thermoelectric Behavior of some Double and Triple half-Heuslers

Abstract

Thermoelectricity is a promising technique to tackle the worldwide energy demand, pollution and resource depletion, which harvests energy by the direct conversion of waste heat into electricity. The limiting factor of this approach to practical power generation remains the low efficiency of the interconversion, which is evaluated in terms of a dimensionless figure-of-merit zT = [(S^2)σT]/[κL + κe], where S is the Seebeck coefficient, σ is the electrical conductivity, κL is the lattice part of thermal conductivity, κe is the electronic part of thermal conductivity, and T is the absolute temperature. The half-Heusler systems are a promising class for mid-to-high temperature range thermoelectric utilization. These are ternary compounds of composition XYZ where X and Y are transition metals and Z is a p-block element. They show high thermal and mechanical stabilities and semiconductor-like transport properties. The aim remains to decrease their inherently high lattice thermal conductivity and improve their electronic transport properties further using various methods of enhancement, thereby improving the material’s zT. The aim of this project was to synthesize and characterize some new double and triple half-Heusler alloys (DHH and THH) of the form XY0.5Y’0.5Z and XY0.33Y’0.33Y”0.33Z, respectively. The synthesis was done by the method of arc melting and then consolidating the ground sample into a high-density pellet utilizing a hot-press. The structural and thermoelectric characterization (electronic transport and thermal properties) were carried out on the synthesized samples. Minimization of lattice thermal conductivity was carried out to reach a minimum of 1.8 Wm^−1K^−1 for the THH. Further tuning of the Fe-Ni ratio was performed to obtain n-type and p-type counterparts. The n-type (zT = 0.40) was found to be superior to the p-type (zT = 0.14) in its thermoelectric properties, and therefore p-type was further improved via a decrease in Co concentration to reach a peak zT of 0.32.