Thermoelectric behaviour of p- and n- type Ti-Ni-Sn half Heusler alloy variants and their amorphous equivalents
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Thermoelectric behaviour of p- and n- type Ti-Ni-Sn half Heusler alloy variants and their amorphous equivalents Song Zhu1, Satish Vitta2 and Terry M. Tritt1 1 Department of Physics & Astronomy Clemson University, Clemson 29634 SC, USA. 2 Department of Metallurgical Engineering and Materials Science Indian Institute of Technology Bombay, Mumbai 400076, India. ABSTRACT Ti-Ni-Sn type half-Heusler alloys which have the versatility to be either p- or n-type depending on the type of substitution, have been synthesized and investigated in the present work. The added advantage of doping them with multiple elements is that they will be amenable to bulk amorphous phase formation. The hole doped alloys were predominantly single phase with a cubic structure, while the electron doped alloys were found to have minor additional phases. All the alloys exhibit extremely weak metallic-like or degenerate semiconductor transport behaviour in the temperature range 20 K to 1000 K. The resistivity of p-type alloys exhibits semi-metallic-to-semiconducting transition at ~ 500 K while the ntype alloys exhibit a weak metallic-like behaviour in the complete temperature range. The Seebeck coefficient has strong temperature dependence with a maximum of 45 μV K -1 in the temperature range 600-700 K in the p-type alloys. The n-type alloys however exhibit a linear variation of the Seebeck coefficient with temperature. The total thermal conductivity of the alloys increases with increasing temperature without any peak at low temperatures indicating significant disorder induced scattering. The p-type alloys have the lowest thermal conductivity compared to the n-type alloys. These alloys become amorphous after pulsed laser deposition except one alloy which exhibits compensated transport behaviour. INTRODUCTION The single most important parameter that has been driving the development of new thermoelectric materials is the ‘figure-of-merit, Z’ although other material parameters such as their ability to be processed into different shapes, mechanical strength, availability of suitable contact materials are also equally important. The figure-of-merit Z defined as α2σ/(κe+κl), where α is the Seebeck coefficient, σ the electrical conductivity and κe and κl are electronic and lattice thermal conductivities respectively and Z depends on two competing phenomena – charge transport and heat transport which have an opposing functional dependence. Hence any materials development strategy needs to be assessed in terms of a balance between the charge carrier mobility, μ or effective mass m*of the charge carrier and the lattice thermal conductivity κl. The thermal conductivity can be significantly reduced by increasing boundary scattering, decreasing the effective grain size,1,2 but this strategy can also lead to an increase in carrier scattering resulting in an insignificant change in Z. This strategy to increase Z however is predicted to be effective in a special class of materials wherein the phonon mean free path becomes significantly larger than the charge carrier
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