Influence of cation and anion substitutions on the thermoelectric properties of pnictide skutterudite compounds.

摘要

A good thermoelectric material is characterized by a high thermoelectric figure of merit (ZT), which involves high Seebeck coefficient, low electrical resistivity and low thermal conductivity. In 1995, it was discovered by a research group at the Jet Propulsion Laboratory that a skutterudite compound, CeFe₄Sb₁₂, had a higher ZT than that of the currently used materials. This discovery was partly based on a suggestion of Dr. Glenn A. Slack that skutterudite compounds might possess good thermoelectric properties because of their ability to accept extra atoms into its open structure which can then rattle and scatter heat-carrying phonons, hence reducing the lattice thermal conductivity. Since then, a number of research studies were initiated to investigate the preparation and thermoelectric properties of the skutterudites and other related compounds.; This study involved the synthesis and thermoelectric property measurements of a number of phosphide and arsenide skutterudite compounds. The goal was to investigate their transport properties and to make a comparison with their antimonide counterparts. It was found that while the binary phosphide CoP ₃ had very low resistivity, it possessed low Seebeck coefficient and high thermal conductivity which resulted in low ZT values. Replacing Co with Ir to form IrP₃ resulted in a larger Seebeck coefficient and lower thermal conductivity, but the electrical resistivity was too high to give a high efficiency. The study of lanthanum filled CoP₃ and a CoP_3−xAs_x solid solution, however, showed an improvement in the thermoelectric figure of merit. This was mainly due to an enhancement in the Seebeck coefficient and lower thermal conductivity, while their electrical resistivities were not significantly affected by the cation or anion substitutions.; The study of ternary compounds, RT₄X₁₂ (R = Ce and Pr, T = Fe and Ru, X = P and As), showed that cerium-phosphorus based compounds exhibited semiconducting properties while the others showed semimetallic behavior. A comparative study between the binary and ternary compounds (COX ₃ and CeFe₄X₃ (X = P, As, Sb)) indicated that the largest reduction in the lattice thermal conductivity occurs for the antimonides (CoSb₃-CeFe₄Sb₁₂), hence providing evidence for the effects of atomic rattling in this class of compounds.