Theoretical Study On The Thermoelectric Properties Of Half-heusler Compounds Containing Rare Earth Element

Owing to serious energy crisis and environmental pollution,the demand for sustainable sources is becoming more and more important.Thermoelectric materials can directly convert waste heat into electricity,which have attracted intensive attention from the science and industry communities.The performance of thermoelectric materials is usually determined by the dimensionless figure of merit:ZT=S2σT/(KL+Ke),where,a,T,k and Ke are the Seebeck coefficient,the electrical conductivity,the absolute temperature,and the lattice and electronic thermal conductivities,respectively.However,these transport coefficients(S,σ,and Ke)are usually coupled with each other,and it is still a great challenge to obtain a high ZT value,especially for the bulk materials.Over the past two decades,the thermoelectric materials of a few half-Heusler compounds have been extensively investigated,especially for the(Ti,Zr,and Hf)NiSb-based and(Ti,Zr,and Hf)CoSb-based compounds.Among them,the maximal ZT value is only 1.5.Moreover,most HH compounds were found to exhibit higher thermal conductivity in the order of magnitude of 10 W/mK,which is undesirable for high performance thermoelectric materials.Recently,Carrete et al.theoretically found that the compound LaPtSb has a particularly low lattice thermal conductivity of 1.7 W/mK at room temperature.It is therefore interesting to ask whether such a compound can exhibit better thermoelectric performance.In my dissertation,we combined the first-principles calculations with Bolzmann transport theory to investigate the thermoelectric performance of half-Heusler compounds LaPtSb and LaPtBi containing rare earth element.The thermoelectric properties of LaPtSb are predicted by multiscale calculations which combine the first-principles calculations,Boltzmann theory,and deformation potential theory.Our calculations show that the compound has a direct band gap of 0.23 eV.Moreover,it was found that LaPtSb has a large power factor at a suitable carrier concentration.For example,the power factor of the n-type system can be optimized to 0.43 W/mK2.Together with the very low lattice thermal conductivity,the optimized ZT value can be enhanced to as high as 2.2 at room temperature,which is much larger than those of many typical half-Heusler compounds.We also investigate the thermoelectric performance of LaPtBi.Compared with LaPtSb,the compound has an inverted band order at Г point.The lattice thermal conductivity decreases monotonously with increasing temperature.The ZT value is estimated to be 0.4 and 1.6 for the p-and n-type system at 1000 K,respectively.In addition,we also compare the effect of three different strains on the band structure of LaPtBi.Our calculations indicate that the lattice thermal conductivity of LaPtBi decreases significantly and the thermoelectric performance of p-type system can be improved through suitable strain.