The Theoretical Studies On The Electrical And Thermal Transport Properties Of Half-Heusler Alloys

Thermoelectric materials refer to the functional materials which could achieve the transformation between thermal energy and electrical energy and could be used in applications including power generation and refrigeration. Euipments using thermoelectric materials for power generation or refrigeration are characterized with quite a few merits, including high reliability, compact structures, long operating life, noise-free operation and non-pollution. Some thermoelectric devices have been successfully applied to a number of the high-tech industries, such as thermoelectric generator that used radioisotopes for heating had been applied to spaceship "Traveler one" and "Galileo Jupiter detector" launched by NASA.In recent years, with energy and environment becoming two major issues of global sustainable development, the exploration for new type of green energy comes up on agenda. As green clean energy materials, thermoelectric materials have received more and more concern and research from scientists and are expected to be widely applied to household refrigeration, heat power generation using vehicle exhaust waste, etc.Compared with other thermoelectric materials, half-Heusler alloys have some unique advantages:the power factor is large and there are holes in its crystal structure. As potential high performance thermoelectric materials, half-Heusler alloys have received a lot of attention. The research object of this paper is the18compounds with half-Heusler structure which have been reported and the number of valence electrons in the primitive cell of which is18. Taking use of density functional theory and density functional perturbation theory, we can calculate the nature of their electric and thermodynamic properties, and then predict excellent thermoelectric properties through the group velocity. Through the systematic study, this paper has reached the following innovative results:(1) Using density functional theory and density functional perturbation theory, we calculate the properties of the electric structure and lattice thermodynamics of10Sb-based half-Heusler alloys with18valence electrons in one primitive cell, and reach the following conclusions:(I) The density of states near the Fermi level of TiCoSb, TiRhSb are steep, while the density of states near the Fermi level of VFeSb and VRuSb are steep, and their band gap near the Fermi level are narrow;(II) The greater the atomic mass, the greater the contribution to the spread of lattice vibration energy; In low temperature area the effect of the value of lattice heat capacity on the thermal conductivity of different materials is significant, while in middle-high temperature area the effect is not so obvious; As for the thermal conductivity, we may conclude that TiCoSb, VFeSb and VRuSb have lower thermal conductivity.(2) Using density functional theory and density functional perturbation theory, we calculate the electric properties, elastic properties and the nature of the phonon of8Sn-based half-Heusler alloys with18valence electrons in one primitive cell, and get the following conclusions:(Ⅰ) They are semiconductor with indirect band gap, and their band gap values are relatively small. Among them, TiPtSn and TiNiSn are special:the Fermi level are in the position of the edge of the band gap, the band gap are relatively the narrowest and the density of states near the Fermi level increase rapidly;(Ⅱ) Through the calculation of the elastic properties, we can conclude that they all satisfy the mechanical stability conditions;(Ⅲ) From the phonon dispersion spectrum analysis, we can arrive at the conclusion that they exist the LO-TO splitting. The analysis of the phonon density of states shows that the greater the atom mass in one primitive cell, the greater its contribution to the acoustic branch; In the temperature range of202.1K-1000K, their heat capacity are very close, so we can predict that they have similar thermoelectric properties from the perspective of thermal conductivity; We find that the group velocity of HfNiSn is the lowest, which means that HfNiSn has lower thermal conductivity and may have better thermoelectric properties than others.