Density Functional Theory Calculation Of Thermoelectric Materials Of (Nb,V)CoSb System

Thermoelectric materials are a class of materials that convert thermal energy into electrical energy and are widely used in portable electronics,aerospace and defense military.Half-Heusler compounds have received special attention due to their high temperature stability,good mechanical stability,low toxicity and good thermoelectric properties,which is very important for practical applications.At present,the 19-electron Half-Heusler material with vacancy doping has also been proved to be a heavily doped 18-electron semiconductor,especially NbCoSb and VCoSb,which has a good prospect in thermoelectric performance.It has been experimentally verified that the single-phase Nb0.8+?CoSb(0???0.25)has an inherent Nb vacancy,which follows the 18-electron rule.Thanks to the elimination of the impurity phase and the optimization of the carrier concentration,the thermoelectric performance was significantly improved by 100%.Nb0.83CoSb achieves a maximum zT value of 0.9 at 1123 K.It is known that the impurity has a great influence on the thermoelectric figure of the Half-Heusler phase.The more impurity phase,the higher formation energy is.Therefore,i used the first principle density functional theory to calculate The formation energy,DOS and band structure of Nb0.75+?CoSb(0???0.25),V0.75+?CoSb(0???0.25)and V?Nb1-?CoSb(0<?<1)by establishing the supercell,to analyze their thermoelectric performance.The lowest point of the formation energy of Nb0.75+?CoSb(0???0.25)is Nb0.83CoSb,which is mutually verified with the conclusion that Nb0.83CoSb is a single-phase component.The DOS,band structure and zT of Nb0.83CoSb and NbCoSb are calculated and compared.Tt is found that the presence of Nb vacancies in Nb0.83CoSb increases the density of states near the Fermi level,optimizes the carrier concentration,and increases its zT value by 100%,which improves the thermoelectric performance.The lowest point of formation energy of V0.75+?CoSb(0???0.25)is V0.91CoSb,which is the most stable component point.Its thermoelectric figure should be the highest in the system of V0.75+?CoSb(0???0.25).Calculate and compare the band structure,DOS and zT of V0.91CoSb and VCoSb,and find that Nb0.91CoSb has higher degeneracy and higher state density than VCoSb,which means it has better electrical transport.Its zT value is 20%higher than VCoSb,which means that it has a better thermoelectric performance.The lowest point of formation energy of V?Nb1-?CoSb(0<?<1)is V0.25Nb0.75CoSb.It has the lowest amount of impurity phase.It is judged that it should be the optimal component point of the thermoelectric performance of V?Nb1-?CoSb(0<?<1)system.Analysis of the state density and band structure shows that the DOS of V0.25Nb0.75CoSb is higher and the band gap is narrower,which indicates a better thermoelectric performance.This paper finds the optimal component points of the system through calculation,and hopes that these results can guide the subsequent experiments.