采用熵工程技术改善SnTe基材料的热电性能

Improvement of thermoelectric performance of SnTe-based solid solution by entropy engineering

SnTe相比于PbTe在热电领域更具有应用潜力,原因是可以减少Pb对环境的毒性作用.但SnTe化合物带隙宽度小,本征Sn空位浓度(nv(Sn))大,因此,本征SnTe具有太大的载流子浓度(~10^(21) cm^(-3)).改善SnTe材料的热电性能有多种方法,但本次工作以熵工程技术为指导分步设计材料成分.第一步,与5%的GeTe形成Sn_(0.95)Ge_(0.05)Te合金以增大外加元素或化合物的固溶度;然后,Sn_(0.95)Ge_(0.05)Te与5%n-型Ag_(2)Se固溶以进一步降低SnTe的p-型载流子浓度;第三步,采用Bi/Sn等摩尔置换形成(Sn_(0.95-x)Ge_(0.05)Bi_(x)Te)_(0.95)(Ag_(2)Se)_(0.05)(x=0—0.1)合金,以进一步增大固溶体的构型熵(ΔS).经过多组元固溶后,共增加构型熵ΔS=5.67 J·mol^(-1)·K^(-1)(x=0.075),从而大大降低了载流子浓度和热导率,使得最大热电优值(ZT)从本征SnTe的约0.22提高到约0.80(x=0.075).证实了熵工程技术是一种能改善SnTe化合物热电性能的有效机制.但实验结果也说明,虽然熵增对材料的热电输运机制有较大影响,但熵增机制需要与其他机制协调才能大幅提高材料的热电优值.

SnTe is a good alternative to PbTe in the thermoelectric(TE)applications,in that it is a compound with no toxic element Pb.Besides,the compound SnTe has a relatively narrow bandgap(0.3-0.4 eV)and high Sn vacancy concentration(Snv)as well.Accordingly,it gives a high carrier concentration(10^(21) cm^(-3))at room temperature(RT),which is not favorable for thermoelectrics,therefore the regulation of both the electronic and phonon scattering mechanisms is strongly required.Up to date,there have been many approaches to improving its TE performance.The typical examples are those involving the valence band convergence,nanostructuring,substitutional and interstitial defects,and lattice softening,which are all practical and effective to improve the TE performance of SnTe.However,in this work the entropy is taken as an indicator to design the SnTe-based TE material with multicomponents and then optimize its TE performance.The detailed scheme involves the chemical composition design step by step.At first,SnTe alloys with 5%GaTe to form a solid solution Sn_(0.95)Ge_(0.05)Te,aiming to increase the solubility of the foreign species.The second step is to form another solid solution(Sn_(0.95)Ge_(0.05)Te)_(0.95)(Ag_(2)Se)_(0.05) via the alloying Sn_(0.95)Ge_(0.05)Te with 5%Ag_(2)Se.The purpose of this step is to reduce the p-type carrier concentration of the system,for the species Ag_(2)Se is a typical n-type semiconductor.The last step is to form a series of solid solutions(Sn_(0.95-x)Ge_(0.05)Bi_(x)Te)_(0.95)(Ag_(2)Se)_(0.05) by substituting different amounts of Bi on Sn in(Sn_(0.95)Ge_(0.05)Te)_(0.95)(Ag_(2)Se)_(0.05),to further enhance the configurational entropy(ΔS).Because of the above approaches,both the carrier concentration and thermal conductivity decrease while the highest TE figure of merit(ZT)increases from 0.22 for the pristine SnTe to~0.8 for the alloy(Sn_(0.95-x)Ge_(0.05)Bi_(x)Te)_(0.95)(Ag_(2)Se)_(0.05)(x=0.075).This result proves that the entropy engineering is a practical way to improve the TE performance of SnTe,and at the same time it illustrates that it is very important to harmonize the entropy engineering with other electronic and phonon scattering mechanisms,in order to improve the TE performance of SnTe effectively.