Solid Solubility,Point Defects And Thermoelectrical Performance Of Mg₂（Si,Sn）Alloies

Mg2(Si,Sn)-based alloy system is a promising mid-temperature TE material and being paid increasing attention because of its low cost, environmental friendliness and high performance. However, compared with the best thermoelectric materials, there are still some problems to be solved in Mg2(Si,Sn)-based materials. Firstly, it’s too hard to synthesize homogeneous and stoichiometric samples, and the miscibility gap of the alloys is still in controversy. Secondly, the high lattice thermal conductivity still suppresses their thermoelectric performance. Thirdly, the unmatched performance of n-type and p-type materials greatly limits their commercial application due to the quite low p-type ZT. Aiming at these problems, we systematically investigated the solid solubility, the point defects of Mg2(Si,Sn) alloys and improved the thermoelectric performance of n-type and p-type alloys. The main results are summarized as follows:(Ⅰ)We employed two preparation methods:B2O3flux-assistant melting and Ta-tube shielded melting followed by quenching to determine the miscibility gap of the solid solutions. Mg2Si1-xSnx(x=0.2,0.4,0.5,0.6,0.8) alloy samples were synthesized, whose XRD patterns showed their phase is strongly dependent on the cooling rate of quenching. Comparing with the previous results, we redetermined miscibility gap of the solid solutions of Mg2S1and Mg2Sn, and their binary phase diagram was reshaped. Two kinds of dominant point defects were found in the system:Mg interstitials and Mg vacancies, which can tune the electrical conductivities and reduce the thermal conductivities, respectively. The two point defects were believed to impose significant and direct impacts on the thermoelectric performance of Mg2(Si,Sn) system, which are more remarkable than those of the phase compositions and microstructures.(Ⅱ) It is first time that the concept of "point defect engineering" is proposed to reduce the thermal conductivities of Mg2Si1-xSnx alloys. We simultaneously controlled the content of defects in the alloys, i.e. Sb substitution, Mg vacancies and Mg interstitials, by adjusting the Mg content and Sb substitution on Si sites. As a result, two desirable effects were realized:the electrical properties were improved due to the optimized carrier concentration and the thermal conductivities were greatly reduced by the enhanced point scattering on phonons. It was proved that unlike Frenkel defects Mg vacancies and Mg interstitials can coexist in Mg2Si1-xSnx solid alloys instead of recombining. In the system, Mg interstitials mainly act as donors to increase carrier concentration while two roles are played by the Mg vacancies, one is to tune the carrier concentration as acceptors and the other one is to reduce the thermal conductivity as point scattering centers. The point defect engineering can be carried out through controlling the content of Sb and Mg. The electrical properties can be optimized by Zn doping and the maximum state-of-the-art figure of merit ZT>1.1was attained at750K in Mg2Si0.4Sn0.5Sb0.1specimen.(Ⅲ) We improved the thermoelectric performance of P-type Mg2X(X=Si,Ge,Sn) based materials by two methods:acceptor doping and pseudo-ternary alloying. It was found that Ag is a more effective acceptor dopant than B. Mg content was controlled to reduce the electron concentration. The ZT of p-type was increased by35%. The results of high temperature Hall measurements showed that the mobility ratio of electrons to holes of Mg2(Si0.33Ge0.33Sn0.33) and Mg2(Si0.2Ge0.2Sn0.6) pseudo-ternary alloys is almost the same. Mg2(Si0.33Ge0.33Sn0.33) is a better P-type thermoelectric system, which exhibits lower thermal conductivity and higher electrical properties. The reduction in thermal conductivities of Ag doped Mg2(Si0.33Ge0.33Sn0.33) resulted in an increase in ZT and the maximum figure of merit was0.4, which was the highest value ever reported.