Preparation And Thermoelectrical Properties Of Mg₂Si-based Semiconductors

Thermoelectric materials are semiconducting functional materials,which can interconvert heat and electricitydirectily.Thermoelectricity is of great interest which offers a reliable,fully solid-state mean of power generation and cooling.Mg₂Si based semiconductors which are nontoxic and cheap,are one of the most promising novel materials among them.In the present work,the traditional method in preparing Mg₂Si based compounds was improved.We added materials step-by-step in inducing melting so as to prepare pure Mg₂Si, Mg₂Ge,Mg₂Sn and ternary,quarternary systems.We firstly used Al₂O₃ as reaction containers so that the effect of different cooling speed to the thermoelectric properties of the materials could be studied.It's not possible to do this by traditional methods.The results showed that the thermal conductivity of the materials reduced markedly by quenching.The figure of merit ZT of Bi-doped sample achieved 0.8 after quenching.Ternary system Mg₂Si_0.6Sn_0.4was prepared and studied.We found that the thermal condudcivities of Mg₂Si based materials reduced remarkably after forming solid solutions. The minimum thermal conductivities of this ternary system was only 1.6 W·m^-1·K^-1,which was 80%lower than that of Mg₂Si,whose was7.9 W·m^-1·K^-1by reports.We doped ternary Mg₂Si_0.6Sn_0.4with all kind of elements in order to improve the electrical conductivity,including Al and rare earth,which were N type dopants and Li and Ag as P type dopants.The experiments and results of Li doped Mg₂Si_0.6Sn_0.4were first reported. Electrical conductivities of all samples increased obviously after doped with Al.The optimized quantity of dopant Li was 1.0 wt%.All kinds of rare earth elements were studied as dopants of Mg₂Si_0.6Sn_0.4.The results showed that the properties of samples reduced as the ordinal of doping elements increased.La doped sample exhibited higher eletrical conductivity and lower thermal conductivity,which led to best thermoelectric property among all rare earth doped samples.Highest ZT value was 0.8 at 800 K,which was about 4 times of undoped sample.Both electrical resistivity and thermal conductivity of Li doped samples Mg_2-xLi_xSi_0.6Sn_0.4(x=0,0.03,0.07,0.15,0.3)increased followed by a decrease tendency. The value of "σ/k" calculated showed that it was an effective way to increase thermoelectric properties of ternary Mg₂Si_0.6Sn_0.4by doping Li.The electrical conductivity of materials decreased after doping Ag.The reason was explained. Quarternary system Mg-Si-Ge-Sn was prepared.The effective of Ge was studied in the aspects of band structure and phonon scattering.The results showed that the electrical conductivity could be improved effectively though the thermal conductivity increased to some extent after adding Ge.Mg₂Sn_0.4Si_0.55Ge_0.05and Mg₂Sn_0.4Si_0.58Ge_0.02exhibited the highest ZT value,which was 0.28 at 600 K.The producing of thermoelectric devices was attempted.Some valuable experience was gained.