The Effect Of Doping With Sb,Bi And La For The Thermoelectric Properties For Mg₂Ge_0.25Sn_0.75 Alloys

Thermoelectric?TE? materials are functional materials, which can achieve mutual transformation between thermal power and electrical energy. It can convert waste heat produced in automobile or industry into electricity, and have been used in automotive waste heat recovery, which was significant for society with the greenhouse effect and energy shortage increasing. Doping heterogeneous atoms or formation of the second phase are effective ways to improve the TE material performance. In this paper, the series alloys of Mg₂Ge_xSn_1-x ? Mg₂Ge_0.25Sn_0.75-xSb_x, Mg₂Ge_0.25Sn_0.75-xBi_x and La_xMg_2-xGe_0.25Sn_0.75 were prepared by high frequency melting, annealing, quenching and spark plasma sintering?SPS? techniques, and their phase consitution and thermoelectric properties were investigated.Different methods were taken to prepare Mg₂ Ge_x Sn_1-x? x = 0.15, 0.2, 0.25, 0.3? series alloys, the results indicated that high frequency melting,annealing,quenching and SPS were effective ways to synthesis Mg₂Ge_xSn_1-x solid solution when the germanium content was between 0.15 and 0.3. When Ge content was 0.25, the reference reported indicated that the alloy showed the highest TE performance.The investigation for the series of Mg₂Ge_0.25Sn_0.75-xSb_x? x = 0.03, 0.05, 0.07, 0.10, 0.15? alloys showed that all the alloys were cubic structure solid solution compounds and showed n-type semiconductor property. The resistivity of undoped sample at room tempertaure was 201.9 ?? m, which was much higher than the value of 3.65 ?? m when Sb content was 0.03. The absolute values of Seebeck coefficient of undoped sample decreased from 320 ?VK^-1 to 85.9 ?VK^-1 as the temperature increasing, while the absolute values for the doped samples increased as Sb content increasing and the maximum absolute value reached to 151.5 ?VK^-1 when the Sb content was 0.15. The dopant of Sb can decrease the ? for the alloys obviously. Compared with 3.48 Wm^-1K^-1 for the undoped sample at room temperature, the minium thermal conductivity decreased to 1.4 Wm^-1K^-1 when the Sb content was 0.1. The maximum ZT of 1.53 was obtained at 623 K as Sb content was 0.07.The investigation for the series of Mg₂Ge_0.25Sn_0.75-x Bi_x? x = 0, 0.005, 0.01, 0.02, 0.04, 0.08? alloys showed that all alloys were cubic structure solid solution compounds. Some small Sn-rich areas appeared with the dopanant of Bi and the areas grew gradually with the Bi content increasing. The substitution of the Bi atoms for Sn atoms can decrease the alloys resistivity and increase the Seebeck coefficient, the resistivity can be decreased to 5.19 ??m when the Bi content was 0.08 compared 201.9 ??m for undoped sample at room temperature. The Seebeck cofficient absolute value increased to 262.2 ?VK^-1 at 773 K when Bi content was 0.005, which is higher than 85.9 ?VK^-1 for undoped sample, the maximun powerfactor can reach to 4335 ?Wm^-1K-2 at 473 K when Bi content is 0.01. The structure defects were increased and phone scattering were enhanced by the Bi dapan, the ? decreased obviously to 1.27 Wm^-1K^-1 as Bi content was 0.02, which was lower than 3.48 Wm^-1K^-1 for undoped sample at room temperature. A high figure of merit was 2.18 at 623 K as Bi content was 0.01.The investigation for the series of La_xMg_2-xGe_0.25Sn_0.75? x = 0, 0.01, 0.015, 0.02, 0.03? alloys showed that all alloys were cubic structure solid solution compound when the La content was lower than 0.01, the second phase of LaMg formed as the La content increased. La doping in the alloy decreased the resistivity on the whole, the resistivity decreased obviously to 32.7 ??m when the La content was 0.03 compared 201.9 ??m with undoped sample at room temperature. The thermal conductivity decreased to 1.68 Wm^-1K^-1when the La content was 0.015 compared 3.48 Wm^-1K^-1 with undoped sample at room temperature. As the results, the thermoelectric properties were improved. The maximum finger of merit was 0.28 at 473 K as La content was 0.015.