Preparation And Thermoelectric Properties Of P-type Mg₂Si_1-xGe_x Based Compounds

Medium-temperature (500-800K) thermoelectric materials could be used for reusing the exhaust heat of automobile and industrial waste heat and converting them into available electricity, and were prospective to increase the conversion efficiency of the fossil energy. Thus the investigation and development of these materials were strategically important for saving energy and reducing carbon emission in our country. Being one type of important medium-temperature thermoelectric materials, Mg2Si1-xSnx-based solid solutions have attracted considerable interest as prospective thermoelectric materials for waste heat recovery because of the features of their abundant and low cost chemical constituents, not containing scarce Te element, environmentally friendly, low density and so on. The n-type Mg2Si1-xSnx based solid solutions have received excellent thermoelectric figure of merit ZT, but the thermoelectric properties of p-type material is still relatively low. Therefore, the investigation and optimization of the thermoelectric performance of p-type Mg2Si based materials have significant effects on the application of thermoelectric power generation. Under this research situation, Mg2Si1-xGex solid solution, as the prospective p-type materials, become focus concern in thermoelectric research field.Focusing on the demands of highly effective n-type and p-type Mg2Si based material for medium temperature thermoelectric power generation, Mg2Si1-xGex solid solutions are promising candidates and are chosen for this research. We adopted a solid state reaction with followed spark plasma sintering (SPS) process for the synthesis of Mg2Si1-xGex solid solutions, and gained optimized parameters for both the solid state reaction and SPS. Based on this, we investigated the influence of single element doping and double doping of elements like Li and Ga, and optimized their ZT values. The main research content and results were listed below.Mg2Si1-xGex solid solutions could be prepared by two step solid state reaction at873-1073K for24h, and then be compacted into dense bulk (relative density higher than99%) by spark plasma sintering through heating at1093-1173K for l0min. Results indicated that, non-doped Mg2Si1-xGex solid solutions exhibited n-type conducion. The electrical conductivity of Mg2Si1-xGex were significantly enhanced with increasing Ge content while Seebeck coefficient remained stable. Meantime, thermal conductivity first increased and then decreased with the increase of Ge content. The largest ZT value of0.32was obtained at650K for Mg2Sio.6Geo.4.The type of conduction of Mg2Sio.6Geo.4solid solution was changed from n-type to p-type through the doping of Li. The hole concentration was improved to-2.5x1019cm-3with the addition of Li which acted as the electro acceptor. The electrical conductivity and the Seebeck coefficient of the sample increased with increasing Li content, and the maximum power factor of0.97x10-3Wm-1K-2and the largest dimensionless figure of merit ZT were obtained at the sample with x=0.035, which wss0.36at700K. This data was one of the best results in the p-type Mg2Si based material system. The addition of Ga can shift the Fermi level to the edge of valence band, thus Ga doped Mg2Si0.6Ge0.4presented a p-type conduction. However, because the solubility limit of Ga in Mg2Si0.6Ge0.4was very low, Ga doped samples displayed a low electrical conductivity. A complex p-n transition was found at x≤0.06which resulted in a very low thermoelectric performance. Due to the presence of Mg2Ga impurity phase that possessed very high electrical conductivity, the sample with x>0.06gained high electrical conductivity and Seebeck coefficient in the same time, and the maximum ZT value of0.09was obtained at650K.The investigation of Li and Ga double doping revealed that, the hole concentration of Mg2-xLix(Sio.6Ge0.4)i-yGay(0.035≤x≤0.05,0<y<0.04) solid solutions was increased through Li doping while Ga addition seemed not to improve the hole concentration that resulted from Li doping. In contrary, the Ga doping decreased the hole concentration, electrical properties and ZT values of the samples. The results indicated that Ga doping could change the existence state of Li and the doping effects, and also reduced the actual doping amount of the samples. The double-doped samples showed the highest power factor and the lowest thermal conductivity when x=0.05, y=0.02, which obtained the maximum ZT value of0.35at800K, comparable to the single Li-doped samples, and it was also one of the best performances of the p-type Mg2Si based materials.