| Mg2Si1-xSnx solid solutions have been regard as an excellent candidate for thermoelectric energy conversion of middle temperature, because of environment friendly and abundant elements. As is known, doping different element to Mg2Si1-xSnx solid solutions can improve the thermoelectric properties. There are many reports on the synthesis processing of Mg2Si1-xSnx (x=0,0.2,0.4,0.5,0.6,0.8, 1.0), and different element doped. But there are no reports about doping on Mg2Si0.3Sn0.7. The preparation of low dimensional or nano-structured materials is an effective way to improve thermoelectric properties. Because the low dimensional materials can regulate the transport of electron and phonon reasonably, enhance the thermoelectric performance.In this study, solid state reaction at low temperature combined with spark plasma sintering (SPS) were used to synthesize Mg2Si0.3Sn0.7 compound. The results showed that the single phase of Mg2Si0.3Sn0.7 compound has been obtained. Combined solid state reaction with SPS, different content of Sb and Bi doped Mg2Si0.3-xSn0.7Sbx(0≤x≤0.015) and Mg2Si0.3-xSn0.7Bix(0<x<0.015) solid solution have been fabricated. XRD shows that all samples are single phase, SEM indicated the grain of Sb doped samples were about 10μm while Bi doped samples' 15-25μm, and were frangibility fracture. The density of the block material was more than 99%.The thermoelectric characters of bulks were measured, included Seebeck coefficient, Electrical conductivity and thermal conductivity. Sb and Bi were benefit for the thermoelectric properties, especially the electrical conductivity (σ) increases. The content of the doped element is very important. For doping Sb to Mg2Si0.3-xSn0.7Sbx(0<x<0.015) compound, when x=0.0075, the highest figure of merit (ZT) of 0.80 was obtained at 590K, on the other hand, Bi doped Mg2Si0.3-xSn0.7Bix(0<x<0.015) compounds, when x=0.015, the sample of shows the maximum value of the figure of merit 0.78 at about 722K. The powder factor of Mg2Si0.3-xSn0.7Bix solid solution increase with the content of Bi atoms, which is the symbol of electrical properties, Bi-doped Mg2Si0.3Sn0.7 can improve the electrical properties. With the rise of temperature,the thermal conductivity of Mg2Si0.3-xSn0.7Sbx and Mg2Si0.3-xSn0.7Bix decrease first, and then increase. The Sb and Bi replaced Si, their atom mass and size is different, which enhance the phonon scatter. And the carrier contribution increase with the doping concentration, so the thermal conductivity increased obviously.The other aspect of this study was discussing the possibility of obtaining dimensional Mg2Si1-xSnx compounds. Firstly, the micron powder was prepared by solid state reaction, the size of Mg2Si0.3Sn0.7 and Mg2Si0.3Sn0.7 was about 50μm. Adopt WC material ball rub pot and hard balls, select 370r/min rotate speed, with n-hexane for ball mill mediμm, the ratio of ball mill to powder was 20:1, feeding on ball mill for 30h, can get sub-micron about 5μm of Mg2Si0.3Sn0.7 compound. Milled for 40h, the particle with 4μm of Mg2Si0.4Sn0.6 powder was obtained. Further extended milling time, XRD test results show that both of these will be oxidated and occur phase separation. The phase is Mg2Si in the further analysis. Maybe the particles which had high surface energy desolventized Mg2Si, Sn and Mg in the milling, and Mg was oxidized to MgO, so the content of MgO increase. Melling for 80h, the peaks of Mg2Si0.3Sn0.7 compound were disordered. Maybe the powder of Mg2Si and Sn reacted to Mg2Si1-xSnx. We draw the conclusion that milling method is not suitable to study the low dimension of this system... |
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