錫摻雜矽化鎂熱電材料之固相合成與分析

Abstract

矽化鎂(Mg2Si)是由價格低廉且無毒之矽與鎂二元素化合所得，其擁有與銻化鉛(PbTe)熱電材料相近之高Seebeck係數與低熱導率，故成為極具潛力的熱電材料之一。而錫的摻雜已証明有助於克服純矽化鎂之低導電度、並更進一步降低熱導率，進而可有效提升整體熱電優值(Thermoelectric figure of merit; ZT)。然而合成特定化合比之無氧化錫摻雜矽化鎂仍被視為一項挑戰，主要是因為錫與鎂均為易氧化之元素，且錫可以任意比例置換矽元素。 本研究以鎂、矽、錫三種粉末為原料，經過長時間球磨細化混合，再藉由固相合成法於不同溫度下合成矽化鎂及錫摻雜之矽化鎂塊材。為防止氧化相產生造成熱電性質下降，本研究嘗試不同製程參數，最終以石英玻璃封管的方式解決反應過程的氧化問題。合成所得最佳化之錫摻雜矽化鎂(Mg2Si0.4Sn0.6)塊材，在470 K下展現最佳熱電性質：Seebeck係數約-145 μV/K，導電率約4.4 S/cm，功率因子約9.310-2 μW/cmK2。此外，利用X光粉末繞射圖譜配合Rietveld精算法，定量分析不同合成條件下，目標錫摻雜矽化鎂(Mg2Si0.4Sn0.6)之合成率，並進而提供錫摻雜矽化鎂，其晶格常數與錫矽成分比之關係。

Magnesium silicide (Mg2Si) which is an inorganic compound consisting of low-cost and nontoxic magnesium and silicon has become one of the most promising thermoelectric materials as lead telluride (PbTe) mainly due to its high Seebeck coefficient and low thermal conductivity. The Sn doping has been proven as an effective strategy not only to overcome the problem of the very low electric conductivity of intrinsic Mg2Si, but to further decrease the thermal conductrivity and thus the overall thermoelectric figure of merit (ZT). In this work, Mg2Si and Sn-doped Mg2Si were synthesized using high-temperature solid-state approaches. The raw materials of Mg, Si, and Sn powders were ball-milled and were then heated at different temperatures. To prevent the formation of oxides which would seriously decrease ZTs, a variety of processing parameters were verified. It has been found that the oxide-less product can be achieved using sealed quartz tubes. The synthesized optimized Mg2Si0.4Sn0.6 bulk exhibits relatively better thermoelectric properties at 470 K where the Seebeck coefficience is -145 μV/K, the electrical conductivity is 4.4 S/cm, and thus the calculated power factor is 9.310-2 μW/cm-K2. In addition, the measured x-ray power diffraction (XRD) patterns comprising multiple Mg2SixSn1-x phases were refined with the Rietveld method not only for quantitatively characterizing the fraction of the targeted Mg2Si0.4Sn0.6 compound, but for providing the experimental relation between the lattice constant and the ratio of Sn/Si of the Mg2SixSn1-x compounds.