Preparation of polycrystalline bulk Mg2Si by using NaSi
Preparation of polycrystalline bulk Mg2Si by using NaSi
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DOI：
10.1007/s10853-009-3773-4
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发表时间：
2009-08
影响因子：
4.5
通讯作者：
Takahiro Yamada;Y. Oishi;H. Morito;H. Yamane
中科院分区：
文献类型：
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作者：
Takahiro Yamada;Y. Oishi;H. Morito;H. Yamane
近年来，热电材料在利用废热有效节能领域引起了人们的关注。 Mg2Si和Mg2Si基材料由于其高热电性能，有望作为PbTe的替代品应用于500至800 K温度范围的热电器件中。此外，Mg2Si在热电材料中密度最低，为2.0 g/cm3，其组成元素Mg和Si含量丰富且无毒[1-4]。根据 Okamoto 等人提出的 Mg-Si 相图[5]，Mg2Si 是该体系中唯一的二元相，熔化温度为 1354 K。由于 Mg2Si 的熔点接近 Mg 的沸点（1363 K），并且在此温度附近 Mg-Si 熔体中 Mg 的蒸发显着，因此在 Mg-Si 熔体中制备 Mg2Si 多晶块体和单晶是在0.2–0.4 MPa 的惰性气体抑制 Mg 蒸发 [6–10]。放电等离子烧结 (SPS) 和热压 (HP) 等高压加工技术也已用于在低于 Mg2Si 熔点的温度下形成 Mg2Si 多晶块体。批量制造的起始材料是通过机械合金化制备的 Mg 和 Si 粉末以及 Mg2Si 粉末的混合物 [11-15]。在我们之前的论文中，我们报道了使用Na-Si熔体在低于传统合成方法的温度下合成b-SiC粉末和多孔b-SiC块体，以及b-FeSi2粉末和块体[16-19]。我们还提出了一个实验确定的 Na-Si 二元系统相图，其中金属间化合物 NaSi 的熔点为 1071 K [20]。由于该熔点高于 Mg，我们尝试使用 NaSi 粉末和 Mg 熔体的致密体来合成 Mg2Si 块体。本文报道了用该方法制备的 Mg2Si 块状样品的 X 射线衍射、形貌和热电性能。起始原料为 Mg（Rare Metallic Co. Ltd.，99.9%，\147 lm）、Si 粉末（Koujundo Chemical Lab. Co. Ltd.，99.999%，\75 lm）、Na 金属（Nippon Souda Co. Ltd.，99.95%）和 NaSi，在充满 Ar 的手套箱（M-Bran、O2\1 ppm、H2O\1 ppm）中进行处理。 NaSi 的制备方法是将等摩尔的 Na 和 Si 在密封在不锈钢管（SUS316，[内部体积 8·9·80 mm）中的 BN 坩埚（内部体积为 6·9·13 mm，Shyowa Denko，99.5%）中于 1073 K 下加热 12 小时。将获得的NaSi用玛瑙研钵和研杵磨成粉末，并通过用矩形模具压制粉末而形成致密体(14mm×9×3mm×9约1mm，8mmol)。将NaSi成形体和Mg粉末(55mmol)放置在BN坩埚中。将坩埚用Ar密封在不锈钢管中，然后在电炉中在973 K下加热24小时。加热后剩余的Na通过与2-丙醇和乙醇反应除去。通过粉末 X 射线衍射（XRD；Rigaku Co.，RINT-2200）和热解石墨单色 CuKa 辐射（k= 1.5418 A）来鉴定在制备的样品中形成的相。 XRD 角度使用标准硅参考材料（NIST SRM 640c，a= 5.431195 A）进行校准。使用 MDI-JADE-6 程序通过最小二乘法细化 Mg2Si 样品的晶格参数。用扫描电子显微镜（SEM；Philips，ESEM XL30）观察样品的形貌。使用能量色散 X 射线分析样品中的元素
Recently, thermoelectric materials have been attracting attention in the field of effective energy saving by utilization of waste heat. Mg2Si and Mg2Si-based materials are expected to be used as an alternative to PbTe for application in thermoelectric devices in the temperature range from 500 to 800 K because of their high thermoelectric properties. Moreover, Mg2Si has the lowest density, ie, 2.0 g/cm3, among thermoelectric materials and its constituent elements, ie, Mg and Si, are abundant and nontoxic [1–4]. According to the Mg–Si phase diagram presented by Okamoto et al.[5], Mg2Si is the only binary phase of this system and melts at 1354 K. Because the melting point of Mg2Si is close to the boiling point of Mg (1363 K) and the vaporization of Mg from the Mg–Si melt is significant around this temperature, preparation of Mg2Si polycrystalline bulk and single crystals from the Mg–Si melt has been performed under 0.2–0.4 MPa of inert gas to suppress Mg vaporization [6–10]. High-pressure processing techniques such as spark plasma sintering (SPS) and hotpressing (HP) have also been utilized to form a Mg2Si polycrystalline bulk below the melting point of Mg2Si. The starting materials for the bulk fabrication were a mixture of Mg and Si powders and Mg2Si powder prepared by mechanical alloying [11–15]. In our previous papers, we have reported the synthesis of b-SiC powder and porous b-SiC bulk, as well as b-FeSi2 powder and bulk by using Na–Si melts at temperatures lower than those of conventional synthesis methods [16–19]. We have also proposed an experimentally determined phase diagram for the Na–Si binary system, where the melting point of an intermetallic compound NaSi was revealed to be 1071 K [20]. Since this melting point is higher than that of Mg, we attempted the synthesis of Mg2Si bulk by using a compact body of NaSi powder and Mg melt. The present paper reports the X-ray diffraction, morphology, and thermoelectric properties of Mg2Si bulk sample prepared by this method. Starting materials of Mg (Rare Metallic Co. Ltd., 99.9%,\147 lm), Si powder (Koujundo Chemical Lab. Co. Ltd., 99.999%,\75 lm), Na metal (Nippon Souda Co. Ltd., 99.95%) and NaSi were handled in an Ar-filled glove box (M-Bran, O2\1 ppm, H2O\1 ppm). NaSi was prepared by heating equimolar Na and Si at 1073 K for 12 h in a BN crucible ([6 9 13 mm in inner volume, Shyowa Denko, 99.5%) sealed in a stainless steel tube (SUS316,[8 9 80 mm in inner volume). The obtained NaSi was powdered with an agate mortar and pestle and formed into a compact body (14 mm 9 3 mm 9 ca. 1 mm, 8 mmol) by pressing the powder with a rectangular die. The NaSi compact body and Mg powder (55 mmol) were placed in a BN crucible. The crucible was sealed in the stainless steel tube with Ar, followed by heating at 973 K for 24 h in an electronic furnace. The Na remaining after heating was removed by reaction with 2-propanol and ethanol. The phases formed in the prepared samples were identified by powder X-ray diffraction (XRD; Rigaku Co., RINT-2200) with pyrolitic graphite monochromatized CuKa radiation (k= 1.5418 A). The XRD angles were calibrated with a standard reference material of silicon (NIST SRM 640c, a= 5.431195 A). The lattice parameters of Mg2Si samples were refined by the least-squares method using the MDI-JADE-6 program. The morphology of the samples was observed with a scanning electron microscope (SEM; Philips, ESEM XL30). The elements in the sample were analyzed with an energy-dispersive X-ray