Preparation of Thermoelectric Material by Reactive Diffusion Method

作为反应扩散形成的金属间化合物的重要特征，我们可以发现以下几点，即，它们不包括任何不需要的相，它们可以在没有坩埚或空气污染的情况下制备，它们具有干净的相边界界面，并且它们可以具有取向的晶粒。当这种方法能够实际应用时，就有可能制备出比目前更优良的热电材料，众所周知，FeSi_2的热电性能不受铁和硅的纯度的影响。然而，目前使用的铁板不到99.99%，它们可能在制备过程中受到污染。此外，还没有使用高纯度铁和硅的研究。本工作的一个目的是通过使用超高纯度的起始材料，考虑到由于熔化的污染，以制备热电材料，并研究热电性能。本文采用反应扩散法，以高纯Fe、Co、Si和Sb为原料，研究了Fe-Si、Co-Si和Co-Sb二元系。研究表明，Fe-Si扩散偶中形成的主要扩散层是Fe_3Si。Fe 3Si的生长速度主要取决于铁的纯度。铁中的氧使Fe 3Si的生长速率减慢。在Co-Si二元系中，我们阐明了CoSi和CoSi 2的生长动力学。FeSi 2和CoSi或CoSi 2都难以通过反应扩散法制备它们。我们尝试了用S-Co/S-Sb、S-Co/L-Sb、S-Co/V-Sb等多种扩散偶在Co-Sb二元系中制备CoSb_3。通过这种技术，我们可以使CoSb 3包括许多小空隙。空隙的存在预计会降低CoSb 3的导热率，因此热电性能也将有望得到改善。

As important characters of intermetallic compounds formed by reaction diffusion, we can pick up the followings, i.e., they do not include any needless phases, they can be prepared without contamination from crucible or air, they have clean phase boundary interface and they can have orientated grains. When we can apply this method practically, there are possibilities to prepare more excellent thermoelectric materials than that so far.It has been well known that the thermoelectric properties of FeSi2 are not affected by the purity of iron and Si. However, iron plates used up to the present were less than 99.99% and they might have been contaminated during the preparing process. Further there is no study in which high purity iron and silicon were used. A purpose of this work is to make thermoelectric material by using ultra high purity starting materials taking the contamination due to melting into account and to study the thermoelectric properties. For the purpose we use reaction diffusion method using high purity Fe, Co, Si and Sb materials.Fe-Si, Co-Si and Co-Sb binary systems have been studied. It has been clarified by this work that main diffusion layer formed in the Fe-Si diffusion couples is Fe3Si. The growth rate of Fe3Si depends significantly on the purity of iron. Oxygen in iron slows down the growth rate of Fe3Si.In the Co-Si binary systems we have clarified the growth kinetics of CoSi and CoSi2. Both FeSi2 and CoSi or CoSi2 are difficult to make them by the reactive diffusion method. We have tried to make CoSb3 in Co-Sb binary system by using various kinds of diffusion couples such as S-Co/S-Sb diffusion couples, S-Co/L-Sb diffusion couples and S-Co/V-Sb diffusion couples. By this technique we could make CoSb3 including many small voids. The existence of voids is expected to decrease thermal conductivity of CoSb3 so the thermoelectric properties will also be expected to be improved.