Preparation of V-based Alloy Membranes using Chemical Transport Process
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Preparation of V-based Alloy Membranes using Chemical Transport Process Tetsuya Ozaki, Yi Zhang, Masao Komaki, and Chikashi Nishimura Ecomaterials Center, National Institute for Materials Science, Sengen 1-2-1, Tsukuba, Ibaraki 305-0047, Japan ABSTRACT Novel preparation process of V-Ni alloys for hydrogen purification membrane using chemical transport was investigated. Vanadium, NH4Cl, and PtCl2 as evaporating source were put in one side of a fused-silica tube, and Ni substrate was put in the other side. The fused-silica tube was sealed in vacuo, and set in a furnace with temperature gradients. Evaporating source temperature was 1173-1273 K. Substrate temperature was 3-100 K higher than the evaporating source temperature. This process consists of formation of HCl from NH4Cl and PtCl2, chemical transport of vanadium under temperature gradient via chlorides in the presence of HCl, and diffusion of the transported V into the Ni substrate. EDX line profile of cross section of the substrates after the process demonstrated that V diffused into the Ni substrates. Distribution of V concentration in the substrates was dependent on the substrate temperature and the temperature difference between the substrate and the evaporating source. Heating condition was optimized to obtain homogeneous V-Ni alloy. When the substrate temperature and evaporating-source temperature were 1228 K and 1223 K, respectively, V diffused homogeneously into the Ni substrate with thickness of 20 µm, and V concentration attained in the substrate was higher than 70at%. INTRODUCTION Metallic membrane to produce high-purity hydrogen increases its importance for fuel cell application and semiconductor manufacturing. Although Pd-Ag alloys are used practically as this kind of membrane, high cost of palladium prevents the alloy from being used widely. V-Ni alloys are promising as the membrane material because of its relatively low cost, excellent hydrogen permeation property, and good resistance to hydrogen embrittlement [1-4]. For its practical use, to be incorporated with module of hydrogen purification apparatus, establishment of preparation process of thin membrane or tube is required. We have tried to cold-roll V-Ni alloys, and succeeded in preparation of thin foils with thickness of less than 40 µm [4]. In order to make still thinner foil, novel approach was attempted in this work. In the presence of Cl2 or HCl, non-volatile metals can be chemically transported in vapor phase along temperature gradient by reversible formation of chlorides [5]. Therefore, starting from pure metal as substrate, preparation of alloy thin foils is possible by combination of chemical transport of alloying element with diffusion of it into the foils. This process has been previously applied to preparation of Ti-Al and Ni-Al alloys using Al as an evaporating source [6]. It was shown by us that V diffused into Ni substrate via chloride in the presence of HCl and that V-Ni layer was formed on the surface of the substrate [7]. In this work, application of this novel pro
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