High Hydrogen Permeability in Nb-Ti-Ni Eutectic Alloys Containing Much Primary (Nb, Ti) Phase
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High Hydrogen Permeability in Nb-Ti-Ni Eutectic Alloys Containing Much Primary (Nb, Ti) Phase Kazuhiro Ishikawa1, Weimin Luo2 and Kiyoshi Aoki1 1 Department of Materials Science, Kitami Institute of Technology, Koen-cho 165, Kitami, Hokkaido 090-8507, Japan 2 Graduate Student of Kitami Institute of Technology ABSTRACT Microstructure, crystal structure and hydrogen permeability (Φ) of Nb-Ti-Ni alloys have been investigated to develop alloys with high hydrogen permeability using a scanning electron microscope (SEM), an X-ray diffractometer (XRD) and a mass flow meter, respectively. The alloys consisting of the B2-TiNi compound and the bcc- (Nb, Ti) solid solution were hydrogen permeable. The values of Φ increased with increasing temperature and Nb content. However, too much Nb content, i.e., too much volume fraction of the primary (Nb, Ti) phase caused the hydrogen embrittlement. The Nb68Ti17Ni15 alloy showed the highest Φ of 4.91 × 10 −8 [molH2m-1s-1Pa-0.5] at 673 K, which was 3.1 times higher than that of pure Pd. The present work demonstrated that the alloys showing high Φ values are obtained on the straight line connecting the compositions of the eutectic and the primary phases. INTRODUCTION Hydrogen gas produced by a steam reforming of hydrocarbons contains much CO gas which damages the catalytic effect of hydrogen dissolution of Pt electrodes for fuel cells, so that hydrogen gas must be purified using hydrogen permeation membrane and so on. Pd based alloys are commercially used as hydrogen permeation membranes for separation and purification of hydrogen gas, but they are too expensive and scarce resources. Then, it is strongly desired to develop low cost and high performance hydrogen permeation alloys [1, 2]. Hydrogen flux (J) through an alloy membrane is described as follows; J=Φ∆P0.5/L (1) 0.5 where Φ, ∆P and L are the hydrogen permeability, the difference of square root of hydrogen pressure at both side of the membrane and the thickness of the membrane, respectively. We can see that there are two possible ways in order to attain large J. The first way is to find out the high Φ alloys. 5A group elements such as V, Nb and Ta have higher Φ than that of pure Pd, but these metals and their alloys suffer from the sever hydrogen embrittlement [3-5]. On the other hand, Equation (1) indicates that large J is obtainable by the large pressure difference and the thin thickness of the membrane, which imply that superior mechanical properties and large resistance to the hydrogen embrittlement are essential for hydrogen permeation alloys. However, high hydrogen permeability is generally incomparable with large resistance to the hydrogen embrittlement in single-phase alloys. Recently, we have demonstrated that the Nb-Ti-Ni alloys consisting of both B2-TiNi and bcc(Nb, Ti) phases and containing much amount of the eutectic {TiNi+(Nb, Ti)} phase show high
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hydrogen permeability without the hydrogen-induced fracture [6-8]. These alloys have been designed on the basis of the new concept that (Nb, Ti
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