Design and microstructural control of new hydrogen permeation alloys fir hydrogen membrane reactor

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Pd-Ag alloys are commercially used as a hydrogen separation and purification alloy, but they are too expensive and scarce in resources. Then, it is strongly desired the development of low cost and high performance hydrogen permeation alloy. Hydrogen permeability φ of 5A metals are higher than that of Pd-Ag alloys, but they suffer from the severe hydrogen embrittlement. That is, the high φ value is incompatible with high resistance to the hydrogen embrittlement in single-phase alloys. The present authors have proposed new hydrogen permeation alloys on the basis of the concept of duplex phases. Firstly, the relation between microstructures and φ has been investigated in the as-cast Nb-Ti-Ni alloys. The Nb_<39>Ti_<31>Ni_<30> alloy consisting of the primary bcc-(Nb, Ti) phase and the eutectic {(Nb, Ti)+TiNi} one shows the highest φ=1.93x10^<-8> [mol H_2 m^<-1> s^<-1> Pa^<-0.5>] at 673 K, which is higher than that of Pd. Furthermore, φ is measurable in the Nb-rich Nb-Ti-Ni alloys on the straight line connecting the eutectic point and the primary phase. φ increases with increasing Nb content and the amount of the primary phase. The most Nb-rich Nb_<68>Ti_<17>Ni_<15> alloy, consisting of the 26vol% eutectic phase and the 74vol% primary phase, shows the highest φ of 4.91x10^<-8> (mol H_2 m^<-1>s^<-1>Pa^<-0.5>). In addition, φ of the eutectic Nb-Zr-Ni alloys containing the primary phases ZrNi or bcc (Nb, Zr) has been measured. φ of the Nb_<20>Zr_<40>Ni_<40> alloy containing the primary bcc-(Nb, Zr) phase (18 vol %) is 2.73x10^<-8> [mol H_2 m^<-1> s^<-1>Pa^<-0.5>] at 673 K. These values are higher than that of pure Pd and of the Nb_<39>Ti_<31>Ni_<30> alloy. The present works strongly indicates that multiphase alloys containing eutectic structures, which suppress the hydrogen embrittlement, are promising as novel hydrogen permeation alloy membranes.