Amorphous alloy membranes

Dolan, M.D., Dave, N.C., Ilyushechkin, A.Y., Morpeth, L.D. and McLennan, K.G. (2006) Composition and operation of hydrogen-selective amorphous alloy membranes. Journal of Membrane Science, 285, 30-55. 

Dolan, M., Dave, N., Morpeth, L. et al. (2009) Ni-based amorphous alloy membranes for hydrogen separation at 400°C. Journal of Membrane Science, 326, 549-555. 

Amorphous alloy membranes are at present being developed to tackle the problems of H2 embrittlement, sintering and cost that occur with crystalline alloy membranes, while still providing H2 permeance comparable to Pd. Fabricated from non-Pd elements, these membranes are low price and inherently resistant to embrittlement because of their amorphous structure. Much development is still required, however, to improve stability and achieve the performance of existing Pd-based crystalline alloy... 

In order to further reduce the use of Pd, Paglieri et have investigated amorphous alloys based on Ni-Nb-Zr. In particular, they have systematically studied the effects of Ta addition on the performance of a series of Ni-Nb-Zr amorphous alloys with various Zr content. A series of amorphous alloy membranes consisting of Ni b2oZr2o, (Nio6Nbo.4)ioo.4Zr and (Nio 6Nbo3Tao.i)ioo- cZr , (where x = 0,10,20 and 30) were prepared using melt... 

The tendency of amorphous alloy membranes to crystallize limits their applications, as this reduces their hydrogen permeabiUty and mechanical... 

Hara, S., N. Hatakeyama, N. Itoh, H.-M. Kimura, and A. Inoue, Hydrogen permeation through palladium-coated amorphous Zr-M-Ni (M=Ti, Hf) alloy membranes, Desalination, 144,115-120, 2002. 

Liu B., Dai W., Wu G., Deng J.-F. Amorphous alloy/ceramic composite membrane preparation, characterization and reaction studies. Catalysis Letters 1997 49 181-188. 

Very little is known about the influence of grain growth, or crystallization if the membrane is composed of an amorphous alloy, on membrane durability. The as-fabricated permselective metal membrane will be polycrystalline or amorphous, depending on the alloy composition and fabrication method. Amorphous, or metallic glass, structures are far less common than are polycrystalline structures. Both amorphous and polycrystalline structures are quasi-stable, meaning that structures are kinetically stabilized and slow to rearrange to the thermodynamically favored structure. In both cases, this would be a single crystal of the metal. 

Moreover, the palladium alloys improve chemical resistance of the membranes. For example, Pd-Cu and Pd-Au increase the resistance to H2S [59] as well as palladium-coated amorphous Zr-M-Ni (M = Ti, Hf) alloy membranes are resistant enough in a hydrogen atmosphere and have stable hydrogen permeability in the range of 200-300" C [60]. 

Excessive costs limit the use of Pd membranes for wide-scale industrial use. So, there is a strong interest in developing non-Pd-based (or, at least, with a low Pd-based content) and low-cost Hj permeation alloys (Luo et aL., 2006 Nishimura et al.., 2002). In particular, Luo et al. (2006) demonstrated that an Nb-Ti-Ni alloy, consisting of only the primary phase and the eutectic phase, shows a high H2 permeability and resistance to H2 embrittlement. Moreover, Kara et al. (2002) reported that Pd-coated amorphous Zr-M-Ni (M = Ti, Hf) alloy membranes were resistant enough in an atmosphere and had stable permeability only to H2 at least in the range of 200-300°C. 

Amorphous alloys generally show higher Hj permeabiUty than the corresponding crystallized ones (Itoh etaL, 1997) and the pressure dependence of H2 permeability follows a relationship described by a simple n" power equation, as for example reported by Itoh et al (1998) in their study on the Hj solubihty in amorphous Pd Si (x = 0.15,0.175,0.2) alloys. In this work, the exponent n joins the H2 flux through the membrane to the H2 partial pressure and is affected by both the temperature and Si content. In the following, the equations, reported in their paper by Itoh et al (1998), are briefly summarized. 

As already said, the power equation was used, for example, by Itoh et al. (1998) in determining the Hj permeability in amorphous alloys, at elevated temperatures up to 390°C. It should also be added that the main conclusions of their work were (a) H2 permeability strongly depends on the Si content and increases with it (b) the amorphous alloy shows higher hydrogen permeability than the crystallized one and shows resistance to hydrogen embrittlement and (c) the amorphous membrane must be used below its crystallization temperature, due to the strong reduction of Hj permeability in the crystallization state. 

---