Concentration Dependence of Hydrogen Diffusivity in Amorphous Zr-Ni Alloys at Elevated Temperatures

Transcription

1 Concentration Dependence of Hydrogen Diffusivity in Amorphous Zr-Ni Alloys at Elevated Temperatures On-line Number 0467 Shigeki Hara, 1 Hong-Xiang Huang, 1 Misaki Ishitsuka, 1 Naotsugu Itoh, 2 Koichi Kita, 3 Komei Kato 3 1 National Institute of Advanced Industrial Science and Technology (AIST), central 5, Higashi 1-1-1, Tsukuba Japan s.hara@aist.go.jp 2 Faculty of Engineering, Utsunomiya University, Yoto 7-1-2, Utsunomiya Japan 3 MITSUBISHI MATERIALS CORPORATION, Non-Ferrous Alloys Research & Technology Laboratories, Shimoishido-Shimo 476, Kitamoto Japan ABSTRACT Amorphous alloys are expected as new cheep membrane materials for hydrogen separation, which is substitutable to expensive palladium alloys, maybe resulting in enlargement of application field. In this study, Zr-Ni based amorphous alloys, typical hydrogen permeable amorphous alloys, were focused on, whose diffusivity was investigated in detail especially with respect to hydrogen concentration dependency. Amorphous alloy membranes µm thick were directly prepared from the melt by rapid quenching. All the membranes were coated with palladium using RF sputtering method to remove the difference in surface activity of each alloy. Hydrogen permeation rate was measured for pure hydrogen at elevated temperatures mainly in the range of K. Hydrogen solubility was investigated using the Sieverts method. Hydrogen diffusivity at these temperatures was derived from the pressure dependence of permeation rate and solubility. Hydrogen permeation rate was proportional to the square root difference of hydrogen pressures on both sides as is often the case with other metal membranes. On the other hand, hydrogen solubility was found to be proportional to about the quarter power of equilibrium hydrogen pressure. This pressure dependency of hydrogen solubility could be explained using Kirchheim s theory, where potential energy distribution for hydrogen sites in amorphous alloys was taken into account. Using the theory, concentration dependence of diffusivity could be also derived, which was well consistent with experimental diffusivity determined by permeability and solubility. KEYWORDS membrane separations, hydrogen permeation, amorphous alloy INTRODUCTION Because dense metal membranes are permeable only to hydrogen, the membranes are expected to be applied for pure hydrogen production for fuel cells to improve their energy efficiency. For such applications, palladium membranes are available today but extremely expensive. Therefore, researches and development on new membrane materials substitutable to palladium have been activated now (Nishimura et al., 1991; Zhang, 2002; Ozaki et al., 2003; Hashi et al., 2004). We have proposed amorphous alloy membranes as such new membrane materials (Hara et al., 2000; 2002; 2003). Their material cost is much lower than that of palladium membranes. Because amorphous alloy membranes can be produced easily and efficiently by rapid quenching, low production cost is also expected. Therefore, amorphous alloy membranes maybe enlarge the application field of metal membranes. In these R & D, hydrogen permeability is generally used as a typical measure. The permeability, P, is defined as follows: 1

2 P J H2 = p, (1) d where J H2 is hydrogen molecule permeation flux, p is difference of square roots of hydrogen pressures on both sides of the membrane, and d is membrane thickness. Because hydrogen permeation mechanism through a metal membrane was based on hydrogen solution and diffusion in the membrane, diffusivity, D, is a very important parameter. To obtain diffusivity, the following equation is commonly used: P = DK 2, (2) where K is a measure of solubility into the membrane, Sieverts constant. The denominator, two, corresponds that one hydrogen molecule is composed of two hydrogen atoms. Here, it should be noted that constant diffusivity is assumed during the derivation of this equation but constant diffusivity is not valid always for metals. For example, at high hydrogen concentrations, hydrogen atoms in the metals interact with each other so that diffusivity changes with concentration. Even for low concentrations, diffusivity in amorphous alloys is known to depend on concentration. Therefore, in such cases, another approach is necessary to estimate diffusivity. However, diffusivity data available for hydrogen permeability analysis have hardly been reported. Diffusivity is often obtained by the time lag method but it is usually an average for a certain range of hydrogen concentration. Diffusivity for high temperatures, that metal membranes are used at, hasn t been reported very much, either. This study was carried out in order to obtain diffusivity useful for development of amorphous alloy membranes. Hydrogen permeation and solution properties were investigated in detail and concentration dependent diffusivity at high temperatures was estimated using obtained data. Thus obtained properties for Zr-Hf-Ni alloys were compared and discussed on the effect of Hf. EXPERIMENTAL The Zr 36 xhf x master alloys were prepared from pure metals by arc melting. The alloy was crushed and put into a fused quartz nozzle with a slit, which was melted by RF induction, and then quenched in an argon atmosphere on a copper roller. Obtained amorphous alloy membranes were µm thick. All the membranes were coated with palladium using the RF sputtering method to remove the difference in surface activity of each alloy. The membrane was held in a permeation cell, whose effective permeation area was 298 mm 2. The cell was set in a thermostat and the temperature was controlled in the range of K. On one side of the membrane, pure hydrogen was introduced and the other side was kept at atmospheric pressure. Hydrogen permeation rate was determined by flow rate of effluent gas from the latter side. Sweep gas was not used in this study. Hydrogen solubility was measured by the Sieverts method. Before every series of measurements for a constant temperature, the sample cell was evacuated by a rotary pump at 573 K for 10 h to obtain a constant initial condition. Solubility was investigated for MPa equilibrium hydrogen pressures in the range of K. Using thus obtained permeability and solubility, concentration-dependent hydrogen diffusivity was estimated using the following equation: 2

3 J c H2 = f ( ) D cf 2d, (3) where c f is concentration in the metal surface on the hydrogen feed side and d is thickness of the membrane. This equation will be explained in detail elsewhere. RESULTS AND DISCUSSION Hydrogen permeability As is often the case with other metal membranes, hydrogen permeation rate was nearly proportional to the square root difference of hydrogen pressures on both sides of the membrane. Therefore, hydrogen permeability was determined by Equation 1. Permeation rate for any feed and permeate pressures can be given using thus determined permeability. This relationship between permeation rate and pressures also means that the membranes had no defect which other gases could pass through as molecules. Arrhenius plot of permeability is shown in Figure 1. Permeability for each membrane was on a straight line in this temperature range, indicating reliable permeability was obtained. Increase in permeability with temperature can be qualitatively explained by temperature dependence in diffusivity greater than that in solubility. Additionally, it was also found that permeability decreased with Hf content. Hydrogen solubility Hydrogen solution properties are shown in Figure 2. For each alloy, all data during desorption and adsorption were on a common line, that is, there was no hysterisis. This means adsorption and desorption rate were relatively fast so that thermodynamical equilibrium was attained in every Permeability [mol/m s P a1/2 ] ,000/T [1/K] Figure 1. Hydrogen permeability of amorphous Zr 36 xhf x membranes. Equilibrium pressure [MPa] Concentration [H/M] 573 K Figure 2. Hydrogen solubility of amorphous Zr 36 xhf x membranes. 3

4 solubility measurement step. As is generally known for amorphous alloys, the amorphous alloys studied here also showed no plateau, suggesting that they had no α-β phase transition in this pressure range. The α-β phase transition often brings spontaneous pulverization or degradation of mechanical strength in hydrogen absorption. No α-β phase transition, therefore, can be considered as an advantage of amorphous alloys in the membrane application. It is said that hydrogen concentration is proportional to square root of hydrogen pressure, which is known as the Sieverts law. This is usually valid at low concentrations for crystalline metals such as crystalline palladium alloys. In Figure 2, however, all the data were on a straight line with a slope of not 2.0 but around 4 in this log-log plot. This means that concentration was proportional to about the quarter power of hydrogen pressure even at these low concentrations. Solution behaviors in amorphous alloys were theoretically investigated in detail by Kirchheim (1982). He assumed that there are various hydrogen sites in an amorphous alloy, the distribution of whose potential energy can be described by a normal distribution function: n G i G = i exp. (4) σ π σ ( G ) The G 0 and σ are parameters determining the average and shape of the distribution function, respectively. From this equation, hydrogen chemical potential, µ H, can be related to hydrogen concentration, c, using the inverse function of the error function, as follows: ( 1 ) The part concerning the error function can be well approximated by: G 0 µ = σerf 1 2c. (5) H erf 1 ( 1 2c) a ln 2c, (6) b 2.3b where a = 0.93 and b = 1.63 (Itoh et al., 1997). From Equation 5 and 6, c = K p 2.3bRT/2σ is derived. This can be regarded as the reason for the quarter power dependency of hydrogen concentration in the amorphous alloys. In this study, G 0 and σ were determined by Equation 5 so as to reproduce the experiment. In Figure 2, equilibrium concentration calculated using thus determined parameters are shown by solid lines. From this figure, it is found that using Equation 5, the solubility can be rather precisely reproduced. Hydrogen diffusivity Hydrogen diffusivity was determined using Equation 1, 3, 5 and obtained parameters, which is depicted in Figure 3. Hydrogen diffusivity depended largely on concentration. This tendency is consistent with a Ström-Olsen s report (1991), where they estimated concentration dependence of diffusivity around room temperature by the time-lag method using an electrochemical cell. Kirchheim derived also another formula for concentration dependent diffusivity (1982). The diffusivity calculated using the formula is represented by solid lines in the figure. Experiment data are well consistent with the lines. This probably suggests that distribution for hydrogen site energy influenced on concentration dependent diffusivity more or less. 4

5 Hydrogen permeation is carried out by controlling not hydrogen concentration in the membrane but pressures on both sides. Therefore, the diffusivity was re-plotted against equilibrium hydrogen pressure (Figure 4). In this figure, diffusivity for 18 % Hf was lower than that for 3 %. This means that decrease in permeability with increasing Hf in this alloys was due to decrease in diffusivity as well as in solubility. Diffusivity [m 2 /s] 5 x K CONCLUSIONS In this study, in order to discuss on hydrogen permeability change with Hf content in amorphous Zr 36 x Hf x alloys, concentration dependent hydrogen diffusivity at elevated temperatures were estimated. Hydrogen permeation rate was proportional to the square root difference of hydrogen pressures on both sides of the membranes. Hydrogen concentration in the amorphous alloys was able to be given using a Kirchheim s formula, where potential energy distribution for hydrogen sites in amorphous alloys was taken into account. Using the theory, concentration dependence of hydrogen diffusivity also could be derived, which was well consistent with experimental diffusivity determined by permeability and solubility. Finally, it was found that hydrogen permeability decrease with Hf was due to diffusivity decrease as well as solubility decrease. This was a trial study for concentration dependent diffusivity at high temperature using permeation and solution properties. An intensive study will gave us more clear information on hydrogen permeation and guidelines to develop new metal membrane materials. ACKNOWLEDGEMENTS Diffusivity [m 2 /s] 5 x x x Equilibrium pressure [MPa] The authors wish to thank Dr. H. Enoki (AIST) for help and discussion in hydrogen solubility experiment. A part of this study is supported by a METI's Regional Consortium Project contracted by the 573 K Concentration [H/M] Figure 3. Concentration dependence of hydrogen diffusivity for amorphous Zr 36 xhf x alloys. Figure 4. Hydrogen diffusivity against equilibrium hydrogen pressure for amorphous Zr 36 xhf x alloys

6 Japan Research and Development Center for Metals (JRCM). REFERENCES Hara, S., K. Sakaki, N. Itoh, H.-M. Kimura and A. Inoue ; An amorphous alloy membrane without noble metals for gaseous hydrogen separation, J. Membrane Sci. 164, (2000) 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, (2002) Hara, S., N. Hatakeyama, N. Itoh, H.-M. Kimura and A. Inoue ; Hydrogen permeation through amorphous-zr 36 x Hf x -alloy membranes, J. Membrane Sci. 211, (2003) Hashi, K., K. Ishikawa, T. Matsuda and K. Aoki ; Hydrogen permeation characteristics of multi-phase Ni Ti Nb alloys, J. Alloys Comp. 368, (2004) Itoh, N., W.-C. Xu, S. Hara, H.-M. Kimura and T. Masumoto ; Solubility of hydrogen in amorphous Pd 1 xsi x alloys as hydrogen permeable membranes, J. Membrane Sci. 126, (1997) Kirchheim, R. ; Solubility, diffusivity and trapping of hydrogen in dilute alloys, deformed and amorphous metals II, Acta metall. 30, (1982) Nishimura, C., M. Komaki and M. Amano ; Hydrogen permeation characteristics of vanadium-nickel alloys, Mater. Trans., JIM 32, (1991) Ozaki, T., Y. Zhang, M. Komaki and C. Nishimura ; Hydrogen permeation characteristics of V-Ni-Al alloys, Int. J. Hydrogen Energy 28, (2003) Ström-Olsen, J. O., Y. Zhao, D. H. Ryan, Y. Huai and R. W. Cochrane ; Hydrogen diffusion in amorphous Ni-Zr, J Less-Common Met (1991) Zhang, Y., T. Ozaki, M. Komaki and C. Nishimura, ; Hydrogen permeation characteristics of vanadium-aluminium alloys, Scripta Mater. 47, (2002) 6