The hydrogen permeability of Pd-Ag alloy membranes has been investigated over a wide temperature range between 100 C and 500 C. The hydrogen permeation coef cient, Φ, for Pd-23mol%Ag decreases with decreasing temperature above 300 C, in good agreement with the previous literature. However, Φ starts to increases below 250 C, and a peak is observed at around 180 C. Considering the silver concentration and operating temperature, the α-α phase transition never occurs in this condition. In other words, the α-α phase transition is not the reason for the anomalous peak behavior of Pd-23mol%Ag alloy at low temperature. In addition, it is con rmed that the diffusion-limiting hydrogen permeation reaction takes place from room temperature up to 500 C. To understand the reason for the peak appearance, the hydrogen permeability has been analyzed in view of the new description of hydrogen permeation based on hydrogen chemical potential. As a result, it is found that the temperature dependence of the PCT factor, f PCT , is dominant for the peak appearance, meaning that the corresponding pressure-composition-isotherms (PCT curves) are essential for the understanding of hydrogen permeability of the alloy. Dependences of the pressure condition and silver concentration on the peak behavior have also been investigated. The peak temperature increases with increasing the hydrogen pressure at feed side. In addition, the peak appears at lower temperature and becomes remarkable with decreasing silver concentration of Pd-Ag alloy membrane. In other words, the composition of Pd-Ag alloy membranes must be designed based on the operating temperature or pressure condition. Thus, this study suggests new possibilities of alloy design for Pd-Ag alloy membranes.
The alloying effects of Ru and W on the hydrogen solubility, the resistance to hydrogen embrittlement and hydrogen permeability are investigated quantitatively for Nb-based hydrogen permeable alloys. It is found that the hydrogen solubility decreases by the addition of alloying element into niobium or by increasing the temperature. As a result, the resistance to hydrogen embrittlement is improved by reducing the hydrogen concentration. On the other hand, the hydrogen flux, J, through the alloy membrane increases linearly with increasing difference of hydrogen concentration, ÁC, between both sides of the membrane. It is shown that the Nb-5 mol%X (X = Ru and W) alloys possess excellent hydrogen permeability without showing any hydrogen embrittlement when used under appropriate permeation conditions, i.e. temperature and hydrogen pressures. Also, the hydrogen diffusion coefficients during the practical hydrogen permeation at high temperature are evaluated from the linear relationship between the hydrogen flux and the hydrogen concentration difference. It is found that the hydrogen diffusion coefficient of pure Nb is much lower than the reported values measured for dilute hydrogen solid solutions. Surprisingly, the hydrogen diffusion is found to be faster in Pd-26 mol%Ag alloy with fcc crystal structure than in pure niobium with bcc structure at 773 K during the hydrogen permeation. It is also interesting that the addition of Ru or W into niobium enhances the hydrogen diffusion of the practical hydrogen permeation at high temperature.
The hydrogen permeation through pure niobium metal was investigated using a gas permeation technique, focusing mainly on the measurements in a highly soluble hydrogen state. The hydrogen diffusion was found to be the rate determining step for the hydrogen permeation reaction. However, the Sieverts law that the hydrogen solubility in a metal is proportional to the square root of hydrogen pressure, was no longer satisfied in the experimental condition of the temperatures of 473 K to 673 K and the applied pressures of 260 kPa at the inlet and 60 kPa at the outlet for hydrogen. This was mainly due to the presence of a large amount of soluble hydrogen in the metal under this condition. As a result, the measured apparent hydrogen permeability decreased monotonously with decreasing temperature in pure niobium. This was completely the reverse of the temperature dependence of the hydrogen permeability in the low solubility state where the Sieverts law was satisfied. It was stressed here that the permeation measurement of hydrogen in the high solubility state was indeed necessary for getting some performance data of metal membranes in practical use.
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