The permeability of hydrogen in bulk palladium at elevated temperatures and pressures

The permeability of hydrogen in bulk palladium membranes (approximately 1-mm thickness) was determined for the first time at conditions of simultaneously elevated temperature (623–1173 K) and hydrogen pressure (0.1×106 to 2.76×106 Pa). When the hydrogen partial pressure exponent value was constrained to a value of 0.5, the permeability was described by an Arrhenius-type relation where the pre-exponential constant and activation energy for this correlation were 1.92×10−7 mol/(m s Pa0.50) and 13.81 kJ/mol, respectively. These Arrhenius values were in good agreement with prior low-pressure correlations. However, the hydrogen flux results of this study were most accurately represented by an Arrhenius permeability expression where 3.21×10−8 mol/(m s Pa0.62), 13.41 kJ/mol, and 0.62 represent the pre-exponential constant, activation energy of permeation and permeability driving force, respectively. Although the partial pressure exponent value of 0.62 was slightly greater than the commonly accepted value of 0.5 (atmospheric and sub-atmospheric pressure studies), the optimal exponent value in this study decreased as the upper limit of pressure employed in the database was reduced. Therefore, the deviation in the partial pressure exponent with increasing hydrogen pressure may be attributed to variances in the product of the diffusion coefficient and Sieverts constant at elevated pressures.