The permeability of hydrogen in bulk palladium membranes (approximately 1mm thick) was determined under conditions of both high temperature (623-1173K) and high hydrogen pressure (0.1 x 106 to 2.76 x 106Pa). When the hydrogen partial pressure exponent was constrained to a value of 0.5, the permeability was described by an Arrhenius-type relationship,

P(mol/msPa0.50) = 1.92 x 10−7exp[-13.81(kJ/mol)/RT]

These Arrhenius parameters were in good agreement with prior low-pressure correlations. However, the hydrogen flux results were here most accurately represented by the Arrhenius permeability expression:

P(mol/msPa0.62) = 3.21 x 10−8exp[-13.41(kJ/mol)/RT]

Although the partial pressure exponent value of 0.62 was slightly greater than the commonly accepted value of 0.5, the optimum exponent value decreased as the upper limit of pressure was reduced. Therefore, the deviation in the partial pressure exponent with increasing hydrogen pressure could be attributed to variance in the product of the diffusion coefficient and Sieverts constant at high pressures.

The Permeability of Hydrogen in Bulk Palladium at Elevated Temperatures and Pressures. B.D.Morreale, M.V.Ciocco, R.M.Enick, B.I.Morsi, B.H.Howard, A.V.Cugini, K.S.Rothenberger: Journal of Membrane Science, 2003, 212[1-2], 87-97