Study of carbon deposition effect on hydrogen permeation through palladium membrane

In view of benefits expected from the employment of membranes for particle control in fusion devices and for separation of hydrogen from its mixtures with hydrocarbons, the behavior of a Pd sample is investigated in a plasma-membrane device with a graphite target. The permeation of hydrogen through a 0.2 mm-thick Pd membrane with clean surfaces was found to be limited by the bulk diffusion. An incident flux of hydrocarbon radicals (approx. 2x10(12) cm(-2)s(-1)) in hydrogen plasma forms no carbon layer on the Pd surface. Applying of a negative bias to the target gives rise to target sputtering, and to the deposition of carbon onto the membrane surface. The formation of carbon layer results in a decrease of the absorption probabilities of both H-2 molecules and H atoms. The effect of the deposition of carbon is found to depend non-monotonically on membrane temperature.