Metal Membrane Durability and Selectivity

There are four principle causes for membrane failure under normal operating conditions (in other words, reduced membrane durability). These reasons are (a) hydrogen-induced embrittlement of the membrane, (b) fatigue fracture due to repetitive swelling and contraction of the membrane, (c) mismatch in the CTE of the membrane and underlying support layer, and (d) defects in the underlying support layer that cause a hole or tear to develop in the membrane. 

Experience has shown that a 25-/rm-thick membrane (2.5 cm in diameter) composed of Pd-25Ag will fail due to fatigue fracture when tested at 400 °C as described above after only 10-50 cycles. In contrast, a 25-/rm-thick membrane (2.5 cm in diameter) composed of Pd-40Cu will survive 1000 cycles without failure and without exhibiting any wrinkles. 

Thermal expansion can lead to membrane failure in the case of supported metal membranes if there is a large difference between the CTE of the support layer and the CTE of the permselective metal layer. If the support layer expands more than the permselective metal layer, then the permselective metal layer will be tom or fractured during heat up. On the other hand, if the support layer expands less than the permselective metal layer, then fatigue fracture may occur by the mechanism described above for hydrogen-induced expansion of the permselective metal layer. Mismatch of CTE is often a cause for poor durability in supported membranes comprised of a palladium alloy deposited onto a porous stainless steel support or a porous alumina support. 

When considering the influence of thermal expansion or hydrogen-induced expansion on membrane durability, it should be remembered that the absolute magnitude of expansion is dependent on the size (dimensions) of the membrane. Generally, smaller membranes will be more durable than larger membranes. 

The last failure mechanism involves defects in the underlying support layer that are transferred to the thin permselective layer. Examples include surface irregularities (such as pits or cracks), and particles on the support surface. Sintered stainless steel supports may be especially prone to residual particles of metal powder that can pierce the thin permselective metal layer during use. Shock and vibration may cause damage (such as cracking) to a ceramic support layer that then results in damage to, and failure of, the permselective layer. 

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