OXYGEN PERMEATION PROPERTIES OF DENSE BI1.5ER0.5O3-AG CERMET MEMBRANES

Oxygen permeation experiments were performed on dense mixed-conducting ceramic-metal composite membranes (thickness 0.2 to 2 mm) Bi1.5Er0.5O 3-Ag with 10.0, 27.8, and 40.0 volume percent (v/o) silver, respective ly, in the temperature range 873 to 993 K and oxygen partial pressure range 10(-3.5) to 1 bar O-2. The oxygen fluxes increased with increasi ng silver content. In the cermets with a nonpercolative silver phase ( 10.0 and 27.8 v/o), the increased oxygen flux relative to that of pure Bi1.5Er0.5O3 was attributed to faster kinetics of surface oxygen exch ange in the presence of silver. Percolativity of the silver phase in t he 40 v/o Ag composition enhances the ambipolar diffusion of oxygen io ns and electrons. High oxygen fluxes (similar to 0.25 mmol m(-2) s(-1) at 873 K) were observed with the latter composition, which were shown to be fully limited by the surface exchange kinetics. The activation energy for oxygen permeation in the temperature range 848 to 1003 K is about 85 to 95 kJ/mol for the compositions without percolativity of s ilver and 115 kJ/mol for the composite with 40 v/o Ag, which reflects a change of the rate-limiting step upon passing the percolation thresh old. Results from both permeation and isotopic exchange measurements o n the composition with Ag percolativity indicated the kinetic order of the surface process in oxygen to be 1/4, indicating a process fundame ntally different from that on pure Bi1.5Er0.5O3.