Barium oxygen fluxes

As noted before, the membrane performance could be affected by the presence of H2O, CO2 or other volatile hydrocarbons in the gas phase of both compartments. As laid down in patent literature [1-3], the oxygen fluxes through Mg-, Ca-, Sr-, and Ba-doped perovskites deteriorated over time, roughly 30-50% over a time period of about 100 h, if the air used as feed gas contained several percent of H2O and amounts of CO2 on a hundreds of ppm level. It was claimed, that either no deterioration is found or the fluxes can be restored to their initial values if the temperature is raised above certain critical values, 500°C for magnesium, 600°C for calcium, 700°C for strontium and 810°C for barium. Though no explanation was given, it is possible that carbonate formation took place. One may further note that the tendency for carbonate formation increases at lower temperatures. 

The picture sequence in Figure 32.4 reveals that the oxygen flux recovery after He treatment is associated with the perovskite reconstruction. Furthermore, the XRD data together with the EDXS study presented in Arnold et al. s paper clearly state the formation of a mixed barium strontium carbonate (Baj Sri j C03) layer about 5 pm thick responsible for the oxygen permeation drop. 

Mallett (31) recommends combustion in a high-frequency furnace in pure oxygen, followed by conductometric determination of carbon dioxide after absorption in barium hydroxide solution. The combustion can only be carried out when fluxing agents are used. For a 1 g sample, 1 g iron and 1 g tin are generally needed. For materials that are difficult to combust the amount of sample is reduced to 0.5 g. If the sample combusts easily, the amount of iron can be reduced to 0.5 g, the amounts of sample and tin being 1 g. 

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