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The oxidation of calcium films by water vapor is the basis of the “Ca test” used to measure low water vapor transmission rates (WVTRs) through gas diffusion barriers. The Ca test assumes that the Ca film oxidation rate is linear with H 2 O flux transmitted through the barrier. However, lag times are often observed during WVTR measurements that could indicate that the Ca film oxidation rate is not linear with H 2 O flux. To explore the oxidation kinetics of Ca films by H 2 O vapor, a new Ca test was developed based on quartz crystal microbalance (QCM) measurements. The QCM measures the mass gain that occurs as the Ca film oxidizes according to the reaction: Ca + 2H 2 O → Ca(OH) 2 + H 2 . The QCM measurements observed a long lag time before the Ca films began to oxidize. The Ca film oxidation was studied using a number of experimental configurations. In all cases, the Ca film oxidation was not linear with H 2 O flux to the Ca film. These results suggest that the nonlinearity of Ca film oxidation in many WVTR experiments may result from the nonlinear oxidation of the Ca film itself. Additional measurements of the times required for complete Ca film oxidation versus H 2 O flux at constant temperatures were consistent with oxidation kinetics that were second-order in H 2 O flux. The nonlinear Ca film oxidation raises doubts about the validity of WVTR measurements using the Ca test. However, modeling based on the measured nonlinear Ca film oxidation kinetics suggests that the Ca test can yield reliable WVTR measurements under special circumstances. These circumstances occur when the accumulation volume between the barrier and the Ca film is small enough to allow for steady state pressure conditions where the H 2 O flux transmitted through the barrier is equivalent to the H 2 O flux removed by oxidation of the Ca film. The lag time is associated with the time required to obtain the steady state pressure in the accumulation volume.
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Higgs et al. (2014) studied this question.
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