Mass transfer in polycrystalline ytterbium disilicate under oxygen potential gradients at high temperatures

To elucidate the mass transfer mechanism in polycrystalline Yb2Si2O7 at high temperatures, the oxygen permeability of Yb2Si2O7 wafers was evaluated under various oxygen potential gradients (dμO) at temperatures up to 1673 K, using 18O2. The 18O was found to concentrate at grain boundaries (GBs), indicating that oxygen preferentially diffuses along the wafer GBs. It was also determined that oxygen permeation is controlled by the mutual GB diffusion of ytterbium and oxide ions, and that ytterbium and oxide ions interdiffuse along the GBs without experiencing any acceleration or retardation effects. The lattice and GB diffusion coefficients for oxygen were directly determined from secondary ion mass spectrometry (SIMS). The oxygen GB diffusion coefficients determined from SIMS-18O line profiles along cross-sections of individual GBs were almost the same as those obtained from SIMS-18O depth profiles. The oxygen GB diffusion coefficient under a dμO was clearly smaller than that in the absence of a dμO (representing self-diffusion data). This work also determined the electronic transference numbers for Yb2Si2O7 with and without the application of a dμO, on the basis of the oxygen GB diffusion coefficients predicted using oxygen permeation data.

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