The sodium diffusion in aluminium-oxide

The transport of Na through the polycrystalline ceramic arc tube of high intensity discharge lamps has been investigated. This complex process consists of several steps: solution in the ceramics, diffusion through the ceramics, leaving the bulk phase, evaporation from the surface. Among the listed processes the kinetics of the diffusion was examined in the temperature range 400-1200 °C, separately from other disturbing effects. X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS) were used to determine the concentration depth profiles. The obtained results confirmed that the grain boundary diffusion plays an important role in the transport process of sodium through the ceramic wall. The bulk and the grain boundary diffusion coefficients and the temperature dependencies of these transport processes have been determined. The activation energy of Na bulk diffusion is 56.5 ± 6.7 kJ/mol at 900-1200 °C, respectively the activation energies of Na grain boundary diffusion amount to 97.5 ± 21.6 kJ/mol in the temperature range 700-1100 °C and 7.7 ± 4.0 × 10−2 kJ/mol at 400-700 °C. The preexponential factor of the bulk diffusion was found to be Do = 5.1 × 10−15 ± 9.5 × 10−17 cm2/s in the temperature range 900-1200°C, whereas the preexponential factors of grain boundary diffusion are Do = 1.1 × 10−10 ± 1.1 × 10−11 cm2/s at 700-1100 °C and Do = 7.5 × 10-15 ± 1.5 × 10−17 cm2/s at 400-700 °C.