The transport of Na through the polycrystalline ceramic arc tube of high intensity discharge lamps was 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 at 400 to 1200C, separately from other disturbing effects. X-ray photo-electron spectroscopy and secondary ion mass spectroscopy were used to determine the concentration depth profiles. The obtained results confirmed that grain boundary diffusion played an important role in the transport process of sodium through the ceramic wall. The bulk and grain boundary diffusion coefficients and the temperature dependences of these transport processes were determined for Na bulk diffusion (900 to 1200C):

D (cm2/s) = 5.1 x 10−15exp[-56.5(kJ/mol)/RT]

for Na grain boundary diffusion at 700 to 1100C,

D (cm2/s) = 1.1 x 10−10exp[-97.5(kJ/mol)/RT]

and for Na grain boundary diffusion at 400 to 700C,

D (cm2/s) = 7.5 x 10−15exp[-0.077(kJ/mol)/RT]

The Sodium Diffusion in Aluminium-Oxide. H.Lovas, V.K.Josepovits, Z.Tóth: Applied Surface Science, 2009, 255[6], 3581-5