The transport of Na through the polycrystalline ceramic arc tubes of high intensity discharge lamps was investigated. The bulk and grain boundary diffusion coefficients and the temperature dependences of the 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]

H.Lovas, V.K.Josepovits, Z.Tóth: Applied Surface Science, 2009, 255[6], 3581-5