The diffusion of W on a (211) plane of a W field emitter was re-examined by using the fluctuation auto-correlation method. It was found that diffusion along channels was described by:

D (cm2/s) = 3.0 x 10-5 exp[-16.8(kcal/mol)/RT]

while diffusion across channels was described by:

D (cm2/s) = 4.0 x 10-9 exp[-6.6(kcal/mol)/RT]

at temperatures below 752K, and by:

D (cm2/s) = 5.0 x 10-4 exp[-24(kcal/mol)/RT]

at temperatures greater than 752K. The results for diffusion along channels yielded activation energies and pre-exponential factors which were intermediate between field ion microscopy results:

D (cm2/s) = 7.7 x 10-3 exp[-19(kcal/mol)/RT]

and results which had been obtained by using the present method, but with a larger slit,

D (cm2/s) = 7.0 x 10-7 exp[-13.3(kcal/mol)/RT]

The new high-temperature results suggested that the postulated channel wall exchange mechanism for cross-channel diffusion at low temperatures gave way to diffusion by climb (higher activation energy but higher pre-exponential factor) over the channel walls at temperatures above 752K.

D.S.Choi, R.Gomer: Surface Science, 1990, 230[1-3], 277-82