INVESTIGATION OF SULPHUR ADSORPTION AND GROWTH ON TUNGSTEN BY FIELD EMISSION MICROSCOPY

Abstract
The interaction of sulphur vapour with tungsten at 295 K and pressures 10^-7 to 10^-4 N m^-2 has been studied by field emission microscopy. Sulphur adsorption at a pressure of 10^-4 N m^-2 produced a large number of brightly emitting spots which moved continually. The emitting region was shown to be distributed at random, but with a noticeable preference for the (011) plane and its vicinals. A field-induced process resulting in a sudden enlargement of the emitting area and consequent mechanical failure of the tip occurred at 295 K provided the sulphur pressure > 10^-4 N m^-2 ; the behaviour was ascribed to the growth of sulphide whisker. With sulphur pressures < 10^-5 N m^-2, a steady sulphur adsorption took place on tungsten resulting in the formation of a smooth chemisorbed sulphur layer on the surface. Complete removal of the chemisorbed sulphur was not possible below 1850 K. Activation energies for both sulphur migration and desorption could be determined. Surface potential changes throughout adsorption, migration and desorption processes could be estimated from the corresponding values of the work functions.