Kinetics of Strontium Oxide on Tungsten

Field electron microscopy has been used to study the kinetics of strontium oxide on tungsten in the range 900–2200°K. Reactions take place on the {111} and {100} which have high concentrations of four nearest neighbor surface atoms. Below 1150°K there is some evidence for the dissociation of SrO and migration of Sr to the {110} edges where clusters of crystallites are formed. Two modifications of field emission patterns associated with the (Sr–O–W) complex within 1150–1550°K are tentatively identified with the basic and normal tungstate; apparently the first goes over to the second with an activation energy of ∼60 kcal as determined from desorption data. An unusually sharp decrease in Fowler-Nordheim work function occurs during this reaction. In the later stages of desorption above 1550°K patterns identifiable with an oxygen-tungsten surface are obtained. Desorption at this stage takes place with an activation energy of ∼140 kcal.