Diffusion and Exchange of Zinc in Crystalline Zinc Oxide

Abstract

The exchange of zinc vapor with crystals of zinc oxide has been measured between 900 and 1025°. The ZnO crystals, incorporating radioactive Zn⁶⁵, were prepared by the reaction of zinc vapor and atmospheric oxygen. A sample of the crystals was weighed into a small quartz bucket suspended in a cylindrical quartz vessel. After addition of enough pure zinc to give the requisite vapor pressure, the vessel was evacuated and sealed. The ZnO sample contained about 1000 acicular crystals, with a mean diameter of 0.01 mm and a selected narrow distribution of diameters. At different times the bucket was withdrawn, and the residual radioactivity of the ZnO was determined. Except for the initial stages, the exchange reaction appeared to be controlled by the diffusion of Zn in ZnO. The diffusion coefficient D was calculated at 1 atmos zinc pressure as D = 4.8 exp(—73.0 kcal/RT) cm² sec^—1. Variation of zinc pressure from 0.2 to 2.0 atmos showed that D varies as P_Zn^0.65, suggesting that the diffusing species is the singly dissociated interstitial zinc ion, Zn_i⁺. These results are consistent with observations on the rate of solution of zinc in zinc oxide, if D = f_iD_i where f_i is the concentration of interstitial zinc and D_i is its mobility, but the possibility that the exchange rate is not diffusion controlled cannot be rigorously excluded.