It was recalled that the models which were used to describe ZnO-based varistors relied upon diffusion and defect data which had been obtained by using techniques that had been superseded by methods such as secondary ion mass spectrometry. Values were reported here for O diffusivities in undoped monocrystalline ZnO as a function of temperature and orientation. Evaporation was taken into consideration when analyzing the experimental results. It was found that, to within experimental error, the energetics of diffusion were isotropic, but were slightly faster in the c-direction. The present results (table 75) could be described by:

a-direction:     D (cm2/s) = 4.0 x 10-7exp[-2.22(eV)/kT]

c-direction:     D (cm2/s) = 9.0 x 10-6exp[-2.52(eV)/kT]

An analysis of previously published data suggested that the intrinsic activation energy was between 3.6 and 4.2eV.

Oxygen Diffusion in Single-Crystal Zinc Oxide. Tomlins, G.W., Routbort, J.L., Mason, T.O.: Journal of the American Ceramic Society, 1998, 81[4], 869-76

Table 75

Bulk Diffusivity of O in ZnO