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ZnO-based varistors relied upon diffusion and defect data which had been obtained by using techniques that had been superseded by methods such as secondary ion mass spectrometry.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 5) 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.

G.W.Tomlins, J.L.Routbort, T.O.Mason: Journal of the American Ceramic Society, 1998, 81[4], 869-76

Table 5

Bulk Diffusivity of O in ZnO