5.3.4.1 Progressive evolution of the breakdown conduction theory
Since the early studies [55], the origin of the non-linearity of conductivity has been
attributed to the regions situated in the ZnO grain boundaries. This was soon confirmed
by thin film experiments [52] and photoconductivity measurements [39].
Several models describing the conduction phenomenon have been developed to give
an account of the non-linear electrical properties of zinc oxide varistors (Table 5.2).
On the other hand, there are other known mechanisms which have been ruled out due
to incompatibility with ZnO properties. The diffusion theory has been excluded [55]
since the current in ZnO is strongly voltage dependent. The avalanche breakdown
mechanism has also been rejected [27, 39, 53, 55] because it results in a positive
temperature coefficient; ZnO is known to exhibit a strong negative temperature
coefficient.