Varistors are electro ceramic components whose electrical resistance strongly depends on the applied voltage. Until a so-called breakdown voltage is reached, the varistor has a high resistance. Above this breakdown voltage the resistance decreases strongly nonlinear. This effect is based on potential barriers located at the grain boundaries. It is reversible and occurs very fast. Therefore these components are used as electrical surge protection elements. Therefore the varistor is connected parallel to the protected device. Because of the ongoing miniaturisation only a few grain boundaries dominates the properties of the whole device. This is why some effects, like the variation of the pressure dependence of the current-voltage-characteristic, are macroscopically measurable, which are averaged in devices dominated by high amount of grain boundaries. The main focus of this work is the analysis of the pressure dependency of individual grain boundaries in multilayer-varistors using lock-in thermography. Due to the piezoelectric properties of ZnO, which the varistor-ceramic is based on, it is expected that the pressure dependency is controlled by the orientation of the grains. For the experimental implementation of static and dynamic compression tests a new testing device was developed. It is required that miniature specimens can be mechanically tested while thermographic recordings are made. The thermographic investigations on statically compressed components show significant change of heated sections. The external mechanical load can increase or decrease the electric conductivity. This causes a local change in the heat generation, which was detected with thermal imaging cameras. The variance of this effect may be caused by a distinct anisotropy of the local behavior on microstructural level. In the mechanically modulated investigations the expected periodic temperature change could not be detected because of the low amplitude.
|Translated title of the contribution||Electro-thermal characterization of multilayer varistors|
|Award date||26 Jun 2015|
|Publication status||Published - 2015|
Bibliographical noteembargoed until null
- Lock-in thermography