Degradation phenomena of ZnO varistors

Abstract

The dc degradation behaviour of ZnO varistors as a function of the variables current, temperature, and time is investigated. The degradation rate is phenomenologically presented by an equation reflecting the observed aging behaviour in dependence on the influencing variables. Accordingly, the degradation rate consists additively of the fraction of degradation during current load and the fraction of recovery. The degradation rate during current load is proportional to current strength. The temperature dependence can be described by an Arrhenius factor (activation energy EA). The degradation rate of recovery is proportional to a nonlinear function of degradation degree. It is directed against degradation rate by current load and is also dependent on temperature (Arrhenius factor with activation energy EX). The varistor-dependent constants of the equation can relatively rapidly be determined a t higher currents (in the mA range) and temperatures (125 t o 165 "C). The applicability of the equation to other loading cases is discussed. Das Gleichstromalterungsverhalten vonZnO-Varifitoren wird in Abhangigkeit von den Belastungs-groBen Strom, Temperatur und Zeit untersucht. Fur die Schadigungsrate wird phanomenologisch eine Gleichung abgeleitet, die das beobachtete Alterungsverhalten in Abhangigkeit von den EinfluBgroBen widerspiegelt. Danach setzt sich die Schadigungsrate additiv aus dem Anteil der Schadigung bei Strombelastung und dem Anteil der Erholung zusammen. Die Schadigungsrate bei Strombelastung ist proportional dem Strom. Die Temperaturabhangigkeit kaun durch einen Arrheniusfaktor ( Aktivierungsenergie EA) beschrieben werden. Die Schadigungsrate der Erholung ist proportional einer iiberlinearen Funktion des Schadigungsgrades. Sie ist der Schadigungsrate durch Strombelastung entgegengerichtet und ebenfalls temperaturabhangig (Arrheniusfaktor mit Aktivierungsenergie ER). Die varistorabhangigen Konstanten der Gleichung lassen sich relativ schnell bei hoheren Stromen (im mA-Bereich) und Temperaturen (125 bis 165 "C) ermitteln. Die Anwendung der Gleichung auf weitere Belastungsfalle wird diskutiert.

Varistor Ceramics

“…This degradation can take a number of forms but usually refers to the steady increase in leakage current when the varistor is subject to a series of pulses or to a constant applied dc or ac voltage. [102][103][104][105][106][107][108] (The steady increase in leakage current with time at fixed voltage led to it being referred to in one of the earliest reports as current creep, presumably by analogy with high-temperature creep of materials under mechanical load.) An example of the degradation is shown in Fig.…”

Microscopic degradation model for ZnO varistors

“…First of all a measure of degradation must be found for a semiquantitative description. I n [9] it has been found practical to define the relative voltage changes of the I-U characteristic at fixed current densities as a measure of degradation D , e.g. 0 -5 = AUlU (at A) or generally D -, = AUlU (at lo-, A).…”

“…It is to be assunied that a degradation of electrical properties due to a change of density and energy of these interface states is caused by the local shift and deformation of concentration profiles of ions or atomic defects being responsible for the states. Therefore a simple, usable microscopic migration and diffusion model is developed, which allows the explanation of experimental results already presented in [9], and the empirical degradation equation (see also [lo]) derived from it.…”

Homogeneity of ZnO Varistors. Distribution of Local Electric Parameters before and after Degradation

“…A local current of 2.5 and 25 pA thus correspond to current densities of 1 and 10 mA/crn2, respectively. After measurement of all individual current-voltage characteristics the raster-like contacted side of the specimen was also contacted by a complete layer of silver paste and the disk subjected to a defined current degradation (experimental details see [4]).…”

“…The degradation rates of electric parameters of ZnO varistors are mainly caused by current flow [3,4].…”