Influence of Trace Eelements on the Electrical Properties of ZnO-based Multilayer Varistors

Nonlinear current-voltage (I-U) characteristics and stability after an I-MAX test of two types of multilayer varistors (MLVs), each type fabricated in two series, were analysed in terms of their structure, microstructure and the presence of trace (i.e., impurity) elements. The structural and microstructural features showed nothing significant that could justify the very different I-MAX characteristics of the MLVs of the same type from the two series. In the larger MLVs, declared for I-MAX 1000A, the most critical factor was found to be the amount of Fe, the source of which was the starting Cr2O3 powder; one batch of Cr2O3 used for their fabrication contained an about 5-times-larger amount of Fe than the other, while the amounts of the other impurity elements (i.e., Al, Si, Mg, Ca, Ti, Na, K) were similar in both. The MLV1000 samples prepared with the Fe-rich Cr2O3 powder failed after a current impulse of 900A, while the samples using the Fe-low Cr2O3 powder withstood even 1400A. In the smaller MLVs, declared for 200A, prepared from Fe-low Cr2O3 and added in half the amount as in the MLV1000 samples, the critical factor was the large addition of SiO2 in the starting composition and the samples failed after a current impulse of 30 A. Amending the composition with the addition of several 100 ppm of Al resulted in an enhancement of I-MAX to 420A, demonstrating the positive effects of Al. The results indicated the need to control the presence of trace elements and showed the complexity of an issue that requires a thorough consideration for each type of MLV to achieve the required electrical characteristics.