Influence of processing on microstructure and electrical characteristics of multilayer varistors

The paper reports on the influence of processing on microstructure and electrical properties of multilayer varistors based on zinc oxide doped with Bi2O3, Sb2O3, Co2O3, MnO, Cr2O3, B2O3, and SiO2. 0.5-1 wt% of AlF3-CaB4O7 was used as a new effective sintering aid. The behavior of green laminates during heating was characterized using differential thermal analysis and a heating microscope. As revealed by XRD, SEM, and EDS methods, the varistor layers are composed of ZnO grains of 1-5 mu m size, submicrometer spinel and pyrochlore grains situated at the ZnO grain boundaries, and nanometric Bi2O3-rich films surrounding ZnO grains. Complex impedance studies carried out in the frequency range of 0.01 Hz-2 MHz at temperatures changing from -30 to 150 degrees C imply the formation of semiconducting grains and insulating grain boundaries. Frequency dependence of dielectric permittivity shows a high plateau at lower frequencies, typical for barrier layer capacitance effect. The fabricated multilayer varistors show nonlinear current-voltage characteristics with a high nonlinear coefficient of 26-38. The breakdown voltage was found to decrease within the range of 66-130 V with sintering temperature increasing from 1000 to 1100 degrees C. Good surge current capability of the varistors was confirmed by the tests using 8/20 mu s pulses.