Abstract
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 µm 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 °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 °C. Good surge current capability of the varistors was confirmed by the tests using 8/20 µs pulses.
Highlights
Varistors which show nonlinear current–voltage characteristics are widely applied for protection of electronic devices against transient overvoltages and current surges caused by lightning, switching of high power devices, and electrostatic discharge (ESD)
The paper describes processing, microstructure, composition, electrical characteristics, and surge current capability of multilayer varistors based on doped zinc oxide
The breakdown voltage of the multilayer varistor with the single ceramic layer thickness of 170 μm remains at a high level of 280 V (Fig. 9(a)), but the nonlinear coefficient decreases to a lower value of 9 (Fig. 9(b))
Summary
Varistors which show nonlinear current–voltage characteristics are widely applied for protection of electronic devices against transient overvoltages and current surges caused by lightning, switching of high power devices, and electrostatic discharge (ESD). Growing application range of low voltage devices and strong demand for miniaturization and enhanced integration scale in electronics stimulate development of multilayer varistors [21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41] These elements destined for surge protection of integrated circuits, hybrid circuits, and surface mount circuits, are fabricated by co-sintering of several tape cast ceramic foils with screen printed internal electrodes connected in parallel. The paper describes processing, microstructure, composition, electrical characteristics, and surge current capability of multilayer varistors based on doped zinc oxide. The influence of sintering temperature and the effect of a sintering aid AlF3–CaB4O7 on microstructure and nonlinear behavior of the fabricated varistors were investigated
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