Microstructure and varistor properties of ZnO–V₂O₅–MnO₂-based ceramics

Excerpt

Recent developments in electronic design have tended toward smaller and higher density packaging of circuitry. This results in a greater susceptibility to surges. Once attacked by surge, electronic circuits can be destroyed in as short as 0.1 μs. ZnO doped with several different metal oxides are semiconducting ceramics possessing varistor behaviors, which exhibit abruptly increasing current in accordance with increasing voltage. This non-ohmicity of current–voltage properties is due to the presence of a double Schottky barrier (DSB) formed at active grain boundaries containing many trap states. Owing to highly non-ohmicity, these ceramic devices are widely used in the field of overvoltage protection systems [1, 2]. ZnO ceramics cannot exhibit a varistor behavior without adding heavy elements with large ionic radii such as Bi, Pr, Ba, etc. Commercial Bi₂O₃- and Pr₆O₁₁-based ZnO varistor ceramics cannot be co-fired with a silver inner-electrode (m.p. 961 °C) in multilayered chip components because of the relatively high sintering temperature above 1000 °C [3, 4]. Therefore, new varistor ceramics are required in order to use a silver inner-electrode. Among the various ceramics, one candidate is the binary ZnO–V₂O₅ system [5‐9]. This system can be sintered at relatively low temperature in the vicinity of about 900 °C. This is important for multilayer chip component applications, because it can be co-sintered with a silver inner-electrode without using expensive palladium or platinum metals. …