Research on a high performance multilayer ZnO varistor and its fabrication with water based tape casting

Abstract

With the rapid development of small size, low working voltage and high reliability of electronic products, the demands for multilayer chip varistors (MLV) which are designed for surge protection and electrostatic discharge (ESD) protection in circuits are increasing rapidly and the requirements for the MLV’s properties become stricter. In order to meet the demands of "green" production methods in an environmental protection modern world, it is important to study environmentally friendly water-based (aqueous) tape casting and the replacement of solvent-based (non-aqueous) tape casting with aqueous tape casting. In this study, using chemically synthesized doped zinc oxide (CS powder) as the starting powder, high performance multilayer ZnO varistors were prepared with water-based tape casting. For the first time, high performance multilayer ZnO varistors were successfully fabricated using both water-based tape casting and a chemically synthesized compound powder. The MLVs made by water-based tape casting exhibit better electrical properties than those made using the solvent-based tape casting. Water-based tape casting reduces MLV’s production costs and reaches the goal of environmentally friendly green production, resulting in great social and economic benefits. The electrical properties of MLVs made by CS powder were compared with those made by metal oxide powder (MO powder). The effect of sintering temperature on the electrical properties of MLVs was studied. With the help of microstructure analysis and electrical property measurement of the MLVs, the MLVs made by CS powder display superior electrical properties as a result of their uniform microstructure throughout. The optimal sintering temperature range for our MLVs is 950℃~1050℃. Water-based tape casting process was studied. The influence of a water-soluble binder and dispersant on the tape casting slurry was analyzed. The aqueous slurry with shear thinning rheological behavior which was desirable for tape casting was successfully developed by the optimization study of slurry composition. The aqueous slurry could be stabilised electrosterically by the dispersant. The MLV green sheets were successfully made with water-based tape casting. Water-based tape casting process was compared with solvent-based tape casting. The microstructure and electrical properties of the MLVs made by water-based tape casting were also compared with those made by solvent-based tape casting. The aqueous green tape is denser than the non-aqueous one. The multilayer ZnO varistors prepared by aqueous tape casting display better electrical properties than those prepared using non-aqueous tape casting. We believe that these superior electrical properties are a direct result of the well dispersed and stable slurry with a shear thinning rheological behavior, which makes dopants distribute more uniformly throughout the MLVs. We conclude that water-based tape casting can replace solvent-based tape casting for the manufacture of high performance multilayer ZnO varistors considering MLV production costs and protection of the environment.