New low-voltage varistor composites

Microvaristors: Functional Fillers for Novel Electroceramic Composites

“…The measurements are tricky and we cannot exclude possible contributions from surface leakage currents or noise pick-up at the lowest current levels. Composites were prepared by mixing appropriate amounts of microvaristors with insulating matrix materials such as oils, gels, epoxies, thermoplastics, elastomers or glass, followed by molding and curing [4,5]. Electrodes were either directly molded into the material or applied after grinding.…”

“…and their interaction with the polymer matrix [2,3]. As a spin-off from the metal-oxide varistor technology, microvaristors [4,5] have been developed and are being introduced in new products [6,7]. When used as a functional filler in an insulating or semiconducting matrix the microvaristors can impart their strong electrical nonlinearity directly to the composite.…”

“…Unlike today's varistor-type composites, the nonlinear behaviour of the new materials is dominated by the intrinsic bulk properties of the microvaristor particles and is not originating from difficult to control and stabilize particle-particle or particle-polymer-particle contacts [2][3][4][5][6][7][8][9][10]. The response of the filler particles is dictated by their grain boundaries and is transferred directly to the behavior of the composite, as will be shown below.…”

The effect of vibratory milling on the powder properties of zinc oxide varistors

“…surrounding each ZnO particle would be expected to be higher than those obtained at zero milling. It is thought that higher homogeneity of the additives around the ZnO crystals would be expected to result in better grain growth control during sintering and thus electrical performance [12][13][14][15][16]. Apart from more efficient milling and obtaining particles closer to 50 nm, a reduction in the primary particle and inter-primary particle bond strength within the aggregates of the received metal oxide additives would allow an even higher uniformity of additives around the ZnO crystals than that obtained with the current additives by vibratory milling.…”

“…All agree that to achieve a uniform sintered microstructure with a high level of grain growth control, a uniform green microstructure consisting of uniformly packed particles with a narrow distribution in size is required. The work can be categorized into four principle approaches taken; chemically derived mixed metal oxide routes [12,13], incorporation of ZnO seeds [14][15][16][17], prior preparation of secondary phases [18] and mechanically derived mixed metal oxide routes [19]. Addition of ZnO seeds was predominantly carried out to improve the manufacturability and the non-linearity of low voltage rated devices.…”