Small-grain ZnO varistors produced by the urea process

A varistor is a nonlinear resistor that becomes highly conducting for voltages greater than a given breakover voltage. Commercially available varistors are produced by ball milling mixtures of oxide powders, followed by sintering. These techniques make it difficult to produce (1) very fine grained varistor ceramics, desirable for high voltage use, and (2) varistors with uniform composition and grain size, desirable for sharp breakover characteristics. Moreover, the ball milling may introduce inadvertent impurities. The authors have developed a procedure that does not rely on physical milling to mix and grind the component powders; homogeneity is ensured since all chemical components are dissolved in nitric acid. Oxide powder then is produced by diluting the nitric acid solution in molten urea and by a series of thermal treatments, removing the contained water and nitrates, solidifying the mixture as melamine, and burning off the organics to leave uniformly mixed, small-grain-sized oxide powders. The resulting oxides are then densified by hot pressing or sintering to produce the desired shape and electrical properties