Low-voltage varistors from ZnO+CaMnO_3 ceramics

摘要

Varistors prepared from ZnO with CaMnO_3 perovskite as the only forming additive, exhibit voltage-limiting current-voltage characteristics with nonlinearity coefficient α up to 380 at low voltages of 1.8-12 V/mm. High nonlinearity is observed only with a suitable combination of processing parameters. The most crucial of them are (ⅰ) initial formulation of ceramics and (ⅱ) the sintering temperature and conditions of post-sinter annealing. An electrically active intergranular phase is formed between ZnO grains with the composition ranging from Ca_4Mn_6Zn_4O_(17) to Ca_4Mn_8Zn_3O_(19), which creates the n-p-n heterojunctions. The low-voltage nonlinearity originates as a result of higher concentration of Mn(Ⅲ)/Mn(Ⅳ) present at the grain boundary layer regions, being charge compensated by zinc vacancies. Under the external electric field, the barrier height is lowered due to the uphill diffusion of holes mediated by the acceptor states. Above the turn-on voltages, the unhindered transport of charge carriers between grains generates high current density associated with large nonlinearity.