Ageing of ZnO-NiO ceramics

Investigation of electrical, mechanical a nd microstructural properties of ZnO-NiO two-phase ceramics with respect to the ageing process of the mate r i a I we re performed. Two constituent phases, ZnOss and NiOss were identif ied in those ZnO-NiO ceramics which exhibit a positive temperature coeffici ent of electrical resistivity (PTCR anomaly). The origin of the PTCR anomal y was explained as a consequence of the different linear thermal expansion of the ZnOss and NiOss constituent phases. The ageing observed in the mater ial is exhibited as an increase in resistivity of the samples and a smaller PTCR anomaly after heating the samples up to the temperature of the resist ivity maximum, and cooling them again to room temperature. According to thi s study, the different linear thermal expansion of ZnOss and NiOss could al so cause ageing of the material. (C) 1998 Kluwer Academic Publishers.