Appearance of remarkable PTCR characteristics of ZnO-TiO₂-NiO ceramics prepared by spark plasma sintering method

We have prepared 85ZnO-10TiO₂-5NiO ceramics by the spark plasma sintering (SPS) method aiming at appearance of remarkable increase in positive temperature coefficient of resistivity (PTCR). The specimens are found to have microstructures composed of small grains and neck-contacts. The PTCR characteristic is obtained when the specimens are oxidized by annealing over about 33 hours at 900°C, and becomes remarkable when the characteristic is measured in a reducing atmosphere of N₂ gas. Especially for annealing during about 75 hours, the resistivity in the N₂ atmosphere is found to increase markedly in a temperature range over 260°C, resulting in a peak value of 1.5 × 10²Ωcm at about 380°C large by 3.5 order of magnitude compared to about 0.04Ωcm in a temperature range below the Curie point of 260°C. In order to study mechanisms by which such remarkable PTCR is observed, we have measured Seebeck coefficients of specimens. When the specimens are annealed, space-charge-layers are formed around the surfaces of neck-channels between grains by trapping of carrier electrons due to absorption of oxygens to ZnO and TiO₂, which are N-type semiconductors. Thickness of the space-charge-layers increases with annealing time. Such growth of layers results in narrowing the parts of channels through which electrons flow, that is, in enhancing the PTCR.