Microstructure and temperature coefficient of resistivity for ZnO ceramics doped with Al2O3

The ZnO doped with Al2O3 showed the minimum electrical resistivity at an Al2O3 content of 0.25%. It was found that the ZnO doped with Al2O3 exhibited the resistance-temperature characteristic reversion from negative temperature coefficient of electrical resistivity (NTCR) to positive temperature coefficient of electrical resistivity (PTCR) at temperatures of about 450 and 250 °C for ceramics sintered at 1400 and 1300 °C, respectively. Both NTCR and PTCR features were weakened for samples with lower Al2O3 content or sintered at a higher temperature. Microstructure studies exposed that the densification process was significantly depressed when the Al2O3 addition levels were over about 1%.