Microstructure and electrical properties of xAl_2O_3-(1-x) LaCrO_3 composite NTC ceramics prepared by microwave sintering

摘要

xAl_2O_3-(1-x)LaCrO_3 (x = 0.3, 0.4, 0.5, 0.7) composite negative temperature coefficient (NTC) ceramics were prepared by microwave sintering. The effects of different atmospheres and sintering temperatures on microstructure and electrical properties of the NTC ceramics were studied. X-ray diffraction results showed the ceramics were composed of a perovskite structure LaCrO_3 and lanthanum hexaaluminate solid solution as well as a separate corundum phase. The existence of Cr~(3+) and Cr~(4+), which resulted in skip conduction, was confirmed by X-ray photoelectron spectroscopy. The resistance of the ceramics sintered in air was lower than that of ceramics sintered in an N_2 atmosphere. Ceramics sintered in air and N_2 showed NTC characteristics starting at 40 °C and 250 °C, respectively. The ρ_(50) and B_(300/700) values of the ceramics sintered in air were in the range of 5.06 × 10~5-2.21 × 10~9 Ω·cm and 4144-6828 K, respectively. The ceramics prepared at 1400 °C with a holding time of 5 min possessed excellent microstructure and electrical properties. Compared with conventional pressureless sintering, the sintering temperature and holding time of the microwave sintering process were decreased by 6.6-12.5% and 95.8-99.0%, respectively.