The Study Of Al₂O₃-doped Ca-Ce-Ti-W-O System High Temperature NTC Materials

With the growing popularity of the vehicle exhaust catalytic converters in recent years,high temperature thermistors are becoming increasingly important. Negative temperaturecoefficient thermistors (NTCR) are widely used, because they afford the best sensitivity andaccuracy at the lowest price. NTC materials that have been put into use are mostly spinelstructure formed by solid solutions of transition metal oxides, such as Mn3O4, Co3O4andNiO, but this type of NTC thermistors is usually applicable for temperature measurementbelow300°C. Scheelite structure NTC materials are found to be expected for hightemperature thermometry recently. Therefore, this paper studied the NTC characteristics inCa-Ce-Ti-W-O system at high temperature carefully, and got stable NTC ceramic materials.CaWO4-CeTi2O6ceramics were prepared by high temperature solid state method, theeffect of the CeTi2O6content on the phase composition, microstructure, electrical propertiesand high temperature stability was systemically investigated. The samples showed goodNTC characteristics over25－800°C after sintering at1300°C and the main crystallinephase was scheelite CaWO4phase. With an increase in CeTi2O6content the average grainsize increased, however the resistivity and aging coefficient decreased. The existence ofelectron hopping between Ce3+/Ce4+, which caused the materials to exhibit semiconductingbehavior, has been confirmed by XPS analysis.The effect of the high resistance phase Al2O3on the phase composition, microstructure,electrical properties and high temperature stability for CaCe0.5TiWO10ceramics wassystemically studied for the first time. All the samples sintered over1300－1360°C haveNTC characteristics. At the same sintering temperature, the resistivity increased with anincrease in Al2O3content, whereas the aging coefficient decreased. The aging coefficient ofthese samples which have same Al2O3content decreased to minimal value1.48－5.10%, andthen began to increase with a rise in sintering temperature. The main crystalline phase ofthese samples was scheelite CaWO4phase and a very small amount of cubic Ca3Al2O6phase when sintering at1340°C. With an increase in Al2O3content, small grains increasedgradually and attached to the large grains, resulting in a decrease of the average grain size ofthe ceramics.