Study On Structure And Property Of The Ca-Ce-Nb-Mo-O Based Thermistor Ceramics

Negative temperature coefficient（NTC）thermistor has the advantages of small size,high sensitivity and low price.It is considered as a potential high temperature sensor to replace precious metal thermal resistance in the future.However,NTC thermistors made of traditional spinel materials are easy to aging at high temperature（above 300℃）and their resistance-temperature characteristics are irreversible.Therefore,it is of great significance to carry out the design synthesis and research of new structure high temperature thermistor materials.Ca-Ce-Nb-Mo-O is a scheelite structure solid solution ceramic material composed of ABO₄ type oxides Ca Mo O4 and CeNbO₄.Its semi-conductivity is dominated by the small polaron hopping conduction of Ce ions in CeNbO₄.Compared with traditional ceramics,Ca-Ce-Nb-Mo-O ceramics have stronger structural tolerance,better electrical properties and high temperature stability.However,the resistivity and B value of Ca-Ce-Nb-Mo-O ceramics at room temperature are too large to be used in industrial field,so it is necessary to further optimize the resistivity and B value.Considering the excellent high temperature resistance and special electronic structure of rare-earth ions,the density,sintering performance,electrical properties and high temperature stability of the ceramic materials can be effectively improved.In this paper,the effects of adjusting the proportion of solid solution phase and rare-earth ion doping on the microstructure and electrical properties of Ce-Nb-Mo-O ceramics have been studied.The results are of great significance to optimize the electrical properties and improve the stability of ceramics.1.The concept of adjusting the negative temperature coefficient（NTC）characteristics of Ca_0.9Y_0.1Mo O₄-x CeNbO₄（1≤x≤3）materials by increasing the content of CeNbO₄ is proposed for the first time.When x≤2.0,the ceramics are single tetragonal Ca_0.9Y_0.1Mo O₄ phase.When x≥2.5,the monoclinic CeNbO₄ phase begins to appear in the ceramics.The ceramics have obvious NTC characteristics in the temperature range of 293 K-973 K.The resistivity,B300℃/500℃constant and activation energy decrease with the increase of CeNbO₄.XPS analysis shows that the increase of Ce³⁺ions content is the main reason for the decrease of resistivity.The B300℃/500℃and Ea values of NTC thermistor are in the range of 5768 K-6583 K and 0.497 e V-0.568e V,respectively.When the thermistor is aged at 500℃for 200 h,the aging coefficient△R/R is less than 2%,suggesting that the material has good stability.2.The microstructure and electrical properties of Ca_0.9R_0.1CeNbMoO₈（R=Y、Sm、Nd or La）ceramics have been systematically studied.The structure of as-sintered ceramics is tetragonal scheelite.With the increase of the radius of rare-earth ions,the lattice distortion and bond interaction increase,and the consistency of grain size decreases.In the temperature range of 473 K-1273 K,the ceramics show NTC characteristics,and the activation energy decreases with the increase of rare-earth ion radius.The doping of rare-earth ions can increase the content of Ce³⁺ions,and thus increasing the conductivity of ceramics.Except Sm³⁺-doped ceramics,the high temperature aging rate of the ceramics is less than 2%.The existence of metastable Sm²⁺ions in Sm³⁺-doped ceramics not only increases the activation energy,but also reduces the high temperature stability.