The Influence Of Ce And Ti Contents On The Property Of Sheelite Type High Temperature NTC Material In Ca-Ce-Ti-W-O System

NTC thermitor has singnicant advantages in high temperature measurement （>300℃）for its cheapness and large output signals. The traditional spinel type NTC thermistorsprepared by the transition metal oxide of Mn, Co and Ni are only applicable fortemperature measurement below300°C for its poor high temperature stability. NTCmaterial with scheelite structure dicovered recently was considered promising in preparinghigh temperature NTC thermistor. In this thesis, Ca-Ce-Ti-W-O system was adopted toprepare the scheelite type high temperature NTC material and enphasis was put on tostudy its resistivity-temperature property, high temperature stability and theory ofconducting and aging.Firstly, the orthogonal experiment design was used to study the change trend of phasecomposition and ionic conductivity ratio of Ca-Ce-Ti-W-O ceramic with the Ce content, Ticontent and sintering temperature. The preparation conditions of scheelite phase andelectronic conductive type NTC material was obtained.Then, according the conclusion of orthogonal analysis, the influence of Ce content onthe crystal structure, resitivity-temperature characteristics, ionic conductivity ratio andhigh-temperature stability of the CaWO₄–xCeTi₂O₆ceramic was studied. When0.2≤x≤0.5,the main phase of CaWO₄–xCeTi₂O₆ceramics sintered over1200–1300°C were scheeliteCaWO₄. The resitivity-temperature property study revealed that the CaWO₄–xCeTi₂O₆ceramics possessed remarkable NTC property over40–800°C. The resitivity decreasedwith the increasing of Ce contents and elevating of sinter temperature. Meanwhile, the DCpolarization test indicated that the NTC effect was pure electronic conductive type. Moreimportant, the study of high-temperature stability showed that it increased with theelevating of sinter temperature and the aging coefficient at500°C for200h of thethermistors sintered at1300°C was1⁸%. The conductivity mechanism of the scheelitetype CaWO_4-xCeTi₂O₆NTC ceramic was explained by the small polaron theory and theanalysis showed that electron hopping between the Ce³⁺/Ce⁴⁺couples at A sites inscheelite structure contributed to generate the NTC property.Finally, the effect of Ti contents on the electric property of Ca₃CeTi_1.0+xW₃O_16+2x（0≤x≤2.0） ceramic was studied. The resistivity droped with the increase of Ti contentwhen0≤x≤1and increased when1.0<x≤2.0. The material constant changed slightly. Theconductive mechanism analysis showed that electron hopping can occur at A and B sites inscheelite CaWO₄structure （ABO₄）, which is very different from the traditional spinel type NTC material.