|MgAl2O4/LaCrO3 双层结构的高温NTC 陶瓷特性研究|
|关键词||高温ntc 热敏电阻 烧结致密性 Mgal2o4 / Lacro3 双层热敏陶 瓷 烧结方式 高温电性能 老化稳定性|
高温（300 oC以上）NTC热敏电阻广泛应用于高温下的温度控制、温度测量与温度补偿。而常温NTC热敏电阻材料在高温下无法稳定工作，因此，如何获得在高温下稳定工作的NTC热敏电阻材料或结构成为高温NTC热敏电阻领域中亟待解决的问题。MgAl2O4的室温电阻率大，适合高温应用，但其结构在高温下易发生阳离子重排，导致其高温下的电性能不稳定。而LaCrO3的高温下的稳定性好，但其电阻率小，无法直接用于高温NTC热敏电阻的应用。本文通过分析高温NTC热敏电阻的发展历史；研究LaCrO3在空气中的烧结性能；研究常规烧结与真空烧结对MgAl2O4-LaCrO3复合陶瓷材料的阻温特性、微观形貌的影响；研究不同摩尔比、不同烧结方式对MgAl2O4 / LaCrO3双层结构热敏电阻陶瓷微观形貌（SEM）、阻温特性和老化稳定性的影响。主要研究结果如下：（1）通过研究改变烧结时的埋烧物（none, Al2O3 , La2O3 , Cr2O3 , LaCrO3）对LaCrO3在空气中的烧结性能的影响。发现：通过埋烧 La2O3，LaCrO3的烧结性能得到极大地提高（线性收缩率为12.5%，相对密度为86.90%）。EDS分析结果显示：在空气中烧结的LaCrO3陶瓷样品的Cr原子百分比均低于La原子百分比，表明LaCrO3在空气中烧结时存在Cr元素的挥发。而使用 La2O3埋烧的LaCrO3的陶瓷样品的Cr原子百分比最高，与致密性最好相符，说明Cr元素的挥发确实影响着LaCrO3的烧结致密性。（2）通过研究常规烧结与真空烧结对MgAl2O4-LaCrO3复合陶瓷材料的阻温特性、微观形貌的影响。发现，摩尔比为3-7、4-6、5-5和6-4的真空烧结的MgAl2O4-LaCrO3复合陶瓷材料的电阻率范围为148—387665 Ω·cm、B100/1000的范围为1666—3400 K；而相同摩尔比的常规烧结制备的MgAl2O4-LaCrO3复合陶瓷材料的电阻率范围为63—122047 Ω·cm，B100/1000的范围为238—2978 K。对于相同摩尔比的MgAl2O4-LaCrO3复合陶瓷材料，真空烧结样品比常规烧结样品的晶粒尺寸更小，电阻率、B值更大。且发现MgAl2O4-LaCrO3复合陶瓷样品的电阻率与EDS中的Cr原子百分比成正比。（3）通过研究不同摩尔比对MgAl2O4 / LaCrO3双层热敏陶瓷的阻温特性和微观形貌的影响。发现，MgAl2O4 / LaCrO3双层热敏陶瓷的电阻率、B值与双层之间的摩尔比有关，且摩尔比为4/6的MgAl2O4/LaCrO3双层热敏陶瓷在900、1000oC下的电阻值最低，与其两相之间元素扩散范围最广的现象相对应。（4）通过研究不同烧结方式对MgAl2O4 / LaCrO3双层热敏陶瓷的阻温特性、微观形貌和老化稳定性的影响。发现，真空热压烧结的MgAl2O4 / LaCrO3双层热敏陶瓷的电阻率最小，如1000 oC下真空热压烧结样品的电阻率为2.5×103 Ω·cm，而常规烧结、真空烧结样品1000 oC下的电阻率为4.1×106 Ω·cm、1×107 Ω·cm。真空热压烧结的MgAl2O4 / LaCrO3双层热敏陶瓷的老化稳定性最好，如在800 oC下老化500 h后，真空热压烧结的MgAl2O4 / LaCrO3双层热敏陶瓷的电阻漂移率为-7.01%，而常规烧结、真空烧结样品的电阻漂移率分别为-75.89%、-72.98%。真空热压烧结的MgAl2O4 / LaCrO3双层热敏陶瓷的B100/1000为1687 K。可以认为，真空热压烧结是制备MgAl2O4 / LaCrO3双层热敏陶瓷最好的烧结方式。
High temperature (above 300 oC) NTC thermistors have important applications in temperature control, compensation at high temperature. However, most NTC thermistors do not operate stably at high temperatures. Therefore, how to obtain NTC thermistor materials or structures that can operate stably at high temperatures has become a problem . MgAl2O4 has high room temperature resistivity and is suitable for application in high temperature regions. However, its spinel structure is prone to cation rearrangement at high temperatures and is unstable at high temperatures. LaCrO3 has good stability at high temperatures, but its resistivity is too small to be used directly in high temperature NTC thermistors. In this paper, the history of high-temperature NTC thermistors, the sintering properties of LaCrO3 in air, the micromorphology (SEM) and temperature-resistance characteristics and aging stability of MgAl2O4/LaCrO3 bilayer thermistors were studied to obtain stable high temperature NTC thermistor. The main findings are as follows:(1) By studying the effect of changing the sinter (none, Al2O3, La2O3, Cr2O3, LaCrO3) on the sintering performance of LaCrO3 in air, it was found that the sinterability of LaCrO3 was greatly improved by buried with La2O3, (linear shrinkage was 12.5% and relative density was 86.90%). The results of EDS analysis showed that the atomic percentage Cr of LaCrO3 ceramics sintered in air was lower than the La atomic percentage, indicating that the Cr element volatilizes when LaCrO3 is sintered in air. The LaCrO3 ceramics buried with La2O3 had the highest Cr atomic percentage, which was in good agreement with the linear shrinkage and relative density, indicating that the volatilization of Cr did affect the sinterability of LaCrO3.(2) The effects of conventional sintering and vacuum sintering on the temperature-resistance characteristics and micro-morphology of MgAl2O4-LaCrO3 composite ceramics were studied. It was found that the resistivity of the vacuum-sintered MgAl2O4-LaCrO3 composite ceramics with molar ratios of 3-7, 4-6, 5-5 and 6-4 were148-387665 Ω·cm, and B100/1000 ranged from 1666- 3400 K. The resistivity of MgAl2O4-LaCrO3 composite ceramic by conventional sintering were 63-122047 Ω·cm, and the B100/1000 were 238-2978 K. For the same molar ratio of MgAl2O4-LaCrO3 composite ceramic, the vacuum sintered sample had smaller grain size and higher resistivity and B value than conventional sintered samples. It was found that the resistivity of MgAl2O4-LaCrO3 composite ceramic samples was proportional to the values of Cr/La and Mg/Al in EDS.(3) The effects of different molar ratios on the temperature-resistance characteristics and microstructure of MgAl2O4-LaCrO3 bilayer thermistors were investigated. It was found that the resistivity and B value of MgAl2O4-LaCrO3 bilayer thermistors were related to the molar ratio between the two layers, and the MgAl2O4-LaCrO3 bilayer thermistors with the molar ratio of 4/6 had the lowest resistance at 900 and 1000 oC and It corresponds to its widest range of element diffusion between the two phases.(4) The effect of different sintering methods on the temperature-resistance characteristics, micro-morphology and aging stability of MgAl2O4-LaCrO3 bilayer thermistors was studied. It was found that the resistance of MgAl2O4-LaCrO3 bilayer thermistors sintered by vacuum hot-pressing was the least. For example, the resistivity of vacuum hot-pressing sintered samples at 1000 oC was 2.5×103 Ω·cm, while the conventional sintering and vacuum sintering samples were 4.1×106 Ω·cm and 1×107 Ω·cm. The aging stability of vacuum hot-pressing sample was best. For example, the resistance drift rate of vacuum hot-pressing sample was -7.01% after aging at 800 oC for 500 h, while the conventional sintered and vacuum sintered samples are -75.89% and -72.98%, respectively. The B100/1000 of the vacuum hot-pressing sample was 1687 K. It can be considered that vacuum hot-pressing sintering is the best sintering method for preparing MgAl2O4-LaCrO3 bilayer thermistors.
|印杏莲. MgAl2O4/LaCrO3 双层结构的高温NTC 陶瓷特性研究[D]. 北京. 中国科学院大学,2018.|
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