|关键词||复合热敏陶瓷 钙钛矿 放电等离子烧结 高温电性能 导电机理|
一般的过渡金属氧化物负温度系数(Negative Temperature Coefficient, NTC)热敏陶瓷材料在300℃以上的高温中使用时，往往会发生严重的老化现象，甚至出现阻温特性的不可逆变化，因而研究和开发新型高温(25~1000℃)热敏陶瓷材料成为目前NTC 热敏陶瓷的重要发展趋势和新的研究热点，这对于提升我国高温热敏电阻器自主研发水平、促进实用化具有重要意义。近年来，以稀土钙钛矿氧化物作为基体NTC 材料，通过复合高阻相来设计合成高温热敏陶瓷材料是比较重要的研究课题。本文以YCr1-xMnxO3(0≤x≤0.5)热敏陶瓷材料的电导率反常机制及导电机理为基础，以MCr0.5Mn0.5O3(M=Y、La)钙钛矿基高温复合热敏陶瓷材料为研究对象，重点研究了复合材料复合度、结构与电性能的关系，提出了复合材料微观结构与NTC 特性关系、高低温缺陷作用机制及导电机理，比较研究了常规烧结和放电等离子烧结(Spark Plasma Sintering, SPS)对复合材料组成、微观结构、电性能和导电机理的影响规律，揭示了复合材料高温老化过程中的缺陷作用机制及老化机理，利用电性能混合原理和渗流理论探讨了两相扩散规律及离子迁移机制；从CaCu3Ti4O12 高的晶界活化能特性出发，系统研究了Mn 离子掺杂对钙钛矿型CaCu3-xMnxTi4O12(0≤x≤1)高温热敏陶瓷结构、微观结构及电性能的影响规律，探讨了材料高低温半导化机理。主要研究结果如下：1. 首次提出了YCr1-xMnxO3 热敏陶瓷材料的电导率反常机制，探讨了该材料的小极化子跳跃电导机理。YCrO3 材料为p 型半导体，电阻率主要由Cr4+离子浓度决定，Mn 离子为n 型掺杂剂，补偿了金属空位，导致Cr4+离子浓度降低，电阻率增大；随着Mn 含量从0.2 增大到0.5， Mn2+/Mn3+向Mn3+/Mn4+混合价态转变，Mn4+离子增多，促进了电子在Mn3+和Mn4+之间的跳跃，导致载流子浓度增大，电阻率降低；YCr1-xMnxO3 系陶瓷材料的lnρ与1000/T 曲线显示了很好的线性关系，符合小极化子跳跃电导的基本特征，电导主要通过电子在Cr3+与Cr4+、Mn3+与Mn4+离子之间传输形成；随着Mn 掺杂量的增大，电导活化能Ea 和材料常数B 值先增大后减小，与电阻率有相同的变化趋势。2. 提出了(Y2O3+CeO2)-YCr0.5Mn0.5O3 复合热敏陶瓷材料微观结构与NTC 特性的关系。复合陶瓷材料晶界电阻随着温度的升高而降低，显示了NTC 特性；lnRgb 与1000/T 的为线性关系，其晶界电导符合小极化子跳跃电导模型；复合陶瓷材料的NTC 特性主要来源于晶界。3. 提出了致使复合热敏陶瓷材料高低温段活化能不同的缺陷作用机制。在低温范围，氧空位易与金属空位发生库仑相互作用，形成缔合缺陷( 'MV · ••O V )，因而使得自由氧空位的浓度减少，Cr4+和Mn4+离子浓度增大，促进了跳跃电导，活化能减小；在高温范围，随着温度的升高，离子的热运动变的剧烈，大量的缔合缺陷发生分解，缔合的氧空位将转变为自由氧空位，使体系中能够迁移的氧空位浓度增大，Cr4+和Mn4+离子浓度减小，跳跃电导受阻，活化能增大。4. 首次系统地研究了xMgAl2O4-(1-x)LaCr0.5Mn0.5O3 复合陶瓷材料的结构、微观结构、相分布及电学性能。复合陶瓷材料主要由立方尖晶石MgAl2O4 相和同构于LaCrO3 的正交晶系钙钛矿LaCr0.5Mn0.5O3 相组成；随着MgAl2O4 含量的增大，LaCr0.5Mn0.5O3 相的晶胞体积减小，这可能是由于更小离子半径的Al3+离子取代了Mn3+和Cr3+离子；较亮的区域主要为钙钛矿LaCr0.5Mn0.5O3 相， 较暗的区域主要为尖晶石MgAl2O4 相，MgAl2O4 和LaCr0.5Mn0.5O3 相之间有相扩散及离子迁移现象；电阻率随着MgAl2O4 含量的增大而增大，这主要是由于作为电绝缘体的MgAl2O4 含量增大，相应的半导相LaCr0.5Mn0.5O3 的量减少，进而导致电阻率增大，当0≤x≤0.8 时，常规烧结的陶瓷材料电阻率ρ300、B400/800 和活化能Ea变化范围分别为：1.76-1.22×108 Ωcm，2646-8711 K，0.228-0.746 eV。5. 首次系统比较研究了常规烧结和SPS 烧结的0.6MgAl2O4-0.4LaCr0.5Mn0.5O3 复合陶瓷材料的微观结构、电性能和老化性能。①SPS 烧结的陶瓷体致密度为98.9%，常规烧结的陶瓷体致密度为84.5%， SPS 烧结过程中高的还原条件、低的氧分压抑制了Cr 的挥发从而促进了材料的致密化烧结。②比起常规烧结，SPS 烧结的样品具有更高的电阻率，其原因主要有： SPS 烧结过程中，短的烧结时间减少了Cr 的挥发，从而导致Cr4+和Mn4+离子减少，最终引起电阻率增大； SPS 烧结过程中，低的氧分压产生了氧空位并释放出电子，产生的电子将补偿部分Cr 空位，从而导致导致Cr4+和Mn4+离子减少，电阻率增加。③SPS 烧结后的样品，高温老化后阻值降低，这可能是由于Cr 挥发增多和氧空位减少引起的；常规烧结的样品，高温老化后阻值轻微地增加，经过250 小时后，其阻值变化率小于8%，这可能是由于常规烧结的材料致密度不够引起晶格氧的损失，进而导致载流子浓度减小，电阻增大。电性能及老化结果表明，传统烧结的复合陶瓷材料稳定性更好，复合陶瓷材料的电阻率和老化可以通过烧结气氛或老化气氛来调节和控制。6. 首次系统地研究了SPS 烧结的MgAl2O4-YCr0.5Mn0.5O3 复合陶瓷材料的结构、微观结构、电学性能及老化性能。复合陶瓷材料由立方尖晶石相MgAl2O4 和同构于YCrO3 的正交晶系钙钛矿YCr0.5Mn0.5O3 相组成；复合陶瓷晶粒尺寸大约在0.5-2 μm，无明显的孔隙存在，致密度在94.1%~97.4%；复合陶瓷材料电阻率随温度升高而减小，显示了很好的NTC特性。在整个温度区间(25-1000℃)，lnρ-1000/T 曲线不是直线，而是以600℃为界，呈现分段稳定的特点；随着MgAl2O4 含量的增大，电阻率增大，这主要是由于MgAl2O4 是电绝缘体，随着MgAl2O4 含量的增大，YCr0.5Mn0.5O3 量减少，即载流子浓度降低，从而导致了电阻率的增大；复合陶瓷材料的ρ25、B25/150、B700/1000、Ea25/150、 Ea700/1000 的范围分别为：1.53×106~9.92×109 Ωcm，3380~5172K，7239~9543K，0.291~0.446 eV，0.624~0.823 eV；复合陶瓷材料高温老化后阻值变小，这可能是由于高温老化过程中，一方面Cr 挥发增多，另一方面氧空位减少，两者均导致Cr4+和Mn4+离子浓度增大，从而引起材料高温老化后阻值降低。7. 采用电性能混合原理和渗流理论结合复合材料电学性能的结果， 探讨了MgAl2O4-YCr0.5Mn0.5O3 复合材料两相扩散规律及离子迁移机制。复合材料的实验电阻率小于通过混合法则计算的电阻率，这可能是由于YCr0.5Mn0.5O3 与MgAl2O4 两相之间的相扩散及离子迁移造成的；当MgAl2O4 的量为0.1 和0.6 时，复合材料的电阻率主要由混合法则控制，当MgAl2O4 的量为0.4 时，复合陶瓷材料的电阻率主要由渗流理论控制。8. 采用Mn 掺杂开发了CaCu3-xMnxTi4O12 新型高温热敏陶瓷材料，并对其结构、微观结构、高温电性能进行了研究。Mn 掺杂的CaCu3-xMnxTi4O12 粉体和陶瓷样品中，主要的相为CaCuTi4O12 相和少量的TiO2 相；随着Mn 含量的增大，晶粒尺寸先减小后增大，当x=0.5时，晶粒尺寸最小，但Mn 掺杂的样品的晶粒均比CaCu3Ti4O12 的小； 在25~800℃，CaCu3-xMnxTi4O12 陶瓷材料的电阻率随温度的升高而降低，显示出良好的负温度系数特性；在整个温度区间(25-800℃)，CaCu3Ti4O12 样品lnρ-1000/T 曲线不是直线，而是以500℃为界，呈现分段稳定的特点，随着Mn 含量的增大，曲线逐渐变为线性，当x=0.5 时，曲线为完全的线性关系；Mn 掺杂将材料高温段B 值从3000 K 提高到了6000 K 左右；其高低温缺陷分别主要来源于氧空位和阳离子化学计量比失衡。
The research and development of new high temperature (25-1000 °C) thermistor ceramics have become an important development trend and new research focus of the negative temperature coefficient (NTC) thermistor materials, as a result of the disadvantages of transition metal oxide such as aging of the electrical properties and the irreversible change of resistance-temperature characteristics in the above 300 °C. This research is significant for improving independent research and development level of high temperature thermistors and promoting their practical application. In recent years, it is possible through associating a high resistance material with a rare earth perovskite oxide (i.e. a NTC material) to design and synthesize high temperature thermistor materials, which is a significant topic in research of thermistor materials. Based on the mechanism of electrical conductivity anomaly and conduction mechanism of YCr1-xMnxO3 (0≤x≤0.5) thermistor ceramics, this thesis selected MCr0.5Mn0.5O3 (M=Y、La) perovskite-based high temperature composite thermistor ceramics as the research object, focused on the relationship between multiplicity, structure and electrical properties of composite ceramics. The relationship between microstructure and NTC properties of composite ceramics, defect interaction mechanism at high and low temperature, and conduction mechanism were put forward. The effects of conventional sintering (CS) and spark plasma sintering (SPS) on composition, microstructure, electrical properties and conduction mechanism were comparatively studied. Defect interaction mechanism and aging mechanism of high temperature aging process were revealed. The electrical properties mixing rule and percolation theory were adopted to probe into the interdiffusion rules and ion migration mechanism. Based on the properties of high grain- boundary activation energy of CaCu3Ti4O12, the influence of Mn doping on the structure, microstructure and electrical properties of high temperature thermistor ceramics CaCu3-xMnxTi4O12 (0≤x≤1)was systemically investigated, and their semiconducting mechanism of high and low temperature was revealed. The main results are summarized as follows. 1. The mechanism of electrical conductivity anomaly of YCr1-xMnxO3 thermistor ceramics was put forward for the first time, and small-polaron hopping conductivity mechanism of the materials was discussed. YCrO3 was a p-type semiconductor, the resistivity was mainly determined by the concentration of Cr4+ ions. Mn ions were acting as an n-type dope, and partly compensated for the effect of metal vacancies, thus leading to an increase in the resistivity. Mn4+ ions increased as Mn content increased from 0.2 to 0.5, which promoted the rise in charge carriers and the electron hopping, thereby resulting in a decrease in the resistivity. The YCrO3 NTC thermistor showed a linear relationship between the lnρ and 1000/T, this relationship was in agreement with the small-polaron hopping mechanism. The electrical conduction in these ceramics could be due to the electron jumps between Cr3+ and Cr4+ ions on one hand, and between Mn3+ and Mn4+ ions on the other hand. The activation energy and B values increased at first and then decreased with increasing Mn content, which had the same varying tendency with resistivity. 2. The relationship between microstructure and NTC characteristics of (Y2O3+CeO2)-YCr0.5Mn0.5O3 composite thermistor materials was put forward. The resistance of the grain boundary decreased with increasing temperature, indicative NTC characteristic. There was a linear relationship between the lnRgb and 1000/T, which indicated that the electrical conduction of grain boundary was in accordance with the small-polaron hopping mechanism. The grain boundary was responsible for the NTC characteristic observed in the composite ceramics. 3. The defect interaction mechanism resulted in different activation energy of high and low temperature was put forward. In the low temperature range, the coulomb interaction between oxygen vacancy and metal vacancy formed defect association ( ? ), which led to a decrease in the concentration of oxygen vacancy and an increase in the concentration of Cr4+ and Mn4+ ions, thus promoting the electron hopping and thereby decreasing the activation energy; In the high temperature range, with the increase of temperature, a large number of associated defects would decompose as the result of thermal motion of ions, then the associated oxygen vacancy would turn into free oxygen vacancy, and the oxygen vacancy that can migrate would increase and thus led to a decrease in the concentration of Cr4+ and Mn4+ ions and thereby increasing the activation energy as a result. 4. The structure, microstructure, phase distribution and electrical properties of xMgAl2O4-(1-x)LaCr0.5Mn0.5O3 composite materials were systemically investigated for the first time. The composite ceramics consisted of two phases: a cubic spinel MgAl2O4 phase and an orthorhombic perovskite LaCr0.5Mn0.5O3 phase isomorphic to LaCrO3; the cell volume of LaCr0.5Mn0.5O3 phase decreased with increasing MgAl2O4 content, which should be ascribed to the substitution of the smaller Al3+ for Mn3+ and Cr3+ ions; The brighter regions were the LaCr0.5Mn0.5O3 and the darker was the MgAl2O4 phase, and there was interdiffusion and ion diffusion between these two phases; The resistivity increased with increasing MgAl2O4 content, which should be attributed to the fact that MgAl2O4 as electrical insulator increased, that of LaCr0.5Mn0.5O3 decreased, thus resulting in an increase in the resistivity; For 0≤x≤0.8, the values of ρ300, B400/800 and activation energy of NTC ceramics were in the range 1.76-1.22×108 Ωcm, 2646-8711 K, 0.228-0.746 eV, respectively. 5. The microstructure, electrical properties and aging characteristics of CS- and SPS-sintered 0.6MgAl2O4-0.4LaCr0.5Mn0.5O3 composite ceramics were comparatively studied for the first time. ① The relative density of the SPS- and CS-sintered composite ceramics was 98.9% and 84.5%, respectively; Highly reducing condition and low oxygen partial pressure during the SPS process were advantageous in suppressing the chromium vaporization, thus facilitating the densification of sintering. ② Electrical properties measurements showed the samples from SPS-sintered ceramics possessed a higher resistivity than that from conventionally sintered ceramics. There were two possible reasons for the increase in the resistivity of SPS-sintered samples: During the SPS process, the short sintering period was advantageous in reducing chromium volatilization, thus leading to a decrease in Cr4+ and Mn4+ ion concentration, thereby increasing the resistivity as a result; Under SPS conditions, low oxygen partial pressure produced oxygen vacancy and caused release of electrons, which would partly compensate for the effect of chromium vacancy, thus causing a reduction of Cr4+ and Mn4+ ion concentrations, and then increasing the resistivity. ③ The resistance of SPS-sintered samples decreased after high temperature aging, which could be due to the increase of chromium volatilization on one hand and the decrease of oxygen vacancy on the other hand in the course of aging; The resistance of CS-sintered samples had a slight increase in comparison to the resistance before aging, and aging coefficient was less than 8% after aging for 250 h, which could be ascribed to the loss of oxygen produced low density, thereby compensating chromium vacancy and increasing the resistance as a result. The aging results indicated that CS-sintered samples had a better electrical stability in comparison with SPS-sintered samples. The resistivity and aging of composite ceramics could be adjusted and controlled by the atmosphere of sintering and aging. 6. The structure, microstructure, electrical properties and aging characteristics of SPS-sintered MgAl2O4-YCr0.5Mn0.5O3 composite ceramics were comparatively investigated for the first time. The composite ceramics consisted of a cubic spinel MgAl2O4 phase and an orthorhombic perovskite YCr0.5Mn0.5O3 phase isomorphic to YCrO3; The grain sizes of the composite ceramics ranged from 0.5 to 2 μm; The composite ceramics had high relative density ranging from 94.1 to 97.4% of the theoretical density, and no obvious porosity was observed in the composites; The resistivity of composite ceramics decreased with increasing temperature, indicative good NTC characteristics; The relationship between the lnρ and 1000/T were not linear over the measured temperature range from 25 to 1000 ?C and changed around 600 ?C, which showed characteristics of segmental stability; The resistivity increased with increasing MgAl2O4 content, which should be attributed to the fact that MgAl2O4 as electrical insulator increased, that of YCr0.5Mn0.5O3 decreased, leading to a decrease in the amount of charge carriers, thus resulting in an increase in the resistivity; The obtained ρ25, B25-150, B700-1000, Ea25/150 and Ea700/1000 of the SPS-sintered composite NTC thermistors were in the range of 1.53×106-9.92×109 Ωcm, 3380-5172 K, 7239-9543 K, 0.291-0.446 eV，and 0.624-0.823 eV, respectively; The resistance of composite ceramic samples decreased after high temperature aging, which could be due to the increase of chromium volatilization on one hand and the decrease of oxygen vacancy on the other hand in the course of aging, thus resulting in an increase in the concentration of Cr4+ and Mn4+ions and thereby decreasing the resistance. 7. The interdiffusion rules and ion migration mechanism of MgAl2O4-YCr0.5Mn0.5O3 composite ceramics were discussed by adopting the electrical properties mixing rule and percolation theory in combination with the results of electrical properties. The interdiffusion and ion diffusion between these two phases led to a decrease in experimental resistivity in comparison to the estimates obtained applying electrical properties mixing rule; When the content of MgAl2O4 was 0.1 and 0.6, electrical properties mixing rules controled the resistivity, whereas the content of MgAl2O4 was 0.4, percolation theory controled the resistivity. 8. The new high-temperature thermistor ceramics CaCu3-xMnxTi4O12 were discovered by doping Mn ions, and their structure, microstructure, and electrical properties of high temperature were investigated. The major phases presented in the Mn-doped powders and ceramics were CaCu3Ti4O12 phase with body-centered cubic structure and TiO2 phase; With the increase of Mn content, the grain size decreased at first and reached a smallest grain at x=0.5, and then increased, but the grain sizes of Mn-doped samples were smaller than those in the CaCu3Ti4O12 samples; The resistivity of CaCu3-xMnxTi4O12 ceramics was decreased with increasing temperature from 25 to 800 ?C, indicative good NTC characteristics; The relationship between the lnρ and 1000/T were not linear over the measured temperature range from 25 to 800 ?C and changed around 500 ?C, which showed characteristics of segmental stability; The relationship between the lnρ and 1000/T gradually changed from the nonlinear to linear with increasing Mn content, and reached a absolute linear dependence at x=0.5; The B values were increased from 3000 K to about 6000 K by doping Mn ions; The defect of high and low temperature in these ceramics may be mainly due to the oxygen vacancy and cation nonstoichiometry, respectively.
|张博. MCr0.5Mn0.5O3 (M=Y、La)钙钛矿基复合热敏陶瓷的制备、结构与导电性能研究[D]. 北京. 中国科学院大学,2014.|
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