|Place of Conferral||北京|
|Keyword||高温ntc热敏电阻 钙钛矿结构 电性能 老化特性 尖晶石结构|
目前能实际应用的高温NTC热敏电阻还较少，因此需研究新型高温NTC热敏电阻材料，以满足应用需求。MgAl2O4是一种典型的尖晶石结构(AB2O4)化合物，其化学性质非常稳定，熔点高(>2200 ℃)。但由于MgAl2O4 电阻率很高，不能直接被应用于高温热敏电阻器。本研究通过离子取代和复合高电导相材料的办法对其电性能进行改进。本文以MgAl2O4为基体材料，研究Cr离子取代部分Al离子对材料的电导率的影响，并研究其导电机理；采用常规烧结与真空热压烧结制备Mg(Al1-xCrx)2O4陶瓷材料，研究不同烧结方法对材料电性能及结构的影响；随后与高稳定性、高电导率的LaCrO3复合，研究复合材料的复合度、结构和电性能之间的关系，并对复合陶瓷材料进行老化试验，研究其老化机理。主要研究结果如下:1. 通过常规烧结方法成功制备了Mg(Al1-xCrx)2O4陶瓷材料，其相结构为与MgAl2O4(JCPDF 21-1152)同构的立方尖晶石相，该材料在500-1000 ℃测试范围内，随着测试温度的升高其电阻率逐渐降低，表现出显著的负温度系数特性。 Mg(Al1-xCrx)2O4陶瓷样品的ρ500、B500/800、Ea 数值，其变化范围分别为2.67×103~3.72×107 Ωcm, 6731~11886 K, 0.581~1.025 eV。当材料中Cr掺杂量的增多时，电阻率逐渐降低，这是由于Cr3+离子取代部分Al3+，在高温烧结过程中有铬挥发，会产生空穴，为达到电荷平衡，部分Cr3+ 变为Cr4+，XPS测试表明样品中确实存在Cr3+ 、Cr4+两种价态。在Cr3+ 和Cr4+之间会发生跳跃导电，随着Cr离子浓度的提高，其电阻率降低。 2. 采用常规烧结和真空热压烧结制备Mg(Al1-xCrx)2O4陶瓷材料，研究不同烧结方法对陶瓷材料微观结构和电性能的影响。两种方法制备的陶瓷材料，其相结构都为与MgAl2O4(JCPDF 21-1152)同构的立方尖晶石相。经真空热压烧结后，其晶粒尺寸仍为纳米级，且烧结致密性有很大提高，在x=0.3时达到最大值92.3 %, 而常规烧结陶瓷材料在x=0.3时为77.7 %。真空热压烧结过程中，外界给予的机械压力为材料中晶界移动、粒子重排提供了驱动力，低氧分压抑制了Cr挥发，从而提高了材料的致密性。当测试温度升高时，材料的电阻率下降，具有负温度系数特性。由于真空热压烧结过程中，Cr挥发量减少，导致Cr4+离子减少，使电阻率增加，使得真空烧结材料样品有更高的电阻率和B值。3. 采用常规烧结法成功制备了LaCrO3-Mg(Al0.7Cr0.3)2O4复合陶瓷材料，该材料为两相复合材料，存在与MgAl2O4 (JCPDS 21-1152)同构的立方尖晶石相和 LaCrO3 (JCPDS 24-1016) 钙钛矿相。复合热敏电阻材料的ρ300、B400/800、Ea 值的变化范围分别为1.55×102~1.41×108 Ωcm、756~11317 K、0.065~0.976 eV。可以通过调节LaCrO3含量，对复合陶瓷材料的电性能进行大幅度的调节。由于渗流现象，复合陶瓷材料在复合度为0.1~0.2 时其电阻率、B值发生剧烈下降。对0.2LaCrO3-0.8Mg(Al0.7Cr0.3)2O4复合陶瓷材料在900 ℃条件下进行700 h的高温老化实验，结果表明在前400 h的老化实验中，其电阻漂移率较大，最高可达11.7 %，随着老化时间延长电阻漂移率下降，老化500 h后其电阻漂移率均在6 %以内。真空热压烧结制备0.2LaCrO3-0.8Mg(Al0.7Cr0.3)2O4复合陶瓷材料，随着烧结温度的升高，其致密度降低，导致电阻率升高。
There are a few of NTC thermistors which can be used in high temperature. In order to meet the application requirements, the new high temperature NTC thermistor should be designed and researched. The MgAl2O4 is a symbol of spinel-type oxides which present a general formula AB2O4. MgAl2O4 has high melting point (>2200 ℃) and excellent chemical stability. Due to the MgAl2O4 has a too high electrical resistivity, it can not be directly used as high temperature thermistors. The resistivity of MgAl2O4 can be reduced by ion substitution or compounding a high conductivity phase. In this paper, substitution of Cr3+ for Al3+ in MgAl2O4 and compounding with LaCrO3 were researched. First of all, the ceramics based on the Mg(Al1-xCrx)2O4 system have been prepared to investigate the effect of the Cr content on the structure and electrical properties. Second, The Mg(Al1-xCrx)2O4 ceramics were produced by conventional sintering and vacuum hot-pressed sintering (VHPS), and the different effect of micro structure and electrical properties of materials were researched. And then, the Mg(Al1-xCrx)2O4 were compounded with LaCrO3 to research the relationship between the complex degree, structure and electrical properties of these composite materials. After that, the composite materials were carried out high temperature aging test to research the aging mechanism. The major results are shown in there:1. The new high temperature NTC ceramics based on the Mg(Al1-xCrx)2O4 have been synthesized by conventional solid state reaction at 1650 ℃. The samples were consisted of a single phase isomorphic to the cubic spinel MgAl2O4(JCPDF 21-1152). The samples exhibited typical NTC characteristic whose resistivity decreases with increasing temperature from 500 to 1000 ℃. The obtained values of ρ500 and B500/800 and Ea are in the range of 2.67×103~3.72×107 Ωcm, 6731~11886 K, 0.581~1.025 eV, respectively. The resistivity decreases with increasing the Cr content. Because of the volatilization of Cr during high temperature sintering, which lead to forming hole, so a part of Cr3+ turn into Cr4+. And the XPS analysis confirms the existence of Cr3+ and Cr4+ ions on lattice sites. As the Cr content increases, the amount of Cr3+ and Cr4+ ions increases, which is in favour of hopping conductivity and thereby decreasing the resistivity. The electrical properties of these ceramics can be adjusted by the Cr content. 2. The effect of micro structure and electrical properties in Mg(Al1-xCrx)2O4 ceramic materials, which were produced by conventional sintering and VHPS, were researched. All of the ceramics were consisted of a single phase isomorphic to the cubic spinel MgAl2O4 (JCPDS 21-1152) too. The sintered samples by VHPS were nanograined. Comparing with conventional sintering, the VHPS can improve the relative density of the ceramics and reached 92.3 % at x=0.3, while the relative density of ceramic sintered by conventional sintering was just 77.7 % at x=0.3. During the VHPS, the pressure can afford driving force to improve crystal boundary migration and particle rearrangement, and the low oxygen partial pressure condition is also advantageous in suppressing the chromium vaporization. So the VHPS can improve the relative density of the ceramics. All the samples show the typical NTC characteristic. Because the chromium vaporization decreased in low oxygen partial pressure condition and leaded to decrease of Cr4+ ions, the ceramics fabricated by VHPS achieved higher resistivity than the conventional sintered.3. The new high temperature NTC composite ceramics LaCrO3----------Mg(Al0.7Cr0.3)2O4 have been prepared using conventional solid state reaction at 1650 ℃. The sintered ceramics are consisted of cubic spinel Mg(Al0.7Cr0.3)2O4 phase and orthorhombic perovskite LaCrO3 phase. The values of ρ300, B400/800 and Ea are in the range of 1.55×102~1.41×108 Ωcm, 756~11317 K, 0.065~0.976 eV, respectively. The electrical properties can be adjusted by changing LaCrO3 content. Because of the percolation effect, the resistivity and B value of composite ceramics observably decreased between the content of LaCrO3 at 0.1 and 0.2. The aging coefficient of 0.2LaCrO3----------0.8Mg(Al0.7Cr0.3)2O4 was higher and irregular, and even reached at 11.7 % in the before 400 h. The aging coefficient was less than 6 % after aging for 500 h. The 0.2LaCrO3-0.8Mg(Al0.7Cr0.3)2O4 composite ceramics sintered by VHPS possess the typical NTC characteristic. The density of these ceramics decreased with the increase of the sintering temperature , and led to the resistivity increase.
|杨田. LaCrO3-Mg(Al1-xCrx)2O4复合热敏陶瓷的制备及性能研究[D]. 北京. 中国科学院大学,2017.|
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