Study On Aging Properties Of 0.4La_1-xSr_xCrO₃-0.6MgAl₂O₄ High Temperature NTC Ceramic And Development Of Components

With the development of science and technology,the electronic components which can be used in the extreme environment have more stringent requirements,including high temperature NTC thermistors.Most NTC thermistor ceramics composed of spinel structure?MMn2O4,where M = Ni,Co?.However,temperature range of the application is relatively narrow of the material,and the internal defects distribution inhomogeneity after as-sintered lead to the electrical property is easy to change,this result in the thermistors is instable at high temperature.Therefore,It is attracted the attention of people that explore new materials with good electrical conductivity and stability in high temperatures.LaCrO₃ material is a perovskite rare earth oxide that has excellent electrical conductivity and stability in high temperature.Therefore,the thesis based on the mechanism of electrical conductivity of La_1-xSr_xCrO₃?0?x?0.1?ceramics,and the composite blended with spinel material of MgAl₂O₄ in proper proportion.The effects of some factors on the crystal structure,microstructure,electrical properties of ceramic materials are discussed,such as the different preparation methods of powders,the doped amount of Sr²⁺,and different of the compound proportion.The main results are summarized as follows:?1?Oxide solid-state reaction and the Pechini reaction processing route wereb selected to fabricate the La_1-xSr_xCrO₃?0?x?0.1?powders.The optimum sintering temperature of La_1-xSr_xCrO₃ ceramic materials were determined by XRD,which was 1600 and 1650 °C.SEM images shown that the ceramic samples has porosity on the surface.This may be due to the chromium volatilization at high temperature in air atmosphere.The ceramic samples obtained by Pechini method are more compactness,which is because the Pechini reaction can improved the activity of the powders,thus improving material sintering activity.The presence of the Cr⁶⁺ is one of the key factors that affect the electrical conductivity of La_1-xSr_xCrO₃.the conductive mechanism can be explained with a small polaron hopping conductivity model,this due to the participation of both Cr³⁺ and Cr⁶⁺ ions in the hopping process.The resistivity decreased with the increased of Sr²⁺ content,which was attributed to the increased of Cr⁶⁺ and the material constants and activation energy La_1-xSr_xCrO₃ ceramics had the same tendency to increased of Sr²⁺ ions content.The aging test shows that the resistance drift rate of samples obtained by vacuum sintering is smaller.?2?Solid-state reaction processing route was selected to fabricate the 0.4La_1-xSr_xCrO₃-0.6MgAl₂O₄?0?x?0.1?powders.The sintering methods are conventional sintering and vacuum sintering.The major phases presented in composite ceramics are cubic spinel MgAl₂O₄ phases and orthorhombic perovskite La_1-xSr_xCrO₃ phases isomorphic to LaCrO₃.SEM and EDX show that the brighter regions are the La_1-xSr_xCrO₃ and the darker regions are the MgAl₂O₄,there has mutual diffusion and ion migration phenomenon between two phase.Compared with conventional sintering,the surface of the sample obtained by vacuum sintering has no porosity,smaller grain size and more compact structure.Resistivity decreased with the increased of content of strontium,this can be explained that the Sr²⁺ to replace the parts of the La3+ lead to generation of holes,that result in the amount of Cr⁴⁺ increased in the perovskite lattice.The electrical conductivity in composite ceramics may be due to the participation of both Cr³⁺ and Cr⁴⁺ ions in the hopping process,and conductive mechanism dependence of the small-polar on hopping transport.?3?Oxide solid-state reaction and the Pechini reaction processing route were selected to fabricate the xLa_0.8Sr_0.2CrO₃-?1-x?MgAl₂O₄?x=0.2,0.3,0.4?powders.Composite ceramic materials mainly include two phases,in which brighter regions were the La_0.8Sr_0.2CrO₃ and the darker regions were the MgAl₂O₄;there has mutual diffusion and ion migration phenomenon between two phase;the compactness improved of increased the MgAl₂O₄ phases;composite ceramic samples resistivity increased with the increased of content of MgAl₂O₄,which should be attributed to the fact that MgAl₂O₄ as electrical insulator increased,the corresponding semiconductor La_0.8Sr_0.2CrO₃ content decreased,thus resulting in an increase in the resistivity.The values of ?400,and B400/800 of 0.2La_0.8Sr_0.2CrO₃-0.8MgAl₂O₄ NTC thermistors obtained by Oxide solid-state method is 423053?.cm and 4317K?...