Room Temperature High TCR [La_0.625(Ca_0.375-xSr_x)MnO₃] Polycrystalline Ceramic Preparation And Research On Its Electromagnetic Transport Performance

As a typical perovskite manganese oxide,La_1-xA_xMn O₃?A=Ca,Sr,Ba,etc.?material has complex physical phenomena and could be used in devices in the electromagnetic field.For the La_1-xA_xMn O₃ material to better meet the conditions of the application,it is necessary to have large magnetoresistance?MR?and temperature coefficient of resistivity?TCR?values,and Metal-Insulator transition temperature?T_p?and the peak temperature of TCR?T_k?to be close to room temperature.However,in the current research,few materials have high MR and TCR values,as well as T_p and T_k are close to room temperature.According to the ferromagnetic phase diagrams of La_1-xCa_xMn O₃ and La_1-xSr_xMn O₃,the Curie temperature?T_c?of both reached the maximum when x=0.375.The maximum T_c value of La_1-xCa_xMn O₃ and La_1-xSr_xMn O₃ is about 275 K and360 K,respectively.If the total doping ratio of Ca and Sr is set to 0.375,the T_p and T_k values of the material could easily reach room temperature.Therefore,this paper chose the La_0.625Ca_0.375-xSr_xMn O₃ system to obtain room-temperature high TCR ceramics.In this thesis,firstly,La_0.625Ca_0.315Sr_0.06Mn O₃ ceramic powders with different sintering temperatures?500-1500°C?were prepared by the sol-gel method.Then La_0.625Ca_0.375-xSr_xMn O₃?x=0?0.13?ceramic bulks were prepared at a sintering temperature of 1450°C.At the same time,La_0.625Ca_0.375-xSr_xMn O₃:Ag_0.2?x=0.09?0.15?ceramic bulks were further prepared.Finally,the electrical properties of the ceramics were optimized by changing Ag doping.X-ray diffraction?XRD?,field emission scanning electron microscope?FESEM?,and transmission electron microscope?TEM?test results of ceramics powder showed that the grain size of the sample gradually increased with the increase of sintering temperature,and the phase formation was incomplete at 500-600°C and completely at 700°C.When the temperature reached 1500°C,the sample was overburned.X-ray photoelectron spectroscopy?XPS?results indicated that the increase in sintering temperature caused more oxygen vacancies to be formed inside the ceramic powder,thereby increasing the Mn³⁺ion concentration,hence,the double exchange effect was enhenced.The UV-Vis absorption results indicated that increased sintering temperature decreased the band gap width of ceramics,which clarified that the higher sintering temperature within a certain range,the better electrical properties of the ceramics.However,the magnetic measurement results showed that the increase of sintering temperature reduced the material's magnetic response.Therefore,to obtain materials with the best electrical and magnetic properties at the same time requires a suitable sintering temperature.La_0.625Ca_0.375-xSr_xMn O₃?x=0?0.13?ceramic bulks obtained an optimal TCR(2.6%K^-1)value of room-temperature?297.3 K?at x=0.10.The La_0.625Ca_0.375-xSr_xMn O₃:Ag_0.2?x=0.09?0.15?ceramics obtained an optimal TCR value(4.9%K^-1)of room-temperature?299.2 K?at x=0.11.The XRD results of both systems showed that the increase in Sr content caused slight distortion in crystal structure of the samples,but did not cause phase transformation.FESEM results showed that Sr promoted the growth of grains and reduced the number of grain boundaries,which weakened the scattering effect on the grain boundaries and improved the electrical properties of ceramics.The optimal ratios of the two systems are different,which may be due to the Jahn-Teller and double exchange effect caused by the change of Sr content were affected by Ag.In view of the effect of Ag on the internal mechanism in the materials,the optimal ratio La_0.625Ca_0.275Sr_0.10Mn O₃ was doped with Ag to further increase the TCR value.When the Ag content was 0.4 mol,the sample obtained a maximum TCR of 8.2%K^-1 at room-temperature 300K.XRD and FESEM results showed that increasing Ag content from 0 mol to 0.5 mol did not cause phase transformation,and the increase in Ag content promoted grain growth and optimized electrical properties.In summary,the ceramic material La_0.625Ca_0.275Sr_0.10Mn O₃:Ag_0.4 with high TCR value at room-temperature was finally obtained by co-doping of Sr and Ag elements in this thesis.This work not only make up for the gap of La_1-xA_xMn O₃ material at x=0.375,but also provide theoretical and experimental reference for the further study of this kind of materials.