Preparation Of La_5/8(Ca_3/8-xSr_x)MnO₃ By Sol-gel Method Polycrystalline Materials And Their Electromagnetic Properties

In recent years,scholars at home and abroad have conducted extensive research on the synthesis and application of LAMO materials,focusing on the improvement of electromagnetic properties for these materials at room temperature,therefore,how to obtain the material with high TCR at room temperature has become the focus of research on this material system.In this paper,since the magnetic phase diagrams of LaCaMnO₃ and LaSrMnO₃ are more likely to make T_p of materials reach room temperature as the content of La is 5/8,La_0.625(Ca_0.375-xSr_x)MnO3:Ag_z polycrystalline ceramics with different Sr content,sintering temperature and Ag doping amount were prepared by sol-gel method.A series of analysis and discussion on the crystal structure,surface morphology,composition and electromagnetic properties of samples were carried out by XRD,SEM,XPS,R-T and other test methods.The experimental results are as follows:All La_5/8(Ca_3/8-xSr_x)MnO₃（x=0,0.03,0.06,0.09 and 0.12）polycrystalline ceramics showed one single phase structure of Pnma,and miscellaneous peaks were not detected in the XRD test results.With the increase of Sr content,the surface compactness of ceramics deteriorated,the average grain size decreased gradually,and the resistivity of materials showed a gradual decreasing trend.Peak TCR gradually decreased and T_k increased gradually（consistent with the trend of T_p）.Consistently,peak MR of the material increased firstly and then decreased.At Sr doping amount of0,peak TCR of the sample reached 13.8%（T_k was 267.43 K）;At Sr doping amount of0.09,T_k of the sample reached 303.26 K（around room temperature range）,and peak TCR reached 3.7%.As Sr content increased to 0.03,peak MR of material increased to36.6%（273 K）;As Sr content increased from 0.03 to 0.12,peak MR of the material decreased to 10.5%（312 K）.In addition,the sintering temperature will also induce a certain impact on the properties of materials.As the sintering temperature increased,all samples showed a single-phase orthogonal structure of Pnma,and no impurities were observed in the sample.The pores on the surface of ceramics reduced,and the density was getting better and better,the average grain size increased,and the crystal quality improved.The resistivity of samples showed a gradual decrease trend,and T_p fluctuated within the range of 300.48 to 314.4 K,peak TCR of the sample showed a gradual increase trend.As the sintering temperature increased from 1000°C to 1400°C,the value of peak TCR increased from 0.45%（T_k was 304.13 K）to 6.5%（T_k was 293.70K）,and T_k varied from 293.70 to 308.19 K,this was consistent with the trend of T_p.The reason for these changes was mainly related to the substitution of Sr²⁺.In order to further improve TCR of the material at room temperature,La_0.625(Ca_0.285Sr_0.09)MnO₃:Ag_z polycrystalline ceramics were prepared at sintering temperature of 1400°C,and structures,surface morphologies,electrical and magnetic properties of obtained LCSMO ceramics were investigated by various analytical techniques.With the increase of Ag doping amount,all the samples showed a single-phase structure of Pnma,without impurities.The average ionic radius increased first and then decreased,and the resistivity of samples showed a gradual decreasing trend.T_p controlled at around room temperature changed very little.With the increase of Ag doping amount,peak TCR and MR of samples increased first and then decreased.As Ag doping amount was 0.3,peak TCR of the sample reached a maximum of 10.27%（T_k was289.56 K）.As z increased from 0 to 0.4,T_k varied from 289.56 to 294.22 K（around room temperature）.As Ag doping amount increased to 0.1,peak MR of the material increased to 15.63%（293.08 K）;As Ag doping amount increaseed from 0.1 to0.3,peak MR of the material decreased to 9.89%（297.67 K）.The reason for these changes was mainly related to the amount of Ag doping.As the amount of Ag doping was low,the incorporation of silver lead to the replacement of A site ions and improvement of the electrical transport properties for the materials.As the amount of silver doping was high,the substitution effect of silver was limited,and the defects of morphology increased,which had a negative impact on its electromagnetic properties.In addition,the influence of sintering temperature on La_0.625(Ca_0.285Sr_0.09)MnO₃:Ag_0.3polycrystalline ceramic target were also studied.Since Ag was easy to volatilize at high temperature,the result of sintering temperature played dominated influence on the samples was obtained by a series of testing methods.