[La_0.67(Ca_0.33-xSr_x)MnO₃]:Ag_{yStudy On The Preparation Of Polycrystalline Materials And Their Electromagnetic Transport Properties}

The synthesis and characterization of perovskite manganites with general formula Re_1-xA_xMnO₃（Re presents rare-earth element and Ae is divalent alkaline-earth metal like Ca,Sr and Ba）have attracted increasing interests,due to their potentials applications in information storage devices,magnetic sensors,magnetic refrigeration,and infrared bolometers.Temperature coefficient of resistivity（TCR）is an essential parameter in metal–insulator transition sharpness,determining the sensitivity of infrared bolometers.Meanwhile,magnetoresistance（MR）defined as relative change in electrical resistivity upon application of external magnetic field.This determines the responsivity of magnetic sensors.Numerous studies demonstrated that TCR could reach values above 10%K^-1,and reasonable MR values were obtained with La_2/3Ca_1/3MnO₃:Ag_x composites.However,the high MR and TCR values of these materials were recorded at below room-temperature.In addition,the large MR was mostly pronounced at elevated magnetic field.This greatly limits the practical application of such materials.Therefore,Obtaining large TCR and LFMR values at room temperature（ie,300 K）has become a research hotspot for such materials.In order to obtain the large room-temperature TCR and MR,this paper combines the characteristic of La_0.67Ca_0.33MnO₃（high TCR and low TCR peak temperature（T_k））and La_0.67Sr_0.33MnO₃（low TCR and high T_k）.Optimizes the T_k of polycrystalline ceramic to room-temperature by co-doping with Ca and Sr.Then,through Ag doping further enhances the TCR and MR of polycrystalline ceramics,and systematically studies the electrical and magnetic properties of[La_0.67(Ca_0.33-xSr_x)MnO₃]:Ag_y polycrystalline ceramics.First,polycrystalline La_0.67(Ca_0.33-xSr_x)MnO₃（x=0,0.03,0.06,0.09,0.12,and 0.15）ceramics were prepared by sol-gel method.As x content further rose,metal-insulator transition temperature（T_p）increased and resistivity decreased.At Sr doping x=0.06,the value of T_p attained room temperature（301.2 K）.At x=0.09,the value of T_k attained room temperature（296.1 K）,the peak TCR value reaches 7.7%K^-1.The La_0.67(Ca_0.27Sr_0.06)MnO₃ material system with T_p reaching room-temperature and T_k close to room-temperature was selected to change the sintering temperature of the sample,and the influence of sintering temperature on the material properties was analyzed.As the sintering temperature increases,the growth of the sample grains is promoted,the grain size is increased,and the number of grain boundaries is reduced,resulting in less grain boundary scattering.Thereby,the resistivity of sample decreased,and the TCR increased with an increase in temperature.According to the analysis of the performance of the sample,1450°C is optimized as the optimum sintering temperature.After the optimum sintering temperature was determined,Ag was added to La_0.67(Ca_0.27Sr_0.06)MnO₃ material system to prepare a series of La_0.67(Ca_0.27Sr_0.06)MnO₃:Ag_y（y=0,0.1,0.2,0.3,0.4,0.5）polycrystalline ceramics.As the Ag content increases,the resistivity of the sample decreases significantly.The results of this experiment show that the addition of Ag does not affect T_p and T_k of the sample,T_p remains at room temperature,and T_k is still lower than room temperature.TCR and MR did not change linearly with the addition of Ag,but increased first and then decreased with the increase of Ag content.When y=0.2,the maximum values were 22.5%K^-1 and 44.5%,respectively.In order to further improve the room temperature TCR,MR and deep analysis of the doping mechanism of Ag.The experiment was based on the T_k of the prepared La_0.67(Ca_0.24Sr_0.09)MnO₃ polycrystalline ceramics reaching room temperature,a series of La_0.67(Ca_0.27Sr_0.09)MnO₃:Ag_y polycrystalline ceramics were prepared.The results confirmed presence of silver at A sites(La³⁺/Ca²⁺/Sr²⁺)as Ag⁺and segregates at grain boundaries（GBs）to form metallic silver.With the incorporation of Ag,the resistivity of the sample drops sharply.The T_p and T_k of the sample remained substantially unchanged with the Ag addition.The peak TCR and MR values of the composites did not change linearly with the addition of Ag,but increased first and then decreased with the increase of Ag content,reaching a maximum of 14.8%K^-1,34.1%at y=0.4.At this time,the peak TCR temperature（T_k=301.0 K）and the peak MR temperature（T_M=300.6 K）of the sample reached the room temperature level.The high room temperature TCR and MR obtained provide the theory and materials foundation for the fabrication of uncooled and highly sensitive bolometer infrared devices,magnetic sensors and other new optoelectronic sensitive devices.