La_1-x(Ca,Sr)_xMnO₃:Ag_y Polycrystalline Ceramic Material And Its Preparation Electrical Performance Studies

The perovskite-structured manganese oxide R_1-xAE_xMnO₃(R is La³⁺,Ce³⁺and AE is Ca²⁺,Sr²⁺,Ba²⁺,Pb²⁺,etc)has excellent properties in terms of electrical and magnetic properties.This can be applied in the preparation of magnetic memory,magnetic detectors,spin electronics and infrared detectors and other devices with important application and research value.However,most metal-insulator transition temperature(T_MI),temperature coefficient of resistivity(TCR),magnetoresistance(MR)of most perovskite materials were obtained at below room temperature,and the maximum performance can be obtained under the externally strengthened magnetic field(greater than 1 T).It undoubtedly limits the application of the material in practical production.So for how to obtain materials with steep metal-insulation transition characteristics and large magnetoresistance effect that can be used in uncooled conditions become the main content of this system research.In this paper,La_1-x(Ca,Sr)_xMnO₃polycrystalline ceramic material samples were prepared by the sol-gel method with reference to the characteristics of La_1-xCa_xMnO₃and La_1-xSr_xMnO₃magnetic phase diagram.First selects the value of x,and then optimize the ratio of Ca and Sr.So that the peak TCR temperature(T_k)and peak MR temperature(T_m)of the sample reach room temperature.Finally,the peak TCR and MR values of samples were further increased by doping with different contents of Ag₂O,and systematically study the electrical and magnetic properties of La_1-xCa_xMnO₃:Ag_ypolycrystalline ceramic materials.A series of La_1-x(Ca,Sr)_xMnO₃samples were prepared by changing the A-site ionic composition.The four-probe method was used to test the?-T curve,and the characteristics of each component sample can be better understood.The experimental results show that the single-phase perovskite crystal structure was generated in all samples.As the content of x gradually increased,the T_MIshifts to the low temperature section.In the test temperature range,when the composition ratio is between 0.6<x?0.9,the samples have no metal-insulation transition characteristics,so the sample under this composition cannot achieve the results expected by the experiment.Therefore,in this experiment,the component ratio mainly studied was between 0.1?x?0.6.By optimizing the composition and changing the doping conditions,polycrystalline ceramic materials with T_MIand T_kat room temperature and high peak TCR and MR values were obtained.La_0.67(Ca_0.3Sr_0.03)MnO₃:Ag_ypolycrystalline ceramics were prepared by changing the content of Ag₂O at the sintering temperature of 1723 K.Ag₂O doping increased the grain size and grain size of the samples,which reduced the number of grain boundaries and scattering at the grain boundary.This reduced the resistivity of the polycrystalline ceramic,which changed the grain size and increased the conductivity of the samples.When y=0.2,the peak TCR reached the maximum of 11.2%K^-1,and T_kwas 281.5 K still below room temperature.Therefore,by optimizing the composition ratio of the material again,the T_kof the sample was kept at room temperature,and the TCR value was further increased.Because the change of the A-site ion composition has a greater effect on the T_ktemperature of the samples,La_0.7(Ca_0.3-xSr_x)MnO₃polycrystalline ceramics were prepared,and the properties of the polycrystalline ceramics were improved by doping Ag₂O.The Sr content is x=0.095,the T_MIreached the room temperature level of 300K;the peak TCR was 13.78%K^-1,T_kwas 299.2 K.The peak MR was 26.41%,and the corresponding T_mtemperature was 302.6 K.When Ag2O was used to dope La_0.7(Ca_0.205Sr_0.095)MnO₃samples,it was found that the TCR and MR values could be greatly improved without changing the T_MI,T_kand T_mtemperature of the samples.The doping amount was y=0.2,the peak TCR was 17.26%K^-1(T_k=299.9 K),the peak MR was 30.34%(T_m=300.93 K).In domestic and foreign researches,there were relatively few studies on the component system with the La=0.8,and most of the work was concentrated on the strong magnetic field to achieve the maximum CMR effect of the samples,which undoubtedly limited its practical application in the field of electromagnetic materials and devices.Therefore,under the low-intensity magnetic field,the electromagnetic properties of the samples were changed by Ag₂O doping to obtain larger TCR and MR to meet the practical application.In the experiment,a series of La_0.845Sr_0.155MnO₃:Ag_ypolycrystalline ceramic samples were prepared on the basis that the T_MIof La_0.8(Ca_0.12Sr_0.08)MnO₃:Ag_0.3sample reached room temperature.The combined test results show that Ag exists at the sample in two forms:elemental and ionic,and Ag⁺ions replaced the A-site ionic,which increases the Mn-O bond length and Mn-O-Mn bond angle,and changes the lattice constant.Ag doping drives the conversion of Mn³⁺to Mn⁴⁺in polycrystalline ceramics,thereby promoting the mobility of electrons between Mn³⁺-O^2--Mn⁴⁺.For the La_0.845Sr_0.155MnO₃:Ag_0.2sample,the peak TCR and MR values reached 17.54%K^-1and 34.58%,respectively,and T_kand T_mwere 300.04K and 303.83 K,respectively.