Preparation Of La_0.7Ca_0.3-xA_xMnO₃:yAg(A=Na , K) Research On Materials And Electromagnetic Properties

Re_1-xAe_xMnO₃(Re:rare earth metal element;Ae:alkali metal element,alkaline earth metal)perovskite manganese oxide is an important representative of strongly related electronic materials.The interaction of carrier ordering,orbital hybridization,lattice distortion,and ferromagnetic coupling in the material of this system results in many unique physical characteristics:Metal-insulator transition,Jahn-Teller effect,Giant magnetoresistance(GMR)effect,and Colossal magnetoresistance(CMR)effect,etc.Therefore,Re_1-xAe_xMnO₃ material has broad application prospects in electronic devices such as infrared detection,thermal radiation measurement,magnetic recording,and removable magnetic sensing.From the published research results,they can be seen that the prepared Re_1-xAe_xMnO₃ material has short plates such as metal-insulator transition temperature(T_p)far from room temperature,low temperature coefficient of resistance(TCR),and low magnetoresistance(MR),which cannot meet practical applications.Therefore,the preparation of Re_1-xAe_xMnO₃ polycrystalline ceramic materials with T_p near room temperature,high TCR and MR is still a hot research direction.In this dissertation,Na and K in monovalent alkali metal elements were selected to be mixed into La_0.7Ca_0.3MnO₃ materials.The effects of different sintering temperatures and doping amounts of Na and K on the properties of ceramic materials were studied,and then Ag powder was added to further improve the electromagnetic properties of ceramic materials.Multiple series of La_0.7Ca_0.3-xA_xMnO₃:yAg(A=Na,K)ceramic samples were prepared.X-ray diffraction(XRD)was used to characterize the crystal structure of the ceramic samples.The surface morphology of the ceramic samples were analyzed by scanning electron microscopy(SEM).The standard four-probe method was used to test the electromagnetic properties of ceramic samples by changing the external magnetic field strength.Multi-series La_0.7Ca_0.3-xA_xMnO₃:yAg(A=Na,K)ceramic samples are all pure perovskite structure.Changing the sintering temperature or the amount of doping could promote the growth of crystal grains,and increase the cell volume and grain size.In La_0.7Ca_0.3-xNa_xMnO₃ series samples,as the doping amount of Na⁺increased,the crystal structure changed from an orthogonal structure to a rhombohedral structure.In La_0.7Ca_0.26A_0.04MnO₃(A=Na,K)ceramic samples prepared at different sintering temperatures,as the sintering temperature increased,T_p gradually decreased and the peak resistivity(?_max)gradually decreased.When the sintering temperature was1200?,the peak resistance temperature coefficient(TCR_max)and peak magnetic resistance(MR_max)of the sample reached their maximum values.In La_0.7Ca_0.3-xA_xMnO₃(A=Na,K)ceramic samples,as the doping amount of Na⁺and K⁺increased,T_p gradually increased and?_max gradually decreased.The TCR_max and MR_max of La_0.7Ca_0.24Na_0.06MnO₃ and La_0.7Ca_0.26K_0.04MnO₃ reached the maximum values in the corresponding series,respectively.To further increase the T_p,TCR_max and MR_max of the samples,Ag powder was mixed into La_0.7Ca_0.24Na_0.06MnO₃ and La_0.7Ca_0.26K_0.04MnO₃for sintering.Two series of polycrystalline targets were prepared.In La_0.7Ca_0.24Na_0.06MnO₃:yAg ceramic samples,when y=0.2,?_max=0.0368?·cm,T_p=271.05 K,the TCR_max and MR_max of the sample reached the maximum,TCR_max=30.57%·K^-1,MR_max=72.28%.In La_0.7Ca_0.26K_0.04MnO₃:yAg ceramic samples,when y=0.2,?_max=0.036?·cm,T_p=274.7 K,the TCR_max and MR_max of the sample reached the maximum,TCR_max=34.29%·K^-1,MR_max=75.71%.The experimental results show that La_0.7Ca_0.3-xA_xMnO₃:yAg(A=Na,K)series materials can be used in electronic devices such as infrared detection,thermal radiation measurement,magnetic recording,and removable magnetic sensing.