Preparation And Properties Of La _1-x (CaSr) _X MnO <3> Polycrystalline Materials

Due to the colossal magneto-resistance (CMR) effect, the rare earth manganese oxides with perovskite structure became the focus of research in the field of condensed state physics. The perovskite manganites, owing to the CMR effect, metal-to-insulator transition temperature (Tp) and temperature coefficient of resistance (TCR), which can be used to prepare nearly room temperature magnetic-electronic devices, colossal magnetoresistance bolometer, infrared detectors, etc. Hence, the preparation of this kind of material and the study of its structure and performances became the focus of our work. In this direction, La1-xCa,Sr)xMnO3:mol%Ag (x= 0.5 and 0.3) materials with variable x composition, different ratio of Ca and Sr co-doping and Ag addition were adopted as the research object. The precursor powders were prepared by sol-gel method, and then the powders pre-sintered, suppressed and sintered to obtain the polycrystalline bulk materials. The structure and properties of the samples were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), magnetic measurement (M-T and M-H), electrical properties test (R-T, MR-T), scanning electron microscope (SEM) and inductively coupled plasma mass spectrometry (ICP).The experimental results show that the single phase of the perovskite structure was synthesised by pre-sintered powders of La0.5CCa0.5-ySry)MnO3 with different ratio of Ca and Sr. With the increasing of Sr content, the XRD diffraction peaks shift down to the low angles, which illustrate that the Sr2+ ions replaced the Ca2+ions and made the cell volumes increased. However, the bulk samples showed high resistivity and low metal-insulator transition temperature (Tp), and some samples also showed the insulator behavior completely. Hence, the La0.5(Ca0.5-ySry)MnO3 (y= 0,0.1,0.2,0.3,0.4 and 0.5) materials cannot arrive nearly room Tp and high TCR values.For the La0.7(Ca0.3-ySr)MnO3 (y= 0,0.05,0.1,0.15,0.2,0.25 and 0.3) samples, there is a structure transition from orthorhombic phase(Pbnm space group) to rhombohedral symmetry (R3c space group), which occure at y= 0.15. The TEM images confirmed the change of crystal structure. With the increasing of Sr content, a significant increase in Tc and Tp for La0.7(Ca0.3-ySr)MnO3 (y= 0,0.5,0.1,0.15,0.2,0.25 and 0.3) composites are observed, and the value of Tp increased from 271 K to 365 K. The enhancement of DE interaction, which caused by the larger radius of cations Sr2+ changed the Mn-O-Mn bonding angles and the Mn-O bonding distance, were proposed to explain the enhancement of magnetic and electrical properties. With the increasing of Sr concentration, the TCR decreased from 3.5 K"1 to 0.4 K"1 and coercivity increased. The increase of spin disorder on surface layers of particles and the scattering of electrons by grain boundaries, which due to the decrease of particle size, were responsible for the increase of coercivity and decrease of TCR, respectively.Based on the nearly room temperature of La1-x(Ca,Sr)xMnO3 materials, the effect of Ag doping on the structure, morphology, electrical transport and magnetoresistance effect for La0.7Ca0.3MnO3 and La0.7(Ca0.25Sr0.05)MnO3 materials were studied. The results indicated that the addition of Ag can significantly degrade the resistivity of the samples, improve the values of Tp, increase the values of TCR and MR at room temperature, enhance the crystallization of the samples, increase particle uniformity and grain size. The optimal doping range of Ag addition is less than 0.1 mol. With the increasing of Ag doping, Ag entered into the lattice and replaced the A site of La3+, Ca2+ and Sr2+ ions. And it was possible that the firstly replaced element was the ionics which have bigger radius. However, the substitution was limited, if continue to increase the amount of Ag, it segregated at grain boundaries or grain surfaces as metal Ag. The substitution effect of Ag enhanced the double exchange interaction between Mn3+-O2--Mn4+, and the distribution of metal Ag at grain surfaces or grain boundaries can improve surface morphology of the matrix particles, reduce the magnetic disorder state at grain boundaries, and provide a second conduction channel for electron transport. Composition analysis results show that Ag in proportion to increase in the samples. However, when the doping amount of Ag is larger, the Ag is more volatile, and the volatilization of Ag will increase the material’s defect, which can reduce the performance of the material. High performance 98%Lao.7Cao.25Sro.o5Mn03:2%molAg materials were obtained (Tp= 301 K, TCR= 3.2%K-1, Tk = 296 K, MR= 20.1%).