Synthesis And Properties Investigation Of La_0.7Ca(0.3)MnO₃Nanoparticles And The Compound Sample

La_0.7Ca_0.3MnO₃, showing various unique properties, including colossalmagnetoresistance effect (CMR), electrical impulses induced variable resistor effect(EPIR) and magnetothermal effect (MCE), has attracted many attentions due to itspotential applications for novel magnetic sensor, volatile resistance-type memory andmagnetic refrigeration devices. In this dissertation, La_0.7Ca_0.3MnO₃nanoparticles withcontrolled size have been prepared and their properties were investigated. On theother hand, La_0.7Ca_0.3MnO₃/BaTiO3composite nanoparticles with core/shell structurewere successfully fabricated. The main points are summarized as follows:(1) A polyacrylamide gel route was used to fabricate La_0.7Ca_0.3MnO₃nanoparticles. Influence of pH values and chelating agents on particle morphologyand properties of products were investigated by X-ray diffraction and scanningelectron microscopy. The optimal pH value for synthesis of perovskiteLa_0.7Ca_0.3MnO₃was obtained to be2. Using acetic acid, oxalic acid, citric acid, andethylenediamine-tetraacetic acid as chelating agents, single-phase La_0.7Ca_0.3MnO₃nanoparticles were prepared at a calcination temperature of600℃. The particlesprepared using the four chelating agents are uniform and regularly-shaped spheres,and have an average size of30,37,45nm, and50nm, respectively. When usingtartaric acid as the chelating agent, a much higher calcination temperature at900℃was required to produce single-phase La_0.7Ca_0.3MnO₃particles. The obtained particlesexhibit an irregular morphology and an obvious adhesive behavior, and have arelatively large average size of170nm. Magnetic hysteresis loop measurementsindicate that the prepared La_0.7Ca_0.3MnO₃samples exhibit paramagnetism at roomtemperature, but their paramagnetic susceptibilities exhibit a decreasing trend withthe decrease of sample particle size. The magnetothermal properties of acetic acid-and EDTA-resulted samples have been investigated. It is demonstrated that withincreasing particle size, the Tcincreases and the magnetic entropy also gets enhanced.(2) The polyacrylamide gel route was developed to prepare core/shell structuredLa_0.7Ca_0.3MnO₃/BaTiO3composite nanoparticles. The morphology of particles wasobserved via SEM and TEM. The results reveal that the prepared particles are nearlyspherical in shape and highly uniform in size, and possess well-defined core/shellstructure. The optical properties of products were studied by UV-Vis diffusereflectance absorption spectra.