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Study on Structural and Magnetocaloric Effect of A-sites Size-disorder La2/3-xRExCa1/3-ySryMnO3 Compounds

Author: LiuGuangHua
Tutor: LuoGuangSheng
School: Nanchang University
Course: Materials Physics and Chemistry
Keywords: Perovskite manganite Crystal structure Doping disorder Magnetocaloric effect Magnetic entropy change
CLC: TB303
Type: Master's thesis
Year: 2007
Downloads: 64
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Abstract


This paper outlines the the magnetocaloric effect generation, the principles and applications of magnetic refrigeration, magnetic refrigeration materials research progress, perovskite manganese oxide magnetic refrigeration materials research progress and magnetocaloric effect thermodynamics theoretical basis; elaborated perovskite manganite oxide crystal structure, the interaction between the electronic structure and double exchange and super-exchange interaction, Jahn-Teller distortion physical mechanism of performance; description of the the Rietveld fitting method structure refinement principle and manganese oxides structure refinement methods. In accordance with the average ionic radius of the A-site of the perovskite manganite and Mn 3 / Mn 4 ratio constant principles calculations to determine the respective elements La, Y, Eu, Ca, Sr, Mn, O content, using a solid-phase sintering method prepared manganese oxide La 2/3-x of R Ex Ca 1/3-y < , of MnO , / sub> Sr y 3 (where R E Y, Eu element), two series of samples; using Rigaku Dmax-rB 12KW turn The target X-ray diffraction (XRD) analysis of the structure of the sample and the phase, structure refinement by the Rietveld method of X-ray diffraction spectra, the structure constant and the Mn-O bond length and Mn-O-Mn bond angle parameters; test sample magnetization curves and magnetization curve, to infer the Curie temperature and calculate the magnetic entropy change near the Curie temperature; by magnetization curves draw Arrot curve (the inflection point) to determine whether using a vibrating sample magnetometer Lakeshore7300 occurrence of one or two magnetic phase transitions. By analyzing the X-ray diffraction spectra and Rietveld refinement results, prepared by solid state reaction of the two series of samples structure parameters do not change much with the increase of the doping concentration x better remain single-phase perovskite orthogonal structure; mine with the doping increase in the degree of disorder, the lattice distortion increases, samples Curie temperature dropped, and the sample La 2/3-x Eu x Ca 1/3-y Sr y MnO 3 Curie temperature monotonic decline, however, the sample La 2/3-x Y x Ca 1/3-y Sr y MnO 3 (x = 0.2) ranks where points instead stretching increased due to a sharp increase of Mn-O-Mn health angle so that double exchange interaction enhanced ferromagnetic exchange interaction enhanced This fully shows that competition leads to a ranking doped disordered double exchange interaction the POINT changes; description the magnetic entropy change and Arrot of curves, with the increase of the doping degree of disorder, the sample giant magnetocaloric effect of a magnetic phase transition shift to the two magnetic phase transition, and samples of La 2/3-x Eu x Ca 1/3-y Sr y MnO 3 of magnetic entropy change almost decreases monotonically; analysis shows that the lattice distortion increases with the increase of the doping degree of disorder, the electronic - lattice coupling effect of reducing the ferromagnetic order transition to the paramagnetic disordered slower heat magnetization curves flattens steep change near the Curie temperature, leading to a magnetic phase transition to the secondary magnetic phase transition transition, Arrot curve inflection point disappears, doping disorder and electron - lattice coupling effect of competition results ; experimental results, the giant magnetocaloric effect, the sample may be used as a specific temperature magnetic refrigeration material generated as a result of a magnetic phase transition.

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