Preparation Of Perovskite Manganese Oxide Magnetic Refrigeration Materials

Magnetic refrigeration is achieved through the magnetic entropy change, which is caused by the magnetic moment order changing due to the applied magnetic field change. Compared to conventional gas compression technology, magnetic refrigeration technology is characterized by high efficiency, saving energy and environmental friendship. Perovskite manganese oxides, as one of the room temperature magnetic refrigeration materials, have many advantages such as good physical properties, steady structure, cheap and simple preparation, etc, which attracts the attention of many researchers.La_0.65Sr_0.35MnO₃ was prepared under different PH conditions by sol-gel method. The phase composition of the as-received powders was indicated using X-ray diffraction. The particle size and morphology were observed by SEM. The results show that the samples called manganites have single perovskite structure and homogeneous particle size distribution when PH is controlled near 0.5 and 8.0 during the sol-gel process. When the PH is between 2 and 7, the sol appears sedimentation and the unhomogeneous sol and gel are achieved which results in the agglomeration of particles without single perovskite structure. As PH increases, the particles grow up with the same appearance. The effect of PH comes from the gel process. The layer-shape gel forms in the system with PH=0.5. However, the three-dimensional reticulate gel occurs when PH=8. These two conditions make perovskite manganese oxides with small size and at the same time avoiding the agglomeration.The effects of 1,2-ethylenediamine (C₂H₈N₂) on the synthesis of manganite La_0.65Sr_0.35MnO₃ using sol-gel method was studied compared with ammonia. The results show the addition of C₂H₈N₃ benefits forming sol and gel when PH is near 8.0. However, the precipitation happens in the system containing ammonia even though at the same PH value. The heated sample with C₂H₈N₂ displays the fine and homogeneous particles, which is different from the aggregation in the sample with ammonia addition. A possible mechanism is proposed that C₂H₈N₂ plays the bridge-linked, cross-linked and chelated roles during gel formation. The bridge-linked and cross-linked benefit the gel stabilization. The chelation makes for the homogeneous distribution of La, Sr and Mn ions in the gel. Therefore, the proper C₂H₈N₂ addition effectively reduces the agglomeration of the powders during the heat treatment due to the bridge-linked, cross-linked and chelated mechanism in the sol-gel process.The influence of particle size and morphology in Tc temperature and the heat treatment process was studied. The result shows that the best heat treatment temperature is at 800°C and best time is 4h. The grains grow coordinately between in a certain temperature range, Abnormal grains grow up when it is higher than 1200°C. Tc and magnetic entropy change are influenced by the grain sizes of Perovskite manganese oxides.Effects of Sr ions and vacancy doped on Tc were studied by preparing of La_0.75-xSr_0.275+xMnO₃ and La(0.925-x)MnO₃. The results show that Tc goes down when the x value rises up. When the x value is between 0 and 0.075, Tc of La_0.725-xSr_0.275+xMnO₃ is affected by the two opposite ways: i) Tc rises up with A> decreasing and ii) Tc rises up with Mn⁴⁺ contents increasing. This produces the unobvious change of Tc. When the x value is between 0.075 and 0.100, Tc is affected by two same ways, in which A> and Mn⁴⁺ contents have the same effects. It can be inferred that Tc is influenced by A> and Mn⁴⁺ contents. Tc of the La_0.925-xMnO₃ sample goes down when the x value rises up. Although Tc is affected by two opposite ways, the density of vacancy makes the serious lattice distortion. The main control factor is determined by A> in this situation.To measure the magnetocaloric effect directly, a system was designed and the deviation was also studied. The results profit the further study.