【作者】 徐天添；

【导师】 姜力强；

【作者基本信息】 浙江工业大学 ， 材料物理和化学， 2009， 硕士

【摘要】 磁制冷材料是通过磁性材料磁矩的有序度在外磁场中发生变化而引起熵变来达到制冷的目的。磁制冷与传统的气体压缩制冷技术相比,具有效率高、耗能小、无污染等特点。钙钛矿锰氧化物室温磁制冷材料由于具有优良的物理特性、制备简单、结构稳定、价格便宜等优势而成为研究关注的重点。采用溶胶-凝胶法制备了钙钛矿锰氧化物La_0.65Sr_0.35MnO₃,通过XRD和SEM研究了不同PH值及乙二胺对合成粉体相组成和微观形貌的影响,结果发现:在PH值为0.5的酸性环境和PH为8.0的弱碱性环境中,获得单纯的钙钛矿结构,粉体颗粒分散良好尺寸均匀。在PH值为2-7之间的弱酸性环境时,有沉淀出现,不能形成均匀的溶胶和凝胶,难以获得单相钙钛矿结构,且粉体中有严重的团聚现象。PH值继续增大,粉体形貌基本没有变化,但颗粒尺寸会有所长大。PH值对合成粉体形貌的影响主要来自于对凝胶化过程的影响。在PH值为0.5的酸性环境中,形成二维的层状凝胶结构,而在PH=8.0的弱碱性环境中,则形成三维的空间网状凝胶结构。这两种结构均有利于获得尺寸细小均匀,团聚较少的钙钛矿锰氧化物粉体。研究了乙二胺对溶胶-凝胶法制备钙钛矿锰氧化物La_0.65Sr_0.35MnO₃显微结构与性能的影响,结果发现:采用乙二胺将溶液pH值调整到8.0附近时,有利于溶胶-凝胶的形成,而采用氨水进行调整,在同样pH值的情况下,有部分沉淀产生,不能很好的凝胶化。添加乙二胺情况下,最终形成的合成产物颗粒细小均匀,而采用氨水调整pH值时,产物团聚现象严重。乙二胺在溶胶-凝胶形成过程中起着桥接、交联和鳌合的作用,桥接和交联有利于凝胶化过程的发生和稳定,鳌合作用有利于金属离子均匀分散在凝胶的三维网络结构中,从而减少后期热处理过程中的团聚。研究了后期热处理工艺以及最终合成纳米颗粒形貌对样品居里温度的影响。得出了最佳热处理温度是800℃,最适合热处理时间是4h。在一定的温度内晶粒长大过程是协调各个晶粒间的位相,当温度高于1200℃时晶粒异常长大。通过改变所合成钙钛矿锰氧化物的晶粒大小,可以改变样品的居里温度以及磁熵变峰宽。研究了La_0.725-xSr_0.275+xMnO₃和La_0.925-xMnO₃的A位浓度掺杂以及空位掺杂对于居里温度的影响,结果发现合成的La_0.725-xSr_0.275+xMnO₃居里温度随着x值的增加而降低。当x值0-0.075之间时,受到<rA>减小引起的居里温度下降与Mn⁴⁺浓度升高引起居里温度上升这两个相悖作用,居里温度降低的幅度不大。当x值0.075-0.100之间时,<rA>减小引起的居里温度下降与Mn⁴⁺浓度升高引起居里温度下降是两个相同过程。x值在0-0.100之间时,A位浓度掺杂和B位Mn³⁺/Mn⁴⁺离子比共同控制居里温度变化。合成的La_0.925-xMnO₃居里温度随着x值的增加而降低。虽受到<rA>减小引起的居里温度下降与Mn⁴⁺浓度升高引起居里温度上升这两个相悖作用,但空位浓度较大引起晶格畸变严重,故<rA>减小引起的居里温度下降是其主要控制过程。自主设计和制造了磁热效应直接测试装置,研究和分析了磁热效应直接测量方法的系统误差以及主要影响,为以后进行该方面的研究进行了探索和尝试。更多还原

【Abstract】 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 <r_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 <r_A> and Mn⁴⁺ contents have the same effects. It can be inferred that Tc is influenced by <r_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 <r_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.更多还原