CaB6粉体的制备及其电子结构研究
发布时间：2018-09-01 12:24
【摘要】：CaB_6具有密度小、熔点高、硬度大、强度高、化学稳定性好等特性,已在耐火材料、炼铜工业、核工业、结构陶瓷等领域获得了应用。由于CaB_6具有优异的特性和广阔的应用前景,有关CaB_6单晶、多晶以及复合材料的研究已经引起国内外很多科研人员的广泛关注。为了制备高性能的CaB_6单晶、多晶及其复合材料,首先需要制备纯度高、粒度分布均匀且具有良好烧结性能的CaB_6粉体。本文基于热力学理论基础,对合成CaB_6的各反应体系的反应焓变和吉布斯自由能进行了理论计算和分析。结果表明:CaO-B203体系的反应焓高达2500kJ/mol,说明反应需要大量的热,反应需要在较高温度和较长的保温时间下进行以提供充足的热量。在常压下,CaO-B203体系的理论起始反应温度为1214.78K。在热力学分析的基础上,采用CaO、B203和C为原料,研究了碳热还原法合成CaB_6粉体的制备工艺。采用XRD、SEM、XRF等测试方法分析了合成粉体的物相组成、显微形貌、元素含量、碳含量以及粒度分布情况,并探讨了碳热还原法合成CaB_6的机理。结果表明:碳热还原法制备CaB_6粉体的最佳合成工艺为物料中CaO与B203的摩尔比为1:3.2,反应温度1600℃,保温时间1h,在此条件下得到的CaB_6粉体纯度和粒径最佳;粉体的含碳量为0.23%、粒径为23.09μm; SEM分析表明其具有立方体结构;XRF分析结果表明粉体中含有的主要元素为B和Ca,二者的原子摩尔比为5.8:1,与CaB_6中硼和钙的化学计量比接近。本文还采用密度泛函理论平面波赝势的方法,对本征CaB_6和C掺杂CaB_6材料的晶胞结构进行了几何优化,在此基础之上系统地研究了碳掺杂量的变化对材料的晶格常数、结合能、能带结构以及态密度影响。计算结果表明:C原子掺杂后,CaB_6的晶格常数在晶轴a、c方向上被拉长,在晶轴b方向上被压缩,晶胞体积分别减小0.719%和1.5%,系统的结合能变小,系统更稳定,但晶体结构不变;本征CaB_6为直接带隙半导体,其禁带宽度为0.149eV;掺杂后的CaB_6具有一定的金属特性;C掺杂CaB_6后,能带整体向低能方向迁移,费米能级进入导带;随着掺杂浓度的提高,由于电子的共有化运动加剧会导致带隙变宽,原子成键变强。
[Abstract]:Because of its low density, high melting point, high hardness, high strength and good chemical stability, CaB_6 has been applied in the fields of refractories, copper smelting industry, nuclear industry, structural ceramics and so on. Due to the excellent properties and wide application prospects of CaB_6, the research on CaB_6 single crystal, polycrystalline and composite materials has attracted the attention of many researchers at home and abroad. In order to prepare high performance CaB_6 single crystal, polycrystalline and its composites, it is necessary to prepare CaB_6 powder with high purity, uniform particle size distribution and good sintering performance. Based on the thermodynamic theory, the enthalpy change and Gibbs free energy of each reaction system for the synthesis of CaB_6 were calculated and analyzed theoretically. The results show that the reaction enthalpy of the system is as high as 2500kJ / mol, which indicates that the reaction needs a large amount of heat, and the reaction needs to be carried out at a higher temperature and a longer holding time to provide sufficient heat. The theoretical initial reaction temperature of CaO-B203 system is 1214.78 K. under atmospheric pressure. On the basis of thermodynamic analysis, the preparation process of CaB_6 powder by carbothermal reduction method was studied by using CaO,B203 and C as raw materials. The phase composition, microstructure, element content, carbon content and particle size distribution of the synthesized powders were analyzed by XRD,SEM,XRF and the mechanism of CaB_6 synthesis by carbothermal reduction method was discussed. The results show that the optimum synthesis process of CaB_6 powder by carbothermal reduction is that the molar ratio of CaO to B203 is 1: 3.2, the reaction temperature is 1600 鈩，
本文编号：2217193
[Abstract]:Because of its low density, high melting point, high hardness, high strength and good chemical stability, CaB_6 has been applied in the fields of refractories, copper smelting industry, nuclear industry, structural ceramics and so on. Due to the excellent properties and wide application prospects of CaB_6, the research on CaB_6 single crystal, polycrystalline and composite materials has attracted the attention of many researchers at home and abroad. In order to prepare high performance CaB_6 single crystal, polycrystalline and its composites, it is necessary to prepare CaB_6 powder with high purity, uniform particle size distribution and good sintering performance. Based on the thermodynamic theory, the enthalpy change and Gibbs free energy of each reaction system for the synthesis of CaB_6 were calculated and analyzed theoretically. The results show that the reaction enthalpy of the system is as high as 2500kJ / mol, which indicates that the reaction needs a large amount of heat, and the reaction needs to be carried out at a higher temperature and a longer holding time to provide sufficient heat. The theoretical initial reaction temperature of CaO-B203 system is 1214.78 K. under atmospheric pressure. On the basis of thermodynamic analysis, the preparation process of CaB_6 powder by carbothermal reduction method was studied by using CaO,B203 and C as raw materials. The phase composition, microstructure, element content, carbon content and particle size distribution of the synthesized powders were analyzed by XRD,SEM,XRF and the mechanism of CaB_6 synthesis by carbothermal reduction method was discussed. The results show that the optimum synthesis process of CaB_6 powder by carbothermal reduction is that the molar ratio of CaO to B203 is 1: 3.2, the reaction temperature is 1600 鈩，
本文编号：2217193
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