Study On Preparation And Electronic Structure Of Calcium Hexaboride

Calcium hexaboride is a material with many good properities,such as low density,high melting point, high hardness, high strength and good chemical stability. It has been applied in some areas, such as refractory industry, copper smelting industry, nuclear industry,structural ceramics industry and so on. some researches on single crystal,polycrystal of CaB₆ have draw the attention of many researchers. For the preparation of high-performance CaB₆ single crystal, polycrystal and composite materials, we need to synthesize CaB₆ powder with high purity, uniform particle size distribution and good sintering performance first.Based on thermodynamic theory, enthalpy of reaction and gibbs free energy were compued and analysised for the synthesis reaction of CaB₆. The results indicate that the reaction enthalpy of CaO-B2O3 system reach up to 2500kJ/mol,this reaction need a lot of heat. At normal pressure, the theory initial reaction temperature of CaO-B2O3 system is 1214.79K.On the basis of thermodynamic analysis, we chose CaO,B2O3 and C as raw material.The synthesis technology of CaB₆ powder by carbothermal reduction reaction was investigated. The phase composition, microtopography, element content and carbon content were analysized by using X-ray diffraction, scanning electron microscope and X-ray fluorescence. The reaction mechanism was also investigated. The results indicated that ultrafine CaB₆ powder was synthesized at 1600℃ for 1 hour when CaO:B2O3=1:3.2. The surface of CaB₆ bulk is loose and easy to be broken. The carbon content of CaB₆ powder is 0.23% and the average particle size is 23.09pm. The SEM analysis indicates that this powder has a cube structure. The XRF analysis shows that the main elements of this powder are Boron and Calcium, the molar ratio of these two elements is 5.8:1, which is approximate to the stoichiometric ratio of CaB₆.By adopting the method of pseudo potential technology of total energy-plane wave based upon the density function theory, the geometry structure of intrinsic and C-doped CaB₆ were optimized. The crystal parameter, cohesive energy, band structure and density of states properties were systematically studied on this basis. The results indicate that the C-doping crystal parameter has been stretched in a and c crystal orientation and compressed in b crystal orientation, the cell volume reduced by 0.719% and 1.5%respectvely, the cohesive energy was smaller than that of CaB₆ without carbon. The system is more stable,but the crystal structure remains the same. The intrinsic CaB₆ is direct-gap semiconductor, its energy gap is 0.149eV, the C-doping has a certain metal property. the fermi level entered into the conduction band and the energy band migration to direction of low energy as a whole.With the increase of doping concentration,the band gap will become wider and atomic bonding stronger because of the aggravation of electronic communization movement.