發(fā)布時間：2018-09-01 12:24

【摘要】：CaB_6具有密度小、熔點高、硬度大、強(qiáng)度高、化學(xué)穩(wěn)定性好等特性,已在耐火材料、煉銅工業(yè)、核工業(yè)、結(jié)構(gòu)陶瓷等領(lǐng)域獲得了應(yīng)用。由于CaB_6具有優(yōu)異的特性和廣闊的應(yīng)用前景,有關(guān)CaB_6單晶、多晶以及復(fù)合材料的研究已經(jīng)引起國內(nèi)外很多科研人員的廣泛關(guān)注。為了制備高性能的CaB_6單晶、多晶及其復(fù)合材料,首先需要制備純度高、粒度分布均勻且具有良好燒結(jié)性能的CaB_6粉體。本文基于熱力學(xué)理論基礎(chǔ),對合成CaB_6的各反應(yīng)體系的反應(yīng)焓變和吉布斯自由能進(jìn)行了理論計算和分析。結(jié)果表明:CaO-B203體系的反應(yīng)焓高達(dá)2500kJ/mol,說明反應(yīng)需要大量的熱,反應(yīng)需要在較高溫度和較長的保溫時間下進(jìn)行以提供充足的熱量。在常壓下,CaO-B203體系的理論起始反應(yīng)溫度為1214.78K。在熱力學(xué)分析的基礎(chǔ)上,采用CaO、B203和C為原料,研究了碳熱還原法合成CaB_6粉體的制備工藝。采用XRD、SEM、XRF等測試方法分析了合成粉體的物相組成、顯微形貌、元素含量、碳含量以及粒度分布情況,并探討了碳熱還原法合成CaB_6的機(jī)理。結(jié)果表明:碳熱還原法制備CaB_6粉體的最佳合成工藝為物料中CaO與B203的摩爾比為1:3.2,反應(yīng)溫度1600℃,保溫時間1h,在此條件下得到的CaB_6粉體純度和粒徑最佳;粉體的含碳量為0.23%、粒徑為23.09μm; SEM分析表明其具有立方體結(jié)構(gòu);XRF分析結(jié)果表明粉體中含有的主要元素為B和Ca,二者的原子摩爾比為5.8:1,與CaB_6中硼和鈣的化學(xué)計量比接近。本文還采用密度泛函理論平面波贗勢的方法,對本征CaB_6和C摻雜CaB_6材料的晶胞結(jié)構(gòu)進(jìn)行了幾何優(yōu)化,在此基礎(chǔ)之上系統(tǒng)地研究了碳摻雜量的變化對材料的晶格常數(shù)、結(jié)合能、能帶結(jié)構(gòu)以及態(tài)密度影響。計算結(jié)果表明:C原子摻雜后,CaB_6的晶格常數(shù)在晶軸a、c方向上被拉長,在晶軸b方向上被壓縮,晶胞體積分別減小0.719%和1.5%,系統(tǒng)的結(jié)合能變小,系統(tǒng)更穩(wěn)定,但晶體結(jié)構(gòu)不變;本征CaB_6為直接帶隙半導(dǎo)體,其禁帶寬度為0.149eV;摻雜后的CaB_6具有一定的金屬特性;C摻雜CaB_6后,能帶整體向低能方向遷移,費米能級進(jìn)入導(dǎo)帶;隨著摻雜濃度的提高,由于電子的共有化運動加劇會導(dǎo)致帶隙變寬,原子成鍵變強(qiáng)。
[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，

本文編號：2217193