【作者】 黄霞；

【导师】 仲剑初；

【作者基本信息】 大连理工大学 ， 功能材料化学与化工， 2009， 硕士

【摘要】 作为一种重要的金属硼化物,CaB₆具有高熔点、高强度、高的化学稳定性、低的电子功函数、较强的中子吸收能力等优异性能,被广泛用作耐火材料、金属脱氧剂、中子吸收剂、耐磨剂及功能陶瓷材料等。一般来说,用来制备CaB₆的硼源为B₄C或B₂O₃,但这两种原料都需在高温下获得,生产成本很高。本研究以价格低廉的六硼酸钙为原料,以金属镁粉为还原剂,采用燃烧合成法成功制备了CaB₆粉末。为了获得较好的硼酸钙原料,对水热合成的五水合六硼酸钙进行了热分析和高温研究。热分析结果表明CaB₆O₁₀·5H₂O分四步脱水,在800℃处有一个相变放热峰,900℃附近出现一个吸热峰;化学分析、XRD分析等表明CaO₁₀·5H₂O在600℃保温2 h结晶水全部脱去,并转化为无定形的无水六硼酸盐CaB₆O₁₀,800℃非晶态CaB₆O₁₀向晶态转变并放出热量,900℃晶态CaB₆O₁₀分解为四硼酸钙CaB₄O₇和玻璃态的B₂O₃。基于热力学理论对CaB₆O₁₀-Mg体系绝热温度和反应吉布斯自由能进行了理论计算和分析。结果表明体系绝热温度高达2406.69 K,高温下CaB₆O₁₀分解为B₂O₃和CaO,体系中B₂O₃-MgO、B₂O₃-CaO、Ca₃（BO₃）₂-B₂O₃-Mg、B₂O₃-CaO-Mg等反应吉布斯自由能均小于0,都有可能发生。XRD分析表明燃烧产物中除CaB₆和MgO外,还存在副产物Mg₃B₂O₆和Ca₃（BO₃）₂,采用1+1盐酸煮沸30 min除去副产物可获得较纯净的CaB₆粉末。在热力学分析、差热分析、燃烧产物XRD分析等基础上建立了CaB₆O₁₀-Mg体系反应模型,探析了燃烧反应机理。工艺规律研究表明,增大原料粒径和制样压力有助于氧化还原反应的发生;Mg粉过量5-15wt.%,原料转化率有所提高,但过量超过15wt.%转化率急剧降低,最佳镁粉使用量为过量5wt.%;添加稀释剂会大大降低原料转化率,当添加量为40wt.%时燃烧反应不再发生;热爆温度高于600℃原料坯体才能被点燃,温度越高越利于燃烧反应的进行;制样时添加粘结剂会带走燃烧体系的热量,降低B₂O₃转化率。酸洗产物电镜分析表明CaB₆颗粒分散均匀,单体呈正方形,颗粒大小不均,一部分颗粒较大,一部分较小,小颗粒因表面能较大,吸附在一起聚集成絮状。更多还原

【Abstract】 As one of important metal-borides,calcium hexaboride（CaB₆） has been attracting much attention for its high melting point,high strength and high Chemical stability,as well as other outstanding properties such as low electronic work function,strong neutron-absorbing.CaB₆ is applied in refractory materials,metal deoxidant,neutron absorbent,wear-resisting agent and functional ceramic materials.Generally,CaB₆ is produced from B₄C or B₂O₃ which is obtained at high temperature and high cost.In this study,the CaB₆ powder was prepared through combustion synthesis by using calcium hexaborate and magnesium as starting materials.In order to gain calcium hexaborate with high quality,thermal analysis and the phase transition of CaB₆O₁₀·5H₂O at high temperature was investigated.It is indicated that CaB₆O₁₀·5H₂O was dehydrated by 4 steps from thermal analysis and there existed one exothermic peak near 800℃and one endothermic peak near 900℃,respectively.Chemical analysis,XRD,TG-DTA analyses showed that CaB₆O₁₀·5H₂O was dehydrated and inverted into amorphous CaB₆O₁₀ at 600℃,amorphous CaB₆O₁₀ transformed into a new anhydrous crystalline hexaborate CaB₆O₁₀ at 800℃,and crystalline CaB₆O₁₀ decomposed and formed CaB₄O₇ and vitreous B₂O₃ at 900℃.Based on thermal dynamic theory,the adiabatic temperature and Gibbs free energies of CaB₆O₁₀-Mg system were calculated and analysized.The calculated adiabatic temperature of CaB₆O₁₀-Mg system reached 2406.69 K and CaB₆O₁₀ was decomposed into B₂O₃ and CaO at high temperature.Gibbs energies of B₂O₃-MgO,B₂O₃-CaO,Ca₃（BO₃）₂-B₂O₃-Mg and B₂O₃-CaO-Mg are negative,so the reactions among them are all possible.The XRD analyses of combustion products showed that there existed a small amount of Mg₃B₂O₆ and Ca₃（BO₃）₂ besides CaB₆ and MgO.The XRD patterns of products leached by 1+1 HCl indicated that the major part of the leached sample is comprised by CAB₆,showing that MgO and by-products were efficiently leached out.The reaction model of CaB₆O₁₀-Mg system was proposed depending on thermal dynamic analysis,DTA,X-ray analysis of combustion products and so on.The mechanism of combustion reaction was discussed simply.The study on technological conditions was also investigated.Large particle size of CaB₆O₁₀ and high compacting pressure is beneficial to the production of CAB₆.The conversion of B₂O₃ increased with the excess quantities of Mg between 5-15 wt.%.However, the conversion of B₂O₃ in materials descends dramatically when the excess of Mg preponderates over 15 wt.%.The optimal raw material ratio is the excess quantity of Mg by 5 wt.%.The add-on of diluent would decrease the conversion of materials.When the adding quantity of MgO reached 40 wt.%,the combustion reaction could not occurred.Only when the explosive temperature is higher than 600℃the material could be iginited,and higher explosive temperature is helpful to combustion reaction.The add-on of binder would carry off the heat of CaB₆O₁₀-Mg system and decreased the conversion of B₂O₃.SEM photos of leached products showed that CaB₆ particles were uniformly distributed, and the single grain appeared square.However,the sizes of CaB₆ particles were not equality, some were lager,and others were too small.Many small CaB₆ grains agglomerated together like spong because of the large surface energy.更多还原