Study On Preparation Of Calcium Hexaboride Powder By SHS

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.