Research On Fast Fabrication Of CaB₆ Ceramics By Self-propagating High Temperature Synthesis And Quick Pressing

Calcium hexaboride(CaB6), due to its particular lattice and electronic structure, has a wild and promising application in structural ceramics, composite, high-temperature materials, metallury, national defense industry, semiconductor and magnetic industry and so on. High purity and small size powders and dense bulk with low cost are the foundament of extensive application. This paper has adopted self-propagating high-temperature synthesis(SHS) and ball-milling methods to synthesize fine powders as well as two quilk sintering methods to fabricate dense bulk ceramics. The comparision of microstructure between different methods was also analysed in this paper.High-purity and homogeneous powders of calcium hexaboride (CaB6) with the small particle size were prepared by self-propagating high-temperature synthesis (SHS) and subsequent acid-cleaning. The phase of the SHS product and the reaction mechanism in Mg-B2O3-CaO system were analyzed; The combustion temperature curves under different molding pressures of the original SHS reaction body were measured; The effects of Mg content and atmosphere on the phase and morphology of the SHS product were discussed; The effect of molding pressure of the original SHS reaction body on the grain size of the final product was investigated. The results show that the argon gas atmosphere restricts the Mg volatilization, and a proper molding pressure favors the synthesis of the homogeneous fine powder. Furthermore, ball-milling method was another method to prepare fine CaB6 powders. And the microstructure and milling mechanism were investigated in this paper. In comparision, SHS is more suitable to prepare high purity and small size calcium hexaboride powders.As to bulk fabrication, Self-propagating High-temperature Synthesis/Quick Pressing (SHS/QP) is mainly used, which is an easy and quick way for ceramic preparation combing SHS and dynamic consolidation to fabricate full-density composites in a single processing operation. The influence of SHS system and weight on microstructure and pressure system on the relative density were discussed in this paper. It's demostrated that proper SHS system favors the maintainance of grain size; different pressure system of the same SHS system has pronounced effect on the relative density but the grain size. The dense bulk with >97% relative density was obtained in 300g SHS system at the pressure of 120 MPa. In addition, the microstructure and sintering mechanism of SPS technology were also investigated.The sintering mechanism and microstructure between SHS/QP and SPS were compared by SEM and Raman analytical method. Both of the above quick methods are liquid sintering mechanism. Compared with the SPS method, owning to instantaneous high temperature, the grain in the SHS/QP method quickly plastic flow and deform to achieve densification and has no extraordinary growth. Moreover the bulk has more residual stress and lattice distortion. But the grain in the SPS method can extraordinarily grow to colomnar under high temperature.