The Study Of SiC-MgAl₂O₄ Composite Ceramic For Immobilizing Simulated Radioactive Graphite

For the safe curing of high-radiation graphite,graphite(¹²C)is used to simulate radioactive graphite(¹⁴C)in this paper.Silicon carbide-magnesium aluminum spinel?Si C-Mg Al₂O₄?composite was synthesized by solid-state reaction using graphite powder?C?,silicon powder?Si?,aluminum hydroxide?Al?OH?₃?and magnesium oxide?Mg O?as raw materials.The Si C-Mg Al₂O₄ceramics were prepared by vacuum pressureless and hot-pressing sintering.X-ray diffraction?XRD?,field emission scanning electron microscopy?FESEM?,differential thermal analysis?DSC-TG?,high resolution transmission electron microscopy?TEM?,energy dispersive spectroscopy?EDS?,X-ray fluorescence analysis?XRF?,infrared spectrum?FTIR?,Raman spectrum?Raman?and other analytical testing methods were adopted to investigate the effectsofthecalcinationschemes,molarratiosofrawmaterials,non-stoichiometric Mg Al₂O₄ and ball-milling process on the preparation,the phase compositions and microstructure of Si C-Mg Al₂O₄ composite.The results showed that the optimal synthesis parameters of Si C-Mg Al₂O₄ composites were as follows:calcination temperature is 1370°C,holding time is 2 h;Si/C molar ratio is 1.3:1;molar fraction of Mg Al₂O₄ is 20%,the molar ratio of Al/Mg is1.7:1;Mg Al₂O₄ is ball-milled for 1 h;the mixing time of C and Si is 5 h;the mixing time of C,Si and Mg Al₂O₄ is 6 min;the mass ratio of pellets is 1:3,and the rotation number of the mill was 350 r/min;Si-Mg interface reaction has a negative effect on the synthesis of Si C-Mg Al₂O₄ composites,and the interface reaction intensity is positively correlated with the reactivity of silicon source and magnesium source.It was showed that the reactivity of elemental silicon and Mg O is stronger than that of Si C and Mg Al₂O₄;Ce³⁺,P⁵⁺,Zr⁴⁺,and Ti⁴⁺easily react with graphite or silicon,which inhibits the synthesis of Si C.Finally,the higher purity Si C-Ca Zr Ti₂O₇-Ca Ti Si O₅,Si C-Ce PO₄,Si C-Zr Si O₄,Si C-Zr O₂composites can not be synthesized.Using XRD,FESEM,EDS,Vickers hardness tester,ceramic bulk density tester,thermal conductivity meter and other analytical means,the phase compositions,microstructure,density,hardness,thermal conductivity of Si C-Mg Al₂O₄ ceramics solidified body prepared by vacuum pressureless sintering and hot-pressing sintering were investigated.The results showed that when Si C-Mg Al₂O₄ ceramic was prepared by vacuum pressureless sintering,for the synthesis of Si C powder:the optimal molar ratio of Si/C was 1.3:1,and the optimal synthesis condition was calcined at 1330°C for 2 h;the addition of Mg Al₂O₄ could effectively improve the sintering of Si C ceramic and reduce the sintering temperature;With proper pressureless sintering temperature and holding time,Si-Mg Al₂O₄ interfacial reaction with certain strength can be obtained.At the same time,a certain amount of glass phase generates,which is beneficial to the sintering of Si C-Mg Al₂O₄ ceramics;the preferred hot-pressing sintering condition for Si C-Mg Al₂O₄ solidified body is 1400°C for 2 h;during the hot-pressing sintering,the interfacial reaction between residual Si and Mg Al₂O₄ is unfavorable for sintering,resulting in a low density of the solidified body.Finally,micro-cracks on the surface of Si C-Mg Al₂O₄ ceramics appears and the thermal conductivity is low.MCC-1 static leaching method was adopted to study the stability of Si C-Mg Al₂O₄ ceramic in deionized water at 90°C and p H=7 by using XRD,SEM,ICP.The results showed that the hot-pressing sintered Si C-Mg Al₂O₄ceramics have good chemical stability.The average normalized leaching rate of element Si,Al and Mg in deionized water at 90°C and p H=7 are respectively0.4270,0.1886,0.3257 g·m^-2·d^-1.