The Study Of SiC-ZCAS Composite Ceramic For Immobilizing Simulated Radioactive Graphite And Chemical Stability

The radioactive nuclear graphite(¹⁴C)were simulated by graphite(¹²C)in the research aiming at the treatment of high-radiation graphite waste.Si powder,graphite powder,SiO₂,Al₂O₃,CaO,and ZnO were used as starting materials.The glass additive of ZnO-CaO-Al₂O₃-SiO₂（ZCAS）was prepared by the melt-quenching method,and the high purity SiC-ZCAS composite powder was synthesized by the solid-phase reaction,the high-performance SiC-ZCAS composite ceramic was prepared by hot-pressing sintering technology.XRD,FT-IR,NMR,SEM,EDS were used to investigate the effects of ZnO content on the network structure,melting characteristics,and chemical stability of ZCAS glass.The results showed that ZnO is less than 0.5mol,the content of Q3 and Q4 increases,and Q3 will transition to Q4 with the increase of ZnO.Most Al was connected to[SiO₄]as[AlO₄],Zn will enter the network structure as[ZnO₄].When ZnO is higher than 0.5mol,excessive Zn starts to depolymerize the network structure to increase the content of NBO.When ZnO=0.5mol,the FT,and E_opt of ZCAS glass were 1449℃and 3.965eV,the network structure had fewer defects and denser,indicated its acid and water-resistance are optimal,the mass-loss rate is 0.035%,0.02811%and 0.006%in pH=3/5/7,respectively.When ZnO=0.3mol,the alkali resistance was reduced,the mass loss per unit area is 1.451*10^-4g/cm² and1.769*10^-4g/cm² in pH=9/11,respectively.XRD,SEM,TEM,EDS,and thermodynamic calculations were used to study the effects of Si/C molar ratio,synthesis temperature,holding time,and content of ZCAS additives on the composition and microstructure of SiC-ZCAS composite powder.The results showed that the optimal synthesis parameters of SiC-ZCAS composite powder were as follows:calcination temperature was 1350°C,holding time was 2 hours;Si/C molar ratio was 1.05:1.The incorporation of the ZCAS glass can reduce the size of SiC grain,and lead the equiaxed ofβ-SiC toward the rod-shapedα-SiC transform at the same time.Thermodynamic calculations showed that Si would react with SiO₂ to generate SiO vapors at 1155℃,and Si will evaporate at 1368℃through Si vapors,which will cause loss of Si.SEM,EDS,bulk density tester,Vickers hardness,thermal conductivity meter were used to investigate the effects of sintering temperature,sintering pressure,and ZCAS content on the relative density,hardness,thermal conductivity and microstructure of the（1-x）SiC-x ZCAS（x=25⁴0wt.%）composite ceramic.The results showed that the relative density,thermal conductivity,and Vickers hardness of the SiC-ZCAS increases with increasing sintering temperature and pressure.The relative density was increased,but the thermal conductivity was decreased,the Vickers hardness value increased first,and then decreased gradually when it reaches the maximum at ZCAS=30wt.%with increasing ZCAS content.The 0.7SiC-0.3ZCAS composites ceramic has choiceness comprehensive properties when sintered at 1550℃and 60 MPa for 1 hour,its relative density,Vickers hardness,and thermal conductivity were 95.5%,1084（HV 10）and 7.104W/m·k,respectively.Using SEM,EDS,ICP,ICP-MS,and PCT,MCC-1 static leaching method,the normalized leaching rate of each element in ZCAS glass and SiC-ZCAS composite ceramic at different pH and temperature were investigated.The results showed that the leaching rate of Ca was highest in acid and neutral conditions,the leaching rate of Si element was highest in alkaline conditions,and the leaching rate of Zn element in alkaline conditions was lower than that of acid and neutral.When ZnO=0.5mol,the glass has better anti-leaching performance,The normalized leaching rate of Ca,Al,Si,Zn were 10^-3g·m-²·d^-1,10^-3-10^-5g·m-²·d^-1,10-²-10^-3g·m-²·d^-1,10^-3-10^-4g·m^-2·d^-1 respectively after leaching 28d in 90℃、pH=3-11 conditions.SiC-ZCAS ceramic has excellent anti-leaching performance under the conditions of200℃,90℃and pH=3/7/11.Temperature and pH had a greater effect on the leaching behavior of each element in the ceramic,the leaching resistance of Ca is poor,and Zn was better.At 200℃,the surface of the ceramic has eroded,and the leaching rate of each element is higher than 90℃.The normalized leaching rate of Ca,Al,Si,Zn were 10^-5-10^-7g·m-²·d^-1,10^-5-10^-6g·m-²·d^-1,10^-5-10^-6g·m-²·d^-1,constant（below instrument detection limit）g·m-²·d^-1 respectively after leaching 35d in 90℃,pH=3-11 conditions.