Research On Surface Modification Of Ultrafine Silicon Carbide Powder And Slip Casting Of Recrytallized Silicon Carbide Ceramic

Recrystallized silicon carbide（RSiC）ceramic has been extensively applied in the fields of high-temperature load-bearing component,thermal exchanger and exhaust filter,due to its excellent high-temperature strength,corrosion resistance and thermal shock resistance.Owing to the special sintering mechanism,RSiC green body hardly shrinks during sintering process so that the mechanical properties of RSiC ceramic are directly related to the density of green body.Therefore,the key to produce high-performance RSiC ceramic is obtaining RSiC green body with high density.Slip casting is the most widely used molding method in industrial production of RSiC ceramic.Raw powders of RSiC ceramic are composed of coarse powder and ultrafine powder with significantly different particle sizes.However,there are many agglomerations in ultrafine SiC powder,resulting in its worse dispersibility.Therefore,to improve the dispersibility of ultrafine powder,surface modification is necessary for ultrafine SiC powder,which is the most effective method to increase the density of RSiC green body.Nowadays,China mainly imports modified ultrafine SiC powder with high performance from Saint-Gobain company in France,and the performance of domestic modified ultrafine SiC powder still needs improvement.Aiming at the above problems,this paper optimized the preparation process of high-density RSiC green body from surface modification of ultrafine SiC powder.Firstly,various modification schemes were used for surface modification of ultrafine SiC powder to improve its dispersibility,and the interaction mechanisms between modifiers and ultrafine SiC powder were revealed.Next,modified ultrafine SiC powder and coarse SiC powder were used as raw materials for slip casting of RSiC.The molding process of RSiC was optimized by adjusting coarse and ultrafine powders’particle gradation and slurry solid loading,and then dense RSiC green body with homogeneous microstructure was prepared.The main study contents and results of this paper were as follows:（1）Polyethyleneimine（PEI）was used for surface modification of ultrafine SiC powder.The adsorption behavior and conformation of PEI on SiC powder surface were revealed at different p H values.With the p H value increasing from5 to 8,the conformation of PEI on SiC powder surface changed from flat train to loop and tail so that PEI saturated adsorption capacity increased from 2.0974mg/g to 5.0589 mg/g,and then the wettability,maximum zeta potential and stability of PEI-SiC powder also increased.However,due to the difference in PEI adsorption conformation,PEI-SiC slurry with 3.0764 mg/g PEI adsorption capacity had the greatest flowability and the slurry viscosity was 157.0 m Pa·s at the solid loading of 50 vol%.The optimal modification process was obtained:p H value was 6.5 and PEI concentration was 0.5 wt%.The prepared PEI-SiC powder had good dispersibility.（2）Aluminum nitrate（Al（NO₃）₃）and sodium polystyrene sulfonate（PSS）were used for composite modification of ultrafine SiC powder.Effects of Al（NO₃）₃ concentration,PSS concentration,p H value and temperature on the viscosity of Al（NO₃）₃-PSS-SiC slurry were explored by orthogonal experiments.The optimal modification process was obtained:Al（NO₃）₃ concentration was0.15 wt%,PSS concentration was 0.3 wt%,p H value was 7.5 and temperature was 90℃.The prepared Al（NO₃）₃-PSS-SiC slurry had the viscosity of 34.5m Pa·s at the solid loading of 50 vol%.Adsorption studies showed the pretreatment of Al（NO₃）₃ could prompt PSS to absorb onto ultrafine SiC powder surface.PSS saturated adsorption capacity increased from 0.5492 mg/g to1.7531 mg/g.After modification,the wettability,zeta potential,stability and dispersibility of Al（NO₃）₃-PSS-SiC powder were improved.（3）Silane coupling agent KH550 and PSS were used for composite modification of ultrafine SiC powder.The optimal process of KH550preliminary modification was obtained:KH550 concentration was 0.65 wt%,p H value was 6.5,ethanol/water ratio was 70 vol%:30 vol%and temperature was 30℃.The optimal process of PSS second modification of KH550-SiC powder was obtained:PSS concentration was 1 wt%and time was 2 h.The prepared KH550-PSS-SiC slurry had the viscosity of 67.2 m Pa·s at the solid loading of 50 vol%.The preliminary modification of KH550 contributed to PSS adsorbing onto ultrafine SiC powder surface.PSS saturated adsorption capacity was 4.4729 mg/g.After modification,the zeta potential,stability and dispersibility of KH550-PSS-SiC powder were improved.（4）Sodium lignosulfonate（SL）was used for surface modification of ultrafine SiC powder.The optimal modification process was obtained:SL concentration was 0.5 wt%and time was 3 h.The prepared SL-SiC slurry had the viscosity of 41.7 m Pa·s at the solid loading of 50 vol%.Adsorption studies showed SL formed monolayer saturated adsorption onto ultrafine SiC powder surface and SL adsorption reached the equilibrium within 180 min.SL saturated adsorption capacity was 1.3263 mg/g.After modification,the zeta potential,stability and dispersibility of SL-SiC powder were improved.（5）Slip casting was carried out with the prepared modified ultrafine SiC powders as raw material.The dispersibility of four modified SiC powders was compared comprehensively by the viscosity of slurry and the density of green body.At p H value of 9,the green body prepared by KH550-PSS-SiC slurry with the solid loading of 55 vol%and the viscosity of 142.3 m Pa·s had the maximum density of 2.2549 g/cm³.Among four modified SiC powders,KH550-PSS-SiC powder had the optimal dispersion stability.Composite modification of KH550and PSS could improve the dispersion stability of ultrafine SiC powder in water in the maximum extent.Afterwards,the slip casting of RSiC was implemented with the prepared modified ultrafine SiC powder and coarse SiC powder as raw materials.At p H value of 9,RSiC slurry with the solid loading of 70 vol%and the viscosity of 636.0 m Pa·s was obtained when the ratio of KH550-PSS-SiC powder to coarse SiC powder was 2:3,and then dense RSiC green body with homogeneous microstructure was prepared.RSiC green body had the maximum density of 2.6603 g/cm³.