Surface Modification Of SiC Powders And Their Slurry Dispersibility In Water

SiC is a kind of very versatile inorganic non-metallic material.Direct Coagulation Casting is the best way to produce high-performance SiC materials.The key of the Direct Coagulation Casting Process is the preparation of a stable slurry with high solid content and low viscosity,but the domestic SiC can not meet requirements.This paper described the surface modifications of Si C powders by the method of acid/base solution washing,heat treatment in the presence of base,graft modification on the surface Si-OH groups.After surface modifications of SiC powders,the surface Si-OH concentration,elements changes and surface morphology change were determined,and in order to prepare SiC slurry with high solid content and low viscosity and explore their mechanism,the solid content and viscosity of modified SiC slurry in water were also determined.1.KF Titrino was used for the first time to measure the concentration of SiC surface hydroxyl groups directly.The RSD value of this method is only 3.8%,better than other methods.The method has the advantages of high precision and easy to overcome the external disturbance factors,and it is an effective method for measuring Si-OH on the surface of powders.The domestic SiC powder(average particle size of about 0.5?m)were washed by different acid or alkali solution for surface cleaning.Results showed that washed by HCl solution can increase the surface Si-OH concentration.After washing by 2 mol/L HCl,The surface Si-OH was found to increase to a maximum concentration(0.18%),28% more than original concentration.While NaOH solution washing can reduce the surface Si-OH concentration.The minimum concentration is about 0.11%,21% less than original concentration.The surface elements of SiC was also analyzed using X-ray energy dispersive spectra(EDS).Results showed that both the acid and base treatment can reduce the C/Si ratio on SiC surface,the most obvious change was observed after washing by 2 mol/L HCl,surface C content decreased by 29%,surface Si content increased by 44.1%.The surface carbon-silicon ratio was changed from 80.4/19.6 to 57.34/42.66.When the solid content of the SiC powder slurry was fixed to 10.7%,the viscosity of the acid/base washed SiC powder slurry was lower than that of the raw material.In the environment with a pH of about 8,the viscosity of the NaOH-treated SiC powder slurry is1.41 mPa.s,which is much smaller than that of the raw material powder slurry.The results showed that acid/base treatment could improve the dispersibility of SiC water slurry.Acid treatment can effectively improve the surface content of Si C powder surface,which is beneficial to the dispersion of Si C powder slurry.Alkali treatment can effectively reduce the surface hydroxyl content of SiC powder,thereby improving the dispersion of SiC powder slurry.The influence of acid/base treatment on the slurry of SiC powder was evaluated by measuring the viscosity of SiC slurry under the same mass fraction,and it was concluded that alkali solution treatment could lead to the better dispersion of SiC powder.2.The surface modification of silicon carbide was accomplished by heating with alkali.The results show that changing the heating temperature will cause the change of surface morphology.The photo shows that heating during 200-250 oC will cause the passivation of SiC powder particles.The best dispersion of SiC powder was achieved by heating at 250 oC with pH value of about 13.When the mass fraction is 50%,the viscosity value of SiC slurry is only 35.6 mpa.s,while the raw material SiC powder slurry viscosity is 905 mpa.s.Heating temperature changes will also cause the SiC powder surface element ratio changes,effectively remove the surface of the excess C element.The alkali treatment can reduce the oxygen content on the SiC powder.When the mass ratio of SiC to NaOH is 1/0.5,the surface oxygen content of the SiC powder is reduced to about 1.08%,about 42% of the raw material level.The change of the ratio of SiC to NaOH resulted in the change of the surface hydroxyl concentration.When the mass ratio of SiC to NaOH was 1/0.5,the silicon hydroxyl concentration on the surface of SiC powder decreased to 0.05%,which was about 45% lower than that of the raw material.The surface modification of SiC powder by heating with alkali can significantly affect the surface hydroxyl content,surface oxygen content,surface carbon and silicon ratio of SiC powder,and finally improve the dispersibility of SiC slurry.3.The surface grafting of SiC powders was carried out by the reactions of surface Si-OH with dicarboxylic acids.The SiC powder before and after heating with alkali was grafted by reacting with oxalic acid to give the SiC slurry with the solid content of 46 vol% and 41vol%(Viscosity 1 pa.s),respectively.The results show that the graft can be prepared directly with the raw material to prepare the better slurry.A slurry with a viscosity of 1 Pa.s and a solid content of 46 vol% was prepared by grafting SiC powder with oxalic acid.A slurry having a viscosity of 1 Pa.s and a solid content of 52 vol% was prepared by grafting the SiC powder with malonic acid.A slurry with a viscosity of 1 Pa.s and a solid content of 45 vol% was obtained by grafting SiC powder with succinic acid.A slurry with a viscosity of 1 Pa.s and a solid content of 46.2 vol% was prepared by grafting Si C powder with adipic acid.A slurry with a viscosity of 1 Pa.s and a solid content of 42 vol% was prepared by grafting SiC powder with pimelic acid.The grafting rates of oxalic acid,malonic acid,succinic acid,adipic acid and pimelic acid were 9.11%,15.18%,5.49%,23.7% and 14.58% respectively by potentiometric titration.The pH value of the slurry in different pH-based water dispersion medium was evaluated.The optimum flowability of the modified SiC powder was the grafted malonic acid.The solid volume fraction of the modified powder in TMAH pH = 13 dispersion medium was up to52vol%,has been initially to achieve the basic requirements of injection molding.In this paper,the grafting rate was quantitatively analyzed by potentiometric titration,and the grafting reaction was strongly demonstrated.The presence of Si-OH and the reactivity of surface Si-OH were also verified from the side.