Surface Modification Of Si₃N₄ Powder By Ball Milling And Its Dispersion In Water

The complex ceramic components required in the high-precision field are mainly prepared by ceramic colloidal molding technology.But the injection molding technology requires relatively high solid content and viscosity of the slurry:that is the solid phase content should not be less than 50 vol%,and the viscosity should not be more than 1 Pa.s.However,the solid content of the silicon nitride suspension produced at the current stage of the plant in China is difficult to directly satisfy the requirements.The research object of this paper is domestic silicon nitride.The powder is subjected to alkali treatment and surface stepwise graft modification treatment by ball milling method.The surface group,viscosity and solid volume fraction of the powder after treatment,slurry Zeta Potential changes,sedimentation conditions,and powder particle morphology were investigated.First:The Si₃N₄ powder was ball milled with NaOH,and the ball milling time and the reaction material ratio?mass ratio of powder to alkali?were changed.From the experimental results,the change of zeta potential of the powder slurry was closely related to the change of solid volume fraction.When the pH of the system is 13,the zeta potential and solid volume fraction of the powder in the suspension will reach the maximum.Suitable milling time can effectively improve the shape of the powder particles,obtain fine and uniform powder particles,which is beneficial to improve the performance of the powder.When the ball milling time is 20 h,the surface hydroxyl group of the powder is the least,and the maximum solid volume fraction of the powder slurry is the highest.After ball milling time is determined,the main factor affecting the surface hydroxyl content of the powder and the zeta potential of the slurry is the reaction material ratio.When the mass ratio of powder to NaOH is 1:0.5,the zeta potential of the powder slurry after treatment is significantly increased to 61.2 mv.The Si-OH content on the surface of the powder was also significantly reduced,which was 0.11%lower than that of the original powder.At this time,the maximum solid volume fraction of the powder slurry reached 44 vol%.The infrared spectrum showed that the characteristic absorption peak of-OH near 3600 cm^-11 disappeared after alkali ball milling,which was consistent with the Karl Fischer measurement.Second:The trimethylchlorosilane reacted with the silanol groups on the surface of the powder by ball milling,and the powder was reacted with trimethylchlorosilane by infrared detection of the powder.From the change of the maximum solid volume fraction of the powder slurry after the grafting reaction under different pH conditions,the highest point of the zeta potential of the system was lowered from 13 to 9 of the pH about the powder slurry compared with the pickling powder.According to the change of solid volume fraction of the suspension,the solid volume fraction of the powder after grafting is obviously improved compared with that of the 6 mol/L nitric acid pickled powder.When the ball milling time is 25 h,the solid volume fraction of the powder is 50 Vol%.From the scanning electron micrograph,the powder particles agglomerated after pickling,and the powder was uniform after grafting,and there was no agglomeration.Third:The dimethyl chlorosilane was initially grafted with the silanol group on the surface of the acid treated by the ball milling method,and further subjected to two-step surface grafting with acrylic acid,methyl acrylate and MSDS?ethyl methacrylate?.The optimum milling time was determined to be 15 h by measuring the hydroxyl groups on the surface of the powder after the reaction.It was found by infrared detection that the characteristic absorption of Si-H bond appeared near the wave number of 2100 cm^-1after the initial grafting,and the secondary graft had the stretching vibration of the carbonyl group around 1700 cm^-1,indicating that the two-step grafting was successful.After changing the pH conditions,it was found that the pH of the maximum solid content of the slurry dropped from around 13 to a pH of about 9.The maximum solids content of the slurry after two-step grafting of acrylic acid,methyl acrylate and MSDS was increased to 44 vol%,46 vol%and 50 vol%.Fourth:The dimethyldichlorosilane is initially grafted with the silanol group on the surface of the powder after acid treatment by ball milling,and then subjected to two-step surface grafting with the diacid substance.The optimum milling time was determined by measuring the change of surface hydroxyl content for 20 h.It was found by infrared detection that the characteristic absorption peak of methyl group appeared in the initial grafting,and the characteristic absorption peaks of hydroxyl group,silicon carbon bond and carbonyl group appeared after the two-step grafting.Compared with the powder after pickling treatment,the maximum solid content of the powder after grafting is lowered from 13 to 5about the pH.In terms of the change in solid content,the powder solid content after the stepwise grafting is significantly higher than that of the powder after the pickling treatment.In particular,the maximum solid content of the powder slurry after the two-step grafting of pimelic acid reached 52 vol%.Finally:The powders of different molecular weight polyethylene glycols?200,1500,4000,8000,10000?were grafted on the basis of preliminary grafting by ball milling to carry out infrared detection after the indicated extraction.The characteristic absorption peaks of-OH,-CH₃ and-CH₂-,C-O-C and Si-O-Si appear after grafting polyethylene glycol in two steps.Because of the overlap of the absorption bands of Si-N-Si and Si-O-Si,the modified nano-Si₃N₄ has a wider absorption peak and a stronger absorption at800¹100 cm^-1.When the pH value of grafted powder size was 5,the maximum solid volume fraction of powder size was the highest After grafting,the maximum solid volume fraction of the powder decreased to about pH 5,and the solid content increased with the increase of PEG molecular chain.The maximum solid volume fraction of the paste was 58 vol%after the powder was connected with peg 10000.Through the analys by EDS,it was found that the content of C element in the powder increased.It was found by sedimentation experiments that the rate of powder growth after grafting was significantly slowed down.From the morphology of the particles before and after grafting,the size of the grafted Si₃N₄ particles is relatively uniform,the agglomeration phenomenon is reduced,and the surface of the particles is coated with a thin film,which is relatively rough,probably due to the hydrophobic organic presence on the surface of the modified Si₃N₄ particles.