| Silicon nitride powder becomes the research object with its excellent physical and chemical properties as a kind of structural material.Colloidal molding technology is the best way to produce high-performance Si3N4 materials,especially gel injection molding and direct coagulation casting.This method is not only cheaper and higher reliability,and could shape various ceramic assembly unit.The key of the colloidal process is the preparation of a stable slurry with high solid content and low viscosity.?Powder slurry viscosity is lower than 1 pa.s,and solid volume fraction is more than 50vol%?.It is very difficult to directly satisfy the requirements of the powders produced by the manufacturers of silicon nitride powders in China.In this paper,domestically produced Si3N4 powder is taken as the research object,followed by pickling,alkaline treatment,and surface graft modification of powders.The hydroxy content on the surface of the powder after treatment,slurry viscosity and solid content,powder size,Zeta potential and powder particle morphology changes of the powder after modification are studied and determined.The powder modification method was investigated by studying the change of surface properties of the powder.Firstly,the initial surface washing treatment of Si3N4 powders with different concentration of acid and alkali solution was prepared.The results show that hydrochloric acid and nitric acid treatment basically have no good effect on the improvement of powder properties.The Si-OH content of the powder after treatment with different concentrations of hydrochloric acid and nitric acid increases,and the solid content decreases,which is not conducive to the improvement of the powder properties.Comparing the solid content of the powder slurry after the hydrochloric acid treatment,the effect after the hydrochloric acid concentration was 1 mol/L was the best,and the solid content of the slurry was 17.5 vol%.The change of the pH of the powder slurry has no obvious effect in the maximum solid volume fraction of the slurry.When the pH is 1314,the solid content of the slurry is 20.5 vol%,which is still lower than the solid content of the slurry after water treated in the same pH environment.The change of the solid content of the slurry after treatment with different concentrations of nitric acid was compared.The performance of the powder after treatment with 6 mol/L nitric acid was relatively good,but the solid content of the slurry was lower than that of the 1 mol/L hydrochloric acid treatment and the powder after wated treated.After adjusting the pH of the slurry,the solid content of the powder slurry after nitric acid treatment has obvious changes.When the pH of slurry is higher than 8,the solid content of the powder slurry after the nitric acid treatment is higher than that of the powder slurry after the hydrochloric acid treatment.And when the pH of slurry is higher than 13,the solid volume fraction of the powder slurry after nitric acid treatment?29vol%?is higher than the solid content?27 vol%?of the powder slurry after water treatment.From the change of powder surface element,the ratio of O/Si in the surface elements of the powder after nitric acid treatment is lower than that of the powder after water treated,and the N/Si ratio is increased.The oxidation degree of the Si3N4 powder surface is reduced and changing the pH of the slurry is conducive to the dispersion of the slurry.Secondly,this paper aims at modifying the surface of silicon nitride powders by NaOH treatment.By this means,the NaOH and nitride powders are fully reacted with each other by changing temperature,material ratio and reaction time.Silicon nitride powders were analyzed and tested by the powder slurry viscosity,the solid volume fraction,the Zeta potential,the content of Si-OH and the particle size.The results show that the maximum absolute value of Zeta potential appeared at pH is 13.NaOH treatment can improve the properties of silicon nitride powders.After modification,the viscosity of silicon nitride powders decreased,the maximum solid volume fraction increased,the absolute value of Zeta potential increased,the content of Si-OH was reduced on the nitride powders surface,and the particle size of powders decreased.When the heating temperature is 200?,the mass ratio of the powders and alkali is 1:0.1,and the reaction time is 2 h,the maximum solid volume fraction is reached to 53 vol%,and the powder slurry viscosity is 56 mpa.s with the pH of 13.This study provides a scientific basis for the preparation of the silicon nitride aqueous slurry with high solid volume fraction and low viscosity.Thirdly,using the activity of hydroxyl on the surface of the powder,carboxyl groups are added to the surface of the powder by reacting hydroxyl groups of the powder with short chain diacids to increase the charge density on the surface of the powder and improve the dispersion of the powder slurry.The Attenuated Total internal Reflectance Fourier Transform Infrared spectroscopy?ATR-FTIR?was used to determine the treated powder.The results of the spectrum analysis showed that the absorption peak near 3300 cm-1 and 1300 cm-1 to 1400 cm-1 was hydroxyl.The peak near the1600 cm-1 to 1700 cm-1 is the absorption peak of the carbonyl group.It is indicated that the presence of acid in the Si3N4 powder.At the same time,the peak near 2900 cm-1 is the asymmetric stretching vibrational absorption peak of methylene,and near 1050 cm-1 is the characteristic peak of Si-O.It showed that-OH on the surface of the powder reacted with the diacid species.Through the measurement of the solid content of the slurry,it can be seen that the highest point of the zeta potential of the powder slurry after grafting with the diacid species is changed,and the solid content of the slurry runs highest when the pH ranges from 5 to 6.The effect of the powder on grafting with adipic acid and pimelic acid is good.The maximum solid content of the powder slurry is 27.5 vol%and 38 vol%respectively.Finally,according to the change of the solid content of the powder after grafting with the diacid,it can be known that the effect after reacting only with the diacid is unsatisfactory,so the powder is further subjected to two-step grafting of dichloromethylsilane and diacid.Hydrophobic silicon-oxygen-silicon bond is formed on the surface of the powder by the reaction of hydroxyl groups and dichloromethylsilane on the surface of the powder,and then another silicon-chlorine bond is used to react with the diacid to exert the dual stabilization mechanism of the powder particles.In the first step of dichloromethylsilane grafting,it can be determined from the change of -OH content on the surface of the powder that the best ball milling time is 5 h.The second step of grafting shows that the solid content of the powder has been significantly improved.Especially when grafting adipic acid and pimelic acid in two steps,the maximum solid content of the powder slurry can reach 51 vol%and 53 vol%respectively. |
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