Preparation Of High Strength Fused Silica Ceramics

Due to their prominent properties of anticorrosion,thermal shock resistance and low thermal expansion coefficient,products of fused silica ceramics are widely used as structural and functional materials in many fields.The traditional preparation methods of fused silica ceramics mainly include slip casting and gelcasting routes.The process of slip casting is simple,but processing green body needs a long molding time.Although gelcasting process can shorten the forming time of the green body,the mechanical strength of product prepared by gelcasting is low.In the thesis,with the purpose of improve the overall uniformity of the product,a high solid phase,low viscosity,and good suspension stability of fused silica slurry has been studied.High strength fused silica ceramics were prepared using glecasting.Meanwhile,the overall uniformity of the product was improved significantly after avoiding the presence of weak microstructure with low density in fused silica ceramics prepared by traditional gelcasting method.In the thesis,the influence of raw materials grain composition,solid content,pH value,vacuum time,the type and amount of dispersants and swing time on the performance of fused silica slurry were investigated.During gelcasting,raw reagents of initiator and monomer,as well as processing parameters of ultrasonic vibration,curing temperature and calcination temperature were varied to prepare fused silica ceramics with high values of density and flexural strength.The samples were characterized by X-ray diffractometry and scanning electron microscopy,values of density and flexural strength were measured to determine the optimum processing conditions.The main conclusions were drawn as below:1.These are the results in the preparation of slip casting fused silica slurry.The different raw materials grain composition can reduce the viscosity of fused silica slurry,improve the bulk density and flexural strength of samples.When the content of fine particles was higher,the slurry has higher viscosity and poor fluidity.Increasing the content of the coarse particles,the viscosity of the slurry was lowered,but the excessive coarse particles were not conducive to the densification of the sample and the formation of the green body.When the raw materials grain composition A:B:C was 5:3:2,the slurry viscosity was 99.55 mPa?s and the bulk density reached 1.92 g/cm~3.The internal particles of the sample were densely packed.The viscosity of the fused silica slurry with different solid phase contents and the density of the green body after solidification molding were investigated.When the solid phase content was 84%,the viscosity of the slurry was 219 mPa?s,and the bulk density of the green body was 1.65 g/cm~3.As the pH increases,the absolute value of the zeta potential of the slurry was increased.When the pH value of the slurry was controlled between 4-5,the slurry had a large zeta potential value and the slurry was the most stable.As the vacuuming time of the slurry was increased,the bulk density of the green body first increases and then gradually stabilizes,and the optimum vacuum time was 15 min.The effects of dispersant polyacrylamide,carboxy methyl cellulose,lactic acid,acetic acid and lactic acid-polyacrylamide on the properties of fused silica ceramic slurry were investigated.It was concluded that the addition amount of polyacrylamide was 0.07 wt.%,the viscosity of the slurry was 425 mPa?s,and the settling height after standing for 24 h was 0.7 cm.When the swing time was 4 h,the viscosity of the slurry was 120 mPa?s,and the slurry had good fluidity.2.Fused silica ceramics were prepared by gelcasting.The curing time of slurry was shortened with the increase of initiator dosage and curing temperature.When the initiator was added in an amount of 0.15 wt.%and the curing temperature was increased to 70?,the curing time of the slurry was 33 min,and the curing time was suitable.When the slurry was ultrasonically vibrated for 15 min,the viscosity of the slurry was 270 mPa?s,the bulk density of the green body was 1.65 g/cm~3,and the particles distribution inside the green body was uniform.When the monomer(N-MAM)was added in an amount of 1.2 wt.%,the flexural strength of the green body was 14.03 MPa,and the strength of the green body was high,and the mechanical processing of the green body can be realized.When the sample was sintered,the heating rate between 200-600 was 3?/min,and between 600-sintering temperature was 6?/min.The optimum sintering temperature was 1180 C,holding time was 3 h,the values of bulk density and bending strength of fused silica ceramics were 1.83g/cm~3 and 43.62 MPa,respectively.The internal particles distribution of the sample was uniform and the bulk density was high.After sintering,the sample had no obvious crystallization peak and remained amorphous.