Study On Gel-casting Molding And Densification Mechanism Of Fused Silica Ceramics With In Situ Silicon Oxynitride Additive

With the rapid development of new energy and information industry,fused silica ceramics have become an important strategic material.In recent years,with the continuous attention and development of environmental protection energy,solar energy as a green energy has been widely valued by countries all over the world,and it has been widely developed and utilized.Quartz ceramic crucible is the key component of solar cell used in polysilicon ingot furnace.Therefore,the amount of large-size and thin-walled fused quartz ceramic crucible is also increasing rapidly.The production technology of quartz ceramic crucible is difficult,the purity of raw materials is high,and the use environment is harsh,and its mechanical properties limit the application of quartz ceramics in various fields,so it is of great significance to improve the mechanical properties of quartz ceramics and improve the compactness of quartz ceramics.As we know,fused quartz ceramic has many incomparable characteristics of other ceramic products,it has a wide application prospect,and it is a kind of ceramic material with great development potential.The design of this project is proposed to meet the preparation of high-performance fused quartz ceramics.By introducing additives into the slurry in situ and analyzing and studying the phase composition,microstructure,mechanical properties,dielectric properties,mechanical properties and thermodynamics,the fused quartz ceramics with high strength,high density and high reliability were prepared by using advanced gel forming technology,and the application range of fused quartz ceramics was broadened.(1)Fused quartz was prepared by dry grinding and wet grinding,and the effects of particle distribution and gradation on the rheological properties of fused quartz slurry and mechanical properties of ceramics were studied.The results show that although there is more silane alcohol on the surface of W-1 powder,it will lead to the increase of slurry viscosity and poor rheological properties.However,because wet ball milling has higher ball milling efficiency,it is more widely used in practical production.In the gradation experiment of powder particles,when the amount of ultra-fine powder is 15 wt.%,the ratio of vibrating density to loose density is the best,and the fluidity of powder filling is the best.According to the shear rate constant n fitted by Herschel-Buckley rheological model,the shear stress and relative viscosity reach the minimum when the amount of ultra-fine powder is 15wt.%.And when the amount of ultra-fine powder is too large,fused quartz is easy to crystallize.So,in this paper,it is considered that the amount of ultra-fine fused quartz powder should be controlled at 10?15 wt.%.(2)In order to obtain fused quartz ceramic slurry with high dispersion,high solid content and low viscosity,this chapter starts with the rheological properties of ceramic slurry and it mainly studies the following contents:the effect of PH value on the dispersion and stability of quartz slurry.The change trend of Zeta potential of quartz particles with different particle size with p H is basically the same,when p H=6,the absolute value of Zeta potential reaches the maximum,and the dispersion is the best.Through the calculation formula of DLVO theory,the force curve between particles of Si O₂ slurry at different PH values can be obtained.The calculated results of the forces between particles are consistent with the previous experimental results.The characterization experiment of dispersant on slurry dispersion was studied.Ammonium citrate was selected as dispersant,PH value of slurry was adjusted to 6,and 0.6 wt.%ammonium citrate was added to make the surface of Si O₂ powder have the maximum absolute value of Zeta potential.Therefore,stable dispersion can be achieved and slurry with high solid content can be prepared.The characterization experiment of solid content on the rheology of slurry was studied.When the solid content was 48 vol.%,the viscosity of slurry increased relatively slowly,the viscosity of slurry was lower,and the flow rate was better.The characterization experiment of ball milling process on the rheological properties of ceramic slurry was studied.With the increase of ball milling time,the viscosity of ceramic slurry decreased,the flow time decreased,and the fluidity became better.When the milling time is 6h,the viscosity and flow time of the slurry decrease to the lowest,and the fluidity is the best.(3)In the densification experiment of fused quartz ceramic billet,the effect of sintering temperature and holding time on the phase composition,surface microstructure and mechanical properties of quartz billet during sintering is studied.The crystallization mechanism of sintering process is analyzed from the point of view of kinetics.The following conclusions are obtained:Cristobalite precipitates from 1300?,and the bulk density increases with the increase of sintering temperature,while the apparent porosity decreases with the increase of holding time.Starting from 1450?,the phase transformation of cristobalite precipitated leads to microcracks on the surface of the material.When the sintering temperature is 1500?,the huge internal stress causes the spalling of the crystallization layer on the surface of the material.The precipitation of cristobalite has little effect on the relative dielectric constant of the material.When the sintering temperature is lower than 1300?,the precipitation of cristobalite is less,and the material value is mainly affected by the bulk density.When the sintering temperature exceeds 1300?,the crystallization amount of cristobalite increases significantly,which leads to the increase of internal stress,the deepening of surface cracks and even the spalling of surface quartz,which finally reduces the value of the material and affects the microwave dielectric properties of the material.After sintering at 1300?for 1 h,the sample has the best microwave dielectric properties at room temperature:?_r=3.72,Q×f=63000 GHz,?_f=–6.6×10^-6/?.Based on the Avrami model of isothermal phase transformation,the crystallization kinetics mechanism of fused quartz,crystal growth factor,crystal nucleation mechanism is the inhomogeneous nucleation type caused by surface structure defects,and the crystallization activation energy E is 928 k J·mol^-1.(4)Fused quartz ceramics were toughened by in-situ synthesis of auxiliaries Si₂N₂O by gel-casting molding.The effects of sintering temperature,holding time and Si addition amount on phase composition,porosity,microstructure and mechanical properties of Si/Si O₂ composite quartz ceramics were studied:with the increase of sintering temperature,the reaction rate of in-situ Si₂N₂O was accelerated,the porosity of SSi₂N₂O ceramics decreased from 45.8%to 11.8%,and the bulk density increased from 1.51 to2.52g/cm³.The mechanical properties of the composite quartz ceramics are also the best at1600?.The bending strength is 192 MPa,and the fracture toughness is 2.25MPa?m^1/2.When the addition of Si is 10%,the sintering temperature is 1600?and the holding time is 1 h,the Si₂N₂O grains grown in situ in the pores are fully developed,and the mechanical properties are the best,and the bending strength and fracture toughness are265 MPa,2.74MPa?m^1/2.If the sintering temperature continues to increase,the Si₂N₂O grains will decompose at high temperature,which will greatly affect the mechanical properties of the samples.The thermokinetics of the synthesis of in-situ Si₂N₂O auxiliaries was studied by non-isothermal thermal analysis.The composite fused quartz ceramic blank was prepared by injection molding method with 10%Si as additive.The composite fused quartz ceramic blank was sintered to 1500?in N₂ atmosphere.According to the thermal analysis curves obtained at different heating rates,the activation energy of reaction was obtained by Kissinger method and Kissinger-Crane method,the activation energy of reaction was E=263.7k J/mol,the frequency factor was A=2.069×10⁷m/s,and the reaction order was n=2.079.Finally,the frequency equation of reaction was obtained as follows:(d?)/(dt)=2.069×10⁷e^{-263.7×103/RT}(1-?)^2.079.