Microstructure Control And Performance Study Of Silicon Carbide Ceramic Membrane

Silicon carbide?SiC?ceramic membrane has an asymmetric gradient structure composed of a support and a membrane.Owing to its high temperature stability,low thermal expansion coefficient,high thermal conductivity,good chemical stability,and excellent mechanical properties,it is widely us ed in high temperature and severe corrosion environment,such as metal melt filtration,hot gas filtration,filtration in chemical production and so on.Based on the pretreatment of SiC raw material powder,the low temperature activation sintering and sintering mechanism of SiC porous support were studied.The microstructure control of SiC ceramic membrane and co-sintering of SiC ceramic membrane and their filtration performance were investigated.The main results are shown as following:?1?The commercial 4000#and 600#SiC powders were used as raw materials,and the powders with spherical sphericity of 0.78 and 0.94 were successfully obtained after treatment at 1750°C and 2050°C,respectively.Spheroidization is mainly carried out by surface diffusion.After spheroidization,the packing style of particles was changed and was attributed to tighter stacking,resulting in a narrower pore size distribution and higher mechanical properties.The bending strength of the4000#SiC sample sintered at 1800°C and the 600#sample sintered at 2200°C was increased from 31 MPa and 23 MPa to 67 MPa and 35 MPa,respectively.?2?The 600#SiC powder and 10000#SiC powder were used as the main raw materials and Al₂O₃-Y₂O₃ was applied as sintering aids.The effects of Al₂O₃-Y₂O₃content and sintering temperature on microstructure,phase composition,mechanical properties and corrosion resistance of SiC porous ceramics were investigated.The results showed that Al₂O₃-Y₂O₃ promoted the sintering of SiC porous ceramics by forming a liquid phase during sintering.It promoted the migration of fine particles to the neck between the coarse particles to form a neck.SiC porous ceramics could form a connected porous structure above 2000 ^oC,and the bending strength increased with the increase of sintering temperature and additive content.When the sintering temperature is increased above 2000°C,the Al ₂Y₄O₉ phase could be transformed into the Y₂O₃ phase and the weight loss occurd.This indicates that Al₂O₃ could react with SiC to form vapor phase SiO,Al₂O and CO.It is beneficial to reduce residual sintering aids and phase of grain boundary,which improves the corrosion resistance of SiC porous ceramics.After corrosion in hot acid solution and hot alkali solution at 90 ^oC for 150 h,the strength of SiC porous ceramics remained at 74%and 94%,respectively.?3?600#SiC powder was used as the skeleton and 10000#SiC powder was used as the binder phase.Besides,B₄C was applied as the sintering aids.The effects of sintering temperature and B₄C content on the properties and microstructure of SiC porous ceramics were investigated.The nucleation and growth mechanism of 6H-SiC transition to plate-like 4H-SiC crystals were discussed.Both the density and the bending strength increased with increasing sintering temperature and B ₄C addition.The conditions for the formation of plate-like 4H-SiC grains are that the sintering temperature is higher than 2200°C and the amount of B₄C addition is greater than0.5%.B₄C is doped into the SiC crystal in the form of a solid solution,which decreases the activation energy of Si and C,thereby transforming its mass transfer mode from the surface diffusion of pure SiC to the lattice diffusion promoted by B ₄C.The doping of B₄C disturbs the energy state structure of 6H-SiC and makes it unstable,which promotes 4H-SiC nucleation.The Si and C atoms diffuse from6H-SiC to the B-rich area containing the crystal nucleus of 4H-SiC,resulting in the growth of 4H-SiC crystals.The strength of solid-phase sintered SiC porous ceramics remained over 92.5%after corrosion for 200 h in hot acid and hot alkali solution at90 ^oC,respectively.The corrosion resistance of SiC porous ceramics is excellent.?4?Using 600#and 10000#SiC powder as raw materials and B₄C as sintering aid,the effects of the ratio of coarse powder to fine powder as well as phase transformation on the microstructure of SiC porous ceramics were studied.The pore size and distribution of the SiC porous ceramic can be controlled in a certain range by adjusting the ratio of the coarse to fine particles.When the coarse particle content was increased from 50%to 80%,the density of the SiC porous ceramic was first increased from 2.03 g/cm³ to 2.06 g/cm³,and then gradually decreased to 1.96 g/cm³.The average pore size of the SiC porous ceramic was increased from 7.9?m to 9.1?m,and the pore size distribution was gradually narrowed.The addition of B₄C had a little effect on the porosity of SiC porous ceramics.The pore size and distribution of SiC porous ceramics can be more effectively regulated by the morphology change of SiC particles induced by B₄C.?5?SiC powder and polymethyl methacrylate?PMMA?was used as raw material and pore formers,respectively.The effects of pore former on the porosity,mechanical properties and microstructure of SiC porous ceramics were investigated.The effect of silica sol addition on the strength of SiC porous ceramics after debinding was discussed.The PMMA was applied as a pore-forming agent to achieve adjustable porosity in the range of 38⁴9%.SiC porous ceramics with a porosity of nearly 70%were preliminarily prepared by gel injection molding.?6?120#coarse SiC powder was used as the skeleton of the support,and 10000#submicron SiC powder as the skeleton filling.B₄C was used as the sintering aids,and the 600#spherical powder as the membrane raw material.SiC porous ceramic membrane with two layers structure was successfully obtained by a co-sintering process at 2200 ^oC after dip coating on the surface of the compacted green body of support.The addition of 1.5 wt%B₄C can decrease the sintering temperature of support from 2350?C to 2200?C,thereby achieving uniformity of the sintering temperature of the membrane and the support.The SiC ceramic membrane prepared by co-sintering process at this way had a nitrogen flux of 19406 m³/m²·h·bar.