Study Of Preparation And Properties For In Situ Fiber Reinforced SiO₂ Aerogel Materials

SiO₂ aerogel is excellent heat insulation materials with high porosity,low density,large specific surface area and other properties,however its weak three-dimensional network lead to poor mechanical properties,which greatly affect its application in many fields.Researchers mainly focused on improving SiO₂ aerogel mechanical properties by introducing reinforcement materials.The reinforcement can be used as a skeleton,effectively supporting the aerogel,inhibiting shrinkage,improving the block integrity,reducing cracks in aerogel during the compression process and improving its strength.In this paper,in-suit reinforced SiO₂ aerogel with only a little fiber was prepared by compounding some fibers and SiO₂ aerogel.The effect of different factors,such as the dispersant,fiber aspect ratio/content/kind,drying method,on the structure and properties of aerogel were studied.Eventually,the optimized result was obtained,which was hoped to have better mechanical strength and thermal insulation.Firstly,it was characterized of that the effect of carboxymethyl cellulosesodium,carboxymethyl cellulose,ceramic dispersant RY-57 and silica nano-powder（nP-SiO₂）on the fiber suspension in sol by sedimentation experiment.The dispersibility of each fiber was improved obviously by silica nano-powder（nP-SiO₂）.When 4wt%（1.6vol%）nP-SiO₂,the mixture settled 3.33%-6.67% after 30 minutes,which other three dispersants could not realize excellent suspension of fibers.The results showed nP-SiO₂ was beneficial to improve the dispersion of fibers in sol.Secondly,the nP-SiO₂/SiO₂ aerogel was prepared by ambient pressure drying and supercritical fluid drying respectively.The effect of nP-SiO₂ content on the structure and properties of SiO₂ aerogels was studied.With nP-SiO₂ content increasing,the shrinkage and cracks in aerogel decreased gradually,and the compressive strength improved first and decreased later.However,the shrinkage rate,integrity and density of the samples by supercritical dry were better than those by ambient pressure drying.When introducing 0.4vol% nP-SiO₂,samples by supercritical drying had the best mechanical properties.And the 0.4vol% nP-SiO₂/SiO₂ aerogel sample was 1.41 times compressive strength,0.76 time shrinkage and 1.16 times density than that of SiO₂ aerogel.The glass fiber（Gf）reinforced SiO₂ aerogels were prepared by ambient pressure drying and supercritical fluid drying,respectively.The effect of fiber aspect ratio,content of Gf and modification of gel on the aerogel was studied.The results showed that the introduction of Gf transferred the aerogels from brittleness to in certain elasticity.With Gf increasing,the specific surface area and average pore size of aerogel remained,but the shrinkage decreased and the compressive strength increased.When introducing 2vol% of Gf with aspect ratio 400,the volume shrinkage of sample by ambient pressure drying was the only 8.95%,which was minimum,and 0.87% volume shrinkage by the supercritical drying.The mechanical properties of samples by ambient pressure drying were far higher than that of samples by supercritical drying.The sample’s strength by the supercritical drying and hydrophobically modifying was better than those of unmodified samples.Its compressive strength was 2.92 times than that of unmodified samples,but this method should be used cautiously because of its detriment to the equipment.Finally,the effect of Gf,silicon carbide-fiber（SiCf）and carbon-fiber（Cf）reinforcing SiO₂ aerogel,dispersant and drying method on the structure and properties of the aerogel were studied respectively.The introducing of fiber improved the shrinkage and the opacity of silica aerogels.The aerogel with 2vol%Cf,had minimum shrinkage and density by ambient pressure and supercritical drying,and the opacity was similar to that of sample with SiCf,better than that of sample with Gf.The nP-SiO₂ greatly reduced the shrinkage of the fiber-reinforced aerogel materials,which maintain the sample surface dense and smooth,improved the opacity of the Gf-reinforced aerogel.Moreover,Cf improved the flexibility of aerogel,and the minimum elastic modulus was 0.2911 MPa.The strength of SiCf-reinforced SiO₂ aerogel material was improved greatly.When 0.025 strain of SiCf-reinforced aerogel by supercritical drying,the load reached 0.019 MPa and load of the sample also with nP-SiO₂ was as high as 0.021 MPa（1.40 times of SiO₂ aerogel）,which was larger than those of Gf and Cf sample.