| Nano-porous SiO2insulation materials were made from three-phase foamedslurry by filter-pressing. The foamed slurry mainly consists of hydrophobic fumedsilica, xonotlite crystal whisker, guartz fiber and water. Certain amounts of waterand gas in the foamed slurry were eliminated before filter-pressing. And the formedsample was dried in the air. A hydrophobic fumed silica particle is a porousaggregate containing many nano particles. The diameter of the silica aggregates isabout10-20micron. Xonotlite crystal whiskers synthesized by dynamichydrothermal process overlap each other in water forming a interwoven porousstructure. Such a structure is important for the fact that xonotlite crystal whiskerencapsulates the hydrophobic fumed silica particles to form a stable three-phasefoam. Besides, the xonotlite crystal whisker, quartz fiber and alumina sol togetherenhance the nano-porous SiO2insulation materials.The main content in the paper includes: process of water based slurry, processof three-phase foamed slurry, concentration of foamed slurry, filter-pressing ofconcentrated foamed slurry, and insulating performance of the nano-porous SiO2insulation materials. In the process of water based slurry, APAM is used as thedispersant agent. The effect of APAM on dispersion of the xonotlite crystal whiskerand quartz fiber in water was studied. The results show that with severe stirring thewater based slurry with mass ratio (xonotlite crystal whisker/quartz fiber) about0.4%/5%can disperse uniformly when the APAM content is about0.06%.Meanwhile, SDBS blister has no significant influence on dispersion of the waterbased slurry.In the process of three-phase foamed slurry, the foaming of SDBA/APAMsolution, the stability of the three-phase foamed slurry with SDBA/APAM solutionand the hydrophobic fumed silica, and the three-phase foamed slurry with the waterbased slurry and the hydrophobic fumed silica were studied. It was found thatsynergism of APAM with SDBS can enhance forming performance of the SDBA/APAM solution. When SDBS/APAM in the solution is about1.2%/0.06%thesolution show the best foaming performance. Same weight of the water based slurryand the SDBA/APAM solution can encapsulate the same weight of the hydrophobicfumed silica. But the three-phase foamed slurry with the water based slurry and thehydrophobic fumed silica is apparently more stabile than that with the SDBA/APAMsolution and the hydrophobic fumed silica. As increasing of the xonotlite crystalwhisker in the water based slurry, the three-phase foamed slurry becomes much morestable. It can be seen that the hydrophobic silica particles are encapsulated in thebubble, and that the hydrophilic xonotlite crystal whisker is dispersed in the liquidmembrane of bubbles in the three-phase foamed slurry. The hydrophilic xonotlitecrystal whisker can adsorb a great amount of liquid to encapsulate the hydrophobicsilica particles. The hydrophilic xonotlite crystal whisker overlap with thehydrophobic silica particles, and the movement between them is restricted.In the course of vacuum filtering, the foamed slurry is concentrated, and thelarge bubbles eliminated by replacing of the large bubbles with many fine bubbles.As increasing solid content in the three-phase foamed slurry, the apparent viscosityof the slurry increases quickly. All of these facts are helpful to improve the stabilityof the foam. At low shear rate, the apparent viscosity of the concentrated slurrydecreases remarkably. At the same time the pourability of the slurry increases, andthe slurry can easily fill the filter-pressing mould. In the course of filter-pressing,part of liquid and gas is discharged, and the large pores among the hydrophobicfumed silica and xonotlite crystal whisker transfer into small pores and disappearfinally. The pore diameters in sample of nano-pore SiO2insulation materials withthe density about0.25g/cm~3mainly are from100to200nm. The pore diameters insample with the density ofabout0.35g/cm~3mainly are from50to150nm. The porediameters in sample with the density o about0.45g/cm~3mainly are around10and30nm.Thermal insulating performance is evaluated by quickly hot plate and thethermal conductivity is also tested. It was found that with increasing of xonotlitecrystal whisker in samples the thermal insulating performance decreases. But the xonotlite crystal whisker improves the strength of samples. In a typical sample theamount of xonotlite crystal whisker is about30wt%of the hydrophobic fumed silica.The typical sample, named NP0, has low thermal conductivity at lower temperatures.However, because of the high radiation transmittance for wavelength from1to5micron, the thermal conductivity of NP0exponentially increases when radiationplays a principal role at high temperatures. To decrease the radiation in nano-porousSiO2insulation materials, some ceramic powder with large absorption or scatteringcoefficient can be used as opacifier. The amount of the opacifier in nano-porous SiO2insulation materials should be optimized for obtaining the best insulatingperformance. The optimized amount of addition of graphite, titania and zirconia areabout20%,25%and35%, respectively.Through filter-pressing of foamed slurry, flexible, rigid and multilayernano-porous SiO2insulation materials are fabricated. In research of nano-porousSiO2multilayer thermal insulation materials, the aluminum foil and flexible graphitepaper are used as heat screen. It was found that nano-porous SiO2multilayer thermalinsulation materials get the best thermal performance when the thickness ofnano-porous SiO2layer in the multilayer thermal insulation is about2millimeters.These nano-porous SiO2insulation materials with different mechanical property canbe used in various fields as high performance insulation materials. |
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