Controlled Preparation And Properties Of Silica Aerogel Composites

As the earliest aerogel studied,silica（SiO₂）aerogel has the characteristics of ultra-low density,large specific surface area,low thermal conductivity,high porosity,etc.,and has promising applications in the fields of high and low temperature adiabatic/adiabatic cooling,waterproofing and thermal insulation,fireproofing,and adsorption.However,the poor mechanical properties and chemical inertness of pure SiO₂ aerogels greatly limit their applications,and compounding them with functional organic or inorganic materials is a common way to solve the above problems.This paper explores composites composed of SiO₂ aerogel with typical organic material polyvinyl alcohol（PVA）and typical inorganic material manganese dioxide（Mn O₂）.PVA has excellent mechanical properties and degradability,and previous studies have shown that the composite of PVA and SiO₂ aerogel can effectively enhance the mechanical properties of the composites,but existing studies often use halogen flame retardants and chemical cross-linking agents to solve the problems of flammability and hydrophilicity of PVA,which have a large impact on the environment.Mn O₂ has more crystalline and active chemical reaction properties,and composites based on Mn O₂ and mesoporous SiO₂ have been applied in the field of catalysis,however,the existing preparation methods are usually template methods with complicated preparation processes,uncontrollable sample morphology and single structure,which limit their scale production and effective performance enhancement.Based on this,this paper explores the environmentally friendly preparation technology of SiO₂ aerogel compounded with PVA and Mn O₂ and its application in the fields of flame retardant,fireproof and dye adsorption.The SiO₂ aerogel-polyvinyl alcohol（PSG）composites were prepared by co-blending,directional freeze-drying method and ambient thermal cross-linking method,and their microstructure,mechanical,thermal insulation,flame retardant and hydrophobic properties were investigated;the SiO₂-Mn O₂ aerogel composites were prepared by sol-gel co-redox reaction,hydrothermal and freeze-drying method,and their morphology,pore structure and methylene blue adsorption properties were investigated.The mechanism of SiO₂ aerogel in the composite system for thermal insulation,flame retardant,hydrophobic and adsorption properties was investigated.The specific research contents and conclusions are as follows.（1）Controlled Preparation and Properties of PSG composites:PSG composites were prepared by blending silica aerogel particles（SA）,glass fibers（GF）and PVA aqueous solution,using directional freeze-drying and thermal cross-linking methods,and the microstructure and physicochemical properties of the composites were investigated.The resulting PSG composites have a density of about～0.133 g/cm³,a specific surface area of～300 m²/g,and an oriented,micro-nano interwoven multistage pore structure（consisting of a PVA backbone with a micron-scale oriented porous structure,an SA/GF three-dimensional interwoven backbone,and an SA with a nanoscale porous structure）.The modulus of elasticity is 0.7～2.1 MPa,which can withstand the weight of more than 75,000 times of its own weight,and exhibits anisotropy of"hard"in the axial direction and"soft"in the radial direction;the thermal conductivity is 35.6 m W/（m·K）,which shows The thermal conductivity is 35.6m W/（m·K）,showing excellent thermal insulation performance in the radial direction than in the axial direction,and stable insulation at ultra-high temperature（150℃）and ultra-low temperature（-75℃）;thermal stability and flame retardancy are significantly improved compared with PVA,which can resist burning by high temperature flame（～940℃）,demonstrating excellent self-extinguishing ability and low heat release（HRR peak 146.5 k W/m²）and low smoke production rate（SPR peak 0.023 m²/s）;the water contact angle is 135.6 m W/（m·K）;with a water contact angle of 135°,it can resist common stains such as cola,milk,ink,coffee,vinegar and mud,demonstrating self-cleaning performance.（2）Preparation and adsorption performance of SiO₂-Mn O₂ aerogel composites:SiO₂-Mn O₂ aerogel composites were prepared by sol-gel combined with hydrothermal and freeze-drying techniques using water glass as the silicon source and divalent manganese salt（manganese acetate）and heptavalent manganese salt（potassium permanganate）as the manganese source.The obtained composite aerogel had a specific surface area of 99.2 m²/g,a pore volume of 0.446 cm³/g,and an average pore size of20.4 nm.The nanoporous SiO₂ phase was interwoven with multilayer stacked ultrathin Mn O₂ nanosheets to form a three-dimensional interpenetrating network,and it was found that SiO₂ in the composite system could promote the development of the Mn O₂phase to the weakly crystalline,δphase.The adsorption performance of the composite material on methylene blue（MB）was tested,and the maximum adsorption capacity reached 538.3 mg/g with a fast adsorption rate,reaching 82.9%of the saturation adsorption capacity in 30 min and complete adsorption equilibrium in 120 min;the adsorption capacity of SiO₂-Mn O₂ composite aerogel was high under strongly acidic（p H=1）or strongly alkaline（p H=13）environment;after alkaline ethanol desorption,the adsorption rate only decreased by 29.5%after 5 cycles of adsorption;the adsorption process was consistent with the pseudo-secondary kinetic model,including three stages of rapid adsorption on the outer surface,adsorption on the inner surface of wider mesopores and adsorption on narrower mesopores,in which the first stage dominated,and the adsorption process also followed the Freundlich adsorption isotherm model;the adsorption mechanism originates from the synergy of multiple modes of physical adsorption,electrostatic interactions,π-πstacking and hydrogen bonding interactions.In summary,in this paper,PSG composites with oriented multistage pore structure and excellent mechanical,thermal insulation,flame retardant and hydrophobic functions were prepared using green raw materials and processes,and SiO₂-Mn O₂ aerogel composites with better organic dye adsorption functions were prepared by in situ introduction of manganese redox reaction in the sol-gel stage of water glass,and hydrothermal and freeze-drying processes.This study is of positive significance to improve the inherent defects of silica aerogels and expand their functions and applications.