Fabrication And Thermal Insulation Properties Of Silica Nanofiber Based Aerogels With Superelasticity

Traditional silica aerogels are attractive candidate for applications in thermal insulation,absorption,and catalysis,due to their appealing characteristics of nano-sized porous structure,high porosity,and high specific area.However,the intrinsic fragile nature of“necklace-like”network consist of nanoparticles also leads to the brittleness of silica aerogels,which has greatly limited their practical application.Nowadays,one of the most effective methods to improve the structural stability of silica aerogels is combining them with polymer binder or commercial microfiber felt.However,the introduction of polymer binders will block the nano-sized porous structre of the aerogel,resulting in poor thermal insulation performance and temperature resistance of the material.And the microfiber felt-based aerogels still suffer from poor mechanical properties,large bulk density,low thermal insulation performance,and the severe powder falling during practical use.To thoroughly address these issues,a series of ceramic fibrous aerogels composed of continuous 1D fibrous materials such as nanowire,nanotube,nanoribbon,and nanofiber were developed,by direct spinning,layer-by-layer stacking,template and self-assembly.In dynamic contrast to the brittleness of traditional silica aerogels,these ceramic fibrous aerogels are compressive and can bear cyclic compressive tests.However,these materials have poor structural adjustability,and the mechanical and thermal insulation properties cannot meet the actual application requirements.Therefore,many efforts dedicated towards developing ceramic aerogels with excellent mechanical properties has been emergened.In this paper,we fouced on the fabrication of silica nanofiber-based aerogels with both excellent mechanical properties and thermal insulation performance.A varity of silica nanofiber-based aerogels with different structures were developed by freeze-drying method and physical/chemical bonding structure construction.And the relationship between the multi-level structure and the thermal insulation and mechanical properties of the aerogels were systematic investigated.Moreover,silica nanofibrous aerogels with both superior bendability and compressibility were developed,by assembling flexible silica nanofibers with a high length-to-diameter ratio into an unique interweaved cellular structure.The main research results are summarized below:（1）We have presented a feasible,synergistic assembly strategy to fabricate silica/carbon dual-fibrous aerogels by using sustainable cellulose nanofiber as dispersant and carbonaceous source.The resultant aerogels exhibited unique honeycomb-like fibrous cellular structure,and the structure stability of the aerogel was enhanced by the ultrafine carbon fibrous network.The resulatant aerogels exhibited an ultralow thermal conductivity of 24.52 W m^-1 K^-1,which could be attributed to the high porosity and the multi-channel heat conduction path of ultrafine carbon fribrous network.In addition,the aerogels showed neglectable plastic deformation after the 500-cyclic compressive recovery test at a large compressive strain of 80%,indicating the superelasticity of the silica/carbon dual-fibrous aerogels.Furthermore,in contrast to the poor fire resistance of cellulose aerogels,the aerogels obtained in this work exhibited excellent fire resistance.（2）We designed and synthesized a hierarchical cellular structured silica nanofibrous aerogel by using electrospun silica nanofibers and silica aerogels as the matrix and silica sol as the high-temperature nanoglue.This pathway leads to the intrinsically random deposited silica nanofibers assembling into stable fibrous cellular structure,which enabling the materials with excellent compressive properties.Meanwhile,the nano-sized porous structure of silica aerogels could effectively improve the thermal insulation performance.The resultant composite aerogels exhibited ultralow thermal conductivity(23.27 m W m^-1 K^-1),large compressive-recovery strain of 80%,and superior 1000-cyclic compressive performance.（3）Superelastic silica nanofibrous aerogels were used to enhance the mechanical properties of silica aerogels,by immersing the nanofibrous aerogels into silica sol.After the following condensation,solvent exchange,and ambient-pressure drying,binary-network structured silica nanofibrous-based aerogels were obtained.The resultant aerogels exhibited temperature-invariant superelasticity and superior fatigue resistance performance during 1×10⁵ compression-recovery cycles tests.Moreover,binary-network structured silica aerogels exhibited excellent high-temperature thermal insulation performance,when exposed the frontside of a piece of aerogel（thickness:20 mm）to the flame of butane blowtorch,the temperature of the backside was below75℃.In addition,a series of 3D simulations was carried out by using Fiber Geo and Conducto Dict software to investigate the relationship between the thermal insulation performance and binary-network structure of the aerogels.（4）Silica nanofibrous aerogels with improved structural continuity were designed and fabricated by assembling silica nanofibers with high length-to-diameter ratio into interweaved cellular structure.The obtained aerogels exhibited enhanced mechanical properties including large compression and buckling strain recovery（85%）,temperature-invariant superelasticity,and robust fatigue tolerance up to 100,000 cycles.In parallel,the bending behavior of a single silica nanofiber was investigated by using focused ion beam-scanning electron microscopy,indicating that the silica nanofiber is flexible.And the deformation of the fibrous cell wall was investigated in situ by SEM.Overall,the flexibility of silica nanofiber,together with the continuity and flexibility of cellular structure enhanced the bendability of aerogels.In addition,when protected by a piece of aerogel with a thickness of 1 cm,the temperature of the outer layer of the 700°C high temperature pipeline is below 110°C,indicating the excellent high-temperature thermal insulation performance of the aerogel.