Atmospheric Preparation And Properties Of Organohybrid Silica Aerogels

Silica aerogel is an emerging nano-functional material,which has broad application prospects in many fields due to its unique three-dimensional porous structure,low density and high porosity.However,due to its poor mechanical properties,complicated and time-consuming preparation procedures and high preparation costs severely limit the application range of silica aerogel.Therefore,how to obtain silica aerogel with good mechanical properties through a simple,safe and low-cost preparation method is an urgent problem to be solved.Introducing hydrophobic groups from the silica source is beneficial to reduce the number of Si-O-Si bonds in the aerogel skeleton and increase the compressible space of the aerogel skeleton,which makes it possible to transform the aerogel skeleton structure from rigid structure to flexible structure.In this paper,organosiloxane containing hydrophobic groups is directly used as the co-precursor to prepare organic hybrid silica aerogel by sol-gel method under the ambient pressure drying.The chemical composition of the obtained aerogel was analyzed by EDS and FTIR.The microstructure and pore structure of aerogel were observed by SEM and TEM.The contact angles,mechanical properties and thermal properties of aerogel were characterized by contact angle measuring instrument,universal material testing machine,synchronous thermal analyzer and thermal conductivity meter,respectively.By analyzing the influence of the number and type of organic groups on the structure and properties of organic hybrid silica aerogel,the silica aerogel with excellent properties are expected to be prepared by simple and low-cost preparation methods.The results of this paper are as follows:(1)Methyl-reinforced silica aerogel: Using tetraethoxysilane(TEOS)and methyltriethoxysilane(MTES)as precursors,after aging and solvent exchange steps,the monolithic methyl-reinforced silica aerogel was prepared under atmospheric drying conditions by acid-base sol-gel method.The influence of the molar ratio of precursors(n(TEOS): n(MTES+TEOS))on the properties of aerogel structure was investigated.The results show that the molar ratio of precursors significantly affects the pore structure of aerogel.With the increase of the molar ratio,the pore structure of aerogel changes from micron size to mesoporous structure.Finally,the number of pores decreases greatly due to volume shrinkage.When n(TEOS): n(MTES+TEOS)= 0.3,the aerogel exhibits a unique dual-mesoporous structure,in which the pore size of the large mesoporous structure is about 17-30 nm,and the pore size of the small mesoporous structure is about 3-6 nm.The structure can absorb both micron and nano-scale oil droplets at the same time,and possesses a good oil-water separation property for a stable emulsion with a separation efficiency of 97.6%.Moreover,the dual-mesoporous structure can also effectively block the conduction of heat insulation in the air inside the aerogel and in the aerogel skeleton,so the thermal conductivity of the aerogel can be as low as 0.0223 W/(m K).In addition,due to the presence of a large number of methyl groups,the methyl-enhanced silica aerogel exhibits excellent hydrophobicity and thermal stability.(2)Dimethyl-reinforced silica aerogel: Based on the methyl-reinforced silica aerogel system,dimethyldiethoxysilane(DEDMS)containing two methyl substituents was introduced as co-precursors,and the dimethyl-reinforced silica aerogel was prepared by simple sol-gel method without solvent exchange and surface modification steps.By adjusting the mole ratio of DEDMS to TEOS,the skeleton morphology of the obtained dimethyl-reinforced silica aerogel was accurately controlled,to explore the influence of skeleton morphology on the mechanical properties,hydrophobicity and adsorption capacity of the aerogel.The results show that with the increase of mole ratio,the morphology of the aerogel skeleton gradually changes from a fragment structure to a coarsened structure,and finally to a chain-like structure.For fragment structure aerogel and chain-like structure aerogel,a large number of weak connections in the skeleton become stress concentration points in the compression process,which makes the aerogel rupture when the compression strain is low.The aerogel skeleton with coarse structure does not have weak connection,so the compression ratio of the aerogel skeleton can reach 78.2%.The large number of hydrophobic groups and rough skeleton surface make the coarsened structure aerogel exhibits superhydrophobicity(contact angle up to 154.8°)and good oil-water separation property,and can be reused for more than 25 times without cracking.(3)Phenyl-reinforced silica aerogel: Based on the dimethyl-reinforced silica aerogel system,TEOS was replaced with phenyltriethoxysilane(PTES),and phenyl-reinforced silica aerogel was prepared by sol-gel method combined with ambient pressure drying.The effects of molar ratio of PTES to MTES(P)on the microstructure,mechanical properties,hydrophobicity,thermal stability and flame retardancy of the obtained aerogel were investigated.The results show that the addition of PTES can form a membrane-like structure on the surface of the original aerogel skeleton,which is helpful to strengthen the connection of aerogel skeleton and enhance the compressive resilience of aerogel.Compared with aerogels without PTES,phenyl-enhanced silica aerogel exhibits significantly improved properties(including mechanical properties,hydrophobicity and thermal stability),and possesses flame retardancy.The increase of P value is helpful for the transformation of aerogel skeleton from a granular structure to a non-granular structure.When P value is 0.12,the aerogel exhibits the best thermal stability,and the maximum decomposition rate temperature of the aerogel in air and nitrogen reaches 586.1? and 742.9?,respectively.In addition,the obtained phenyl-enhanced silica aerogel also exhibits excellent adsorption capacity(9.7-17.6 g/g)and oil-water separation ability,and can be reused by extrusion to remove oil pollution.This paper uses three simple and low-cost methods to strengthen the framework of silica aerogel and improve its mechanical properties,which provides a reference for the large-scale preparation and industrialized application of silica aerogel.The reinforced silica aerogel not only exhibit good agglomeration properties,it can be restored to its original state even after 80% compression strain,showing excellent mechanical properties.In addition,the silica aerogel possesses excellent hydrophobicity,thermal stability,flame resistance and adsorption capacity,which can be applied in fire resistance and oil-water separation.