Study On Reinforcing Hydrophobic Silica Aerogels And Their Combustion Performance

Silica aerogels（SA）are regarded as a typical nanoporous insulation materials which have been widely appreciated in the field of thermal insulation.However,the practical applications are hindered due to the poor mechanical properties of silica aerogels.So improving the mechanical properties becomes the prerequisites for further developing various applications of silica aerogels.Currently,the primary silica aerogels are hydrophobic which is beneficial to maintain the outstanding performance for a long term.For hydrophobic silica aerogels,the organic groups introduced by surafce modification is not only the chemical fundamentals for the hydrophobicity,but also becomes the basic reason for the flammability.However,the potential fire risk of hydrophobic silica aerogels has not been recognized properly so far.Inversely,they are usually mistaken as noncombustible materials.Therefore,this work is devoted to exploit the mechanically improved silica aerogel insulation materials,study the flammability of hydrophobic silica aerogels and seek the methods to reduce the combustion performance of hydrophobic silica aerogels.This work mainly includes four parts as follow.From the perspective of silica aerogel themselves,tetraethoxysilane（TEOS）,methyltrimethoxysilane（MTMS）used as silicon sources,co-precursor silica aerogel insulation blocks were prepared under ambient pressure.The research found the keypoint of synthesizing co-precursor silica aerogels was to control the condensation rate of different precursors by adjusting NH3 H2O concentration.Besides,the research revealed the effects of NH3 H2O concentration on the microstructure and macroscopic properties of co-precursor silica aerogels and realized the regulation of their thermal properties.For further improving the mechanical properties of silica aerogel materials,aramid fibers and aramid pulps used as reinforcements respectively,the generated aramid fiber/silica aerogel composites obvously improved the elasticity and flexibility while the obtained aramid pulp/silica aerogel composites markedly enhanced the compressive strength.Simultaneously,the research provided new ideas for fabricating the high-performance fiber/silica aerogel composites.For the first time,the combustion characteristics of hydrophobic silica aerogels were studied systematically.The results showed that the ambient dried silica aerogels had smaller apparent activation energy and higher fire hazard than the supercritical dried silica aerogels.When the radiant heat flow exceeding 25 kW/m2,these two type silica aerogels were prone to flashover.The research broke the fallacy that hydrophobic silica aerogels were noncombustible materials,and pointed out that the organic groups introduced by surface modification is the origin of fire risk.Based on the pyrolysis and combustion characteristics,the combustion mechanism of hydrophobic silica aerogels is preliminarily revealed.On the aspects of reducing the flammability of hydrophobic silica aerogels,sodium silicate（SS）and phosphoric acid（PA）were used as silicon source and acidic catalyst respectively to prepare hydrophobic silica aerogels（SS/PA）in order to decrease the combustible components in silica aerogels.Compared to the silica aerogels prepared by TEOS as silicon source and hydrochloric acid（HA）as acidic catalyst,the thermal stability of SS/PA was distinctly improved and the tested results displayed that the fire risk was TEOS/HA>SS/HA>SS/PA.Furthermore,Al（OH）₃,Mg（OH）.,used as dopants,were introduced into siliac aerogels.The results showed that the thermal stability of doped silica aerogels was improved obviously and the gross calorific values and fire hazards of doped silica aerogels were clearly decreased,which indicated that the flammability of doped silica aerogels was lessened.These two strategies can effectively lower the combustion performance which were of great importance for improving the security of hydrophobic silica aerogels during usage.