Effects Of Heat Treatment In Different Atmospheres On The Properties Of Hydrophobic SiO₂ Aerogels

Hydrophobic SiO₂ aerogel is a new type of nano-porous material,which is widely used in the field of thermal insulation.Hydrophobicity not only gives SiO₂ aerogels excellent waterproof properties,but also brings thermal safety risks to SiO₂ aerogels.Heat treatment is one of the measures to control the thermal safety of hydrophobic SiO₂ aerogels.At present,heat treatment is mainly carried out in air atmosphere,which is not conducive to maintaining its hydrophobicity.In this study,hydrophobic SiO₂ aerogels were heat treated in air atmosphere and argon atmosphere respectively,and the effects of heat treatment in two kinds of atmosphere were compared and analyzed,and the evolution of surface functional groups of hydrophobic SiO₂ aerogels was characterized by quantitative solid-state nuclear magnetic resonance（quantitative solid-state NMR）.The main research contents are as follows:1.Three kinds of hydrophobic SiO₂ aerogels modified by methylchlorosilane were treated in air atmosphere.The results show that when the heat treatment temperature exceeds 350℃,the silyl methyl on the surface of hydrophobic SiO₂ aerogel will disappear due to thermal oxidation,which can effectively improve its thermal safety,but also make it lose its hydrophobicity.When the heat treatment temperature is 800℃,the hydrophobic SiO₂ aerogel basically loses the three-dimensional network skeleton structure,the pore volume and specific surface area decrease sharply,the number of clusters increases obviously,and the density and thermal conductivity increase sharply,which makes it lose the characteristics of aerogel.2.Three kinds of hydrophobic SiO₂ aerogels modified by methylchlorosilane were heat treated in argon atmosphere.The results show that the functional groups on hydrophobic SiO₂ aerogels undergo thermal decomposition.Even if the heat treatment temperature is as high as 700℃,some silyl methyl remains on the surface,which makes it maintain good hydrophobicity.After heat treatment at 700℃,the peak heat flux of hydrophobic SiO₂ aerogels reached 590℃,which was about 300℃higher than that of untreated SiO₂ aerogels,and the gross calorific value decreased to about 2 MJ/kg.Compared with untreated SiO₂ aerogels,the thermal hazard of hydrophobic SiO₂ aerogels was reduced by about 70%.After heat treatment in argon atmosphere at 1100℃,Si-O-C ceramic fibers appeared in all three kinds of hydrophobic SiO₂ aerogels.3.The surface functional groups of hydrophobic SiO₂ aerogels were quantitatively measured by solid state nuclear magnetic resonance,and the changes of surface functional groups before and after heat treatment were studied.The test results show that the solid-state NMR technique could accurately measure the content of functional groups in SiO₂ aerogel.The elements on the hydrophobic SiO₂ aerogels are mainly silicon and oxygen elements,and the proportion of carbon and hydrogen elements is very small;when the heat treatment temperature reaches 600℃,the ethoxy groups on the three hydrophobic SiO₂ aerogels disappear completely,and the hydrogen element is only approximately 1%and the carbon element is only about 3%.Test and analysis show that the pyrolysis of silicon methyl group in argon atmosphere is a step-by-step process.