The Roles Of Density And Oxygen Concentration On The Structure And Mechanical Properties Of Silica Aerogel

The structure and mechanical properties of silica aerogels has strong dependence on the densityρand oxygen concentration.In this study,molecular dynamic simulations are performed to study the roles ofρand O/Si molar ratio(OSMR)onthe structure,tensile properties and compression properties of silica aerogels.This study explains the correlation between theρ,OSMR and the structure,tensile properties and compression properties of silica aerogel,which can further guide to tune the structure of silica aerogels in experiments.First,the silica aerogel models with sixρincluding 60 kg/m~3,100 kg/m~3,180kg/m~3,250 kg/m~3,320 kg/m~3 and 430 kg/m~3 are generated,and their structures were analyzed.The average pore size and porosity of silica aerogels increase with decreasingρ.The OSMR influences the structure by changing the forming ability of tenuous ligaments in silica aerogels.The forming ability of tenuous ligaments decreases with decreasing OSMR from 2.0 to 1.9;then increaseswith further decreasing OSMR to 1.5.The increasing tenuous ligaments in silica aerogel leads to the dispersed ligaments and small average pore size.Secondly,the silica aerogels with OSMR=1.5、1.6、1.7、1.8、1.9 and 2.0,ρ=100、180、250、320 and 430 kg/m~3 were performed uniaxial tensile simulations.The silica aerogels at 1.5≤OSMR≤1.7 have dispersed skeleton structure and the skeleton structure stretch,which cause the elastic modulus increase.The silica aerogels at 1.5≤OSMR≤1.7 have dense skeleton structure and the skeleton structure can’t stretch,as a result,the bonds are elongated and a large number of Si-O bonds break during tensile process.The main coordination form is Si(O)3 in silica aerogels at 1.5≤OSMR≤1.7 and Si(O)4 in silica aerogels at 1.8≤OSMR≤2.0.The length of the Si-O bond in Si(O)3 and Si(O)4 is 1.575(?)and 1.605(?),respectively.The length of the Si-O bond in Si(O)3 is less than that in Si(O)4,so the bond in Si(O)3 is difficult to elongate during stretching,resulting in a larger tensile stress of 1.5≤OSMR≤1.7silica aerogels.The silica aerogels of 1.5≤OSMR≤1.7 failed mainly by tensile force and 1.8≤OSMR≤2.0 by shear force.Finally,the silica aerogels with OSMR=1.5、1.6、1.7、1.8、1.9 and 2.0,ρ=100、250 and 430 kg/m~3 were performed uniaxial compression simulations.The silica aerogels at 1.5≤OSMR≤1.7 also have toughening effect in the compression,the mechanism is the same as the tensile test.Moreover,the silica aerogels at 1.5≤OSMR≤1.7 have the large stress,which is also due to the Si(O)3 as the main coordination form,the bond length is short and it is not easy to be compressed.During the failure process of compression,1.5≤OSMR≤1.7 silica aerogels fail mainly by pressure,and 1.8≤OSMR≤2.0 silica aerogels fail mainly by shear force.