The Multi-scale Simulation And Experiments Of Silica-based Aerogels

As an important method in the fields of scientific research,computer simulation has become a helpful support for scientific experiments and theoretical research.Aerogels,seen as crosslink polymers,are a kind of nano-porous solid materials,whose network skeletons can span microcosmic,mesoscopic and macroscopic scales.Because of unique structures of aerogels,computer simulation is a suitable method to study their preparing principles,the process of structural formation and properties.In this paper,“Materials Studio” was used to model the diversified silica-based aerogels in molecular level.The network-structure models of different systems were analyzed.With the application of those structures,mechanical properties and thermal conductivities were calculated and compared with experiment al data.Mesoscopic simulations were also proceeded to study accumulational status of the solution.The vacuum impregnated aerogels composites were prepared and analyzed.A dynamic and cyclic method was adopted to construct the cross-linked structures of aerogels by using Perl scripts and COMPASS force field.Two ways were used to construct the three unitary systems of different densities,i.e.Si(OH)4(T-system),CH3Si(OH)3(M-system)and(CH3)2Si(OH)2(D-system).The better method was found and applied to model different binary systems consisting of M-system and D-system under different proportions.The results show that the reaction distances between –Si OH groups are 3 ?~4.5 ?.The bond lengths of cross-linked structures of different system remain relatively unchanged.With the increase of the quantity of-CH3 groups in systems,the conversions decrease.the ring structures was less,the entire system becomes looser due to the reducing of the cyclic structures.The strength of networks is weaker and the elasticity is higher.The agglomeration of gel particles in different solvents environments under mesoscopic scale were built based on the theory of dissipative particle dynamics(DPD).The calculated results in molecular level were used during modeling process.The results show that the gel particles are aggregated to form the ball regions in water solution,but homogenously dispersed in dimethylsulfoxide(DMSO)solution.With the increase of D-system,the agglomeration regions decrease.The gel particles tend to uniform distribution,when the repulsive force parameters between solute and solvent are approximate.The thermal conductivities of different aerogel systems were calculated based on the low density model and compact one by using non-equilibrium molecular dynamics(NEMD)theory.The temperature gradients are liable to convergent in the compact model,which benefits to obtain correct theoretical value of thermal conductivity.The results show that the calculated data matched well with experimental ones.The several different aerogel composites using M3D2 aerogel sample as the template were prepared by the method of vacuum impregnation.All of composites possess the microscopic characteristics of the temple aerogels and impregnating ones.With the increase of D-system,the gel particles of the impregnating aerogels change from the microscale to the macroscale.The specific surface areas and porous structures are controlled by the impregnating aerogels.