Research On Molecular Dynamics Simulation Of Composite Aerogels And Heat Transfer Model

Aerogel is a nano-porous lightweight insulation material with high porosity,high specific surface area and other excellent properties.It can be widely used in energy-saving construction,aerospace,catalytic adsorption and other fields.In this paper,the self-assembly processes of nano-porous structure of aerogels were simulated by molecular dynamics method,and the structures and properties of aerogels with different density were compared and analyzed.Then the thermal conductivity of aerogels was obtained by establishing heat transfer model of units,and the characteristics of heat transfer were analyzed.Firstly,to study the self-assembly process of nano-porous structure of SiO₂aerogels,the models were dynamics calculated in atomic scale.And energy,nano-porous structure,the binding force between atoms and mechanical property were analyzed in different systems.The results show that van der waals interaction of non-bonding dominates the self-assembly process.The structure is mainly made of nano-clusters when the density is 0.078 g·cm^-3,and an amorphous skeleton structure with silicon and oxygen can be formed when the density exceeds 0.172 g·cm^-3.With the incensement of binding force between atoms,the activity and free-moving space decrease.The elastic modulus changes from 0.126 5 to 0.788 9 MPa when densities range from 0.078 to 0.443 g·cm^-3,and elastic modulus is linear with density.Results of simulation are in good agreement with experimental values.Secondly,the self-assembly processes of nano-porous structure of polyurea/SiO₂composite aerogels were simulated.And energy changes,mean square displacement,the porous structure and mechanical properties were compared.It can be shown that the electrostatic interaction is the dominant force in the self-assembly process,and the diffusion motility of polymer chain decreases with the incensement of density.The network skeleton can be formed when the density exceeds 0.236 g·cm^-3,the skeleton gradually becomes thicker with the incensement of density.Elastic modulus rise from85.39 to 213.94 MPa when densities increase from 0.236 to 0.521 g·cm^-3.Compared to pure SiO₂ aerogels,the connection of secondary particles are enhanced after cross-linked and elastic modulus of materials are improved by 190 to 240 times.Finally,the nano-pore model based on the microstructure of nano-porous insulation materials was built.The coupled conduction of gas and solid of nanoparticles,gas-phase heat transfer in pores and radiation heat transfer were considered comprehensively and the calculation formula of effective thermal conductivity was deduced.The thermal conductivity of aerogels was calculated by using this heat transfer model,and the results are in good agreement with pre-experimental values.By analyzing the effects of temperature,density,specific surface area and other factors of thermal conductivity of aerogels,it shows that there is an optimal density where the thermal conductivity of aerogel is minimum.Radiation heat transfer is the main method of heat transfer at high temperature,and adding fibers can effectively decrease the thermal conductivity of aerogel at high temperature.