The Study Of SiO₂ Aerogel Adsorption And Capillary Condensation And Simulation Of Mechanical Properties

Silica aerogels are nanostructured porous solid materials with three-dimensional network structure.Due to the highly porous structure,silica aerogels have unique characteristics such as low density,high surface area,high porosity,good chemical stability and low intensity,etc.Because of these excellent properties,silica aerogels have broad potentials to be applied in many areas such as thermal,acoustic,optical,electrochemical,adsorption and new energy development,etc.In the Inertial Confinement Fusion experiment,silica aerogels can be used as an ideal cryogenic target carriers for liquid hydrogen isotopes fuels,achieving the homogenize the frozen fuel ice layer and effectively controlling the implosion shrinkage,etc.It is important to study the self-supporting mechanical properties of silica aerogel material as a target material,and hydrogen,deuterium and tritium adsorption and condensation properties,which is of great significance for the study of ICF.In this study,the mechanical properties,adsorption properties and the capillary condensation phenomenon during adsorption process of silica aerogels were studied by experiments and calculations,which in turn might provide insights into the future directions on synthesis of silica aerogels and their application in inertial confinement fusion experiments.The main work of this paper is as follows:(1)Simulation and analysis of mechanical properties of silica aerogels.Firstly,a similar architecture of silica aerogels is generated by Voronoi tessellation based on its micro structural parameters(i.e.,relative density,pore size,ligament diameter and particle size),which are derived from the characterization of SEM images.The compressive experiments and finite volume method(FVM)simulations demonstrates the constitutive materials parameters of bulk aerogels,i.e.,the Young’s modulus,yield stress are 0.042 GPa,0.023 GPa,respectively.In the next stage,we focus on the scaling laws for the mechanical properties of silica aerogels.Two types of silica aerogels are modeled by Voronoi tessellation,namely SA-I(with Gaussian distributions of pore size and ligament diameter)and SA-II(with constant pore size and ligament diameter).FVM simulations results suggest that both the Y oung’s modulus and yield strength show the power-law relations with relative density,and the silica aerogel with ultra-low relative density can also perform higher stiffness and strength,if the ligament diameter is a constant value,making it could be the target material with good self-supporting property in ICF experiment.(2)The hydrogen and deuterium adsorption simulation of silica pore structures: The cylindrical silica pore structure models are constructed and the adsorption / desorption behaviors of hydrogen and deuterium are simulated by using the Grand Canonical Monte Carlo simulation.In the simulation,the effects of temperature and pore size on the hydrogen and deuterium adsorption/desorption properties are investigated,obtaining and the adsorption/desorption isotherms,density distribution and configuration analysis under different conditions.The research results show that as the decrease of temperature,the pressure at which the gas-liquid phase transition is starting becomes lower,the pressure required is much smaller than the pressure required at the flat liquid level.Moreover,a small increase in pressure can aggravate the phenomenon of capillary condensation,and the amount of fluid adsorbed increases rapidly.In the process of the pressure increasing,the hydrogen and deuterium adsorption position changes from the pore wall to the whole pore.Hydrogen and deuterium show the same law in the process of temperature and pressure changes.(3)Deuterium adsorption experiment of silica aerogels at low temperature: three kinds of silica aerogels with different density are prepared by sol-gel method,and SEM analysis and nitrogen adsorption tests were performed on these samples,the deuterium adsorption of three samples at different temperatures was measured.The effects of density,pore size and temperature on the deuterium adsorption and capillary conden sation are studied,systematically,compared with the simulation results.The results shows that the samples with smaller densities,the capillary condensation of deuterium exhibit obvious.However,for the sample with the largest density,the capillary condensation hardly happened.With the increase of temperature,the amount of deuterium adsorption in silica aerogels is greatly decreased,the pressure required for capillary condensation increases.These results are consistent with the simulation effects of the temperature on the hydrogen adsorption and capillary condensation,which are obtained in the previous section.