| The long-term heat protection is an important technical problem which must be solved properly in the development of hypersonic vehicle.Materials with low density and high thermal insulation performance become the first choice of insulation materials.Silica aerogel is a nano-porous three-dimensional network structure with very low thermal conductivity and density.As a kind of heat insulation materials for aircraft,silica aerogel has a broad application prospect.However,because the production cost of silica aerogel is high and the experimental operation is complex,the simulation calculation method can be used to study the thermodynamic properties of silica aerogel,which can save the cost,save the experimental time and greatly improve the design speed.Firstly,solid phase thermal conductivity and gas-solid coupling thermal conductivity of silica aerogel are simulated based on non-equilibrium molecular dynamics method.The atomic structure model of aerogel solid skeleton was established and characterized by random porosity method.The thermal conductivity of aerogel in solid phase was calculated by non-equilibrium molecular dynamics method,and the variation of the thermal conductivity with structural parameters such as pore size,porosity and set temperature was studied.The results show that the solid thermal conductivity decreases with the increase of porosity.When the porosity reaches 94%,the solid thermal conductivity of several samples decreases to less than0.02 W/（m·K）,and the solid thermal conductivity of the aerogel with a pore size of 20 nm is less than 0.01 W/（m·K）.The thermal conductivity of solid phase decreases first and then increases with the increase of pore size,and the minimum value is obtained at the pore size of20 nm.The thermal conductivity of solid phase increases with the increase of temperature.The pores of the solid skeleton model were filled with nitrogen molecules to obtain the overall structure model of aerogel.Non-equilibrium molecular dynamics was used to simulate the overall structure model,and the gas-solid coupling thermal conductivity was calculated.The results show that the thermal conductivity of gas-solid coupling decreases first and then increases with the increase of pore size.When the pore size is larger than 20 nm,the thermal conductivity of gas-solid coupling has a higher growth rate than that of solid phase.The thermal conductivity of gas phase increases with the increase of porosity,and the attenuation of solid phase thermal conductivity is suppressed under the condition of high porosity.The gas-solid coupling thermal conductivity of the sample model increases from 0.012 W/（m·K）to more than 0.04 W/（m·K）with increasing temperature.Secondly,the bolometric thermal conductivity of aerogel is calculated by theoretical formula.The results show that with the increase of temperature,the bolometric thermal conductivity increases greatly.When the temperature is above 800 K,the bolometric thermal conductivity far exceeds the gas-solid coupling thermal conductivity.In order to reduce the radiant thermal conductivity at high temperature by carbon black doping,a model of multilayer mask was proposed.The calculation showed that the carbon black with a radius of1μm was doped with an aerogel with a thickness of 2 mm.The radiation thermal conductivity could be reduced to a very low level with only 0.2%doping rate.Finally,the specific heat capacity and bulk expansion coefficient of silica aerogel solid skeleton are calculated based on equilibrium molecular dynamics.The specific heat capacity and bulk expansion coefficient change similarly with temperature.With the increase of temperature,the specific heat capacity changes from 600～700 J/（kg·K）to 800～1150 J/（kg·K）.For aerogel with the same porosity,the increase of pore size will significantly increase the specific heat capacity of aerogel.At 300K,the bulk expansion coefficient of the sample is concentrated in 1.2～1.4×10-6/K,and increases to 1.7～2.3×10-6/K when the temperature reaches 900K. |
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