Study On Multi-scale Thermodynamic Properties Of SiO₂ Aerogel Micropheres In Waterborne Thermal Insulation Coatings

This study investigates the thermodynamic behavior of SiO₂ aerogel microspheres in water-based thermal insulation coatings.The thermal conductivity of the microspheres and their insulation coatings is examined using experimental,theoretical,and numerical simulation approaches at the micro,meso,and macroscales.The results from these three aspects are compared and validated to reveal the insulation mechanism of SiO₂ aerogel microspheres in different systems.This study provides a theoretical basis and experimental reference for developing more efficient insulation fillers and coatings.The main research contents of this paper are as follows:（1）Preparation and characterization of SiO₂ aerogel microspheresSmooth-surfaced SiO₂ aerogel microspheres were synthesized via sol-gel method followed by atmospheric drying process.The impact of different synthesis procedures on the surface morphology,pore structure,and thermal insulation properties of SiO₂ aerogel microspheres was investigated.Results indicate that the SiO₂ aerogel microspheres exhibit a smooth surface and good spherical structure with a low thermal conductivity of 0.0228 W/（m·K）.When the volume ratio exceeds 7:3,the prepared SiO₂ aerogel microspheres exhibit a good spherical structure,while samples prepared with a volume ratio less than 7:3 demonstrate irregular block-like structures.At a volume ratio of n-hexane and precursor solution of7:3,both NH₃-catalyzed and NH₄OH-catalyzed systems can produce similar spherical structures.However,SiO₂ aerogel microspheres synthesized by the NH₃-catalyzed system possess a smooth surface morphology,while those synthesized by the NH₄OH-catalyzed system exhibit an apparent rough surface morphology.（2）Structure and thermal properties of SiO₂ aerogel microspheres at microscaleA numerical model was developed using finite element software COMSOL to simulate the microstructure of SiO₂ aerogel microspheres.This study investigates the impact of various parameters,such as aerogel density,porosity,specific surface area,environmental temperature,and pressure,on the thermal conductivity of SiO₂ aerogel materials,and analyzes the heat transfer mechanism of SiO₂ aerogel at the microscale.The findings indicate that under ambient conditions,the average effective thermal conductivityλ_e of SiO₂ aerogel increases with density,while it decreases with specific surface area and porosity.Additionally,theλ_e of SiO₂ aerogel increases with temperature,and the impact of pressure is observed to be on the gas-phase heat transfer process of the aerogel.Higher pressure results in a higher gas-phase thermal conductivity,leading to an increase in theλ_e of SiO₂ aerogel.Moreover,the study reveals that at high temperatures,theλ_e of SiO₂ aerogel first decreases and then increases with increasing material density.This suggests the presence of an optimal density that results in the lowestλe of SiO₂ aerogel,and this optimal density increases with increasing environmental temperature.（3）Thermodynamic Behavior of SiO₂ Aerogel Microspheres in Coatings in the Mesoscale RegimeSiO₂ aerogel microspheres were used as insulation fillers and added to a styrene-acrylic acid ester emulsion to prepare a two-component barrier-type thermal insulation coating.The thermal conductivity of the two-component barrier-type thermal insulation coating was measured and calculated by experimental,numerical simulation,and theoretical calculation methods.The differences between the three results were compared,and the reasons for the differences were analyzed.The heat transfer mechanism of SiO₂aerogel microspheres in coatings was revealed,and new ideas and methods for studying the insulation performance of composite materials were provided.The addition of SiO₂ aerogel microspheres in a certain mass fraction can effectively reduce the thermal conductivity of the insulation coating.Compared with the pure styrene-acrylic acid ester coating,when the mass fraction of SiO₂ aerogel microspheres reaches 10%,the thermal conductivity of the coating can be reduced to a minimum of0.125 W/（m·K）,which is reduced by more than 40%.The thermal conductivity of the SiO2 aerogel microsphere-styrene-acrylic acid ester insulation coating is independent of the particle size and distribution of SiO₂ aerogel microspheres but is only related to the mass/volume fraction of SiO₂ aerogel microspheres.The thermal conductivity decreases linearly with the increase in the mass/volume fraction of SiO₂ aerogel microspheres.（4）Thermal insulation performance of SiO₂ aerogel microsphere thermal insulation coatings in macroscopic systemsThe thermal insulation effect of SiO₂ aerogel microspheres in a potassium silicate-styrene-acrylic emulsion coating system was studied.SiO₂ aerogel microspheres with excellent thermal insulation performance were selected as thermal insulation fillers.The coating thermal conductivity and thermal insulation temperature difference were used as the evaluation criteria for thermal insulation performance in macroscopic systems.The influence of the amount of SiO₂ aerogel microspheres in the system on the coating performance was investigated.Combining with finite element software COMSOL,the influence of coating thermal conductivity and thickness on the thermal insulation performance of walls was studied,and the optimal amount of SiO₂ aerogel microspheres was determined.Based on the Fourier law,the formula for calculating the equivalent thermal conductivity of multilayer composite materials was derived,and the influence of coating and wall thermal conductivity and thickness on the equivalent thermal conductivity of the composite thermal insulation system was studied based on this formula.The results showed that the coating performance of SiO₂ aerogel microsphere thermal insulation coatings met the requirements of the national standard for inorganic architectural coatings for exterior walls.SiO₂ aerogel microspheres can significantly reduce the thermal conductivity of the coating.When the mass fraction of SiO₂ aerogel microspheres is 4%,the thermal conductivity of the coating reaches the lowest value,as low as 0.0511 W/（m·K）.The thicker the coating,the lower the thermal conductivity,the larger the thermal insulation temperature difference,and the better the thermal insulation performance of the coating.Among them,reducing the thermal conductivity of the coating is a more efficient way to improve the thermal insulation performance of the coating.The theoretical calculation obtained the same conclusion as the numerical simulation,that is,the thicker the substrate and the larger the thermal conductivity,the larger the equivalent thermal conductivity of the multilayer composite material.The thicker the coating and the lower the thermal conductivity,the smaller the equivalent thermal conductivity of the multilayer composite material.