Prediction And Optimization Of Thermal Conductivities Of Silica Aerogel Composites For Thermal Insulation

SiO₂ aerogel owing to its refractory non-combustible and ultra-low thermal conductivity(about 0.02 W·m^-1·K^-1), excellent insulation properties and broad prospect in energy saving. The volume fraction and diameter of fiber are key parameters having significant effect on the thermal conductivity of the fiber/silica aerogel composites. The fused silica fiber/silica aerogel composites with different fiber volume fractions were prepared via ambient pressure drying under controlling the amount of the fused silica. The scanning electron microscope, analyzer of surface area and pore size, measuring instrument of optical contact angle and transient plane source method were used to obtain the volume fraction, micro structure, surface area and pore size, hydrophobic angle and thermal conductivity of the composites, respectively.Based on the microstructure characteristics of fused silica fiber/silica aerogel composites, a theoretical model by fractal cross-sphere and series was built to calculate the conductive thermal conductivity. A theoretical model by Mie scattering theory, Reighlay scattering theory, Bridging technical and Rosseland function was built to predict the radiative thermal conductivity. Combining the theoretical models of conductive and radiative thermal conductivity, the total thermal conductivity was calculated. And the feasibility of the model was verified by comparison between the predicted values and measured values.Based on theoretical model of the thermal conductivies of the composites, the effects of volume fraction and diameter of fused silica on the thermal conductivities were investigated. The results indicated that the conductive thermal conductivity increases while radiative thermal conductivity decreases, as the volume fraction is increased. Thus, the total thermal conductivity appears firstly decreased and then increased. Therefore, the total thermal conductivities of the fiber/silica aerogel composites at high temperatures can be decreased effectively by increasing the volume fraction and decreasing the diameter.Based on the effects of fiber volume fraction and fiber diameter on the thermal conductivities of fiber/silica aerogel composites, the optimizations of fiber volume fraction and fiber diameter were made to obtain the minimum total thermal conducitivies. For instants at the ambient temperature 300 K, the minimum total thermal conductivities were decreased from 0.0182 to 0.0172 W·m^-1·K^-1, when individually optimizing volume fraction comparing to parameters of measurement at fiber diameter of 8.0 μm. And the minimum total thermal conductivities were decreased from 0.0184 to 0.0182 W·m^-1·K^-1 for individually optimizing fiber diameter at fiber volume fraction of 3.0%.Based on the individually optimizing results for volume fraction and diameter, the simultaneous optimization on volume fraction and diameter was then made. Compared to individual optimization, the simultaneous optimization was to obtain the minimum total thermal conductivities. For instants at the ambient temperature 300 K, the minimum total thermal conductivity by a simultaneous optimization was 6.6% lower than that by individually optimizing the fiber volume fraction at diameter of 8 μm, 1.1% lower than that by individually optimizing the fiber diameter at volume fraction of 3%. And the more considerable decrease of thermal conductivies could be made at high ambient temperature. The results of optimization provide guidance for structure design and properties optimization of fused silica/silica aerogel composites for promoting energy conservation application of the composite in aerospace, industrial, construction and other fields.