Mechanism And Characteristics Of Heat Conduction In Nano-insulation Material

Heat transfer with nano- or micro-scale specialties in silica aerogel will affect its macroscopical thermophysics properties. The thesis is to research the heat transfer mechanism on silica aerogel and further to provide the theoretical directions for thermal design and application of compound insulating materials.Applied the kinetic theory of gases, the expression of the thermal conductivity of gas in nanopore is derived. The results indicte that with the nanopore size becoming smaller, the thermal conductivities of a gas in nanopore are reduced and much smaller than that in free space under the same conditions. In nanopore, the attraction between molecules influences the size of the gas molecular mean free path and the collision diffuse scattering status among the gas molecules and between the gas molecules and pore wall, futher the thermal conductivity of gas in nanopore.A heat transfer model of solid matrix unit obtained from the structure of silica aerogel is developed. According to the kinetic theory, considered the different limitations of phonon mean free paths, the influencing factors on thermal conductivity of solid matrix unit in silica aerogel are analysed. Theoretical results indicate that the thermal conductivity in the solid matrix unit is determined by both the diameter of the particle and the diameter of the cross section of the connected particles. When the two diameters are comparable to, or smaller than the phonon mean free path of the bulk silica, the thermal conductivity of solid matrix unit is reduced rapidly.According to the microstructure of silica aerogels, nanopore model is built. The apparent conductivity in several conditions is calculated. This paper has also studied the effect of the physical parameters of silica aerogels on the apparent conductivity, then compared the apparent conductivity in several nano-pore models, and finally compared with the experimental values. The results showed that, in low density silica aerogels, using the cube model and a smaller diameter, the calculated apparent conductivity is closer with the experimental data; in high density silica aerogels using the eight prism model and a larger diameter, the calculated apparent conductivity is closer with the experimental data.