The Investigation Of Low Dimensional Thermal Insulations Based On Silica Powder

In this paper, the micro-sized and nano-sized silica were used as raw materials and ceramic fiber as reinforcing additives to prepare low-dimensional thermal insulation composites characterized with nano-sized pores, high porosity, low thermal conductivity and excellent heat resistance. The effects of particle sizes, the amounts of ceramic fiber and formation pressure on the mechanical properties, thermal properties and shaping behavior was investigated. Finally, the finite element method was adopted to numerically simulate the impact of pore sizes and porosity on the thermal properties.Fumed silica with sizes of 30nm, 100nm and 1000nm were used as the starting materials. The influence of particle sizes of fumed silica on the mechanical and thermal properties of low dimensional thermal insulating composites was studied by changing the proportional ratio of silica with different sizes.The results showed that with the increasing of big-size particles, the bulk density and cold crushing strength of samples were enhanced, while the permanent linear shrinkage and thermal conductivity of samples reduced under the same conditions of temperature. As the thermal conductivity of matrix insulation materials, fumed silica, was lower than that of the still air, with the the bulk density increasing, thermal conductivity was becoming lower.In the meantime, the microscopic mechanism of the effect of particle sizes of fumed silica on the thermal properties of the insulating materials was inspected. As the accumulation density of the insulating materials became larger, the particle contact point numbers per unit length would be larger and consequently the thermal resistance created by the grain boundary contacts would be higher. The more grain boundary contacts between particles, the smaller the pores fenced by the particles becomes, which will lower the heat convection of air. The role of this resistance becomes more significant when the remaining pore sizes are smaller than 100nm.The influence of the addiction amount of ceramic fiber on thermal and mechanical properties of insulating materials was investigated by examining by examining distribution of the ceramic fiber and the binding state between the ceramic fiber and micro-sized particles in the materials. The experimental results demonstrated that the ceramic fiber addition can improve the integrity and enhance the cold crushing strength of the samples. While the materials in the samples not containing ceramic fiber presented a lot of cracks and behaved as brittle as glass, it would be quite difficult to be dispersed when the amount of fiber is superior to 10wt%. As the ceramic fiber increases, the bulk density and cold crushing strength of samples become higher, but the increasing extent of the permanent linear shrinkage and thermal conductivity get to be lessened. The ceramic fiber transferred more heat directly as a result of its relative large section area and long conductive line.The influence of forming pressure on the integrity, microstructure, pore shape, pore distribution and thermal properties of samples formed at different pressures was investigated. The results revealed that the inclusive pores were connective and the pore sizes were bigger with lower molding pressure, resulting in the lower bulk density and cold crushing strength. With the compacting pressure increasing, the inclusive pores became smaller and most of them were sealed ones, leading to the higher bulk density and cold crushing strength. It was found that the higher forming pressure increased the density and decreased the thermal conductivity of the insulating materials.The samples were fired at different temperatures to determine the maximum service temperature. The results showed that the finer the grain particles of fumed silica tend to be crystallized more easily. The fumed silica would transform into quartz completely above the temperature of 1100℃and the composites would fail to act as insulating materials. Therefore, the long term service temperature of the insulating materials should be lower than 1000℃The finite element method was used to simulate the influence of pore size and porosity on the temperature-dependent thermal conductivity of the insulating materials. The analysis of simulation results was given that thermal conductivity of samples with smaller inclusive pores was lower, but samples with higher porosity in the range of 25vol%-75vol% were characterized with higher thermal conductivity.