Preparation Of Nano Silica Powder Based Thermal Insulating Composites And Construction Of Micro/nano-scale Heat Transfer Calculationmodel

Nanoporous silica thermal insulating material is one of super thermal insulating material with excellent performance including low density,high porosity,low thermal conductivity,et al,of which the most representative one is silica aerogel.However,numerous studies have demonstrated that silica aerogel is difficult to achieve large-scale production for some disadvantages including brittleness,complicated processing method,long production cycle,high production cost and insecurity of supercritical drying method with high temperature and high pressure equipment,et al.Therefore,nono silica powder based thermal insulating composites were prepared by drying molding technology,which were composed of fumed silica and reinforced fiber and properties of the composites were characterized and analyzed,meanwhile micro/nano-scale heat transfer calculation model of the composites were established in this paper.The operation parameters of high-speed powder mixing modification machine were controlled reasonably,which could mix materials evenly and improve the surface coating effect of fiber.Moreover,suitable molding pressure and reasonable pressing process were also the key to achieve excellent performance of the composites.When the rotational speed is 1200r/min,time is 30min,molding pressure is 2MPa,the flexural strength of the composites could reach to 0.3MPa and the thermal conductivity of the composites at 100? is only 0.0205W/(m·K).The introduction of reinforced fiber could significantly improve mechanical performance and thermal insulation property of the composites.Alkali free ultra-fine glass fiber,zirconia fiber,polycrystalline mullite fiber,ceramic fiber and quartz fiber were selected as reinforced fibers in this paper.When alkali free ultra-fine glass fiber was adopted,the mechanical performance and thermal insulation property of the composites were the best and they were proportional to the mass ratio of fiber.When the mass ratio of alkali free ultra-fine glass fiber increased from 0%to 15%,the flexural strength of the composites increased from 0.0423MPa to 0.3698MPa and the thermal conductivity decreased from 0.1592W/(m-K)to 0.0683W/(m·K)at 50? Moreover,the length of fiber should be controlled reasonably,the flexural strength increased with the increase of the length of fiber,but the thermal insulation performance turned worse due to the increase of aggregates.The results showed that the length of fiber was suitable for 6mm.Micro/nano-scale heat transfer calculation model was established in this paper,and the effective thermal conductivity of the composites was calculated.The results showed that the change rules of thermal conductivity of calculation and experiment were the same basically and the values of calculation and experiment were little difference.