Micro-nano scale heat transfer mechanisms for fumed silica based thermal insulating composite

Abstract

Developing nanoporous thermal insulation materials is an effective method for solving energy consumption problems. In this study, fumed silica based thermal insulating composite was prepared using a novel dry molding method. Thermal conductivity was measured using a guarded hot plate method in the steady state. The results indicated that this composite exhibited excellent thermal insulating performance, achieving an ultra-low thermal conductivity of 0.0205 W/mK at 100 °C. By analyzing the microstructure using field emission scanning electron microscopy (FESEM), we found that the fiber surface was modified by a 12 μm-thick layer of fumed silica particles that reduced solid heat transfer. However, at high temperatures, thermal insulating performance decreased rapidly, and thermal conductivity reached up to 0.119 W/mK at 500 °C. Based on this problem, a heat transfer model was constructed to explain micro-nano scale heat transfer mechanisms of this composite. The results indicated that gas-solid coupled thermal conductivity occupied a large proportion of 78.3% at 100 °C, but the value was only 0.015 W/mK. As the serving temperature increased, radiative heat transfer gradually occupied the dominant position in total heat transfer. Radiative thermal conductivity reached up to 0.101 W/mK at 500 °C, which was approximately 93.3% of total thermal conductivity. Therefore, it was apparent that the increase in radiative heat transfer was the main cause of the decline in thermal insulating performance at high temperatures. Additionally, the effect of the fiber mass ratio on thermal conductivity was investigated at various temperatures. Empirical and simulated data were compared, and the results revealed that the heat transfer model was basically consistent with experimental findings, achieving a mean error of only 4.78%.