Identification And Control Of Key Structural Characteristics Of Thermal Conductivity In Aerogels

As a kind of typical nanoporous materials, aerogels have obtained extensively attentions in aerospace insulation field and building insulation field due to their unique nanostructured framework and superior properties.The prominent insulation performance makes aerogels be one of the most promising insulation materials in the future. At present, the investigations about aerogels are primarily focused on the development and application of new aerogel system and optimization and large-scale application of traditional aerogel system. The massive existence of macropores in network structures hasn't received much attention. Lowering the macropore fractions makes the thermal conductivity of aerogels keep further reduction, thus the thermal insulation performance could be improved. On the basis of fully understanding the existence of macropore structures in aerogel materials, the research idea that improving the thermal insulation performance through reducing the macropore fraction and the characterization method that can fully describing the macropore fractions in structures are innovatively proposed in this study. This paper aims to explore the regularity of reducing the macropore fraction and improving the thermal insulation performance and the feasibility of processing techniques for improving the thermal insulation performance through adding nano materials in aerogel structure or implementing additional processing to the as-formed network. This paper broadens the research ideas on the structure and thermal insulation performance and has the pioneering significance in study and application of aerogel materials.The technique that adding lamellar nano materials graphene oxide(GO) into the structure of aerogels in sol-gel process is used in this work.This study investigates the influence of varying GO additions on the nano structures, the macropore fractions changing with GO additions, and the change regulation of macropore fraction variation and thermal insulation performance of aerogels. Moreover, this study also investigates the influence of GO additions on the mechanical strength and thermal stability.The macropore fractions decrease from 63.05% to 38.18% and density increases from 0.074 to 0.189 g/cm3 with the increase of GO contents. The thermal conductivity of aerogels decreases from 0.0090 W/(mK) to 0.0075 W/(mK) due to the restriction degree of the gaseous heat transfer being enhanced after the whole space of the structure is divided into many smaller spaces by GO sheets. The thermal conductivity of aerogels doesn't show continuous decline with macropore fractions decreasing because the GO increases the contribution of solid heat transfer to the final thermal conductivity of aerogels. In addition, the mechanical strength is enhanced in the presence of GO. The compression modulus increases from 0.238 Mpa to 0.394 Mpa and the materials exhibit toughness.In order to enhance the interfacial compatibility and avoid the introduction of extra macropores due to the poor interfacial compatibility of two counterparts, the granular nano Al2O3 is chosen as additive in this work. After the introduction of nano-sized clusters into the network of aerogels, the density increases from 0.074 to 0.304 g/cm3 and the macropore fractions are reduced from 63.05% to 23.12%. The thermal conductivity of aerogels is lowered with addition of Al2O3 except the maximum addition, and the minimum value is 0.0074 W/(mK). The contribution of solid heat transfer is increased ow ning to the Al2O3 contents increasing, thus resulting in the increasement of thermal conductivity.However, the thermal conductivity of Al2O3-added aerogels is still lower than that of pure aerogels (except the maximum addition). This is because the reduction of gaseous heat transfer being severely limited along with the macropore fractions decreasing can offset the increasing of solid heat transfer. Taking the sharp increasement of density into account, the phenomenon that the thermal conductivity of aerogels with maximum Al2O3 addition (0.0097 W/(mK)) is not much higher than that of pure aerogels (0.0090 W/(mK)) can also prove the importance of reduction of macropore fraction in improving the thermal insulation performance. In addition, the enhancement of density helps the improvement of mechanical strength when taking the thermal insulation performance into consideration.This is important to practical application of aerogel-based materials.We innovatively put forward the multiple impregnation technique to treat the as-formed structure of aerogels by hydrolyzed sols. This work studies the influence of different impregnation times on the micro structures of aerogels and lowering the macropore fractions by adding new network into the as-formed network and increasing the crosslink density. Exploring the feasibility of improving the thermal insulation performance via multiple impregnation technique. Impregnation treatment betters the pore structure of aerogels and fills the pores, and the macropore fractions are significantly reduced from 63.05% to 18.76%. The density of aerogels increases from 0.074 to 0.218 g/cm3 with the increase of impregnation times, however, the thermal conductivity doesn't increase but shows a downward trend. The minimum value of thermal conductivity reaches 0.0081 W/(mK). This phenomenon suggests that the decrease of macropore fractions enhances the degree of restriction on gaseous heat transfer. This makes the aerogels break through the limitation of obtaining high thermal insulation performance only by lowering the density, which drives the application of aerogels.Aging treatment to wet gel using tetraethoxysilane (TEOS) as mother liquid is explored in this work. By introducing effective solid content into the network of wet gel during aging, this work investigates the influence of varying mother liquid contents on the micro morphology and pore structure of aerogels. The macropore factions are obviously lowered from 63.05% to 38.00% after aging treatment by the mother liquid. However,the decline of macropore fractions in this way is mainly due to most pores being blocked, which can be demonstrated by the sharp decline of the pore volumes (from 4.25 to 1.40 cm3/g) and specific surface areas (from 837.4 to 313.6 m2/g). Moreover, the uniformity of pores gets bad. This makes the restriction of pore structure on the gaseous heat transfer weak.Meanwhile, the density of aerogels experiences a clear rise from 0.074 g/cm3 to 0.33 g/cm3, which enhances the solid heat transfer. As a result, the thermal conductivity are remarkably increased from 0.0090 W/(mK) to 0.068 W/(mK). The investigation shows that the density should be taken into consideration in order to improve the thermal insulation performance by lowering the macropore fractions in aerogels.Generally, thermal treatment is used to study the thermal stability of materials. From the view of adjusting the as-formed pore structure, this work explores the influence of pore structure variation during appropriate thermal treatment on lowering the macropores fractions and improving the thermal insulation performance. Through thermal treatment on the as-prepared aerogels, this work studies the effect of different thermal treatment conditions on the microstructure and the change regulation between macropore fractions and thermal conductivity. The density of aerogels increases from 0.074 to 0.176 g/cm3. Below the sintering temperature of silica aerogels (650?), appropriate temperature is good for improving the microstructure, enhancing the homogeneity of pores, and maintaining the pore volumes. Thus the macropore fractions are reduced from 63.05% to 35.83%. The thermal conductivity doesn't increase along with the enhancement of density but slightly decreases from 0.0090 W/(mK) of pure aerogels to 0.0080 W/(mK) of thermal-treated aerogels.Above sintering temperature, the pore structures are badly damaged and the nano structure characteristics are weakened due to the appearance of sintering. Although the macropore fractions still present decreasing to 24.82%, the solid heat transfer obviously increase due to the enlargement of skeleton particles and melting of pore structure. Hence the thermal conductivity is significantly increased, and the corresponding value is 0.030 W/(mK). Therefore, appropriate thermal treatment below the sintering temperature to the aerogels contributes to promoting the thermal insulation performance of aerogels to some degree.