What is aerogel

       Aerogel is a new generation of energy-efficient thermal insulation material. Aerogel is a kind of solid material with nano porous network structure and filled with gaseous dispersion medium in the pores. It is the lightest solid in the world. Due to its unique structure, aerogels exhibit excellent performance in many fields such as heat, acoustics, optics, electricity, mechanics, etc. At present, the commercial application of aerogels mainly focuses on their efficient heat resistance capability, and downstream applications include petrochemical industry, heat pipe network, lithium battery, building materials, outdoor clothing, aerospace, military industry and many other fields.

Figure 1: Physical picture of aerogel

Table 1: Aerogel parameters

       The thermal barrier principle of aerogel is the non convection effect, infinite baffle effect and infinite path effect brought by its independent structure. The thermal conductivity of aerogel is 0.012~0.024W/(m · K), which is 2~3 orders of magnitude lower than that of traditional thermal insulation materials. Its thermal insulation principle is that uniform and dense nano pores and multi-level fractal pore channel microstructure can effectively prevent air convection, reduce heat radiation and heat conduction: 1) No convection effect: the pores of aerogel are nano, and the internal air loses free flow ability; 2) Infinite shielding effect: nanoscale pores with infinite pore walls, minimizing radiative heat transfer; 3) Infinite path effect: Heat conduction occurs along the pore wall, and the nanoscale pore wall is infinitely long.

Figure 2: Thermal insulation principle of aerogel

      Compared with traditional thermal insulation materials, the thermal insulation performance of silica aerogel thermal insulation blanket is 2-8 times that of traditional materials, so the amount of aerogel is less under the same thermal insulation effect. Taking pipes as an example, if pipes with a diameter of 150mm need to achieve the same thermal insulation effect, the thickness of corresponding thermal insulation materials such as expanded perlite, calcium silicate, rock wool and aerogel felt are 90mm, 76mm, 64mm and 20mm respectively. According to the calculation of Sinopec Tahe Refining&Chemical Co., Ltd., after transforming the atmospheric coking unit from the traditional thermal insulation material to the combined thermal insulation method of "silica aerogel thermal insulation felt+single-sided aluminum foil glass fiber cloth thermal insulation material", the heat loss was reduced by 34.7%, and the thickness of the thermal insulation layer was reduced by more than 50% compared with the traditional thermal insulation material.

Figure 3: Under the same thermal insulation performance, the amount of aerogel felt is the least

       In addition, aerogel has the advantage of long service life, which is about 4 times of that of traditional thermal insulation materials. Traditional insulation materials such as rock wool and polyurethane are prone to absorbing water during long-term use, which affects the insulation effect. On the other hand, the uneven distribution of insulation materials due to gravity after absorbing water, especially in the use of pipeline insulation, can easily cause insulation materials to accumulate at the bottom of the pipeline, ultimately affecting its service life. Aerogel has excellent waterproof effect, and its hydrophobicity rate is more than 99%. It can still maintain a stable structure and heat insulation effect during long-term use.

      At present, commercial aerogels are usually composite products with various forms. Aerogels have the disadvantages of low strength and poor toughness, so it is necessary to increase the strength and toughness by adding particles, fibers and other reinforcements, or by adding carbon black, ceramic fibers and other shading agents to improve the radiation shielding ability. Therefore, currently on sale aerogel products are often made of aerogel materials and base materials. According to the product form, aerogel products can be divided into aerogel felt, aerogel paper, aerogel cloth, aerogel plate, aerogel powder, aerogel gel slurry, aerogel gel coating, etc.

       There are many kinds of aerogel materials, among which the commercial application of SiO2 aerogel is the most mature. According to the precursors, aerogels can be divided into seven categories: oxides, carbides, polymers, biomass, semiconductors, non oxides and metals. Many different precursors can prepare aerogels with different properties, which greatly enriches the diversity of aerogels and expands the application range of aerogels. At present, the application of SiO2 aerogel is the most mature in the market. In 2019, the proportion of silica aerogel in the world will reach 69%.

      The precursor of silica aerogel can be divided into organic silicon source and inorganic silicon source. The commonly used organic silicon sources are functional silanes such as methyl orthosilicate and ethyl orthosilicate, while inorganic silicon sources include silicon tetrachloride and water glass. Compared with inorganic silicon sources, organic silicon sources are more expensive, but have high purity and good process adaptability, and can adapt to supercritical drying and atmospheric drying. Although inorganic silicon source water glass has a lower price, it contains more impurities and is currently mainly used in atmospheric drying.

         The preparation process of aerogel mainly includes sol gel, aging, modification and drying of wet gel. Sol gel process refers to the process of precursor sol aggregation and condensation to form gel. However, due to the insufficient three-dimensional strength of the newly formed wet gel, it is easy to break and collapse, so it is necessary to age in the parent solution for a period of time to improve the strength or use surface modification to reduce or eliminate drying stress. The drying process is to replace the solution in the pores of wet gel with air and discharge it.

The drying process is the key to the synthesis step. During the drying process, wet gel needs to bear a drying stress up to 100Mpa-200MPa, which will make the gel structure shrink and crack continuously, and easily lead to structural collapse. At present, the mainstream drying process routes include supercritical drying and atmospheric pressure drying.

The principle of supercritical drying is that when the temperature and pressure reach or exceed the supercritical value of the liquid solvent medium, the liquid in the pores of the wet gel is directly transformed into the fluid without gas-liquid phase zone, the gas-liquid interface on the surface of the pores disappears, and the surface tension becomes very small or even disappears. When the supercritical fluid is discharged from the gel, it will not lead to the contraction of its network skeleton and collapse of its structure, thus obtaining the blocky nano porous aerogel material with the original structure of the gel. Early drying media mainly used methanol, ethanol, isopropanol, benzene, etc., but this technology has certain risks and complex equipment. Therefore, in recent years, a low-temperature environment supercritical drying process using carbon dioxide as the drying medium has been developed to improve drying conditions and reduce risks by reducing the critical temperature and pressure during drying.

The principle of normal pressure drying is to use a drying medium with low surface tension and related modifiers to replace the solvent in the wet gel, so as to reduce the capillary force generated during drying and avoid the destruction of the gel structure when removing the solvent, so as to achieve normal pressure drying. Long time dialysis and solvent replacement treatment are usually required for wet gel before normal pressure drying. The cost of atmospheric drying equipment and energy consumption is relatively low, and the equipment is simple, but the requirements for formula design and process combination optimization are high, and the preparation of non silica aerogels is immature.