Carbon aerogels are one of the most promising materials for high-temperature thermal insulators under inert atmosphere owing to their properties of low density, low thermal conductivity, ultra-high temperature resistance in inert atmospheres, and high infrared specific extinction coefficient. At present, there have been some studies on carbon aerogel thermal insulations at home and abroad,but most of them focus on the improvement of mechanical, antioxidant and ablation resistance properties, the simplification and low cost of the preparation process, as well as the ultralight and super-elastic properties of carbon aerogel. There is few study about how to further optimize the high-temperature thermal insulation performance of carbon aerogel thermal insulations. The excellent thermal insulation properties of carbon aerogels are inseparable from their special nanopore and skeleton structures, therefore, realizing the controllable preparation of the structures of carbon aerogels is the key point to further optimize the thermal insulation properties and to facilitate the practical applications. This paper introduces the relationship between the structure and thermal insulation property of aerogel, summarizes the research progress on the effect of sol-gel preparation process on the structure of carbon aerogel thermal insulation, and provides an appropriate outlook on the research direction of the preparation process.
The thermal conductivity of carbon aerogel represents its thermal insulation performance to a certain extent, and can be used as an indicator for the initial screening of carbon aerogel thermal insulation. Generally, the lower the thermal conductivity of the material, the better the thermal insulation performance is expected. Based on heat transfer theory, the thermal conductivity of carbon aerogel is represented as the sum of contributions from solid thermal conductivity, gaseous thermal conductivity, radiative thermal conductivity, and coupling thermal conductivity. Under certain environmental conditions, the thermal conductivity is mainly closely related to the structure (such as particle size, pore size and porosity, etc.) and density of carbon aerogel. Therefore, mastering the correspondence between structure and thermal conductivity, and then guiding the design of the preparation process of carbon aerogel, is the key point to obtain carbon aerogel thermal insulation materials with low thermal conductivity. Therefore, mastering the correspondence between structure and thermal conductivity and then guiding the design of the preparation process of carbon aerogel is the key to obtain low thermal conductivity carbon aerogel thermal insulations. In order to obtain low thermal conductivity, the pore size (at least less than the average free range of gas molecules) and particle size of carbon aerogel should be as small as possible, uniformly distributed, a density of not too large (about 0.10–0.15 g/cm3, and combined with the requirements for the mechanical strength of the material).
The structure of carbon aerogel is mainly determined by its preparation process, and the preparation of organic aerogel by sol-gel method is the first step to obtain carbon aerogel. Therefore, the structure of carbon aerogel depends on the structure of organic aerogel,which is affected by the sol-gel reaction and its proceeding conditions, mainly including the concentration of reactants, the type and concentration of catalysts, the parameters of gel-aging and the carbonization process. Among them, the concentration of reactants and the type and concentration of catalysts have the most significant effect on the structure of carbon aerogels. The particle size of the aerogel is mainly affected by and inversely proportional to the concentration of catalyst. The density is mainly affected by and proportional to the concentration of reactants. The pore size distribution and the volume of mesopores are jointly affected by both of them. In order to prepare carbon aerogel thermal insulation with high specific surface area, low density and abundant mesopores, base catalyst is generally selected, at a concentration of not too low. Besides, the specific surface area and the volume of mesopores of carbon aerogels are larger than those of their corresponding organic aerogels at low catalyst concentrations. Additionally, the structural properties of the aerogels can be further improved by introducing surfactants.
Summary and prospects Although the potential application of carbon aerogels as high-temperature thermal insulators has been confirmed in some fields, they still face great challenges in the research of many basic problems and practical applications. (1) Establish theory and simulation techniques, to predict the chemical reaction and material migration in the sol-gel process. Develop characterization method including observation of the sol-gel process, such as nucleation, growth and aging. Investigate the influence of process parameters on the nucleation, growth and aging process and analyze the corresponding reaction mechanisms. (2) Enrich the characterization methods for microstructures and develop the three-dimensional reconstruction of microstructure and the prediction of thermal insulation performance, so as to better understand the influence of microstructure on thermal insulation performance. (3) Establish the influence of process parameters on microstructure, so as to realize the controllable preparation of carbon aerogel with low thermal conductivity.