Microstructure Of Silica Aerogels Research Articles

Synthesis of silica aerogel monoliths with controlled specific surface areas and pore sizes

To replace traditional preparation methods of silica aerogels, a small-molecule 1,2-epoxypropane (PO) has been introduced into the preparation process instead of using ammonia as the cross-linking agent, thus generating a lightweight, high porosity, and large surface area silica aerogel monolithic. We put forward a simple solution route for the chemical synthesis of silica aerogels, which was characterized by scanning electron microscopy (SEM), TEM, XRD, FTIR, thermogravimetric analysis (TGA) and the Brunauer–Emmett–Teller (BET) method In this paper, the effect of the amount of PO on the microstructure of silica aerogels is discussed. The BET surface areas and pore sizes of the resulting silica aerogels can be freely adjusted by changing the amount of PO, which will be helpful in promoting the development of silica aerogels to fabricate other porous materials with similar requirements. We also adopted a new organic solvent sublimation drying (OSSD) method to replace traditional expensive and dangerous drying methods such as critical point drying and freeze drying. This simple approach is easy to operate and has good repeatability, which will further facilitate actual applications of silica aerogels.

A parametric study on mass diffusion coefficient of desiccants for dehumidification applications: Silica aerogels and silica aerogel coatings on metal foams

A separate sensible and latent cooling air-conditioning system integrated with a solid desiccant-based dehumidification device provides potential energy savings as compared to a conventional vapor compression system. Silica aerogels are hygroscopic materials that are deployed as solid desiccants for dehumidifying devices, such as an enthalpy wheel. These hygroscopic materials are highly porous and have good moisture adsorption and desorption characteristics due to the microstructure. Mass diffusivity is an important parameter that relates to the rate of the adsorption or desorption process in silica aerogels. The current study is focused on the evaluation of the mass diffusivity of solid silica aerogels and silica aerogel coatings on metal foam substrates. The porous structure of silica aerogels affects the mass diffusion coefficients. The synthesis process to manufacture the silica aerogels causes the porous structure to differ and thus impacts the mass diffusivity. The sol-gel process is used to prepare silica aerogels using various basic (ammonium hydroxide, sodium hydroxide, potassium hydroxide) and acidic (hydrofluoric acid, steric acid, hydrogen peroxide) catalysts with the same precipitator (tetramethyl orthosilicate) and solvent (methanol). The wet samples have been dried using a super-critical process to avoid internal cracking due to the capillary force. Scanning electron microscopy is used to analyze the microstructure of super-critically dried silica aerogels. The dynamic vapor sorption method is used to determine the effective mass diffusivity for the different silica aerogels. It is found that the mass diffusivity is related to the microstructure of silica aerogels, which depends on the catalysts used in the sol-gel process; however, the values for mass diffusivities for solid desiccants and desiccant coatings are similar. In addition, a parametric study is conducted to determine the effect of relative humidity and temperature on the adsorption and desorption mass diffusivity. Furthermore, the impact of the drying process and aging on the mass diffusivity of aerogels has been investigated.

Simulation of the tensile properties of silica aerogels: the effects of cluster structure and primary particle size.

A new two-level model is proposed to investigate the relationship between the mechanical properties and microstructure of silica aerogels. This two-level model consists of the particle-particle interaction model and the cluster structure model. The particle-particle interaction model is proposed to describe interactions between primary particles, in which the polymerization reaction between primary particles is considered. The cluster structure model represents the geometrical structure of silica aerogels, and it is established using a modified diffusion-limited colloid aggregation (DLCA) algorithm. This two-level model is used to investigate the tensile behavior of silica aerogels based on the discrete element method (DEM). The numerical results show that the primary particle size has significant effects on the elastic modulus and tensile strength of silica aerogels. Moreover, the power-law relationships between tensile properties and aerogel density are dependent on the variation of the primary particle radius with density. The present results can explain the difference among different experimental exponents to a certain extent. In comparison with experimental data within a wide density range, this two-level model provides good quantitative estimations of the elastic modulus and tensile strength of silica aerogels after the size effects of the primary particle are considered. This paper provides a fundamental understanding of the relationship between the mechanical properties and microstructure of silica aerogels. The two-level model can be extended to study the mechanical properties of other aerogels and aerogel composites.