Atomic-level sintering mechanism of silica aerogels at high temperatures: structure evolution and solid thermal conductivity.

• Molecular potential functions are examined for silica simulations under high temperature. • Molecular Dynamics simulations illustrate the sintering mechanism of silica nanoparticles. • The proposed method of "solid-liquid" coexistence can effectively analyse sintering process. • High temperature effects on nanoparticle contact are substantially identified and quantified. • The proposed model of thermal conductivity can accurate capture available experimental data. Silica aerogels are extensively used for thermal insulation at high temperatures. However, it has been experimentally found that high temperature can change the pore structure and undermine thermal insulation. The mechanism of high temperature influence is unclear due to the limited resolution of experimental observations. In the present work, molecular dynamics simulations and theoretical modeling were combined to investigate the effects of high temperature on pore structures and heat conduction. At the scale of nanoparticles, the structural evolution induced by the sintering process of silica aerogels was numerically investigated. The melting process of a single silica nanoparticle can be divided into three stages, and it was found that extending the stage of surface diffusion can improve the thermal stability of silica nanoparticles. At the scale of particle assembly, the contact diameter between adjacent particles was identified and numerically evaluated. At the scale of bulk material, a theoretical model was developed to predict the solid thermal conductivity by considering both the temperature effect on the intrinsic thermal conductivity of backbone and the effect of nanoparticle size. A good agreement was found between the present model and available experimental data. An insightful discussion of the existing theoretical models and experimental data was also presented. The present work can inspire further cross-scale modeling of silica aerogels and may pave the way for the development of silica aerogels with high stability and thermal insulation at high temperatures. [ABSTRACT FROM AUTHOR]