Anomalous Sintering and Structural Evolution of APD Silica Aerogels Under Extreme Thermal Conditions

ABSTRACT

This study characterizes the sintering behavior of ambient‐pressure‐dried (APD) silica aerogels under a high‐vacuum environment (1 × 10 ⁻⁵  Torr) over a temperature range of 400°C–900°C. Isothermal treatments conducted for 1–30 min demonstrate that residual organic species are completely removed by 600°C, leading to a temporary increase in specific surface area (SSA) to 468 m ² /g through pore reopening. Analysis of the sintering kinetics reveals an anomalous, non‐monotonic temperature dependence; the rate constant ( K ) follows the sequence 600°C > 900°C > 800°C. In addition, x‐ray diffraction analysis indicates that the kinetic retardation observed at 800°C coincides with a peak in structural disorder, as evidenced by the significant broadening (FWHM) of the amorphous halo. Between 800°C and 900°C, densification resumes via a surface diffusion–dominated mechanism, yielding an estimated apparent activation energy of ∼58 kJ/mol over this specific interval. These findings establish a definitive correlation between the degree of amorphous structural disorder and the densification kinetics in nanoporous silica frameworks.