Microstructure control of the silica aerogels via pinhole drying

Pinhole drying process is a slowly dryness in the air caused by capillary force. It was found useful to improve the transparency of silica aerogel. So in this work, pinhole drying was considered as a modification on the microstructure. We studied the effect of pinhole drying on the light transmission of aerogel. The effect of pinhole drying on the microstructure of aerogel was also investigated. The spherical transmittance (τ _s) and directional transmittance (τ _d) were measured by ultraviolet–Visible–infrared spectrophotometer, the scattering light at the 90° direction of the gel was measured by a nephelometer, and the microstructure of the aerogels were analyzed applying small angle X-ray scattering, nitrogen adsorption porosimetry, and scanning electron microscope. The results showed that pinhole dried silica aerogels and non-pinhole dried aerogels exhibited different microstructure and optical properties. The directional transmittance of the pinhole dried aerogel and non-pinhole dried aerogel at 600 nm wavelength was 57.9 and 40.9%, respectively, an improvement of 17.0%. The transparency ratio (τ _d/τ _t) of pinhole dried aerogel and non-pinhole dried aerogel at 400 nm wavelength was 0.26 and 0.07, respectively, an improvement of 0.19. Small angle X-ray scattering measurements showed that these improvements can be attributed to the smaller mean cluster size of pinhole dried aerogels than that of non-pinhole dried aerogels. 90° scattering light experiments indicated that the clusters of the gel were becoming smaller during the drying process caused by capillary force according to Rayleigh scattering theory. It is the reason why pinhole dried aerogels have smaller mean cluster size and relatively transparent.