Influence of temperature on the physical properties of citric acid catalyzed TEOS silica aerogels

In the present paper, the experimental results on the influence of temperature on such physical properties as bulk density, percentage of porosity, pore size distribution and optical transmission of silica aerogels are reported. The aerogels were heated at different temperatures ranging from room temperature up to 1000 degrees C. It was found that highly transparent (> 90%), monolithic and pure silica glasses were obtained at temperatures as low as 1000 degrees C with a heating rate of 1.5 degrees C min(-1). It has been observed that initially within the temperature range 25-250 degrees C, the bulk density decreased from 0.21 to 0.19 g cm(-3) owing to weight loss without any volume and porosity changes of the aerogels, while above 250 degrees C the bulk density, percentage of volume shrinkage and weight loss increased with increase in temperature and at the same time the porosity of the aerogels decreased. The-optical transmission of the aerogel increased (up to approximate to 70%) with an increase in temperature up to around 400 degrees C and then the transparency decreased (to approximate to 20%) up to a temperature of 950 degrees C before increasing further to approximate to 90% at 1000 degrees C. A sudden change of pore size distribution of the aerogels was observed in the temperature range of 800 to 975 degrees C. The pore size distribution and the radii of pores increased up to a temperature of 800 degrees C and above this temperature the size distribution narrowed with decrease in pore radii. The results have been supported and discussed by considering the particle and pore sizes observed by scanning electron microscopy.