Silica aerogels have been prepared through sol-gel process by polymerization of TEOS in the presence of NH4F and NH4OH as catalysts. The solvent present in the gel is replaced by ethanol followed by a non-polar solvent such as n-hexane prior to solvent modification step. Gels are made hydrophobic by treating them with HMDZ to prevent rupture during drying, which has been confirmed by FTIR. Gels are then washed and dried carefully in a PID controlled oven at atmospheric pressure. The ageing duration and solvent exchange combinations are optimized to yield crack-free gels prior to drying. Aerogels are characterized for density, specific surface area, pore volume, pore size, thermal stability and contact angle. Hydrophobic, high surface area (570 m2/g), low density (0.07 g/cm3) silica aerogels are synthesized by using optimized mole ratio of precursors and catalysts. Silica aerogel granules (1-3 mm) as well as monoliths (Ф~35 mm) could be produced through ambient pressure drying of gels.

Materials with relative permittivity or dielectric constant near to that of air (εr~1) are known as ‘ultra-low k’ materials. They find a number of applications in inter-connects of micro-electronic circuits, antennae, high-speed communication substrates etc. Among the inorganic materials, porous silica is the widely studied candidate. Porous silica can be of many types depending upon the extent of porosity and size and connectivity of pores. This paper presents the details of measurement of permittivities and the results of silica beads and silica aerogels. Silica beads, prepared by microwave heating of silica gels, are spherical beads of average 1mm size. Hydrophobic silica aerogels, prepared by ambient pressure drying of silica gels, are irregular chunks of 5-10 mm size. Both are potential bulk fill insulation materials and hence the permittivity can be measured as an aggregate filling a definite volume. The permittivities of these have been measured upto 1 MHz by 3-terminal method using a precision LCR meter and a powder-paste cell as per ASTM-D150-11. The εr values of silica aerogels and silica beads in 20 Hz-1 MHz range could be measured and are less than 1.6 at 1 MHz.

Conventional methods of processing hollow silica granules are tedious, expensive and time consuming. The present work aims at the production of hollow silica granules by a rapid, simple & cost-effective process, by employing microwave heat treatment which reduces the time factor drastically. Microwave processing is an emerging field and is fast catching up as an excellent alternative to the electrical heating methods. Spherical hollow silica granules with in-situ fibrous network were obtained by firing the sol-gel derived silica gel in a microwave furnace. Sol was synthesized using TEOS, distilled water, HCl & Ethanol and was allowed to age in a wide tray at ambient temperature & pressure. The ageing time of the gel was varied from 1-80 days and the characteristics of the granules hence obtained were studied. The temperature of heat treatment was also varied and it was observed that spherical granules form in the temperature range of 1250-1400°C. For comparison, the same gel was heat treated in a conventional furnace and the characteristics of the granules hence obtained were also studied. It was observed that conventionally sintered granules were less dense than the microwave derived ones but were not hollow. This paper describes the processing of hollow silica granules and the effect of gel ageing duration and temperature of sintering on the final product obtained.