Monolithic silica aerogel - material design on the nano-scale

By means of a production process in two major steps - a sol/gel process and a supercritical drying – open-cell, monolithic silica aerogel can be made. This material can have a density in the range of 30- to 300 kg/m3, corresponding to porosities between 86 and 98 %. The solid structure has characteristic dimensions of 2 – 5 nm and the typical pore diameter is about 20 nm.

A number of different applications of aerogel have been proposed: - Because of the large internal surface of aerogel (up to 1000 m²/g), the material is proposed to serve as substrate for catalytic materials. - The special pore structure of aerogel could be used for gas filters in the 20 to 100 nm region. - The sound velocity within aerogel is in the range of 100 to 300 m/s, which should be one of lowest for an inorganic material.

Due to the low density, low acoustic impedance of aerogel could help boost the efficiency of piezoelectric transducers. - Other applications could be; waste encapsulation, spacers for vacuum insulation panels, membranes, etc. Department of Civil Engineering is co-ordinator of a current EU FP5 research project1, which deals with the application of aerogel as transparent insulation materials in windows.

Due to the excellent optical and thermal properties of aerogel, it is possible to develop windows with both high insulation and high transmittance, which is impossible applying the conventional window techniques, i.e. extra layers of glass, low-e coatings and gas fillings. It can be shown that an aerogel window, North facing and for a Danish location, will have a positive energy balance over the heating season, i.e. the aerogel window will contribute to the space heating, whereas a conventional super low-energy window cannot perform beyond a neutral energy balance for the same conditions2.

Finally, it can be mentioned that one of the partners, the Swedish company Airglass AB, in the above-mentioned EU project, has produced the aerogel material used for a dust-analyser on the Stardust satellite, which collected dust from the comet “Wild 2” in January 2004. It is expected that Stardust will return to Earth on January 15, 2006 (more information on the NASA website: http://stardust.jpl.nasa.gov/). 1”Highly Insulating and LIght Transmitting aerogel glazing for Super Insulating Windows” (HILIT+), contract ENK6-CT-2002-00648. 2 K.I.

Jensen, J.M. Schultz & F.H. Kristiansen, J. of Non-Crystalline Solids, Vol. 350, 2004, pp 351-357