A Sol-Gel Route to Very High Porosity Silica Aerogels
- Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
There is interest at Lawrence Livermore National Laboratory in very high porosity (ie. porosity > 99%), transparent silica aerogels for applications involving high energy charged particle detection via the Cerenkov effect, and for targets used in inertial confinement fusion experiments. Conventional silica sol-gel chemistry is limited for the production of transparent aerogels having porosities greater than 99% because gelation is either so slow or unachievable, and because, even when gelation is achieved, the large pore sizes result in loss of transparency. We have developed a two-step sol-gel process that bypasses the limitations of the conventional process and allows the formation of very high porosity silica aerogels which are transparent up to the highest achieved porosities. Silica aerogels characteristically have a high porosity (defined as the pore volume fraction within the bulk) because their nanometer sized particles link together to form a gel with an open three-dimensional structure which survives the removal of the liquid phase with minimal collapse. The transparency of aerogels also results from the smallness of the scattering objects (particles and pores) within the aerogel. According to Rayleigh's theory, transparency is assured when the average scatterer size is a fraction of the transmitted wavelength. This requires that the average scatterer size is less than 50nm as a condition for transparency of aerogels at visible wavelengths. Considering an ideal model for the aerogel as consisting of uniform spherical particles making up unit cells and the 50nm pore size limit, it can be shown that the average particle size must be less than 5nm to achieve porosities greater than 99% and still maintain transparency. However, the average particle and size pore measured for conventional aerogels is found to increase as the porosity is increased above 97%.
- Research Organization:
- Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 6191997
- Report Number(s):
- UCRL-JC-103740; CONF-910291-2; ON: DE91008501
- Resource Relation:
- Conference: 5. Ultrastructure Processing Conference, Orlando, FL (United States), 17-21 Feb 1991
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
GELS
MICROSTRUCTURE
SILICA
POROSITY
SOL-GEL PROCESS
SPECTROSCOPY
CHALCOGENIDES
COLLOIDS
CRYSTAL STRUCTURE
DISPERSIONS
MINERALS
OXIDE MINERALS
OXIDES
OXYGEN COMPOUNDS
SILICON COMPOUNDS
SILICON OXIDES
360602* - Other Materials- Structure & Phase Studies
360601 - Other Materials- Preparation & Manufacture