Light scattering and haze in TMOS-co-APTES silica aerogels
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TMOS-co-APTES wet-gels have been used as templates for polymer crosslinked aerogels, which in turn have been used as strong materials and as starting materials for ceramic aerogels. Such wet-gels are prepared in acetonitrile (CH3CN) via co-gelation of tetramethylorthosilicate (TMOS) and 3-aminopropyl silane (APTES) in a 3:1 v/v ratio. Qualitatively, native monolithic TMOS-co-APTES aerogels appear more clear than the corresponding straight TMOS-derived materials, suggesting applications in thermally insulating daylighting. For that application, it is important to understand and minimize haze. Haze from the bulk of silica aerogels may be due to scattering from particles or pores, but study of its origin may be complicated by scattering and reflections from the surface of the monolith. In this report, using wet-gels as proxy materials for aerogels, haze from TMOS-co-APTES was investigated as a function of density, which is controlled by the amount of TMOS + APTES in the sol. The wet-gel approach to the study of haze of aerogels has the advantages of eliminating surface scattering issues from aerogel monoliths, and owing to better refractive index matching between the skeletal framework and the surrounding medium (CH3CN), bulk scattering is also reduced, facilitating study of lower-density materials. Haze from the bulk was traced to Rayleigh scattering down to 0.017 g cm−3 aerogels. The opacity of even lower-density samples (down to 0.006 g cm−3) was attributed to the Mie scattering. The scatterer size was obtained via Rayleigh’s law, and matched the secondary particle sizes of silica. The size of those particles increased as the density decreased, producing higher haze. Results suggest that in order to keep haze low at lower densities, one must keep the particle size low, for example, by increasing the rate of gelation.
The haze of the title silica aerogels increases as their density decreases.
Wet-gels are good proxy materials for measuring haze in lower-density silica aerogels.
Wet-gels do not have surface scattering issues, and better index matching reduces haze, allowing measurements at lower densities.
The haze of the title silica aerogels was traced to Rayleigh scattering from secondary skeletal silica particles to densities as low as 0.017 g cm−3.
Increased haze at lower densities was attributed to increasing particle size.
KeywordsHaze Clarity Rayleigh scattering TMOS APTES Aerogels Wet-gels
We thank ArpaE under Award Number 1425-1506 and the NSF under Award No. 1530603 for financial support. We also thank the Materials Research Center of the Missouri University of Science and Technology for support with materials characterization.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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