The optical properties of silica aerogels are best described by the phrase “silica aerogels are transparent”. This may seem obvious, as silica aerogels are made of the same material as glass. However, the situation is not as simple as that comparison. While distant objects can be viewed through several centimeters of silica aerogel, the material displays a slight bluish haze when an illuminated piece is viewed against a dark background and slightly reddens transmitted light. These effects are a result of Rayleigh scattering effects. The various aspects of optical transmission through silica aerogel are discussed below.
The vast majority of the light that we see when we look at objects is scattered light (light that reaches our eyes in an indirect way). The phenomenon of scattering leads to several well known natural effects, such as blue skies, red sunsets, the white (or gray) color of clouds, and poor visibility on foggy days. Scattering results from the interaction of light with inhomogeneities in solid, liquid, or gaseous materials. The actual entity that causes scattering, called the scattering center, can be as small as a single large molecule (with an inherent inhomogeneity) or clusters of small molecules arranged in a non-uniform way. However, scattering becomes more effective when the size of the scattering center is similar to the wavelength of the incident light. This occurs in small particles (~400-700 nm in diameter for visible light) that are separated from on another, or by larger, macroscopic, particles with inherent irregularities. When scattering centers are smaller in size than the wavelength of the incident light, scattering is much less effective. In silica aerogels, the primary particles have a diameter of ~2-5 nm, and do not contribute significantly to the observed scattering. However, scattering does not necessarily arise from solid structures. There is in silica aerogels, a network of pores which can act, themselves, as scattering centers (see the section on the pore structure of aerogels). The majority of these are much smaller (~20 nm) than the wavelength of visible light. There are, however, invariably a certain number of larger pores that scatter visible light. Control of the number an size of these larger pores is, to a certain degree, possible by modifying the sol-gel chemistry used to prepare the aerogel. As scattering efficiency is dependent on the size of the scattering center, different wavelengths will scatter with varying magnitudes. This causes the reddening of transmitted light (red light has a longer wavelength, and is scattered less by the fine structure of aerogels) and the blue appearance of the reflected light off silica aerogels.
A simple method can be used to quantitatively measure the relative contributions of Rayleigh scattering and the wavelength-independent transmission factor (due to surface damage and imperfections) for silica aerogels prepared with different recipes and/or drying procedures. Briefly, the transmission spectrum of an aerogel slab of known thickness is measured and the transmission is plotted against the inverse fourth power of the wavelength. These data are fit to the equation:
where T = transmittance, A = wavelength independent transmission factor, C = intensity of Rayleigh scattering, t = sample thickness, and Lambda = wavelength. From this plot A and C can be determined. Aerogels with a high value of A and a low value of C will be the most transparent. Scattering may also be accompanied by absorbance which will further attenuate the transmitted light.
VISIBLE TRANSMISSION SPECTRUM
The intrinsic absorbance of silica is low in the visible region. Therefore the tranmittance in this region is primarily attenuated by scattering effects. As wavelengths become progressively shorter, scattering increased, eventually cutting off transmission near 300 nm. Weak absorbances begin to appear in the near infrared, and again cuts off transmission around 2700-3200 nm.
UV-VIS-NIR Spectrum of Silica Aerogel
There is then a “visible window” of transmission through silica aerogel that is an attractive feature of this material for daylighting applications.
As the spectrum moves into the infrared, scattering becomes less important, and standard molecular vibrations account for the spectral structure. A strong, broad absorbance band is usually observed at 3500 cm-1, due to O-H stretching vibrations. A weaker O-H bending vibration band is seen at 1600 cm-1. Both adsorbed water and surface -OH groups contribute to these bands. Thoroughly drying the sample before analysis will eliminate vibrations due to water, while surface -OH groups can be significantly eliminated by firing the aerogel at 500 degrees C. The Si-O-Si fundamental vibration gives the strong band at ~1100 cm-1. There is a region of high infrared transparency between 3300 and 2000 cm-1. This allows a certain amount of thermal radiation to pass through silica aerogel and lower its thermal insulative performance. Addition of additives that absorb radiation in this region can remedy this problem (see the section on Thermal Conductivity).
Infrared Spectrum of Silica Aerogel
Special thanks to the Lawrence Berkeley Laboratory’s Microstructured Materials Group for permission to use this paper.