Arlon J. Hunt

Ambient-temperature supercritical drying of transparent silica aerogels☆

Abstract A novel technique to supercritically dry alcogels to transparent silica aerogels at near ambient temperature is described. This supercritical drying occurs at ⩽ 40°C instead of at ⩾ 270°C, by substituting liquid CO2 for the alcohol in the alcogel. The time required for drying is reduced from 2–3 days to 8–10 h. Light scattering, optical and micro-structural properties of aerogels produced by this process and by high-temperature supercritical drying are compared and shown to be similar.

Transport properties of gas in silica aerogel

Abstract The motion of gas molecules in silica aerogel is restricted by the solid silica matrix. As a result, the mean free path, velocity distribution function, diffusivity, viscosity and thermal conductivity are changed. In this work, relations for these quantities are derived for a gas in silica aerogel using an approach similar to that used for a gas in free space. Results for the mean free path predict that, for p ≥ 10 bar, the mean free path of the gas molecules in aerogel will be almost the same as in free space. However, as the pressure is reduced, the mean free path reaches a constant finite value instead of increasing as in free space. The thermal conductivity of a gas in aerogel starts to decrease at p = 10 bar and is almost negligible at 0.01 bar, while the thermal conductivity of a gas between parallel walls 1 cm apart starts to decrease at p = 10 −4 bar and is almost negligible at p = 10 −7 bar. The predicted thermal conductivity of a gas in aerogel is in good agreement with experimental results.

Thermal characterization of carbon-opacified silica aerogels

A new method to introduce carbon into aerogels to block the infrared component of radiant heat transfer within the aerogel was developed to improve the thermal properties of silica aerogel. Chemical vapor infiltration and heat treatment were used to deposit and grow nanophase carbon deposits inside the aerogel. Infrared measurements were performed on the doped material to determine the absorption. A vacuum thermal tester was used to measure the thermal conductivity as a function of air pressure and specimen temperature on flat plate aerogel samples.

Theoretical modeling of carbon content to minimize heat transfer in silica aerogel

Abstract Silica aerogel has a small absorption coefficient over the range 3–8 μm where significant thermal energy is transferred by radiation. Adding carbon to silica aerogel reduces thermal radiation but increases solid conduction. Whether the total energy transfer increases or decreases depends on the carbon content. This paper presents a theoretical method for determining the optimal carbon-loading level in silica aerogel to minimize the energy transfer. This method includes calculation of heat transport by coupled conduction and radiation through aerogel which is optically thin in some spectral ranges and thick in others, and the calculation of solid conductivity and spectral absorption coefficient, both of which vary with the carbon content. At ambient temperature, about 8% carbon in silica aerogel can lower the total energy transfer by about 1 3 . At temperatures as high as 600 K, non-opacified aerogel has a total energy transfer that is 10 times bigger than that of opacified aerogel with optimal carbon content. The optimal carbon content that minimizes total energy transfer increases linearly with temperature.

Effective optical constants n and κ and extinction coefficient of silica aerogel

In this work the normal reflectance, {ital R}, at a planar silica aerogel interface and the normal transmittance, {ital T}, of a silica aerogel slab were measured using a Fourier Transform Infrared Spectrometer. Two procedures were used to obtain the effective optical constants, i.e., the refractive index {ital n} and the absorption index {kappa}, of silica aerogel. One procedure determined {kappa} from the measured transmittance {ital T} and then determined {ital n} from the results for {kappa} and from the measured reflectance {ital R} using the Kramers-Kronig relation; the other procedure determined {ital n} and {kappa} of silica aerogel from {ital n} and {kappa} of fully dense silica glass by using the Clausius-Mossotti equation, Maxwell Garnett formula, and Bruggeman formula. The first procedure has a relatively large error due to the inaccuracy of the transmission and reflection measurements. The second procedure, especially the Clausius{endash}Mossotti equation, yields values of {ital n} that are consistent with experiments and may be used for the calculation of the effective optical constants and the extinction coefficient of silica aerogel. {copyright} {ital 1996 Materials Research Society.}

Improving the visible transparency of silica aerogels

Abstract Silica alcogels have been prepared in two steps, first in an acid and then in a base to control the relative rates of hydrolysis and condensation of TEOS. Increasing the H2O/TEOS ratio in the base-catalyzed step was the most important factor in reducing light scattering of the alcogel. The two-step gel could be practically made with H2O/TEOS = 30, achieving a scattering intensity less than 1 5 that of the best one-step gel. Lower scattering of the gel results in an aerogel with improved visible transparency and more homogeneous microstructure. TEM observation reveals that the two-step aerogel has a microstructure containing both linear polymers and colloidal particles that are cross-linked to a gel network.

Aerogel composites using chemical vapor infiltration

A new method to produce novel composite materials based on the use of aerogels as a starting material is described. Using chemical vapor infiltration, a variety of solid materials were thermally deposited into the open pore structure of aerogel. The resulting materials possess new and unusual properties including photoluminescence, magnetism and altered optical properties. An important characteristic of this preparation process is the very small size of the deposits that gives rise to new behaviors. Silicon deposits exhibit photoluminescence, indicating quantum confinement. Two or more phases may be deposited simultaneously and one or both chemically or thermally reacted to produce new structures.

Photoluminescence of chemically vapor deposited Si on silica aerogels

We have prepared in situ porous Si by the decomposition of SiH4 at 500 °C on an aerogel substrate. Electron microscopy studies indicate that the as‐deposited Si is primarily amorphous while the sample annealed in Ar at 800 °C has various nanometer‐sized crystalline Si particles. Visible photoluminescence (PL) can be observed only from the annealed sample and the PL peak red shifts with the annealing temperature from 800° to 1000 °C. The results support the quantum confinement theory as the luminescence mechanism in porous Si.

Carbon nanostructures in silica aerogel composites

A new method of preparing carbon nanotubes and their derivatives using silica aerogels as a matrix for the deposition of carbon is repeated. We present results of observations of graphite tubes and rings including nested structures in nanometer dimensions using high resolution transmission electron microscopy. Furthermore, we propose a model for the growth of carbon nanotubes in three steps including nucleation, growth, and closure of tubes.

SILICA AEROGEL, A TRANSPARENT HIGH PERFORMANCE INSULATOR

ABSTRACT Aerogel is a transparent, low density, insulating solid useful for a variety of applications. We have been investigating silica aerogel for use as an transparent insulator between double glazings to form a high thermal performance window or solar collector cover that is capable of increasing the thermal resistance of conventional double glazing by a factor of 5 times or more. We report recent progress in the preparation of clearer silica aerogel and the status of scale up and commercialization activities.