Science of Silica Aerogels
Nanocomposites
It was readily apparent to early researchers working on aerogels that they were ideal for use in composite materials. However, with much fundamental research needed into the preparation and properties of aerogels, this area has only recently been explored. In this discussion "composite" is used in the broadest sense of the term, where the final product consists of a "substrate" (the silica aerogel) and one or more additional phases (of any composition or scale). As all the materials considered here have a silica aerogel substrate, there is always at least one phase with physical structures with dimensions on the order of nanometers (the particles and pores of the aerogel). The additional phases may also have nanoscale dimensions, or may be larger (up to centimeters). Because of this the materials can legitimately be classified as "nanocomposites". There are generally three routes to aerogel nanocomposites: addition of the second component during the sol-gel processing of the material (before supercritical drying), addition of the second component through the vapor phase (after supercritical drying), and chemical modification of the aerogel backbone through reactive gas treatment.
Aerogel Nanocomposites Through Sol-Gel Processing
These composites were prepared by adding metal salts, or other compounds to a sol before gelation. The deep blue aerogel contains nickel; the pale green, copper; the black, carbon and iron; the orange, iron oxide; and the remaining aerogels, organic compounds.
This approach is the logical first route to aerogel nanocomposites and can produce many varieties of composites. There are, however, limitations to these procedures. Simply stated, a non-silica material is added to the silica sol before gelation. This added material may be a soluble organic or inorganic compound, insoluble powders, polymers, biomaterials, bulk fibers, woven cloths, or porous preforms. In all cases, the additional components must withstand the subsequent process steps used to form the aerogel (alcohol soaking, and supercritical drying). The conditions encountered in the CO2 drying process are milder than the alcohol drying process and are more amenable to forming composites. If the added components are bulk, insoluble materials (such as carbon fibers or mineral powders), steps must be taken to prevent the settling of the insoluble phase before gelation. This can often be accomplished by gently agitating the mixture until gelation is imminent. The silica aerogel with the best thermal properties results from the addition of a small amount of carbon black to the sol using this technique.
The addition of soluble inorganic or organic compounds to the sol provides a virtually unlimited number of possible composites. There are two criteria that must be met to prepare a composite by this route. First, the added component must not interfere with the gelation chemistry of the silica precursor. This is difficult to predict in advance, but rarely a problem if the added component is reasonably inert. The second problem encountered in this process is the leaching out of the added phases during the alcohol soak or supercritical drying steps. This can be a significant impediment if a high loading of the second phase is desired in the final composite. When the added component is a metal complex, it is often useful to use a binding agent, such as (CH3O)3SiCH2CH2NHCH2CH2NH2. This can bind with the silica backbone through the hydrolysis of its methoxysilane groups and chelate the metal complex with its dangling diamine. This general approach has been used by several research groups to prepare nanocomposites of silica aerogels or xerogels. After the gel has been dried, the resulting composite consists of a silica aerogel with metal ions atomically dispersed throughout the material. Thermal post-processing induces thermal diffusion and reduction of the metal ions, forming nanometer-scale metal particles within the aerogel matrix. These composites are being extensively studied for use as catalysts for gas-phase reactions.
Aerogel Nanocomposites Through Chemical Vapor Infiltration
This composite was prepared by chemical vapor infiltration. The dark spots are Fe3O4 nanocrystals. Many of these are single domain.
The open pore network of silica aerogels allows for easy transport of vapors throughout the entire volume of the material. This provides another route to an aerogel nanocomposite. Virtually any compound with at least a slight vapor pressure can be deposited throughout a silica aerogel. In fact, silica aerogels should be stored in a clean environment to prevent the unwanted absorption of volatile pollutants. To prevent subsequent desorption of the added phase, it is useful to convert the adsorbed material into a non-volatile phase by thermal or chemical decomposition. This can be done during, or after the initial deposition. The Microstructured Materials Group has prepared a wide variety of aerogel nanocomposites using this process, including:
- Silica aerogel-Carbon composites
- These have been prepared through the decomposition of various hydrocarbon gases at high temperatures. However, due to the fine structure of silica aerogels, the decomposition take place at a much lower temperature (200-450 degrees C) than the corresponding decomposition in the absence of the aerogel. Carbon loadings ranging from 1-800% have been observed. Surprisingly, at lower loadings, the carbon deposition is relatively uniform throughout the volume of monolithic aerogel slabs. At higher loadings, the carbon begins to localize at the exterior surface of the composite monolith. Interesting aspects of these composites include electrical conductivity at higher loadings, and mechanical strengthening of the composite relative to the original aerogel.
- Silica aerogel-Silicon composites
- The thermal decomposition of various organosilanes on a silica aerogel forms deposits of elemental silicon. In this case the rapid decomposition of the silane precursor leads to deposits localized near the exterior surface of the aerogel substrate. Thermal annealing of the composite induces crystallization of the silicon. The resulting composite, with 20-30 nm diameter silicon particles, exhibits strong visible photoluminescence at 600 nm.
- Silica aerogel-Transition Metal composites
- Organo-transition metal complexes are ideal precursors for this type of composite. Even the least volatile of these possesses a sufficient vapor pressure to be deposited within an aerogel. Under controlled conditions, these deposit uniformly throughout the entire volume of the aerogel monolith. Typically, the metal compounds are then thermally degraded to their base metals. These intermediate composites are generally highly reactive, due to the disperse nature of the metallic phase, and can be easily converted to metal oxides, sulfides, or halides. This process can be repeated several times to increase the loading of the metallic phase. Typically composites prepared in this way possess crystals of the desired metal compound on the order of 5-100 nm in diameter.
The graphic below displays the magnetization/demagnetization curve for a silica aerogel/Fe3O4 composite prepared by this process. The curve shows that the composite is a "soft" ferromagnetic material. The magnetite crystals in this composite are 20-60 nm in diameter. Many appear to be single domain, as observed by electron microscopy.
The Microstructured Materials Group has a patent pending on various aspects of this process, which is available for Technology Transfer.
Aerogel Nanocomposites Through Energized Gas Treatment
These pieces of silica aerogel have been coated with silicon nanoparticles using chemical vapor methods. The composites emit red light when excited with ultraviolet light.
The Microstructured Materials Group has recently discovered a simple process that can alter the chemical structure of the silica (or other oxide) backbone of an aerogel. This process utilizes an energized reducing (or other) gas to form thin films of new material on the interior surface of the aerogel. The techniques used in this case are similar to standard plasma methods. However, the nanoscale pore structure of silica aerogels prohibit the formation of a plasma within an aerogel. Nevertheless, the centers of thick monoliths are affected by this process. In the simplest case, silica aerogel monoliths are partially reduced by energized hydrogen. The resulting composite consists of a silica aerogel with a thin layer of oxygen-deficient silica (SiOx) on the interior surface. As with other reduced silica materials, this material exhibits strong visible photoluminescence at 490-500 nm when excited by ultraviolet (330 nm) light. However, the process used in this case is relatively gentle, and does not alter the physical shape or optical transparency of the original aerogel. This composite is the basis for the aerogel optical oxygen sensor.
The Microstructured Materials Group has a patent pending on this process, which is available for Technology Transfer.
As noted above, several aerogel nanocomposites exhibit strong visible photoluminescence. The spectra shown below are for the silicon nanoparticles/silica aerogel (red emitter) and gas-treated reduced silica aerogel composite (blue-green emitter)
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