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Material Preparation and Processing
by Dr. Michael Droege, Ocellus Tecnologies, CA
I. Technology Overview
A. Aerogel Materials
Porous solids such as wood, coral, and bone are common structural materials found in nature. These materials have evolved a cellular structure in which the material of the cell wall surrounds an open pore space. This porous, cellular structure, imparts unique properties such as stiffness and strength while simultaneously being a low density (lightweight) material.
Synthetic foams can be prepared from a wide variety of metals, glasses, and polymers. These foams also possess a cellular structure in which the cells are either isolated (closed-cells) or interconnected (open-cells), depending upon the production process. Organic polymer foams are well known and commonly used in the insulation and construction industries. Examples of commercially available organic polymer foams include polyurethane seat cushions and polystyrene coffee cups.
Aerogels are a unique class of polymer foams that can be prepared from either organic or inorganic polymers. Aerogels typically possess a nanostructure that is composed of extremely small interconnected particles (like pearls on a pearl necklace) that have diameters of about 100 angstroms and which surround pore spaces that are typically less than 100 nanometers. This structure results in a solid material with ultrafine pore size, low density, and open-cell porosity. It is these structural features that impart the unusual acoustic, mechanical, optical, and thermal properties observed for aerogels. For example, the nanostructure is responsible for the high surface area of aerogels (350-1500 m2/g) and its very low sound propagation. The ultrafine pore size minimizes the scattering of visible light, thus aerogels can be prepared as transparent, porous solids. The high porosity and small pore size of aerogels makes them lightweight, superinsulating materials. It is the combination of these unique properties that is now being exploited by the artist.
B. Aerogel Preparation and Processing
A two-step process typically produces aerogels. Step one is a chemical reaction known as a "sol-gel" reaction that produces the aerogel structure in solution. Step two is a special drying procedure called a "supercritical extraction" that results in the final dried solid. The sol-gel reaction is well known and used commercially to produce, for example, soft contact lens. The supercritical extraction process is used commercially to purify a wide array of products, such as decaffeinating coffee, extracting fats, refining drugs, etc.
The term sol-gel describes the change from a colloid-containing liquid solution to a semisolid gel (jelly). In the case of an aerogel, the starting materials are mixed in alcohol and water; they then undergo a chemical reaction that results in transformation from a liquid to a jelly-like solid. This gel consists of the aerogel's continuous, three-dimensional solid framework while the pores of the gel hold the liquid. These gels typically have very low strength and are soft, flexible, and do not flow.
The objective of the drying step is to remove the liquid from the pores while maintaining the aerogel's nanostructure. Because of the very small pore size, simple evaporation of the liquid results in very large internal forces (capillary pressure) that cause shrinkage and cracking of the gel. The use of supercritical extraction overcomes this drying problem. Under specific temperature and pressure conditions, the liquid becomes gas-like, with little surface tension, and can be readily removed from the pores of the gel without developing destructive internal forces. The result of supercritical drying is that the aerogel maintains the original size and shape of the gel, but now the pores contain only air (hence the term aerogel).
II. aer( )sculpture Materials and Processing
A. aer( )sculpture Gel Formation
To begin this artistic endeavor, we are using a classic aerogel of silica (silicon dioxide or glass). This material, originally described by Kistler in the 1930's, is well known and possesses a large body of scientific knowledge. The sculpture gel is produced by the reaction of a silane tetralkoxide with water in an alcohol solvent to produce a clear liquid. The amount of alcohol solvent in the formulation can be varied to obtain a range of densities of the gel. Control over this parameter allows the artist to achieve different visual effects such as color and translucency. This liquid is then poured into a mold created by the artist. After some time has past, the liquid transforms to a solid gel that now possesses the physical characteristics (size, shape, surface texture) of the mold. The solvent-laden sculpture is now ready for the drying process.
B. aer( )sculpture Drying Process
The solvent is removed by supercritical extraction. In this case, the highest quality sculptures are obtained by processing the gel at the supercritical point of the alcohol solvent (greater than about 250 șC and 1200 psi). To successfully process these gels and produce an
aer( )sculpture that is true to the artist's vision, we use a new, patented aerogel processing technology that allows the fabrication of precise, net-shaped, monolithic aerogels. Our method allows the simple production of monolithic aerogel with irregular shapes and surface features. This method is excellent for producing monolithic aerogels possessing artistic content. In addition, our process technology allows a significant increase in the speed at which the supercritical extraction can be performed. For example, a high quality, monolithic
aer( )sculpture can be produced in only a few hours.
A key benefit of this technology is that aerogel processing time can be significantly reduced and throughput increased without sacrificing quality or appearance.