Aerogel materials deliver thermal insulation in both hot and cold applications.

Moving a concept from prototype to production is often the most challenging part of innovation, not unlike building a really cool boat in your garage, only to find you can’t get it out the door. Aerogel materials represent a classic example.

The first aerogel was created by a scientist named Steven Kistler in 1931, yet aerogel materials have only recently made it out of the lab and into the market. The reason is that an aerogel is a devilishly difficult thing to produce. By way of analogy, imagine making a bowl of Jell-O, then, afterward, finding a way to extract all the water and replace it with a gas, such that the level of Jell-O in the bowl has not diminished — no shrinkage. Then, after performing that trick, figure out how to do it repeatedly on a production scale in a cost-effective manner so that potential customers find it economically feasible to use.

The successful execution of such magic delivers an extremely low-density material, so lightweight it’s often called “frozen smoke.” Moreover, the ratio of gas to solid, and the exceedingly small scale by which the gas becomes entrapped, make aerogels excellent thermal insulating materials. The promise of super-insulation inspired many to pursue research and development of aerogels over a period of decades, but only in the past few years have commercial aerogel products become available. One of the few producers of aerogel products is Aspen Aerogels, Northborough, Mass., which discovered a solution to the challenge of manufacturing them on a production scale.

Aerogel material shown in its monolithic form.

Aerogels are made by taking a gel, then removing the liquids by a process called supercritical drying. Using a combination of pressure and temperature to push the fluid to its supercritical state between a gas and a liquid removes the surface tension of the liquid that would otherwise destroy the lattice-work of solids as it exits the pores. This approach allows the structure of suspended solids to remain intact. The resulting material is akin to a sponge, or a meringue, only one where the lattice-work of pores is sized on the nanoscale level.

The first aerogels were made from silica gels. Eventually, aerogels were also developed using other materials, including alumina, chromia and carbon. In the early days of their development, one the biggest problems with aerogels was that, after the liquid was evaporated, the resultant material tended to be brittle and difficult to work with.

Aspen Aerogels leaped this hurdle by developing a breakthrough, proprietary process for creating aerogels within a matrix of non-woven fabric. It begins by impregnating blankets of the textile with silica gel. The material is then pressurized to take the gel’s liquid to a supercritical state, where it is then extracted, leaving the resulting aerogel integrated into the matrix of the fabric. This yields a product that is roughly 95 percent air and possesses both thermal and acoustic insulation properties anywhere from two to eight times greater than competing insulation materials.

Enlarge this picture Table 1. Thermal conductivity comparison of various insulating materials.

The patented Aspen Aerogels process allows the company to economically produce the aerogel blanket material on a high-volume, mass-production basis. Prices for the material vary by thickness and composition, but are now within the realm of feasibility for many applications. And though the prices are still significantly higher than conventional materials such as fiberglass and foams, they are expected to fall in the future as production capacity increases. In many applications, some of the added cost can be recouped by cost-saving design changes that a higher insulating value permits. Aspen’s goal is to make the aerogel blankets price-competitive with fiberglass and polyurethane foam.

Importantly, the process of impregnating fabric with aerogel yields a product that is flexible, durable, waterproof, and easy to fabricate with simple cutting tools.

The flexibility of the product has attracted NASA’s interest as a possible insulating material for space suits. Another key attribute of the aerogel blanket is that, unlike traditional fiber-based or flexible-foam insulating materials, the aerogel textile is not dependent upon loft for its insulating value. It still retains its insulating properties even when compressed.

That’s why the material can already be found as the basis for Polar Wrap’s Toasty Feet inserts for footwear. Aspen’s current products can handle compressive loads up to 150 psi, but the company expects to push that much higher in the future, all the way to 4,000 psi.

Enlarge this picture Fig. 1. Thermal conductivity comparison between different insulation materials at ambient temperature and pressure. Values represent general averages for various product forms.

Aspen’s aerogels are also chemically treated at the molecular level to repel water, which further broadens their application range. Using ASTM C-1104 and third-party testing protocol C-1104, the Aspen aerogel blankets were shown to absorb less than 1 wt% water.

The most significant property of the aerogel materials, however, is their extremely low thermal conductivity. (See comparisons in Table 1, Fig. 1 and Fig. 2.) Aerogels achieve this by means of low-solid content, high pore volume, low pore size averaging 10 nm, and an amorphous structure, which creates a tortuous path for heat.

Enlarge this picture Fig. 2. Thermal resistance (expressed as R-value per inch) for different insulation materials at ambient temperature and pressure.

The low thermal conductivity of aerogel materials allows appliance designers to achieve higher insulating values while devoting less space to insulation. This gives them the option of maintaining current insulation values while reducing the insulating cavity, or maintaining current insulating cavity and increasing the insulating value. The former approach could be used to increase interior dimensions of a refrigerator. The latter might be used to meet an increased refrigeration efficiency standard without resorting to more expensive compressor technology. The temperature resistance of aerogels permits them to provide thermal insulation for both hot and cold applications. The materials can withstand the extremes of cryogenic temperatures, but also heat up to 1,200 DegF.

Enlarge this picture Fig. 3. Pressure dependence of Aspen Aerogels’ Spaceloft 6250 at 23 DegC after conditioning in a dry nitrogen atmosphere.

Aspen Aerogels materials are already being used in industries such as apparel, aerospace, transportation, and oil & gas. Developments are also underway for many other industries, including residential and commercial appliances, where applications might include ovens, ranges, cooktops, water heaters, refrigerators, and freezers. The aerogel materials will help appliance makers meet energy efficiency standards and achieve Energy Star ratings more easily. The materials can also save on production costs, because they are easily installed and easy to handle, not requiring complex equipment associated with blown-polyurethane foam systems.

While trying to increase the market penetration of its current products, Aspen Aerogels continues research and development on aerogels in an effort to lower costs, discover new materials and form factors, and find new applications.

For more information email: mfinkle@aerogel.com

SIDEBAR: Aerogel Properties

Below are some general properties of silica aerogel blankets made by Aspen Aerogels.

· Thermal Conductivity: 0.011-0.013 W/m-K at 38 DegC (100 DegF) and 760 torr. Conductivity decreases to 0.004 W/m-K at 10 torr.

· Constant Use Temperatures: From –273 DegC/-459 DegF to 650 DegC/1,200 DegF.

· Density: Currently available in densities from 0.10 g/cm3 to 0.12 g/cm3 (6 pcf to 8 pcf). Nominal surface areas are between 400 m2/gram and 1,000 m2/gram depending on formulation.

· Surface Area, Pore Size, Morphology, and Distribution: Open celled structure (2 nm to 50 nm pores) with an average pore size approximately 10 nm.

· Flexibility: Conformable at 0.25 in. thickness, drapeable at 0.125 in. thickness.

· Compressive Strength: Aerogels typically have excellent compressive strength compared to microcellular foams and fibrous insulations.

· Hydrophobicity: Unencapsulated materials will float on pure water indefinitely and can resist liquid water infiltration at hydrostatic pressures up to 750 psi.

· Acoustic Properties: At 100 m/s, aerogels have an extremely slow speed of internal sound propagation. Sound transmission through aerogel is significantly retarded with fiber reinforcement.

· Toxicity: Aspen Aerogels materials are based on amorphous silica gel, which is considered safe and non-toxic. In typical handling, unencapsulated materials will generate nuisance dust.