Aerogels

Location:

Concepts Shown:

Inorganic polymer chains

Equipment:

The various sol-gels that Prof. Laine has, and possibly the sample aerogel that he is supposed to receive.

Procedure:

The knowledge of aerogels is limited. Thus there is a chance that a majority of the students will not be aware of aerogels. Passing the mystery material will arouse curiosity. Telling people that aerogels are the best solid thermal insulators and that they are almost as light as air, raises interest. The certain wow factor can be utilized to talk about the more scientific aspects of the aerogel. For instance, its unique pore structure and silica base. Most people associate the word 'organic' (carbon backbone) with the word 'polymer'. It is important to know about inorganic polymers as well, which have silica backbones.

Science:

Aerogels Most gels are made out of an amorphous form of silica. Ordinary gels consist of networks of particles suspended in a liquid. Jell-o gelatin is the best example of a simple gel. An aerogel is a silica gelatin with one difference. It has had all its water removed without destroying its solid structure. The result is an airy solid whose particles and pores are only a few hundred angstroms across, far smaller than the shortest wavelength of visible light. A result of its tiny structure is the bluish tint it emits under certain lighting conditions. This tint is caused by the Rayleigh scattering of light waves. Silica gels form the raw materials for aerogels. Aerogels are extremely poor conductors of heat. This property is directly related to the fact that any gas that would try to permeate an aerogel, would first have to negotiate a tortuous path through its pores. In fact, as a result, aerogels are the best solid insulators in existence. Supercritical extraction is the process used to make aerogels. In this process, the water in the gel is replaced with alcohol that is then heated far above its boiling point under great pressure. At this point the alcohol is termed supercritical. Usually, removing a fluid from a gel's pores would cause collapse of its solid structure under the force of surface tension . Since the supercritical alcohol lacks surface tension, it can be extracted from the gel without destroying it. This process is however, both expensive and dangerous. Alcohol is comparable in chemical energy to dynamite or gasoline. The lightest aerogel has twice the density of air.

SEAgels

SEAgels are a family of solids made from agar, a protein found in kelp. SEAgels (Safe Emulsion Agar gels) are easier and cheaper to make than aerogels. Starting with agarose, a white powder derived from seaweed, a gel is made by dissolving the powder in water and then cooling the solution. The water is easily removed by freeze drying. Since the pores of the SEAgels are 100 times the size of those in aerogels they can accommodate ice crystals that would destroy the structure of silica gels. The ice is then drawn off in a vacuum. The foamy white solid left behind is so light, that only the weight of the air in its pores keeps it on the ground. If the air could be removed, it would be a lighter that air solid. SEAgels are quite fragile, by by increasing the amount of agarose, stronger SEAgels can be made.

Remarks:

The one sample gel brought to class was an example of a xerogel. It was made from a 1:1:1 ratio of sodium alumina silicate. The solvents used were ethylene glycol and water. The usual ceramic yield of the evaporation process (excess solvent is removed) is about 2% of the initial weight. The xerogel can be used as a starting material for the aerogel process.

There have been numerous inquiries made as to the uses of aerogels in industry. Some aerogels are unparalleled at blocking sound. This feature has made them very interesting to the Navy, which has envisioned quieter submarines. Conventional thermal insulation utilized in refrigerators will be banned by the year 2000, as they release CFC's, which are detrimental to the ozone layer. Aerogel insulation would increase efficiency and pose no environmental threat. Additionally, most silica aerogels would turn to sand when exposed to moisture, making them biodegradable. As a result of an aerogel's enormous surface area, (a piece the size of a sugar cube, may contain as much surface areas as four or five basketball courts) it could be used to make a supercapacitor, that will solve the problem of the electric car - how to accelerate a car from standstill without draining the battery. At present however, aerogels are used to snare micrometeoroids. Because of its minute pores and low density the aerogel offers little resistance to the cosmic dust.

References:

Author:
Rahul Pinto
Credits:


Related Equipment
Related Supplies