Aerogel



Aerogels are extremely light, low density substances able to withstand extreme temperatures and often hold large amounts of weight. They are derived from a gel, in which the liquid component of the gel has been replaced with a gas - thus the name aerogel. They are also known as frozen smoke, solid smoke, and solid air. NASA utilizes aerogels in space research, as well as spacesuits. Aerogels also have many other uses, as well as potential ones. Aerogels can come in many different colors, and there are multiple forms of this substance.

Production
Aerogels begin as a gel, a colloidal dispersion of solid particles in a liquid medium. A gel has a density like a liquid, as well as a fixed shape, as in solids.

The chemical reactions forming the gel leave impurities behind within the gel’s interior, which interferes with the aerogel’s drying process. Because of this, a gel needs to be purified before any processing takes place. A gel is purified by being soaked in a pure solvent, which causes any impurities to diffuse out of the gel, while the solvent diffuses into the gel. A fresh solvent is often switched multiple times throughout a few days. This purification process can take anywhere from a few hours to multiple weeks.

The next step is to dry the gel. However, if the gel were allowed to simply dry by being left uncovered until dry, the liquid evaporating would cause capillary action to occur, shrinking the gel. The result would be called a xerogel. Xerogel retains the original shape, but it shrinks considerably, and often cracks. The critical point of a substance is the specific temperature and pressure where the liquid and gas phases combine into a singular phase that acts like a gas, but also has the density and thermal conductivity of a liquid. Another name for this merged phase is a supercritical fluid. Essentially, a supercritical fluid is a substance above both its critical pressure and critical temperature, which means that it possesses properties of both gases and liquids.

To achieve this supercritical fluid, a gel is first placed in a pressure vessel with an amount of whatever liquid is inside the gel’s pores. This vessel is heated gradually until it reaches the liquid’s critical temperature. As this occurs, the vapor pressure of the liquid begins to increase, which causes the vessel’s pressure to increase, climbing towards the liquid’s critical pressure. Once the critical point is exceeded, the liquid inside the gel’s pores is changed into a supercritical fluid. After this occurs, the fluid’s capability of exerting capillary pressure onto the gel’s form has been completely depleted. The supercritical fluid is present inside the whole of the vessel, as well as the pores of the gel. Because of this, the fluid in the gel is able to be removed without any danger to the gel itself. To do so, the vessel is slightly depressurized, still above the critical point, and the temperature is maintained above the critical point. Enough liquid needs to be removed from the vessel so when the vessel is completely depressurized and cool, there won’t be enough liquid left to recondense. This step may need to be repeated multiple times. The fluid returns to a gas phase, rather than to a liquid phase.

Properties
The term aerogel is often used lightly, not specifically describing one substance but the shape and geometry that a substance takes on. Aerogels can be made out of silica (either hydrophilic or hydrophobic), transition metal oxides, lanthanide oxides, actinide oxides, main group oxides, and mixed matrix oxides, polymers like phenolics and polyamides, amorphous carbon, matels and carbides, among others. Silica aerogels have an apparent density (mass per unit volume ) of 0.003-0.35 g/cm3. They have an internal surface area of 600-1000 m2/g. These aerogels are 0.13-15% solid, although most often they are 5% solid and 95% open space. They have a mean pore diameter of ~20 nm. As well, silica aerogels have a thermal tolerance ranging up to 500 °C, at which point shrinkage begins to occur. Their melting point is 1200 °C.

Aerogels are extremely low-density substances. In fact, aerogels are all of the lowest density materials ever produced. One silica aerogel produced weighed only three times as much as air, and could be made even lighter than air if the air within it’s pores were to be removed by vacuum. If aerogel were to be cut into average-sized bricks, it would require 150 of these bricks to weigh as much as a single gallon of water. The lowest density aerogel produced had a volume of 99.98% air. As another example, if Michelangelo's David was carved out of aerogel, it would weigh no more than four pounds. Aerogels can come in a variety of colors. From silica and alumina aerogels, with their transparent blue cast, to carbon aerogel’s shiny black color, to dark red-brown, colorless, dark green or blue, the colors are determined by the primary substance in the aerogel. Aerogel’s transparency can also vary, from clear to slightly foggy to completely opaque.

Applications


Aerogels have many applications, and innumerable potential applications. Aerogels have been used sice the 1960’s for insulating spacesuits for astronauts. This is because they are extremely strong and are able to survive the takeoff conditions of the rockets with ease. Early in the 21st century, aerogels started to be used by NASA in an attempt to capture space dust. With the Stardust mission, NASA hopes to retrieve particles from space beyond the moon for the first time in history. Aerogels were exactly what was needed to achieve this goal, because they are able to trap the particles without altering them in any way. More recently, aerogels have become essential in the insulation industry. For several years, aerogels have been present in cavity injected insulation and insulating boards. Switzerland recently utilized aero-based plaster to insulate historical buildings. Labs in Switzerland have worked in conjunction with a manufacturing company in order to develop an insulating material based off of aerogels that they believe will offer twice the insulating ability of average insulator materials.

It is projected that aerogels will eventually work themselves into areas such as clothing and heat proofing. Scientists are currently looking into applications for aerogels such as a new, improved spacesuit and cryogenics. Polymer aerogels are perfect for vacuum conditions such as space, as well as other low gravity conditions like that of the moon or other planets. Several United States government agencies are currently looking into a concept of utilizing thin polymer aerogels for shelters, like insulated tents. Other foreseeable applications include refrigeration, construction, historical restoration, and insulation.

Inventor
Aerogels were invented by Samuel Stephens Kistler, between the years 1929 and 1930. The first commercial aerogels were sold as early as the 1950’s. Kistler spent a substantial percentage of the previous years studying both aerogel’s properties and uses. He received his bachelor’s from Stanford University in 1922 and then spent a small amount of time working for the Standard Oil Company in California. After a while, he grew tired of working here and took up a teaching role at the College of the Pacific. He stayed here for several years. Additionally,in 1927, Kistler began to work on his doctorate at Stanford during his summers so he was not teaching. Next he accepted a position with University of Illinois as an assistant professor. He stayed there from 1931 until 1935.

Kistler moved to Salt Lake City after accepting the position of the Dean of Engineering at the University of Utah. He continued to earnestly research multiple topics, as well as teach and consult in the industry world until he retired. He was very interested in supercritical fluids, even from the beginning of his education. He wrote his master’s thesis in 1922 (at Stanford) on the topic of proposing the crystallization of amino acids from supercritical fluids. In the late 1920’s, Kistler worked closely with Professor J.W. McBain to publish multiple papers on the subject of utilizing wet gels as ultra-filters. In the four years spent at Illinois, Kistler seemed to focus mostly on researching aerogels. His work there led to the publication of several major primary publications discussing the properties of aerogels. These papers include findings from studies on the thermal conductivity of silica aerogel, as well as the catalytic properties of different oxide aerogels. Even today, Kistler’s research intrigues researchers. Kistler created a license agreement with Monsanto Corporation to produce silica aerogel commercially in the early 1940’s. However, Monsanto dumped Kistler’s aerogel around 1970 because of high manufacturing cost and competition. Kistler also worked on the original synthetic diamond project. He lived until 1975, at 75 years old, and a few years after his death, aerogel science and technology was revived.

Video
Even a thin piece of silica aerogel is capable of withstanding immense heat. |5sw1tNeJ0Rw