The Sunday Metallurgist:
Hello friends. I am Aditya, a graduate in Metallurgy and Material Technology. I blog to kindle the spirit of Science, Enquiry and the spirit of appreciation for the latest technical and scientific advances in Materials Science and Metallurgy.
This weekly is intended to inspire and educate young metallurgists, and keep veterans and busy industrial practitioners informed about latest discoveries in Metallurgy and Materials Science.
Edition 1:
This week we shall dwell on Aerogel:
Ever wondered about the existence of materials with 99.5% porosity? That is transparent to light, can bare load up to 1000 times its weight, completely insulating Heat and Cold alike, and ultra light?
Yes. This material has as-many as 15 Guinness. The material is Aerogel.
This how an Aerogel looks like. It is ultra light and transparent
The production of an aerogel involves two main steps
The preparation of a wet gel, and the drying of that gel. The second step is the most critical, as the fluid must be removed without destroying the solid framework of the gel. This is achieved by the process of supercritical fluid drying.
Preparation of Gel:
The standard procedure is Sol-Gel technique. The term sol-gel refers to a process in which solid particles dispersed in a liquid (a sol) agglomerate together to form a continuous three-dimensional network extending throughout the liquid (a gel). The term sol-gel is sometimes used as a noun to refer to gels made through the sol-gel process, but this is somewhat of an abuse of the term, since pretty much all gels are made through the sol-gel process.
Now, to manufacture an Aerogel, the connecting 3 D network has to be retained and the matrix of the liquid has to be evaporated. The crux of the problem lies here. For example, if a sponge is wet with water – a very common example, it expands in volume. If it is left over for a few hours, the water in the sponge slowly gets evaporated and the sponge contracts to its original volume. Meaning, when a liquid is evaporated from a liquid – solid interface, an un-avoidable shrinkage occurs in the material. It can be observed with many other systems, Like apples – moisture is the liquid between protein molecules, leave a slice of apple in air and it will contract.
Now if by any process, entire liquid in the system could be evaporated by not causing any volumetric changes what so ever, that would produce – an Aerogel.
That process is Super-critical-Drying.
In general, supercritical drying is used when liquid needs to be removed from a sample that would be damaged by evaporative or other drying techniques. Biological specimens, for example, are often preserved through supercritical drying. As mentioned earlier, capillary action induced by liquid evaporating from a
gel’s pores causes the gel to shrink. So what if there was some way to avoid capillary forces to begin with? This is where supercritical drying comes in.
All pure substances (that won’t decompose) have what’s called a critical point–a specific and characteristic pressure and temperature at which the distinction between liquid and gas disappears. For most substances, the critical point lies at a fairly high pressure (>70 atmospheres) and temperature (>200°C). At the critical point, the liquid and vapor phases of a substance merge into a single phase that exhibits the behavior of a gas (in that it expands to fill the volume of its container and can be compressed) but simultaneously possesses the density and thermal conductivity of a liquid. This phase is called a supercritical fluid. Say we have a sealed container containing a liquid below its critical point inside and equipped with a pressure gauge on top. In fact, a certain amount of liquid will evaporate in the container until the vapor pressure of the liquid is reached in the container, after which no more liquid will evaporate and the gauge will read a corresponding stable pressure.
Now if we heat this container, we will notice the pressure in the container increases, since the vapor pressure of a liquid increases with increasing temperature. As the critical point draws near, the pressure in the container squeezes molecules in the vapor close enough together that the vapor becomes almost as dense as a liquid.
At the same time, the temperature in the container gets high enough that the kinetic energy of the molecules in the liquid overwhelms the attractive forces that hold them together as a liquid. In short, as the pressure and temperature in the container get closer to the critical point, the liquid phase becomes more gas-like and the vapor phase more liquid-like. Finally, the critical point is reached and the meniscus dividing the two phases blurs away, resulting in a single supercritical phase. As this occurs, the surface tension in the fluid gradually drops to zero, and thus the ability of the fluid to exert capillary stress does too.
Aerogelification:
· In the case of making aerogels, a gel is placed in a pressure vessel under a volume of the same liquid held within its pores (lets say ethanol for example).
· The pressure vessel is then slowly heated to the liquid’s critical temperature. As this happens, the vapor pressure of the liquid increases, causing the pressure in the vessel to increase and approach the critical pressure of the liquid. The critical point is then surpassed, gently transforming the liquid in the gel (as well as the liquid and vapor surrounding the gel) into a supercritical fluid.
· Once this happens, the ability of the fluid in the gel to exert capillary stress on the gel’s solid framework structure of the gel has decreased to zero.
· With supercritical fluid now present throughout the entire vessel and permeating the pores of the gel, the fluid in the gel can be removed.
· This is done by partially depressurizing the vessel, but not so much as to cause the pressure in the vessel to drop below the critical pressure.
· The temperature of the vessel must also remain above the critical temperature during this step. The goal is to remove enough fluid from the vessel while the fluid is still supercritical so that when the vessel is fully depressurized/cooled down and drops below the fluid’s critical point, there will simply not be enough substance left in the vessel left for liquid to recondense.
· This might require several cycles of heating (and thus pressurizing) followed by depressurization (again all done above the critical point). Once enough fluid has been removed from the vessel, the vessel is slowly depressurized and cooled back to ambient conditions.
· As this happens, the fluid in the vessel passes back through the critical point, but since much of the fluid has been removed and the temperature is still elevated as the vessel depressurizes, the fluid reverts to a gas phase instead of a liquid phase. What was liquid in the gel has been converted into a gas without capillary stress every arising, and an aerogel is left behind.
What is left out after this drying is a solid mass, with as high as 99.95% porosity, with a mild blue color.
Uses:
Now, because the material, that is Silica is a refractory material in nature, and the air, that the inter-connecting media in side the solid Aerogel matrix, are both bad conductors, this makes Aerogel an excellent insulator.
The mass of this only 0.05% and rest all is void, hence it is extremely light. It is infact so light that it can be supported over the flaming gasses of a Bunsen flame.
Transparent silica aerogel would be very suitable as a thermal insulation material for windows, significantly limiting thermal losses of buildings.
Its high surface area leads to many applications, such as a chemical adsorber for cleaning up spills (adsorption). This feature also gives it great potential as a catalyst or a catalyst carrier
Aerogel performance may be augmented for a specific application by the addition of dopants, reinforcing structures, and hybridizing compounds. Using this approach, the breadth of applications for the material class may be greatly increased.
NASA used aerogel to trap space dust particles aboard the Stardust spacecraft. The particles vaporize on impact with solids and pass through gases, but can be trapped in aerogels. NASA also used aerogel for thermal insulation of the Mars Rover and space suits.
The US Navy is evaluating aerogel undergarments as passive thermal protection for divers. For deep divers, the temperatures in deep sea are too low, and the diver is surrounded by cold water, especially in
The scope of this material is wide. This is just a pointer. If this article enthuses you, keep looking for it, you will learn a lot. keep learning. that is the secret of being youthful - atleast at heart.
Have a nice weekend.
The Sunday Metallurgist.
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