Aerogels

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Aerogel, also known as solid smoke has about 99% air  and is the lightest known substance.  The term aerogel might make us think that it is a gel like substance but its properties don't come close to a gel's physical properties. It is a  hard transparent substance which can shatter like glass when considerable pressure is applied but yet is very strong.

A human size aerogel would weigh about a pound and yet support a weight of 1000 pounds. That is amazing for something so light. Due to its high porosity, very large surface area and low solid density, aerogels have become the product of choice in the field of adsorbents and  insulators. Aerogels have extensive uses as support material in packaging, construction, space modules. As they are non toxic, aerogels can be pretty much used anywhere.

How is  Aerogel made?

Aerogels are wet gels which do not have a liquid entrained in them but replaced with air. If this gel is formed from the water phase it is termed, hydrogel. Hydrogels have to be exhanged with organic solvents before drying them out. If the gel is alchohol based it is dried directly. The process of making an aerogel has  two steps. The making of the gel and the second step is the drying of the gel.

To make the wet gel, an aqueous condensation of sodium silicate was used but this posed  problems as the gels had to be washed repeatedly to take away the salts formed in the gel.  With the technology of solgels growing considerably, the way gels are perepared now, is by using silicon alkoxide precursors such as, tetramethyl orthosilicate, Si-(OCH₃)₄. The organic groups can be changed in the alkoxide precursors with other organic groups to give the aerogels different properties.

Si(OCH₂CH₃)₄ + 2H₂O _{------------------------} SiO₂  + 4HOCH₂CH₃

The whole reaction is performed in ethanol. The equation shows two molecules of water for one alkoxide monomer. More water is used to make a good aerogel, in the absence of which the resultant aerogel formed would be weak. Different acid or base catalysts are used to speed up the formation of aerogels. If an acid catalyst is used, it results in the aerogel drying faster in the supercritical drying part of the process, resulting in a less transparent  aerogel. The gels have to be left in the ethanol. Failure to do so will make the aerogel dried out and an inferior product is obtained. At least 48hrs is given for all the silicon to form the chains in the gel and the pH and water content is monitored during this time. The longer it takes, the better the gel forms and the silicon network is more extensive. 

The next step is to remove the water content in the gel. This is done by soaking the gel in alcohol a number of times. As the gel thickens the process of removing water takes longer. Once this is achieved the gel enters the second phase of the process, the "supercritical drying".

What is supercritical drying?

In this phase the liquid in the gel is removed leaving only the silica chains. This is done in an autoclave where ethanol is vented  above its critical temperature and then supercritical venting again at lower temperatures. The autoclave is pressurized by pumping CO₂ to 750-800 psi and then the temperature is decreased 5-10° C. Liquid CO₂ is flushed through the gel to remove all ethanol from it. Once the gel becomes ethanol free the autoclave is heated to a  critical temperature of  CO₂ which is above 31° C. CO₂ is slowly released as the temperature increases. All this time the pressure is maintained above the critical pressure of CO₂ at 1050psi and slowly released until it reaches an ambient pressure. The process takes anywhere from a few hours to a week depending on the thickness of the aerogel. The aerogel is now ready.

How to handle Silica gels?

• Silical gels are more durable if sealed under pressure. 

• Silica gels get easily destroyed  by coming into contact with other liquids as they will try to enter the pores of the gel.

• Wearing gloves while handling to avoid getting dry skin when working with aerogels.

What are the properties of Aerogels?

According to Aspen Aerogels,the company that makes highly specialized aerogels taht are used by NASA for their space craft in thermo insulation, a silica aerogel material with a density of about 100 kg/cubic meters can have surface areas of around 800 to 1500 m2/g. This is equal to 3 to 5 football fields per gram of solid material. From these figures one can see  the amount of surface area embedded within the aerogel.

• Have very low sound velocity

• Can make excellent sound barriers

• Act as good energy absorbers, which is why are used in space to absorb high velocity dust particles that can penetrate even steel

What are the different kinds of Aerogels?

• Silica gel:  Silica gel is the one discussed in this article. It has excellent thermal insulating properties and therefore used in constructions to trap heat which is a good way of saving on energy costs. It is also used to trap space dust
• Carbon gel: Is aerogel made from carbon. Carbon aerogels have high porosity and are used in composite paper. They are also used in making super capacitors due to the high surface area
• Alumina gel: Alumina which is alumnium oxide and nickel are mostly used to make this aerogel. Its used by NASA scientists to collect high velocity particles
• Chaclogels: These use chalcogens such as those belonging to the sulfur family which is oxygen,selenium,tellurium to make the gels. these gels are used in pollution absorption specially heavy metals

What are the uses of Aerogels?

• Used while doing stunts in movies.
• Mostly used in space crafts. It insulates against sound,temperature and electricity.
• The insulation and low thermal conductivity is one of the main applications of aerogels.Structure of the aerogels show that it has many dead ends which make temperature transfer very difficult. Aerogelsa re used to manufacture winter clothing due to their low thermal conductivity properties.