AEROGELS : THE LIGHTEST SOLID

AEROGELS : THE LIGHTEST SOLID

-      By Divya Rajan

-      Batch 2k18 , Deptt. Of Chemical Engg.

BIT  Sindri , Dhanbad

Hydrogen is the lightest gas while Mercury is the lightest liquid. But have you ever wondered that which is the lightest solid in the universe!

Well! In this presentation, we will learn about the lightest solid.

Aerogels are the world's lightest solids known to man. They are also called excellent thermal insulators and have been used in numerous Mars mission and the Stardust particle return mission. 

Aerogel is a synthetic porous ultralight material  derived from a gel , in which the liquid component for the gel has been replaced with a gas without significant collapse of the gel structure. The result is a solid with extremely low density and extremely low thermal conductivity. 

 

Despite the name, aerogels are solid, rigid, and dry materials that do not resemble a gel in their physical properties: the name comes from the fact that they are made from gels. 

DISCOVERY

Aerogels are considered to be a recent discovery.  However, the first aerogel was created by Samuel Stephens Kistler in 1931, at the college of the Pacific in Stockton, California, as a result of a bet with Charles Learned over who could replace the liquid in "jellies" with gas without causing shrinkage.

Fig. Samuel Stephens Kistler

If a wet gel is allowed to simply dry, it would shrink and crack. Kistler correctly deduced that the solid component of the gel has micropores and that the liquid vapour interface of the evaporating liquid exerted strong surface tension forces that collapsed the pore structure of the gel. He then discovered the crucial aspect of aerogel production.

HOW ARE THEY MANUFACTURED?

Silica aerogels are typically synthesized by using a sol-gel process.

1.      A “Sol” is made by mixing tiny particles with a liquid solvent.

2.     The Sol is made into a gel by adding a catalyst that bonds the particles to one another, causing the mixture to solidify.

3.     The liquid solvent is removed by drying, leaving only the solid aerogel behind.

The precursors are a liquid alcohol such as ethanol which is mixed with a silicon alkoxide, such as tetramethoxysilane (TMOS), tetraethoxysilane (TEOS), and polyethoxydisiloxane (PEDS).

The solution of silica is mixed with a catalyst and allowed to gel during a hydrolysis reaction which forms particles of silicon dioxide. The oxide suspension begins to undergo condensation reactions which result in the creation of metal oxide bridges linking the dispersed colloidal particles. These reactions generally have moderately slow reaction rates and as a result either acidic or basic catalysts are used to improve processing speed.

 

Fig. Aerogels are created by removing moisture from a gel while maintaining the gel structure. The resulting material provides very effective insulation.

 NICKNAMES

Owing to its translucent nature and the way light scatters in the material, aerogel has got various nicknames. Let’s have a glance –

v  Frozen Smoke

v  Solid smoke

v  Solid air

v  Solid cloud

v  Blue smoke

PROPERTIES

1)     SHAPE: - Aerogels are among the most fascinating materials. They almost look like holograms yet they feel like hard foam, this is the reason why they are often called frozen smoke or solid smoke.

2)    CONDUCTIVITY: - Aerogels are good thermal insulators because they almost nullify two of the three methods of heat transfer.

They are good conductive insulators because they are composed almost entirely of gases, which are very poor heat conductors.

They are good convective inhibitors because air cannot circulate through the lattice. Aerogels are poor radiators because infrared radiation (which transfers heat) passes through them.

Fig. Insulation in an aerogel                  Fig. Insulation Sheets of an aerogel

Fig. A flower is on a block of aerogel suspended over a flame from Bunsen burner explaining the insulating properties of aerogel.

3)   HYGROSCOPIC: - Owing to its hygroscopic nature, aerogel feels dry and acts as a strong desiccant. People handling aerogel for extended periods should wear gloves to prevent appearance of dry Brittle spots on their skin.

  Fig. Silica gel is an excellent desiccant and chemically safe to use with foods.

4)    KNUDSEN EFFECT: - Aerogels may have a smaller thermal conductivity than that of the gas they contain. This is caused by the Knudsen effect, a reduction of thermal conductivity in gases when the size of the cavity encompassing the gas becomes comparable to the mean free path. Effectively, the cavity restricts the movement of the gas particles, decreasing the thermal conductivity in addition to eliminating convection. For example, thermal conductivity of air is about 25 mW/m·K at STP and in a large container, but decreases to about 5 mW/m·K in a pore 30 nanometers in diameter.

5)    WATERPROOFING: - Aerogel contains particles that are 2–5 nm in diameter. After the process of creating aerogel, it will contain a large amount of hydroxyl group on the surface. The hydroxyl groups can cause a strong reaction when the aerogel is placed in water, causing it to catastrophically dissolve in the water.

     One way to waterproof the hydrophilic aerogel is by soaking the aerogel with some chemical base that will replace the surface hydroxyl groups (–OH) with non-polar groups (–OR), a process which is most effective when R is an aliphatic group.

APPLICATIONS

 Owing to its properties, Aerogels have myriad applications in variety of fields.

v Let’s start with the fascinating one – NASA used an 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.

 Fig. 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.

v A chemical adsorber for cleaning up spills.

v A catalyst or a catalyst carrier.

v Silica aerogels can be used in imaging devices, optics, and light guides.

v  Thickening agent in some paints and cosmetics.

v As components in energy absorbers.

v A material used in impedance matchers for transducers, speakers and range finders. 

AEROGELS AND FUTURE :-

Hence, we conclude that aerogels are one of the revolutionizing materials.

Due to their unique combination of low thermal conductivity, low density, high surface area, and translucence, aerogels are evolving into a diverse array of cutting – edge material technologies.

Polymer-enhanced aerogels and polymer-based aerogels have numerous potential applications, both in space, on distant planets and on our own Earth. They are light, durable and extremely effective at insulating and preventing heat transfer. NASA has taken aerogels to the next level, beyond what was previously imagined, and uncovered a world of possibilities for this versatile material.

Related Post