Though measuring just one billionth of a meter, nanomaterials find large number of industrial applications due to their unusual mechanical, electrical, optical and magnetic properties, says Dhirajlal K Chauhan.

 

In day to day life, we come across numerous materials which have the same properties and characteristics during their use unless operating environment changes drastically. However, there are some materials which have very different properties as compared to their bulk size form. These are known as nanomaterials. Nanomaterials are finding large number of applications due to their unusual mechanical, electrical, optical and magnetic properties. 

 

These materials are characterised by at least one dimension in the nanometer size range. A nanometer (nm) is one billionth of a meter, or 10-9m. One nanometer is approximately the length equivalent to 10 hydrogen or 5 silicon atoms aligned in a line.

 

Although nanotechnology is a new area of research, nanomaterials are known to be used for centuries. For example, the Chinese used gold nano particles as an inorganic dye to introduce red colour into their ceramic porcelains more than thousand years ago. 

 

Nanomaterials have a much greater surface area to volume ratio than their conventional forms, which leads to greater chemical reactivity and strength. Also at the nano scale, quantum effects can become much more important in determining the materials’ properties and characteristics, leading to novel optical, electrical and magnetic behaviours.

 

They can be found in sunscreens, cosmetics, sporting goods, stain resistant clothing, tyres, electronics, as well as many other everyday items and are used in the field of medicine for the purpose of diagnosis, imaging and drug delivery.

 

Nanocoatings and nanocomposites are finding uses in diverse consumer products, such as windows, sports equipment, bicycles and automobiles. There are novel UV-blocking coatings on glass bottles which protect beverages from damage by sunlight, and longer-lasting tennis balls using butyl-rubber/nano-clay composites. Nano scale titanium dioxide, for instance, is finding applications in cosmetics, sun-block creams and self-cleaning windows, and nano scale silica is being used as filler in a range of products, including cosmetics and dental fillings. 

 

According to Siegel, nanostructured materials are classified as zero dimensional, one dimensional, two dimensional, three dimensional nanostructures (refer Figure 1).

 

One of the nanomaterials is produced from carbon. It is known as carbon nanotube. These are long, thin cylinders of carbon and are large macromolecules that are unique for their size, shape, and remarkable physical properties. They can be thought of as a sheet of graphite (a hexagonal lattice of carbon) rolled into a cylinder.

 

Nanotubes have a very broad range of electronic, thermal, and structural properties that change depending on the different kinds of nanotube (defined by its diameter, length, and chirality, or twist). To make things more interesting, besides having a single cylindrical wall (SWNTs), nanotubes can have multiple walls (MWNTs)-cylinders inside the other cylinders.

 

Arc discharge method is one of the methods to manufacture carbon nanotubes. In this method a chamber containing a graphite cathode and anode contains evaporated carbon molecules in a buffer gas such as helium. The chamber also contains some amount of metal catalyst particles (such as cobalt, nickel, and/or iron). DC current is passed through the chamber while the chamber is also pressurised and heated to approximately 4000ºK. In the course of this procedure, about half of the evaporated carbon solidifies on the cathode tip into a ‘cylindrical hard deposit’. The remaining carbon condenses into ‘chamber soot’ around the walls of the chamber and ‘cathode soot’ on the cathode. The cathode soot and chamber soot yield either single-walled or multi-walled carbon nanotubes. The cylindrical hard deposit doesn't yield anything particularly interesting.

 

Applications of nanomaterials 

Given below are some of the applications of nanomaterials: 

 

Mechanical grinding

Mechanical attrition is a typical example of ‘top down’ method of synthesis of nanomaterials, where the material is prepared not by cluster assembly but by the structural decomposition of coarser-grained structures as the result of severe plastic deformation. This has become a popular method to make nanocrystalline materials because of its simplicity, the relatively inexpensive equipment needed, and the applicability to essentially the synthesis of all classes of materials. Major advantage of this process is the possibility for easily scaling up to tonnage quantities of material for various applications.

 

The serious problems associated with this method are contamination from milling media and/or atmosphere, and to consolidate the powder product without coarsening the nanocrystalline microstructure.

 

Sol-gel process

The sol-gel process, involves evolution of inorganic networks through the formation of a colloidal suspension (sol) and gelation of the sol to form a network in a continuous liquid phase (gel). The precursors for synthesising these colloids consist usually of a metal or metalloid element surrounded by various reactive ligands. The starting material is processed to form a dispersible oxide and forms a sol in contact with water or dilute acid. Removal of the liquid from the sol yields the gel, and the sol/gel transition controls the particle size and shape. Calcination of the gel produces the oxide. 

 

The reactions involved in the sol-gel chemistry based on the hydrolysis and condensation of metal alkoxides M(OR)z can be described as follows:

MOR + H2O ? MOH + ROH (hydrolysis)

MOH + ROM ? M-O-M + ROH (condensation)

Sol-gel method of synthesising nanomaterials is very popular among chemists and is widely employed to prepare oxide materials. 

 

Gas-phase synthesis methods

These are of increasing interest because they allow elegant way to control process parameters in order to be able to produce size, shape and chemical composition controlled nanostructures. The techniques that are most frequently used to produce this type of microstructure are chemical vapour deposition (CVD), physical vapour deposition (PVD), various aerosol techniques, and precipitation from the vapour, supersaturated liquids or solids.

 

Summing up

From the discussions above it is clear that Nano technology is gaining great importance now a day due to typical characteristics and properties of nano materials in various forms. In particular these materials contribute significantly in decreasing the size of various electronic components such as transistors, capacitors, printed circuit boards etc.

 

References:

• https://www.researchgate.net/publication/259118068_Chapter_-_Introduction_to_nanomaterials

• ‘Introduction to nanomaterials and nanotechnology’ by Vladimir Pokropivny, Rynno Lohmus, Irina Hussainova, Alex Pokropivny, Sergey Vlassov, Tallinn Technical University -2007, Institute of Mechanical Engineering

 

Dhirajlal K Chauhan is a metallurgical engineer with B Tech and M Tech degrees from IIT Bombay. He has extensive experience in the areas of quality assurance in cold rolling mills, heat treatment of various alloy tool steels and heat treatment equipments. He also worked as Assistant Professor at Indus University near Ahmadabad and at ITM Universe, Vadodara, (Gujarat) under Gujarat Technological University. He can be reached on email: chauhan_dhirajlal@yahoo.com and Cell: 089803 94971