Nanostructures can be made in numerous ways.A broad classification divides methods into either those which build from the bottom up, atom by atom, or those which construct from the top down using processes that involve the removal or reformation of atoms to create the desired structure.
In the bottom-up approach, atoms, molecules and even nanoparticles themselves can be used as the building blocks for the creation of complex nanostructures; the useful size of the building blocks depends on the properties to be engineered. By altering the size of the building blocks, controlling their surface and internal chemistry, and then control- ling their organization and assembly, it is possible to engineer properties and function- alities of the overall nanostructured solid or system. These processes are essentially highly controlled, complex chemical syntheses.
Schematic representation of the top-down and bottom-up processes and their relationship to biological processes and structures
On the other hand, top-down approaches are inherently simpler and rely either on the removal or division of bulk material, or on the miniaturization of bulk fabrication processes to produce the desired structure with the appropriate properties. When controlled, both top-down and bottom-up methods may be viewed as essentially different forms of microstructural engineering.
Ball Milling Technique:
One nanofabrication process of major industrial importance is high-energy ball milling, also known as mechanical attrition or mechanical alloying. The coarse-grained materials (usually metals but also more recently ceramics and polymers) in the form of powders are crushed mechanically in rotating drums by hard steel or tungsten carbide balls, usually under controlled atmospheric conditions to prevent unwanted reactions such as oxidation.
This repeated deformation can cause large reductions in grain size via the formation and organization of grain boundaries within the powder particles. Different components can be mechanically alloyed together by cold welding to produce nanostructured alloys. A nanometre dispersion of one phase in another can also be achieved.
Microstructures and phases produced in this way can often be thermodynamically metastable. The technique can be operated at a large scale, hence the industrial interest.
Generally any form of mechanical deformation under shear conditions and high strain rates can lead to the formation of nanostructures, since energy is being continu- ously pumped into crystalline structures to create lattice defects. The severe plastic deformation that occurs during machining, cold rolling, drawing, cyclic deformation or sliding wear has also been reported to form nanostructured material.