SYNTHESIS AND PROCESSING OF NANO POWDERS
Zero dimensional nano structures are the three dimensional relatively equi axed materials having their largest dimension in between ‘0’ to ‘100’ nm. These are also called as nano particles or nano powders. Nano particles have created a high intrest in recent years by virtue of their unusual mechanical, electrical, optical and magnetic properties. Due to their special properties, nano particles are finding their wide applications in all fields of engineering, medicine like longer lasting medical implants of biocompatible nanostructured ceramic and carbides, biocompatible coating for medical applications, drug delivery, protection coatings, composite materials, anti fogging coatings for spectacles and car windows etc. Many techniques, including both top-down and bottom-up approaches, have been developed and applied for the synthesis of nanoparticles. This chapter deals with some of the common as well as some of the more obscure processes that have been employed in recent years with an emphasis on production of nano particles.
Processes for producing ultrafine powders
There are two approaches to the synthesis of nanomaterials and the fabrication of nanostructures; viz top-down and bottom-up. Top down approach involves the breaking down of the bulk material into nano sized structures or particles. These techniques are an extension of those that have been used for producing micron- sized particles. An example of such a technique is high-energy wet ball milling. The alternative approach, which has the potential of creating less waste and hence the more economical, is the ‘bottom- up’. Bottom up approach refers to the build up of a material from the bottom: atom-by-atom, molecule-by-molecule, or cluster-by-cluster. Many of these techniques are still under development or are just beginning to be used for commercial production of nano powders.
Figure: Schematic representation of the ‘bottom up’ and top down’ synthesis
processes of nanomaterials with the popular techniques that are used.
Characteristics of Nano particles that should posses by any fabrication technique:
Getting merely a small size is not the only requirement. It should have
i. Identical size of all particles (also called mono sized or with uniform size distribution.
ii. Identical shape or morphology.
iii. Identical chemical composition and crystal structure that are desired among different particles and within individual particles, such as core and composition must be the same.
iv. Individually dispersed or mono dispersed i.e., no agglomeration.