Nanoparticles are formed through the natural or human mediated disintegration of larger structures or by controlled assembly processes. The associated processes occur either in the gas phase, in a plasma, in a vacuum phase or in the liquid phase. Particles are classified according to size: in terms of diameter, fine particles cover a range between 100 and 2500 nanometers. Nanoclusters have at least one dimension between 1 and 10 nanometers and a narrow size distribution. Nanopowders ,nanoparticles, or nanoclusters. Nanocrystals are nanometer sized single crystals. Many scientist researches have been done in this field due to a wide variety of potential applications in biomedical, optical and electronic fields.
Inert-gas condensation is frequently used to make nanoparticles from metals with low melting points. The metal is vaporized in a vacuum chamber and then supercooled with an inert gas stream. The supercooled metal vapor condenses into nanometer-sized particles, which can be entrained in the inert gas stream and deposited on a substrate.
A thermal plasma provides the energy necessary to do evaporation of small micrometer size particles and its temperatures are in the order of 10000k. Nanoparticles are formed upon cooling while exiting the plasma region. Silica sand can be vaporized with an arc plasma at atmospheric pressure. The resulting mixture of plasma gas and silica vapour can be rapidly cooled by quenching with oxygen, thus ensuring the quality of the fumed silica produced. Energy coupling to the plasma is done through the electromagnetic field generated by the induction coil. The plasma gas does not come in contact with electrodes, thus eliminating possible sources of contamination and allowing the operation of such plasma torches with a wide range of gases including inert, reducing, oxidizing and other corrosive atmospheres.
Attrition and pyrolysis are two methods to generate nanoparticles. In attrition, macro or micro scale particles are ground in a ball mill, a planetary ball mill, or other size reducing mechanism. The resulting particles are air classified to recover nanoparticles. In pyrolysis, a vaporous precursor is forced through an orifice at high pressure and burned. The resulting solid is air classified to recover oxide particles from by-product gases.