27-02-2016, 04:16 PM
large surface area of nanoparticles also results in a lot of interactions between the intermixed materials in nanocomposites, leading to special properties such as increased strength and/or increased chemical/heat resistance.The transition from classical mechanics to quantum mechanics is less gradual. Once particles become small enough they start to exhibit quantum mechanical behavior. The properties of quantum dots (covered in a separate report) is a case in point. These are sometimes called artificial atoms because free electrons in them start to behave in a way similar to electrons bound by atoms in that they can only occupy certain permitted energy states.
Additionally, the fact that nanoparticles have dimensions below the critical wavelength of light renders them transparent, a property which makes them very useful for applications in packaging, cosmetics and coatings.Some of the properties of nanoparticles might not be predicted simply by understanding the increasing influence of surface atoms or quantum effects. For example, it was recently shown that perfectly-formed silicon 'nanospheres', with diameters of between 40 and 100 nanometers, were not just harder than silicon but among the hardest materials known, falling between sapphire and diamond.
Nanoparticles have been used for a very long time, probably the earliest use being in glazes for early dynasty Chinese porcelain. A Roman cup, called the Lycurgus cup, used nanosized gold clusters to create different colors depending on whether it was illuminated from the front or the back. The cause of this effect was not, of course, known to those who exploited it.
Carbon black is the most famous example of a nanoparticulate material that has been produced in quantity for decades. Roughly 1.5 million tons of the material is produced every year. Nanotechnology, though, is about deliberately and knowingly exploiting the nanoscale nature of materials, which would, for many, exclude early use of carbon black from being given the nanotechnology label. However, new production and analysis capabilities at the nanoscale and advances in theoretical understanding of the behavior of nanomaterials certainly mean nanotechnology can be applied to the carbon black industry.
Nanoparticles are currently made out of a very wide variety of materials, the most common of the new generation of nanoparticles being ceramics, which are best split into metal oxide ceramics, such as titanium, zinc, aluminum and iron oxides, to name a prominent few, and silicate nanoparticles (silicates, or silicon oxides, are also ceramic), generally in the form of nanoscale flakes of clay. According to the most widely-accepted definitions, at least one of their dimensions must be less than 100 nm, but some interesting new applications use particles of a few hundred nanometers, so this report will not be overly strict about the 100 nm limit. The nanoparticles in metal and metal oxide ceramic nanopowders tend to be roughly the same size in all three dimensions, with dimensions ranging from two or three nanometers up to a few hundred (one might expect such fine particles to stay suspended in air but in fact they settle out into a very fine powder, drawn together by electrostatic forces).
Silicate nanoparticles currently in use are flakes about 1 nm thick and 100 to 1 used being pontmorillonite, a layered alumino-silicate. The nanoparticles can be incorporated in a polymer either during polymerization or by melt compounding (mixing in with a plastic 'melt'—for thermoset plastics this is a one-time process since they become set by heat, and cannot be re-melted. Thermoplastics, by contrast, can be repeatedly softened by heating).
Pure metal nanoparticles can be induced to merge into a solid, without melting (a process called sintering) at lower temperatures than for larger particles, leading to improved and easier-to-create coatings, particularly in electronics applications such as capacitors. Metal oxide ceramic nanoparticles can also be used to create thin layers, whether crystalline or amorphous.
Ceramic nanoparticles, like metallic nanoparticles, can also be formed into coatings and bulk materials at lower temperatures than their non-nano counterparts, reducing manufacturing costs. Superconducting wires have been made out of ceramic nanoparticles, creating a material that is relatively flexible where traditional ceramic materials are far too brittle. A very active area of research and development on nanoparticles surrounds their use to make nanocrystalline coatings, the novel properties of which are covered in a separate report. Nanocrystalline ceramics are already in use, by the US Navy, for example.
Although metal oxide ceramic, metal, and silicate nanoparticles constitute the majority of nanoparticles with current and expected applications, there are others too. A substance called chitosan, used in hair conditioners and skin creams, has been made in nanoparticle form and the process was patented late in 2001. These nanoparticles improve absorption.
Production techniques
There is a wide variety of techniques for producing nanoparticles. These essentially fall into three categories: condensation from a vapor, chemical synthesis, and solid-state processes such as milling. Particles can then be coated, with hydrophilic (water-loving) or hydrophobic (water-hating) substances, for example, depending on the desired application.
Vapor condensation
This approach is used to make metallic and metal oxide ceramic nanoparticles. It involves evaporation of a solid metal followed by rapid condensation to form nanosized clusters that settle in the form of a powder. Various approaches to vaporizing the metal can be used and variation of the medium into which the vapor is released affects the nature and size of the particles. Inert gases are used to avoid oxidation when creating metal nanoparticles, whereas a reactive oxygen atmosphere is used to produce metal oxide ceramic nanoparticles. The main advantage of this approach is low contamination levels. Final particle size is controlled by variation of parameters such as temperature, gas environment and evaporation rate.