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Nanoparticles Synthesis

Nanoparticles' synthesis can be done using several different methods such as attrition/milling, pyrolysis, sol gel fabrication, inert gas condensation, solvothermal reaction and structured media. Let's delve into some of these methods in brief. While the attrition/milling is referred to as the top-down process, the other ones fall in the bottom-up category.

Nanoparticles synthesis using attrition or milling

In this particular method of nanoparticles' synthesis, micro or macro scale particles are grounded in a planetary ball mill, a general ball mill or with the help of any other size-reduction mechanism. The particles received are then air classified in order to recover nanoparticles. The process involves mechanical thermal cycles and the yields have the following characteristics:

  • Broad size distribution, varying from 10 nm to 1000 nm
  • Varying particle shapes or geometries
  • Impurities

The particles obtained from this nanoparticles' synthesis method generally find their application in nano-grained bulk materials and nanocomposites.

Nanoparticles synthesis using pyrolysis

In the pyrolysis method of nanoparticles' synthesis, a vaporous precursor (in the form of gas or liquid) is forced through an opening or a hole at a very high pressure, and is then burned. The produced solid is then air-classified for recovering oxide particles from the byproduct gases. This method usually leads to agglomerate and aggregates instead of singles and primary nanoparticles.

Thermal plasma may also be used instead of gas for introducing the energy needed for evaporating micrometer-sized particles. Please note, the thermal plasma temperature is usually around 10,000 K, which is good enough for easy evaporation of solid powder. The nanoparticles are obtained upon cooling as they're moving out of the plasma region. Radio Frequency induction plasmas, DC arc plasma and DC plasma jet are few examples of the plasmas employed for this purpose.

The primary advantage of using this nanoparticles' synthesis method is that it's fairly simple, cost-effective and can be operated continuously with high yield.

Nanoparticles synthesis using solvothermal method

This is one of the three liquid phase fabrication methods (involving the wet chemistry route) of preparing nanoparticles; the other two being - synthesis in structured media method and sol gel method. It involves dissolution of the precursors in hot solvents like n-butyl alcohol and solvents other than water, as they can provide friendly and milder reactive conditions. On the other hand, if water is used, the synthesis process is termed as a hydrothermal method.

Nanoparticles synthesis using the sol gel process

This is another wet chemical technique for nanoparticles' synthesis, which is used widely in ceramic engineering and materials science. The steps involved in creation of nanoparticles are:

  • A stable sol (chemical solution) is formed first, followed by
  • Gelation through polyesterification or polycondensation reaction.
  • Then the gel is aged into a solid mass, leading to the gel network's contraction, phase transformations and Ostwald ripening.
  • The gel is then dried-up for removal of liquid phases (solvents), possibly resulting in fundamental changes to the gel's structure.
  • Thereafter, dehydration is carried out at temperatures around 8000°C, for removal of M-OH groups, resulting in stabilization of the gel, thus protecting it from rehydration.
  • Then the gel is densified and decomposed at temperatures over 8000°C, leading to collapse of the pores inside the gel network and removal of the remaining organic contaminants.

The final component's microstructure will be strongly impacted by the changes brought about during the processing phase. The precursor solution (sol) can be used for synthesizing powders like nanospheres and microspheres, or deposited on a substrate to create a film (through spin-coating or dip-coating), or cast into an appropriate container in a required shape (for obtaining aerogels, membranes, glasses, monolithic ceramics or fibers).

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