Plasma Synthesis of Nanoparticles for Nanocomposite Energy Applications Page: 3 of 12
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shape, size and chemistry. Liquid methods can be expensive because of starting materials,
multi-step processing, and liquid waste, and generally result in undesirable surface chemistry.
Another drawback of liquid synthesis is that, often, entirely new processes must be developed
for each new material. Current gas phase processes generally utilize relatively expensive liquid
feedstock, do not have the particle size and morphology control afforded by the plasma
technology and generally result in undesirable surface chemistry. Both liquid-phase and gas-
phase processes generate pollutants (e.g., waste water, carbon dioxide, NOx, chlorine,
hydrogen chloride, etc). The high temperature and anhydrous conditions of plasma synthesis
allow formation of particles with unique and desirable surface properties and morphology
unachievable with other methods. These particle properties are critical to their performance in
Aerosol and Gas-Phase Methods
The aerosol methods for production of material particles can be divided into the gas-to-particle
and the liquid/solid-to-solid routes. In the liquid/solid-to-solid route, the product particles are
formed from droplets of reactant particles via intraparticle reactions. Using this method it is
possible to produce single and multicomponent materials of controlled homogeneous
compositions. It is a continuous process that can be scaled up. However, this is a complicated
technique since many different physical and chemical phenomena (e.g. evaporation of the
solvent, chemical reactions) can occur simultaneously. The gas-to-particle route is a common
method when nanoparticles are produced through gas-phase processes. In this route, particles
nucleate from a supersaturated vapor. Supersaturation can be achieved by physical processes
such as cooling of a hot vapor or through chemical reactions of gaseous precursors, which
results in the formation of condensable species. Very small particles on the order of nanometers
can be produced, and the final products often have high purity. However, the production rates
may be low in some processes and multicomponent materials may be difficult to produce. There
may also be problems with hazardous reactants and by-products. Varying the process
conditions (e.g. the initial concentration of precursor, maximum temperature, residence time,
and cooling rate) can control the degree of agglomeration of the synthesized nanoparticles. In
the gas-to-particle synthesis there are two routes, physical and chemical, depending on how the
vapor supersaturation needed for particle nucleation comes about. However, these processes
are identical in terms of the aerosol dynamics that occur once the condensable species have
formed. In the physical vapor process, the solid precursor and the final product are the same
material. This route is simple since no chemical reactions occur in the gas phase. However,
temperatures high enough to vaporize the precursor are needed, and this limits the materials
that can be processed. In general the yield in physical vapor condensation of nanoparticles
tends to be low. In the chemical vapor route, the production rates can be significant if a
precursor with high volatility is available. Therefore, chemicals such as metal chlorides or
metalorganic compounds are often used, for example, titania is produced on an industrial scale
from TiCI4 using this method. However, in using metal chlorides or metalorganic compounds
there is a possible risk of contaminating the final product. Composite particles can be produced
using the chemical vapor route. Recently, smaller nanoparticles ranging from 1 to 10 nm with
consistent crystal structure, specific surface structures or properties, and a tight particle size
distribution have been produced by both gas-phase techniques. The particles produced by
these processes have typical size variances of about 20%. However, for measurable
enhancement of the quantum effect, the size variances must be reduced to less than 5% .
Initial development of new crystalline materials was based on nanoparticles generated by
evaporation and condensation (nucleation and growth) in a subatmospheric inert-gas
environment [2,3,4]. Various aerosol-processing techniques have been reported to improve the
production yield of nanoparticles [5,6]. These include synthesis by combustion flame [7,8,9,10],
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Kong, Peter C. & Kawczak, Alex W. Plasma Synthesis of Nanoparticles for Nanocomposite Energy Applications, article, September 1, 2008; [Idaho Falls, Idaho]. (digital.library.unt.edu/ark:/67531/metadc897497/m1/3/: accessed October 16, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.