Biomedical Engineering Reference
In-Depth Information
In the case of chemical vapor deposition, feed gases are reacted
in a chamber and the resulting species are attracted to a substrate.
Once again the reaction products can be controlled and not only in
terms of composition but also in terms of how they are deposited.
The substrate effectively provides a template from where the
deposited coating can grow in a very well controlled manner. Electro-
deposition involves a similar process; however, the controlled coating
is deposited from solution by the application of an electric ield.
4.1.2.3 Gas phase synthesis
These include gas condensation processing (GPC), lame pyrolysis,
electro-explosion, laser ablation, high-temperature evaporation,
and plasma synthesis techniques. Gas condensation was the irst
technique used to synthesize nanocrystalline metals and alloys
[9−11]. In this technique a metallic or inorganic material is vaporized
using thermal evaporation sources such as Joule heated refractory
crucibles, electron beam evaporation devices or sputtering sources in
an atmosphere of 1-50 mbar He (or another inert gas like Ar, Ne, Kr).
Cluster form in the vicinity of the source by homogenous nucleation
in the gas phase and grow by coalescence and incorporation of atoms
from the gas phase. The cluster or particle size depends critically on
the residence time of the particles in the growth regime and can be
inluenced by the gas pressure, the kind of inert gas, i.e. He, Ar or
Kr, and on the evaporation rate/vapor pressure of the evaporating
material.
The average particle size of the nanoparticles increases with an
increasing gas pressure, vapor pressure, and mass of the inert gas. A
rotating cylindrical device cooled with liquid nitrogen was employed
for the particle collection. The nanoparticles in the size range from
2-50 nm are extracted from the gas low by thermophoretic forces
and deposited loosely on the surface of the collection device as
a powder of low density and no agglomeration. Subsequently, the
nanoparticles are removed from the surface of the cylinder by means
of a scraper in the form of a metallic plate. In addition to this cold
inger device, several techniques known from aerosol science have
now been implemented for the use in gas condensation systems such
as corona discharge, etc. These methods allow for the continuous
operation of the collection device and are better suited for larger
scale synthesis of nanopowders. However, these methods can only
be used in a system designed for gas low, i.e. a dynamic vacuum is
