Biomedical Engineering Reference
In-Depth Information
Metal deposition can be carried on under two major techniques (Madou
2002
) : with
current (pulsed or DC) or electroless (via catalytic, exchange, or electrophoretic
reactions). The fi rst method takes place in an electrolytic cell and involves a reaction
under an imposed bias and current fl ow. In this case, important process parameters
to control are pH, current density, temperature, agitation, and solution composition.
The second technique, the electroless deposition, is based on a substantial oxidation
reaction that replaces the dissolution of a sacrifi cial substrate.
1.2.6
Spraying Synthesis
Spraying processes are a viable technology for coating large area devices, and rep-
resent a simple and inexpensive alternative to produce sensing devices and nano-
sized coating of surfaces (Mooney and Radding
1982
) . Spraying processes take
place in a chamber containing a high-energy fl ame produced, for example, using
plasma spraying equipment or using a carbon dioxide laser. A fl ow of reactants (gas
or liquid in form of aerosols or a mixture of both) is forced into the fl ame and
decomposes, forming particles by homogeneous nucleation and growth. The subse-
quent rapid cooling of the material completes the formation of nanoscale particles.
In its simplest form, the reagent is simply dissolved in a carrier liquid and sprayed
on a hot surface in the form of tiny droplet of ~100-nm diameter. In this case, the
spray is formed from a liquid pressurized by compressed air or mechanically com-
pressed through a small nozzle.
1.2.7
Atomic or Molecular Condensation
Condensation is a well-known technique used to produce primarily metal-based
nanoparticles. The process takes place in a vacuum chamber: fi rst, a bulk piece of
metallic material is heated, melted, and vaporized to produce a stream of atomized
matter. A gas fl ow (either inert or reactive) is then introduced into the reaction
chamber to rapidly cool the metallic vapor. The cooled metallic atoms form con-
densed liquid nanoparticles, which then coalesce in a controlled environment, pre-
serving a spherical shape with smooth surfaces. As the liquid particles are further
cooled, they solidify and stop growing. The so-formed nanoparticles are very reac-
tive and sensitive to agglomeration, which is prevented by adding coating or surfac-
tants to keep them separated. If oxygen is present in the gaseous stream of the
second chamber, the reaction results in the formation of metal oxide nanoparticles.
1.2.8
Supercritical Fluid Synthesis
Supercritical fl uids are materials forced in a state above their critical point by
regulating their pressure and temperature. They are obtained by vaporizing a liquid
in a closed chamber, until the vapor phase becomes as dense as the liquid phase.
