Biomedical Engineering Reference
In-Depth Information
One of the basic bottom-up techniques used to fabricate nanomaterials is chemi-
cal precipitation. This method allows producing nanoparticles of metals, alloys,
oxides, etc. from aqueous or organic solutions following relatively simple and cost-
effective steps. The drawback of this synthesis method, however, is the diffi culty in
the ability to control the distribution of particle size and shape, which limits the use
of the produced nanomaterials in applications where a random particle distribution
is undesired. Nanoscale precipitates can be obtained with various techniques, for
example by a controlled phase transformation guided by the free energy diagrams
or by controlling the solid-state diffusion, following a composite route approach, for
example mixing two different materials and stirring them mechanically. Other
approaches can be found in exploiting internal oxidation of materials, in thin fi lm
deposition of coatings, or sputtering.
The phase transformation approaches rely mostly on the rapid cooling of a solu-
tion that, not given enough time to reestablish equilibrium transformations, becomes
supersaturated and precipitates into more stable and fi ne solid states. Common
methods for rapidly cooling a solution (achieving cooling rates between 10
5
and 10
8
degrees per second) are splat cooling, quenching in salt baths, or melt spinning.
These techniques are often used to obtain thin fi lms and bulk materials with an
extremely fi ne microstructure that infl uences positively its mechanical and electri-
cal properties. Another common example of nanofabrication of nanoparticles or
nanostructured fi lms based on phase transformation of materials is called “spinodal
decomposition.” It is a spontaneous reaction driven by the free energy minimization
of the material that guarantees the formation of a periodic structure of nanoparticles
of equal sizes.
In order to control or limit the growth of nanomaterials produced with bottom-up
methods, two techniques are conventionally used: arrested precipitation and physi-
cal restriction. Arrested precipitation relies on the exhaustion of one of the reactants
or on the introduction of chemicals that would block the reaction. The physical
restriction method relies on the confi nement of the volume available during growth
of the individual nanoparticles. Example of physical restriction includes using the
micelle-reverse-micelle reactions or using templates (like, for example, the porous
alumina described in the Sect.
1.1.3
above).
There are numerous bottom-up methods used to produce nanomaterials; we limit
the rest of the chapter to the description of a few: sol-gel processing, CVD, self-
assembly and bio-assisted synthesis, laser pyrolysis, electroplating, plasma or fl ame
spraying synthesis, atomic or molecular condensation, and supercritical fl uid
synthesis.
1.2.1
Sol-Gel Processing
The sol-gel process in general is based on the transition of a system from a liquid
“sol” (mostly a colloidal suspension of particles) into a gelatinous network “gel”
phase. With this, it is possible to create at low temperature ceramic or glass mate-
rials in a wide variety of forms. It is a long-established industrial process that is
