Materials Engineering and Metals
While all the high-tech promise of nanotechnology lies in
medical and electronic applications, the field of materials engineering is also
catching the nano wave.
Metals have always featured some form of micrometer-sized
features. They are not perfect lattices
of atoms like semiconductors and other crystals. Instead, they have what is known as short-range order. This means that a metal is made up of
adjacent grains. Each grain has a
perfect lattice except at the grain boundary, where it meets up with the
adjacent grain that has the same lattice but in a different orientation.
To make a long story short, a variety of experiments have
demonstrated that man key metal properties, like hardness, follow what is known
as the Hall-Petch relationship. From
grain sizes with diameters from micrometers to nanometers, hardness varies inversely
to root diameter. This means that the
smaller that the grain size of a metal gets, the harder and stronger it will
be.
This relationship has been proven to be precise to a certain
extent. Below a threshold, the
relationship begins to break down and can go in a variety of directions
depending on the test apparatus and material in question. The point is, however, that significant
gains in material strength can be had if you bring the grain size down below
100 nm.
There are several ways to do this. The best way so far is electrodeposition with pulse plating. In normal grain growth, there are two
competing factors: grain nucleation and grain growth. Nucleation defines the rate at which disordered areas become
ordered into lattices. Grain growth
effects how quickly these areas spread and eventually meet each other to form
grain boundaries. In order to create
smaller grain sizes, it’s important to minimize grain growth and maximize
nucleation. One way to do this is to
use a modified electrodeposition technique where the applied voltage is pulsed
instead of held constant. This can be
further enhanced with the introduction of dopants, like phosphorus in nickel.
Other techniques like mechanical attrition and fixed-channel
compression take a more active approach to beat down the metal and accumulate
many defects in the material until it has reached the desired grain size.
The result of these techniques is what is known as
nano-crystalline metals. They typically
exhibit higher strengths, but at the cost of increased brittleness and lowered
ductility. Many applications for such
metals are in the defense industry where thin, high-strength metal armor is
important.
Other research in the nanomaterials realm include composite
materials with enhance resistance to wear and corrosion. One instance of this is the famous nano-pants
that resist stains by repelling the absorption of water.
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