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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.