Saturday, April 16, 2011

In MatSciEng: Metal Glass, Tiny Diamonds, and Fruit Fibers

Limited time to write about all new things Materials Science and Engineering has left me with a number of items I've been meaning to write about - or at least bring to light.  This post will be an update of sorts, as well as a trial run for a new format - one which I aim to use in the future in order to focus on newly-published developments for a more current and broad scope.  (Because hey, this is the internet after all.)

It's a Metal; It's a Glass

The atoms in metals are typically arranged in an ordered lattice structure, for example residing on the corners of cubes placed side-by-side.  In contrast, the molecules in glass have a much more random structure ("amorphous"), not keeping to any particular ordered or repeated lattice arrangement.  Bulk metallic glass (BMG) is a material which consists of metal atoms but without the lattice.  In most cases, BMGs possess material properties much different than the lattice metal counterpart.  For example, while strength is typically increased, ductility is decreased - which limits the ability for the BMG to be formed into shapes.   

However, as recently reviewed by Schroers et al [1] (also see ScienceDaily), BMGs can be formed using a specific method (thermoplastic blow molding) which utilizes specifically the BMG's limited ductility mechanisms.  The end result is an apparent ductility (or formability) which is significantly greater.  And because this process is inexpensive, complex shapes (particularly, shells or hollow containers) can be fabricated for a wide range of commercial products where previously only low-strength, cheap, and/or poor quality metals have been allowed.

Diamonds: Not Just for Expensive Jewelery Anymore

Keeping on with metals and glass, diamond has a number of properties advantageous to materials engineers.  For example, it has a thermal conductivity more than twice that of silver and about three times that of copper. Diamond also offers high hardness, low coefficient of thermal expansion, high melting point, and high wear resistance.  And in particulate form, diamond is relatively inexpensive: you can purchase 5 grams of sub-micron-sized synthetic diamond particulate from Alfa Aesar for about $200 (5 g is roughly 1.5 cubic centimetres).  This makes diamond particulate a good candidate for composite materials.

Which are exactly what Nadler's group from Georgia Tech Research Institute have made (from ScienceDaily).  By processing films of silver containing diamond particulate, and tailoring the amount of diamond included in the composite, the thermal properties (e.g. conductivity 25% greater than in copper, and the coefficient of thermal expansion) could be engineered for critical defense-oriented electronics.

"Ductile" Concrete

Continuing with the composites focus, I've been recently directed to the Journal of Advanced and High-Performance Materials (site here), a periodical dedicated to the advancement of materials in building sciences.  In the current issue Li [2] outlines the development and state-of-the-art of Engineered Cementitious Composite (ECC) - a composite consisting of concrete reinforced with internal fibers.  The fibers act to interrupt and distribute large cracks into smaller ones as well as keep micro-cracked concrete together (Figure 1).  The end result is a concrete material - which can also be sprayed on existing concrete as a protective layer - possessing greater damage tolerance, toughness, resilience, and energy absorption capacity (particularly in dynamic or blast loading).

Figure 1.  "Ductile" ECC in bending.  Image taken from Li [2].

I Love to Drive, Drive, Drive ...

We now interact with a variety of plastic/polymer products made from corn, potato, bamboo, as well as other "grown" materials.  Also care of ScienceDaily, fibers from pineapples and bananas are now being used to produce plastic parts and components for automotive, defense, and other high-performance applications.  The Brazilian researchers report their "nano"-cellulose polymer fibers have a stiffness almost as great as Kevlar - known for its use in bullet-proof vests - with the added benefit that the parts can be bio-degradable. 

On the other end of the spectrum, a "smart material" is a catch-all phrase that describes a material that possesses added "active" functionality as opposed to being passive only.  For example, when electric currents are passed through piezo materials, they can oscillate or vibrate.  In this fashion, automotive noise (or noise of any kind) can be dampened and eliminated altogether, thereby reducing energy losses.  On the other hand, these same materials can be used to capture the oscillation or vibration energy in a car and turn it into useful electrical energy.  Work with these kinds of materials is on-going at the Fraunhofer Institute Adaptronics Alliance in Germany (also see ScienceDaily).  Also there, safety devices are being designed consisting of magneto-rheological fluids - in other words, fluids which can change their state (from solid to water and everywhere in-between) by the strength of a magnetic field passed through them.  A large number of applications are currently in the conceptual stage, such as safety mechanisms for machinery or gears.

Take-Away Materials, Today

Finally, I'll end with a take-away message which you can literally take with you.  Materials Today has a great collection of podcasts reviewing current work being done in materials science and engineering.  In particular is a March 16 2011 podcast by Prof. James Tour at Rice University who discusses "Developments in Graphene".  Graphene - an atomically-thick version of graphite - has powerful potential in strength, conductivity, electronics, and computing.  It's a thesis topic on its own, so I'll leave that for a later post.  You can also find a number of recent articles over at Nature Materials and Science concerning recent developments with graphene.  In May of this year will be a conference in Boston regarding the "road to applications" for this powerful material in the next 5-10 years.

[1] J. Schroers et al. Thermoplastic blow molding of metals.  Materials Today, Vol. 14(1-2) (2011), pp. 14-19.
[2] V.C. Li.  High-ductility concrete for resilient infrastructures.  Journal of Advanced and High-Performance Materials, Winter 2011, pp. 16-21. (Available On-Line.)

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