Processing Limitations in Nano-Materials

Example Nanomaterials from Talapin et al at the University of Chicago (link).

In the recent issue of the journal Nanotechnology (also covered at ScienceDaily), Kelly [1] has written a very interesting article concerning processing limitations of some small-scale engineering designs and constructions.  From ScienceDaily:

The overall goal when entering nanotechnologies into the market is low-cost, high-volume manufacturability, but at the same time, the materials' properties must be highly reproducible within a pre-specified limit, which Kelly states cannot happen below the 3 nm limit when trying to make arrays. 

The top-down approach to manufacturing, which Kelly states is limited, uses external tools to cut and shape large materials to contain many smaller features.  Its alternative, the bottom-up approach, involves piecing together small units, usually molecules, to construct whole materials -- much like a jigsaw puzzle -- however this process is too unpredictable for defect-free mass production of arrays.

As outlined by Kelly, standard manufacture operates under what is called 6-sigma yield - which essentially means there is a manufacturing defect rate of only 0.00034%. Those extremely tight tolerances are simply not met by the respective manufacturing processes of most nanotechnologies.  However, there can be two ways to (re)consider these limitations.

The first is that - given the need for the technology - the scales of production will follow.  Moore's Law is a well-known example.  In short, due to continuous improvement in fabrication equipment and manufacturing technology - much of which was not around when Moore's "Law" was first proposed - the number of transistors possible on a single chip has doubled every two years from the 1970's to the present day.  Based on this, it is reasonable to propose that additional improvements in manufacturing technology will help raise future nanotechnologies out of the "unmanufacturable" category.     

The second way to consider these limitations is with reference to another complex system - the human body - which also possesses a great deal of complex functionality and manufacturing.  For example, our own DNA replication and copying processes would not meet 6-sigma yield if not for clever DNA proof-reading and correcting mechanisms.  In addition, some degree of DNA error is manageable because redundancy exists.  Redundancy might not sound like an ideal situation for large-scale fabrication, but it is a useful tool to help maintain proper system functionality even if a small percentage of components are defective.  And, thermodynamically speaking, if there will be an unavoidable amount of error, then redundancy may be the path of least resistance.  Furthermore, if you accept defects will exist - if not from manufacturing, then due to in-situ operation - the design process changes from a defect-free nanotechnology to one which may perhaps even benefit from a certain concentration of them - much like the way doped semiconductors operate and interact.  But of course, this may not be possible for some nano-materials and their applications, e.g. quantum dots requiring precise confinement energies.

Ultimately, as Kelly writes, some nanotechnologies are unfortunately bound to remain on a lab bench:

Many results in nanoscience can be shown to be intrinsically unmanufacturable in terms of ideas for applications in electronic or optoelectronic components, and so will remain as scientific curiosities.

Time will tell which are the most manufacturable and which are so in a sustainable manner.  

[1] MJ Kelly. Intrinsic top-down manufacturability. Nanotechnology, Vol. 22 (2011), 245303.

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.)

Lightweight Cellular Materials and Lessons from Nature

New hybrid or composite materials can be considered a combination of two or more constituent parts.  The end result is a material that possesses a set of properties not offered by either individual part alone.  A basic example, concrete reinforced with rebar is a composite material possessing both strength (provided by the concrete) and toughness (provided by the steel).

The concept of a "constituent part" can be extended further to include liquids and gases.  This is the approach taken by Ashby [1], who has considered cellular materials as hybrids of 1) solid material and 2) air.  Like composite materials consisting of two interwoven solid materials, the arrangement of the "air" component in cellular materials can also be designed through the wide range of manufacturing techniques [2]:

Variety of metal foams and their fabrication methods (from Wadley [2] - click to enlarge).

As the architecture of these materials is designed, so are the resultant characteristics (e.g. density, strength, energy absorption, resonant frequency, and the list goes on).  There is a great deal of very interesting on-going work which focuses on processing, properties, and applications of these cellular materials - most of which focuses on the light-weight structural benefits.  For more information, see bi-annual conferences Cellular Materials (CellMat) and Porous Metals / Metalic Foams (MetFoam).    

Natural Structures and Sustainable Design

Some of you may have already noticed now closely the above cellular materials mimic those of natural structures, of which a well-known example is spongy (trabecular) bone:

The internal cellular structure of spongy (trabecular) bone.  (Source: National Cancer Institute.)

The employment of this cellular structure illustrates mechanical benefits such as light weight and high load-bearing structural efficiency.  However, as recently studied by Doube et al [3] (also from ScienceDaily), this cellular design is not restricted to select organisms; in fact, the adoption of a cellular bone structure is wide-spread and is allometric (i.e. scalable with organism size) across a large number of mammals and birds from elephant to shrew.

Such a prevalent example of natural materials design is therefore not as much a niche evolutionary development as it is a generally-adopted (and powerful) design tool.  And because tools such as these came about through evolutionary means, they therefore also offer insight into new dimensions for human-powered sustainable materials design and development.

[1] M.F. Ashby.  Hybrids to fill holes in material property space.  Philosophical Magazine, Vol. 85 (2005), pp. 3235-3257.
[2] H.N.G. Wadley.  Cellular metals manufacturing.  Advanced Engineering Materials, Vol. 4 (2002), pp. 726-733.  (Available On-Line.)
[3] M. Doube, et al.  Trabecular bone scales allometrically in mammals and birds.  Proceedings of the Royal Society B, Vol. 278 (2011), DOI: 10.1098/rspb.2011.0069