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Nanomaterials:
It's a Small, Small World

(Released February 2002)

 
  by Kathleen Hickman  

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Over the past decade, nanomaterials have been the subject of enormous interest. These materials, notable for their extremely small feature size, have the potential for wide-ranging industrial, biomedical, and electronic applications. As a result of recent improvement in technologies to see and manipulate these materials, the nanomaterials field has seen a huge increase in funding from private enterprises and government, and academic researchers within the field have formed many partnerships.

Nanomaterials can be metals, ceramics, polymeric materials, or composite materials. Their defining characteristic is a very small feature size in the range of 1-100 nanometers (nm). The unit of nanometer derives its prefix nano from a Greek word meaning dwarf or extremely small. One nanometer spans 3-5 atoms lined up in a row. By comparison, the diameter of a human hair is about 5 orders of magnitude larger than a nanoscale particle. Nanomaterials are not simply another step in miniaturization, but a different arena entirely; the nanoworld lies midway between the scale of atomic and quantum phenomena, and the scale of bulk materials. At the nanomaterial level, some material properties are affected by the laws of atomic physics, rather than behaving as traditional bulk materials do.

Although widespread interest in nanomaterials is recent, the concept was raised over 40 years ago. Physicist Richard Feynman delivered a talk in 1959 entitled "There's Plenty of Room at the Bottom", in which he commented that there were no fundamental physical reasons that materials could not be fabricated by maneuvering individual atoms. Nanomaterials have actually been produced and used by humans for hundreds of years - the beautiful ruby red color of some glass is due to gold nanoparticles trapped in the glass matrix. The decorative glaze known as luster, found on some medieval pottery, contains metallic spherical nanoparticles dispersed in a complex way in the glaze, which give rise to its special optical properties. The techniques used to produce these materials were considered trade secrets at the time, and are not wholly understood even now.

Development of nanotechnology has been spurred by refinement of tools to see the nanoworld, such as more sophisticated electron microscopy and scanning tunneling microscopy. By 1990, scientists at IBM had managed to position individual xenon atoms on a nickel surface to spell out the company logo, using scanning tunneling microscopy probes, as a demonstration of the extraordinary new technology being developed. In the mid-1980s a new class of material - hollow carbon spheres - was discovered. These spheres were called buckyballs or fullerenes, in honor of architect and futurist Buckminster Fuller, who designed a geodesic dome with geometry similar to that found on the molecular level in fullerenes. The C60 (60 carbon atoms chemically bonded together in a ball-shaped molecule) buckyballs inspired research that led to fabrication of carbon nanofibers, with diameters under 100 nm. In 1991 S. Iijima of NEC in Japan reported the first observation of carbon nanotubes1, which are now produced by a number of companies in commercial quantities. The world market for nanocomposites (one of many types of nanomaterials) grew to millions of pounds by 1999 and is still growing fast.

The variety of nanomaterials is great, and their range of properties and possible applications appear to be enormous, from extraordinarily tiny electronic devices, including miniature batteries, to biomedical uses, and as packaging films, superabsorbants, components of armor, and parts of automobiles. General Motors claims to have the first vehicle to use the materials for exterior automotive applications, in running boards on its mid-size vans. Editors of the journal Science profiled work that resulted in molecular-sized electronic circuits as the most important scientific development in 20012. It is clear that researchers are merely on the threshold of understanding and development, and that a great deal of fundamental work remains to be done.

What makes these nanomaterials so different and so intriguing? Their extremely small feature size is of the same scale as the critical size for physical phenomena - for example, the radius of the tip of a crack in a material may be in the range 1-100 nm. The way a crack grows in a larger-scale, bulk material is likely to be different from crack propagation in a nanomaterial where crack and particle size are comparable. Fundamental electronic, magnetic, optical, chemical, and biological processes are also different at this level. Where proteins are 10-1000 nm in size, and cell walls 1-100 nm thick, their behavior on encountering a nanomaterial may be quite different from that seen in relation to larger-scale materials. Nanocapsules and nanodevices may present new possibilities for drug delivery, gene therapy, and medical diagnostics.

Surfaces and interfaces are also important in explaining nanomaterial behavior. In bulk materials, only a relatively small percentage of atoms will be at or near a surface or interface (like a crystal grain boundary). In nanomaterials, the small feature size ensures that many atoms, perhaps half or more in some cases, will be near interfaces. Surface properties such as energy levels, electronic structure, and reactivity can be quite different from interior states, and give rise to quite different material properties.

Let us examine in particular nanocomposites based on polymeric materials, keeping in mind that this is but one small division of nanomaterials. There are several varieties of polymeric nanocomposites, but the most commercially advanced are those that involve dispersion of small amounts of nanoparticles in a polymer matrix. Those most humble of materials, clays, have been found to impart amazing properties. For example, adding such small amounts as 2% by volume of silicate nanoparticles to a polyimide resin increases the strength by 100%. One should keep in mind, of course, that 2% by volume of very small particles is a great many reinforcing particles. Addition of nanoparticles not only improves the mechanical properties, but also has been shown to improve thermal stability, in some cases allowing use of polymer-matrix nanocomposites an additional 100 degrees Centigrade above the normal service conditions. Decrease in material flammability has also been studied, an especially important property for transportation applications where choice of material is influenced by safety concerns. Clay/polymer nanocomposites have been considered as matrix materials for fiber-based composites destined for aerospace components. Aircraft and spacecraft components require lightweight materials with high strength and stiffness, among other qualities. Nanocomposites, with their superior thermal resistance, are also attractive for such applications as housings for electronics.

Others have examined the electrical properties of nanocomposites, with an eye to developing new conductive materials. The use of polymer-based nanocomposites has been expanded to anti-corrosion coatings on metals, and thin-film sensors. Their photoluminescence and other optical properties are being explored. Polymer-matrix nanocomposites can also be used to package films, an application which exploits their superior barrier properties and low permeability.

Although some nanomaterials require rather exotic approaches to synthesis and processing, many polymer-matrix nanocomposites can be prepared quite readily. Clay/polymer nanocomposites have been made by subjecting a clay such as montmorillonite to ion exchange or other pretreatment, then mixing the particles with polymer melts. There are also a number of other ways to fabricate the materials, including reactive processes involving in situ polymerization. The low volume fraction of reinforcement particles allows the use of well-established and well-understood processing methods, such as extrusion and injection molding. Ease of processing and forming may be one explanation for the rapidly expanding applications of the materials. Automotive companies, in particular, have quickly adopted nanocomposites in large scale applications, including structural parts of vehicles.

The most energetic research probably concerns carbon nanotubes. Nanoparticles of carbon - rods, fibers, tubes with single walls or double walls, open or closed ends, and straight or spiral forms - have been synthesized in the past 10 years. There is good reason to devote so much effort to them: carbon nanotubes have been shown to have unique properties, stiffness and strength higher than any other material, for example, as well as extraordinary electronic properties. Carbon nanotubes are reported to be thermally stable in vacuum up to 2800 degrees Centigrade, to have a capacity to carry an electric current a thousand times better than copper wires, and to have twice the thermal conductivity of diamond (which is also a form of carbon). Carbon nanotubes are used as reinforcing particles in nanocomposites, but also have many other potential applications. They could be the basis for a new era of electronic devices smaller and more powerful than any previously envisioned. Nanocomputers based on carbon nanotubes have already been demonstrated.

It is not so amazing, then, that government bodies, companies, and university researchers are joining forces or competing to synthesize, investigate, produce, and apply these amazing nanomaterials.

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