ON THE face of it, metal and glass are very different substances. Metal is malleable and opaque; glass is brittle and transparent. It is hard to imagine a transparent metal, or a missile made of glass. But those are just two of the strange possibilities raised by researchers working in the intriguing experimental no man's land between the two materials.
In the past few weeks, researchers at Rice University in Houston, Texas, and Pennsylvania State University have been making transparent metals using a new technique borrowed from the fast-moving field of “mesoporous” solids. As their name suggests, these are materials with an orderly structure of tiny pores, just a few tens of nanometres (thousand-millionths of a metre) across. They are made using a process that employs tiny “sacrificial” silica spheres to define the porous structure.
The silica spheres, 40 nanometres in diameter, are packed together in a regular pattern (like stacked fruit) and heated to 800°C so that they stick together. Then, through a series of chemical steps, the even tinier gaps between the spheres are filled with a second material. Finally, the silica spheres are dissolved away using hydrochloric acid.
The result is a solid shot through with tightly packed spherical pores, so that 75% of it is empty space. Since the spacing of the pores is much less than the wavelength of visible light (which ranges from 400 to 750 nanometres), the resulting materials scatter light only very slightly. In other words, they are transparent.
Although this method was originally devised to make porous polymers, Thomas Mallouk and his colleagues at Pennsylvania State University have been trying it with other materials, including metals. They have yet to publish their results, so they are keeping quiet about exactly which metals they are working with. But, according to Dr Mallouk, the transparent metals they have made so far are coloured, and resemble slightly cloudy glass.
Vicki Colvin at Rice University has also been making pieces of porous metal with unusual optical properties, including transparency, using a similar technique. Dr Colvin says that depending on the pore-size, which can be varied by using different sacrificial spheres, her metals resemble slightly coloured smoked glass, are translucent like opal, or flash iridescent colours like butterfly wings (whose colour depends on minute surface features, not pigments).
Despite the implausibility of these materials—science-fiction fans will recall the “transparent aluminium” that features in one of the Star Trek films—they actually promise to prove rather useful. Transparent metals might, for example, be handy in flat-panel displays and other devices where it is desirable to be able to see through an electrical conductor. They could be useful as catalysts, in place of the metal sponges currently used in everything from cars to oil refineries. Diffraction gratings made of porous metal could also be used to manipulate the beam of an X-ray microscope, something that cannot be done with lenses or mirrors.
By adjusting the pore size so that a particular type of charged atom (ion) can just fit inside each pore, it might also be possible to control the flow of ions through a porous metal by applying a voltage to it. This would be useful for applications including filtering and separation, removing heavy ions from water, drug delivery, and sensing very low concentrations of ions in solutions. Charles Martin, a researcher at Colorado State University, has made an experimental sensor out of porous gold which can detect certain ions in concentrations as low as one part in ten billion.
Metal, but not as we know it
While some researchers investigate the properties of glass-like metals, others are working with metallic glass. Technically, a glass is any material that has been cooled from a liquid to a solid without crystallising. Most metals crystallise as they cool, assuming a random pattern of grains within which the atomic arrangement is regular. But rapid cooling can prevent the crystals from forming. The result is a solid with a random, amorphous arrangement of atoms that has more in common with a very viscous liquid than a solid.
Metallic glass was first discovered in 1959 by Pol Duwez of the California Institute of Technology (Caltech). He cooled a mixture of gold and silicon at a rate of one million degrees centigrade per second to make a metallic-glass alloy. Since then researchers have devised new alloys, containing four or more metals, which form metallic glass at much slower cooling rates.
The resulting material looks much like any other metal, but it has some unusual properties. Normal metals have defects in their crystalline structures, and in the boundaries between grains. One type of defect, called a dislocation, behaves much like a ruck in a piece of carpet, and allows the metal to slip and deform internally when a load is applied. Since it lacks these defects, metallic glass can support much greater loads without deformation, and is therefore much stronger than its crystalline counterpart.
Todd Hufnagel of Johns Hopkins University in Baltimore, Maryland is working with the American army to make a metallic-glass alloy for use in armour-piercing projectiles. A study carried out at Caltech found that unlike a conventional warhead, which flattens into a blunt mushroom-shape on impact, the sides of a metallic-glass projectile sheer off, in effect making it even sharper. (Depleted uranium has similar properties, but has environmental drawbacks.) According to William Johnson, a pioneering researcher into metallic glass at Caltech, its strength also makes it suitable for use in aircraft structures, armour-plating, and biomedical implants.
But of all the possible applications for these exotic forms of metal, one at least is a little more down to earth. As a result of his research, Dr Johnson has set up a company to sell pricey golf clubs made of a metallic glass called LiquidMetal. Lacking the crystalline defects found in conventional metal clubs, his golf clubs absorb less energy on impact—so that more energy is transferred to the ball.