Atom smith – Dick Siegel constructs materials one molecule at a time
In another era, Dick Siegel mihgt have been a blacksmith. He is a big man, with a big head and thick hands that seem perfect for gripping a forge hammer, and his voice is a deep baritone. These being the very late days of the twentieth century, however, Siegel spends his days not hunched over an anvil in a smithy but behind a desk at Rensselaer Polytechnic Institute in Troy, New York, dressed not in a heavy leather apron but in a blazer and a blue shirt open at the collar. He seems more like an engineer or a doctor than a smith – jovial, outgoing, conventional.
Down in his basement, though, in a space about the size of a small motel room, Siegel practices a peculiar brand of metallurgy. There are no hammers, no anvil or bellows, no horseshoes hanging on the walls. But Siegel proudly displays a contraption made of vacuum chambers, liquid-nitrogen tanks, and electron beams, all interlinked by hoses and stainless-steel pipes and tethered to computers. He and his colleagues refer to it as the gas condensation unit, or just the unit, but you might just as well call it a forge.
In many ways, Siegel is like the smiths of Damascus, who 1,400 years ago learned how to make a steel unlike any that they had made before. The sword made from that steel would become the stuff of legend – the acme of the blademaker’s art and the bane of generations of Crusaders. Christian soldiers returned from their wars with tales of the sword’s toughness and razor-sharp edges; they told how it could cut a silk scarf in midair yet never break in battle. The best swords of the Crusaders could never do both: they were either sharp but brittle or tough but dull. The Damascus smiths, however, knew how to tickle and cajole and LAY steel until it seemed to become another metal altogether.
Like the old wizards of Damascus, Siegel has learned how to transform substances from ordinary to extraordinary. As he stands in front of his forge, he displays a small metal disk no larger than some of the small batteries that power watches and other electronic gadgets. It ‘is pure copper, Siegel says, and indeed it looks and feels like the copper in a freshly minted penny. It is not. After passing through his forge, it is five times harder than normal – an even bigger improvement over ordinary copper than Damascus steel was over its weaker cousins. Siegel made this measurement himself, using a standard test of hardness called a microindentation test, in which a diamond stylus is driven down onto a sample; the smaller the resulting dent, the harder the metal.
Next Siegel takes out a pill-size piece of titania, a common ceramic. At one time the sample was shaped like a miniature soup can, but now it looks as if someone stepped on it, squeezing the two ends together and mashing out the middle. Strange behavior for a ceramic. These nonmetallic substances are usually so stiff that they respond to pressure by fracturing into a thousand pieces. Drop a tin cup on the floor, and it will flex a little bit and bounce back into shape, small dents notwithstanding. Drop a ceramic cup and you’ll still be picking up bits of it next Christmas. A ceramic that mashes instead of smashes – like the one that has come out of Siegel’s forge – is as rare as a metal cup that shatters when it hits the floor.
Finally Siegel brings out a disk of yttria, another ceramic. Like most ceramics, yttria is normally as white and opaque as Grandma’s china. This disk, however, is a ghost of its former self. It is completely transparent. Grandma would be able to see the pattern of her tablecloth right through dishes made of this material.
What’s going on? How does a mild-mannered bit of copper or yttria step into a phone booth and emerge with superpowers like ultrastrength or transparency? The short answer is that Siegel has developed a new way of manipulating matter, on an atomic level. The long answer requires explaining how Siegel’s methods takes advantage of physical principles that scientists still do not completely understand.
The traditional route to modifying materials is to use chemistry. Adding a little carbon to iron creates a much stronger metal: steel. Or, as anyone who has played with a chemistry set knows, a few drops of the right chemical can turn a white liquid clear, or vice versa. Siegel doesn’t follow this path. The superstrong copper from his forge is chemically identical to normal copper. Nothing has been added. Similarly, the mashable titania and the transparent yttria have the same chemical formulas as the normal forms of those ceramics. Instead of changing the chemical compositions, Siegel alters the way in which the atoms in these materials organize themselves.
If you take a piece of normal copper, slice it, and examine the cross section with a 100-power microscope, you’ll see a crazy patchwork quilt, with no two patches the same shape or size. These patches are two-dimensional slices of the three-dimensional building blocks that make up the copper. Each of these building blocks – or grains, as they’re usually called – is a single crystal in which the atoms are stacked in an orderly fashion, like the oranges in a grocer’s display. The grains themselves, however, are far from orderly. They take on irregular shapes and sizes, and they’re jumbled together, like fieldstones in a farmer’s wall, except that they’re packed so closely that, for the most part, less than an atom’s width separates any two of them.
The attributes of that piece of copper – everything from its strength and flexibility to its color and its electrical conductivity – depend on two things: the properties of the individual grains, and the relationships and interactions between the grains. Copper is a good conductor, for instance, because electrons – the carriers of electric current – can move freely inside the grains and can jump easily from one grain to another.
Now suppose you take that bit of normal copper, run it through Siegel’s forge, and peer at it through the same microscope. In place of the patchwork quilt, there is a smooth and featureless nothingness. Increase the magnification tenfold, and still there is nothing. Jack it up by another factor of ten, and a bit of graininess appears. (By now you’ve had to switch to an electron microscope because the details have gotten too small for a conventional optical microscope.) A third factor of ten and then a fourth, and finally you see it: Siegel’s copper has a patchwork structure very similar to that of the standard sample, but the individual grains are 10,000 times smaller. A trillion grains of the superhard copper could fit inside one average-size grain of the normal stuff. Proportionally, the size difference is about the same as that between a grain of sand and a Volkswagen Beetle. Copper grains this small contain only a few thousand atoms, or maybe a few tens of thousands of atoms, rather than the thousands of trillions in conventional copper grains.
The same tiny grains account for Siegel’s mashable titania and transparent yttria. And this is the secret of Siegel’s forge. It can produce materials – metals, ceramics, semiconductors, and nearly any other solid – in which the grain sizes are smaller than those in the more common versions by a factor of a thousand or more. “If you shrink matter below a critical length scale,” Siegel explains, “it changes properties. What used to happen doesn’t happen as easily or at all, and completely new things can happen.” This sensitive dependence on the grain size opens up an entirely new way of manipulating matter. Instead of using chemical reactions, Siegel says, “you can control a material’s traits by controlling the size of the building blocks. I think it’s an absolute revolution.”
Yttria, for example, in its usual state is white, for the same reason that snow is white: the jumble of crystalline grains and of pores between the grains scatters and reflects visible light. Although a single crystal of yttria – like a single ice crystal – is essentially transparent, a collection of them will bounce light around and spit it back out. The yttria produced by Siegel’s forge, however, has grain sizes (and pore sizes) that are too small to scatter the fight. Light waves ignore features smaller than one-quarter of a wavelength – roughly 100 nanometers, or 100 billionths of a meter, or one-thousandth the width of a human hair. That is truly tiny. But the grains in Siegel’s transparent yttria sample are tinier still – a mere 8 nanometers across. “The wavelength of the light is so long with respect to these features that it doesn’t see them,” Siegel says.
For other traits, such as hardness or electrical and magnetic properties, the critical lengths generally range from 10 to 100 nanometers. By making materials with grain sizes smaller than this, Siegel can get them to behave in novel and unusual ways.
One of the most curious things about these “nanocrystalline” materials is how their behavior continues to change as the grains get smaller and smaller. Siegel has observed this in his copper samples. If the average grain size is 50 nanometers, the copper is twice as hard as conventional copper, whose grains are a thousand times larger. Take the grain size down to 25 nanometers, and the copper is three times as hard; at 15 nanometers it is four times; and when the grains reach 6 nanometers, the copper is an amazing five times harder than normal. But for a truly dramatic demonstration of how properties depend on grain size, Siegel’s copper is only a poor second to an exhibit devised by chemists Louis Brus and Michael Steigerwald, at. AT&T Bell Laboratories in Murray Hill, New Jersey.
Brus and Steigerwald have captured the rainbow in several jars. One shines bright red, another is orange, a third glows pale green, while a fourth is black. The jars contain a series of powders, Brus explains. The powder in the first jar is cadmium selenium, a semiconductor with few commercial applications but whose optical properties are useful for this particular experiment. The powder in the second jar is also cadmium selenium. The powder in the third jar is – you guessed it – cadmium selenium. The only difference between the powders is the size of their grains. The grains in the orange powder, for example, are 30 percent smaller than those in the red powder: 3.5 versus 5 nanometers. Shrinking the grains constrains the space in which the material’s electrons can move, which in turn alters their energy levels, and since the wavelengths – or colors – that a material will absorb are determined by the energy levels of its electrons, changing the grain size alters the color. By choosing the grain size carefully, Brus says, he can produce any color in the spectrum.
With their focus on manipulating grains at the level of nanometers, Siegel and his collaborators have given a new look to what is actually a very old field. The smiths who forged Damascus swords, for example, practiced a similar technique, albeit on a scale a thousand times larger and without knowing exactly what they were doing to the grains. Researchers believe that a key to Damascus steel’s hardness is the tiny grains of iron carbide dispersed throughout the metal. Stanford metallurgist Oleg Sherby has hypothesized that the Damascus smiths hardened the swords by hours of hammering. This pounding broke up networks of large iron carbide grains into smaller, isolated grains, which made the swords hard – and thus able to hold a sharp edge – without being brittle.
In Mexico, Mayan and other pre-Columbian cultures put small-grain materials to work in a more peaceful way. When Siegel and Diana Magaloni of Yale analyzed those cultures’ paintings, many of which are still brilliantly colored after more than a millennium, they found that the grain sizes in the pigments were only a small fraction of the expected size. Siegel and Magaloni suspect that the colors’ vividness is at least partly a result of the grain size: since the small grains don’t scatter as much light, colors appear more intense. The researchers also suspect that the vividness has something to do with more pigment grains’ being crammed into a given space. In any case, Siegel notes, grinding the pigments so finely would not have been easy with the tools available to these ancient artists. If they were willing to work hours with a mortar and pestle, they must have known that their payoff would be a more powerful palette.
These days, controlling grain size and structure is a big business. Steel mills use giant rolling presses that squeeze sheets of steel to break the grains into smaller pieces – the smaller the grains, the tougher the steel. Conversely, the computer chip industry depends on making grains as large as possible. Tiny, complex circuits demand a uniformity and consistency that only single crystals can provide, so they are built on single crystals of silicon, several inches across, that are manufactured specifically to avoid the jumble of grains that normal silicon contains. In general, any manufacturer of metals, ceramics, or semiconductors must take grain structure into account in one way or another.
However, creating and controlling nanometer-scale grains – grains a thousand times smaller than those in modern steels and ceramics – demands some new methods. Over the past decade or so, materials scientists have developed a number of ways to get down into the required range. Some of them are little more sophisticated than the hammering of the Damascus smiths and essentially use brute force to break large grains into smaller ones. The device of choice is often the high-speed ball mill, a distant relative of the paint shakers that hardware stores use to mix up a gallon of midnight blue or sunset pink. You put a powder of, say, copper into the machine’s steel canister, add several tungsten carbide balls, and flip the switch. The can shakes violently, tossing the balls around and crushing the powder between them and the steel sides. Ten or 20 hours later, the grains of the pulverized powder are reduced to fragments a mere hundredth their previous size.
The technique works, but the results are crude. The fragments have irregular shapes and sizes, and they are often contaminated by atoms scraped off the tungsten carbide balls and the steel walls. Furthermore, the pounding can shrink the grains only so far. Once they reach a certain average size, no amount of brute force will reduce them further. This limitation does not prevent steel mills from using the technique to make alloys from metals that wouldn’t normally combine, but for Siegel’s purposes it is less than ideal.
The alternative to breaking the grains down is to build them up from scratch, atom by atom. In theory, this makes it possible to create grains of virtually any size, from those containing only a few atoms to those with thousands, millions, or billions. Putting the theory into practice, however, demands more than a little cleverness.
It’s actually not hard to assemble atoms into grains – or clusters, as they’re usually called while they are still individual collections of atoms and haven’t yet been packed together into one solid piece. Under the right conditions, atoms will naturally congregate. First a pair of atoms attract each other, then a third arrives, and a fourth, and soon atom after atom is joining the rapidly growing cluster. Crystals form in this way. The growth of the grains stops only when no more atoms remain to join the crowd, or when the grains get so big that they start crowding one another. The challenge for Siegel and other researchers is to stop the growth once the clusters reach the desired size and, furthermore, to carry out this controlled growth on a massive scale. If the clusters are 10 nanometers across, it will take a quadrillion – a million billion – of them to make a pellet a millimeter across, which is not much larger than a grain of sand.
Brus grows his colorful clusters in solution. He mixes two reagents, one containing cadmium and the other selenium, which combine to create cadmium selenium crystals. To control the sizes of the clusters, he grows them inside tiny micelles, spherical drops of water that are suspended in a beaker of the organic solvent heptane. The clusters can get no bigger than the micelles because the reaction that produces the cadmium selenium will not proceed in heptane. Once the clusters reach their full extent, Brus adds a solution of organic molecules that are attracted to the surface of the clusters. These molecules form an organic skin that will keep the clusters at the desired size after they’re removed from the confines of the micelles.
This approach works well when the goal is to study single clusters – which indeed is Brus’s major interest – but the organic coatings make it difficult to assemble the clusters into solid objects. And even if they are solidified, the result is not pure cadmium selenium but rather a collection of cadmium selenium bricks with an organic mortar between them.
Siegel has taken a different tack. As he explains it, his technique for creating solids with nanometer-size grains is much like boiling a pot of water on a cold day in a room with a window. As water molecules steam out of the pot, they aggregate into clusters, many of which freeze when they reach the windowpane. Scrape these ice crystals off and you can pack them into a snowball.
To do the same thing with copper, for example, takes $100,000 worth of equipment instead of an old pot and a windowpane, but the principle is the same. First Siegel heats the copper. He does this either by putting the copper in a tungsten pot – a “boat” – and heating the boat with an electric current, or by zapping the copper with a beam of high-energy electrons. As the temperature rises, the copper atoms boil off into a chamber filled with helium gas. Collisions with the helium atoms slow the copper atoms enough so that when two of them collide they stick together instead of bouncing apart. Gradually, atom by atom, the copper “steam” condenses into clusters. The helium gas carries the clusters to a “cold finger” – a cylinder cooled by liquid nitrogen to -319 degrees Fahrenheit – which plays the role of the windowpane. A Teflon blade scrapes the copper clusters from the walls of the cylinder, and they fall through a funnel into a compactor, where the clusters are in turn shaped into little pellets of pure nanograin copper.
By adjusting the evaporation rate of the copper, the pressure of the helium gas, and how quickly the flow of helium carries the clusters to the cold finger, Siegel can control the size of the clusters. The control isn’t perfect, but it is pretty good. If Siegel wants clusters that are 8 nanometers across, his machine generally produces clusters between 6 and 10 nanometers wide.
Over the past decade, as Siegel has studied the properties of nano-materials, he has pieced together a pretty good picture of why they act as they do. The explanations underline just how differently matter behaves when it is chopped into such small bits.
Why, for instance, do copper and other metals get so hard when their grains get small? Normal metals are relatively soft because metal atoms can move past one another easily. That allows a dislocation in a grain – a structural “Kink” – to shift around easily from place to place. Siegel likens the process to moving a heavy rug across a carpeted floor. Dragging the rug is next to impossible, but the rug can be moved easily, a little at a time, by creating a bump – a dislocation – in one end and pushing the bump across to the other end. When a diamond stylus presses down on a piece of metal, it similarly causes dislocations, which shift around in response to the movement of the stylus and forth a dent in the metals surface.
Strengthening a metal has traditionally involved adding impurities that disrupt the normal crystalline structure and make it harder for dislocations to move – for example, putting carbon in iron to produce steel. As Siegel says, this is like “putting a chair on the rug.” Shrinking the grains hardens the metal in a different way: it makes it much more difficult for dislocations to form. Dislocations that occur near the edge of a grain are inherently unstable because the atoms tend to shift around in such a way that they quickly push the dislocations out of the grain, at which point they dissipate. In large grains, most dislocations are too far from the grain boundary to be eliminated in this way. Nanometer-size grains, however, are small enough that any dislocation is relatively close to the edge, and thus close to immediate elimination. The smaller the clusters, the fewer the dislocations – and the harder the metal.
On the other hand, decreasing grain size makes nanocrystalline ceramics easier to deform. Why the contradiction? A ceramic is stiff, Siegel explains, because the atoms in its grains cannot move past one another as easily as the atoms in a metal. Put too much stress on a ceramic plate – by, say, dropping it from a height of several feet – and cracks will open up between the grains. The ceramic has no other way to relieve the stress than to shatter into tiny bits.
Ceramics with nanometer-size grains, however, have an alternative: individual grains can shuffle around, sliding past one another like grains of sand in a sand pile. The trick is to perform this shuffle without opening up large gaps between the grains, gaps that will break the grain-to-grain bonding and create cracks. Siegel’s ceramics manage to pull this off because atoms at the edges of the grains can shift slightly as the grains move, filling up any gaps before cracks form. This doesn’t happen with large-grain ceramics, Siegel says, because the atoms at the interfaces move very slowly, and if the grains are large, it takes an impossibly long time for the atoms to move far enough to fill in the gaps between the grains. Even with nanometer-size grains, the process is not quick. It took Siegel 15 hours of steady pressure at high temperatures to mash his titania soup can. In principle, he could do the same with any large-grained ceramic. But the process would take 100 million times longer – some 170,000 years.
Eventually, Siegel predicts, industry will recognize the value of his nanocrystalline materials. Powerful magnets might have coils of superhard copper that resist being stretched by the strong magnetic field. Since nanoscale powders of palladium and other metals have lots of surface area upon which to support chemical reactions, they might serve as catalysts in the chemical industry. Siegel eventually expects to see nanocrystalline semi-conductors and composites, and perhaps even nanoplastics.
But he’s not waiting for others’ recognition. Six years ago Siegel formed Nanophase Technologies Corporation to commercialize his work. (He put phase in the name because it refers generally to any form of matter, crystalline or liquid or amorphous, and because it has a zippy sound to it.) So far the company has built a production version of Siegel’s forge that can churn out tens of tons of ceramic powder a year. It is also working with Caterpillar and Lockheed-Marietta to develop a process for making ceramic parts for a combustion engine, which could run hotter and more efficiently than a wholly steel one. Since ceramics tend to shrink during baking, ceramic engine parts have to be made too large and then machined down to size, a prohibitively expensive process for mass production. Nanophase ceramics, however, can be deformed, which means the parts could in theory be pressed into shape in a die. The finished pieces would be indistinguishable from a normal ceramic except that they would be much cheaper.
Even if the ceramic engine project pans out, it won’t be the first application of Siegel’s forge. Nanophase Technologies already has a product on the market, and it’s available at most department stores, in the cosmetics section. The first visible sign of the materials revolution, it turns out, is makeup containing nanophase iron oxide. Since the ingredient is a trade secret, Siegel asks that the brand remain anonymous. Whereas most coloring agents in makeup are somewhat opaque and give the skin a painted look, the nanophase iron oxide absorbs light without scattering it, thereby providing color but not opacity. As a result, according to vice president for technology John Parker, the makeup “imparts a rich, transparent color” to the skin. To be sure, that’s an odd product to come from a blacksmith’s forge, and perhaps not something the old smiths of Damascus would appreciate. But who said that a revolution has to begin with a sword?
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