A cup of coffee and a piece of wire started Kaushik Bhattacharya on a research path into the world of smart materials. A graduate student at the time, Bhattacharya was in the office of
University of Minnesota materials science professor Richard James 15 years ago discussing potential research topics. James bent a small strand of nickel titanium in half, then dropped it into a mug of warm Joe.
The wire snapped back to its original form, a nifty characteristic of this class of alloys that can "remember" a physical shape with the help of external stimuli. Because of that ability, such materials have been dubbed "smart." The wire also snapped the young doctoral candidate to attention. "It was fascinating. It was one of those things that grabs you intensely, and you have to find out why it does that," recalls Bhattacharya, who went on to study with James and specialize in memory alloys.
Today a professor at CalTech in Pasadena, Calif., the 37-year-old Bhattacharya is one of the most promising young theoreticians in the field of smart materials. These substances hold significant promise for everything from creating smaller and more efficient optical switches to building highways that can report when their surface needs repair. The materials cut a broad swath across chemistry, engineering, and physics -- and have such a wide range of uses that they're difficult to pigeonhole.
EARLY START. Which is why Bhattacharya hates the term smart materials. "In anything I write, I would never use the word. The connotation is that these materials are smart and other materials are stupid," he says. Rather, Bhattacharya defines these alloys as "active materials -- because they actively change their microstructures."
Bhattacharya heard plenty about microstructure and molecules over the dinner table as a youth and through his undergraduate years. The son of a chemist, he grew up in the southern Indian city of Mysore. In 1986, he entered the prestigious Indian Institutes of Technology to
study mechanical engineering as an undergraduate at the Madras (now Chennai) campus.
Still, his immersion in the field of active materials didn't occur until after he watched James dunk that wire, a stunt that effectively dissuaded him from pursuing a doctorate in more staid branches of material science. Today, his choice looks wise. The burgeoning field attracts hundreds of millions in research dollars and is a darling of everyone from the Defense Dept. to IBM.
THINKING SMALL. Indeed, active materials have ended up in everything from specialized coatings on silicon transistors to exotic carbon materials in fighter planes, and their total annual sales reach well into the billions of dollars. A Japanese company has designed a type of paint that incorporates active materials able to emit an electrical signal when a building is in danger of collapsing. Remember that nickel titanium wire? It's now used in cardiac stents and cell-phone antennas, both of which need to recover their shape if they're bent.
The wealth of new materials comes courtesy of rapid advances in pure research. Scientists have begun to think in terms of atomic-level manufacturing, thanks to experiments in which electron-scanning and -tunneling microscopes have been used to form tiny switches from atoms. Silicon chips are now measured at molecular levels of thickness and coating. "We're at a period of extraordinary convergence if you look at the kinds of fabrication techniques available today," Bhattacharya says. "You can make [micro]structures that you couldn't make five years ago. You can visualize and measure [materials] at scales that were unimaginable."
In 5 to 10 years, that same ability will fuel a new wave of designer materials designed for specific properties and tendencies much in the same way that scientists now design hybrid genes for agricultural products. Those materials will be manufactured or altered to augment or incorporate active behaviors that could, say, cause a surface to change angle if an electrical charge is passed through it.
SIZE MATTERS. That's precisely what NASA hopes to do with future space telescopes, which it may build with shiny surfaces coated with thin films of materials that, when an electrical charge passes through them, will subtly bend and focus the mirror. The benefit? No glass means lighter payloads, a significant concern with large telescopes.
One of the most interesting new insights, according to Bhattacharya, is that materials have various characteristics at different sizes, a fact that sounds self-evident but that scientists are now beginning to understand more fully. They've already learned that the size demarcations for atomic structures are far more complex than previously thought.
"Take a simple granular crystal," says Bhattacharya. "Individual grains might be of the order of microns. Within each grain you might find [so-called] domains, of the order of hundreds of nanometers -- billionths of a meter. Then there are domain walls, which have breadth of a few nanonmeters, and the crystal structure molecular unit cell, which has a length scale on the order of one nanometer."
At each of these increments, the material might exhibit a different behavior. Scientists are now getting closer to understanding how all of these behaviors interact, from the atomic level to the street level, where humans can perceive material characteristics. That knowledge, in turn, promises some interesting advances.
PHYSICAL LIMITS. Consider advanced optical switches. These devices currently rely on so-called electrostatic comb drives to position sets of mirrors, which in turn reflect and target beams that carry data and voice information. Researchers now foresee that they'll soon bump up against the physical limits of these tiny motors. They draw too much power and take up too much space, explains Bhattacharya. "The geometry of the mirror is intricate, and we are beginning to arrive at the smallest size for this technology," he says.
That's where he believes "ferro-electric" materials might come in. These crystalline structures have electric poles, similar to magnetic poles as in iron. The difference, however, is that they react to electric fields (voltage) rather than magnetic fields and thus are far easier to use. Scientists envision coating the backs of these mirrors or connected actuator arms with thin films of ferro-electric material. Applying an electrical charge to them would cause a repulsive or attractive force at the atomic level. That in turn, could translate into more finely tuned movements of tiny optical mirrors in the switches. And it would use a fraction of the energy. The upshot? Faster, cheaper optical switches.
For all the promise of active materials, Bhattacharaya doubts that they'll replace traditional ones anytime soon. Rather, he thinks they'll augment existing materials -- and make big differences in niche areas. "If you look at the average automobile," he says by way of analogy, "the materials that go into it today are dramatically different than what you saw a decade ago, but the basic structure is still steel."
Further, some theoretical heavy lifting remains before it'll be possible to make designer materials. That's where smart guys like Bhattacharaya come in, even if they hate to be called smart material scientists. By Alex Salkever