MIT lab uses computer simulations to understand materials.

Neil Savage

You would think that by now, well into the 21st century, scientists would know all the basics about common materials such as cement and steel. Unfortunately, you’d be wrong.

“People right now don’t even know, is cement a crystal? Is it a paste?” says Sidney Yip, a professor of nuclear engineering and materials science at MIT. We know cement is a mix of calcium, silicon, and water. Scientists know there are crystals in its structure, but it contains other components as well. And certainly we know how to build bridges and skyscrapers out of it.

But perhaps if we had a more complete understanding of the atomic and molecular structure of cement and other common materials—ceramics, plastics, steel—we could build safer bridges, sturdier buildings, cars that last longer, or lighter, safer airplanes.

That’s the aim of Yip’s lab: to use computer simulations to understand materials, from the subatomic level all the way up to individual ball bearings or big slabs of concrete. If the researchers can predict how a crack might start and spread through cement, maybe they can stop or divert it. If they can discover where carbon atoms are likely to cluster among iron atoms, causing steel to weaken, they can add elements to the steel to prevent this clustering, making it stronger. “If you do the right chemistry, you can turn your steel into something really magical,” Yip says.

The lab relies on high-powered computers and mathematical modeling to look at the quantum mechanical properties of the electrons in the atoms of a particular material, say silicon. Then they examine the atomic structure of that material to understand how it interacts with other atoms or molecules, such as calcium ions. Next they look at the microstructures in the resulting new material—for example, how calcium ions and silicon crystals cluster in a block of cement. Finally, they study the properties of the material in bulk, for example, the degree to which a slab of concrete deforms when force is applied to it.

This sort of simulation has become possible only in recent years, as computers have become powerful enough to handle the complex mathematical representations of all the variables Yip studies. Dieter Wolf, a physicist at Argonne National Laboratory in Illinois, says Yip’s work linking the atomic structure of a material to how it behaves in bulk is helping to move materials science forward. “He’s definitely a leader in the field.”

A few months ago, one of Yip’s postdoctoral researchers, Xi Lin, demonstrated the difference these new computer simulations can make. He’s been modeling artificial “muscle” made of polymer, which flexes when a charge is applied to it. The muscle could be useful in robotics, but it moves at only about one one-hundredth of the speed of human muscle. That’s because sodium ions—added to the material to improve its conductivity—also slow down the muscle’s contraction.

Thanks to Lin’s simulation of the interaction between the electrons flowing through the muscle during flexing and the polymer itself, he predicts that a laser beam can trigger the contraction without the use of sodium and make the muscle move a thousand times faster than human muscle. While fast-acting artificial muscle remains years away, Yip hopes that experimentalists will use the results from his lab to guide their future development work.

Another project, being funded by the U.S. Air Force, is trying to identify ceramics that are lightweight enough to be used in airplane engines but strong enough to withstand the high temperatures the engines generate.

The applications of his simulations should be widespread, Yip says. “A lot of science and technology all goes back to materials,” he says. “Materials are an enabling thing.”

Neil Savage is a freelance writer in Lowell, MA.