Mıt Research: Making Materials To Order

A team of researchers at MIT has found a way to make complex composite materials whose attributes can be fine-tuned to give various desirable combinations of properties such as stiffness, strength, resistance to impacts and energy dissipation. The key feature of the new composites is a "co-continuous" structure of two different materials with very different properties, creating a material combining aspects of both. The co-continuous structure means that the two interleaved materials each form a kind of three-dimensional lattice whose pieces are fully connected to each other from side to side, front to back, and top to bottom.

The research — by postdoc Lifeng Wang, who worked with undergraduate Jacky Lau and professors Mary Boyce and Edwin Thomas — was published in April in the journal Advanced Materials. The research was funded by the U.S. Army through MIT's Institute for Soldier Nanotechnologies.

The initial objective of the research was to "try to design a material that can absorb energy under extreme loading situations," Wang explains. Such a material could be used as shielding for trucks or aircraft, he says: "It could be lightweight and efficient, flexible, not just a solid mantle" like most present-day armor.

In most conventional materials — even modern advanced composites — once cracks start to form they tend to propagate through the material, Wang says. But in the new co-continuous materials, crack propagation is limited within the microstructure, he says, making them highly "damage tolerant" even when subjected to many crack-producing events.

Some existing composite materials, such as carbon-carbon composites that use fibers embedded in another material, can have great strength in the direction parallel to the fibers, but not much strength in other directions. Because of the continuous 3-D structure of the new composites, their strength is nearly equal in all dimensions, Wang says.

Thomas, the Morris Cohen Professor of Materials Science and Engineering and head of MIT's Department of Materials Science and Engineering, says that in most existing composite materials, the fibers form disordered mass with "zero continuity," while the other material — typically a resin that fills the space and then hardens — is continuous and connected in three dimensions. The material that forms the continuous structure "tends to dominate the properties" of the composite, he says. "But when both materials are continuous, you can get benefits that are surprisingly synergistic, not just additive."