Tuesday, October 1, 2013

How to make ceramics that bend without breaking

New materials developed at MIT could lead to actuators on a chip and self-deploying medical devices
October 1, 2013
When subjected to a load, the molecular structure of a zirconia ceramic material (austenite) deforms (in its martensite phase) rather than cracking,. When heated, it then returns to its original shape. (Credit: Lai et al./MIT)
Ceramics tend to crack under stress. But researchers from MIT and Nanyang Technological University in Singapore have developed a way of making minuscule flexible ceramic objects that also have a “memory” for shape (when bent and then heated, they return to their original shapes).
The surprising discovery is reported this week in the journal Science, in a paper by MIT graduate student Alan Lai, professor Christopher Schuh, and two collaborators in Singapore.
Shape-memory materials, which can bend and then snap back to their original configurations in response to a temperature change, have been known since the 1950s in metals, and some polymers, but not in ceramics,” explains Schuh, the Danae and Vasilis Salapatas Professor of Metallurgy and head of MIT’s Department of Materials Science and Engineering.
The team accomplished this in two key ways.
  • Created tiny ceramic objects, invisible to the naked eye: “When you make things small, they are more resistant to cracking,” Schuh says.
  • Made the individual crystal grains span the entire small-scale structure, removing the crystal-grain boundaries where cracks are most likely to occur.
Those tactics resulted in tiny samples of ceramic material with deformability equivalent to about 7 percent of their size. “Usually if you bend a ceramic by 1 percent, it will shatter,” Schuh says. But these tiny filaments, with a diameter of just 1 micrometer — one millionth of a meter — can be bent by 7 to 8 percent repeatedly without any cracking, he says.
Ceramic-like strength, but metal-like ductility
These materials could be important tools for those developing micro- and nanodevices, such as for biomedical applications, Schuh says, such as microactuators to trigger actions within such devices — the release of drugs from tiny implants, for example.
Compared to the materials currently used in microactuators, Schuh says, the strength of the ceramic would allow it to exert a stronger push in a microdevice.
The ceramics used in this research were made of zirconia, but the same techniques should apply to other ceramic materials. Zirconia is “one of the most well-studied ceramics,” Lai says, and is already widely used in engineering. It is also used in fuel cells, considered a promising means of providing power for cars, homes and even for the electric grid. While there would be no need for elasticity in such applications, the material’s flexibility could make it more resistant to damage.
The material combines some of the best attributes of metals and ceramics, the researchers say: Metals have lower strength but are very deformable, while ceramics have much greater strength, but almost no ductility — the ability to bend or stretch without breaking. The newly developed ceramics, Schuh says, have “ceramic-like strength, but metal-like ductility.”

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