Tough Ceramic Emerges From Ice

Materials scientists have drawn inspiration from nature to enhance the properties of ceramics. Antoni P. Tomsia, Sylvain Deville, and colleagues at Lawrence Berkeley National Laboratory have used nacre, or mother-of-pearl, as a model for the production of improved ceramic materials.

Ceramics tend to be strong but brittle. On the other hand, nacre, a natural ceramic composite, is both strong and tough enough to reinforce mollusk shells. Nacre is constructed of layers of calcium carbonate platelets held together with a small amount of protein "mortar." Tomsia's team created a synthetic mimic of this structure, using freezing water as a template (Science 2006, 311, 515).

The researchers knew that when seawater freezes, it can form tiny plates of ice. Impurities in the water are squeezed out as the plates form and become trapped in the spaces between them. Tomsia's group exploits this behavior using concentrated suspensions of ceramic particles in water. As the water freezes, it pushes the ceramic particles into the layers between the ice plates. The rate of freezing determines the thickness of the resulting ceramic layers, which can range from 1 to 200 µm.

The ice is removed by freeze-drying, leaving behind a ceramic scaffold with pores shaped like the ice plates. The scaffold is fired to stabilize the structure. Finally, the pores in the scaffold are filled with a second compound such as epoxy or aluminum alloy to create a dense composite that resembles nacre.

University of Michigan materials scientist John Halloran says the method yields materials with "remarkably improved mechanical properties" compared with traditional ceramics. The technique can also be applied to other materials. For instance, Tomsia's team used it to produce a bone substitute four times stronger than the material currently used for implantation.