Queen Conch Shell Suggests New Structure For Ceramics

The Queen conch shell, CWRU researchers say, is a natural brittle composite that defends against predatory attacks. It is made of 99 percent aragonite -- a white to pale-yellow mineral with a vitreous luster that is often found in limestone caverns and near hot springs and geysers -- and less than 1 percent protein.

"The shell contains a protein matrix that surrounds and separates three layers of mineral," said Roberto Ballarini, CWRU professor of civil engineering and a lead researcher on the project. "It is also 10 to 30 times stronger and as much as 1,000 times tougher than its principal constitute aragonite (CaCO3), because it has a brick-and-mortar-like microarchitecture. The shell makes an excellent model for load-bearing ceramics that are suitable for aerospace and computer industry applications," he added.

Click here to view illustrations of the conch shell's structure and the researchers' experiments.

Other researchers reporting on the Nature article -- "Structural basis for the fracture toughness of the shell of the conch Strombus gigas" -- were Arthur Heuer, principle researcher and Kyocera Professor of Ceramics in the Case School of Engineering, and Shekhar Kamat and Xiaowei Su, a doctoral student and a post-doctoral fellow, respectively, in CWRU's Department of Materials Science and Engineering.

"A jet engine generates a lot of heat and pressure and the lining can be made from a ceramic composite that won't crack and can withstand very high heat," Ballarini said. "This research tells us to be mindful of the manner in which we distribute the materials of our inorganic ceramic compounds so that they mimic those found in nature. Biomimetics, the interdisciplinary study of nature as a model for inorganic materials, is becoming more and more popular as high technology grows," he added.

CWRU researchers used a room-size atomic resolution electron microscope -- one of just six in U.S. academic institutions nationwide -- to see infinitesimally small layers of the shell, as small as one micron or one thousandth of a millimeter thick. The Queen conch shell, Ballarini explained, contains many cracks along the interfaces of its crossed lamellar structure that dissipate energy, making it strong and fracture resistant.

"The multiple cracks enable the shell to survive a crab attack better than if it had large cracks because the load bearing is distributed and the microcracks in the outer layers shield each other. Furthermore, crack propagation in the middle layer is suppressed by lamellae that act as reinforcing bars," he said.

Ceramics are used in such an array of products from abrasives to blast furnace linings. "We see myriad applications for this new information," Ballarini said. "Mother Nature has been at the design table far longer than we have, and this shell gives us insight into how to distribute our materials across a structure to design the kind of materials that high-tech products demand. This research can be valuable to a range of industries from aerospace to medical imaging equipment where ceramics play a critical role in the success of a product."

Research on the Queen conch shell began in 1997.