Caltech materials scientists develop non-brittle ceramics with the help of Alumina

Caltech materials scientist Julia Greer and her colleagues are on the path to developing a material that is thermally insulating but also extremely lightweight, or one that is simultaneously strong, lightweight, and nonbreakable—properties that are generally thought to be mutually exclusive.

In a paper published in the September 12 issue of the journal Science, the Caltech researchers explain the method to produce a ceramic (e.g., a piece of chalk or a brick) that contains about 99.9% air yet is incredibly strong, and that can recover its original shape after being smashed by more than 50%.

"Ceramics have always been thought to be heavy and brittle," says Greer, a professor of materials science and mechanics in the Division of Engineering and Applied Science at Caltech. "We're showing that in fact, they don't have to be either.”

The researchers use a direct laser writing method called two-photon lithography to "write" a three-dimensional pattern in a polymer by allowing a laser beam to crosslink and harden the polymer wherever it is focused. The parts of the polymer that were exposed to the laser remain intact while the rest is dissolved away, revealing a three-dimensional scaffold. That structure can then be coated with a thin layer of just about any kind of material—a metal, an alloy, a glass, a semiconductor, etc.

In the latest work, Greer and her students used the technique to produce what they call three-dimensional nanolattices that are formed by a repeating nanoscale pattern. After the patterning step, they coated the polymer scaffold with a ceramic called alumina (i.e., aluminum oxide), producing hollow-tube alumina structures with walls ranging in thickness from 5 to 60 nm and tubes from 450 to 1,380 nm in diameter.

Greer's team found that the alumina structures with a wall thickness of 50 nm and a tube diameter of about 1 micron shattered when compressed. That was not surprising given that ceramics, especially those that are porous, are brittle. However, compressing lattices with a lower ratio of wall thickness to tube diameter—where the wall thickness was only 10 nm—produced a very different result.

"You deform it, and all of a sudden, it springs back," Greer says. "In some cases, we were able to deform these samples by as much as 85%, and they could still recover."

"One of the benefits of using nanolattices is that you significantly improve the quality of the material because you're using such small dimensions," Greer says. "It's basically as close to an ideal material as you can get, and you get the added benefit of needing only a very small amount of material in making them."

The Greer lab is now aggressively pursuing various ways of scaling up the production of these so-called meta-materials.

The lead author on the paper, "Strong, Lightweight and Recoverable Three-Dimensional Ceramic Nanolattices," is Lucas R. Meza, a graduate student in Greer's lab. Satyajit Das, who was a visiting student researcher at Caltech, is also a coauthor.