Ceramics don't have to be brittle: Materials scientists are creating materials by design

September 11, 2014 by Kimm Fesenmaier
This sequence shows how the Greer Lab's three-dimensional, ceramic nanolattices can recover after being compressed by more than 50 percent. Clockwise, from left to right, an alumina nanolattice before compression, during compression, fully compressed, and recovered following compression. Credit: Lucas Meza/Caltech

Imagine a balloon that could float without using any lighter-than-air gas. Instead, it could simply have all of its air sucked out while maintaining its filled shape. Such a vacuum balloon, which could help ease the world's current shortage of helium, can only be made if a new material existed that was strong enough to sustain the pressure generated by forcing out all that air while still being lightweight and flexible.

Caltech materials scientist Julia Greer and her colleagues are on the path to developing such a material and many others that possess unheard-of combinations of properties. For example, they might create 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.

Greer's team has developed a for constructing new by taking advantage of the unusual properties that solids can have at the nanometer scale, where features are measured in billionths of meters. In a paper published in the September 12 issue of the journal Science, the Caltech researchers explain how they used the method to produce a ceramic (e.g., a piece of chalk or a brick) that contains about 99.9 percent air yet is incredibly strong, and that can recover its original shape after being smashed by more than 50 percent.

"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. This very clearly demonstrates that if you use the concept of the nanoscale to create structures and then use those nanostructures like LEGO to construct larger materials, you can obtain nearly any set of properties you want. You can create materials by design."

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. Then the researchers use another method to etch out the polymer from within the structure, leaving a hollow architecture.

The applications of this technique are practically limitless, Greer says. Since pretty much any material can be deposited on the scaffolds, the method could be particularly useful for applications in optics, energy efficiency, and biomedicine. For example, it could be used to reproduce complex structures such as bone, producing a scaffold out of biocompatible materials on which cells could proliferate.

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 with a ceramic called alumina (i.e., aluminum oxide), producing hollow-tube alumina structures with walls ranging in thickness from 5 to 60 nanometers and tubes from 450 to 1,380 nanometers in diameter.

Greer's team next wanted to test the mechanical of the various nanolattices they created. Using two different devices for poking and prodding materials on the nanoscale, they squished, stretched, and otherwise tried to deform the samples to see how they held up.

They found that the alumina structures with a wall thickness of 50 nanometers 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 nanometers—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 percent, and they could still recover."

To understand why, consider that most brittle materials such as ceramics, silicon, and glass shatter because they are filled with flaws—imperfections such as small voids and inclusions. The more perfect the material, the less likely you are to find a weak spot where it will fail. Therefore, the researchers hypothesize, when you reduce these structures down to the point where individual walls are only 10 thick, both the number of flaws and the size of any flaws are kept to a minimum, making the whole structure much less likely to fail.

"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.

Explore further: Nanoscale Structures with Superior Mechanical Properties Developed

More information: "Strong, Lightweight and Recoverable Three-Dimensional Ceramic Nanolattices," Science, 2014. www.sciencemag.org/lookup/doi/10.1126/science.1255908

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not rated yet Sep 11, 2014
I wonder how easy their method is to scale up to centimeter...meter scale.
Sep 11, 2014
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not rated yet Sep 12, 2014
> The Greer lab is now aggressively pursuing various ways of scaling up the production of these so-called meta-materials.

If they can figure it out, *that* would be the news. Because frankly there have been quite a few similar articles these days. They are all pretty much pointless until someone manages to produce samples on macroscopic scale.
not rated yet Sep 12, 2014
The mention of lack of defects - if a defect is introduced to the ceramic under stress, does it fail catastrophically?
not rated yet Sep 15, 2014
The reason why vacuum balloons don't exist today is due to the elasticity of the skin materials which are structually unstable when the envelope is made sufficiently big so as to displace enough air for static lift. So a flexible ceramic material is no way to go here, despite all the good work done on the neo-technology, this material would not find use for this particular application!
not rated yet Sep 15, 2014
"So a flexible ceramic material is no way to go here, despite all the good work done on the neo-technology, this material would not find use for this particular application!"

Remember that these are nano-materials whose properties can be adjusted by the structure of how they are assembled. One can get a flexible membrane, or a stiff one of high strength, with different architectures in the nano scale structure. That means the general technology, which can be applied to metals, and perhaps carbon structures like diamond and graphene, is able to do what is needed, including a floating vacuum balloon.

With high enough strength/mass ratios in a sealed nano-material it is not impossible it would float without a vacuum on the inside of the sphere. After all, these structured nano-materials are presently up to 100 times lighter than solid equivalents, and if Dr. Greer is right, may go as high as 1,000 times lighter.

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