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Materials

Whipping Up Metal Foams

Combustion synthesis yields unprecedented nanoporous transition-metal materials

by Bethany Halford
July 10, 2006 | A version of this story appeared in Volume 84, Issue 28

COLORFUL CHEMIST
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Credit: Courtesy of Bryce Tappan
Tappan prepares a pellet of energetic material.
Credit: Courtesy of Bryce Tappan
Tappan prepares a pellet of energetic material.

Having literally risen from the ashes of a ruined experiment, Bryce C. Tappan's nanoporous metallic foams are veritable phoenixes of chemical research. With their ultralow density and remarkably high surface area, these nanostructured materials could accelerate development in areas as diverse as catalysis and bone replacement (J. Am. Chem. Soc. 2006, 128, 6589).

Tappan, a materials chemist at Los Alamos National Laboratory, created the material accidentally when he was carrying out a routine burning-rate experiment to study the decomposition of the nitrogen-rich iron complex ammonium tris[bi(tetrazolato)-amine]ferrate(III). He ignited a small pellet of the metal complex, expecting it to either explode or slowly burn. Instead, smoke filled the reaction chamber, obscuring Tappan's view and making it impossible to record the material's decomposition.

Thinking his experiment had been a "complete failure," Tappan set about cleaning out the reaction chamber so he could start over. "But when I opened up the vessels there was this long column of material," he recalls. "As I was sweeping it out, I thought 'What possibly could this consist of?'"

The Styrofoam-like column of material reminded Tappan of an aerogel. Just by handling it, he says, he could tell the mystery material was something interesting. Because of the reaction chamber's inert atmosphere, Tappan knew it had to be some form of iron. Following his hunch, he ignited the column in air. A flash of orange sparks shot out of the foam, followed by a slow burn front that left behind a telltale rust-colored material in its wake.

Further analysis revealed that the mysterious column was a foam composed mostly of iron metal. The material is riddled with pores between 10 and 20 nm in diameter, resulting in ultralow density and high surface area, which are unprecedented for metallic foam, according to Tappan.

FROTHY
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Credit: Courtesy of Bryce Tappan
The finer structure of Tappan's iron foam can be seen in this scanning electron micrograph. 10 µm
Credit: Courtesy of Bryce Tappan
The finer structure of Tappan's iron foam can be seen in this scanning electron micrograph. 10 µm

That the metal would assume the shape of nanoporous foam came as a surprise to Tappan and his Los Alamos colleagues. Nanoporous metals usually exist as either powders or thin films. Although there are a few examples of metal foams, these tend to have large pores and low surfaces areas.

The handful of metallic foams that have been prepared were usually made using template-directed synthesis, a method that is compatible with just a few metals. Hoping they'd struck upon a general method for making nanoporous transition-metal foams, Tappan and his coworkers tried the combustion synthesis with other nitrogen-rich transition-metal complexes. So far, they've had success with cobalt, copper, and silver.

The researchers aren't certain precisely how the foams form. They reason that as the complexes burn, they release hot metal atoms. These atoms coalesce into larger particles. At the same time, the nitrogen and hydrogen gases generated in the complexes' decomposition blow tiny holes in the coalescing metal. What results is foam; it's not all that different from a barista blowing air through hot milk to make the foam atop a latte.

"It is remarkable that the metal centers are capable of finding binding sites while large amounts of gas are evolved," Tappan points out. Even more unpredictable, he says, is that a nanoporous, three-dimensional network is produced instead of a disperse powder.

With their exceptionally large surface area, the ultra-lightweight metal foams could be used for a variety of applications. Membranes, catalysis, gas storage, thermal insulation, electrodes, tissue engineering, and drug delivery are among those that spring to mind for Dennis Wilson, chief executive officer of New Mexico-based Energetic Materials & Products.

"This is the first time that both a practical and scalable process has become available for producing a variety of metal foams," Wilson says. "This seminal work represents an advanced technology platform to produce a wide variety of metal foams with control over morphology, such as pore size. It represents a breakthrough that could enable numerous applications in the rapidly growing green tech marketplace."

Ensuring that each nitrogen-rich metal complex is safe enough for the combustion synthesis takes a considerable amount of time. Nevertheless, Tappan hopes to mine the periodic table for other interesting nanoporous transition-metal foams. "In terms of reactivity, you gain a lot with such high surface areas," he explains. "One huge advance with metal foams could be getting a real base metal like nickel to behave more like platinum or palladium."

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