A well-known method of making heat sinks for electronic devices is a process called sintering, in which powdered metal is formed into a desired shape and then heated in a vacuum to bind the particles together. But in a recent experiment, some students tried sintering copper particles in air and got a big surprise.

Instead of the expected solid metal shape, what they found was a mass of particles that had grown long whiskers of oxidized copper. “It was sort of serendipitous,” says Kripa Varanasi, d’Arbeloff Assistant Professor of Mechanical Engineering at MIT. “We got this crazy stuff, particles covered in nanowires,” he says.

The resulting process could turn out to be an important new method for manufacturing structures that span a range of sizes down to a few nanometers (billionths of a meter) in size. “You go in one step from solid spherical powder to very complex structures,” says Christopher Love, a mechanical engineering graduate student who is lead author on the paper. “The process is very simple, and the structures are durable,” he says. These new structures could be used for managing the flow of heat in various applications ranging from powerplants to the cooling of electronics.

Not only were the particles covered with fine wires, but the abundance of the wires turned out to be dependent on the size of the original copper particles. “We are the first to observe a size-dependent oxidation in copper,” Varanasi says. That means researchers can easily synthesize porous structures at various scales, in bulk, by selecting the particles they start out with: Particles smaller than a certain size sinter, while larger particles grow nanowires. 

The discovery is reported in a paper being published in the journal RSC Nanoscale. In addition to Varanasi and Love, the paper’s authors are mechanical engineering graduate student J. David Smith and postdoc Yuehua Cui of the Laboratory for Manufacturing and Productivity.

Such hierarchical structures can be very effective for thermal management, cooling everything from microprocessors to the boilers of huge powerplants. They might even prove useful in engineered geothermal power, which holds great promise as a system for providing clean, renewable power. Because the resulting structures are so easily controlled, “you can optimize them to control phenomena taking place at different length and time scales,” Varanasi says.

(via Bristly particles could be boon for powerplants)