Images taken with a field emission scanning electron microscope show the nanowire bristles that form on copper particles of different sizes. At top right, a cross-section of one of the particles reveals its hollow interior. Image: Varanasi Laboratory |
Sometimes,
a simple decision to try something unconventional can lead to a significant
discovery.
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 the Massachusetts Institute of Technology (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 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 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.
While
the growth of nanowires on bulk copper sheets had been observed before, Varanasi says, this is
the first time it has been observed across a variety of size scales at once,
and the first time the process has been analyzed and explained. “There have
been a bunch of different theories about how these nanowires grow,” he says.
But now, “this paper established thoroughly” what the mechanism is for copper
particles: The bristles grow outward through diffusion, leaving the particles
hollow in the middle as the metal migrates outward.
The
team is now testing the same process with other materials. For example, if it
works with zirconium—the metal now used as the cladding for fuel rods in
nuclear reactors—it might help improve heat transfer. In a nuclear reactor,
where this process drives turbines and produces power, such an advance could
boost the reactors’ overall efficiency.
In
addition to thermal management, these results could help to optimize certain
catalytic processes, Varanasi
says.
