Christopher Love - Bristly particles could be boon for powerplants
Multi-scale material may have applications
in heat transfer, potentially helping
powerplants be more efficient.
David L. Chandler, MIT News Office
October 17, 2011
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 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
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.
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’
In addition to thermal management, these results could help to optimize certain catalytic processes, Varanasi
Suresh Garimella, a professor of mechanical engineering at Purdue University who was not involved in this
research, says the “simple and potentially cost-effective nature of the method” for growing copper nanowires
“makes the findings significant,” with potential applications including catalysis and thermal management.
Brent Segal, chief technologist at Lockheed Martin Nanosystems in Billerica, Mass., says this is “significant work
on controlling the electrical properties and thermal properties” of materials, and possibly their optical properties as
well. Such control, from the microscopic to the nanoscopic scale — a thousand-fold difference in size — “has not
been seen before” in a single process, he says.
Upon seeing the team’s description of this new technique, Segal says, “You immediately think, ‘I want to go try 75
other materials’” to see if they work in a similar way. “I think 100 different labs around the country will try
everything they’ve got on the shelf” using this technique, he adds.
The work was supported by the MIT Deshpande Center, a DARPA Young Faculty Award, the MIT Energy Initiative,
and a National Science Foundation graduate research fellowship.