Mechanical engineer Deyu Li in the laboratory. Photo: Daniel Dubois/Vanderbilt University |
The
surprising discovery of a new way to tune and enhance thermal
conductivity—a basic property generally considered to be fixed for a
given material—gives engineers a new tool for managing thermal effects
in smart phones and computers, lasers, and a number of other powered
devices.
The
finding was made by a group of engineers headed by Deyu Li, associate
professor of mechanical engineering at Vanderbilt University, and
published online in Nature Nanotechnology.
Li
and his collaborators discovered that the thermal conductivity of a
pair of thin strips of material called boron nanoribbons can be enhanced
by up to 45% depending on the process that they used to stick the two
ribbons together. Although the research was conducted with boron
nanoribbons, the results are generally applicable to other thin film
materials.
An entirely new way to control thermal effects
“This
points at an entirely new way to control thermal effects that is likely
to have a significant impact in microelectronics on the design of smart
phones and computers, in optoelectronics on the design of lasers and
LEDs, and in a number of other fields,” says Greg Walker, associate
professor of mechanical engineering at Vanderbilt and an expert in
thermal transport who was not directly involved in the research.
A pair of boron nanoribbons stuck together on a microdevice used to measure thermal conductivity. Image: Courtesy of the Li Laboratory |
According
to Li, the force that holds the two nanoribbons together is a weak
electrostatic attraction called the van der Waals force. (This is the
same force that allows the gecko to walk up walls.)
“Traditionally,
it is widely believed that the phonons that carry heat are scattered at
van der Waals interfaces, which makes the ribbon bundles’ thermal
conductivity the same as that of each ribbon. What we discovered is in
sharp contrast to this classical view. We show that phonons can cross
these interfaces without being scattered, which significantly enhances
the thermal conductivity,” says Li. In addition, the researchers found
that they could control the thermal conductivity between a high and a
low value by treating the interface of the nanoribbon pairs with
different solutions.
The enhancement is completely reversible
One
of the remarkable aspects of the effect Li discovered is that it is
reversible. For example, when the researchers wetted the interface of a
pair of nanoribbons with isopropyl alcohol, pressed them together and
let them dry, the thermal conductivity was the same as that of a single
nanoribbon. However, when they wetted them with pure alcohol and let
them dry, the thermal conductivity was enhanced. Then, when they wetted
them with isopropyl alcohol again, the thermal conductivity dropped back
to the original low value.
“It
is very difficult to tune a fundamental materials property such as
thermal conductivity and the demonstrated tunable thermal conductivity
makes the research especially interesting,” Walker says.
One
of the first areas where this new knowledge is likely to be applied is
in thermal management of microelectronic devices like computer chips.
Today, billions to trillions of transistors are jammed into chips the
size of a fingernail. These chips generate so much heat that one of the
major factors in their design is to prevent overheating. In fact, heat
management is one of the major reasons behind today’s multi-core
processor designs.
“A
better understanding of thermal transport across interfaces is the key
to achieving better thermal management of microelectronic devices,” Li
says.
Discovery may improve design of nanocomposites
Keeping integrated chips that contain billions of transistors, like the one pictured, from overheating has become a major challenge for the semiconductor industry. Image: Travelin’ Librarian/Flickr |
Another
area where the finding will be important is in the design of “nanocomposites”—materials made by embedding nanostructure additives
such as carbon nanotubes to a host material such as various
polymers—that are being developed for use in flexible electronic
devices, structural materials for aerospace vehicles, and a variety of
other applications.
Collaborators
on the study were post-doctoral research associate Juekan Yang,
graduate students Yang Yang and Scott Waltermire from Vanderbilt;
graduate students Xiaoxia Wu and Youfei Jiang, post-doctoral research
associate Timothy Gutu, research assistant professor Haitao Zhang, and
Associate Professor Terry T. Xu from the University of North Carolina;
Professor Yunfei Chen from the Southeast University in China; Alfred A.
Zinn from Lockheed Martin Space Systems Company; and Ravi Prasher from
the U.S. Department of Energy.
The
research was performed with financial support from the National Science
Foundation, Lockheed Martin’s Engineering and Technology University
Research Initiatives program and the Office of Naval Research.
Enhanced and switchable nanoscale thermal conduction due to van der Waals interfaces
