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Mechanical engineering doctoral student Baratunde
A. Cola, from left, looks through a view port in a plasma-enhanced
chemical vapor deposition instrument while postdoctoral research
fellow Placidus Amama adjusts settings. The two engineers recently
have shown how to grow forests of tiny cylinders called carbon
nanotubes onto the surfaces of computer chips to enhance the flow
of heat at a critical point where the chips connect to cooling
devices called heat sinks. The carpetlike growth of nanotubes has
been shown to outperform conventional "thermal interface
materials." The research is based at the Birck Nanotechnology
Center in Discovery Park at Purdue.
(Purdue News Service photo/David Umberger)
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The method developed by the Purdue researchers
enables them to create a nanotube interface that conforms to a heat
sink's uneven surface, conducting heat with less resistance than
comparable interface materials currently in use by industry, said
doctoral student Baratunde A. Cola.
Findings were detailed in a research paper that appeared in
September's issue of the journal Nanotechnology. The paper was written
by Amama; Cola; Timothy D. Sands, director of the Birck Nanotechnology
Center and the Basil S. Turner Professor of Materials Engineering and
Electrical and Computer Engineering; and Xianfan Xu and Timothy S.
Fisher, both professors of mechanical engineering.
Better thermal interface materials are needed either to test computer
chips in manufacturing or to keep chips cooler during operation in
commercial products.
"In a personal computer, laptop and portable electronics, the better
your thermal interface material, the smaller the heat sink and overall
chip-cooling systems have to be," Cola said.
Heat sinks are structures that usually contain an array of fins to
increase surface contact with the air and improve heat dissipation,
and a fan often also is used to blow air over the devices to cool
chips.
Conventional thermal interface materials include greases, waxes and a
foil made of a metal called indium. All of these materials, however,
have drawbacks. The greases don't last many cycles of repeatedly
testing chips on the assembly line. The indium foil doesn't make good
enough contact for optimum heat transfer, Fisher said.
The Purdue researchers created templates from branching molecules
called dendrimers, forming these templates on a silicon surface. Then,
metal catalyst particles that are needed to grow the nanotubes were
deposited inside cavities between the dendrimer branches. Heat was
then applied to the silicon chip, burning away the polymer and leaving
behind only the metal catalyst particles.
The engineers then placed the catalyst particle-laden silicon inside a
chamber and exposed it to methane gas. Microwave energy was applied to
break down the methane, which contains carbon. The catalyst particles
prompted the nanotubes to assemble from carbon originating in the
methane, and the tubes then grew vertically from the surface of the
silicon chip.
"The dendrimer is a vehicle to deliver the cargo of catalyst particles,
making it possible for us to seed the carbon nanotube growth right on
the substrate," Amama said. "We are able to control the particle size
- what ultimately determines the diameters of the tubes - and we also
have control over the density, or the thickness of this forest of
nanotubes. The density, quality and diameter are key parameters in
controlling the heat-transfer properties."
The catalyst particles are made of "transition metals," such as iron,
cobalt, nickel or palladium. Because the catalyst particles are about
10 nanometers in diameter, they allow the formation of tubes of
similar diameter.
The branching dendrites are tipped with molecules called amines, which
act as handles to stick to the silicon surface.
"This is important because for heat-transfer applications, you want
the nanotubes to be well-anchored," Amama said.
Researchers usually produce carbon nanotubes separately and then
attach them to the silicon chips or mix them with a polymer and then
apply them as a paste.
"Our direct growth approach, however, addresses the critical heat-flow
path, which is between the chip surface and the nanotubes themselves,"
Fisher said. "Without this direct connection, the thermal performance
suffers greatly."
Because the dendrimers have a uniform composition and structure, the
researchers were able to control the distribution and density of
catalyst particles.
The research team also has been able to control the number of "defect
sites" in the lattice of carbon atoms making up the tubes, creating
tubes that are more flexible. This increased flexibility causes the
nanotube forests to conform to the surface of the heat sink, making
for better contact and improved heat conduction.
"The tubes bend like toothbrush bristles, and they stick into the gaps
and make a lot of real contact," Cola said.
The carbon nanotubes were grown using a technique called microwave
plasma chemical vapor deposition, a relatively inexpensive method for
manufacturing a thermal-interface material made of carbon nanotubes,
Fisher said.
"The plasma deposition approach allows us great flexibility in
controlling the growth environment and has enabled us to grow carbon
nanotube arrays over a broad range of substrate temperatures," Fisher
said.
The research has been funded by NASA through the Institute for
Nanoelectronics and Computing, based at Purdue's Discovery Park. Cola
also received support through a fellowship from Intel Corp. and Purdue.
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