Editor's note:
The following article describes an ORNL technology development that
won an R&D 100 Award in 2000. The awards are presented annually
by R&D Magazine
in recognition of the year's most significant technological innovations.
ORNL's 107 R&D 100 awards place it first among DOE laboratories
and second only to General Electric.
Transportation
Applications |
Bake It Like A Cake
Artificial diamonds. Buckyballs.
Carbon foam with high thermal conductivity. These products are the results
of serendipitous discoveries made by researchers working with carbon.
Carbon foam was discovered in 1998 at ORNL by James W. Klett, a carbon
researcher in ORNL's Metals and Ceramics Division. The foam received
an R&D 100 Award in 2000 from R&D magazine for being one of 100
most significant innovations in the past year. In June 1999 a patented
ORNL method for making this special graphite foam was licensed exclusively
to Poco Graphite in Decatur, Texas, which calls the product PocoFoam™.
Currently, materials
research on the carbon foam at ORNL is being funded by DOE's Office
of Transportation Technologies. "This is a truly revolutionary material
that will find uses in many applications," says Patrick Davis of the
Office of Transportation Technologies. "Specifically, we believe carbon
foam is an enabling technology that will solve critical heat rejection
problems we must overcome before fuel-cell and advanced power electronics
technologies can be introduced into automobiles."
Klett says he
made his fortunate discovery of graphite foam by accident while changing
a fabrication process. "We had been making carbon-carbon composites,
which are carbon fibers embedded in a carbon matrix," he explains. "Because
of their heat transfer abilities, such composites show promise for making
better brakes and heat shields. But we were trying to find a cheaper
way to make the composites."
Klett was experimenting
with a process he modified by eliminating a couple of steps. He noticed
that a carbon foam had formed as a result.
"We usually heat
treat a carbon material to very high temperatures to develop a graphite
product," he says. "I took the carbon foam and heat treated it to make
it a graphite foam. I then noticed that it transferred heat remarkably
fast. When I held the sample in the palm of one hand and pressed an
ice cube in tweezers against the top, the heat from my hand caused the
ice to melt quickly, cooling my hand.
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|
James
Klett shows that ice held against the graphite foam will melt
quickly because the heat from the hand holding the foam is transferred
rapidly through the foam. As a result, this hand feels the cold
fast. (Photo by Curtis Boles.)
|
"So Tim Burchell
and I made more foam samples, ran heat conductivity tests on them, and
confirmed they had a special property. We were the first to recognize
that a carbon foam could be made that has unusually high thermal conductivity."
Klett and his
colleagues found that the key to the foam's conductivity is its unusual
graphite crystal structure. It has a skeletal structure full of air
pockets, making it only 25% dense and lightweight. The network of ligaments
in the foam wicks heat away from its source almost better than do high-performance
graphite fibers. PocoFoam™, which is three to nine times more thermally
conductive than typical lightweight carbon foams, conducts heat better
than aluminum but at one-fifth the weight. Moreover, the open porosity
allows air, water, or some other fluid to pass through the foam. This
property, combined with the high surface area, leads to very high heat
transfer coefficients in heat exchangers made of PocoFoam™.
 |
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Colorful
micrographs show skeletal structure and air pockets of PocoFoam™.
|
Transportation
Applications
After the discovery, Klett
and Burchell began thinking up applications for the foam. They determined
that because the foam is lightweight and transfers heat rapidly, it
could improve the efficiency of transportation vehicles. For example,
the foam could be used to make a smaller, lighter car radiator that
might be placed away from the front of a car to give it an energy-saving
and less-polluting aerodynamic design. If the size of the front of the
car is reduced, the car will not have to push as much air in its forward
motion, allowing it to use fuel more efficiently. Because a smaller
radiator can make a car lighter and faster since it allows a more aerodynamic
design, automotive racing teams are interested in foam radiators.
This radiator
would quickly transfer heat from the engine to air blowing through its
foam components. In a heat sink made of the foam, hot air or water coolant
may pass through a tube penetrating a foam block, and cold air or water
would pass through the porous foam. Conversely, the foam can be machined
into fins (vertical parallel plates) or vertical pins on a base, as
in metal heat sinks.
"A subcontractor
is now building car radiator prototypes from PocoFoam™," Klett
says. "Small radiators could remove heat from fuel cells that will be
used to power electric cars and buildings." In cars, the heat from the
car radiator could be transferred to warm the passenger compartment.
In buildings the waste heat could be recaptured for additional power
production.
The Department
of Defense is interested in smaller radiators for personnel carriers,
Klett says. A smaller radiator would present less of a target to the
enemy, making it less likely that it would be hit, effectively stopping
the vehicle.
 |
|
James
Klett holds a block of graphite foam. A prototype radiator in
the background is made partly of this type of foam. (Photo by
Curtis Boles.)
|
PocoFoam™
could be used also for cooling brakes (which develop hot spots), as
well as oil and transmission fluid in automotive systems. It could find
a place in lithium-ion batteries being developed to power electric cars.
"Our preliminary studies show that carbon foam may work better than
the typical carbon fibers in the anode of a lithium-ion battery," Klett
says. "Our evidence suggests that a carbon foam anode would discharge
ions to the electrolyte faster than a graphite fiber anode."
PocoFoam™ could be
used to replace aluminum blocks as heat sinks to cool power electronic
modules in hybrid vehicles now being developed in the United States.
A hybrid vehicle has a gasoline engine and an electric motor/generator.
The vehicle requires small, lightweight power electronic modules, such
as inverters, to convert direct current (dc) from batteries to alternating
current (ac) for use by the electric motor in accelerating the wheels.
The electric motor becomes a generator when it captures energy from
the wheels to slow or stop them. The inverter then converts ac to dc
when it takes power from the generator to recharge the batteries.
At ORNL a heat
sink with fins made of the special graphite foam has been cooling a
Pentium 133 chip in a desktop computer since December 12, 1998. "If
you reduce an electronic chip's temperature by 10°C (20°F),"
Klett says, "a rule of thumb is that its lifetime will double before
it fails."
The foam could
also be used in pistons because it can withstand temperatures as high
as 500°C in air. Robert Kirk, manager of DOE's Office of Advanced
Automotive Technology, observes, "When I review the list of recurring
areas of automotive interest, I see the thermally conductive carbon
foam most frequently. I am pleased with this high level of industry
interest because it provides convincing evidence that our research funds
are well placed and effectively used."
There are other
possible uses for foam heat sinks, as well. They may prove useful in
protecting electronic components on space satellites from heat damage.
Another possible application would be in evaporative cooling, in which
the high specific surface area (>4m2/g) combines with the
high thermal conductivity to produce very efficient cooling as water
evaporates from the foam surfaces.
"The aerospace
industry is interested in coating carbon foam with titanium to make
a high-strength material with high thermal conductivity," Klett says.
"The Navy is considering using it for various components to make its
boats lighter and smaller."
Bake
It Like a Cake
Klett compares the batch
method he cultivated to produce graphite foam production to baking a
cake. "You put the batter in a pan, stick it in the oven, and heat it
to the right temperature until you get a foam," he says, explaining
that in essence a cake is a foam. "In our method, we put pitch—tar—in
a vessel, pressurize it, heat it, and let it decompose. The gas evolved
by decomposition during pyrolysis bubbles through the viscous heated
pitch, producing a foam."
Poco Graphite
has an exclusive license to produce graphite foam using ORNL's patented
batch method. The company has started pilot production of PocoFoam™
in the form of sheets and blocks. In May 2000 the first PocoFoam™
product was sold. Poco Graphite also has the machining capability to
produce finished parts made of the foam.
The PocoFoam™
production process is expensive, but the manufacturer says the price
of the product should come down as demand increases. "Remember VCRs"
says Klett. "They cost more than $1200 apiece when they first came out
and now they're much less expensive." In the meantime, Klett, Burchell,
Claudia Walls, Claudia Rawn, and Marie Williams (all part of the graphite
foam development team at ORNL) are testing and evaluating another process
in search of a cheaper production method.
"We're looking
at methods that will allow continuous production of the foam," Klett
says. "This approach should reduce production time and increase throughput."
Klett concedes
that PocoFoam™ is not exceptionally strong; its tensile and compressive
strength and its other mechanical properties are not as good as those
of aluminum and copper. "But its compressive strength compares well
with some aluminum honeycombs used for heat sinks," Klett says. "When
we impregnated foam samples with epoxy resin, we found that the foam's
compressive strength increased ten times. Nevertheless, we believe that
if we tweak the fabrication process to improve the foam's mechanical
properties, we would probably sacrifice its high thermal conductivity."
Meanwhile, Klett
looks forward to making more discoveries as he searches for "out-of-the-box"
applications for PocoFoam™.
Beginning
of Article
Related Web
sites
PocoFoam™
Web Site
ORNL's
Metals and Ceramics Division
DOE's Office of Transportation
Technologies
DOE's Office
of Advanced Automotive Technology
