Research Horizons Magazine
Carbon nanotubes (CNTs) are a hexagonal network of carbon atoms rolled
to form a seamless cylinder - a sort of "chicken wire" lattice
of graphite. "This material has tremendous electrical, thermal and
structural properties, however, few products utilizing CNTs have hit the
commercial market," says Jud Ready, a research engineer in GTRI's
Environment and Materials Laboratory.
Ready is developing a CNT-based electrochemical double-layer capacitor, a project sponsored by the U.S. Army Space and Missile Defense Command. Such supercapacitors would provide more power, increased energy density (more charge per gram of weight) and longer life than traditional batteries and capacitors that store electrical energy.
Ready's supercapacitors are made of two CNT-based active electrodes immersed in an electrolyte and separated by an ion-permeable membrane that prevents electron transfer. "CNTs are ideal to use as the active electrode material because their nanoscale dimensions provide more surface area for storing charge," Ready says. That extra surface area exponentially increases capacitance - the amount of power that can be stored.
Ready began work on the project last spring, aided by Stephan Turano,
a materials science graduate student at Georgia Tech, and Charlie Higgins,
a computer science major from Georgia State University. The team has already
produced dozens of CNT supercapacitors, which have been used for electrical
Feedback from those tests helps improve the manufacturing process. For
example, the researchers have learned that when pressure is applied to
electrodes during testing, the supercapacitor performs better. With that
in mind, Ready is trying to incorporate a clamping or bolting between
the two electrode plates during production to increase pressure.
The next step is reliability testing to see how the CNT supercapacitors
hold up under different environments, which is especially important for
space-based applications. The devices are placed in a chamber that exposes
them to extreme temperature and humidity, accelerating the aging process.
"We can simulate 20 years of life in about 1,000 hours instead of
having them sit around for 20 years," Ready says.
Initially, Ready obtained CNTs from NASA's Johnson Space Center. But
with a new piece of equipment, a chemical vapor deposition furnace, the
researchers can now produce CNTs on site. "This will enable us to
try a different manufacturing technique - chemical vapor deposition versus
the HiPCO (high pressure carbon monoxide) process - and compare and contrast
the two methods," Ready says.
The CNTs from NASA come in a bottle, which the researchers mix into a
paste and then apply between the two electrodes. "The contact between
the paste and electrodes is important," Ready explains. "By
using the chemical vapor deposition furnace, we can actually grow CNTs
in situ on copper foil electrodes, which will provide a better connection."
To produce the CNTs, gases are fed into a sealed quartz tube (about 2 inches by 18 inches), which contains a substrate, such as copper foil or silicon wafers. A catalyst is required to help attach the carbon to the substrate, and Ready has been using nano-sized islands of nickel. The furnace is heated to about 900 degrees Celsius, and the CNTs self-assemble from there, Ready says. The entire process, from closing the furnace door to opening it, takes about three hours, but much of that time involves cooling as the CNTs form in about 30 minutes.
Chirality: the Holy Grail
Besides providing an alternative manufacturing technique, the new furnace
enables researchers to produce CNTs in a controlled manner: They can alter
the temperature and flow-rates of gases (hydrogen, methane and ethylene)
used to form the CNTs. Varying these factors will affect both the quantity
and quality of CNTs produced.
One of the biggest challenges is controlling the physical dimensions
of CNTs, as their electrical properties vary depending on length, diameter
and chirality (how the graphite rolls up). Controlling chirality is by
far the most daunting task, which Ready calls "the Holy Grail"
of CNT production.
Some chiral arrangements yield CNTs with semi-conducting properties,
while others have metallic properties. "If you could control chirality,
you could control the 'flavor' of the CNT," Ready explains, noting
that his team wants to produce CNTs with 100 percent metallic properties.
Although Ready focuses on electronic and power applications, CNTs hold
potential for a wide variety of uses, including flat-panel displays, electric
field generators, solar cells and loss-less motor windings.
Yet a consistent manufacturing method is the key to introducing CNT-materials
into real-world devices. "Producing one CNT-based supercapacitor
with exceptional capabilities is one thing," Ready says. "Yet
producing hundreds or thousands of supercapacitors that perform identically
and reliably enough to be operationally viable is quite another thing
- and our ultimate goal."
With that in mind, Ready is trying to establish partnerships with large
manufacturers that could aid in testing and production, and recently signed
an agreement with Maxwell Technologies
Inc., a San Diego-based manufacturer of supercapacitors. "Working
with external industry partners like Maxwell Technologies will help us
get CNTs out of the lab and into products that can actually be used,"
"Our strategy is to create strong win-win relationships focused on commercializing breakthrough technologies," says Richard Smith, executive vice president at Maxwell Technologies. "The potential for CNTs in ultracapacitors is a multibillion dollar business, and it's exciting to team with such a prestigious group as GTRI."
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