Commercial applications of (CNTs) soon to be realised

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First discovered in 1991, carbon nanotubes have remained largely a laboratory 

curiosity. Now, researchers at the Georgia Tech Research Institute (GTRI) are 

out to change that by producing carbon nanotube-based devices for 

commercial applications. 

 

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 Electro-Optics, 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. 

 

In-house production

 

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, aided by Stephan Turano, a materials 

science graduate student at Georgia Tech, and Charlie Higgins, a computer 

engineering major from Georgia State University. The team has already 

produced dozens of CNT supercapacitors, which have been used for 

electrical tests.

 

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," he explains.

 

"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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