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ORNL researchers are seeking ways to reduce the costs of making lightweight carbon-fiber composites for use in advanced vehicles.

Carbon-Fiber Composites for Cars

To make a vehicle that gets 80 miles per gallon of gasoline to satisfy one goal of the U.S. Partnership for a New Generation of Vehicles (PNGV), the automobile industry is seeking a lighter structural material. Steel is the material of choice today because of its strength and low cost. But steel is heavy, so the industry is starting to use lighter materials instead. Fiberglass has long been used extensively in the Chevrolet Corvette and more recently in some body panels of the Saturn car. Audi's A8 automobile and the hood and engine parts of the Ford F150 pickup are made of aluminum.

To meet the ultimate PNGV mileage goal, one potentially enabling technology is to use carbon-fiber composites, which form the structure of U.S. fighter jets. Carbon-fiber composites weigh about one-fifth as much as steel, but can be comparable or better in terms of stiffness and strength, depending on fiber grade and orientation. These composites do not rust or corrode like steel or aluminum. Perhaps most important, they could reduce vehicle weight by as much as 60%, significantly increasing vehicle fuel economy.

The problem is that carbon-fiber composites cost at least 20 times as much as steel, and the automobile industry is not interested in using them until the price of carbon fiber drops from $8 to $5 (and preferably $3) a pound. Production of carbon fibers is too expensive and slow. The raw material is typically pitch, or polyacrylonitrile (PAN) precursor. It is converted to carbon fibers using thermal pyrolysis, a slow, energy-consuming process that is combined with stressing to achieve the right properties. The precursor, the energy needed to heat it to make fibers, and the large ovens and other capital equipment required in the process contribute to the high cost. As a result, carbon-fiber composites cannot compete economically with steel in the auto industry.

Researchers Alicia Compere and Bill Griffith in ORNL's Chemical and Analytical Sciences Division and several industrial teams are exploring alternative precursors to reduce carbon fiber raw material costs. One promising candidate is lignin, a waste produced during pulping to make paper. This is one project in a joint program of research between ORNL and North Carolina State University (NCSU). The program was recently formalized in a memorandum of understanding between the UT-Battelle management team and NCSU, one of the team's six core universities.

Photomicrograph of a carbon fiber precursor
Photomicrograph of a carbon fiber precursor produced by firing a fiber that is 99% lignin. Lignin is dissolved out of wood to separate it from the cellulose used to make paper.

The Composite Materials Technology Group in ORNL's Engineering Technology Division (ETD) is collaborating with the automobile industry to improve the processes of manufacturing and characterizing carbon-fiber composites under program manager Dave Warren. This group, led by Bob Norris, is also developing materials for NASA's Advanced Space Transportation Program, armor protection for Army aviation and the Federal Aviation Administration, and high-temperature shafting for the Comanche helicopter.

Felix Paulauskas is leading a team of ETD and Fusion Energy Division investigators and industrial collaborators who are working to demonstrate that microwave heating of PAN precursor in a plasma instead of using less-energy-efficient thermal processing increases the speed and reduces the cost of producing carbon fibers. The project showed that a properly designed and implemented microwave-assisted plasma energy delivery system might quadruple production speed and reduce energy needs and fiber price by up to 20%.

Photo and inset of a microwave/plasma processing device
Terry White (left), Ken Yarborough, and Felix Paulauskas examine carbon fiber produced rapidly by the microwave/plasma processing device they developed. A carbon-containing, stabilized or partially stabilized polyacrylonitrile (PAN) precursor fiber is fed into a large tube containing nitrogen gas. When heated by microwaves from the generator in the back-ground, the gas becomes a hot plasma that burns away all the PAN fiber's constituents except carbon, producing the carbonized fiber shown below. Inset: Ultraviolet light is emitted from the hot nitrogen plasma.
(Photos by Curtis Boles.)

Several methods for fabricating carbon-fiber composites have been developed by ORNL researchers and others. For most automotive composite applications, carbon fibers are aligned into a preform, which is placed in a mold. The resin is then injected into a mold and preform and heated to activate and cure the resin. As a result, the fibers are glued together, providing tremendous strength. ETD researchers are working with the auto industry to develop techniques that will automatically align fibers for the preform and will infuse resin effectively into the preform to create finished composites.

Several ETD mechanical test machines will be moved to the National Transportation Research Center. One device being built by ETD specifically for this program is an intermediate strain rate test machine. The device requirements were developed by ETD's Ray Boeman in collaboration with the automobile industry. In this machine, samples will be compressed at a very fast rate, and measurements will be made to determine the effect of the speed of deformation on the material's properties.

"It's like silly putty," says Dick Ziegler, manager of ORNL's Transportation Technology Program. "If you pull it in two directions slowly, it simply stretches. If you pull it fast, it breaks." The data from this device will be valuable for computer simulations of crashes involving cars made of carbon-fiber composites. (See Supercomputers Help Model Cars in Collisions.)

Because of their high strength, carbon-fiber composites could make cars safer. But they won't be used in cars until ways are found to reduce this low-weight material's high cost.

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Related Web sites

ORNL's Chemical and Analytical Sciences Division
ORNL's Engineering Technology Division
ORNL's Fusion Energy Division
Partnership for a New Generation of Vehicles
NASA's Advanced Space Transportation Program

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