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 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%.
 |
|
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.
Beginning
of Article
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
