OAK RIDGE NATIONAL LABORATORY--TECHNICAL HIGHLIGHTS
This article also appears in the Oak Ridge National Laboratory
Review (Vol. 25, No. 2), a quarterly research and development
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WELD ANALYSIS ADVANCE AT THE HFIR
The stresses formed in a complex multipass weld as a result of the
welding process have been mapped for the first time by a team of
scientists using neutron scattering at ORNL. A multipass weld is a
weld formed by passing a welding instrument several times and
adding filler metal between two metallic plates to completely join
them together.
Residual stresses are stresses remaining in an object when no
external force is applied. Knowledge of residual stresses is very
important because when the combination of residual stress and
applied stress exceeds the yield stress of a material, the object
will likely deform or fail.
Members of the research team are Camden Hubbard, Stan David, and
Tad Dodson, all of ORNL's Metals and Ceramics Division; Steve
Spooner of the Solid State Division; and three researchers from
Chalk River Nuclear Laboratories--J. H. Root, J. M. Holdens, and J.
Schroder. The team used a specially modified spectrometer system at
the High Flux Isotope Reactor (HFIR), built cooperatively by the
Solid State and Metals and Ceramics divisions.
"We have shown that the HFIR is a valuable tool for understanding
the magnitude and distribution of residual stresses in a weldment,"
says Hubbard. "This knowledge is important because large tensile
residual stresses may lead to catastrophic failure of a weld in
service. Because ORNL has a special X-ray facility, the
steady-state neutron source offered by the HFIR, and plans for an
Advanced Neutron Source, ORNL has the capability of becoming a
`residual stress center' that can nondestructively determine
whether various materials have unacceptable levels of residual
stress."
The stresses were mapped by studying how the neutrons from the HFIR
were scattered from the atoms in a solid weld metal. The mapping
was performed on a 30-cm (12-in.) square welded plate of ferritic
steel. The sample consisted of two 1.27 cm (1/2-in.)-thick steel
plates welded together using a matching ferritic steel wire and gas
tungsten arc welding process. This multipass weld required about 12
welding passes to completely join the two plates of steel
together.
Using neutron scattering, the researchers obtained a
three-dimensional map of strains in a series of "volume elements"
in the weld, the heat-affected zone, and the base metal of the
steel plate. Each volume element was several cubic millimeters.
From the measurements of the strains (which extend along the
length, width, and height of each volume element), the researchers
calculated the residual stresses in each of the three principal
directions.
Stresses (or forces) on a crystalline material cause lattice
strains. The size of a strain can be determined by measuring the
distance between the planes of atoms in the material. In a
crystalline material containing residual stresses, compressive
stresses cause these distances to decrease, and tensile stresses
cause them to increase in comparison with lattice spacings in a
stress-free material.
The scientists irradiated the welded sample with a neutron beam
having a fixed wavelength of 1.65 angstroms, selected to make the
scattering angle 90ø for a stress-free ferritic steel sample.
However, when residual stresses exist, the separation between the
atomic planes is smaller or larger than the separation in the
stress-free part of the sample, so the neutrons are scattered at
slightly different angles. From the observed diffraction angles,
the lattice spacings and, therefore, the strains are determined.
By changing the sample position and using a special detector to
accurately count the neutrons scattered at various angles, the
researchers obtained a detailed map of residual stresses in the
weld sample. The key to their success was the computer-controlled
sample stage, beam collimators, and detector attachment added to
the existing triple-axis neutron spectrometer at the HFIR. These
modifications and this study were made possible by support from the
ORNL Director's R&D Fund.
In the experiment on the ferritic steel weld sample, the
researchers found large tensile stresses parallel to the weld line
in the weld metal and out into the steel plate where the stresses
eventually became compressive.
"We found that the tensile stresses approach the yield stress of
the material and may nearly exceed this stress in the weld zone,"
Hubbard says. "If these stresses are not minimized, such welds
could show signs of cracking after exposure to applied loads. These
residual stresses can be reduced in the weld by post-weld,
stress-relief heat treatments. Our ability to map residual stresses
in welds will allow us to evaluate the effectiveness of such
treatments."
Multipass welds are commonly used in many industries. "Our data,"
says Hubbard, "will be useful in predicting the properties of
multipass welds in ferritic plates and will provide a basis for
modeling multipass welds." Hubbard adds that the HFIR instrument
could also be used to map residual stresses in industrial objects
such as ceramic-coated turbine blades or oil well casings.
--Carolyn Krause
BOOK PRESENTS INTEGRATED FOREST STUDY RESULTS
A landmark book edited by an ORNL scientist and a former ORNL staff
member contains valuable information for researchers and
environmental managers. The new book examines the effects of
atmospheric pollutants on forests and their implications for forest
management and pollution control. One of its surprising findings
suggests that some American forests in the Southeast may be more
vulnerable than those in the Northeast to acidity from the
atmosphere.
The book _Atmospheric Deposition and Nutrient Cycling in Forests_
(Springer-Verlag Publishers, New York, 1992) was edited by Dale W.
Johnson, a former ORNL soil scientist now with the Desert Research
Institute of the University of Nevada, and by Steven E. Lindberg,
leader of the Atmospheric and Geochemical Processes Group of ORNL's
Environmental Sciences Division (ESD). Lindberg was recently
elected a fellow of the American Association for the Advancement of
Science, which cited him "for outstanding research elucidating
mechanisms, pathways, and interactions of atmospheric pollutants in
forests and for unique creativity in developing techniques to
quantify their biogeochemical cycles." He was the first ORNL
scientist specializing in atmospheric and hydrospheric sciences to
be named an AAAS fellow.
The book is a detailed synthesis of the recently completed
Integrated Forest Study (IFS), which was begun in 1985 and was an
interdisciplinary project involving experts in the atmospheric,
ecological, and soil sciences. IFS studies were carried out at 17
forest research sites in Canada, Norway, and the United
States--specifically, in Tennessee, North Carolina, Georgia,
Florida, Maine, New York, and Washington. ORNL scientists designed
the project, obtained the funding, and managed the work of more
than 50 scientists representing 25 institutions and universities in
North America and Europe. The data obtained make possible
far-reaching conclusions, the editors say, because all sites were
monitored over the same period, using comparable instruments and
standardized protocols.
For the first time, the IFS accurately measured the total
deposition on forests of both wet and dry pollutants from the
atmosphere. The IFS measurements were made using a number of
remote, automated collectors and sensors, some of which were
developed at ORNL, that took samples either during precipitation or
after it stopped; in this way, some of the samplers collected rain,
snow, and cloudwater and the others collected only dry pollutants.
According to Lindberg, "The IFS estimates of the total atmospheric
deposition of acidity far exceed those of the National Atmospheric
Deposition Program, which used only rain samplers to collect wet
deposition for the National Acidic Precipitation Assessment
Program."
The primary goal of the IFS was to determine the effects of
atmospheric deposition of sulfur and nitrogen oxides, acidity, and
ozone on the movements and availability of nutrients that trees
take up from forest soils. These nutrients include nitrogen,
phosphorus, sulfur, potassium, calcium, and magnesium.
Ions of some of these nutrients can be replaced at their soil sites
by ions from deposited atmospheric acids, making these leached
nutrients unavailable to trees as they are washed away in soil
water. Another element of interest is aluminum, a toxic metal found
in the soil. It can be made available for uptake by trees if the
soil becomes acidified by the deposition of sulfate and nitrate.
"The IFS findings," says Johnson, "could improve the accuracy of
forecasts of nutrient deficiencies in forests that may result from
atmospheric deposition or harvesting."
Among the important findings of the IFS study are these:
- Atmospheric deposition of acidity, sulfate, and nitrate was
greatest at the high-elevation sites affected by cloudwater.
It was higher in the southeastern United States (e.g., Smoky
Mountains) than in the Northeast (e.g., Whiteface Mountain
in New Hampshire). This finding is unexpected and could not
have been revealed by traditional bulk collector
measurements.
- Except perhaps for mountain forests, acidic deposition
combined with ozone exposure is unlikely to cause tree
leaves to lose significant amounts of their nutrients.
- Deposition of sulfate and nitrate has increased the rate of
nutrient leaching from most of the forest soils at IFS sites
by 20 to 60%, making these nutrients unavailable for use by
trees and forest vegetation. Levels of potentially toxic
aluminum rose in the most acidic soils.
- Atmospheric deposition may have contributed to the declining
health of some red spruce stands in the Smoky Mountains,
particularly Clingmans Dome, by increasing the leaching of
both nitrate and aluminum from the soil. As a result,
nitrate levels increase in streams and aluminum becomes more
available for uptake by trees (in preference to the nutrient
calcium) and for transport to streams, where it can harm
fish.
According to the IFS, an Oak Ridge loblolly pine site is considered
vulnerable to total nutrient leaching by deposited acids because
its highly weathered soils are low in minerals that could
neutralize these acids. However, because of the high tolerance of
loblolly pine to leached aluminum in soil solution, decline of the
loblolly pine forest around Oak Ridge is deemed unlikely.
One of the outside reviewers who praised the book said, "This
synthesis has provided convincing evidence of many issues in
atmospheric deposition, and it will be an important text for
years."
Although completed in 1989, the IFS has resulted in spin-off
research projects at ORNL and elsewhere. Sandy McLaughlin of ESD
built on IFS findings in his proposal of continued studies of the
effects of aluminum and calcium on the physiology of spruce trees
in the Smoky Mountains. Helga Van Miegroet and Chuck Garten have
received ORNL seed money to use nitrogen isotopes to determine the
critical sources and sinks for atmospheric and soil nitrogen in
Smoky Mountain forests. Supported by the U.S. Forest Service and
the National Park Service, Lindberg and Jim Owens have designed and
conducted a study of sulfur deposition on forests at different
elevations on Clingmans Dome. "Because of the IFS findings in the
Smokies," says Lindberg, "the National Park Service has committed
to establishing a long-term air pollution, water quality, and
forest health monitoring study near the IFS site on Clingmans Dome.
Clearly, the IFS results continue to influence future research on
the effects of air pollution on forests."
--Carolyn Krause
NEW ELECTRON MICROSCOPE IMAGES SHOW HOW SUPERCONDUCTORS GROW
High-temperature superconductors can be made by using lasers to
deposit films on a substrate, one atomic layer at a time. But the
ability of the superconductor to carry useful amounts of electrical
current depends on its structure, and achieving the best
microstructure could require some fine-tuning of the film
deposition process.
Steve Pennycook of ORNL's Solid State Division has developed an
imaging process using an electron microscope that can reveal in
detail how the individual atomic layers of superconducting and
insulating material actually grow. This information is crucial for
researchers trying to make superconducting films that have an
improved ability to carry current.
Pennycook says his imaging process is the only electron microscopy
technique that reveals "the fossil record" of the growth process by
which superconducting films are formed. It enabled him to discover
that the deposited material forms "islands" that eventually combine
into a smooth layer upon which new islands form to make the next
layer, and so on. Pennycook reported his findings in an invited
talk at the 1992 spring meeting of the Materials Research Society
in San Francisco.
"Our technique," he says, "is the only one that shows the size,
shape, and rate of growth of the islands. Knowing the details of
how these films grow helps us determine and control the
microstructure to improve the ability of the film to carry
current."
The films are actually "superlattices," alternate layers of
superconducting and insulating materials on a magnesium oxide
substrate, which serves as a template to properly align the first
layer of deposited atoms to obtain the desired microstructure. The
superconducting layers are made of an oxide of yttrium, barium, and
copper (YBCO). The insulating layer is composed of an oxide of
barium, copper, and the rare earth praseodymium.
Pennycook says that an island of YBCO is six atomic layers high,
the height of the smallest possible amount of YBCO, known as one
unit cell. The typical island is 50 unit cells long. "This material
likes to be flat," he says, "and most of its energy is directed
toward spreading it out."
Pennycook's special imaging process uses a 100-kilovolt scanning
transmission electron microscope (STEM) and a ring-shaped detector
to detect electrons scattered at large angles after they penetrate
the sample (rather than those passing straight through it). This
"Z-contrast" imaging process, which images atoms directly, is so
named because atoms having the highest atomic number (number of
protons, or Z) scatter more electrons and, therefore, are seen as
bright, whereas the lighter atoms are shown as gray or black.
"Z-contrast electron microscopy gives a direct view of materials on
an atomic scale," Pennycook says. "It reveals previously unknown
atomic arrangements, and the relative brightness of each atomic
column indicates composition. It gives us snapshots of each
superconducting layer and can reveal problems in layer structure,
such as surface roughness."
Because barium atoms are the heaviest in YBCO, they are bright
white in Z-contrast micrographs; however, in images of both the
insulating and superconducting layers, praseodymium is the
brightest of all imaged atoms because it is the heaviest element
present. In some of Pennycook's images, the barium atoms resemble
the vertebrae in an X ray of part of a human spinal column. Other
images show steps between layers of YBCO unit cells, a record of
the presence of islands at that stage of the film's growth.
The details of film growth obtained by Pennycook help Douglas
Lowndes and his Solid State Division associates determine how to
alter laser ablation to obtain superconducting films that can carry
useful amounts of electrical current. Laser ablation is a technique
in which bits of superconducting material are evaporated and
deposited in a controlled way as a layered film on a substrate.
Pennycook's images indicate whether they should consider changing
the oxygen pressure, temperature, and rate of film deposition to
improve the film microstructure. "If the material is deposited too
fast, there is less time to form islands, and the growth mode
changes," Pennycook says. "Increased surface roughness may result,
causing short circuits in device structures."
One of the results of the collaboration between Lowndes and
Pennycook was the discovery that making a "superlattice" by
depositing two layers (instead of one) of a superconducting
material between each pair of insulating layers results in a film
that is better able to carry electrical current without losing its
superconductivity.
Pennycook's imaging capability will improve dramatically later this
year with the installation of a newly built 300-kilovolt STEM. This
technology will allow him to achieve a resolution of 1.3 angstroms,
the highest direct resolution in the world. With his current
microscope, he can easily image a YBCO unit cell, which is 12
angstroms high, and a layer of barium atoms, about 2 angstroms
high. An angstrom is about one millionth the diameter of a human
hair.
With the new capability, Pennycook will obtain sharp images not
only of the barium atoms but also of the yttrium and copper atoms
in YBCO. He plans also to extend these studies of growth mechanisms
to other materials, including semiconductors, metals, and ceramics.
--Carolyn Krause
ORNL RESEARCHERS CULTIVATE BIOMASS ENERGY PROGRAM
For thousands of years, trees and grasses have been burned to
produce heat. But only recently have scientists recognized that
large-scale cultivation of trees and grasses could meet a
significant portion of the country's future energy needs.
Growing concern over increased levels of carbon dioxide in the
atmosphere, resulting from burning fossil fuels, and their effect
on global warming has aroused considerable interest in biomass
crops. Janet Cushman, a staff member of ORNL's Environmental
Sciences Division and manager of DOE's Biofuels Feedstock
Development Program, said when biomass crops are burned for energy,
the carbon dioxide produced is merely the recycled carbon dioxide
removed from the atmosphere during plant photosynthesis, "a vast
improvement over emitting carbon dioxide from fossil stores as you
do when coal and oil are burned."
Cushman said biomass is an especially promising option for reducing
carbon dioxide emissions from transportation systems because it is
the only renewable energy source that can produce liquid fuels,
such as methanol and ethanol, for vehicles. "All the
others--hydroelectricity, direct solar, geothermal, and
wind--produce only electricity or heat," she said. The main
obstacle to the widespread use of biomass-derived fuels has been
their cost. However, by developing more productive crops and by
continuing to decrease production and conversion costs, ORNL
researchers believe biomass-derived ethanol costs could be lowered
to 60 to 70 cents per gallon within the next 5 to 10 years, a price
competitive with $25-per-barrel oil. In this way, biomass fuels
could reduce U.S. dependence on unreliable supplies of foreign
oil.
Biomass research was first sponsored by DOE after the oil crises of
the early 1970s. The original goal of the research was to increase
the productivity and lower the cost of growing and harvesting woody
plants for energy uses. In 1978, DOE's Biofuels Feedstock
Development Program began working with universities and
agricultural facilities across the country to identify the tree
species most appropriate for biomass fuel crops and to select the
types of land most suitable to cultivate those crops. Of 140 tree
species screened by 1986, six were targeted as models--poplar,
eucalyptus, black locust, sycamore, sweetgum, and silver maple.
Lynn Wright, deputy manager of the Biofuels Feedstock Development
Program, said that considerable information about the procedures
needed for successful tree farming--site selection and preparation
and a variety of weed-control measures--came from this
silvicultural research on about 25 species. ORNL has also supported
successful research in genetic improvement and tissue culture. "In
fact," Cushman said, "essentially all the noncommercial hardwood
tissue culture research being done in the United States now is
funded through this program." In 1984 DOE expanded the scope of
its energy crop research to include nonwoody, herbaceous plants
that could supplement woody crops. Cushman said the original idea
was to compare nonwoody species and to select the best for
development. However, the researchers soon realized that different
nonwoody species are best for different situations. "A suitable
grass crop can be developed for any kind of land that becomes
available for energy feedstock production," she said "and grass
crops can also be grown and harvested with standard farm equipment,
which lowers the cost for interested farmers." Annual crops can be
used in rotation with conventional agricultural crops. Perennial
crops form dense stands that can protect erosion-prone soils.
Legumes can fix nitrogen, reducing fertilizer use. All of these
types of crops could have a place in energy production.
After National Energy Strategy activities in 1991, DOE proposed an
aggressive biofuels initiative with a goal of having a proven
technology for producing ethanol from plants' cellulosic
materials--not just the starchy parts of the grain now being
used--by the year 2000.
Cushman believes that to arrive at this target, several pilot-scale
facilities that process between 40 and 100 tons per day, 300 days
per year for 2 years will be needed, so the facilities must be
operational no later than 1998. "And if genetically improved trees
are to be a part of the project, they must be planted by 1992,
since in most parts of the country it takes six years to grow a
tree," she said.
"We've received several calls from people interested in biofuels
crops. In California there is interest in putting together a
consortium of farmers and industries. And in Minnesota, several
groups have already been thinking about what it would take to get
dedicated energy crop production facilities going," Cushman said.
Enthusiasm is clearly building. The next step is to develop the
needed support.
Besides the environmental value of carbon dioxide recycling, Wright
reported that biomass crops offer other benefits. "A lot of
cropland is easily eroded. If we could plant perennial grasses or
trees on that land, it would reduce soil loss and at the same time
offer an economically viable crop for farmers," Wright said.
Biomass crops also offer economic benefits to the United States by
reducing dependence on foreign oil and by providing alternative
crops that could boost the nation's farm economy.
Wright said it is also important to have a strong scientific
understanding of the environmental issues related to biomass crop
development. "Of course, environmental issues depend on who you're
talking to. If you're from an agricultural background, you want to
know that soil productivity can be maintained," she said, noting
that some people looking at biomass from an environmental
standpoint consider the habitat and biodiversity issues to be most
critical. "Some groups are concerned that if new markets are
created for agricultural products, additional cropland might be
cleared, encroaching into sensitive habitats, like wetlands or
stream buffers," she said. To address those issues, such groups as
the U.S. Environmental Protection Agency and the Audubon Society
are assessing policy options that would encourage environmentally
acceptable, large-scale biomass energy systems.
Wright said another environmental concern is the chemicals needed
for energy crops, even though they require fewer chemicals than
traditional food crops. "The trend is to use fewer chemicals," she
said. "We're learning when and how to apply chemicals
conservatively--in narrow strips, for example, rather than
broadcast over the entire site."
Wright believes the amount of readily available cropland will
continue to increase. "Given predictions that in the future more
crops will be produced on less land and that export demands will
decline, then less land will be needed for food crops," Wright
said. "If we can develop fuel crops that produce an average of 10
dry tons per acre, we could ultimately supply half the country's
transportation fuel needs on 125 million acres. Although these
goals are very optimistic, research could make them achievable
within 30 to 40 years."
ORNL researchers make a point of keeping all the groups involved in
this research talking to each other. Cushman believes this is one
reason the program has been so successful. "Even though most of the
crop development research takes place at different universities and
agricultural facilities, we feel it's crucial to have a strong
central organization that keeps people talking and that cuts across
state, regional, and institutional boundaries," she said. As an
example of fruitful interaction, researchers from ORNL and the
National Renewable Energy Laboratory of Golden, Colorado, are
working together on a project to examine the chemical changes that
take place in biomass crops during storage and how these changes
might affect the quality of the alcohol produced.
America's farmlands have long been considered the breadbasket of
the world. And within the next few years, if ORNL researchers are
successful, these farmlands may also become known as an important
energy resource for the nation.
--Karen Bowdle
ATF EXPERIMENT UNDER REPAIR
ORNL's Advanced Toroidal Facility (ATF), the world's largest fusion
experiment of its type, ceased experiments on November 15, 1991,
after nearly four years of operation (and over 20,000 "plasma
shots"). The $20-million ATF is a stellarator, which, along with
the better-known tokamak, belongs to the family of toroidal
(doughnut-shaped) confinement devices that dominate modern
controlled fusion research.
Fusion energy is produced by fusing high-energy nuclei of hydrogen
isotopes. Magnetic fields can be used to confine these nuclei in a
hot ionized gas, or plasma. In this way, loss of plasma energy to
the vessel wall is substantially reduced. It is predicted that an
improved toroidal device that effectively confines a fusion plasma
could lead to a practical source of electricity by the middle of
the next century.
The ATF stellarator (shown in the figure) uses helical windings to
create the magnetic field that contains and stabilizes the plasma
used in experiments; the circular coils position and shape the
plasma. By contrast, for plasma confinement the tokamak requires a
plasma current that must be driven for steady-state operation.
Other comparably sized stellarators operate in Germany, Japan, and
Ukraine. (For background information on the ATF and magnetic fusion
research at ORNL, see the Number Four, 1987, issue of the Review.)
Since it started operation in January 1988, the ATF has had a
distinguished history. The ATF's unique design allowed creation of
a wide range of plasma configurations for study of fundamental
physics issues relevant to both tokamaks and stellarators.
During the first year, ATF researchers studied a regime of plasma
operation that has the potential for confining plasma having more
energy content for a longer time at a given magnetic field
strength. An unexpected magnetic field perturbation had allowed
access to this "second-stability" regime at much lower power than
had been otherwise needed.
After compensating for this field perturbation, ATF researchers
studied other basic confinement physics issues including the
"bootstrap current," an internal self-generated current in toroidal
plasmas that must be maximized in tokamaks and minimized in
stellarators for optimum performance. The ATF group found that its
experimental measurements of this current agreed with theoretically
predicted values, giving confidence that the bootstrap current can
be used for optimizing future tokamak and stellarator designs.
The ATF researchers have also made significant contributions to
understanding confinement in toroidal plasmas. Comparison of
measured fluctuations in the edge-plasma parameters in the ATF and
in a comparably sized tokamak allowed researchers to gain a better
understanding of the role of these fluctuations in causing
deterioration in plasma confinement. These studies have been
extended to the plasma core in the ATF in a collaborative
experiment between ORNL scientists and researchers from the
Institute of General Physics in Moscow. Results of preliminary
analyses show evidence of a particular plasma instability that was
sought in this experiment. Throughout its history, the ATF program
has featured collaborations of this type with U.S. universities and
fusion laboratories in Japan, Spain, Russia, Ukraine, and Germany.
Although the ATF was developed as an experiment to test
optimization principles and steady-state operation, budget
restrictions have prevented the ORNL stellarator from reaching its
full potential. A DOE decision in late 1990 to focus on tokamaks
altered the ATF research program and resulted in operation of the
ATF at a reduced level in fiscal year 1991 and a plan to mothball
it in fiscal year 1992.
At the end of May 1991, an electrical short caused extensive damage
to part of one of the large helical windings shown in the schematic
above. A limited repair was implemented, and the ATF resumed
operation on September 25, making it possible for the ATF group to
accomplish most of the goals planned for the summer of 1991.
The ATF is currently being repaired to preserve the option of
restarting the ATF program. In addition, the ATF group hopes to
double the plasma heating on the ATF and to extend operation to the
long-pulse, steady-state regime for which the device was designed.
Whether this restart occurs depends on a review of the U.S. fusion
program strategy now under way. Encouraging support from the U.S.
fusion community and from DOE gives reason for optimism that the
ATF program will restart in 1993.
--James F. Lyon, ORNL's Fusion Energy Division
ORNL WINS FOUR R&D 100 AWARDS
In the fall of 1992, four ORNL developments received R&D 100 Awards
from Research & Development magazine. The magazine's editors
selected these developments to be on their list of the top 100 new
technology advances in the world. The winning entries were ion
implantation to produce hard-surfaced polymers, a method of
accurately blending new compounds with ozone-depleting
chlorofluorocarbons (CFCs) in air conditioners and refrigeration
units to cut down on the use of CFCs, the surface-enhanced Raman
optical data storage system, and a computer technology for
identifying the genes in sequences of genetic information.
Since 1967 when the Laboratory first entered the competition, ORNL
has received 69 R&D 100 Awards (originally called I-R 100 Awards)
and Energy Systems has received 79, placing Oak Ridge first among
DOE sites in this competition.
John Henry could have been a polymer-drivin' man. According to song
and story, America's hammer-swinging, spike-pounding railroad
builder, John Henry, was a "steel-drivin' man." Of course, that's
only because he didn't have the benefit of recent developments at
ORNL's Triple Ion Irradiation Facility (TIF). If he had, he might
have traded in his steel spikes for a set made of ion-implanted
Kapton, a light, durable polymer with a surface that is three times
as hard as stainless steel.
Created using TIF's three accelerators, this potentially
revolutionary material, along with other "hard-surfaced" polymers,
is the brainchild of Eal Lee, Monty Lewis, and Lou Mansur,
researchers in ORNL's Metals and Ceramics Division. Polymers are
chemical "strings," or linear combinations of repeating molecules
used in a variety of products, from soft drink bottles to machine
gears. Most ion-beam surface modification research has been aimed
at improving the physical characteristics of semiconductors, metals
and ceramics to broaden their applications. The use of polymers in
severe environments has been limited by their inherent softness and
poor resistance to wear, abrasion, and other factors. Researchers
hope to overcome these traditional weaknesses by using ion
implantation to combine the most useful features of polymers, such
as light weight, flexibility, and corrosion resistance, with the
hardness of metals and ceramics.
To accomplish this, ORNL researchers are trying to increase the
number of bonds between molecules by firing new atoms in the form
of positive ions into the polymer's molecular matrix. The new
structures created through this process couldn't be achieved using
traditional polymer processing techniques because they don't rely
on normal chemical reactions; instead, the ions are wedged into the
polymers' molecular structure by sheer force. "The ability to
create new materials that cannot be synthesized by conventional
chemical means," says Mansur, "opens up entirely new areas of
materials science and engineering."
This violent union produces new materials, often having physical
properties far different from those of the original polymers. "Part
of our work is looking at the atomic and molecular basis of the
changes in the properties of these materials," says Lee. "Another
part is applying what we've learned."
During the studies, polymers are implanted with boron, carbon,
silicon, and iron ions by the TIF facility's three accelerators
using either single beams or two or three beams simultaneously. The
use of multiple beams produces reactions not only between the beams
and the target, but also between the ion species of the beams
themselves. This approach yields different, and in some cases
better, results than using the three beams separately.
To determine the effect of the ion treatments, the implanted
materials are analyzed using a number of techniques, including
microscopic methods, hardness and wear testing, X-ray analysis, and
Raman spectroscopy.
The primary effect of high-energy ion-beam processing on polymers
is to produce materials having an increased number of
interconnections between molecules in all three dimensions. This
increased molecular interaction often results in several enhanced
surface features, such asHardness. Depending on the type of ion
beams used, ion implantation studies at ORNL have produced polymers
that are 13 to 41 times as hard as their untreated counterparts.
Wear Resistance
Improvement of wear resistance varies with the polymer-ion beam
combination used. The most impressive increase in wear resistance
resulted when samples of Kapton were treated with single or
multiple beams of boron, nitrogen, and carbon ions. These samples
showed no signs of wear in abrasion tests with a nylon ball; tests
with a chromium-steel ball produced a high degree of wear on the
ball and scratches on the polymer surface.
Surface Smoothness
Ion-beam treatment generally produced a much smoother surface when
compared optically with an untreated surface.Oxidation resistance.
When exposed to an oxygen plasma, untreated Kapton lost six times
as much material to oxidation as ion-beam-treated Kapton.
Chemical Resistance
Untreated samples of Lexan and polystyrene dissolve in benzene, but
treated samples do not.
Conductivity
Implanting certain polymers with metal ions increased their
electrical conductivity by a factor of 1010.
Wear-resistant polymers could be used to manufacture high-speed
moving parts and lightweight, load-bearing components. Polymer
films can be applied to any surface and ion-beam treated, producing
hard, wear-resistant protective coatings. Ion beam processing also
improves resistance to oxidation and chemical attack, suggesting
that hard-surfaced polymers may be useful in low-earth orbit, where
atomic oxygen severely erodes traditional polymers.
An important finding of the project is that the degree of molecular
interaction of the treated polymers is closely correlated with
their hardness, indicating that ion implantation has increased the
carbon content of the backbones of the polymer molecules, producing
a more rigid structure. Spectrographic analysis of the samples
confirmed that ion-beam treatment increases the number of new bonds
between molecules. Because the degree of hardness is dependent upon
the type of ions implanted, new types of bonding, not present in
the untreated material, are thought to play an important role in
increasing hardness.
Currently, ion-implanted polymers are still considered high-tech
materials, and their cost of production is too high for the mass
market. This will limit initial applications to high-value-added
products and critical components; however, costs could come down as
demand increases.
Other technologies, such as oxide coating and mineral treatments
have been used in the past to improve the surface properties of
polymers, and more recently, ultraviolet-cured, scratch-resistant
polymers have come onto the market. However, no commercially
available product comes close to the hardness achieved by ion-beam
processing at ORNL.
Now, just imagine what Paul Bunyan could have done with a
hard-surfaced polymer axe....
CFC/HFC RATIOMETER EASES MOVE TO OZONE-FRIENDLY COOLANTS
Chlorofluorocarbons, or CFCs, are gaseous compounds commonly used
in automotive, industrial, and residential air conditioning and
refrigeration systems. When CFCs are released or leak from these
systems, some of them eventually make their way into the upper
atmosphere, where they break down the layer of ozone that screens
out sunlight's ultraviolet rays. Overexposure to these rays is
linked to a variety of ailments, including skin cancer.
To combat the problem of ozone depletion, beginning in 1993, the
use and production of CFCs in the U.S. will be drastically
curtailed. The good news is that researchers have found ozone-safe
substitutes for CFCs. The bad news is that these chlorine-free
replacement gases, called hydrofluorocarbons (HFCs), aren't
compatible with the existing systems, so they can be used only in
new ones.
So, what can be done with existing systems when they need to be
recharged with refrigerant? Well, the good news is that several
mixtures of HFCs and CFCs have been developed that are both easier
on the ozone than CFCs alone and compatible with existing systems.
Since CFC/HFC blends are being developed for use in automobile air
conditioners over the next few years, it will become increasingly
important to identify the kinds of refrigerant used because
charging these devices with the wrong kind of refrigerant could
cause irreversible damage. Also, the refrigerants must be mixed at
specific ratios to function effectively. Unfortunately, some
compounds in the mixture are more likely to leak from the system
than others. So, the bad news is that, in order to get the correct
mixture, either the ratio of gases present in the system must be
known before it is recharged, or the refrigerant must be replaced
entirely.
Traditional methods of determining this ratio include analyzing a
sample of the system's contents in a gas chromatograph or an
infrared spectrometer. However, the high cost of this equipment and
the need for specially trained operators rule these options out for
most air conditioning or refrigeration repair contractors. To make
this capability more widely accessible, Fang Chen of ORNL's Energy
Division and Steve Allman and Winston Chen, both of the Health and
Safety Research Division, have developed a CFC/HFC ratiometer,
which is capable of identifying the refrigerants present in a given
sample and determining the concentration of each component when
mixtures are used.
This instrument takes advantage of differences in the electron
drift velocities of different gas mixtures in an electric field.
That is, under certain conditions electrons travel much more
rapidly through some gases than others--at a characteristic rate
for each gas. To determine the drift rate for a particular mixture
of gases, a sample is drawn into the ratiometer, where an electric
field is applied to the mixture. Then a flash from an ultraviolet
light source causes a beryllium plate to produce low-energy
electrons to drift a known distance across an electric field.
"Depending on the blend of materials," says Chen, "the speed of the
electrons will be retarded by a given amount compared to electrons
moving through air." The ratiometer then compares these data to
that gathered from analyses of known gas mixtures to determine the
composition of the sample.
"We envision that the final product will be a briefcase-sized
device that is lightweight, relatively inexpensive--about $1000--
and easily used by an auto mechanic or refrigeration technician,"
says Chen. "And because this instrument is designed specifically
for refrigerants, results can be obtained immediately without
complex data analysis."
SERODS COULD HOLD 18,000 ENCYCLOPEDIAS ON A CD
The surface-enhanced Raman optical data storage (SERODS)
technology, which was developed by Tuan Vo-Dinh of ORNL's Health
and Safety Research Division and David L. Stokes, a University of
Tennessee graduate student, may have large computer memory
applications. It is based on the recently discovered principle that
the enhanced light-emitting properties of certain molecules
embedded in an optical medium can be altered at the molecular level
to store information.
The SERODS system can achieve 1 terabyte (1012 bytes) of storage
capacity in a 12-in. disk, thus achieving about 100 times greater
storage capacity than other conventional optical storage systems.
For example, a 12-inch SERODS disk could store 18,000 sets of the
Encyclopedia Britannica, or 450 million pages of text.
Several organizations in both government and private industry have
expressed interest and are pursuing licensing agreements for using
SERODS technology for supercomputer memories, health care (e.g.,
medical data banks for hospitals and medical imaging), space
satellite data storage for global environmental studies, integrated
data storage systems for "paperless" navy ships, and entertainment
(optical disks for movie rental companies).
"The ORNL system could be used for virtually any activity requiring
large-memory data storage," Vo-Dinh said. "Examples are archive
storage for the insurance industry, data banks for financial firms
and banks, global data storage for the proposed U.S. orbiting space
laboratory of the National Aeronautics and Space Administration,
optical archives for the Library of Congress, and DNA information
storage for the Human Genome Project."
SERODS is the only system available that offers three-dimensional
data storage for replacing conventional two-dimensional disk
surfaces, thus greatly increasing data storage capacity. It also
provides protection for sensitive and proprietary information
because such data can be accessed only if the frequency of the
vibrating molecules used for the optical layer is known.
The SERODS system uses a writing laser, a reading laser, a
photometric detector, and an optical disk or a three-dimensional
multilayer optical storage medium. A sample SERODS optical disk
contains a plastic or glass substrate covered with silver-coated
polystyrene microspheres and an optical layer containing
light-emitting molecules. When they are close to such a surface,
molecules in the optical layer interact with its microstructures,
causing the molecules to scatter more and emit what is known as
surface-enhanced Raman light.
A writing laser is used to encode bit information by altering the
light-emitting properties of specific clusters of molecules on the
disk while leaving other molecules intact. A reading laser excites
all the molecules in the optical layer, inducing specific
microregions of the disk to produce altered light signals and
unaltered signals that correspond with "one" bits and "zero" bits,
respectively. A photometric detector tuned to the frequency of the
Raman emissions is used to retrieve the stored information (for
details, see "Technical Highlights," ORNL Review, Vol. 23, No. 1,
1990, p. 85).
GRAIL SEPARATES GENETIC WHEAT FROM CHAFF
In ancient times, kernels of wheat for making flour were gathered
by hand--a slow and inefficient process. Then, an innovative
approach was developed. First, the grain was crushed to loosen the
kernels of wheat from their husks, or chaff. Then this mixture of
wheat and chaff was thrown into the air. The slightest breeze would
blow the light chaff to the side, and the heavier wheat would fall
down in a pile.
Genetic researchers face a similar dilemma as they try to separate
areas of biologically relevant information from the rest of the
sequence of DNA bases that makes up the human genome. What makes
their job even more difficult is that about 95% of the genetic
information they look at is chaff. Only 3-5% of the DNA bases
contain instructions for manufacturing proteins, the molecules that
govern the chemical processes necessary for life.
Not surprisingly, developing the technology to locate genes in DNA
sequences is a primary goal of DOE's Human Genome Project. This
challenge has been largely met by ORNL researchers through the
construction of the Gene Recognition and Analysis Internet Link
(GRAIL) system. Ed Uberbacher, Reinhold Mann, and Ralph Einstein of
the Engineering Physics and Mathematics Division and Richard Mural
and Xiaojun Guan of the Biology Division have integrated biological
insight about genes and the proteins they code for into this
state-of-the-art, computer-based artificial intelligence system.
Using neural networks, which simulate the function of neurons in
the brain, and other intelligent machine principles being developed
at ORNL's Center for Engineering Systems Advanced Research, GRAIL
"learns" to recognize the characteristics of genes in a DNA
sequence. Combining the information it gathers from examining known
genes with biological knowledge provided to it, GRAIL develops
principles that it uses to locate new genes. Each party contributes
expertise in this partnership between human and machine, resulting
in new insights into the problem of gene recognition.
The system is already being used to analyze the DNA sequence for
the genes responsible for Huntington's chorea, various muscular
dystrophies, and a number of other genetic diseases. Once the gene
or genes responsible for a particular disease have been located and
analyzed, the potential exists for the development of genetic
screening tests for prospective parents and prenatal diagnosis of
the disease. The degree to which these relevant portions of the
genome can be identified and understood will largely determine
whether knowledge of the human genome can be applied to problems in
biotechnology, gene therapy, and the development of
pharmaceuticals--technologies which are based on the manipulation
of genes and their protein products.
About 400 biotechnology companies and research laboratories
currently use GRAIL to analyze DNA sequences, primarily to locate
genes that may play a role in human disease or have applications in
medicine, biotechnology, or pharmaceuticals. In the long term, it
is likely that gene therapies or cures for some of the thousands of
genetic diseases that affect humans will be developed from genetic
insights provided by GRAIL.
(keywords: weld analysis, forests, pollutants, superconductors,
electron microscopy, biomass, fusion energy, ion implantation,
hydrofluorocarbons, SERODS, human genome)
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Date Posted: 2/7/94 (ktb)