ORNL researchers have developed a method of imaging heat-induced damage in epoxy-resin composites. These fiber-reinforced plastics, called polymer matrix composites, are used in high-performance aircraft because they can be easily fabricated into parts of high strength and low weight. The only problem is that they may degrade when exposed to elevated temperatures from sources such as lightning, misdirected jet exhaust, and burning of spilled fuel.
Now, for the first time, early signs of thermal damage in advanced aircraft materials can be pinpointed quickly using ORNL's Composites Damage Imager (CDI). This system consists of a video camera, laser, computer, and special optics.
Eric Wachter (right) and Walt Fisher show the composite damage imager they developed for imaging thermal damage in structural components of aircraft.
"The system can quickly and easily detect and display thermal damage to polymer matrix composites before measurable physical defects appear," says Eric A. Wachter, a scientist in ORNL's Health Sciences Research. "It offers a fast, flexible, accurate, and easy-to-use means for imaging damage to aircraft wings, flaps, and doors.
"These composites," Wachter explains, "can appear undamaged to the naked eye and even under a microscope. Yet, invisible thermal damage may have caused a composite material to lose more than half of its strength."
Tests show that CDI can image critical thermal damage in composites that cannot be detected by traditional nondestructive evaluation techniques, such as ultrasonic and X-ray imaging.
When laser light is shone on an undamaged region of a composite, the resin emits light in a specific pattern of intensities and wavelengths. The pattern of this laser-induced fluorescence changes in a recognizable way when the laser light illuminates a damaged region of the composite. A camera that detects these "spectral shifts" can be used to image damaged areas.
"The fluorescence from a suspect region of a composite can be imaged instantly," Wachter says. "The high-resolution image is automatically recorded using a video camera containing a charge-coupled device and special optical filters. The device enables the production of a false-color image that highlights the damaged areas of the material."
The technology was developed to support work being conducted by the Oak Ridge Centers for Manufacturing Technology for the U.S. Air Force and Navy to
detect heat-damaged composites in aircraft.
Eric Kaufman, a researcher at the Bioprocessing Research and Development Center in ORNL's Chemical Technology Division, and P. T. Selvaraj, a postdoctoral scientist at the center, are using bacteria to convert sulfur dioxide and other sulfur oxide products into hydrogen sulfide, which they then convert chemically or biologically into elemental sulfur. They have also found that these bacteria can live off sewage, making the process economical.
"A 500-megawatt coal-fired power plant that burns coal that is 3.5% sulfur with no sulfur dioxide control," Kaufman says, "has the potential of releasing up to 360 metric tons of sulfuric acid per day." Sulfuric acid, a component of acid rain, is formed in the atmosphere when sulfur dioxide reacts with moisture there.
American utilities must comply with increasingly stringent requirements to reduce emissions of sulfur dioxide to the atmosphere. These requirements are being phased in under the Clean Air Act Amendments of 1990 to protect the public from this air pollutant, which can cause potentially fatal respiratory illnesses, and to reduce acid precipitation, which can slow the growth of forests and fish populations.
Utilities can meet these requirements in several ways. They can burn lower sulfur coal or they can treat flue gas containing sulfur dioxide as it is produced at coal power plants. The treatment is called flue gas desulfurization (FGD) because it removes sulfur from the gas.
In the more commonly used treatment, called limestone-forced oxidation, the sulfur dioxide in the gas is passed through a slurry of water and limestone (calcium carbonate). The reaction yields the product calcium sulfate, commonly known as gypsum.
Although this FGD waste gypsum is used for wallboard in Japan, in the United States it cannot compete economically with mined gypsum. While 1% is used for cement and building materials in this country, most gypsumsome 20 million tons a yearis buried or stacked in landfills.
A second, emerging flue gas treatment is the use of regenerable sorbents. Here, sulfur dioxide is absorbed onto a catalyst and then is freed to generate a concentrated sulfur dioxide stream. This stream is run through a hydrotreatment process, which requires methane and water to add hydrogen to sulfur dioxide. Hydrogen sulfide formed in the hydrotreating process is then combined with additional sulfur dioxide in a Claus plant to produce elemental sulfur.
To conduct their bioprocessing experiments, Kaufman and Selvaraj introduced into a vertical glass cylinder, or bioreactor, two types of bacteriasulfate-reducing bacteria (SRBs), which cannot tolerate oxygen, and heterotrophs, which remove oxygen to help the SRBs survive. The bacteria are fed a sewage digest that provides the bacteria with carbon, their main food source.
P. T. Selvaraj, a postdoctoral scientist with the Bioprocessing Research and Development Center at ORNL, adjusts controls on a bioreactor. The glass vessel contains bacteria in beads suspended in sewage media. Sulfur dioxide, a coal power plant pollutant, is passed through the bioreactor, and the bacteria convert the pollutant to hydrogen sulfide for subsequent conversion to sulfur, a useful product.
"Use of pretreated sewage as the source of carbon makes this process economical," Kaufman says. "Without this approach, we'd have to buy expensive organic acids to serve as the food source for the bacteria."
In the bioreactor, gelatin-like beads containing SRBs are suspended in sewage media through which sulfur dioxide flows. The SRBs convert the sulfur dioxide into hydrogen sulfide.
"We got the idea that SRBs can also break down the sulfate in calcium sulfate, which is gypsum," Kaufman says. "Gypsum is accumulating at many coal-fired steam plants as an end product of flue gas desulfurization. By dissolving the gypsum in water or sewage media, we can make a slurry that can be passed through our bioreactor, where it is converted to hydrogen sulfide."
To reduce the amount of gypsum accumulating at coal plants, Kaufman and Selvaraj propose a process to reclaim sulfur from calcium sulfate and regenerate the limestone (calcium carbonate) used to produce gypsum from sulfur dioxide. Their research suggests this scheme could reduce carbon dioxide emissions from coal-fired power plants.
"While the sulfur from the waste gypsum is reclaimed as elemental sulfur, calcium can be precipitated to form the original calcium carbonate using waste carbon dioxide from the coal plant," Kaufman says. "Thus, by making the process more efficient, we can at the same time reduce the coal plant's emissions of carbon dioxide, which can increase the greenhouse effect and possibly alter the climate."
The hydrogen sulfide also could be passed through a Claus plant. But a bioreactor might be preferable because it would use less energy.
The researchers also conducted experiments that confirmed that an electrical
current could be used to separate sulfur from oxygen in sulfur dioxide.
However, Kaufman says, electrochemical reduction of sulfur dioxide is more
expensive than the biological approach because it uses considerable
This micrograph shows tungsten carbide-tungsten needles produced in the chemistry laboratory of Carlos Bamberger from needle-shaped crystals of sodium tungsten bronze. These needles could be used to make ceramic composites and ceramic-metal composites for use in engines.
Carlos Bamberger, an ORNL chemist, has discovered a chemical method for producing tungsten carbide needles from needle-shaped crystals of sodium tungsten bronze, an inexpensive material. The chemical reaction employed is a "pseudomorphic reaction" in which the final product has the same shape as the material initially used in the reaction.
"The performance of a material is often strongly dependent on its shape, or morphology," Bamberger says. "For example, whiskers of silicon carbide, a very hard and durable material, have been shown to strongly reinforce other ceramics.
"Because the morphology of a material during synthesis cannot be predicted, this work is valuable because it shows for the first time that needles of tungsten carbide can be made from needles of another starting material."
Bamberger, a senior staff member of ORNL's Chemical and Analytical Sciences Division, believes that properly shaped tungsten carbide could be used to form ceramic composites and ceramic-metal composites that have industrial use.
"This experiment," Bamberger concludes, "showed that, by a simple reaction
with a common gas, an inexpensive material could be converted into an
industrial product of potential high value."
Scientists at ORNL may have a solution to this problem. In work sponsored by DOE and the U.S. Forestry Service, they have developed an advanced method for detecting hidden cannabis gardens. Their method "sniffs" trace vapors from the pot smoker's plant.
"The field test was very successful," says Marcus Wise, an MTA developer and scientist in ORNL's Chemical and Analytical Sciences Division. "The MTA detected a wide range of volatile organic compounds of various molecular weights.
"It appears that the higher-molecular-weight compoundshydrocarbons called terpenes that are found in oils, resins, and balsamsmay be unique enough to the cannabis plant to be used as a signature for detection."
In the test, Wise says, the MTA was operated in the field to determine if it could detect organic vapors from the cannabis plant in real time. Also, vapor samples were collected on sorbent tubes and later analyzed in the laboratory.
The data collected and displayed as spectra by the MTA indicated that the organic compounds in the vapors were related to terpene. These findings at the garden site were confirmed in the laboratory by gas chromatography and mass spectrometry of the sorbent tube samples.
"When we operated the MTA in a conventional mode at the garden," Wise says, "we found we could detect several molecules of organic compounds for every billion molecules of air in our samples. Newer ion traps, which can be operated in an advanced mode, will allow us to detect organic compounds at levels lower than one part per billion in air as they are emitted from cannabis plants."
"Work is scheduled to continue on this project," Wise says, "The plan is to optimize MTA operating conditions for maximum sensitivity and to survey other vegetation for the presence of the target terpenes."
In the late 1980s, Wise, Michelle Buchanan, and Mike Guerin, then of ORNL's Analytical Chemistry Division, developed new sample-handling equipment and computer software to enable an ion-trap mass spectrometer to sample and monitor air directly. Today this system can detect and measure trace levels of organic compounds in air, water, soil, and body fluid samples within minutes.
Participants in the marijuana research include Wise from the
Instrumentation Group; Jan Ma from the Analytical Methods Group; and Rob Smith
from the Environmental Monitoring Group, all in ORNL's Chemical and Analytical Sciences Division.
|A permeable wall of iron filings, serving as an in situ reactive barrier in groundwater,
removes chlorine from a contaminated groundwater plume. An iron-palladium
preparation was found to be even more effective in achieving total dechlorination.
Groundwater at DOE's uranium enrichment facilities is contaminated with trichloroethylene (TCE), a chlorine-containing organic solvent once commonly used to remove dirt and grease from metals. Because the toxic material is present in concentrations exceeding Environmental Protection Agency (EPA) limits, DOE Office of Environmental Management is interested in the possibility of using iron wall technology for in situ treatment of groundwater. Korte and Liang were asked to evaluate the effectiveness of this technology for use at two enrichment facilities, the Paducah, Kentucky, and Portsmouth, Ohio, gaseous diffusion plants.
In the ORNL studies, it was found that dechlorination of TCE occurs on the surface of the iron filings. Also, the researchers observed that the by-products of the reaction include partially dechlorinated compounds, such as dichloroethene and vinyl chloride, and completely dechlorinated hydrocarbons, such as ethene and ethane, which are not of a great concern at trace levels.
"We found that in a batch reactor, about 20% of the by-products contain chlorine, indicating that complete dechlorination had not occurred with elemental iron alone," Liang says. "One of these byproductsabout 8% of the totalis vinyl chloride, which must be kept at trace levels1 part per billion or belowto meet EPA or state regulations."
To achieve complete rather than partial dechlorination, the ORNL and University of Arizona researchers jointly designed and tested an iron-based material containing a trace amount of palladium. The presence of the palladium, a catalyst, increases the rate of dechlorination 10 to 100 times and minimizes the production of undesirable vinyl chloride (reducing it from 8% to less than 1% of total by products).
"Because of the effectiveness of this bimetallic system," Liang says, "we
can achieve total dechlorination in less time."
To determine the exact proportions of by-products formed in the reaction between TCE and iron, the researchers used zero-headspace extractors and a purge-and-trap concentrator with a gas chromatograph (GC) in the laboratory. The extractor is a closed cylinder that contains the sample solution but excludes air space (head space), preventing the organic compounds from volatilizing from the water. Lack of control in keeping organic compounds from escaping from water could cause inaccuracies in the measurements of the amounts and proportions of TCE by-products in the treated water.
Liyuan Liang and John Goodlaxson adjust the zero-headspace extractors for studies of the effectiveness of iron and palladium in removing chlorine from organic compounds found in contaminated groundwater.
A water sample for GC analysis is obtained from the top of the apparatus by forcing the internal piston up, using pressurized gas introduced at the bottom. "Because the batch reactor is a closed system," Liang says, "all by-products and their concentrations in water during treatment can be precisely measured and a good mass balance can be achieved."
The researchers also studied the rates of the dechlorination reactions using fine iron filings and iron filings coated with palladium. "One flow-through column study," Liang said, "showed that removal of half of the TCE in a pure iron system occurred in about 10 minutes. With the addition of the palladium in the amount of about 0.05% of the iron, the removal time is reduced to seconds."
The detailed pathways and mechanisms for the dechlorination reactions involving the metals are still unknown. In the dechlorination reaction, iron atoms donate electrons, breaking the bonds between the chlorine atoms and the carbon atoms in the organic compounds. Most chlorine is released as a chloride ion or combines with ferrous iron to precipitate out as green rust.
The research was sponsored by DOE's Office of Environmental Management.
Korte and Liang's collaborators in the research include John Goodlaxson and
Abinash Agrawal, both of ORNL's Environmental Sciences Division; Rosy
Muftikian, Carina Grittini, and Quintus Fernando, all of the University of Arizona in Tucson; and Jay Clausen of DOE's Paducah Gaseous Diffusion Plant.
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