At first they were considered laboratory-created freaks. Then some of them turned up in outer space. Now they're being sent to ORNL from the frozen reaches of northern Russia. What's going on here?
RNL's Bob Hettich was on the case. He analyzed. He checked. He double checked. His conclusion?
"Buckyballs. Definitely buckyballs."
These enigmatic clusters of carbon atoms have been puzzling scientists since 1985 when they were discovered in a research laboratory among the by-products of laser-vaporized graphite. Their hollow spherical structure, reminiscent of the geodesic domes of eccentric architect Buckminster Fuller, earned them the names "buckyballs" and "fullerenes."
Qualities, such as their unique structure, heat resistance, and electrical conductivity, have fueled speculation about their possible applications in high-temperature lubricants, microfilters, more efficient semiconductors, and manufacturing processes.
To learn more about buckyballs and how they are formed, researchers began to look for naturally occurring fullerenes, particularly on the earth. The first evidence that fullerenes occur naturally on the earth came to light when Arizona State University researchers Semeon Tsipursky and Peter Buseck examined a sample of shiny black rock, known as shungite, from northeastern Russia. Shungite is a rare, carbon-rich variety of rock believed to have been formed between 600 million and 4 billion years ago, although how it was formed is debatable. Electron microscopy of the shungite samples revealed a pattern of white circles with black centers--similar to micrographs Tsipursky had seen of laboratory-produced fullerenes.
To confirm their suspicions, Buseck and Tsipursky sent a trace of powdered rock between two glass slides to Bob Hettich of ORNL's Chemical and Analytical Services Division for examination by mass spectroscopy, a technique that sorts molecules by weight and electric charge. Hettich had previously worked with Buseck to analyze samples from both meteorites and terrestrial rocks for evidence of fullerenes, but they had found none. The shungite sample was different, however; Hettich's analysis confirmed the presence of fullerenes in the rock.
"We wanted to make sure we were looking at only what was in the sample and not distorting it in any way," says Hettich. So, he conducted two separate analyses of the sample. In the initial analysis, he used a pulsed laser to vaporize and ionize the sample, preparing it for analysis by mass spectroscopy. Hettich also analyzed carbon samples known not to contain fullerenes to ensure that none were being created by the laser vaporization process itself. The initial analysis confirmed the presence of both C60 and C70, two common fullerenes, in the shungite sample.
To dispel any lingering doubt, Hettich repeated the analysis without a laser, this time using a 400°C stainless steel probe to vaporize the sample and introduce it into the mass spectrometer for ionization. This technique, known as thermal desorption, cannot create fullerenes in fullerene-free graphite material, yet it yielded identical results, confirming the presence of the two types of buckyballs in the sample.
When Buseck and Tsipursky told Hettich that the rock had come from Russia and not a meteorite, he was somewhat surprised. "In the laboratory," says Hettich, "fullerenes are created in an atmosphere of inert gases, like helium, because common diatomic gases, like nitrogen and oxygen inhibit fullerene growth. This is why fullerenes are not found in ordinary soot, like that in household fireplaces. It seemed more likely to find naturally occurring fullerenes in meteorites, where interaction with these gases would be less of a problem."
The discovery of fullerenes in the shungite sample has provided some hard information for buckyball hunters who have been working mostly on educated guesses and speculation. "We've been working with Peter Buseck for quite a while analyzing various samples, but until now we hadn't found any fullerenes," Hettich notes, "This discovery helps us redefine where to look." More recently, C60 and C70 have also been found in a sample of glassy rock from the mountains of Colorado. Known as a fulgurite, this type of rock structure is formed when lightning strikes the ground. Busek, Tsipursky, and Hettich speculated in a 1992 paper that lightning strikes could provide conditions that are favorable for the formation of buckyballs.
The shungite fullerenes are notable not only for their earthly origin, but also because they may have been formed as solids--most laboratory-created fullerenes are grown in the gas phase. "This is the first example of solid-phase fullerene growth," says Hettich, "It has raised a lot of questions about how the rock was formed, how old it is, and how its composition may have changed over time. Because the shungite sample may be volcanic in origin, you can imagine conditions, like those in a volcano, that would be hot enough to form fullerenes and, at the same time, have little or no oxygen or nitrogen present. But right now, no one is sure exactly how these fullerenes were produced."
"This kind of discovery raises more questions than it answers," says Hettich, "but that's not necessarily a bad thing."--Jim Pearce
Sizing Up Fullerenes--"SANS Doute"
"Sans doute!" a confident Frenchman might say--"without a doubt!" But in the brand new world of fullerenes, this sort of certainty is sometimes in short supply. Much of the uncertainty surrounding these newly discovered carbon clusters stems from their size--you could line up 25 million C60 molecules on a ruler before passing the inch mark.
So, although tools like mass spectrometers can be used to distinguish heavier fullerenes from lighter ones--separating C120 from C180, for instance--researchers still have trouble answering some of the most basic questions about them. How big are they? Are they shaped like spheres, dumbbells, or what? How and where do other atoms bond to their inner and outer surfaces?
Using a time-tested analysis technique of small-angle neutron scattering, appropriately labeled SANS, a team of researchers from ORNL's Biology, Chemical Technology, Health Sciences Research, and Solid State divisions is working to dispel some of the mystery surrounding fullerenes, including how they interact and bond with other elements and with each other.
The preferred method of studying the structure of most materials is crystallography. This technique enables researchers to pinpoint the location of every atom in a sample. "Even though C60 has been crystallized, this is not always possible with other materials," says Stephen Henderson of ORNL's Biology Division. "Other techniques, like SANS, are more accessible, though they give less structural information." SANS requires only that the material be dissolved, rather than crystallized; then scattered neutrons are counted for several hours and the data are analyzed.
The SANS research facility, located at ORNL's High Flux Isotope Reactor, is operated by George Wignall of the Solid State Division. There, dissolved fullerene samples are placed in the path of a neutron beam. As the beam passes through the sample, neutrons are deflected, or scattered, by carbon molecules in the solvent. This scattering is recorded by a detector, providing a two-dimensional pattern, or "signature," for the material, which can then be analyzed to determine the size and shape of the dissolved molecules.
"The greatest significance of using SANS to analyze fullerenes is its ability to discern shapes," says Bob Haufler, a postdoctoral fellow in the Health Sciences Research Division (HSRD). "This is clearly fertile ground for new chemistry. I think it will be especially helpful in situations where atoms of hydrogen or metals are attached to the inside of the fullerenes." "It's also interesting to see how the fullerenes interact with the solvents," says Kathleen Affholter of the Solid State Division, "to see if polymers are forming, for example."
The SANS facility "sees" objects in its neutron beam by keeping track of the neutrons the objects scatter. This scattering varies with the square of an object's volume, so when its diameter decreases by half, it scatters only one-quarter as many neutrons. As a result, the smallest fullerenes are near the lower limit of what the SANS can see--a factor in the past reluctance of researchers to use SANS in this type of research.
Even though the outcome was in doubt, Wignall encouraged Affholter and others to pursue the project because the potential scientific payoff was so high. "If it hadn't been for George," Affholter says, "the project wouldn't have started in the first place." When Affholter introduced the idea of using the SANS facility for fullerene studies to Bob Compton of HSRD, he said his group was making some C120 and C180 molecules and they didn't know what they looked like--whether they were dumbbell-shaped or just bigger versions of the more or less round C60 and C70 balls. "We decided to look at the C60 and C70 balls first," says Affholter, "because they were available and their sizes and shapes were already known."
Because of the relatively small sizes of these fullerenes, the researchers sought to optimize several factors in the experiments. First, the distance the neutron beam traveled through the fullerene solution was increased from a typical 2 mm to 20 mm, increasing the likelihood of interactions between the fullerenes and the neutron beam. "As a result," says Henderson, "we got incredibly good statistics after an hour or so. Often, for solution work, differences are hard to see even after 10 hours of counting."
Second, the fullerenes were dissolved in a solvent that is relatively transparent to neutrons to maximize the contrast between the two. "A visual analogy," says Henderson, "would be observing blue balls in a transparent solvent, rather than in a blue solvent." Fortunately, the solvent that provided the best contrast also dissolved C60 and C70 most effectively, again putting more molecules in the path of the beam.
The team hopes to expand its work to include further explorations of the basic chemistry of buckyballs, including imaging fullerenes that have been combined with other elements, such as hydrogen, fluorine, and various metals. They expect to be able to determine how many and where these "piggyback" atoms are attached to the inner and outer surfaces of the fullerenes. They also expect to be able to produce and analyze larger fullerenes.
"It is difficult to get this kind of information from other techniques," Henderson says. "We also expect to be able to see whether these additional atoms have expanded the structure of the fullerenes. The actual mechanics and chemistry of adding other atoms to these molecules helps us understand how they react and combine with other elements. It could be that these materials--hydrogenated fullerenes, for instance--are better starting points for making other products."
"This project was an excellent example of cooperation among four research divisions," says Affholter. "Everybody had something to add to the project. Everybody talked and pulled together to make it work." The success of this group bodes well for the future of the informal collaboration. "We've gotten together to determine what we want to do next," Affholter says. "We like doing this kind of work, and if we don't do it, people at other labs will."
Evaluating Stealthier Submarine Designs
Now that the "new world order" left in the wake of the Cold War has had a couple years to take shape, several trends are clear. One is that, despite predictions of a new prosperity and a peace dividend, conflicts, both ancient and new, continue to make the world a very dangerous place. Another more disturbing trend is that unfamiliar variables must be figured into the world's balance of power--an equation that had remained relatively unchanged for four decades.
Far from causing peace to break out across the world, the end of the Cold War and the collapse of the Soviet Union have actually increased the number of nations with access to sophisticated military hardware. In addition, the economic instability that accelerated the Communist bloc's demise continues unabated in its successor states, pressuring some of them to offer high-tech armaments to any government that can afford them.
Among the weapons finding their way into the burgeoning international arms market are submarines. For example, last November The New York Times reported deployment of a U.S. nuclear submarine in the Persian Gulf following Russia's $600 million sale of three submarines to Iran. The deal went through despite U.S. objections, with the former Soviet republic citing its obligation to fill arms contracts and its need for hard cash.
The U.S. submarine deployment, ostensibly conducted to check out the sonic properties of the Gulf, highlights the importance of acoustic stealth to effective submarine operations. The obvious advantage a submarine has over a surface ship is its ability to travel, conduct surveillance, or initiate an attack without attracting the attention of other vessels. However, in recent years the technology used to detect sound underwater has improved dramatically, rendering older, noisier subs ineffective for many tasks and putting a premium on developing new "silent-running" ships.
Designing a submarine that can evade a state-of-the-art acoustic dragnet is no small trick, especially given the immense size of these ships. The pride of the U.S. fleet, the 171-meter-long (560-foot-long) Ohio class vessels, displace nearly 19,000 tons of seawater. Their counterparts in the former Soviet navy are the 171-meter-long Typhoon-class ships. These seagoing behemoths displace 25,500 tons of water, making them, by far, the largest underwater vessels ever built.
To ensure that U.S. submarines maintain the upper hand in these underwater games of cat and mouse, the U.S. Navy, with help from ORNL, has constructed the David Taylor Research Center in Bethesda, Maryland, and the Naval Sea Systems Command.
The product of this long-term joint effort is the Large Cavitation Channel Data Acquisition and Analysis System (LCCDAAS). By incorporating instrumentation that can be electronically steered to look at all aspects of a propeller, hull, or other component, this system is 5 times more sensitive than any other noise-detection facility and focuses on noises in three dimensions, instead of one or two. This capability allows researchers to more effectively isolate unwanted noises in the components being tested.
The method of noise detection and isolation used by the LCCDAAS is called beamforming. Beamforming is a way of combining individual sensor data to enhance the signal-to-noise ratio for noise levels below that of the background noise in the ocean or test facility. Beamforming also enables researchers to determine where the noise is coming from.
The beamformers use 95 sensors, or hydro-phones, in four frequency bands, from 1250 Hz to 20 kHz, covering four octaves of sound. A single hydrophone would pick up noise from all directions, but by using an array of hydrophones, beamforming "aims" the array at the target and keeps extraneous noise to a minimum.
When a beamformer is operating in a large body of water, the distance between the beamformer and its target is much larger than the distance between hydrophones, so the sound waves reaching the hydrophones are assumed to be traveling in the same plane--arriving simultaneously at all of the sensors when the array is steered broadside to the target. In reality, sound waves are spherical, but for closely spaced hydrophones at a great distance from the target, the difference is negligible. When the target is much closer to the hydrophone array, as close as 2 meters (6 feet) in the LCC facility, the sound waves reach different hydrophones at different times, and the computer must make instantaneous time-delay adjustments to determine the source of the sound.
To accomplish this feat in real time, the system's sensors funnel 4.6 million measurements each second to a bank of eight computers operating in parallel. The system can beamform 45 acoustic data channels simultaneously as well as monitor 128 facility-related sensors measuring parameters such as water temperature, pressure, and acidity level. This amount of computational firepower produces an instantaneous acoustic analysis of each component being tested. The computer system then integrates acoustic data with the facility measurements to give researchers a picture of the environmental conditions being simulated as the data are gathered.
In a time when both national security and government spending often dominate the news, the LCCDAAS is proving to be doubly beneficial. Not only do the data the system produces translate into stealthier submarines with greater versatility and effectiveness, but they also streamline the design process and help avoid cost overruns by enabling designers to correct potential problems on scaled-down hulls and other components before they go into production.
Future plans for the system include expanding capabilities at the LCCDAAS to decrease analysis time and possibly developing a more compact, shipboard version of the system to monitor experiments at sea. In the meantime, the LCCDAAS is helping ensure that U.S. forces can keep both a low profile and a watchful eye on things as the new world order evolves.--Jim Pearce
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