Chemicals

U.S. troops blew up enemy bridges with explosives in World War II to slow the advance of supplies or enemy forces.

In modern times, patrollers use explosives at ski resorts to purposely create avalanches so the runs are safer when skiers arrive.

Other than creating the desired effect (a destroyed bridge or avalanche), the users didn’t exactly know the microscopic details and extreme states of matter found within a detonating high explosive.

In fact, most scientists don’t know what happens either.

But researchers from Lawrence Livermore National Laboratory and the Massachusetts Institute of Technology have created the first quantum molecular dynamics simulation of a shocked explosive near detonation conditions, to reveal what happens at the microscopic scale.

What they found is quite riveting: The explosive, nitromethane, undergoes a chemical decomposition and a transformation into a semi-metallic state for a limited distance behind the detonation front.

Nitromethane is a more energetic high explosive than TNT, although TNT has a higher velocity of detonation and shattering power against hard targets. Nitromethane is oxygen poor, but when mixed with ammonium nitrate can be extremely lethal, such as in the bombing of the Alfred P. Murrah Federal Building in Oklahoma City.

“Despite the extensive production and use of explosives for more than a century, their basic microscopic properties during detonation haven’t been unraveled,” said Evan Reed, the lead author of a paper appearing in the Dec. 9 online edition of the journal, Nature Physics. “We’ve gotten the first glimpse of the properties by performing the first quantum molecular dynamics simulation.”

In 2005 alone, 3.2 billion kilograms of explosives were sold in the United States for a wide range of applications, including mining, demolition and military applications.

Nitromethane is burned as a fuel in drag racing autos, but also can be made to detonate, a special kind of burning in which the material undergoes a much faster and far more violent type of chemical transformation. With its single nitrogen dioxide (NO2) group, it is a simple representative version of explosives with more NO2 groups.

Though it is an optically transparent, electrically insulating material, it undergoes a shocking transformation: It turns into an optically reflecting, nearly metallic state for a short time behind the detonation shock wave front.

But further behind the wave front, the material returns to being optically transparent and electrically insulating.

“This is the first observation of this behavior in a molecular dynamics simulation of a shocked material,” Reed said. “Ultimately, we may be able to create computer simulations of detonation properties of new, yet-to-be synthesized designer explosives.”

Other Livermore researchers include M. Riad Manaa, Laurence Fried, Kurt Glaesemann and J.D. Joannopoulos of MIT.

The work was funded by the Laboratory Directed Research and Development program.

In work that may help law enforcement officials better identify terrorists, researchers at the University of Missouri-Rolla are using glass microspheres -- each about the width of a human hair -- to trace explosives back to their manufacturers.

Dr. Delbert Day, Curators’ Professor emeritus of ceramic engineering and Dr. Paul Worsey, professor of mining engineering at UMR, are combining their talents in materials and explosives to create a more effective way for munitions manufacturers to identify where and when their explosives are made.

“With explosives we want to find out who is using them illegally by having some way to track them,” says Day. “If you know where and when the explosive was made, you can narrow down the number of suspects that might have purchased this explosive.”

The glass microsphere’s chemical composition becomes a signature and can provide the name of company, the plant location, and the day it was manufactured. “By controlling the chemical composition you can put this information inside the glass microsphere and only the manufacturer of the explosive has the code,” Day says.

The tag’s safety is a big concern as explosives are made of dangerous materials, Worsey says. Any material added to an explosive is carefully evaluated for fear that it may cause it to detonate prematurely or make it more dangerous.

“Our idea of using glass microspheres was based on the premise that they are already deliberately added to explosives to improve performance and have not caused a safety problem so they must be safe to use, so why not use them for a tag?” says Day.

Glasses are durable materials with a high melting temperature. Day says it’s unlikely that glass would react in a dangerous way because it’s an inorganic material -- unlike polymers -- which makes it less reactive. Polymers -- tiny chips of plastic -- are currently used to tag some explosives, but Day says they are less durable than glass microspheres.

“A glass is stable to much higher temperatures than most polymers, so there is a higher likelihood that a glass microsphere will survive an explosion compared to most polymers,” says Day.

The glass spheres already added to explosives are hollow, whereas the tags would be solid. Day says there is no evidence that using a solid sphere instead of a hollow one would cause any additional risk. The hollow glass spheres, or “microballoons,” are added to emulsion explosives because they make the explosive more effective, Days says.

After an explosive detonates, the tags -- solid glass microspheres -- are found in and around the explosion site. The microspheres are then detected using a variety of methods depending on what was added to the spheres when they were formed. For example, if a certain chemical is added, the microspheres will glow when exposed to low-level radiation. They can also be made magnetic. Then a magnet can be used at the explosion site to pick up the microspheres, says Worsey.

“In the field of taggants there are a lot of applications, with tagging explosives being just one. There is more interest in it now because of the terrorist activity,” says Day. Other possible applications include tagging the chemicals used to make drugs, land mines, credit cards, jewelry and electronic products.

The first systematic study of a new group of explosives has concluded that the materials are so shock sensitive -- apt to detonate if struck or heated -- that the legendarily touchy nitroglycerin seems a pillar of stability by comparison.

Conducted by Thomas M. Klapötke and colleagues in Germany, the study is scheduled for the May 30 issue of the Journal of the American Chemical Society, a weekly publication.

In the study, researchers focus on newly developed chemical analogues, or variants, of two common high explosives in which carbon atoms have been replaced by atoms of silicon, the element in ordinary beach sand. Because of the extreme sensitivity of the compounds, which the researchers did not expect, only a limited number of tests could be performed before samples exploded.

A sample of one compound, for instance, exploded when touched gently with a small plastic laboratory spatula. Another sample exploded under a microscope. Measurements showed that the silicon analogue was more than 3 times more sensitive to impact than the parent compound.

The report states that the compound is "one of the most dangerous materials, and tends to explode on the slightest impact."

Scientists in the United States and China are reporting development of a new type of fluorescent sensing material that could lead to innovative devices for rapid detection of explosives in security screening, criminal investigations, and other applications.

In the study, Southern Illinois University's Ling Zang and colleagues at the University of Illinois at Urbana-Champaign and the Chinese Academy of Sciences point out that fluorescent-based sensors signal the presence of explosives by losing their glow. Such existing devices, however, have serious limitations, which created the need for a new generation of sensor materials.

The new fluorescent film, made from nanofibrils, overcomes those disadvantages. In laboratory tests, it sensed the presence of vapors from TNT and a related explosives compound with greater effectiveness than existing materials. After sensing the compounds and losing its fluorescence, the material recovered its ability to fluoresce repeatedly during the tests. The experiments suggested that sensors made from the material would resist deterioration from exposure to sunlight, another drawback with existing sensor materials.

Their report is scheduled for the June 20 issue of the Journal of the American Chemical Society, a weekly publication.

Scientists in Germany are reporting development of a new generation of explosives that is more powerful than TNT and other existing explosives, less apt to detonate accidentally, and produce fewer toxic byproducts.

Their study of these more environmentally friendly explosives is scheduled for the June 24 issue of ACS’ Chemistry of Materials, a bi-weekly journal.

In the new study, Thomas M. Klapötke and Carles Miró Sabate point out that conventional explosives such as TNT, RDX and HMX, widely-used in military weapons, are rich in carbon and tend to produce toxic gases upon ignition.

In addition to polluting the environment, these materials are also highly sensitive to physical shock, such as hard impacts and electric sparks, making their handling extremely dangerous. Greener, safer explosives are needed, the researchers say.

To meet this need, Klapötke and Sabate turned to a recently explored class of materials called tetrazoles, which derive most of their explosive energy from nitrogen instead of carbon. They identified two promising tetrazoles: HBT and G2ZT. The researchers developed tiny “bombs” out of these materials and detonated them in the laboratory. The materials showed less sensitivity to shock than conventional explosives and produced fewer toxic products when burned, the researchers say.

Stanford chemists have developed a new way to make transistors out of carbon nanoribbons. The devices could someday be integrated into high-performance computer chips to increase their speed and generate less heat, which can damage today's silicon-based chips when transistors are packed together tightly.

For the first time, a research team led by Hongjie Dai, the J. G. Jackson and C. J. Wood Professor of Chemistry, has made transistors called "field-effect transistors"—a critical component of computer chips—with graphene that can operate at room temperature. Graphene is a form of carbon derived from graphite. Other graphene transistors, made with wider nanoribbons or thin films, require much lower temperatures.

"For graphene transistors, previous demonstrations of field-effect transistors were all done at liquid helium temperature, which is 4 Kelvin [-452 Fahrenheit]," said Dai, the lead investigator. His group's work is described in a paper published online in the May 23 issue of the journal Physical Review Letters.

The Dai group succeeded in making graphene nanoribbons less than 10 nanometers wide, which allows them to operate at higher temperatures. "People had not been able to make graphene nanoribbons narrow enough to allow the transistors to work at higher temperatures until now," Dai said. Using a chemical process developed by his group and described in a paper in the Feb. 29 issue of Science, the researchers have made nanoribbons, strips of carbon 50,000-times thinner than a human hair, that are smoother and narrower than nanoribbons made through other techniques.

Field-effect transistors are the key elements of computer chips, acting as data carriers from one place to another. They are composed of a semiconductor channel sandwiched between two metal electrodes. In the presence of an electric field, a charged metal plate can draw positive and negative charges in and out of the semiconductor. This allows the electric current to either pass through or be blocked, which in turn controls how the devices can be switched on and off, thereby regulating the flow of data.

Researchers predict that silicon chips will reach their maximum shrinking point within the next decade. This has prompted a search for materials to replace silicon as transistors continue to shrink in accordance with Moore's Law, which predicts that the number of transistors on a chip will double every two years. Graphene is one of the materials being considered.

David Goldhaber-Gordon, an assistant professor of physics at Stanford, proposed that graphene could supplement but not replace silicon, helping meet the demand for ever-smaller transistors for faster processing. "People need to realize this is not a promise; this is exploration, and we'll have a high payoff if this is successful," he said.

Dai said graphene could be a useful material for future electronics but does not think it will replace silicon anytime soon. "I would rather say this is motivation at the moment rather than proven fact," he said.

Although researchers, including those in his own group, have shown that carbon nanotubes outperform silicon in speed by a factor of two, the problem is that not all of the tubes, which can have 1-nanometer diameters, are semiconducting, Dai said. "Depending on their structure, some carbon nanotubes are born metallic, and some are born semiconducting," he said. "Metallic nanotubes can never switch off and act like electrical shorts for the device, which is a problem."

On the other hand, Dai's team demonstrated that all of their narrow graphene nanoribbons made from their novel chemical technique are semiconductors. "This is why structure at the atomic scale—in this case, width and edges—matters," he said.

Carbon nanotubes, described as the reigning celebrity of the advanced materials world, are all the rage. Recently researchers at Rice University and Rensselaer Polytechnic Institute used them to make the “blackest black” — the darkest known material, reflecting only 0.045 percent of all light shined on it.

Sandia National Laboratories is also in on the carbon nanotube game, with research led by physicist François Léonard. Léonard has considerable experience in the subject, so much that he wrote the book on it — literally. He’s the author of a forthcoming work, Physics of Carbon Nanotube Devices, which could become the definitive text on the topic.

Carbon nanotubes are long thin cylinders composed entirely of carbon atoms. While their diameters are in the nanometer range (1-10), they can be very long, up to centimeters in length. The carbon-carbon bond is very strong, making carbon nanotubes very robust and resistant to any kind of deformation. The properties of other single-element materials are obvious — gold is a metal and silicon is a semiconductor, for example. Carbon nanotubes, on the other hand, have a sort of dual personality not found in other materials made from a single element. They’re special because they can be either metallic or semiconducting.

Léonard explains that this results from the actual structure of a carbon nanotube; the way the atoms are arranged around the tube determines its electronic properties. To explain this concept to a group of undergraduates at the University of California, Berkeley, he uses three rolls of chicken wire, each cut at a different angle. The chicken wire represents the sheet of graphene from which the nanotube is cut. The angle of that cut creates a different bond geometry along the nanotube, which results in different properties.

Working in uncharted territory

Léonard’s experience with carbon nanotubes began when the field was just emerging. While the discovery of carbon nanotubes is credited to Japanese physicist Sumio Iijima in 1991, work on applications didn’t begin until the late 1990s. Léonard was at IBM as a postdoc when researchers there built the first transistor from carbon nanotubes.

As a theoretical physicist, Léonard was working in uncharted territory. From the beginning, he worked on modeling approaches to understand how carbon nanotubes might behave in certain applications. He joined Sandia in 2000, where he has continued his carbon nanotube research.

The semiconducting side of carbon nanotubes holds a lot of promise for the development of new nanoelectronic devices. “A carbon nanotube creates a transistor that is only one nanometer wide,” says Léonard. “This makes it possible, in principle, to achieve very high device densities compared with the current state of the art.” The field emission properties of carbon nanotubes are also exciting. Flat panel displays are typically made from a high density of sharp tips, to which high voltage is applied to extract electrons. These electrons strike and activate the pixels in the screen. Carbon nanotubes can serve this purpose because they are very sharp, long, and can sustain high fields and high temperatures.

‘Layla’ on a nanotube receiver

Researchers have demonstrated the ability to assemble such devices with a single carbon nanotube. At a recent conference, one scientist played Eric Clapton’s “Layla” on a carbon nanotube device acting as a radio receiver.

Another potential use is in chemical and biological sensors. Carbon nanotubes, because of their small diameter, can serve as very sensitive detectors, with the ability to detect a single molecule of a target substance. DNA detection has also been demonstrated. Currently, Léonard is leading a team to develop optical detection using carbon nanotubes. The project is a partnership with Lockheed Martin.

Unique electronic properties

Semiconducting carbon nanotubes have many properties that make them attractive for optical detection. They have unique electronic properties that favor light absorption. In addition, the wavelength over which light is absorbed can be controlled with nanotubes of different diameters. Importantly, the device fabrication process could be entirely compatible with fabrication processes used by the semiconductor industry. In addition to carbon nanotubes, Léonard is interested in electronic transport in other nanostructures — carbon nanotubes as well as nanowires and single molecules. The question, he says, is how does current pass across nanostructures? How is transport of electrons different than in conventional materials?