Focus on Research
Penn State Intercom......October 18, 2001

Supercomputers reveal
strongest carbon nanotubes

A team of researchers lead by Vincent Crespi, Downsborough associate professor of physics, has used computer simulations to discover carbon fibers with mechanical strength comparable to that of diamond.

Crespi, graduate student Dragan Stojkovic and recent doctoral degree graduate Peihong Zhang report that they have discovered incredibly strong and stiff carbon tubes about 0.4 nanometers in diameter. The so-called nanotubes could theoretically be made from simple starting materials.

"This new fiber hasn't been synthesized yet," Crespi said, "but several physicists and chemists are interested in making them, and they may prove very useful in nanotechnology applications."

Using supercomputers at the San Diego Supercomputer Center, the University of Michigan and the University of Texas, Crespi's team simulated the electronic states and total energies of various carbon molecules. The nanotube discovery by Crespi's team was made serendipitously while its members were studying unrelated features of carbon compounds.

"This is one of those sideways inspirations that comes when you're looking at one thing and you suddenly realize it has a different application," Crespi said.

He immediately adjusted the focus of his simulations.

"Actually, I was motivated to make this strong nanotube the moment I realized it could be done."

Commercially available "carbon fiber" is six to 10 micrometers thick, or one-fifth the thickness of a human hair, and made of carbon-containing polymers. It is used to make items ranging from golf clubs and tennis rackets to bicycle frames and racing yachts. While this type of carbon fiber is weaker than carbon nanotubes, it is easy to produce in large quantities. Manufacturers weave it into sheets, bars, tubes and other shapes -- often in several overlapping layers to increase their strength. Binders such as epoxy resins are often applied to the sheets to connect the fibers to one another for additional strength.

Carbon nanotubes are 10,000 times thinner than commercial carbon fiber. Researchers make them using chemical vapor deposition, a standardized industrial technology in which simple ingredients self assemble. Crespi said vapor deposition also would most likely be used to make the much stronger version of nanotube that his group discovered.

Not all nanotubes have the same properties. The smallest diameter nanotubes created to date have a circumference of about 10 carbon atoms. These tubes are not stable and must be grown within larger-diameter carbon tubes or in tiny cylindrical holes in special crystals known as zeolites.

The team recently made a key discovery that a particular type of tetrahedral carbon atom -- one with three weakly bonded groups and a relatively tightly bonded group -- had special properties. When connected to one another, these molecules have carbon-carbon bonding angles of about 109.5 degrees, which also is the ideal bonding angle of carbon atoms with tetra hedral symmetry. In addition, the stiff, small-diameter and chemically stable carbon nanotube discovered by the researchers has a circumference of only six carbon atoms, or about 0.4 nanometers, the smallest diameter theoretically possible.

"Based on our calculations, these new nanotubes are about 40 percent stronger than the other nanotubes formed using the same number of atoms," Crespi said. "In fact, the nanotubes we simulated may well be the stiffest one-dimensional systems possible."

Geoscientists present clearer
picture of pre-oxygen atmosphere

By A'ndrea Elyse Messer
Public Information

Methane and carbon dioxide, not ammonia, were the greenhouse gases that compensated for our less energetic sun during the pre-oxygen Archean, University geoscientists said.

"We are looking at what the Earth's atmosphere was like prior to 2.3 billion years ago, before the rise of oxygen," said James F. Kasting, professor of geosciences and meteorology. "We believe it is likely that methane was the major component of the atmosphere then and the major greenhouse gas."

Kasting and geosciences graduate student Alexander A. Pavlov looked at a variety of ways to estimate the methane concentrations in the Archean atmosphere. Methane is produced by methanogenic bacteria, organisms that create methane from organic material or hydrogen and carbon dioxide.

"One way to estimate methane in the Archean atmosphere is to take today's methane production by organics and lower today's oxygen to that of the Archean, arriving at about 1,000 times higher than today's methane value," Kasting said. "However, other methods of estimation are better."

Looking at the way methanogenic bacteria produce methane, researchers noted that given abundant nutrients, these bacteria will convert hydrogen until insufficient energy exists to continue. According to thermodynamic analysis, this means that 90 to 95 percent of the hydrogen would be converted to methane.

"This analysis also produces a methane level in the Archean of 1,000 times today's level," Kasting said.

According to Kasting, this level of methane would compensate for the sun which produced only 80 percent of the energy during the Archean that it does today. Previous suggestions for greenhouse gases to compensate for the sun's lower energy include carbon dioxide and ammonia. In the Archean, while Cyanobacteria, bacteria capable of photosynthesis similar to algae, are producing some oxygen, the oxygen is quickly reduced. Ammonia, which some, including Carl Sagan, thought a likely greenhouse gas for this stage in Earth's history, could not have served that purpose, according to Kasting and Pavlov.

"The problem with ammonia is that it will not persist in the atmosphere because sunlight easily breaks it apart into nitrogen and hydrogen," Kasting said. "In 1997, Sagan and others proposed that smog like that on Saturn's moon Titan could have protected the ammonia from photolysis."  

A'ndrea Elyse Messer can be reached at aem1@psu.edu.

Segregation increasing in
public schools in the suburbs

As suburban schools nationwide are experiencing increasing enrollments of blacks, Hispanics and Asian students, reflecting national population trends, there has been a concurrent rise in the levels of school segregation between white and minority students among suburban schools, a University researcher said.

"Racial school segregation, long an urban phenomenon, has in recent decades appeared in the suburbs, primarily as a result of increasing residential segregation between suburban school districts," according to Sean F. Reardon, assistant professor of education and sociology. "Over a quarter of suburban students are nonwhite, and these students are increasingly concentrated in schools and school districts with disproportionately few white students, as compared to the overall suburban student population."

Reardon and John T. Yun, doctoral student at Harvard University's Graduate School of Education, analyzed data on the racial enrollments of all public schools in suburban areas from 1987 to 1995. They measured segregation by determining the degree of unevenness in the racial compositions of suburban schools. They investigated the relationship between rates of minority population growth and rates of change in segregation during those years.

"Although racial school segregation is still lower among suburban schools than among urban schools, we found that suburban areas with the most rapidly growing minority student populations are, on average, experiencing relatively rapid increases in segregation levels," they noted. "This is primarily due to the fact that the most rapid growth of minority student enrollments has been in suburban school districts with low or declining white student enrollments."

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