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Lab News -- April 25, 2008

April 25, 2008

LabNews 04/25/2008PDF (500 kb)

Sandia simulations may explain nanoparticles ‘pinned’ to graphene

By Neal Singer

Graphene flakes are notoriously difficult to work with. Still, they are stronger than diamond, better heat-shedders and conductors than silicon, and thought to have great potential in the worlds of microelectronics and sensors.

In 2005, a German team discovered a new wrinkle in the battle to harness them. A graphene flake lying atop an iridium crystal unexpectedly caused new iridium atoms, deposited on top of the flake, to arrange themselves into equally sized, equally spaced clusters. Not only that — the cluster arrays remained stable even as the temperature was raised into the 400 to 500 kelvin range.

Imagining a whole new set of possible applications, people wanted to know why.

It was hard to understand how a graphene sheet — a featureless, flat sheet of carbon atoms — lying on an equally featureless iridium surface, somehow converted itself into a kind of muffin tin that drew newly arrived iridium atoms into equally spaced, equally sized clusters (“muffins”).

“At the outset,” writes Sandia researcher Peter Feibelman (1130), “this seemed quite a mystery.”

Sherlock Holmes himself, looking for clues to why the iridium quantum dots so mysteriously attached, would have found little to go on.

The iridium support layer was flat as could be. The same was true of the graphene layer that formed on top of it, which sported neither hooks nor ports for nanoparticle docking.

Graphite itself — merely a group of sheets of graphene — is so slippery it can be used as a lubricant. Why would nanodots attach to the completed graphene layer instead of just sliding away?

Even granted an attachment mechanism, why would newly introduced iridium atoms form a moiré — a regular, ordered array — atop the graphene instead of a planar second surface — a sandwich where the iridium was the bread and graphene the meat?

The explanation for the template effect would be almost impossible to see by direct examination.

But Peter’s computational simulations, detailed in a paper published electronically last week by Physical Review B, produced a plausible explanation.

His work demonstrates that in regions where half the graphene flake’s carbon atoms sit directly above iridium atoms of the underlying crystal, iridium atoms added on top of the graphene flake make it buckle. These regions do not occur randomly, and in fact form the regular array needed to explain the nanodot moiré.

The buckling weakens tight links between the graphene’s neighboring carbon atoms, freeing them to attach to the added iridium atoms. Furthermore, buckling not only allows the carbon atoms that buckle upward to capture deposited iridium atoms, but also causes the carbon atoms that buckle down to attach firmly to the metal below, explaining the remarkable thermal stability of the nanodot arrays.

This orderly nanoscopic arrangement appeals to scientists trying to understand aspects of catalysis, Peter says. The atoms that make up tiny nanodots are expected to be in direct contact with inserted materials, important for speeding up desirable chemical reactions. The regular arrangement of the nanodots makes the science relatively simple, because every catalyst particle is the same and sits in the same environment.

“The rigorous periodicity of the nanodot arrays is a huge advantage compared to amorphous or ‘glassy’ arrangements where everything has to be described statistically,” says Peter.

Similar quantum dot arrangements on electrically insulating graphene could keep information packets separate and “addressable” for data storage, or provide superior conditions for quantum computing. -- Neal Singer

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The physics of carbon nanotubes

By Patti Koning

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 is, naturally, in on the carbon nanotube game, with research led by physicist François Léonard (8756). François 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.

François says he hadn’t thought of authoring a book on carbon nanotubes until he was approached by publishers, a result of a review article he wrote. “I was intrigued,” he says. “It seems like there is a need for a book like this to explain the physics behind the applications.”

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.

“Carbon nanotubes have very intriguing properties, both from a scientific perspective and for applications,” says François.

Carbon nanotubes have a sort of dual personality not found in other materials made from a single element. The properties of other single-element materials are obvious — gold is a metal and silicon is a semiconductor, for example. Carbon nanotubes are special because they can be either metallic or semiconducting.

François 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

François’ 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. François was at IBM as a postdoc when researchers there built the first transistor from carbon nanotubes.

As a theoretical physicist, François 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.

A 2007 paper that he coauthored, “Optically Modulated Conduction in Chromophore-Functionalized Single-Wall Carbon Nanotubes,” received a lot of attention, including a write-up in Nature. The paper detailed the incorporation of a photosensitive dye with a single carbon nanotube, so that its electrical connectivity can be controlled by light. This approach allows the detection of light at intensities thousands of times less than previously accomplished.

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 François. “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. Recently Samsung prototyped a 40-inch color display made with carbon nanotubes as emitter tips.

The applications just get wilder. Carbon nanotubes can convert MEMS (microelectromechanical systems) into NEMS (nanoelectromechanical systems).

‘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, François is leading a team to develop optical detection using carbon nanotubes. The project is a partnership with Lockheed Martin under its Shared Vision program.

Unique electronic properties

“This project fits into many of Lockheed Martin’s and Sandia’s missions. In addition to national security applications, optical detectors are used extensively in basic science, for everything from looking at nanoscale materials to galaxies,” says François.

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, François 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?

François’ book is expected to be out by the end of April. See the publisher’s website here for details. — Patti Koning

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Team developing viable biofuel for military aircraft

By Michael Padilla

Coming soon: New biofuel for military jets.

That’s what Sandia researchers are working on as part of a Defense Advanced Research Projects Agency (DARPA) funded team led by UOP LLC, a Honeywell company.

The team is looking at the production of military Jet Propellant 8 (JP-8) fuel based on renewable biomass oil feedstocks, including oil crops, unconventional sources like algae, and various forms of waste vegetable and animal oils.

The goal of the 18-month effort, backed by a $6.7 million project award from DARPA, is to develop, demonstrate, and commercialize a process by October to produce the JP-8 fuel used by US and NATO militaries.

Sandia researchers are working with team members at UOP and Cargill to evaluate technical, economic, and environmental interdependencies. The team is conducting comparative life-cycle analyses and tradeoff assessments and assessing the scale-up feasibility of high-volume bio-oil feedstock and JP-8 fuel production from suitable oil crops and other sources.

At the same time, Sandia, UOP, Honeywell Aerospace, Cargill, and Southwest Research Institute researchers are working to evaluate, develop, and commercialize the processes and biofeedstock and biofuel production scale-up pathways needed to enable reliable, high-volume, competitively priced jet fuel production based on feedstock rather than petroleum.

A new complementary DARPA biofuel program announced in November is specifically focused on the production of JP-8 from algae and lignocellulosic materials. Sandia partnered on six different teams that submitted proposals to this program. Proposal funding decisions are expected to be made in late April, with funded projects expected to begin by late summer.

Systems analysis

According to Sandia project leader Ron Pate (6313), Sandia researchers are addressing issues and options for the necessary expansion of reliable and cost-competitive oil crop production and oil feedstock processing. This includes evaluation of promising oil crops that will not directly compete with food and feed markets, can avoid the use of higher-quality agricultural land, and may also allow for reduced demand for energy, fresh water, and other inputs.

“National scale-up of oil crop-based aviation fuel production at the volumes, supply availability, reliability, and competitive costs desired is a complex and dynamic ‘system of systems’ challenge,” says Ron. “We are leveraging our capabilities and expertise in systems dynamics modeling, simulation, and assessment to help provide insight and decision support to the project.”

Several key issues and interdependencies for bio-oil feedstock and biofuel production scale-up include land use, water demand and availability, soil and climate conditions, energy, and other critical inputs.

Conversion processes under development are expected to yield high fractions of liquid biofuel product in the form of JP-8 and green diesel, along with other useful coproducts. Mass conversion yields to JP-8 are process- and feedstock-dependent, but can be well above 50 percent, says Ron.

Oils derived from plants like soy, oil palm, sunflower, and numerous others provide an easy-to-handle material with high energy density and chemical structures that can more easily be converted into high-performance liquid fuels than other forms of biomass. Production of conventional oil crops for biofuel will face limits due to competing markets for oil crop products and competing uses for the land and water required to grow the crops, Ron says.

Algae that create oil in the form of triacylglycerols (TAGs) and fatty acids have long been seen as a promising option for producing liquid transportation biofuels, Ron says. Algae can be grown using land not otherwise suitable for agriculture, and can use lower quality water sources such as inland brackish ground water, various waste waters, desalination concentrate, by-product water from oil, gas, and coal-bed-methane energy mineral extraction, and coastal sea water.

Despite the high productivity potential of algae, Sandia’s preliminary techno-economic assessment reveals several major areas where innovation will be required before affordable algal biofuel production is possible.

These include less energy-intense processes associated with algal biomass harvesting, dewatering, and neutral lipid extraction. Costs of algal oil production need to be brought down by at least an order of magnitude to be competitive with other alternatives, says Ron. Currently, Sandia has several internally funded projects underway to address issues associated with algae for biofuel.

Lignocellulosic biomass represents a widely available biofuel feedstock source. Lignocellulosic materials come from forest industry residues, including sawmill and paper mill discards, municipal solid waste that includes discarded wood and paper products, agricultural residues, including corn stalks, straw, and sugarcane bagasse, and biomass from dedicated energy crops that include fast-growing herbaceous grasses and woody trees.

Outlook

Fuel produced using the new processes will have to meet stringent military specifications.

The processes are expected by the military to achieve high-energy efficiency in the conversion of renewable bio-oil feedstock to JP-8 fuel and other valuable coproducts that can include green diesel fuel and other industrial chemicals, Ron says.

The biorefinery output of high-quality biofuels and other coproducts will combine to reduce waste and production costs. UOP expects the technology will be viable for future use in the production of fuel for commercial jets. -- Michael Padilla

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DOE program (with Sandia help) reaches out to touch you with solar

By Neal Singer

DOE photovoltaic funding for years has gone to programs that promise more efficient conversion of sunlight to electricity, or in aiding solar start-up companies. It’s called “technology push.”

Now for something different. For the past year, an unusually innovative DOE program called Solar America Cities has focused on reaching out to formerly ignored, sometimes low-profile city decision makers who administer large chunks of urban real estate. It’s called “technology pull.”

The insight at DOE management was that these key folk could purchase enough solar to make its installation as common and ordinary as curbside recycling.

DOE encouragement would include matching funds, technical support, free policy analyses, and public relations suggestions to help educate relevant political participants as well as the public.

Bringing practical savvy to the table

“Tiger Teams” play a large role in underpinning the program. Personnel from Sandia, the National Renewable Energy Lab (NREL), the Florida Solar Energy Center, New Mexico State University, and private sector partner CH2M Hill aid city managers and staff with practical savvy as DOE personnel push the higher vision of “making solar mainstream.”

Tiger Teams are assemblages of experts put together for a particular purpose, says Tiger Team group leader Vipin Gupta (6337). They disband once the mission is completed, only to reassemble elsewhere.

“Tigers are an appropriate metaphor,” Vipin says. “Our people are independent-minded and driven. You can’t just issue an order to them. They’re decentralized, creative, and getting more and more disciplined.”

As Tiger Team member Jeannette Moore (2734) puts it more viscerally, “People in city agencies have been talking about solar for years. Usually, they’ve gotten a little sleepy. Then we show up. We tell them, we’re going to do solar right now. That wakes everyone up.”

Program started as a casual drawing on a piece of paper

The program was conceived by DOE acting program manager Tom Kimbis in a casual drawing on a piece of paper on an airplane trip. He passed it back to a colleague who thought it was a nice idea but saw no reason why anyone would participate.

They would participate, Kimbis decided, because “Cities are strapped for cash,” he said in an interview. “We’d give them money. But we’d give more than money. Two hundred k [dollars] is a lot in Ann Arbor but nothing in New York. We’d give them wording for legislation, when legislators call us for advice. A website where they could exchange ideas, so that New York could see what San Francisco was doing. And we’d give it a name — Solar America Cities — because for some reason cities like names [like Sunbelt Cities]. Amazingly, it makes people want to move there.”

The remarkably energetic effort celebrated its first anniversary in Tucson April 14-16.

One-hundred twenty involved participants from 25 selected cities (chosen competitively from among 50 to 75 applicants, says Kimbis) explained or absorbed lessons of success or failure in attempts to use solar not only to save energy and lower greenhouse gases but generate low-interest loans, foster start-up companies, attract technically educated personnel, create high-paying jobs, and develop solar education courses. Other areas under discussion included solidifying local political support, writing workable inspection codes, supplying wording for appropriate legislation when asked, and choosing appropriate and sometimes “out-of-the-box” materials and locations for various forms of solar.

“I’m amazed this is a DOE project,” says Mustapha Beydoun, a research scientist at the Houston Advanced Research Center. “It’s so inclusive. It’s good to see who’s tried what and what works and what doesn’t,” he said of the conference, “so you don’t have to reinvent the wheel. Problems often come up exactly where you never expected them to.”

Cost of solar coming down as other energy prices go up

While from a flat financial viewpoint, the dour view is correct that solar is still too expensive to be practical — in some areas, three times the cost of generating electricity from coal — some attendees pointed out that solar power is strongest when the demand for electricity is greatest, at the hottest part of summer days. Thus, it could be used to lower the number of power plants needed to meet air conditioning and other power needs of these peak hours.

Solar electricity also requires no water to convert its fuel into electricity — a possible problem for other methods of generating power as fresh water becomes scarcer.

An oft-repeated mantra, often in the form of graphs, at the convention was that the costs of other fuels are rising while the cost of converting sunlight to electricity is declining.

Rick Scheu, CEO of Portland, Ore.-based King Solar Products, said that administrators in Germany had decided it was useless to compare the various subsidies for different forms of energy production: “About solar, they decided, ‘We need it and we’re putting it in.’”

Austin Mayor Will Wynn (“That’s really my name. My parents did it to me,”) said that city buildings will all be 100 percent alternative-energy run by Jan. 1, 2009, with 15 megawatts of solar online by 2012 and 100 solar megawatts by 2020.

“I tell people that Texas was America’s number-one energy state in the 20th century, and if we want to remain that in the 21st, we need to work on starting up companies that harness the sun,” he said.

So the enthusiasm was there, along with more practical motives like the need to meet legislated requirements on alternative energy production, the carrot of tax incentives, and the funding and technical assistance provided by the DOE program.

The program distributed $200,000 cash to each chosen city for the execution of its developing citywide solar adoption plan, and also makes available a kind of gift certificate of $200,000 drawn on DOE that pays for work by Sandia and other labs for solar technical assistance. The cities contribute, on average, $200,000 of their own, though larger cities like Boston and New York contribute far more. The city population must be at least 100,000.

Asked by Lab News what will bring other cities to the table once the two-year DOE-funded program ceases in fiscal 2009, DOE program “market transformation director” Charlie Hemmeline said that the agency’s solar programs weren’t going away, and suggested that cities later interested in getting help to follow the path laid out by the 25 chosen cities might not find a deaf ear at DOE.

And there’s more. The Solar American Showcase program and the Government Solar Installation Program are less publicized but equally real parts of DOE’s solar effort.

The showcase program provides $200,000 and Tiger Team technical assistance to companies, universities, cities, or states interested in trying new solar technologies. The winners include Forest City Military Communities in Hawaii, the city of San Jose, the Orange County Convention Center in Orlando, Fla., Montclair State University in New Jersey, and a Housing Authority project in northeast Denver.

The government installation program provides solar technical assistance to federal entities.

Sandia provided two Tiger Team members for these projects last year, says Vipin: one at the Smithsonian Zoo last spring and summer, figuring out the photovoltaic needs for the elephant house (3,000 square feet of photovoltaics would do the job for cooling and lights) and for the US Capitol Building complex. “The Tiger Team did a comprehensive study there on creative ways to adopt solar without running against the stringent historic architecture restrictions there,” Vipin says. -- Neal Singer

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NASA tests HYTHIRM systems for thermal, optical imaging

By Stephanie Holinka

NASA has once again turned to Sandia’s National Solar Thermal Test Facility — aka the Solar Tower — to help evaluate new technology for future space shuttle missions. NASA’s most recent testing series in early March evaluated the HYpersonic AeroTHermodynamic InfraRed Measurements (HYTHIRM) systems.

HYTHIRM is a collection of systems NASA wants to use to plan and execute missions. It includes a suite of radiometric infrared imaging systems, mission planning capabilities such as a radiance prediction methodology, and an understanding of atmospheric effects. NASA test director Kamran Daryabeigi says planners willuse HYTHIRM to evaluate the performance of the participating sensor systems and associated image-processing algorithms.

Sandia test engineer Cheryl Ghanbari (6337) says NASA expects the new instruments to provide more accurate thermal and radiological monitoring data on the conditions on the shuttle’s surface during reentry. NASA also hopes, Cheryl says, that these monitoring systems will help scientists understand overheating of the shuttle surface due to unexpected boundary layer transition from laminar to turbulent flow caused by anomalies (like protruding gap filler).

The Solar Tower testing involved the coordination of infrared imaging assets from five locations — three land imagers, one flown on a Navy P2 aircraft, and one space-based.

A four-foot-by-four-foot array of 64 shuttle LI-900 ceramic tiles was placed on the test arm at the top of the 200-foot-high tower, Cheryl says. Then thermocouples internally installed in some of the tiles in the array and an infrared imager located close to the test target provided actual surface temperature conditions.

A solar beam, made from reflections of 40 of the field heliostats, was focused on the test array of shuttle tiles. The array was subjected to uniform and nonuniform heating to obtain radiometric data of a known radiation source at temperatures up to 2,000°F. This high temperature, Cheryl says, simulated surface temperatures of the shuttle during reentry into Earth’s atmosphere, allowing NASA to calibrate and evaluate HYTHIRM.

Cheryl says the facility provided an ideal test bed for an unobstructed view of the heated panel from the infrared imaging assets.

The test results will be used to evaluate the readiness of multiple monitoring systems and will help NASA determine their relative priority for deployment in support of future hypersonic boundary layer transition flight experiments on the shuttle orbiter in 2009.

Subsequent analysis of the imagery will be used to evaluate the performance of the participating sensor systems and associated image-processing algorithms. -- Stephanie Holinka

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