Posts Tagged ‘Oak Ridge National Laboratory’

Wednesday, May 7, 2014 @ 12:05 PM gHale

There is a new way to build nanowires just three atoms wide that should help scientists eventually create paper-thin, flexible tablets smartphones.

To create these new nanowires, you have to use a finely focused beam of electrons to build what some of the smallest wires ever made, said Junhao Lin, a Vanderbilt University doctoral student and visiting scientist at Oak Ridge National Laboratory in Tennessee who made the discovery. The tiny metallic wires are one-thousandth the width of the microscopic wires used today to connect the transistors in integrated computer circuits.

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“It’s at the cutting edge of everything,” said Sokrates Pantelides, Lin’s adviser and the university’s Distinguished Professor of Physics and Engineering. “People have obviously made nanowires, but they often might be 50 or 100 nanometers across. We have nanotubes one nanometer across. These are 0.4 nanometers. I would expect them to be fragile but they’re not at all. They are extremely robust.”

Lin made the nanowires using semiconducting materials that naturally form monolayers, which are layers one molecule thick, Pantelides said.

The materials, called transition-metal dichalcogenides, are the combination of the metals molybdenum or tungsten with either sulfur or selenium, university researchers said. The best-known member of the family is molybdenum disulfide, a common mineral used as a solid lubricant.

Scientists have used transition-metal dichalcogenides to build an atomic-scale honeycomb lattice of atoms that have shown important properties, like electricity, strength and heat conduction, Pantelides said.

Researchers have already created functioning transistors and flash memory gates out this material. By creating tiny nanowires out of this same material, the transistors and flash memory gates can end up connected.

The new nanowires are not stand-alone wires. They end up built into the honeycomb lattice, along with the transistors and gates. It’s all one thin, flexible material, which could end up used to build thin electronics, like smartphones and tablets.

“Looking to the future, we can create a flexible two-dimensional material,” said Pantelides. “You could potentially have screens or pages that are flexible like a sheet of paper. You might be able to fold them and then open them up to see the screen. The material is flexible already because it’s just one layer of atoms.”

Scientists around the world are working on the thin, flexible material, Pantelides said. The tiny nanowires are a key piece that has been missing in this scenario.

“This will likely stimulate a huge research interest in monolayer circuit design,” Lin said.

Friday, April 25, 2014 @ 11:04 AM gHale

Treating cadmium-telluride (CdTe) solar cell materials with cadmium-chloride improves their efficiency, but how that happens remained a mystery – until now.

After an atomic-scale examination of the thin-film solar cells, the light bulb has turned on and this decades-long debate about the materials’ photovoltaic efficiency increase after treatment appears solved.

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A research team from led by the Department of Energy’s (DoE) Oak Ridge National Laboratory (ORNL), the University of Toledo and DoE’s National Renewable Energy Laboratory used electron microscopy and computational simulations to explore the physical origins of the unexplained treatment process.

Thin-film CdTe solar cells are a potential rival to silicon-based photovoltaic systems because of their theoretically low cost per power output and ease of fabrication. Their comparatively low historical efficiency in converting sunlight into energy, however, has limited the technology’s widespread use.

Research in the 1980s showed that treating CdTe thin films with cadmium-chloride significantly raises the cell’s efficiency, but scientists have been unable to determine the underlying causes. ORNL’s Chen Li, first author of a study on the subject, said the answer lay in investigating the material at an atomic level.

“We knew that chlorine was responsible for this magical effect, but we needed to find out where it went in the material’s structure,” Li said. “Only by understanding the structure can we understand what’s wrong in this solar cell — why the efficiency is not high enough, and how can we push it further.”

By comparing the solar cells before and after chlorine treatment, the researchers realized atom-scale grain boundaries ended up implicated in the enhanced performance. Grain boundaries are tiny defects that normally act as roadblocks to efficiency, because they inhibit carrier collection which greatly reduces the solar cell power.

Using state of the art electron microscopy techniques to study the thin films’ structure and chemical composition after treatment, the researchers found chlorine atoms replaced tellurium atoms within the grain boundaries. This atomic substitution creates local electric fields at the grain boundaries that boost the material’s photovoltaic performance instead of damaging it.

The research team’s finding, in addition to providing a long-awaited explanation, could help guide engineering of higher-efficiency CdTe solar cells. Controlling the grain boundary structure is a new direction that could help raise the cell efficiencies closer to the theoretical maximum of 32 percent light-to-energy conversion, Li said. Currently, the record CdTe cell efficiency is only 20.4 percent.

“We think that if all the grain boundaries in a thin film material could be aligned in same direction, it could improve cell efficiency even further,” Li said.

Friday, April 25, 2014 @ 10:04 AM gHale

A fuel cell catalyst that converts hydrogen into electricity needs to rip open a hydrogen molecule.

By capturing a view of how the catalyst does this gives insight into how to make the catalyst work better for alternative energy uses.

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This study is the first time scientists have shown precisely where the hydrogen halves end up in the structure of a molecular catalyst that breaks down hydrogen. The design of this catalyst ended up inspired by the innards of a natural protein called a hydrogenase enzyme.

“The catalyst shows us what likely happens in the natural hydrogenase system,” said Morris Bullock of the Department of Energy’s (DoE) Pacific Northwest National Laboratory. “The catalyst is where the action is, but the natural enzyme has a huge protein surrounding the catalytic site. It would be hard to see what we have seen with our catalyst because of the complexity of the protein.”

Hydrogen-powered fuel cells offer an alternative to burning fossil fuels, which generates greenhouse gases. Molecular hydrogen — two hydrogen atoms linked by an energy-rich chemical bond — feeds a fuel cell. Generating electricity through chemical reactions, the fuel cell spits out water and power.

If renewable power can store energy in molecular hydrogen, these fuel cells can be carbon-neutral. But fuel cells aren’t cheap enough for everyday use.

To make fuel cells less expensive, researchers turned to natural hydrogenase enzymes for inspiration. These enzymes break hydrogen for energy in the same way a fuel cell would. But while conventional fuel cell catalysts require expensive platinum, natural enzymes use cheap iron or nickel at their core.

Researchers have been designing catalysts inspired by hydrogenase cores and testing them. In this work, an important step in breaking a hydrogen molecule so the bond’s energy can end up captured as electricity is to break the bond unevenly. Instead of producing two equal hydrogen atoms, this catalyst must produce a positively charged proton and a negatively charged hydride.

The physical shape of a catalyst – along with electrochemical information — can reveal how it does that. So far, scientists found the overall structure of catalysts with cheap metals using X-ray crystallography, but for hydrogen atoms X-rays won’t cut it. Based on chemistry and X-ray methods, researchers have a best guess for the position of hydrogen atoms, but imagination is no substitute for reality.

Bullock, Tianbiao “Leo” Liu and their colleagues at the Center for Molecular Electrocatalysis at PNNL, one of DoE’s Energy Frontier Research Centers, collaborated with scientists at the Spallation Neutron Source at Oak Ridge National Laboratory in Tennessee to find the lurking proton and hydride. Using a beam of neutrons like a flashlight allows researchers to pinpoint the nucleus of atoms that form the backbone architecture of their iron-based catalyst.

To use their iron-based catalyst in neutron crystallography, the team had to modify it chemically so it would react with the hydrogen molecule in just the right way. Neutron crystallography also requires larger crystals as starting material compared to X-ray crystallography.

“We were designing a molecule that represented an intermediate in the chemical reaction, and it required special experimental techniques,” Liu said. “It took more than six months to find the right conditions to grow large single crystals suitable for neutron diffraction. And another six months to pinpoint the position of the split H2 molecule.”

Crystallizing their catalyst of interest into a nugget almost 40 times the size needed for X-rays, the team succeeded in determining the structure of the iron-based catalyst.

The structure, they found, confirmed theories based on chemical analyses. For example, the barbell-shaped hydrogen molecule snuggles into the catalyst core. Once split, the negatively charged hydride attaches to the iron at the center of the catalyst; meanwhile, the positively charged proton attaches to a nitrogen atom across the catalytic core. The researchers expected this set-up, but no one had accurately characterized it in an actual structure before.

In this form, the hydride and proton form a type of bond uncommonly seen by scientists — a dihydrogen bond. The energy-rich chemical bond between two hydrogen atoms in a molecule is a covalent bond and is very strong. Another bond called a “hydrogen bond” is a weak one formed between a slightly positive hydrogen and another, slightly negative atom.

Hydrogen bonds stabilize the structure of molecules by tacking down chains as they fold over within a molecule or between two independent molecules.

The dihydrogen bond in the structure is much stronger than a single hydrogen bond. Measuring the distance between atoms reveals how tight the bond is. The team found the dihydrogen bond was much shorter than typical hydrogen bonds but longer than typical covalent bonds. In fact, the dihydrogen bond is the shortest of its type so far identified, the researchers report.

This unusually strong dihydrogen bond likely plays into how well the catalyst balances tearing the hydrogen molecule apart and putting it back together. This balance allows the catalyst to work efficiently.

“We’re not too far from acceptable with its efficiency,” said Bullock. “Now we just want to make it a little more efficient and faster.”

Friday, February 21, 2014 @ 09:02 AM gHale

A new suite of computer codes that closely model the behavior of neutrons in a reactor core, called neutronics, could give a more accurate way to analyze nuclear power reactors.

Technical staff at Westinghouse Electric Company, LLC, supported by the research team at the Consortium for Advanced Simulation of Light Water Reactors (CASL), used the Virtual Environment for Reactor Applications core simulator (VERA-CS) to analyze its AP1000 advanced pressurized water reactor (PWR). The testing focused on modeling the startup conditions of the AP1000 plant design.

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“In our experience with VERA-CS, we have been impressed by its accuracy in reproducing past reactor startup measurements. These results give us confidence that VERA-CS can be used to anticipate the conditions that will occur during the AP1000 reactor startup operations,” said Bob Oelrich, manager of PWR Core Methods at Westinghouse. “This new modeling capability will allow designers to obtain higher-fidelity power distribution predictions in a reactor core and ultimately further improve reactor performance.”

The AP1000 reactor is an advanced reactor design with enhanced passive that builds on decades of Westinghouse’s experience with PWR design. The first eight units are currently under construction in China and the United States, and represent the first Generation III+ reactor to receive Design Certification from the U.S. Nuclear Regulatory Commission.

CASL is a U.S. Department of Energy (DoE) Innovation Hub established at Oak Ridge National Laboratory (ORNL), a part of DoE’s National Laboratory System. The consortium core partners are a strategic alliance of leaders in nuclear science and engineering from government, industry and academia.

“At CASL, we set out to improve reactor performance with predictive, science-based, simulation technology that harnesses world-class computational power,” said CASL Director Doug Kothe. “Our challenge is to advance research that will allow power uprates and increase fuel burn-up for U.S. nuclear plants. In order to do this, CASL is meeting the need for higher-fidelity, integrated tools.”

During the first generation of nuclear energy, performance and safety margins ended up at conservative levels as industry and researchers gained experience with the operation and maintenance of what was then a new and complex technology. Over the past 50 years, nuclear scientists and engineers gained a deeper understanding of the reactor processes, further characterizing nuclear reactor fuel and structure materials.

By making use of newly available computing resources, CASL’s research aims for a step increase in the improvements in reactor operations that have occurred over the last several decades.

“CASL has been using modern high-performance computing platforms such as ORNL’s Titan, working in concert with the INL Fission computer system, for modeling and simulation at significantly increased levels of detail,” said CASL Chief Computational Scientist John Turner. “However, we also recognized the need to deliver a product that is suitable for industry-sized computing platforms.”

With that understanding, CASL designed the Test Stand project to try out tools such as VERA-CS in industrial applications. CASL partner Westinghouse ended up selected as the host for the first trial run of the new VERA nuclear reactor core simulator (VERA-CS). Westinghouse chose a real-world application for VERA-CS: The reactor physics-analysis of the AP1000 PWR, which features a core design with several advanced features. Using VERA-CS to study the AP1000 provides information to further improve the characterization of advanced cores compared to traditional modeling approaches.

Westinghouse’s test run on VERA-CS focused on modeling one aspect of reactor physics called “neutronics,” which describes the behavior of neutrons in a reactor core. While neutronics is only one of VERA’s capabilities, the results provided by VERA-CS for the AP1000 PWR enhance Westinghouse’s confidence in their startup predictions and expand the validation of VERA by incorporating the latest trends in PWR core design and operational features.

The CASL team now is working on extending the suite of simulation capabilities to the entire range of operating conditions for commercial reactors, including full-power operation with fuel depletion and fuel cycle reload.

Wednesday, November 20, 2013 @ 04:11 PM gHale

By identifying fundamental forces that change plant structures during pretreatment processes used in the production of bioenergy, there may be more effective ways to convert woody plant matter into biofuels.

Pretreatment subjects plant material to extremely high temperature and pressure to break apart the protective gel of lignin and hemicellulose that surrounds sugary cellulose fibers.

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“While pretreatments are used to make biomass more convertible, no pretreatment is perfect or complete,” said Department of Energy’s Oak Ridge National Laboratory’s (ORNL) Brian Davison, who was a coauthor of a research study and paper on the subject.

“Whereas the pretreatment can improve biomass digestion, it can also make a portion of the biomass more difficult to convert,” he said. “Our research provides insight into the mechanisms behind this ‘two steps forward, one step back’ process.” Also, pretreatment is the most expensive stage of biofuel production.

The team’s integration of experimental techniques including neutron scattering and X-ray analysis with supercomputer simulations revealed unexpected findings about what happens to water molecules trapped between cellulose fibers.

“As the biomass heats up, the bundle of fibers actually dehydrates — the water that’s in between the fibers gets pushed out,” said ORNL’s Paul Langan. “This is very counterintuitive because you are boiling something in water but simultaneously dehydrating it. It’s a really simple result, but it’s something no one expected.”

This process of dehydration causes the cellulose fibers to move closer together and become more crystalline, which makes them harder to break down.

In a second part of the study, the researchers analyzed the two polymers called lignin and hemicellulose that bond to form a tangled mesh around the cellulose bundles. According to the team’s experimental observations and simulations, the two polymers separate into different phases when heated during pretreatment.

“Lignin is hydrophobic so it repels water, and hemicellulose is hydrophilic, meaning it likes water,” Langan said. “Whenever you have a mixture of two polymers in water, one of which is hydrophilic and one hydrophobic, and you heat it up, they separate out into different phases.”

Understanding the role of these underlying physical factors — dehydration and phase separation — could enable scientists to engineer improved plants and pretreatment processes and ultimately bring down the costs of biofuel production.

“Our insight is that we have to find a balance which avoids cellulose dehydration but allows phase separation,” Langan said. “We know now what we have to achieve — we don’t yet know how that could be done, but we’ve provided clear and specific information to help us get there.”

Wednesday, October 30, 2013 @ 07:10 PM gHale

There could soon be new types of nuclear fuel pellets that would be safer in the event of a nuclear disaster.

New materials could end up encasing uranium-bearing fuel as an alternative to zirconium alloys, which have seen use as the outer layer of nuclear fuel pellets for the last 50 years, said a team of scientists from the University of Tennessee (UT) and Oak Ridge National Laboratory (ORNL), who will present their work at the AVS 60th International Symposium and Exhibition in Long Beach, CA.

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Using sophisticated computer analyses, the UT and ORNL team identified the positive impact of several possible materials that exhibit resistance to high-temperature oxidation and failure, on reactor core evolution, thereby buying more time to cope in the event of a nuclear accident.

“At this stage there are several very intriguing options that are being explored,” said Steven J. Zinkle, the Governor’s Chair in the Department of Nuclear Engineering at the University of Tennessee and Oak Ridge National Laboratory. There is evidence that some of the new materials would reduce the oxidation by at least two orders of magnitude.

“That would be a game-changer,” he said. The materials examined include advanced steels, coated molybdenum and nuclear-grade silicon carbide composites (SiC fibers embedded in a SiC matrix).

The next step, he added, involves building actual fuel pins from these laboratory-tested materials and exposing them to irradiation inside a fission reactor. Once they perform as desired, the new fuel concepts would likely end up tested in a limited capacity in commercial reactors to then enable larger deployment possibilities.

Though it would take years before any new fuel concepts end up used commercially, given the rigorous and conservative qualification steps required, Zinkle said, these new materials may eventually replace the existing zirconium alloy cladding if they prove to be safer.

The typical core of a nuclear power plant uses the heat generated by fission of uranium and plutonium in fuel rods to heat and pressurize water. Steam then generates to drive steam turbines for electricity production. Water continuously circulates as a coolant to harness the thermal energy from the fuel and to keep the core from overheating.

The cooling pumps are a critical part of the reactor design because even when a nuclear reactor shuts down, the power it generates from radioactive decay of fission products remains at 1 percent of its peak for hours after shutdown. Given that nuclear power plants generate a staggering sum of energy under their nominal operating conditions (~4 GW of thermal energy), even one percent power levels after shutdown prove substantial. That’s why it’s essential to have cool water circulating continuously even after the shutdown occurs. Otherwise you risk overheating and ultimately melting the core – like leaving a pot boiling on the burner.

That’s basically what happened at Fukushima. On March 11, 2011, engineers at the plant managed to initially safely shut down the plant following a massive earthquake, but then a large tsunami knocked out the backup generators running the water pumps an hour later. What followed were explosions associated with hydrogen generated from the reduction of steam during high temperature oxidation of core materials, and releases of radioactive fission products. The accident displaced the local population and will take years and require a significant cost to clean-up.

Fukushima has had a profound impact on the safety culture of the industry, said Zinkle. Despite the fact that not a single U.S. nuclear power plant was unsafe or shut down in the wake of the accident, the Nuclear Regulatory Commission has issued new requirements to enhance their safety, including increasing the requirements for backup power generation on site.

Monday, August 27, 2012 @ 05:08 PM gHale

Knowing the position of missing oxygen atoms could be the key to cheaper solid oxide fuel cells with longer lifetimes.

That means new microscopy research could enable scientists to map these vacancies at an atomic scale.

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Although fuel cells hold promise as an efficient energy conversion technology, they have yet to reach mainstream markets because of their high price tag and limited lifespans.

Overcoming these barriers requires a fundamental understanding of fuel cells, which produce electricity through a chemical reaction between oxygen and a fuel. As conducting oxygen ions move through the fuel cell, they travel through vacancies where oxygen atoms used to be. The distribution, arrangement and geometry of such oxygen vacancies in fuel cell materials should affect the efficiency of the overall device, researchers said.

“A big part of making a better fuel cell is to understand what the oxygen vacancies do inside the material: How fast they move, how they order, how they interact with interfaces and defects,” said Department of Energy’s Oak Ridge National Laboratory’s (ORNL) Albina Borisevich. “The question is how to study them. It’s one thing to see an atom of one type on the background of atoms of a different type. But in this case, you want to see if there are a few atoms missing. Seeing a void is much more difficult.”

ORNL scientists used scanning transmission electron microscopy to determine the distribution of oxygen vacancies in a fuel cell cathode material below the level of a single unit cell. The team verified its findings with theoretical calculations and neutron experiments at the lab’s Spallation Neutron Source.

“Even though the vacancy doesn’t generate any signal in the electron micrograph, it’s still a big disturbance in the structure,” Borisevich said. “You can see that the lattice expands where vacancies are present. So we tracked the lattice expansion around vacancies and compared it with theoretical models, and we were able to develop a calibration for this type of material.”

By providing a means to study vacancies at an atomic scale, the ORNL technique will help inform the development of improved fuel cell technologies in a systematic and deliberate fashion, in contrast to trial and error approaches.

Beyond its relevance to applications in fuel cells and information storage and logic devices, ORNL coauthor Sergei Kalinin said the team’s research is also building a bridge between two scientific communities that traditionally have had little in common.

“From my perspective, it is physics marrying electrochemistry,” Kalinin said. “The idea is that vacancies are important for energy, and vacancies are important for physics. The materials that physicists like to study are exactly the same as the materials used for fuel cells, and unless we understand how vacancies behave at interfaces, ferroic domain walls, and in thin films, we will not be able to fully appreciate the physics of these systems.”

Thursday, August 23, 2012 @ 07:08 PM gHale

A new material in development may make fueling nuclear reactors with uranium harvested from the ocean more feasible.

By combining Department of Energy’s Oak Ridge National Laboratory’s (ORNL) high-capacity reusable adsorbents and a Florida company’s high-surface-area polyethylene fibers creates a material that can rapidly, selectively and economically extract valuable and precious dissolved metals from water.

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The material, HiCap, outperforms today’s best adsorbents, which perform surface retention of solid or gas molecules, atoms or ions. HiCap also effectively removes toxic metals from water, according to results verified by researchers at Pacific Northwest National Laboratory.

“We have shown that our adsorbents can extract five to seven times more uranium at uptake rates seven times faster than the world’s best adsorbents,” said Chris Janke, one of the inventors and a member of ORNL’s Materials Science and Technology Division.

HiCap effectively narrows the fiscal gap between what exists today and what they need to economically extract some of the ocean’s estimated 4.5 billion tons of uranium. Although dissolved uranium exists in concentrations of just 3.2 parts per billion, the sheer volume means there would be enough to fuel the world’s nuclear reactors for centuries.

The goal of extracting uranium from the oceans began with research and development projects in the 1960s, with Japan conducting the majority of the work. Other countries pursuing this goal include Russia, China, Germany, Great Britain, India, South Korea, Turkey and the United States. Researchers developed adsorbent materials, but none has emerged as being economically viable.

What sets the ORNL material apart is the adsorbents are made from small diameter, round or non-round fibers with high surface areas and excellent mechanical properties. By tailoring the diameter and shape of the fibers, researchers can significantly increase surface area and adsorption capacity. This and ORNL’s patent pending technology to manufacture the adsorbent fibers results in a material able to selectively recover metals more quickly and with increased adsorption capacity, thereby dramatically increasing efficiency.

“Our HiCap adsorbents are made by subjecting high-surface area polyethylene fibers to ionizing radiation, then reacting these pre-irradiated fibers with chemical compounds that have a high affinity for selected metals,” Janke said.

After the processing, scientists can place HiCap adsorbents in water containing the targeted material, which ends up quickly and preferentially trapped. Scientists then remove the adsorbents from the water and the metals end up extracted using a simple acid elution method. They can then regenerate and reuse the adsorbent after conditioning it with potassium hydroxide.

In a direct comparison to the current state-of-the-art adsorbent, HiCap provides significantly higher uranium adsorption capacity, faster uptake and higher selectivity, according to test results. Specifically, HiCap’s adsorption capacity is seven times higher (146 vs. 22 grams of uranium per kilogram of adsorbent) in spiked solutions containing 6 parts per million of uranium at 20 degrees Celsius. In seawater, HiCap’s adsorption capacity of 3.94 grams of uranium per kilogram of adsorbent was more than five times higher than the world’s best at 0.74 grams of uranium per kilogram of adsorbent. The numbers for selectivity showed HiCap to be seven times higher.

“These results clearly demonstrate that higher surface area fibers translate to higher capacity,” Janke said.

ORNL researchers conducted field tests of the material at the Marine Sciences Laboratory of Pacific Northwest National Laboratory in Sequim, WA, and at the Rosenstiel School of Marine & Atmospheric Science and Broad Key Island in collaboration with the University of Miami.

Tuesday, May 15, 2012 @ 03:05 PM gHale

There is now a new way to accurately measure gas bubbles in pipelines.

In the end, it all comes down to safety as the ability to measure gas bubbles in pipelines is vital to the manufacturing, power and petrochemical industries.

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In the case of harvesting petrochemicals from the seabed, warning of bubbles present in the harvested crude is crucial because otherwise when these bubbles come up from the seabed (where pressure is very high) to the surface where the rig is, the reduction in pressure causes these bubbles to expand and could cause a “blow out.” A blow out is the sudden release of oil and/or gas from a well and issues with the blow out preventer were key in Deepwater Horizon oil spill in the Gulf of Mexico in 2010.

Currently, the most popular technique for estimating the gas bubble size distribution (BSD) is to send sound waves through the bubble liquid and compare the measured attenuation of the sound wave (loss in amplitude as it propagates) with that predicted by theory.

The key problem is the theory assumes the bubbles exist in an infinite body of liquid. If in fact the bubbles are in a pipe, then the assumptions of the theory do not match the conditions of the experiment. That could lead to errors in the estimation of the bubble population.

There is now a new method, which takes into account that bubbles exist in a pipe, said Professor Tim Leighton from the Institute of Sound and Vibration Research at the University of Southampton, who is leading a project team.

Leighton and his team started the work as part of an ongoing program to devise ways of accurately estimating the BSD for the mercury-filled steel pipelines of the target test facility (TTF) of the $1.4 billion Spallation Neutron Source (SNS) at Oak Ridge National Laboratory (ORNL), one of the most powerful pulsed neutron sources in the world.

The research explores how measured phase speeds and attenuations in bubbly liquid in a pipe might invert to estimate the BSD (independently measured using an optical technique). This new technique, appropriate for pipelines such as TTF, gives good BSD estimations if the frequency range is sufficiently broad.

“The SNS facility was built with the expectation that every so often it would need to be shut down and the now highly radioactive container of the mercury replaced by a new one, because its steel embrittles from radiation damage,” Leighton said. “However, because the proton beam impacts the mercury and generates shock waves, which cause cavitation bubbles to collapse in the mercury and erode the steel, the replacement may need to be more often than originally planned at full operating power. Indeed, achieving full design power is in jeopardy.

“With downtime associated with unplanned container replacement worth around $12 million, engineers at the facility are considering introducing helium bubbles, of the correct size and number, into the mercury to help absorb the shock waves before they hit the wall, so that the cavitation bubbles do not erode the steel. Oak Ridge National Laboratory (ORNL) and the Science and Facilities Research Council (Rutherford Appleton Laboratory, RAL) commissioned us as part of their program to devise instruments to check that their bubble generators can deliver the correct number and size of bubbles to the location where they will protect the pipelines from erosion.

“This paper reports on the method we devised half-way through the research contract. It works, but just after we designed it the 2008 global financial crash occurred, and funds were no longer available to build the device into the mercury pipelines of ORNL. A more affordable solution had to be found, which is what we are now working on. The original design has been put on hold for when the world is in a healthier financial state. This has been a fantastic opportunity to work with nuclear scientists and engineers from ORNL and RAL.”

Monday, May 14, 2012 @ 02:05 PM gHale

A carbon nanotube sponge that can soak up oil in water with incredible efficiency is now in the works.

Carbon nanotubes, which consist of atom-thick sheets of carbon rolled into cylinders, have captured scientific attention because of their high strength, potential high conductivity and light weight. However, producing nanotubes in bulk for specialized applications often faced limitations because of difficulties in controlling the growth process as well as dispersing and sorting the produced nanotubes. Not anymore.

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That is because Bobby Sumpter at the Department of Energy’s (DOE’s) Oak Ridge National Laboratory (ORNL) was part of a multi-institutional research team that set out to grow large clumps of nanotubes by selectively substituting boron atoms into the otherwise pure carbon lattice.

Sumpter and Vincent Meunier, now of Rensselaer Polytechnic Institute (RPI), conducted simulations on supercomputers, including Jaguar at ORNL’s Leadership Computing Facility, to understand how the addition of boron would affect the carbon nanotube structure.

“Any time you put a different atom inside the hexagonal carbon lattice, which is a chicken wire-like network, you disrupt that network because those atoms don’t necessarily want to be part of the chicken wire structure,” Sumpter said. “Boron has a different number of valence electrons, which results in curvature changes that trigger a different type of growth.”

Simulations and lab experiments showed the addition of boron atoms encouraged the formation of “elbow” junctions that help the nanotubes grow into a 3-D network.

“Instead of a forest of straight tubes, you create an interconnected, woven sponge-like material,” Sumpter said. “Because it is interconnected, it becomes three-dimensionally strong, instead of only one-dimensionally strong along the tube axis.”

Further experiments showed the team’s material, which is visible to the human eye, is extremely efficient at absorbing oil in contaminated seawater because it attracts oil and repels water.

“It loves carbon because it is primarily carbon,” Sumpter said. “Depending on the density of oil to water content and the density of the sponge network, it will absorb up to 100 times its weight in oil.”

The material’s mechanical flexibility, magnetic properties, and strength lend it additional appeal as a potential technology to aid in oil spill cleanup, Sumpter said.

“You can reuse the material over and over again because it’s so robust,” he said. “Burning it does not substantially decrease its ability to absorb oil, and squeezing it like a sponge doesn’t damage it either.”

The material’s magnetic properties, a result of the team’s use of an iron catalyst during the nanotube growth process, means a magnet can easily control it or remove it in an oil cleanup scenario. This ability is an improvement over existing substances used in oil removal, which usually stay behind after cleanup and can degrade the environment.

The experimental team submitted a patent application on the technology through Rice University. A research paper entitled “Covalently bonded three-dimensional carbon nanotube solids via boron induced nanojunctions,” is also available.

The research team included researchers from ORNL, Rice University; Universidade de Vigo, Spain; RPI; University of Illinois at Urbana-Champaign; Instituto de Microelectronica de Madrid, Spain; Air Force Office of Scientific Research Laboratory; Arizona State University; Universite Catholique de Louvain, Belgium; The Pennsylvania State University; and Shinshu University, Japan.

 
 
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