David Kirkpatrick

April 9, 2010

Graphene plus substrate still great thermal conductor

A graphene two-fer this evening. This news is another important finding toward commercializing graphene.

The release:

With support, graphene still a superior thermal conductor

Super-thin material advances toward next generation applications

IMAGE: A one-atom thick sheet of graphene (highlighted in the circular window) on top of a silicon dioxide support proves to be an excellent thermal conductor, according to new research published…

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CHESTNUT HILL, MA (4/8/2010) – The single-atom thick material graphene maintains its high thermal conductivity when supported by a substrate, a critical step to advancing the material from a laboratory phenomenon to a useful component in a range of nano-electronic devices, researchers report in the April 9 issue of the journal Science.

The team of engineers and theoretical physicists from the University of Texas at Austin, Boston College, and France’s Commission for Atomic Energy report the super-thin sheet of carbon atoms – taken from the three-dimensional material graphite – can transfer heat more than twice as efficiently as copper thin films and more than 50 times better than thin films of silicon.

Since its discovery in 2004, graphene has been viewed as a promising new electronic material because it offers superior electron mobility, mechanical strength and thermal conductivity. These characteristics are crucial as electronic devices become smaller and smaller, presenting engineers with a fundamental problem of keeping the devices cool enough to operate efficiently.

The research advances the understanding of graphene as a promising candidate to draw heat away from “hot spots” that form in the tight knit spaces of devices built at the micro and nano scales. From a theoretical standpoint, the team also developed a new view of how heat flows in graphene.

When suspended, graphene has extremely high thermal conductivity of 3,000 to 5,000 watts per meter per Kelvin. But for practical applications, the chicken-wire like graphene lattice would be attached to a substrate. The team found supported graphene still has thermal conductivity as high as 600 watts per meter per Kelvin near room temperature. That far exceeds the thermal conductivities of copper, approximately 250 watts, and silicon, only 10 watts, thin films currently used in electronic devices.

IMAGE: Boston College physicist David Broido worked with colleagues from the University of Texas at Austin and France’s Commission for Atomic Energy to determine why graphene maintains its superior thermal conductivity…

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The loss in heat transfer is the result of graphene’s interaction with the substrate, which interferes with the vibrational waves of graphene atoms as they bump against the adjacent substrate, according to co-author David Broido, a Boston College Professor of Physics.

The conclusion was drawn with the help of earlier theoretical models about heat transfer within suspended graphene, Broido said. Working with former BC graduate student Lucas Lindsay, now an instructor at Christopher Newport University, and Natalio Mingo of France’s Commission for Atomic Energy, Broido re-examined the theoretical model devised to explain the performance of suspended graphene.

“As theorists, we’re much more detached from the device or the engineering side. We’re more focused on the fundamentals that explain how energy flows through a sheet graphene. We took our existing model for suspended graphene and expanded the theoretical model to describe this interaction that takes place between graphene and the substrate and the influence on the movement of heat through the material and, ultimately, it’s thermal conductivity.”

In addition to its superior strength, electron mobility and thermal conductivity, graphene is compatible with thin film silicon transistor devices, a crucial characteristic if the material is to be used in low-cost, mass production. Graphene nano-electronic devices have the potential to consume less energy, run cooler and more reliably, and operate faster than the current generation of silicon and copper devices.

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Broido, Lindsay and Mingo were part of a research team led by Li Shi, a mechanical engineering professor at the University of Texas at Austin, which also included his UT colleagues Jae Hun Seol, Insun Jo, Arden Moore, Zachary Aitken, Michael Petttes, Xueson Li, Zhen Yao, Rui Huang, and Rodney Ruoff.

The research was supported by the Thermal Transport Processes Program and the Mechanics of Materials Program of the National Science Foundation, the U.S. Office of Naval Research, and the U.S. Department of Energy Office of Science.

November 18, 2008

Nanocoating improve industrial energy efficiency

The release from today:

TOUGH NANOCOATINGS BOOST INDUSTRIAL ENERGY EFFICIENCY

Ames Laboratory project seeks to reduce friction and extend tool life

AMES, Iowa – Friction is the bane of any machine.  When moving parts are subject to friction, it takes more energy to move them, the machine doesn’t operate as efficiently, and the parts have a tendency to wear out over time.

But if you could manufacture parts that had tough, “slippery” surfaces, there’d be less friction, requiring less input energy and the parts would last longer.  Researchers at the U.S. Department of Energy’s Ames Laboratory are collaborating with other research labs, universities, and industrial partners to develop just such a coating.

“If you consider a pump, like a water pump or a hydraulic pump, it has a turbine that moves the fluid,” said Bruce Cook, an Ames Laboratory scientist and co-principal investigator on the four-year, $3 million project. “When the rotor spins, there’s friction generated at the contacting surface between the vanes and the housing, or stator.  This friction translates into additional torque needed to operate the pump, particularly at start-up.  In addition, the friction results in a degradation of the surfaces, which reduces efficiency and the life of the pump.  It takes extra energy to get the pump started, and you can’t run it at its optimum (higher speed) efficiency because it would wear out more quickly.”

Applying a coating to the blades that would reduce friction and increase wear resistance could have a significant effect in boosting the efficiency of pumps, which are used in all kinds of industrial and commercial applications. According to Cook, government calculations show that a modest increase in pump efficiency resulting from use of these nanocoatings could reduce U.S. industrial energy usage by 31 trillion BTUs annually by 2030, or a savings of $179 million a year.

The coating Cook is investigating is a boron-aluminum-magnesium ceramic alloy he discovered with fellow Ames Laboratory researcher and Iowa State University professor of Materials Science and Engineering Alan Russell about eight years ago. Nicknamed BAM, the material exhibited exceptional hardness, and the research has expanded to include titanium-diboride alloys as well.

In many applications it is far more cost effective to apply the wear-resistant materials as a coating than to manufacture an entire part out of the ceramic.  Fortunately, the BAM material is amenable to application as a hard, wear-resistant coating.  Working with ISU materials scientist Alan Constant, the team is using a technique called pulsed laser deposition to deposit a thin layer of the alloy on hydraulic pump vanes and tungsten carbide cutting tools. Cook is working with Eaton Corporation, a leading manufacturer of fluid power equipment, using another, more commercial-scale technique known as magnetron sputtering to lay down a wear-resistant coating.

Pumps aren’t the only applications for the boride nanocoatings. The group is also working with Greenleaf Corporation, a leading industrial cutting tool maker, to put a longer lasting coating on cutting tools.  If a tool cuts with reduced friction, less applied force is needed, which directly translates to a reduction in the energy required for the machining operation.

To test the coatings, the project team includes Peter J. Blau and Jun Qu at one of the nation’s leading friction and wear research facilities at DOE’s Oak Ridge National Laboratory, or ORNL, in Tennessee.  Initial tests show a decrease in friction relative to an uncoated surface of at least an order of magnitude with the AlMgB14-based coating.  In preliminary tests, the coating also appears to outperform other coatings such as diamond-like carbon and TiB2.

In a separate, but somewhat related project, Cook is working with researchers from ORNL, Missouri University of Science and Technology, the University of Alberta, and private companies to develop coatings in high-pressure water jet cutting tools and severe service valves where parts are subject to abrasives and other extreme conditions.

“This is a great example of developing advanced materials with a direct correlation to saving energy,” Cook said. “Though the original discovery wasn’t by design, we’ve done a great deal of basic research in trying to figure out the molecular structure of these materials, what gives them these properties and how we can use this information to develop other, similar materials.”

Funding for both projects is provided by the DOE’s Office of Energy Efficiency and Renewable Energy.  BAM is licensed to Newtech Ceramics, an Iowa based startup company located in Des Moines. The ISU Research Foundation provided nearly $60,000 in funding for development of material samples for marketing as part of the startup effort.

Ames Laboratory is a U.S. Department of Energy Office of Science laboratory operated for the DOE by Iowa State University.  Ames Laboratory creates innovative materials, technologies and energy solutions. We use our expertise, unique capabilities and interdisciplinary collaborations to solve global challenges.

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A photograph of an AlMgB14 coating on a steel substrate.  The substrate is the mottled structure on the left-hand side of the photo and the coating is the thin, darker strip running along the edge of the steel.  (The blemishes on the steel are carbide inclusions)  The coating has a thickness of approximately 2 to 3 microns (about 1 ten thousandths of an inch)

A photograph of an AlMgB14 coating on a steel substrate. The substrate is the mottled structure on the left-hand side of the photo and the coating is the thin, darker strip running along the edge of the steel. (The blemishes on the steel are carbide inclusions) The coating has a thickness of approximately 2 to 3 microns (about 1 ten thousandths of an inch)