David Kirkpatrick

September 8, 2010

Graphene research may lead to electronics improvement

A fairly radical improvement. Try highly efficient, very-low-heat producing and smaller electronics devices. I enjoy blogging about nanotech research with real promise for market applications.

From the link:

NIST recently constructed the world’s most powerful and stable scanning-probe microscope, with an unprecedented combination of low temperature (as low as 10 millikelvin, or 10 thousandths of a degree above absolute zero), ultra-high vacuum and high . In the first measurements made with this instrument, the team has used its power to resolve the finest differences in the electron energies in graphene, atom-by-atom.

“Going to this resolution allows you to see new physics,” said Young Jae Song, a postdoctoral researcher who helped develop the instrument at NIST and make these first measurements.

And the new physics the team saw raises a few more questions about how the electrons behave in graphene than it answers.

Because of the geometry and electromagnetic properties of graphene’s structure, an electron in any given energy level populates four possible sublevels, called a “quartet.” Theorists have predicted that this quartet of levels would split into different energies when immersed in a magnetic field, but until recently there had not been an instrument sensitive enough to resolve these differences.

“When we increased the magnetic field at extreme low temperatures, we observed unexpectedly complex quantum behavior of the electrons,” said NIST Fellow Joseph Stroscio.

What is happening, according to Stroscio, appears to be a “many-body effect” in which electrons interact strongly with one another in ways that affect their energy levels.

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…

Click here for more information.

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…

Click here for more information.

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.

August 12, 2008

Green roofs

I might as well stick with a theme here and cover a release on “green roofs.” Green roofing is vegetated, that is covered in plants, to help insulate buildings and collect water.

Certainly a specialty item in the various energy efficiency options, but not quite as hippie-fied as it might seem at first glance. I’m not going to check the specs, but I bet a green roof gets a building LEED points.

This is a release out of the University of Texas at Austin and the Lady Bird Johnson Wildflower Center. The project was sponsored by the City of Austin, Roof Consultants Institute Foundation, Austin Energy, TBG Partners and involved labor donated by local roof contractors.

The release:

Green Roofs Differ in Building Cooling, Water Handling Capabilities

July 28, 2008

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AUSTIN, Texas — The first study to compare the performance of different types of green roofs has been completed by the Lady Bird Johnson Wildflower Center at The University of Texas at Austin and suggests that buyers shouldn’t assume these roofs are created equal.

 

Interest in vegetated roofs has increased as water and energy conservation becomes more important to property owners. Yet the study of six different manufacturers’ products found the green roofs varied greatly in capabilities such as how much they cooled down a building’s interior and how much rainwater they captured during downpours.

“Just having a green roof may not mean anything in terms of preventing water from reaching the street level, for instance,” said Dr. Mark Simmons, a center ecologist and the lead investigator on the study. “Green roofs have to be done right, and our hope is to help manufacturers understand how to improve their designs.”

Simmons and collaborators published their findings online Friday on the Web site of the journal Urban Ecosystems. The researchers will continue to collect real-time temperature and other data from the study.

Wildflower Center staff designed the first commercial green roof in Austin at the Escarpment Village Starbuck’s. Simmons, center colleagues and Brian Gardiner from Austech Roof Consultants Inc. simulated green roof conditions by studying the manufacturers’ roofs atop metal insulated boxes. The study of 24 experimental roof tops during fall 2006 and spring 2007 at the center suggested a green roof could reduce a building’s air conditioning bills about 21 percent compared to traditional, tar-based black-top roofs.

During one 91-degree day of the study, for example, a black topped box without air conditioning reached 129 degrees inside. Meanwhile, the green roof replicas produced indoor temperatures of 97 to 100 degrees Fahrenheit.

“That’s a huge difference to have a 20-or-so degree temperature drop,” Simmons said, noting that green roofs’ temperature-lowering capabilities are also believed to double the lifespan of roofing material.

An even greater temperature difference was found on roof surfaces, where black-top roofs reached 154 degrees Fahrenheit on that 91-degree day. By comparison, the soil temperature of the green roofs was between 88 and 100 degrees Fahrenheit.

Part of the rooftop differences, Simmons noted, resulted from the native plants used on the green roofs. Each had 16 different types of plants native to Texas in a similar arrangement as part of this first-ever study of their use on green roofs. The study didn’t directly measure their cooling impact. However, plants cool surfaces by providing shade, and by shedding water to cool down, like humans do by sweating.

States such as Texas that experience flash flooding may benefit even more from the ability of green roofs to capture water, lessening runoff onto streets and storm drains. Yet this feature varied the most among the six manufacturers. The better green roofs retained all of the water during a ½-inch rainfall, and just under half the water when 2 inches of rain fell. Some roofs, however, only retained about a quarter of the water in a light, ½-inch rain and as little as 8 percent during deluges.

The presence of native plants likely helped all the green roofs capture water better. In comparison to sedums, a type of succulents traditionally used on most green roofs, native plants can take in more water and release more of it to the atmosphere. The center will study these factors in future green roof research.

Regardless of those findings, Simmons doesn’t expect to be giving blanket recommendations about green roof manufacturers because of the variability in their products. That variability is the reason that some of the green roofs in the study that captured water well didn’t have the best plant growth, for example.

“After you choose a manufacturer, tell them what kind of plants and what other features you want,” Simmons said. “It’s up to them to then tailor the green roof to your needs.”

Note: This project was sponsored by the City of Austin, Roof Consultants Institute Foundation, Austin Energy, TBG Partners, and involved labor donated by local roof contractors. Learn more about the Lady Bird Johnson Wildflower Center.

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