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Wang and colleagues used small angle X-ray diffraction and wide-angle X-ray diffraction to observe changes in the molecular structure of wurtzite crystal under pressure.

It may come as a bit of a surprise, but the strongest material in the world isn’t very strong. Subject it to ultra-high pressure, though, and graphite becomes the hardest substance known.

Most materials that transform under high pressure revert to their original structure when the pressure is lifted, losing any useful properties they may have gained when the pressure was on.

Now, by understanding the process behind the transformation itself, researchers have taken a step toward creating a new class of exceptionally strong, durable materials that maintain their high-pressure properties - including strength and superconductivity - in low-pressure environments.

Cornell  University reported that staff scientist at the Cornell High Energy Synchrotron Source Zhongwu Wang and his team focused on wurtzite, a cadmium-selenium crystal in which atoms are arranged in a diamond-like structure and molecules are bonded on the surface. When thin sheets of wurtzite are squeezed under 10.7 gigapascals of pressure, or 107,000 times the pressure on the Earth’s surface, their atomic structure transforms into a rock salt-like structure.

As pressure was applied, Wang and colleagues integrated two X-ray diffraction techniques (small- and large-angle X-ray diffraction) to characterize changes in the crystal’s surface shape and interior atomic structure, as well as the structural change of surface-bonded molecules.

They first discovered that the nanosheets required three times the pressure to undergo the transformation as the same material in larger crystal form.

And adding a bonding molecule called a soft ligand to the surface of the high-pressure nanosheets, the researchers observed the effect of that bonding to the nanosheets’ internal structure, transformation pressure, and spacing.

They also tested the material’s yield strength, hardness and elasticity during the transformation. Understanding how those properties change as the molecules interact could help researchers design stronger, tougher materials, Wang says.

“Now we know how the atoms move. We understand the intermediate procedure,” says Wang. His experimental process could hold promise for understanding the transformation pathway for other compounds as well.

The research appears in the Proceedings of the National Academy of Sciences.

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ORNL’s James Klett holds an LED streetlamp. The lamp will use heat sinks of graphite foam
(samples in his left hand) to extend the life of the LEDs and cut operating costs.

Around 1997, Oak Ridge National Lab’s James Klett and Timothy Burchell discovered how to make graphite foam, a material that had at least one amazing property: It transfers heat like crazy.

If this property of the foam seems a little counter-intuitive, that’s because foam materials are often associated with with heat insulation properties. But in this case, the foam acts as a super heat radiator. A story in an ORNL newsletter said the stuff worked so well that if you put an ice cube on a hockey puck-sized chunk of the graphite foam, and put the foam on you hand, “the cube melts from your body heat as if it were on a hot griddle.”

At the time, Klett, a researcher in the lab’s Metals and Ceramics Division, noted that, “Graphite foam is as thermally conductive as aluminum at one-fifth the weight. It has a very high surface-area-to-weight ratio and a high heat transfer coefficient. This interests engineers and designers because products that use energy wage an ongoing battle with heat,” he says.

He said the key to the foam’s conductivity is its unusual graphite crystal structure that is full of air pockets, making it only 25% dense and lightweight. A network of graphite “ligaments” in the foam wicks heat away from its source.

Klett shows that ice held against the graphite foam will melt quickly because the heat from the hand holding the foam is transferred rapidly through the foam. As a result, this hand feels the cold fast.

When they made their discovery, Klett and Burchell were building on a legacy of carbon innovations that go back to at least the 1960s when Johhn Googin developed the first method to produce carbon foams was used as high-temperature furnace insulation. Klett and Burchell also developed a commercial carbon-carbon disk brakes system.

Over the past decade, Klett, Burchell and ORNL have licensed the special foam for numerous applications – especially with mechanical and electronic heat-transfer applications – and the material garnered an R&D 100 award.

Now, the foam’s ability to act as an efficient heat sink is being put to new uses in the world of energy-efficient lighting. On Friday, ORNL announced that it has licensed the foam to LED North America for use in commercial LED lighting systems such as in the large arrays now being manufactured for street lamps and parking garages.

The lab says passive cooling materials, such as the foam, are needed to increase LED efficiency and lifetime. ORNL reports that each 10° decrease in temperature can double the life of the lighting components. “While this technology will reduce temperatures and increase the life of the LED lighting systems, what it will really do is save municipalities millions of dollars every year in replacement fixture costs as well as maintenance,” Klett said.

Besides being lightweight, Klett says the foam is easy to machine and use in manufacturing. These advantages give it a growing edge compared to traditional heat transfer materials, such as copper or aluminum.

LED North America president Andrew Wilhelm predicts that the foam will double the life of the LED units. He also says the first lamps using the foam will be installed later this year in an ORNL parking lot.

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Scientists worldwide are probably hitting their heads wondering, “Why didn’t I think of this!”

The idea for a simple new process came in a burst of inspiration: Can a camera flash instantly heat up graphite oxide and turn it into graphene?

Researchers simply hold a consumer camera flash over the graphite oxide and, a flash later, the material is now a piece of fluffy graphene. Awesome!

Previous processes to reduce graphite oxide relied on toxic chemicals or high-temperature treatment.

The process, discovered by Jiaxing Huang, assistant professor of materials science and engineering at Northwestern’s McCormick School of Engineering and Applied Science, and his graduate student Laura Cote, was published in the Aug. 12 issue of the Journal of the American Chemical Society.

Check it out:

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