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Smashing Silver Microcubes Toughens Up Materials

Pat Waurzyniak
By Patrick Waurzyniak Contributing Editor, SME Media
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Rice University researchers (from left) Olawale Lawal, Ramathasan Thevamaran, Edwin Thomas and Sadegh Yazdi hold clay models of deformed cubes that represent the results of their microscale experiments. The researchers smashed silver microcubes at near supersonic speeds to see how deforming their crystalline structures could make them stronger and tougher.
Photo courtesy Jeff Fitlow/Rice University

Scientists at Rice University (Houston) are smashing tiny silver cubes into a hard target in order to make these metallic microcubes ultrastrong and tough by rearranging their nanostructures upon impact.

The Rice team reported in Science magazine that firing a tiny, nearly perfect cube of silver onto a hard target turns its single-crystal microstructure into a gradient-nano-grained (GNG) structure. The experiment aimed to learn more about how materials deform under overwhelming stress, as might be experienced by a bulletproof vest or a spacecraft that encounters micrometeorites.

Creating a gradient nanostructure in materials by way of deformation will make them more ductile, the researchers believe, and therefore less likely to fail catastrophically when subsequently stressed. Ultimately, they want to develop nano-grained metals that are tougher and stronger than anything available today.

Led by materials scientist Edwin Thomas, the William and Stephanie Sick Dean of Rice’s George R. Brown School of Engineering, the team used its advanced laser-induced projectile impact test (LIPIT) rig to shoot microcubes onto a silicon target. The mechanism allowed them to be sure the cube hit the target squarely. The Thomas lab developed the LIPIT technique several years ago to fire microbullets to test the strength of polymer and graphene film materials. This time the researchers were essentially testing the bullet itself.

“The high-velocity impact generates very high pressure that far exceeds the material’s strength,” Thomas said. “This leads to high plasticity at the impact side of the cube while the top region retains its initial structure, ultimately creating a grain-size gradient along its height.”

The original projectiles needed to be as perfect as possible. That required a custom fabrication method, Thomas said. The cubes for the study were synthesized as single crystals via bottom-up seed growth to about 1.4 microns per side, about 50 times smaller than the width of a human hair. LIPIT transformed laser power into the mechanical energy that propelled the cubes toward a target at supersonic velocity. The cubes were placed on top of a thin polymer film that thermally isolated them and prevented the laser itself from deforming them. When a laser pulse hit an absorbing thin-film gold layer underneath the polymer, the laser energy caused it to vaporize. That expanded the polymer film, which launched the microcubes.

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Microscopic silver cubes were the bullets in Rice experiments to show how deformation upon impact can make materials stronger and tougher.
Photo courtesy the Thomas Group/Rice University

The distance covered was small—about 500 micrometers—but the effect was large. While the experiments were carried out at room temperature, the cube’s temperature rose by about 350ºF (177ºC) upon impact at 400 m/sec and allowed dynamic recrystallization.

The one-step Rice process makes such structures with a range of grains from about 10 to 500 nanometers over a distance of 500 nanometers. That produces a gradient at least 10 times higher than the other techniques, the researchers reported.

For more information, an abstract of the paper, published in the Oct. 21 issue of Science, is available at http://science.sciencemag.org/content/354/6310/312.

New Way of Tuning Thermal Conductivity Cuts Heat in Devices

Researchers have found an unexpected way to control the thermal conductivity of 2D materials, which will allow electronics designers to dissipate heat in electronic devices that use these materials.

To better understand the thermal conduction properties of 2D materials, a team of researchers from North Carolina State University (Raleigh, NC), the University of Illinois at Urbana-Champaign (UI) and the Toyota Research Institute of North America (TRINA) began experimenting with molybdenum disulfide (MoS2), which is a 2D material, or TMD.

2D materials have a layered structure, with each layer having strong bonds horizontally, or “in plane,” and weak bonds between the layers, or “out of plane.” The materials have unique electronic and chemical properties, and hold promise for use in creating flexible, thin, lightweight electronic devices. In most electronics applications, it’s important to be able to dissipate heat efficiently, which can be tricky, as in 2D materials heat is conducted differently in plane than it is out of plane.

For example, in one class of TMD, heat is conducted at 100 W per meter per Kelvin (W/mK) in plane, but at only 2 W/mK out of plane. That gives it a “thermal anisotropy ratio” of about 50. The researchers found that, by introducing disorder to the MoS2, they could significantly alter the thermal anisotropy ratio.

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Introducing lithium ions between layers of molybdenum sulfide can tune the thermal conductivity of the material.
Image courtesy NC State University

The researchers created this disorder by introducing lithium ions between the layers of MoS2. The presence of the lithium ions does two things simultaneously: it puts the layers of the 2D material out of alignment with each other, and it forces the MoS2 to rearrange the structure of its component atoms.

When the ratio of lithium ions to MoS2 reached 0.34, the in-plane thermal conductivity was 45 W/mK, and the out-of-plane thermal conductivity dropped to 0.4 W/mK—increasing the material’s thermal anisotropy ratio from 50 to more than 100. In other words, heat became more than twice as likely to travel in plane—along the layer, rather than between the layers. And that was as good as it got. Adding fewer lithium ions made the thermal anisotropy ratio lower. Adding more ions also made it lower. In both cases, the ratio was affected in a predictable way, meaning that the researchers could tune the material’s thermal conductivity and thermal anisotropy ratio.

“This finding was very counterintuitive,” says Jun Liu, an assistant professor of mechanical and aerospace engineering at NC State and co-corresponding author of a paper describing the work. “The conventional wisdom has been that introducing disorder to any material would decrease the thermal anisotropy ratio. But based on our observations, we feel that this approach to controlling thermal conductivity would apply not only to other TMDs, but to 2D materials more broadly.

“We set out to advance our fundamental understanding of 2D materials, and we have,” Liu adds. “But we also learned something that is likely to be of practical use for the development of technologies that make use of 2D materials.”

For more information, see an abstract of the paper, “Tuning Thermal Conductivity in Molybdenum Disulfide by Electrochemical Intercalation,” at http://www.nature.com/articles/ncomms13211 in the journal Nature Communications.

2D Material May Be Too Brittle for Flexible Electronics, Optical Use

A team of researchers at Rice University (Houston) has found that an atom-thick material under consideration for use in flexible electronics and optical devices may be more brittle than previously believed.

The team led by Rice materials scientist Jun Lou tested the tensile strength of 2D semiconducting molybdenum diselenide and discovered that flaws as small as one missing atom can initiate catastrophic cracking under strain. A link to the report in the journal Advanced Materials is available at http://onlinelibrary.wiley.com/doi/10.1002/adma.201604201/abstract.

“It turns out not all 2D crystals are equal,” said Lou, a Rice professor of materials science and nanoengineering. “Graphene is a lot more robust compared with some of the others we’re dealing with right now, like this molybdenum diselenide. We think it has something to do with defects inherent to these materials.”

The finding may cause industry to look more carefully at the properties of 2D materials before incorporating them in new technologies, he said.

These defects could be as small as a single atom that leaves a vacancy in the crystalline structure, he said.

“It’s very hard to detect them,” he said. “Even if a cluster of vacancies makes a bigger hole, it’s difficult to find using any technique. It might be possible to see them with a transmission electron microscope, but that would be so labor-intensive that it wouldn’t be useful.”

Molybdenum diselenide is a dichalcogenide, a 2D semiconducting material that appears as a graphene-like hexagonal array from above but is actually a sandwich of metallic atoms between two layers of chalcogen atoms, in this case, selenium. Molybdenum diselenide is being considered for use as transistors and in next-generation solar cells, photodetectors and catalysts as well as electronic and optical devices.

Lou and colleagues measured the material’s elastic modulus, the amount of stretching a material can handle and still return to its initial state, at 177.2 (plus or minus 9.3) gigapascals. Graphene is more than five times as elastic. They attributed the large variation to pre-existing flaws of between 3.6 and 77.5 nanometers. Its fracture strength, the amount of stretching a material can handle before breaking, was measured at 4.8 (plus or minus 2.9) gigapascals. Graphene is nearly 25 times stronger.

“The important message of this work is the brittle nature of these materials,” Lou said. “A lot of people are thinking about using 2D crystals because they’re inherently thin. They’re thinking about flexible electronics because they are semiconductors and their theoretical elastic strain should be very high. According to our calculations, they can be stretched up to 10%. But in reality, because of the inherent defects, you rarely can achieve that much strain. The samples we have tested so far broke at 2-3% [of the theoretical maximum] at most,” Lou said. “That should still be fine for most flexible applications, but unless they find a way to quench the defects, it will be very hard to achieve the theoretical limits.”

Co-authors of the paper include Rice graduate students Emily Hacopian, Weibing Chen, Jing Zhang and Bo Li, alumna Yongji Gong and Pulickel Ajayan, chair of Rice’s Department of Materials Science and NanoEngineering, the Benjamin M. and Mary Greenwood Anderson Professor in Engineering and a professor of chemistry; Xing Li of Rice and Peking University, China; Minru Wen of Tsinghua University, China, and the Georgia Institute of Technology; Wu Zhou of Oak Ridge National Laboratory.

The research was supported by the Air Force Office of Scientific Research, the Welch Foundation, the Department of Energy Office of Basic Energy Sciences, the National Science Foundation and the National Science Foundation of China.

Edited by Senior Editor Patrick Waurzyniak.

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