Nanotechnology Helps Scientists Make Bendy Sensors for Hydrogen Vehicles
In recent years, Americans have been intrigued by the promise of hydrogen-powered vehicles. But experts have judged that several technology problems must be resolved before they are more than a novelty.
Recently, scientists at the US Department of Energy's Argonne National Laboratory have used their insights into nanomaterials to create bendy hydrogen sensors, which are at the heart of hydrogen fuel cells used in hydrogen vehicles.
In comparison to previously designed hydrogen sensors, which are rigid and use expensive, pure palladium, the new sensors are bendy and use single-walled carbon nanotubes (SWNTs) to improve efficiency and reduce cost. The development of these hydrogen sensors will help to ensure economical, environmental and societal safety, as the nation is realizing the potential for a more hydrogen-based economy.
Yugang Sun and H. Hau Wang, researchers in Argonne's Center for Nanoscale Materials and Materials Science Division, respectively, fabricated the new sensing devices using a two-step process separated by high and low temperatures. First, at around 900°C, researchers grow SWNTs on a silicon substrate using chemical vapor deposition. Then, researchers transfer the SWNTs onto a plastic substrate at temperatures lower than 150°C using a technique called dry transfer printing.
This precise process is what allows the film of nanotubes to form on the plastic, after which the palladium nanoparticles can be deposited on the SWNTs to make the sensors. The palladium nanoparticles play an important role in increasing the interaction between hydrogen and the SWNTs to enhance the change of resistance of the device when it is exposed to hydrogen molecules.
According to Sun, these sensors exhibit excellent sensing performance in terms of high sensitivity, fast response time and quick recovery, and the use of plastic sheets reduces their overall weight and increases their mechanical flexibility and shock resistance. The sensors are also able to be wrapped around curved surfaces, and this proves useful in many applications, notably in vehicles, aircraft and portable electronics.
“The leakage of hydrogen caused by tiny pinholes in the pipe of a space shuttle, for example, could not be easily detected by individual rigid detectors because the locations of pinholes are not predetermined,” said Sun. “However, laminating a dense array of flexible sensors on the surfaces of the pipe can detect any hydrogen leakage prior to diffusion to alert control units to take action.”
Flexible hydrogen sensors show a change of 75 percent in their resistance when exposed to hydrogen at a concentration of 0.05 percent in air. The devices can detect the presence of 1 percent hydrogen at room temperature in 3 seconds. Even after bending, with a bending radius of approximately 7.5 mm, and relaxing 2,000 times, the devices still perform with as much effectiveness.
Tightly Packed Molecules Lend Unexpected Strength to Nanothin Sheet of Material
Scientists at the University of Chicago and Argonne National Laboratory have discovered the surprising strength of a sheet of nanoparticles that is merely 50 atoms thick.
“It's an amazing little marvel,” said Heinrich Jaeger, professor in physics at the University of Chicago. “This is not a very fragile layer, but rather a robust, resilient membrane.”
Even when suspended over a tiny hole and poked with an ultrafine tip, the membrane boasts the equivalent strength of an ultrathin sheet of plexiglass that maintains its structural integrity at relatively high temperatures.
“When we first realized that they can be suspended freely in air, it truly surprised all of us,” said Xiao-Min Lin, a physicist at Argonne's Center for Nanoscale Materials.
The material's characteristics make it a promising candidate for use as a highly sensitive pressure sensor in precision technological applications. "If we use different types of nanoparticles to make the same kind of suspended membrane, we can even imagine using these devices as chemical filters to promote catalytic reactions on a very small length scale," Lin said.
As artificial atoms, the nanoparticles might also serve as building blocks in assembling specially designed nano-objects. “This is the ultimate limit of such a solid. It's just one layer,” Jaeger said. “What is interesting is that already one layer is so resilient and has these interesting properties.”
But the payoff is scientific as well as technological. Scientists had already discovered that the electronic properties of semiconductor material can change dramatically when its tiniest metallic components are tightly packed between organic molecules, a phenomenon called nano-confinement. “But, now we find that mechanical properties can also change dramatically. On a basic science level, that's why this is exciting,” Jaeger said.
The experimental material consisted of gold particles separated by organic "bumpers" to keep them from coming into direct contact. The research team suspended this array of nanoparticles in a solution, and then spread the solution across a small chip of silicon, a popular semiconductor material. When the solution dried, it left behind a blanket of nanoparticles that drape themselves over holes in the chip, each hole measuring hundreds of nanoparticles in diameter. Then the researchers probed the strength of the freely suspended nanoparticle layer by poking it with the tip of an atomic force microscope.
Plexiglass draws its strength from the nature of its polymers, long chains of molecules that become entangled with one another. But the short-chain polymers the research group used to link the nanoparticles were scarcely long enough to qualify as polymers at all.
“They probably do not have the chance to entangle like a ‘card-carrying’ polymer would do,” Jaeger said. “The molecules are anchored to the gold particles, but only on one end. The strength comes from compressing them between the gold particles.” The research team also found that the material held together when heated until reaching temperatures of 210 degrees and higher.
While the Chicago-Argonne experiments focused on two-dimensional sheets, they generally agree with computer simulations on similar three-dimensional assemblies of smaller nanoparticles conducted by Uzi Landman's team at the Georgia Institute of Technology.
“The behavior of these systems is sensitive to dimensionality, and this is a subject that should be explored in the future,” said Landman, the Fuller Callaway Chair in Computational Materials Science at the Georgia Institute of Technology. “This actually brings another control parameter into question. Change the dimensionality, you change the properties.”
Flexible Electronics Could Find Applications as Sensors, Artificial Muscles
Flexible electronic structures with the potential to bend, expand and manipulate electronic devices are being developed by researchers at the US Department of Energy's Argonne National Laboratory and the University of Illinois at Urbana-Champaign.
These flexible structures could find useful applications as sensors and as electronic devices that can be integrated into artificial muscles or biological tissues. In addition to a biomedical impact, flexible electronics are important for energy technology as flexible and accurate sensors for hydrogen.
Argonne scientist Yugang Sun and a team of researchers at the University of Illinois led by John A. Rogers created a concept that led to the development of these structures. The concept focuses on forming single-crystalline semiconductor nanoribbons in stretchable geometrical configurations with emphasis on the materials and surface chemistries used in their fabrication and the mechanics of their response to applied strains.
“Flexible electronics are typically characterized by conducting plastic-based liquids that can be printed onto thin, bendable surfaces,” Sun said. “The objective of our work was to generate a concept along with subsequent technology that would allow for electronic wires and circuits to stretch like rubber bands and accordions leading to sensor-embedded covers for aircraft and robots and even prosthetic skin for humans. We are presently developing stretchable electronics and sensors for smart surgical gloves and hemispherical electronic eye imagers.”
The team of researchers has successfully fabricated thin ribbons of silicon and designed them to bend, stretch and compress like an accordion without losing their ability to function.
The Center for Nanoscale Materials at Argonne integrates nanoscale research with Argonne's existing capabilities in synchrotron X-ray studies, neutron-based materials research and electron microscopy with new capabilities in nanosynthesis, nanofabrication, nanomaterials characterization as well as theory and simulation.
Carbon Nanotubes Could Improve Thermal Management in Electronics
As the electronics industry continues to churn out smaller and slimmer portable devices, manufacturers have been challenged to find new ways to combat the persistent problem of thermal management.
New research suggests carbon nanotubes may soon be integrated into ever-shrinking cell phones, digital audio players and personal digital assistants to help ensure the equipment does not overheat, malfunction or fail.
The chips inside an electronic device give off heat as a byproduct of power consumption when the object is on or being used. To reduce high temperatures, heat sinks, finned devices made of conductive metal such as aluminum or copper, are attached to the back of the chips to “pull” thermal energy away from the microprocessor and transfer it into the surrounding air. Fans or fluids are sometimes used to improve the cooling process, but they increase the device weight, size and bulk.
Using microfin structures made of aligned multiwalled carbon nanotube arrays mounted to the back of silicon chips, researchers from Rensselaer Polytechnic Institute and the University of Oulu in Finland have proven that nanotubes can dissipate chip heat as effectively as copper, the best known, but most costly, material for thermal management applications. And the nanotubes are more flexible, resilient and 10 times lighter than any other cooling material available.
“As devices continue to decrease in dimension, there is a growing need for miniature on-chip thermal management applications,” said Robert Vajtai, a researcher with the Rensselaer Nanotechnology Center. “When reduced to sub-millimeter sizes, the integrity of materials typically used for cooling structures breaks down. Silicon becomes very brittle and easily shatters, while metallic structures become bendable and weak.”
Carbon nanotubes, however, maintain their impressive combination of high strength, low weight and excellent conductivity, and the carbon nanotube coolers can be manufactured very cost effectively, Vajtai said.
The researchers have developed a simple and scalable assembly, using an innovative processing and transfer technique to integrate the nanotube structures on the chip. Thick films consisting of 1.2 millimeter long multi-walled carbon nanotubes were grown and detached from silicon/silicon oxide templates, and a laser was used to carve out freestanding 10 by 10 fin array blocks. The bottom of the nanotube cooler blocks were then soldered onto the backside of a thermometer test chip that was mounted on a silicon substrate. This technique employs conventional manufacturing methods, providing an easy protocol to transfer and integrate nanotube arrays onto the silicon platforms currently used in electric circuits consisting of miniaturized components, according to the researchers.
Compared to a chip with no cooling source, 11 percent more power was dissipated from the chip mounted with the nanotube cooler. Under forced nitrogen flow, the cooling performance with the fins was improved by 19 percent.
"These numbers are consistent with the heat dissipated by the best thermal conductors, and demonstrate the possibility of a lightweight, solid-state add-on structure for an on-chip thermal management scheme which works without involving heavy metal block and fan or fluid-flow procedures for heat removal which can greatly increase the weight of electronic devices," Vajtai said.
The researchers are continuing to explore a variety of techniques to further optimize the nanotube's cooling capabilities by improving the thermal interface between the chip and the nanotube, enlarging the cooler's surface area, and perfecting the fin-array geometry.
The research is funded by the Academy of Finland, the Nokia Scholarship, and the Focus Center New York for Electronic Interconnects.
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