The Sensitive Side Of Carbon Nanotubes: Makes Powerful Pressure Sensors

When the block is compressed, individual carbon nanotubes start to buckle, which in turn decreases the block's electrical resistance. Researchers can measure this resistance in order to determine precisely how much pressure is being placed on the blockBlocks of carbon nanotubes can be used to create effective and powerful pressure sensors, according to a new study by researchers at Rensselaer Polytechnic Institute.

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Taking advantage of the material's unique electrical and mechanical properties, researchers repeatedly squeezed a 3-millimeter nanotube block and discovered it was highly suitable for potential applications as a pressure sensor. No matter how many times or how hard they squeezed the block, it exhibited a constant, linear relationship between how much force was applied and electrical resistance.

"Because of the linear relationship between load and stress, it can be a very good pressure sensor," said Subbalakshmi Sreekala, a postdoctoral researcher at Rensselaer and author of the study.

A sensor incorporating the carbon nanotube block would be able to detect very slight weight changes and would be beneficial in any number of practical and industrial applications, Sreekala said. Two potential applications are a pressure gauge to check the air pressure of automobile tires, and a microelectromechanical pressure sensor that could be used in semiconductor manufacturing equipment.

Despite extensive research over the past decade into the mechanical properties of carbon nanotube structures, this study is the first to explore and document the material's strain-resistance relationship. The paper, titled "Effects of compressive strains on electrical conductivities of a macroscale carbon nanotube block," was published in a recent issue of Applied Physics Letters.

Over the course of the experiment, the researchers placed the carbon nanotube block in a vice-like machine and applied different levels of stress. They took note of the stress applied and measured the corresponding strain put on the nanotube block. As it was being squeezed, the researchers also sent an electrical charge through the block and measured its resistance, or how easily the charge moved from one end of the block to the other.

The research team discovered that the strain they applied to the block had a linear relationship with the block's electrical resistance. The more they squeezed the block, the more its resistance decreased. On a graph, the relationship is represented by a neat, straight line. This means every time one exposes the block to a load of X, they can reliably expect the block's resistance to decrease by Y.

This reliability and predictability of this relationship makes the carbon nanotube block an ideal material for creating a highly sensitive pressure sensor, Sreekala said.

The pressure sensor would function similarly to a typical weight scale. By placing an object with an unknown weight onto the carbon nanotube block, the block would be squeezed down and its electrical resistance would decrease. The sensor would then send an electrical charge through the nanotube block, and register the resistance. The exact weight of the object could then be easily calculated, thanks to the linear, unchanging relationship between the block's strain and resistance.

A study published earlier this year, written by Rensselaer senior research specialist Victor Pushparaj, who is also an author of the pressure sensor paper, showed that carbon nanotubes are able to withstand repeated stress yet retain their structural and mechanical integrity. Electrical resistance decreases as the block is squeezed, as the charged electrons have more pathways to move from one end of the block to the other.

In the new study, Sreekala and the research team found that the nanotube block's linear strain-resistance relationship holds true until the block is squeezed to 65 percent of its original height. Beyond that, the block's mechanical properties begin to fail and the linear relationship breaks down.

The team is currently thinking of ways to boost the nanotubes' strength by mixing them with polymer composites, to make a new material with a longer-lived strain-resistance relationship.

"The challenge will be to choose the correct polymer so we don't lose efficiency, but retain the same response in all directions," Sreekala said.

Technorati : Carbon Nanotube, Nanotechnology

Three Atomic Nuclei Created;Super-heavy Aluminum Isotopes May Exist

Researchers at Michigan State University's National Superconducting Cyclotron Laboratory, NSCL, have created three never-before-observed isotopes of magnesium and aluminum. The results not only stake out new territory on the nuclear landscape, but also suggest that variants of everyday elements might exist that are heavier than current scientific models predict.

It's been a longstanding project since the beginning of nuclear science to establish what isotopes can exist in nature," said Dave Morrissey, University Distinguished Professor of chemistry and one of the paper's authors. "This result suggests that the limit of stability of matter may be further out than previously expected; really, it shows how much mystery remains about atomic nuclei."

Particles that comprise atomic nuclei, protons and neutrons, are held together by the nuclear force. One of the four fundamental forces that collectively describe the interactions of all matter in the cosmos, the nuclear force, has been the subject of scientific inquiry since the 1930s.

Despite much progress in nuclear physics during the subsequent decades, understanding of how the nuclear force and other effects play out inside nuclei is far from complete. For example, even today scientists aren't sure exactly what combinations of protons and neutrons can make up most atomic nuclei.

One way experimental nuclear physicists explore this issue is by using accelerator facilities to create reactions that, in effect, kluge together piles of protons. An element is defined by its number of protons. For example, hydrogen has one proton; helium, two protons; oxygen eight protons, uranium, 92 protons. Whenever physicists establish a new proton limit, they invariably garner attention for conjuring new elements. In October 2006, a team of Russian and American scientists generated worldwide headlines for creating an element with 118 protons, the most protons ever recorded in a single nucleus.

Another way to probe nuclear stability is to see how many neutrons can be loaded onto nuclei of more quotidian elements, which is the focus of much of the work at NSCL. Elements can exist as different isotopes, which contain the same number of protons but different numbers of neutrons. As an example, the most abundant stable isotope of carbon has six protons and six neutrons. However, trace amounts of carbon-13 and carbon-14 -- with seven and eight neutrons respectively -- also can be found on Earth.

The neutron-limit, referred to as the neutron-dripline, is a basic property of matter. Yet remarkably, despite more than a half-century of inquiry, scientists know the dripline location only for the eight lightest elements, hydrogen to oxygen. So one very basic question -- what's the heaviest isotope of a given element that can exist" -- remains unanswered for all but eight of the hundred or so elements on the Periodic Table.

In an experiment that ran earlier this year at NSCL, researchers successfully created and detected three new super-heavy isotopes of magnesium and aluminum: magnesium-40, with 12 protons and 28 neutrons; aluminum-42, 13 protons and 29 neutrons; and aluminum-43, 13 protons and 30 neutrons. If the everyday version of aluminum were a 160-pound adult, aluminum-43 would be a muscular, 255-pound heavyweight.

"Evidence of particle stability for magnesium-40 obtained at NSCL is a major step in the field of rare isotope physics," said Hiro Sakurai, chief scientist at RIKEN in Japan, who was not involved in the research. The RIKEN research institute in Saitama, Japan, is home to the world's most powerful accelerator facility for creating radioisotope beams.

The fleeting appearance of these three nuclear newcomers is significant for several scientific and technical reasons.

First, when is comes to magnesium, the results indicate that the dripline extends at least as far as, and possibly beyond, magnesium-40. The isotope wasn't detected in several dripline-focused experiments conducted around the world since 1997 and the research community had begun to suspect that it was beyond the bounds of stability. Though it's difficult to compare across disciplines, physicists' success in detecting three magnesium-40 isotopes in the course of an 11-day experiment is roughly similar to the achievement of biologists who finally snap an image of an elusive and thought-to-be-extinct animal after years of traipsing through the jungle.

"The discovery of the hitherto unknown heaviest magnesium and aluminum isotopes at NSCL is a milestone in rare isotope research and is a great accomplishment for the worldwide scientific community exploring unstable nuclei close to the so-called neutron dripline," said Horst Stocker, director of Gesellschaft fur Schwerionenforschung, GSI, who was not involved in the research. Darmstadt, Germany-based GSI is one of the world's top accelerator facilities for producing heavy-ion beams for research.

Second, aside from being a similarly interesting outlier, aluminum-42 carries added importance since it is a near-dripline nucleus with an odd number of neutrons. Isotopes of lighter elements that toe the edge of existence generally have even numbers of neutrons due to the fact that neutrons naturally pair up inside nuclei. With an even number of neutrons, the nuclei in effect have a tidy, complete set of such pairs that collectively form a sort of energetic scaffolding that increases stability.

According to one of the leading theoretical models, aluminum-42 shouldn't exist. That it does suggests that the dripline may in fact tilt in the direction of more novel, neutron-rich isotopes, an implication that will help to extend nuclear theory and point the way to future experiments.

The NSCL result "alters the landscape of known nuclei, it alters our understanding of the forces that bind nuclei into stable objects, and it has important implications for future attempts with next-generation facilities to map the evolution of nuclear structure and existence into the most weakly bound nuclei," said Rick Casten, D. Allan Bromley Professor of Physics at Yale University, also not involved in the research.

The experimental technique itself also is noteworthy. Creating and measuring rare isotopes is always needle-in-a-haystack work that requires researchers to hunt for a few desired nuclei from a swarm of fast-moving and mostly known and therefore less interesting particles. But in this experiment, NSCL researchers achieved a hundred- to thousand-fold boost in their ability to filter out what can be thought of as junk. They did so by essentially jury-rigging the facility to filter the beam twice. The result was an ability to detect and measure isotopes so rare that they represent less than one in every million billion particles that passed by the detectors.

The dual filtering process, more properly known as two-stage separation, is a fixture in most new and planned facilities for rare isotope beam research, including the proposed upgrade of NSCL. This experiment marks one of the first uses of two-stage separation in the world and the first time the technique has been tried at NSCL, which typically filters and purifies particles only once in its A1900 separator.

NSCL detectors returned just one blip of data consistent with the existence of aluminum-43. This generally isn't enough to count as a discovery, according to the conventions of nuclear science. However, more than 20 instances of its immediate neighbor, aluminum-42, were observed. Because of this relative abundance and the fact that, due to pairing, the 30 neutrons in aluminum-43 should prove more stable than the 29 neutrons in aluminum-42, the solitary signature of aluminum-43 etched in the data logs carries more than usual amount of credibility.

"Experiments such as these are paving the way into the new era of nuclear structure studies that technological developments are opening to investigation for the first time ever," said Yale's Casten.

The findings appear in the October 25 issue of the journal Nature.

Technorati : Atomic Nuclei, Super-heavy Aluminum Isotopes