Foamy Sensor Detects Explosives

Tuesday, November 29, 2011 @ 11:11 AM gHale


Graphene foam can outperform leading commercial gas sensors in detecting potentially dangerous and explosive chemicals.

The discovery opens the door for a new generation of gas sensors for bomb squads, law enforcement officials, defense organizations, and in various industrial settings.

The new sensor successfully and repeatedly measured ammonia (NH3) and nitrogen dioxide (NO2) at concentrations as small as 20 parts-per-million. Made from continuous graphene nanosheets that grow into a foam-like structure about the size of a postage stamp and thickness of felt, the sensor is flexible, rugged, and finally overcomes the shortcomings that prevented nanostructure-based gas detectors from reaching the marketplace.

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“We are very excited about this new discovery, which we think could lead to new commercial gas sensors,” said Rensselaer Polytechnic Institute Engineering Professor Nikhil Koratkar, who co-led the study along with Professor Hui-Ming Cheng at the Shenyang National Laboratory for Materials Science at the Chinese Academy of Sciences. “So far, the sensors have shown to be significantly more sensitive at detecting ammonia and nitrogen dioxide at room temperature than the commercial gas detectors on the market today.” Click here to review the paper, entitled “High Sensitivity Gas Detection Using a Macroscopic Three-Dimensional Graphene Foam Network.” There is also a video of Koratkar talking about this research.

Over the past decade researchers knew individual nanostructures were extremely sensitive to chemicals and different gases. To build and operate a device using an individual nanostructure for gas detection, however, has proven to be far too complex, expensive, and unreliable to be commercially viable, Koratkar said. Such an endeavor would involve creating and manipulating the position of the individual nanostructure, locating it using microscopy, using lithography to apply gold contacts, followed by other slow, costly steps. Embedded within a handheld device, such a single nanostructure can suffer damage and become inoperable. Additionally, it can be challenging to “clean” the detected gas from the single nanostructure.

The new postage stamp-sized structure developed by Koratkar has all of the same properties as an individual nanostructure, but is much easier to work with because of its large, macroscale size. Koratkar’s collaborators at the Chinese Academy of Sciences grew graphene on a structure of nickel foam. After removing the nickel foam, what’s left is a large, free-standing network of foam-like graphene. Essentially a single layer of the graphite found commonly in our pencils or the charcoal we burn on our barbeques, graphene is an atom-thick sheet of carbon atoms arranged like a nanoscale chicken-wire fence. The walls of the foam-like graphene sensor consist of continuous graphene sheets without any physical breaks or interfaces between the sheets.

Koratkar and his students developed the idea to use this graphene foam structure as a gas detector. As a result of exposing the graphene foam to air contaminated with trace amounts of ammonia or nitrogen dioxide, the researchers found the gas particles stuck, or adsorbed, to the foam’s surface. This change in surface chemistry has a distinct impact upon the electrical resistance of the graphene. Measuring this change in resistance is the mechanism by which the sensor can detect different gases.

Additionally, the graphene foam gas detector is very convenient to clean. By applying a ~100 milliampere current through the graphene structure, Koratkar’s team was able to heat the graphene foam enough to unattach, or desorb, all of the adsorbed gas particles. This cleaning mechanism has no impact on the graphene foam’s ability to detect gases, which means the detection process is fully reversible and a device based on this new technology would be low power — no need for external heaters to clean the foam — and reusable.

Koratkar chose ammonia as a test gas to demonstrate the proof-of-concept for this new detector. Ammonium nitrate is present in quite a few explosives and can gradually decompose and release trace amounts of ammonia. As a result, ammonia detectors often see use to test for the presence of an explosive. A toxic gas, ammonia also goes in a variety of industrial and medical processes, for which detectors are necessary to monitor for leaks.

Results of the study show the new graphene foam structure detected ammonia at 1,000 parts-per-million in 5 to 10 minutes at room temperature and atmospheric pressure. The accompanying change in the graphene’s electrical resistance was about 30 percent. This compared favorably to commercially available conducting polymer sensors, which undergo a 30 percent resistance change in 5 to 10 minutes when exposed to 10,000 parts-per-million of ammonia.

In the same time frame and with the same change in resistance, the graphene foam detector was 10 times as sensitive. The graphene foam detector’s sensitivity is effective down to 20 parts-per-million, much lower than the commercially available devices. Additionally, many of the commercially available devices require high power consumption since they provide adequate sensitivity only at high temperatures, whereas the graphene foam detector operates at room temperature.

Koratkar’s team used nitrogen dioxide as the second test gas. Different explosives including nitrocellulose gradually degrade, and can produce nitrogen dioxide gas as a byproduct. As a result, nitrogen dioxide also can be a marker when testing for explosives. Additionally, nitrogen dioxide is a common pollutant found in combustion and auto emissions. Many different environmental monitoring systems feature real-time nitrogen dioxide detection.

The new graphene foam sensor detected nitrogen dioxide at 100 parts-per-million by a 10 percent resistance change in 5 to 10 minutes at room temperature and atmospheric pressure. It showed to be 10 times more sensitive than commercial conducting polymer sensors, which typically detect nitrogen dioxide at 1,000 part-per-million in the same time and with the same resistance chance at room temperature. Other nitrogen dioxide detectors available today require high power consumption and high temperatures to provide adequate sensitivity. The graphene foam sensor can detect nitrogen dioxide down to 20 parts-per-million at room temperature.

“We see this as the first practical nanostructure-based gas detector that’s viable for commercialization,” said Koratkar, a professor in the Department of Mechanical, Aerospace, and Nuclear Engineering at Rensselaer. “Our results show the graphene foam is able to detect ammonia and nitrogen dioxide at a concentration that is an order of magnitude lower than commercial gas detectors on the market today.”

The graphene foam can detect different gases beyond ammonia and nitrogen dioxide, he said.



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