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Remote Sensing with Soft Magnetic Alloy Strips |
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Researchers at Penn State University are developing passive sensors using strips of soft magnetic material that could be used to detect temperature, stress, corrosion, viscosity and a host of other properties. The sensors are made from the same materials that are used in antitheft tags for shops and so should prove to be inexpensive monitoring devices. ‘These materials typically cost about $100 a mile and each sensor is about an inch long,’ says Dr Craig A Grimes, associate professor of electrical engineering and a member of Penn State’s Materials Research Institute. ‘Consequently, the sensors would cost just about nothing.’ The sensor material is an amorphous ribbon of alloy that is manufactured to be softly magnetic by quick cooling - one example is an iron, molybdenum, boron, silicon alloy. The material is activated prior to use as a sensor so that it generates a characteristic ‘fingerprint’ response when stimulated by an external magnetic field. ‘These magnetoelastic thin-film sensors are the magnetic analogue of an acoustic bell,’ says Grimes. ‘When an externally applied magnetic field reaches the sensors, they ring like a bell, emitting both magnetic flux and acoustic energy with a characteristic resonant frequency’. When heated or cooled, or subjected to other changes in their immediate environment, the sensors’ response is altered, and this is detected via external analysis with an applied magnetic field. One of the key benefits of the sensors is that they operate remotely and passively, and so do not require any special built-in wiring. This means that they can be simply embedded in the material or structure that they are sensing. And because the materials are softly magnetic, the orientation of the sensors in the structure is unimportant. The sensors could have a range of different applications, with obvious uses being in roads, bridges and buildings. Sensor strips embedded in a road surface could indicate when the temperature is low enough for the application of salt to be required (or indeed, too low for the salt to do any good). In bridges and buildings, the sensors could be used to indicate a change in the stresses inside the concrete of the structure, to indicate whether, say, a building is still stable following an earthquake. In most applications, the sensors would need coating with a polymer to prevent corrosion, but uncoated sensors could also be used to monitor the extent of corrosion. Alternatively, sensors could be coated with an analyte responsive layer. The sensors can also be immersed in water and other liquids, and so could provide not only temperature readings but also information on viscosity, liquid density and surface tension. In experiments to date, the sensors have proved extremely accurate. Reporting the work in a recent issue of Applied Physics Letters, Grimes, together with Dale M Grimes, professor emeritus of electrical engineering, and Keat G Ong, postdoctoral fellow at the Materials Research Institute, say that, ‘We found the temperature response of 40 sensors to be experimentally identical.’ With such a consistent response from these simple strips, they could be finding their way into a variety of materials to sense a range of properties in the future. Grimes has a patent on the work, which was supported by NASA and the National Science Foundation. |
