Passive sensors remotely monitor temperature and stress

The same material that makes the theft detectors go off in a department store when the salesperson forgets to remove the anti-theft tag, may make inexpensive, passive temperature and stress sensors for highways, concrete buildings and other applications possible, according to Penn State researchers.

“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 member of Penn State’s Materials Research Institute. “Consequently, the sensors would cost just about nothing, or about a half cent apiece.”

The material used in these sensors 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. Magnetically soft materials have no strong fixed magnetic fields, even though they contain iron. In magnetically soft materials, the magnetic field switches back and forth depending on the environment and can generate many higher order harmonic frequencies.

“These magnetoelastic thin-film sensors are the magnetic analog 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.” Just as a bell changes pitch and overtones when heated or cooled, the magnetoelastic thin-film changes magnetic response.

When a customer walks out of a department store with an anti-theft device still on a purchase, these metal strips set off an alarm because the sensors at the door sense the soft magnetic field. To use these strips as temperature and stress sensors, an activator must be passed near the sensor strips. Because the sensors operate passively and remotely, there are no wires or connectors required, so the sensors are simply and even randomly imbedded in the material to be sensed.

A simple loop that generates a magnetic field activates the sensor from a distance. This magnetic field is not blocked by the materials in the road surface or concrete and is not altered by any iron, such as rebar in construction concrete. Rebar does not have the magnetic properties needed to support the higher frequency harmonics. A figure-eight loop senses the strips’ response, reading the harmonics of the strips magnetic field. These harmonics are like the overtones of the bell and change as the environment of the strip changes.

On roadways, sensor strips embedded in the road surface could indicate when temperatures are low enough for salt application, but not too low for the salt to do any good. In the case of buildings involved in earthquakes or other structurally altering events, the sensors can indicate a change in the stresses inside the concrete and help to determine if the building is safe for occupancy.

The strips need to be coated with a polymer to avoid corrosion, although if corrosion is the property to be sensed, then the strips should be either left uncoated to corrode or coated with an analyte responsive layer.

In a recent issue of Applied Physics Letters, Grimes, with Dale M. Grimes, professor emeritus of electrical engineering, and Keat G. Ong, postdoctoral fellow at Materials Research Institute, say that “we found the temperature response of 40 sensors to be experimentally identical.” These simple sensor strips provide a consistent temperature reading.

Because the sensors are softly magnetic, their orientation in the materials relative to the activatoris unimportant.

These sensors can also be immersed in water or other liquids and provide not only temperature but also viscosity, liquid density and surface tension. Because the sensors are so inexpensive, their use in a wide variety of materials, sensing a wide number of properties, may be possible in the future.

Grimes has a patent on this work, which was supported by NASA and National Science Foundation.

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