Remotely queried magnetoacoustic sensors for monitoring starch concentrations, pH, and polymer curing

Although acoustic wave sensors that use piezoelectricity have many advantages, all of them need electrical connections to excite the piezoelectric crystals with an alternating voltage. This paper presents a new type of continuously operating, in-situ, and remotely monitored sensor that doesn't require electrical connections. The new sensor is comprised of a magnetoelastic metallic glass ribbon. Its sensing principle is similar to acoustic wave sensors. An externally applied alternating current (ac) magnetic field is used to excite magnetoelastic waves inside the magnetoelastic thin ribbon. Frequency responses are monitored with a pickup coil located outside the test area and the resonant frequencies are measured. The sensor responds to mass loading as a microbalance by decreasing its resonant frequency. When immersed in liquid, its resonant frequency is correlated with the square root of the product of liquid viscosity and density. We studied the relationship between the frequency shift and the square root of the product of viscosity and density of a starch solution. We found that the frequency shift was linearly proportional to the starch concentration. After bonding a poly-hydroxyethyl acrylate (poly-HEA) membrane to the magnetoelastic ribbon, the sensitivity of the sensor to water loading was greatly increased. The new sensor has also been used to monitor polymer curing. After bonding the ribbon with a pH sensing membrane, it was used to monitor pH. Because the sensor does not require electrical connections, it can remotely monitor concentrations in situ in a sealed container.