A Spanish research institution has developed a multifunctional sensor device based on a multilayer magnetic microwire. This microwire is basically made of four layers: a metallic ferromagnetic soft nucleus, a glass coating layer over which a nanometric layer of gold is sputtered, followed by electrodeposition of a layer of magnetic material.
The use of this devise as multifunctional sensor is based on magneto-elastic coupling, which is manifest in the magnetic layers as a consequence of the presence of an external stress, and which depends on the composition of the various layers of the microwire.
This multifunctional sensor device can be used as:
- Magnetic sensor, when the microwire is mounted onto a rigid support and is able to detect very small changes in the external magnetic field. This is due to the magneto-elastic coupling between the external layers and the nucleus, since the difference in magnetostriction (properties of a material to change its dimension under the action of an applied external field) between the outer layer and the inner nucleus will result in stress build-up in the latter and therefore a different output signal.
- Temperature sensor, when an external change of temperature will induce stresses in the nucleus because of the difference in thermal expansion coefficient between the inner nucleus and the outer core. The output signal therefore modified.
- Position sensor based on the effect of the Earth’s magnetic field, the value of which varies according to the orientation of the sample, and its action on the microwire produces a variable output from the magnetic nucleus. Also in this case the sample is fixed onto a rigid support so that the change in the magnetic properties of the nucleus is a direct consequence of the change in position only.
The stress exerted onto the nucleus by the presence of an external applied force modifies the output signal of the nucleus. The stress can be tensile, compressive, bending or torsion. In the first two cases the microwire is mounted onto a rigid support, while in the second case a flexible set-up is used, allowing the measurement of the pressure generated by the flow of a fluid (liquid or gaseous) or the curvature of a sensor element. For the detection of the tension and/or torsion, two small rigid supports, placed at the ends of the microwire and leaving the wire length free, are needed.
The introduction of an additional layer in the microware makes it possible to be used in other applications. For instance, when the external outer shell includes an additional layer of thermochromic material, which, in presence of light, would generate heat in the microwire, the output will be modified by the stresses generated on the nucleus by the difference in thermal expansion coefficient of the various layers. Similarly, the addition of an organic layer, sensible to the external condition, (such as the presence of CO, gas, or the presence of particular elements in a liquid) will induce stresses in the metallic nucleus because of the linear changes of this layer as a consequence of absorption or reaction with the selected element, thus making it suitable as multifunctional chemical sensor in the detection of smokes, and various kinds of gases.
This technological offer includes a new concept of sensor by using a novel element: a multilayer magnetic microwire. This new kind of microwire can act as a sensor itself, and its reduce dimensions allow the development of different kind of sensors in miniaturised devices.
The sensor device is multifunctional, this being an important advantage of this device over other commercial counterparts. Other advantages of the multifunctional sensor reside in its low cost, high sensitivity, fast response time, and easy integration of the element in miniaturised devices due to its small mass and reduced dimensions.