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Magnetostrictive linear position sensors with display. Image Credits: Sergey Ryzhov/shutterstock.com
Magneto-strictors are magnetic analogs of piezoelectric materials. They change dimensions whenever they become magnetized, making them intrinsically-ferromagnetic.
In a magnetostrictive material, intense magnetic fields (the saturation flux density is typically around 1T) will distort the shape of the electron orbitals to the extent that the material itself will undergo a dimensional change. This distortion can be directly related to the intensity of the magnetic field but is not dependent on field polarity.
Materials have positive magneto-striction when they expand in the direction of the magnetic field, resulting in a transverse constriction. In these materials, the magnetization can be increased by applying tensile stress. The converse applies to negatively magnetostrictive materials.
The induced strain has relatively low hysteresis (typically 2-3%), and magnetostrictive elements can exert high forces (e.g. a 10mm rod operated at 0.1% strain requires a clamping force of around 4kN to produce zero displacement). A 0.1% strain is typically about the maximum strain that can be achieved. Response times of about 1ms can also be achieved. This behavior can be used to convert electrical energy into sound energy and vice versa.
A disadvantage to these materials is the high cost of production due to the cost of raw materials such as terbium. Furthermore, the method of delivering the magnetic field for actuation is not readily compatible with embedding as in the case of composite materials. These materials are also brittle and difficult to shape.
Magnetostrictive Materials
The best known magnetostrictive material is Terfenol, a compound consisting of Terbium (Te) and iron (Fe). These elements became an inspiration for the name of the material, and researchers only added the suffix ‘NOL’ to denote the Naval Ordinance Laboratory where the material was studied. Nickel, cobalt, and iron are also known to exhibit magneto-striction as do some recently discovered rare earth elements such as dysprosium (Dy). Nickel and cobalt are negatively magnetostrictive, while iron is positively magneto-strictive in the presence of a weak magnetic field and negatively magnetostrictive when subjected to stronger magnetic fields.
Some examples of electrostrictive ceramics include lead magnesium niobate, lead titanate, and lead lanthanum zirconate titanate. These exhibit strain deformation in the presence of an electric field and hysteresis in the order of 2-3 percent. They can also achieve positional feedback of around 10nm and work at frequencies as high as 40kHz. The only disadvantage to these materials is their dependence on temperature. For example, a material designed to operate at room temperature may experience a deterioration of up to 50% at 50°C.
Key Properties
- Ability to undergo a longitudinal distortion (or strain) in response under the influence of a magnetic field
- Relatively low hysteresis
- Expensive to produce
Applications and Current Research
Studies on the use of magneto-strictors have been focused on their applicability as magnetostrictive transducers or for any sonar application. These devices can be used to convert an electrical input to vibratory mechanical energy, or conversely to convert vibratory mechanical energy into an electrical output.
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