Magnetostriction (cf. electrostriction) is a property of ferromagnetic materials that causes them to change their shape or dimensions during the process of magnetization. The variation of materials’ magnetization due to the applied magnetic field changes the magnetostrictive strain until reaching its saturation value, λ. The effect was first identified in 1842 by James Joule when observing a sample of iron. This effect causes energy loss due to frictional heating in susceptible ferromagnetic cores. The effect is also responsible for the low-pitched humming sound. You can hear this sound coming from transformers. On the other hand, oscillating AC currents produce a changing magnetic field.
The principle of Magnetostrictive transducers
Magnetostrictive transducers work on the principle that iron-rich metals expand and contract in a magnetic field. Thin plates of these metals stack up to make a core. Copper wire is then wrapped cylindrically around the core, and the assembly is placed in a canister. When an electrical current goes through the copper coil, it creates a magnetic field, and the core changes shape. When you turn off the electrical current, the coil returns to its original shape. This expansion and contraction cause the device’s canister to resonate and generate ultrasonic waves.
They are remarkably durable, though its overall competitive advantages are quickly decreasing as the technology around piezoelectric transducers evolves and improves. The use of iron-based metals means there is no degradation over time; metals like nickel maintain their magnetostrictive properties on a constant level for the lifetime of the ultrasonic cleaning unit. The direct weld connects the metal stack and the unit’s diaphragm ensuring the bond will never loosen. Also, diaphragms used with this type of transducer are usually 5 mm thick or more, eliminating the risk of cavitation erosion wear-through.
While magnetostrictive transducers have a higher mass and can drive more power into the tank than piezoelectric transducers, their use is limited. They can only generate a frequency between 22 and 30 kHz. So, its use is restricted to cleaning applications where the parts are large, the contaminants are difficult to remove, and complete removal of microscopic particles isn’t required. Their energy generation process has three steps in energy conversion—electric energy to magnetic energy to mechanical energy—making it less efficient overall.