Better energy harvesting with Galfenol

Better energy harvesting with Galfenol

Galfenol, composed primarily of gallium and iron, could be the next big thing for harvesting energy from vibrations.

Galfenol was first developed in 1999 but has seen few commercial applications. Now, scientists at the U.S. Dept. of Energy's Ames Lab say they have perfected methods of producing the material in the form of rolled sheets and wires, making it possible to use Galfenol-based smart parts in a variety of new applications, but particularly those in military and commercial vehicles.

Galfenol is similar to another material developed at Ames called Terfenol-D, which changes shape when subjected to a magnetic field. A key difference, though, is that Terfenol-D is brittle and will shatter like glass. In contract, Galfenol behaves more like a metal in that it can be welded and machined. Its magnetostriction, however, is only a third to a quarter that of Terfenol-D, but it can operate at significantly lower drive fields. Recent research has also shown that Galfenol can operate in tension and compression, something no other high frequency smart material can do.

Terfenol-D is an alloy of terbium, dysprosium, and iron. It is said to produce "giant" magnetostriction, strain much greater than any of the other magnetostrictive materials. In practice, Terfenol-D usually takes the form of an actuator rather than an energy harvester. It is typically configured as a rod surrounded by electrical coil in a way resembling the configuration of a solenoid. Electrical current flowing through the coil induces a magnetic field which acts to realign the magnetic domains in the Terfenol-D. As the domains rotate, they distort the atomic structure, causing the material to grow and contract as the current is applied and removed, efficiently converting electricity into motion. One application for Terfenol-D configured this way has been in fuel-injector applications.

At frequencies in the range of about 10 to 100 Hz, Terfenol-D actuators can provide repeatable displacements in the range of hundreds of micrometers or even greater. This makes them candidates for high precision motion necessary to realize various state-of-the-art manufacturing processes. The material can also respond at very high frequencies, in excess of 20 kHz, while still producing a large amount of force.

Terfenol-D is often compared to piezoceramics such as lead-titanate-zirconate (PZT). But PZT materials typically have a lower energy density, the amount of power produced in a given size, than Terfenol-D. And because PZT is a ceramic, it must be specially designed to withstand harsh environments. In addition, PZT loses its magnetostrictive properties over time. Terfenol-D's magnetostrictiveness is inherently tied to its crystal structure, and so it does not fade over time or with usage. Maximum displacements available from Terfenol-D actuators range from 50 to 250 microns.

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