An alloy first made nearly two decades ago by the U. S. Navy could provide an efficient new way to produce electricity. The material, dubbed Galfenol, consists of iron doped with the metal gallium. In new experiments, researchers from UCLA, the University of North Texas (UNT), and the Air Force Research Laboratories have shown that Galfenol can generate as much as 80 megawatts of instantaneous power per square meter under strong impacts.
Galfenol converts energy with high efficiency; it is able to turn roughly 70 percent of an applied mechanical energy into magnetic energy, and vice versa. (A standard car, by contrast, converts only about 15 to 30 percent of the stored energy in gasoline into useful motion.) Significantly, the magnetoelastic effect can be used to generate electricity. “If we wrap some wires around the material, we can generate an electrical current in the wire due to a change in magnetization,” Domann said.
Galfenol in experiments using a device called a Split-Hopkinson Pressure Bar to generate high amounts of compressive stress (e.g., powerful impacts). They found that when subjected to impacts, Galfenol generates as much as 80 megawatts of instantaneous power per cubic meter.
By way of comparison, a device known as an explosively driven ferromagnetic pulse generator produces 500 megawatts of power per cubic meter. However, as their name implies, such generators require an explosion—one that destroys the ferromagnet, even as it produces power.
Among the potential applications, Galfenol-powered devices could be used as wireless impact detectors. “Essentially, we could fabricate small devices that send out a detectable electromagnetic wave when a mechanical pulse moves through it,” Domann said. These devices could be embedded in vehicles—military or civilian—to detect collisions. Because electromagnetic waves travel three orders of magnitude faster than mechanical waves, information about the impact could be transmitted ahead of the waves created by the impact.
This picture is of the experimental setup showing the Hopkinson bar surrounded by a water-cooled electromagnet. A cylinder of Galfenol is inside of the electromagnet, sandwiched between the Hopkinson bars. The magnet was used to apply a wide range of static magnetic fields to Galfenol while it was mechanically impacted. Credit: John Domann/UCLA
This paper presents the experimental measurements of a highly magnetoelastic material (Galfenol) under impact loading. A Split-Hopkinson Pressure Bar was used to generate compressive stress up to 275 MPa at strain rates of either 20/s or 33/s while measuring the stress-strain response and change in magnetic flux density due to magnetoelastic coupling. The average Young’s modulus (44.85 GPa) was invariant to strain rate, with instantaneous stiffness ranging from 25 to 55 GPa. A lumped parameters model simulated the measured pickup coil voltages in response to an applied stress pulse. Fitting the model to the experimental data provided the average piezomagnetic coefficient and relative permeability as functions of field strength. The model suggests magnetoelastic coupling is primarily insensitive to strain rates as high as 33/s. Additionally, the lumped parameters model was used to investigate magnetoelastic transducers as potential pulsed power sources. Results show that Galfenol can generate large quantities of instantaneous power (80 MW/m3 ), comparable to explosively driven ferromagnetic pulse generators (500 MW/m3 ). However, this process is much more efficient and can be cyclically carried out in the linear elastic range of the material, in stark contrast with explosively driven pulsed power generators.
A Kolsky bar was used to generate large constant strain rates in Galfenol, and measure the stress-strain response, as well as the change in magnetic flux density due to magnetoelastic coupling. The experimental results indicate that the average Young’s modulus of Galfenol is invariant with increasing strain rates of up to 33/s. The measured voltage was proportional to strain rate, with a more rounded appearance, attributed to dynamic magnetic effects. Furthermore, the measured voltage and change in flux were highly dependent on bias field strength. A lumped parameters model was created that effectively simulates the measured pickup coil voltages in response to an applied stress pulse. The model suggests that magnetoelastic coupling is relatively insensitive to strain rates as high as 33/s. The model also suggests that Galfenol can generate large quantities of instantaneous power, comparable to those created by explosively driven ferromagnetic pulse generators. However, this process is much more efficient and can be cyclically carried out in the linear elastic range of the material, in stark contrast with explosively driven pulsed power generators.
SOURCES- UCLA, Phys.org, Journal of Applied Physics
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