Terbium (Tb), dysprosium (Dy) and Terfenol-D, which is an iron-based super-magnetostrictive material with a magnetostriction effect of about 1,000ppm, have large magnetostriction effects. But they are also brittle materials.
This time, Ueno created a structure that is small but can generate a large amount of electricity by taking advantage of the ductility of Galfenol. Specifically, one ends of two long and thin magnetostrictive elements are fixed, and weights are added to the other ends (parallel beam structure). To output electricity, coils are made by winding thin electric wires around the magnetostrictive elements.
When the weights vibrate up and down, bending deformation occurs to the two magnetostrictive elements. At this point, a compressive force is applied to one element, and a tensile force is applied to the other element.
When the weights are moving up and down, the compressive force and the tensile force are alternately applied to each of the two elements, periodically changing the magnetic flux. This time variation generates induced voltage on the coils, and electric power can be efficiently output.
"Because Galfenol is a ductile material, even the thin rod-shaped structure does not easily break," Ueno said.
The dimensions of the magnetostrictive element are 1.0 x 0.5 x 10mm, and it is equipped with a magnet that is 2mm in diameter and 2mm in length for bias magnetization.
Moreover, Galfenol features an excellent thermal property. Its Curie temperature is as high as 700°C, and its performance does not change much within the temperature range of -200 to 200°C, Ueno said. Therefore, Galfenol is suited for use in places where change in environment is fast and violent.