Abstract
Magnetostrictive Fe-Ga and Fe-Al alloys are promising materials for use in bending-mode vibrational energy harvesters. For this study, 50.8 mm × 5.0 mm × 0.5 mm strips of Fe-Ga and Fe-Al were cut from 0.50-mm thick rolled sheet. An atmospheric anneal was used to develop a Goss texture through an abnormal grain growth process. The anneal lead to large (011) grains that covered over 90% of sample surface area. The resulting highly-textured Fe-Ga and Fe-Al strips exhibited saturation magnetostriction values (λsat = λ∥ − λ⊥) of ∼280 ppm and ∼130 ppm, respectively. To maximize 90° rotation of magnetic moments during bending of the strips, we employed compressive stress annealing (SA). Samples were heated to 500°C, and a 100-150 MPa compressive stress was applied while at 500°C for 30 minutes and while being cooled. The effectiveness of the SA on magnetic moment rotation was inferred by comparing post-SA magnetostriction with the maximum possible yield of rotated magnetic moments, which is achieved when λ∥ = λsat and λ⊥ = 0. The uniformity of the SA along the sample length and the impact of the SA on sensing/energy harvesting performance were then assessed by comparing pre- and post-SA bending-stress-induced changes in magnetization at five different locations along the samples. The SA process with a 150 MPa compressive load improved Fe-Ga actuation along the sample length from 170 to 225 ppm (from ∼60% to within ∼80% of λsat). The corresponding sensing/energy harvesting performance improved by as much as a factor of eight in the best sample, however the improvement was not at all uniform along the sample length. The SA process with a 100 MPa compressive load improved Fe-Al actuation along the sample length from 60 to 73 ppm (from ∼46% to ∼56% of λsat, indicating only a marginally effective SA and suggesting the need for modification of the SA protocol. In spite of this, the SA was effective at improving the sensing/energy harvesting performance by a factor of ∼2.5 in the best sample. As with the Fe-Ga strip, improvement in performance was quite varied along the strip length.
Highlights
Increasing usage of low energy fixed and portable electronics and environmental concerns are driving the development of technologies that harvest energies present as an untapped ambient background resource
We focus on the efficacy of the stress annealing process and material characterizations for a general energy harvesting application
Stress was gradually applied to achieve a compressive stress of ∼150 MPa in the three Fe-Ga samples and ∼100 MPa in the three Fe-Al samples. (A holder redesign is needed to achieve 150MPa without buckling of the Fe-Al samples.) The annealing temperature of 500 ◦C and loads were held for 30 minutes, after which the heating chamber was slowly cooled while maintaining the compressive stress on the samples
Summary
Increasing usage of low energy fixed and portable electronics and environmental concerns are driving the development of technologies that harvest energies present as an untapped ambient background resource. There are several technologies for conversion of ambient vibrational energy into electrical power. These include piezoelectric materials,[1,2] magnetostrictive materials[3,4] and electromagnetic induction.[5,6]. For the energy harvesting application being considered we use a thin cantilevered beam (strip) of magnetostrictive material. The strips were stress annealed using the protocol presented by Yoo et al.[18] This was done to maximize 90◦ rotation of magnetic moments when the strip bends by providing an initial condition in which magnetic moments start aligned with the thru-thickness easy axes, i.e. perpendicular to the length of the strip. We focus on the efficacy of the stress annealing process and material characterizations for a general energy harvesting application
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