Magnetics, Magnetostriction, Energy Harvesting

This paper presents a summary of research to utilise the massive amount of low grade heat energy, for instance which exists in the worlds oceans, by a new type of magnetic cycle. Developed herein are methods based on 2nd order phase changes that make it possible to irrefutably build a Maxwell Demon. Ferrofluids displaying temporary magnetic remanence are an almost perfect embodiment of the working substance for these cycles. Standard Kinetic Theory, Thermodynamic, Electrodynamic analysis and experiment validates the new cycle.

We report neutron diffuse scattering measurements on highly magnetostrictive Fe1−xGax alloys (0.14<x<0.20) with different thermal treatments. This diffuse scattering scales with magnetostriction and exhibits asymmetric peaks at the (100) and (300) reciprocal lattice positions that are consistent with the coexistence of short-range ordered, coherent nanometer-scale precipitates embedded in a long-range ordered, body-centered cubic matrix. A large peak splitting is observed at (300) for x=0.19, which indicates that the nanoprecipitates are not cubic and have a large elastic strain. This implies a structural origin for the enhanced magnetostriction.

Under the excitation of high-frequency AC longitudinal magnetic field, a rotation can be induced in a magnetic wire
under particular circumstances. In the present work latest results on amorphous, nanocrystalline and polycrystalline
wires are reported. In particular, it is concluded that a minimum magnetostriction value is required to observe such an
effect, irrespective of the structural nature of the wire.

Magnetostrictive behaviour of the electrical steel cores of transformers and electrical machines has been studied under wellknown excitation waveforms, e.g. purely sinusoidal. However, the magnetisation may contain higher harmonics due to the grid harmonics for the application in transformers or due to PWM excitation in the case of induction machines. The presence of a single low-order harmonic in the magnetisation has been reported by the authors, but in reality a superposition of several low and high-order harmonics are present, which needs to be studied. Thus, in this work the magnetostrictive behaviour of electrical steel is measured under a sinusoidal magnetisation with high-order harmonics. A macroscopic description of the magnetostrictive behaviour is presented afterwards.

The new surprising effect consisting on the mechanical rotation of magnetic wires is reviewed: they rotate (tens of Hz) when subjected to AC magnetic field (tens of kHz). This phenomenon, originating  in the generation of a magnetoelastic standing wave, is observed in FeSiB and CoSiB, (amorphous), FeNbCuSiB (nanocrystalline) and Ni and NiFe (microcrystalline) wires with diameter ranging from 0.1 to 3 mm with the only restriction of being magnetostrictive. But a new mechanical experiment is here introduced giving a new insight on the phenomenon: similar response is observed when samples (even non-magnetostrictive) are mechanically excited by the vibrations induced by a loudspeaker membrane.

High-pressure x-ray diffraction measurements were performed at room temperature on single crystals of the highly magnetostrictive alloy Fe0.81Ga0.19 (galfenol). This alloy has a bcc crystal structure at ambient pressure but undergoes a bcc-to-hcp phase transition at 24 GPa on compression. A large hysteresis loop is observed in which the reversed transition occurs at 13 GPa on decompression. The midpoint of this transition is 18.5 GPa. The measured bulk modulus of this material is 182 (±17) GPa, which is comparable to that of pure iron. As with iron, the hcp structure of the alloy can be derived from a compression of the bcc lattice along [001] that is accompanied by shearing along [11¯0]. Our results indicate that the addition of Ga shifts the bcc-to-hcp transition from 13 GPa in pure iron to 18.5 GPa, and we speculate that this is due to the larger atomic radius of Ga. A uniaxial loading of 3 GPa completely suppresses the diffuse scattering in Fe0.81Ga0.19. We ascertain that the magnetostrictive properties of the alloy are reduced under pressure.