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In this paper we investigate the capability of harvesting the electric energy from mechanical vibrations in a dynamic environment through a unimorph piezoelectric membrane transducer. Due to the impedance matrices connecting the efforts and flows of the membrane, we have established the dynamic electric equivalent circuit of the transducer. In a first study and in order to validate theoretical results, we performed experiments with a vibrating machine moving a macroscopic 25 mm diameter piezoelectric membrane. A power of 1.8 mW was generated at the resonance frequency (2.58 kHz) across a 56 k optimal resistor and for a 2 g acceleration.

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We have investigated the dc current–voltage characteristic of high temperature superconducting microbridges. When a dc voltage is applied to a microbridge, it switches to a lossy state due to the formation of a hotspot in the bridge. We have measured the length and temperature of the hotspot as a function of the applied voltage, and have developed a thermal model to explain its steady state behavior. The hotspot has a flat-topped temperature profile, with the maximum temperature independent of the applied voltage. The length of the hotspot, and hence the bridge resistance, increases linearly with the applied bias, so the current is independent of the applied voltage once switching has occurred.

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We present a quantitative analysis of the response of a thin superconducting film to an applied nonuniform ac magnetic field. The analysis is directly applicable to the single-coil inductive measurement technique where a small coil driven by an ac current produces a nonuniform field, with Jz determined by the appearance of a third-harmonic component of the voltage generated across the coil. We derive a simple model to explain the response of the film to the applied magnetic field in the absence of weak links. This model is used to predict the third-harmonic voltage generated across the coil. The derivation of the model explains why superconducting films having thicknesses even less than the penetration depth screen out the nonuniform ac magnetic fields generated by the coil. A simplified version of the model yields analytic expressions that describe the magnitude and phase of the third-harmonic component at high drive currents while the full model yields excellent agreement with experimental measurements for our highest quality epitaxial films. In other films, the presence of weak links leads to a characteristic signature in the harmonic response at low drive currents. We have also found that ion irradiation can reduce the critical current densities by significant amounts without introducing weak links into the film.

International audienceFerrimagnetic GdCo5-xCux alloys exhibiting the effect of Gd- and 3d-sublattice magnetization compensations at defined temperatures were studied with respect to their use as thermoreversible permanent magnets PMs. Coercive fields 0Hc in the range of 0.3–1.6 T were measured for annealed single crystals with x=1–2.2 having compensation points in the vicinity of room temperature. Two applications of such a thermoreversible PM, namely, a thermally controlled actuator and a contactless temperature sensor, are demonstrated