In this paper, harvesters coupling magnetostrictive and piezoelectric materials are investigated. The energy conversion of quasi-static magnetic field variations into electricity is detailed. Experimental results are exposed for two macroscopic demonstrators based on the rotation of a permanent magnet. These composite/hybrid devices use both piezoelectric and magnetostrictive (amorphous FeSiB ribbon or bulk Terfenol-D) materials. A quasi-static (or ultra-low frequency) harvester is constructed with exploitable output voltage, even in quasi-static mode. Integrated micro-harvesters using sub-micron multilayers of active materials on Si have been built and are currently being characterized.

The increasing demand in alternative energy sources for low-power electronics gives rise to substantial research activity in the field of energy harvesting devices in recent years. Among the different energy sources, thermal sources can be used to produce electrical energy by means of thermoelectric materials, which exploit Seebeck effect [1], or pyroelectric materials [2]. However, thermoelectric devices require large spatial temperature gradients in order to be efficient. In much the same way, pyroelectric materials are inefficient with slow variations of ambient temperature. Nevertheless there is a possibility to convert indirectly thermal energy into electrical energy through mechanical transformations [3]. Taking advantage of the large mechanical deformation (up to 10% [4]) of shape memory alloys (SMA) in the vicinity of their thermally induced phase transition one can convert this mechanical energy into electrical energy with relatively good efficiency by coupling SMA with piezoelectric materials.In this paper we present experimental proof of thermal energy harvesting using SMA/piezoelectric laminated composite. For the prototype we used 30 µm thick Ti 50 Ni 25 Cu 25 ribbons as SMA layer, prepared by melt spinning technology. After annealing ribbons exhibit a microcrystalline structure. TiNiCu ribbons are characterized by a very high energy density (2-6 J/cm 3 ) associated with first-order martensitic transformation. The piezoelectric layer is a Micro Fiber Composite (MFC) based on a layer of PZT fibers with push-pull-type symmetric polarization units [5]. Figure 1 illustrates the fabricated prototype of the thermal energy harvester. A 2x40 mm 2 piece of TiNiCu ribbon was glued onto the MFC with cyanoacrylate. Before gluing the ribbon was pre-stressed in order to define a preferential deformation direction [6].For real ambient applications it is possible to adjust the transition temperature simply by varying the SMA composition. The presented composite's transition temperature is around 70 o C.For our experiments the working temperature definition was performed by direct Joule heating. Frequency dependence measurements of the composite output voltage were carried out by heating current modulation. Figure 2 shows the voltage response as a function of time for different current modulation frequencies. The peak-to-peak temperature variation is estimated as ∆T = 10°C. The output response is maximum up to 1 Hz with an average peak-to-peak value of about 3V. When increasing further the frequency, the output decreases but even at relatively high frequencies (100Hz) we still observe perceptible non-zero response. In order to estimate the harvested energy, a chemical condenser was charged from the output of the MFC via a diode bridge and then discharged through a resistor. The results show a maximum harvested energy of 30 mJ/cm 3 per phase transition. This value is comparable with the commercial thermoelectric devices. The principal advantage of our composite is to be effective for smaller temperature gradients. Fig...