Power generation with laterally packaged piezoelectric fine wires

Abstract

Converting mechanical energy into electricity could have applications in sensing, medical science, defence technology and personal electronics 1 , and the ability of nanowires to 'scavenge' energy from ambient and environmental sources 2-4 could prove useful for powering nanodevices [5][6][7][8] . Previously reported nanowire generators [9][10][11] were based on vertically aligned piezoelectric nanowires that were attached to a substrate at one end and free to move at the other. However, there were problems with the output stability, mechanical robustness, lifetime and environmental adaptability of such devices. Here we report a flexible power generator that is based on cyclic stretching -releasing of a piezoelectric fine wire that is firmly attached to metal electrodes at both ends, is packaged on a flexible substrate, and does not involve sliding contacts. Repeatedly stretching and releasing a single wire with a strain of 0.05 -0.1% creates an oscillating output voltage of up to 50 mV, and the energy conversion efficiency of the wire can be as high as 6.8%.In existing piezoelectric nanowire generators, a zigzag electrode $50-100 nm above an array of ZnO nanowires 9-11 is forced to move by external forces or disturbances, thus bending the nanowires and inducing a voltage. However, the deliberate rubbing together of the electrode and the nanowire array results in wear, increased contact resistance/instability, and infiltration of vapour and liquid. In the design reported here, a piezoelectric fine wire (PFW) lies flat on a flexible substrate and is fixed to electrodes at both ends (Fig. 1a). When the substrate bends and stretches the wire, a tensile strain of 0.05 -0.1% is induced in the wire (see Fig. 1b and Supplementary Information), leading to a drop in the piezoelectric potential along the wire, and forcing electrons to flow along an external circuit to charge the wire. And when the substrate is released, electrons flow back in the opposite direction. Periodically bending and releasing the PFW therefore generates an alternating current, and generators based on multiple PFWs can be integrated to raise the output voltage. The entire structure is packaged inside a thin layer of insulating wax or flexible polymer to maintain its physical stability (Fig. 1c).Electrical measurements have shown that effective single-wire generators (SWG) all exhibit Schottky behaviour at one end (Fig. 1d). The short-circuit current (I sc ) and open-circuit voltage (V oc ) were measured to characterize the performance of an SWG. To verify that the measured signal was generated by an SWG rather than the measurement system, we have developed two criteria: the 'switching-polarity' test 10 ( Fig. 1e) and a number of 'linear superposition' tests (see Supplementary Information). When the current meter was forward connected to an SWG, a positive voltage/current pulse was recorded during fast stretching of the substrate (Fig. 2a), and a corresponding negative pulse for fast release (where 'fast' means an angular bending rate of $2608 sec 2...

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