A low frequency MEMS energy harvester scavenging energy from magnetic field surrounding an AC current-carrying wire

This paper reports on a low frequency piezoelectric energy harvester that scavenges energy from a wire carrying an AC current. The harvester is described, fabricated and characterized. The device consists of a silicon cantilever with integrated piezoelectric capacitor and proof-mass that incorporates a permanent magnet. When brought close to an AC current carrying wire, the magnet couples to the AC magnetic field from a wire, causing the cantilever to vibrate and generate power. The measured average power dissipated across an optimal resistive load was 1.5 μW. This was obtained by exciting the device into mechanical resonance using the electro-magnetic field from the 2 A source current. The measurements also reveal that the device has a nonlinear response that is due to a spring hardening mechanism

Comment on "Comparison of air breakdown and substrate injection as mechanisms to induce dielectric charging in microelectromechanical switches" [Appl. Phys. Lett. 92, 043502 (2008)]

The purpose of this comment is to provide additional insight into the reliability of microelectromechanical capacitive switches (MEMSs) investigated by Molinero and Casta er [Appl. Phys. Lett. 92, 043502 (2008)]. We show that the presence or absence of ambient humidity determines whether the shift in the capacitance-voltage (C-V) curve of oxide-based MEMS occurs as a result of voltage stress. In humid air, negative and positive shifts in the C-V curve are observed after negative and positive bias stress. In dry air no such shifts in the C-V curve are seen. These shifts are similar to those reported on oxide-based switches by Molinero and Castaner [Appl. Phys. Lett. 92, 043502 (2008)] where they show shifts occurring in room ambient pressure, but not in vacuum. This indicates that not only air pressure but also air humidity can be responsible for shifts in MEMS. (C) 2009 American Institute of Physics. (doi: 10.1063/1.3255008

Ferroelectricity and Large Piezoelectric Response of AlN/ScN Superlattice

Based on density functional theory, we investigate the ferroelectric and piezoelectric properties of the AlN/ScN superlattice, consisting of ScN and AlN buckled monolayers alternating along the crystallographic <i>c</i>-direction. We find that the polar wurtzite (w-ScAlN) structure is mechanically and dynamically stable and is more stable than the nonpolar hexagonal flat configuration. We show that ferroelectric polarization switching can be possible for an epitaxially tensile-strained superlattice. Because of the elastic constant <i>C</i>₃₃ softening, together with an increase in <i>e</i>₃₃, the piezoelectric coefficient <i>d</i>₃₃ of the superlattice is doubled compared to that of pure w-AlN. The combined enhancement of Born effective charges (<i>Z</i>₃₃) and sensitivity of the atomic coordinates to the external strain (∂u3∂η3) is the origin of the large piezoelectric constant <i>e</i>₃₃. Moreover, we show that the epitaxial biaxial tensile strain significantly enhances the piezo-response, so that <i>d</i>₃₃ becomes 7 times larger than that of w-AlN at 4% strain. The tensile strain results in a huge enhancement in <i>e</i>₃₃ by increasing <i>Z</i>₃₃ and ∂u3∂η3, which boost the piezoelectric

A new nonlinear compliant mechanism for harvesting energy from ocean waves

Traditonal linear oscillators, such as cantilevers or pendulums, are only sensitive to specific resonant frequencies. They have then very narrow frequency bandwidths when harvesting energy from ocean waves. In order to enhance the dynamic performance of the wave energy converters (WECs), used to expand the autonomy of Lagrangian Drifters, a statically balanced compliant mechanism (SBCM) is investingated. It is based on finite element analysis (FEA) simulaitons. The design of the SBCM is introduced and its static force-displacement curve is obtained in FEA. The dynamic response of the SBCM to harmonic base excitiaons at low frequencies and low accelerations is investigated based on time-domian FEA simulations. The close agreement between simulations, numerial and analytical results verifies that the SBCM is sensitive to ultra-low frequencies with weak accelerations in a wide frequency range. The applicability of the SBCM in WECs is demonstrated by adding PVDF films in the FEA model. In the time-domian simulation, the SBCM-based WEC is excited by the drifter motion pattern obtained from Orcaflex and corresponding to two typical ocean waves (i.e. synthesized Airy and Jonswap models). Relative displacement between the base and mass and the electric outputs are obtained. According to this work, the SBCM provides a structural solution for WECs with enhanced energy harvesting performance.Postprint (published version

Experimental isolation of degradation mechanisms in capacitive microelectromechanical switches

DC and bipolar voltage stresses are used to isolate mechanical degradation of the movable electrode from charging mechanism in microelectromechanical capacitive switches. Switches with different metals as the movable electrode were investigated. In titanium switches, a shift in the pull-in voltages is observed after dc stressing whereas no shift occurs after the bipolar stressing, which is to be expected from charging theory. On switches with similar dielectric but made of aluminium, the narrowing effect occurs regardless if dc or bipolar stressing is used, which indicates the mechanical degradation as the mechanism responsible. (C) 2012 American Institute of Physics. (http://dx.doi.org/10.1063/1.4726116

A simple electrical test method to isolate viscoelasticity and creep in capacitive microelectromechanical switches

A bipolar hold-down voltage was used to study mechanical degradation in radio-frequency microelectromechanical capacitive shunt switches. The bipolar signal was used to prevent the occurrence of dielectric charging and to isolate mechanical effects. The characteristics of material stress relaxation and recovery were monitored by recording the change of the pull-in voltage of a device. The creep effect in movable components was saturated by repeated actuation to the pulled-in position, while comparison with a theoretical model confirmed the presence of linear viscoelasticity in the devices. (C) 2014 AIP Publishing LLC

Broadening the Bandwidth of Piezoelectric Energy Harvesters Using Liquid Filled Mass

AbstractA narrow bandwidth is one of the most challenging issues that vibrational energy harvesters have to overcome. This paper demonstrates a novel method of broadening the bandwidth without significantly reducing the peak output voltage. The method uses a liquid filled mass to create a sliding mass effect in order to broaden the bandwidth. The fluid mass increased the full-width-half-maximum (FWHM) value from 1.6Hz to 4.45Hz with no significant decrease in peak-to-peak voltage when compared to an empty mass. The fluid filled mass has a non-linear mass distribution during low frequency, high acceleration applications

Location dependence of a MEMS electromagnetic transducer with respect to an AC power source

A MEMS, silicon based device with a cantilever oscillationsand an integrated magnet is presented for magnetic to electrical transduction. The cantilever structure can be configured either as an energy harvester to harvest power from an AC power line or as an AC current sensor. The positioning of the transducer with respect to the AC conductor is critical in both scenarios. For the energy scavenger, correct positioning is required to optimize the harvested power. For the current sensor, it is necessary to optimise the sensitivity of the sensor. This paper considers the effect of the relative position of the transducer with respect to the wire on the resulting electromagnetic forces and torques driving the device. It is shown here that the magnetic torque acting on a cantilever beam with an integrated magnet and in the vicinity of an alternating electromagnetic field is a very significant driver of the cantilever oscillations

Identification of the transient stress-induced leakage current in silicon dioxide films for use in microelectromechanical systems capacitive switches

Dielectric charging at low electric fields is characterized on radio-frequency microelectromechanical systems (RF MEMS) capacitive switches. The dielectric under investigation is silicon dioxide deposited by plasma enhanced chemical vapor deposition. The switch membrane is fabricated using a metal alloy which is shown to be mechanically robust. In the absence of mechanical degradation, these capacitive switches are appropriate test structures for the study of dielectric charging in MEMS devices. Monitoring the shift and recovery of device capacitance-voltage characteristics revealed the presence of a charging mechanism which takes place across the bottom metal-dielectric interface. Current measurements on metal-insulator-metal devices confirmed the presence of interfacial charging and discharging transient currents. The field-and temperature-dependence of these currents is the same as the well-known transient stress-induced leakage current (SILC) observed in flash memory devices. A simple model was created based on established transient SILC theory which accurately fits the measured data and reveals that charge exchange at the bottom metal-dielectric interface is responsible for charging currents and pull-in voltage changes in these MEMS devices. (C) 2015 AIP Publishing LLC

Shock-induced aluminum nitride based MEMS energy harvester to power a leadless pacemaker

The next generation of implantable leadless pacemakers will require vibrational energy harvesters in order to increase the lifetime of the pacemaker. This paper reports for the first time the use of a piezoelectric MEMS linear energy harvester device that fits inside a pacemaker capsule. The silicon based MEMS cantilever device uses CMOS compatible Aluminum Nitride as the piezoelectric layer. The developed harvester operates based on a shock-induced vibration that is generated from the low frequency (60–240 beats per minute) high acceleration (>1 g) vibration of the heart. The off-resonance, high g impulses force the high-frequency harvester to oscillate at its resonant frequency. A power density of 97 and 454 μW cm−3 g−2 was achieved for a heart rate of 60 and 240 beats per minute respectively. The forced oscillation causes the linear harvester to dampen after 100–200 ms which reduces the average power compared to a typical sinusoidal excitation. A two and four cantilever system occupies 35% and 70% of the overall volume of the capsule while obtaining 2.98 and 5.96 μW respectively at a heart rate of 60 bpm respectively and 1 g acceleration. The results in this paper demonstrate that a shock-induced linear MEMS harvester can produce enough electrical energy from the vibration of a heart to power a leadless pacemaker while maintaining a small volume