Volume 3, Issue 4 -

This paper reports for the first time the experimental demonstration of a wireless sensor node only powered by an aeroacoustic energy-harvesting device, meant to be installed on an aircraft outside skin. Aeroacoustic noise is generated on purpose to serve as a means of converting mechanical energy from high velocity airflow into electrical energy. Results related to the physical characterization of the energy conversion process are presented. The proposed aeroacoustic transducer prototype, consisting in a rectangular cavity fitted with a piezoelectric membrane, is shown to deliver up to 2 mW AC power under Mach 0.5 airflow. Optimized power management electronics has been designed to interface with the transducer, including a self-powered Synchronized Switch Harvesting on Inductor (SSHI) interface circuit and an efficient buck-boost DC/DC converter. The design of micropower auxiliary circuits adds functionality while preserving high efficiency. This circuit stores energy in supercapacitors and is able to deliver a net output DC power close to 1 mW. A fully autonomous system has been implemented and tested, successfully demonstrating aeroacoustic power generation by supplying a battery-free wireless datalogger in conditions representative of an actual flight.

This work focuses on a polymer based capacitive harvester which can be fabricated with help of roll-to-roll and low cost printing methods. In contrast to electrostatic MEMS based parallel plate transducers or dielectric elastomer systems here, the capacitance is varied as function of the mechanical load by changing of the top electrode area with help of an electrically conducting composite elastomer. In case of a composite elastomer electrode the maximum capacitance in compressed state does not only depend on the thickness and permitivity of the dielectric but first of all on the quality of the interface and the micro structure of the conducting phase in the composite electrode at the interface which was investigated by FEM Maxwell simulation. An equivalent circuit model is used to study the influence of the leakage current inside the dielectric and the bulk resistivity of the elastomer electrode. First experiments with state of the art polymer, thin film and novel printed dielectrics in contact with elastomer electrodes have been performed to prove the harvesting principle at low frequencies. Charges between 25 and 70 nAs per cm 2 have been transferred per cycle at 100 V/200 V while the maximum capacity was between 0.4 and 0.8 nF/cm 2 .

The present discussion shows a perspective about using graphite as a current collector in order to achieve high energy density in a symmetric supercapacitor system. Several electrochemical modes (such as rest potential analysis, CV, PEIS, GCPL) were carried out to evaluate the electrochemical characteristics of graphite in aqueous 6 mol/L KOH. And, the resulting performance was compared to an another conventional current collector system based on nickel-stainless steel. Interestingly, widening of cell voltage was observed for graphite when compared to nickel-stainless steel. The discussion reveals the reasonable influences and validations of widening in cell voltage towards the values in energy densities. This perspective also highlights some issues related to carbon (graphite) current collectors and encloses with some promising strategies in overcoming these issues, not limiting the domain of application (either micro or macro supercapacitor devices).

Relative motion is required for vibration energy harvesting, such as magnet moving past the coil in inductive approach and tip-mass motion in piezoelectric approach. Typically, relative motion is created by amplifying the source displacement and storage of mechanical energy in an auxiliary vibrating mass. In this study, we propose a novel technique to create the relative motion without amplification of the original source displacement. The technique relies on cancelling the vibration at one location and transferring the source vibration directly to another location through combination of a vibration isolator with a vibration absorber. In this multi-degree of freedom configuration, the power is harvested from the displacement of the vibrating source rather than the displacement of an auxiliary mass. This configuration eliminates the need to capture relative motion with respect to an externally fixed component. A prototype was designed and fabricated based on this concept which was found to harvest 45 mW at 0.9 G base acceleration and weighed 462 g. Through analytical modeling it was determined that the prototype could generate 87 mW @ 1 G base acceleration, while weighing only 243 g. Also, an optimal balance between the bandwidth and the maximum power harvested was identified through parametric analysis.

We have previously proposed a bi-stable nonlinear oscillator based piezoelectric energy harvester to address the challenges of low-frequency, low-g vibration energy harvesting at MEMS scale. The compressive residual stress in micro-fabricated thin films could be employed to induce buckling in doubly clamped beams. The buckled beams with two equilibriums can snap through at low frequencies and enhance the power generation. This paper focuses on the modeling of the dynamics of the bi-stable oscillator based energy harvesters. An electromechanical lumped model has been built by Lagrange’s formulation and lumped parameters have been solved explicitly. More particularly, the model considers multi-layer thin films and the residual stress in these films, to be readily applicable to MEMS device design. Harmonic functions as the approximate solutions have been used to analytically solve for the frequency responses of the nonlinear oscillations. Two modes of oscillations predicted by the analytical model have been validated by testing a meso-scale prototype.

Energy filtering by energy barriers has been proposed to interpret observations on large thermoelectric power factor (TPF) enhancement in highly doped nanocrystalline Si (nc-Si). Previous Boltzmann transport equation (BTE) modeling indicated that high TPFs could be explained as the result of the presence of energy barriers at the grain boundaries, the high Fermi energy due to the high doping level, and the formation of a low thermal conductivity second phase. To test the assumptions of the BTE modeling and provide more realistic simulations, we have performed Monte Carlo (MC) simulations on the transport properties of composite nc-Si structures. Here, we report on (i) the effect of an energy barrier, and (ii) the effect of multiple barriers on the conductivity and the Seebeck coefficient. In short structures, a TPF enhancement was found and it has been attributed to energy filtering by the energy barrier. The MC indicated that the TE performance can be improved by multiple barriers in close separation. It has been shown that TPF enhancement is possible even when the condition for thermal conductivity non-uniformity across the composite structure is not-fulfilled.

Silicon is the reference material of microelectronics, is readily available, relatively unexpensive, and its use may take profit of a fantastic technology. This may explain why a substantial effort has focused on improving its thermoelectric efficiency, either by top-down nanostructuring or through suitable processing. In this paper we report an analysis of the electronic transport properties of heavily boron-doped nanocrystalline silicon films. High-temperature thermal treatments are confirmed to remarkably increase its thermoelectric power factor. Electrical conductivity and Hall effect measurements were carried out over the temperature range 20–300 K along with Seebeck coefficient measurements. We provide evidence of the occurrence of low-temperature hopping conduction between impurity subbands. Dopant ionization was studied as a function of temperature. Freeze-out temperature was found to correlate with the Seebeck coefficient in agreement with Pisarenko equation. This brings to the conclusion that, while untreated samples are weakly degenerate, the thermal processing reverts them into non-degenerate semiconductors, in spite of the high doping level.

A thermoelectric microgenerator based on multiple silicon nanowire arrays is fabricated and its performance evaluated for different convection regimes. Mature silicon microfabrication technology is used to fabricate the device structure. As a post-process, a bottom-up approach is used to grow silicon nanowires by a VLS-CVD mechanism. The thermal design of the microgenerator features a thermally isolated silicon platform which is connected to the bulk silicon rim through several arrays of silicon nanowires. Simulations are carried out to evaluate the need of an external heat sink to improve the thermal gradient seen by the nanowires and the power output of the microgenerator. Results show a significant improvement with a heat sink raising the thermal gradient from 3 K to approximately 100 K when the external temperature gradient is 300 K. Experimental measurements with different convection regimes also show a radical improvement on the power output comparing natural convection and two different forced convection regimes. The first forced convection regime is a broad airflow from a commercial CPU fan placed on top of the device, while the second (air jet forced convection) uses a syringe to focus the airflow from the compressed air line to the platform. The maximum output power for a natural convection regime is 2.2 nW for a hotplate temperature of 200 °C, while the air jet forced convection regime generates up to 700 nW, which correspond to 35 µW/cm 2 considering a device footprint of 2 mm 2 .