This paper presents an experimental piezoelectric energy harvester exhibiting strong mechanical nonlinear behavior. Vibration energy harvesters are usually resonant mechanical systems working at resonance. The resulting mechanical amplification gives an output power multiplied by the mechanical quality factor Q when compared to non-resonant systems, provided that the electromechanical coupling k 2 is high as well as the mechanical quality factor Q. However, increasing the Q value results in a narrowband energy harvester, and the main drawback is the difficulty of matching a given vibration frequency range to the energy harvester's resonance frequency. Mechanical nonlinear stiffness results in a distortion of the resonance peak that may lead to a broadband energy harvesting capability while keeping a large output power as for high Q systems.This paper is devoted to an experimental study of a Duffing oscillator exhibiting piezoelectric electromechanical coupling. A nonlinear electromechanical model is first presented including piezoelectric coupling, a nonlinear stiffness as for a Duffing oscillator, and an additional nonlinear loss term. Under harmonic excitation, it is shown that for a particular excitation range, the power frequency bandwidth is multiplied by a factor of 5.45 whereas the output power is decreased by a factor of 2.4. In addition, when compared to a linear system exhibiting the same power bandwidth as for the nonlinear one (which is here 7.75%), the output power is increased by a factor of 16.5.Harmonic study is, however, partially irrelevant, because Duffing oscillators exhibit a frequency range where two stable harmonic solutions are possible. When excited with sine bursts or colored noise, the oscillator remains most of the time at the lowest solution. In this paper, we present a technique-called fast burst perturbation-which consists of a fast voltage burst applied to the piezoelectric element. It is then shown that the resonator may jump from the low solution to the high solution at a very small energy cost. Time-domain solution of the model is presented to support experimental data.

Vibration energy harvesters are usually resonant mechanical systems working at resonance. The subsequent mechanical amplification results in output powers multiplied by the mechanical quality factor when compared to non-resonant systems. The main drawback is the difficulty of matching a given vibration frequency range to the energy harvester's resonance frequency. Among several techniques, the use of nonlinear mechanical resonators was proposed in several studies for enlarging energy harvester power bandwidth. In addition, microelectromechanical systems become nonlinear when driven even at moderate levels due to their small size. This paper is devoted to a theoretical study of a Duffing oscillator exhibiting piezoelectric electromechanical coupling. After presenting the dimensionless model, it is solved both in the frequency domain and in the time domain. The frequency-domain simulations show that a huge gain in bandwidth is possible when the resonator is highly nonlinear. Special attention has been paid to the influence of electromechanical coupling. However, this encouraging result is counterbalanced by the difficulty to make the resonator reach high level vibration. Indeed, the Duffing oscillator exhibits a frequency range where two harmonic solutions are possible. When excited with sine bursts or colored noise, the oscillator remains most of the time on the lowest solution. From simulations in the time domain, it is shown that fast burst perturbation (FBP) applied to the piezoelectric voltage may induce the jump from lowest solution to highest solution. Consequently a huge gain may be expected in output power. Finally, the resonator is excited with colored noise and the previously developed strategy is applied. Once again, the mean output power may be greatly enhanced.

Lithium-ion conduction in complex hydrides LiAlH4 and Li3AlH6 was investigated using ac complex impedance measurements. The conductivities at room temperature were 8.7×10−9 S/cm in the case of LiAlH4 and 1.4×10−7 S/cm in the case of Li3AlH6. To enhance the conductivity of Li3AlH6 having good thermal stability in heating/cooling cycles, mechanical milling, and addition of lithium halides (LiCl, LiI) were implemented. The maximum value of 2.5×10−4 S/cm at 393 K was observed when 0.33 M ratio of LiI was added to Li3AlH6. This study demonstrated two research directions to enhance the lithium-ion conductivity in a variety of complex hydrides.

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We have studied sodium-ion conduction in complex hydrides NaAlH4 and Na3AlH6. The electrical conductivities of these complex hydrides were studied using ac impedance measurements at temperatures up to 363 K for NaAlH4 and 433 K for Na3AlH6. Nyquist plots obtained by the measurements indicated the sodium-ion conduction. Also, dc measurements showed that sodium-ion transference numbers of NaAlH4 and Na3AlH6 were almost unity. Na3AlH6 exhibited the sodium-ion conduction of 4.1 × 10−4 S/cm at 433 K. This study will open up research on complex hydrides as solid-state sodium-ion conductors.

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We conducted a high-throughput investigation of the fundamental properties of (Mg,Hf)AlN thin films (0 < x < 0.24) aiming for developing high-performance AlN-based piezoelectric materials. For the high-throughput investigation, we prepared composition-gradient (Mg,Hf)AlN films grown on a Si(100) substrate at 600 °C by cosputtering AlN and MgHf targets. To measure the properties of the various compositions at different positions within a single sample, we used characterization techniques with spatial resolution. X-ray diffraction (XRD) with a beam spot diameter of 1.0 mm verified that Mg and Hf had substituted into the Al sites and caused an elongation of the c-axis of AlN from 5.00 Å for x = 0 to 5.11 Å for x = 0.24. In addition, the uniaxial crystal orientation and high crystallinity required for piezoelectric materials to be used as application devices were confirmed. The piezoelectric response microscope indicated that this c-axis elongation increased the piezoelectric coefficient almost linearly from 1.48 pm/V for x = 0 to 5.19 pm/V for x = 0.24. The dielectric constants of (Mg,Hf)AlN were investigated using parallel plate capacitor structures with ∼0.07 mm electrodes and showed a slight increase by substitution. These results verified that (Mg,Hf)AlN is a promising material for piezoelectric-based application devices, especially for vibrational energy harvesters.

SUMMARYIn this paper, we clarify the use and potential of the electret, in the context of vibration-driven micro-energy harvesting that seeks to obtain energy from a vibrating environment to power small electronic devices. An electret is an insulator containing a permanent electric charge or dipole. We make broad comparisons against other methods such as electromagnetic methods and methods using piezoelectric materials, in order to identify the advantages and disadvantages of the electrets. In particular, we introduce a new general model that covers both the electrets and piezoelectric materials to enable comparisons on a common basis. This is followed by a review of a recent method, developed by our group to make full use of the implanted charge inside an electret. Furthermore, we investigate other ways to realize the full potential of the electret as a key material for electromechanical energy conversion in applications where only small forces are involved.