Gen Hashiguchi

As the Internet of Things (IoT) ecosystem continues to expand, energy harvesting (EH) has become increasingly important for realizing self-powered sensor nodes. In this paper, we propose a vibration energy harvester based on a flexible printed circuit (VEH-FPC) that effectively utilizes the substrate area in IoT devices. To achieve both a lower resonance frequency and higher output power, an auxetic mechanical metamaterial (MM) with a negative Poisson’s ratio was incorporated into the elastic layer. In addition, to suppress the delamination of electronic components induced by the large deformation of the MM structure, a hybrid design was developed in which only the mounting area for electronic components was selectively implemented as a solid-core (SC) structure. Finite element method (FEM) analysis and experimental validation demonstrated that while preserving the electrical connectivity of the sensor, which is essential for proper FPC functioning, the hybrid design achieved a 20% reduction in resonance frequency and 1.8 times higher output power compared to the conventional SC design. Furthermore, the hybrid design prevented sensor delamination even under severe bending tests, demonstrating the coexistence of mechanical stability and enhanced power-generation performance.

As the Internet of Things (IoT) ecosystem continues to expand, energy harvesting (EH) has become increasingly important for realizing self-powered sensor nodes. In this paper, we propose a vibration energy harvester based on a flexible printed circuit (VEH-FPC) that effectively utilizes the substrate area in IoT devices. To achieve both a lower resonance frequency and higher output power, an auxetic mechanical metamaterial (MM) with a negative Poisson’s ratio was incorporated into the elastic layer. In addition, to suppress the delamination of electronic components induced by the large deformation of the MM structure, a hybrid design was developed in which only the mounting area for electronic components was selectively implemented as a solid-core (SC) structure. Finite element method (FEM) analysis and experimental validation demonstrated that while preserving the electrical connectivity of the sensor, which is essential for proper FPC functioning, the hybrid design achieved a 20% reduction in resonance frequency and 1.8 times higher output power compared to the conventional SC design. Furthermore, the hybrid design prevented sensor delamination even under severe bending tests, demonstrating the coexistence of mechanical stability and enhanced power-generation performance.

Potassium-ion electrets, which mainly consist of amorphous silica and permanently store negative charge, are vital elements of small autonomous vibration-powered generators. However, negative charge decay is sometimes observed to cause issues for applications. In this study, we propose an improved guideline for the fabrication of potassium-ion electrets by first-principles calculations based on density functional theory, which shows that yield reduction can be suppressed by additional oxidation. Moreover, we experimentally confirm that the additional oxidation step effectively extends the lifetime of potassium-ion electrets.

Potassium-ion electrets, which are key elements in vibration-powered microelectromechanical generators, can store negative charge almost permanently. However, our experiments show that the charge storing capability of potassium ion electrets is severely degraded when a SiC plate instead of the usual SiO ₂ plate is used in the fabrication process, indicating the negative influence of carbon incorporation. We theoretically studied the atomic and electronic structures of amorphous silica (a-SiO ₂ ) with and without potassium and carbon atoms using first-principles molecular-dynamics calculations. Our calculations show that negative charge accumulates in fivefold-coordinated Si atoms without carbon contamination. However, carbon contamination of the potassium-ion electret causes oxygen deficiencies in SiO ₂ , hindering the formation of fivefold-coordinated Si atoms. As a result, carbon contamination causes degradation of the negative charge storing capability of potassium-ion electrets.

We report on a DC/DC step-down switching regulator using a microelectromechanical systems (MEMS) switch for the first time. The output voltage developed in a storage capacitor is used as a feedback control voltage for the electrostatic actuator that drives a micromechanical contact switch inserted between the rectifier and the capacitor. The switch is configured to be normally-on owing to the electrostatic potential remaining in the electret film on the electrodes. When the output voltage exceeds and the feedback control voltage cancels the built-in potential of the electret, the differential voltage acting on the actuator becomes small and the contact is opened, thereby decoupling the rectifier from the storage capacitor. When the output voltage becomes low, on the other hand, the net differential voltage becomes large and the contact is closed again, thereby reconnecting the rectifier to the capacitor and recovering the voltage. Fast repetition of this sequence maintains the DC voltage of the storage capacitor at a constant level. For demonstration, an electret film that is pre-charted to 240 V is used to regulate the output voltage in a range between 12 V and 14 V. Different from the switching regulators using transistors, the MEMS regulator does not require any bias current for operation, being suitable as a low-power voltage regulator for environmental energy harvesters. [2021-0220]

An imaging method for a highly charged material using a Kelvin probe force microscope (KFM) was developed. Energy harvesters generate electric energy from the surrounding energy sources for a stand-alone power supply for Internet-of-Things devices. An electret film is a key component in an electrostatic induction-type energy harvester. However, the evaluation method of the electret film remains a problem for microscale measurements. KFM can provide a microscopic surface potential image corresponding to charge distribution. KFM is generally used to measure low potential distributions below a few volts because the electrostatic force between the cantilever that holds the tip and the sample is negligible. A 50- $\mu \text{m}$ -long tip probe was fabricated, which has a longer tip than any commercially available probes. The surface potential difference of 50 V with a step distribution was measured using the developed KFM with a long tip probe. A potassium ion electret film formed on the sidewall of a comb electrode in an energy harvester fabricated by microelectromechanical systems’ technology was measured using the developed KFM. The measured charging potential distribution was from −30.1 to −28.9 V in a measured area, which was close to the mean charging potential measured using a conventional method of −29.2 V. The developed KFM can contribute to the evaluation of highly charged micro-sized samples.

In this paper, we propose a vibration energy harvester (VEH) based on a flexible printed circuit (FPC) capable of power generation using a polymer-based piezoelectric material, and designed a metamaterial elastic layer suitable for the VEH-FPC, which is both an FPC and a VEH. As a result of FEM analysis, we were able to reduce the resonance frequency by 20% and increase the power generation by 1.8 times compared to the conventional solid elastic structure while suppressing the stress generated at the sensor junction.

This paper proposes and demonstrates an electret MEMS switching device in purpose of voltage regulation of the charging voltage to be used in a vibration power generator system. The device can be operated with almost zero current consumption; hence it will be applicable to a vibration power generating system under low current generation conditions. We have successfully regulated the charging voltage around 14V with the MEMS switching device having 240V electret potential.

A high-speed measurement method has been developed to electrically determine the potential voltage of the permanent charge or electret formed in a MEMS (microelectromechanical systems) vibrational energy harvester. While the conventional admittance method requires long time to observe the waveforms of the output power in time domain that diminishes when the built-in electret potential is compensated by an external voltage, this work proposes a new method to uniquely and promptly determine the electret potential by using the FFT (fast Fourier transform) analysis on the vibrational energy harvester excited by the white-noise voltage; the resonant peak in the frequency domain diminishes when the electret potential is compensated by the applied external bias voltage. White noise has been used in conventional method to measure the frequency response of devices of linear characteristics. However, due to the randomness of the input signal, random noise is inherently introduced to the measurement result, which urges us to repeat the measurement and to perform time-averaging to smooth out the noise. Therefore we used a newly developed white noise so-called “pure” white noise, which has a totally flat power spectrum, as it is synthesized from waveforms of a constant magnitude and random phases. We compute the inverse Fourier transform of such pure-white noises to instantly obtain the power spectrum without using mathematical averaging, and thus the throughput of measurement is enhanced ten-times faster.