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Publications (7)6.11 Total impact

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    ABSTRACT: This paper describes an analysis and a performance limit of a vibrational energy harvester with a novel slit-and-slider structure. This structure has a separable electret and microelectromechanical systems (MEMS) parts. In the MEMS parts, movable electrodes slide due to external vibration and receive electrical field that is periodically modulated by slits of fixed electrodes. The structure was fabricated based on MEMS technology and produced an ac current of 170 pA with an external vibration of amplitude of 1 m/s2 at a frequency of 1166 Hz. Since the structure is separable, individual characterization of the electret and movable electrodes was performed. On the basis of their quantitative analyses, a structural model was constructed and validated. The model showed a way to optimize structural and material parameters for enhancement of output power and predicted a performance limit of 2.5 × 10-3 μW and 6.1% as output power and harvester effectiveness, respectively. This value of effectiveness is comparable to that of conventional non-MEMS-based large energy harvester around 1 cm3, which indicates feasibility of MEMS-based small energy harvesters around 0.01 cm3 by appropriate designing.
    Journal of Microelectromechanical Systems 01/2012; 21(5):1218-1228. · 2.13 Impact Factor
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    ABSTRACT: This paper describes an RF-MEMS switch structure and its fabrication process for developing low-loss multiport RF switches that integrate multiple RF MEMS switches and CMOS control circuits. In our structure, RF MEMS switches and coplanar waveguides are seamlessly integrated, and they are suspended above a CMOS LSI to reduce the loss due to the lossy Si substrate. A gold multilayer stacking technique was used to fabricate the structures, and the STP technique was used to seal them for damage-free packaging. Switching operation of RF MEMS switches was achieved and low insertion loss of 0.07 dB/mm at 5 GHz was obtained for the suspended coplanar waveguides.
    01/2011;
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    ABSTRACT: This paper describes a millimeter-sized electret vibrational energy harvester fabricated by a MEMS-based process for power generation enhancement. To increase current generation, we developed a new “slit-and-slider” structure with horizontal arrays and vertical protrusions. This structure enables us to increase an area and to narrow a gap of energy conversion in the limited size. The structure was fabricated by gold-electroplating. A lower chip with a movable part and an upper chip with a fixed part were aligned by chip-on-chip mounting. The device resonated around 1430 Hz with an acceleration amplitude of 6 m/s 2 . With an ethylene-tetrafluoroethylene
    01/2011;
  • Y. Sato, T. Ono, N. Sato, R. Fujii
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    ABSTRACT: This is a brief report on the first observation of polarization features and the source region of MF/HF auroral radio emissions emanating from the topside ionosphere. These emissions are called “Terrestrial Hectometric Radiation (THR)” and regarded as a counterpart of auroral roar and MF burst which are observable from the ground. THR typically occurs in either or both of two frequency bands near 1.5–2.0 MHz and 3.0–4.0 MHz, whose polarization features correspond to the L-O and R-X mode. The R-X mode, which has never been reported as auroral roar and MF burst, can be attributed to nonlinear coupling of two upper hybrid waves. The Akebono satellite observation shows THR emissions merge with upper hybrid waves in a frequency-time diagram under the matching condition fUH ∼ 2fce. This observation suggests that plasma instability enhances the upper hybrid waves under this condition, and then they are converted into MF/HF auroral radio emissions.
    Geophysical Research Letters 01/2010; 37(14). · 3.98 Impact Factor
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    ABSTRACT: This paper presents experimental proof of energy harvesting with MEMS-based vibrational devices. To clarify the effect of the coupling of vibration of MEMS devices and electrical field of electrets for current generation, a slit-and-slider structure was developed. This structure enables to combine the MEMS devices and electrets after their vibrational and electrical properties have been characterized separately. In the fabrication, the movable gold electrodes were released and then aligned by chip-on-chip mounting to face the fixed electrodes. A 1-mm-square electrode resonated around 1166 Hz with external vibration of a magnitude of acceleration of 1 m/s<sup>2</sup>. The surface and rear potentials of the electret were around -150 and +100 V, respectively. The coupling of the mechanical and electrical properties produced an AC current of 170 pA. The results pave the way to designing and fabricating small-sized vibrational energy harvesters.
    Solid-State Sensors, Actuators and Microsystems Conference, 2009. TRANSDUCERS 2009. International; 07/2009
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    ABSTRACT: Recent ground-based observations have detected MF auroral radio emissions called MF burst and auroral roar. It is interpreted that origin of both emissions is upper hybrid waves generated in the ionosphere by auroral electrons. Some theoretical studies [e.g. Weatherwax et al., 2002] have proposed that MF burst which has a broad band spectrum is generated at an altitude of few hundred kilometers over a wide altitude range while auroral roar is excited in altitudes where a condition of fuh ~ nfce (n=2, 3) is met, which leads to its narrow band spectrum. The polarization spectrum observation at the Husafell observatory in Iceland, which started in 2005, revealed the polarization characteristics of 3fce roar and the relationship with energy of precipitating electrons [Sato et al., 2008]. However, occurrence rate is low because the observation site is located in the auroral zone. We install a new instrument for MF auroral radio emissions at Longyearbyen in Svalbard. The observation at Longyearbyen has several advantages: higher occurrence rate can be expected in the polar cap region, and simultaneous observations with EISCAT Svalbard Radar (ESR) and other ground-based observations can be easily obtained. The instrument consists of two types of observation systems. One is designed for the continuous observation of spectrum in a frequency range below 6 MHz. The other is designed to obtain waveform data in a frequency range below 4 MHz by an A/D converter with a sampling speed of 10MSPS. By using the wave form data, the arrival direction angle of a received radio wave can be estimated. In this presentation, we will introduce the purpose of the observation and the detail of the instrument, and report some initial results.
    AGU Fall Meeting Abstracts. 12/2008;
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    ABSTRACT: In order to study the generation and propagation processes of the MF auroral radio emissions (auroral roar and MF burst), a radio spectrograph ststem was installed at Husafell in Iceland (invariant latitude: 65.3deg). Within the observation period since late 2006, several MF auroral radio emissions have been detected. Based on our observation results, the polarization character of the MF bursts was consistent to the previous result by Shepherd et al. [1997], and identified that 3fce roar is L-O mode wave. It is suggested that auroral roar appears during magnetic storm recovery phase while MF burst is associated with aurora breakup. We compared the 3fce roar detected on May 23, 2007 with auroral image observed by the Polar/UVI and auroral particle observed by the DMSP/SSJ4 in the southern hemisphere. These image data show that the auroral oval has multilayer structure and the Husafell station is located between enhanced layers. In a region with relatively low energy electrons' precipitation (several 100 eV), MF radio waves can propagate to the ground because there is weak ionization in the D and E regions. Therefore, it is suggested that the auroral roar is generated by low energy electrons precipitating near the observation site and propagates downward to the ground. To understand the mechanism, the energy spectrum of precipitating electrons is the key parameter not only for plasma instability but also for ionization of low altitude region. This observation result shows important evidence. On the hypothesis that the frequency of auroral roar coincides with harmonics of fce in the source region, the observation frequencies of 3.1-3.5 MHz correspond to the altitude range of 550-880 km, and electron density should be more than 105/c.c. in this altitude range. It is less likely to occur in the polar ionosphere. When we introduce alternative hypothesis that the plasma instability occurs at (n+1/2)fce ESCH waves, the estimated altitude of the source region becomes 250-350 km. Thus, we have to reexamine the proposed generation mechanism of the MF auroral radio emissions.
    AGU Fall Meeting Abstracts. 12/2007;