A detailed study of the electrical transport properties of gate tunable graphene lateral tunnel diodes is presented. The graphene-Al 2 O 3 -graphene lateral tunnel diodes are fabricated on Si/SiO 2 substrates, and the fabricated devices show rectifying characteristics at the low voltage below 1 V. The rectifying behavior can be controlled by applying back gate voltages. As a result, the devices show high asymmetry and strong nonlinearity current-voltage (I-V ) characteristics, which are desirable properties for applications such as optical rectennas and infrared detectors. The electrical transport mechanism of the graphene lateral diodes is analyzed by extracting parameters from the measured I-V characteristics. We find that trap-assisted tunneling from the defect levels in the Al 2 O 3 layer is the most likely mechanism of the forward current of the fabricated graphene lateral diodes.
A near-infrared telescope with an effective aperture diameter of 30 mm has been developed. The primary objective of the development is to observe northern bright stars in the J, H, and Ks bands and provide accurate photometric data on those stars. The second objective is to observe a belt-like region along the northern Galactic plane (|b| ≤ 5° and δ ≥ −30°) repeatedly, to monitor bright variable stars there. The telescope has been in use since 2016 December. The purpose of this paper is to describe the design and operational performances of the telescope, the photometric calibration methods, and our scientific goals. We show that the telescope has the ability to provide photometry with an uncertainty of less than $5\%$ for stars brighter than 7, 6.5, and 6 mag in the J, H, and Ks bands, respectively. The repeatability of the photometric measurements for the same star is better than $1\%$ for bright stars. Our observations will provide accurate photometry on bright stars that are lacking in the Two Micron Sky Survey and the Two Micron All-Sky Survey. Repeated observations at a good cadence will also reveal their nature in terms of variability in the near-infrared.
Polymer electrolyte fuel cells (PEFCs) are expected to be applied in wider range of applications including automobiles, and the operation at higher current density has been required. However, in the operation at high current densities, the performance of PEFCs is degraded due to liquid water accumulation in the gas diffusion layers (GDLs) which prevents the supply of oxygen to the catalyst. Furthermore, heat is generated in catalyst layer due to electrochemical reaction, leading temperature rise inside PEFCs. Since liquid water accumulation is closely related to temperature distribution, it is expected that temperature distribution alters the spots of liquid water accumulation. Therefore, it is necessary to understand the relationships between the liquid water distribution and the temperature distribution in the GDLs to improve the performance at high current density. This study intended to investigate the effects of temperature distribution on liquid water accumulation by in-situ X-ray visualization. Liquid water distribution of 60, 70, and 80 ℃ cell temperature cases were measured by X-ray radiography, and current density was kept 1.5 A/cm2 in all cases. We used a serpentine type cell with ribs and flow channels of 300 µm width with 1.5 x 3.0 mm2 active area. As for GDLs, SIGRACET 29BC was used. The humidity of hydrogen in the anode was fixed at 100%. While, the humidity of air in the cathode was adjusted so that the gap between water vapor pressure and saturation pressure becomes 4.0 kPa. Fig. 1 (a), (b) and (c) shows the liquid water distribution of the 60 ℃, 70 ℃, 80 ℃ cases. It is shown that the liquid water distribution in a region near the MPL/substrate boundary is clearly altered by the cell temperature. This effect is also observed in the averaged profiles of liquid water volume under rib areas (Fig. 1(d)). In Fig. 1(d), liquid water volume in the region near the MPL/substrate boundary (the region between 50 µm to 80 µm) of the 70 and 80 ℃ cases are clearly reduced compared to the 60 ℃ case. Since the overvoltage of each case is almost the same, the resultant temperature gradient inside GDLs due to the heat production is considered to be the same. Meanwhile, the saturated vapor pressure increases more abruptly at higher temperature, since the saturated vapor pressure is more affected by the initial cell temperature. The saturated vapor pressure of the 80 ℃ case is much larger than the other cases, therefore it is suggested that the lower amount of liquid water accumulation of the 80 ℃ case is caused by the larger saturated vapor pressure. Figure 1
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