Shuichi Akamine

Application of chemical dosimetry to hydroxyl radical measurement during underwater discharge

The pulsed streamer discharge was generated in the liquid by using point-to-sphere electrode geometry. In this study, the OH radicals were measured indirectly by a chemical dosimetry. Terephthalic acid (TA) was used as an OH radical scavenger and fluorescence of the resulted 2-hydroxyterephthalic acid (HTA) was measured. Instead of the use of conventional fluorescence spectrophotometer, a novel fluorescence observing system by using a light-emitting diode (LED) as a light source and a simplified spectrometer as a detector was developed. As a result, the fluorescence intensity increased with time elapsed during the discharge process. The production rate of OH radicals in the liquid was estimated to be of the order of 10−10 M s−1 to 10−9 M s−1 under the present experimental conditions. As an advantage of terephthalate dosimetry, it was found that the discharge system in the small reactor used in our experiment was a useful means for in-situ evaluating formation of OH radicals in an underwater discharge.

Streamer Corona Discharge Induced by Laser Pulses During LIF Measurements in a DC Non-thermal Plasma Reactor for NO Oxidation

Abstract A possible interference of the light emitted by the laser induced streamer coronas and the laser induced fluorescence (LIF) signal during the measurements of NO concentration in a DC positive corona discharge was experimentally investigated. For in-situ NO measurement a LIF system consisting of a XeF excimer laser, dye laser and BBO crystal, generating a tuned laser line at 226 nm was employed. From the measured occurrence timing between the regular streamer coronas, laser pulse, LIF signal and laser induced streamer it was found that the LIF signal appears almost immediately after the laser incidence and lasts over about 30 ns, while the induced streamer starts about 35 ns after the LIF signal and lasts about 350-500 ns. Due to the 5 ns interval between the LIF signal and the laser induced streamer the undisturbed detection of the LIF signal can be possible with a properly adjusted timing of the ICCD camera (the gate opening and exposure time). Two-dimensional distribution of NO molecules concentration in the discharge gap was measured using the LIF technique. The time-resolved evolution of the laser induced streamers was visualized using the ICCD camera with the proper timing adjustment. This resulted in determining the velocity of propagation of the streamer (about 2.5 × 105 m/s) and the averaged diameters of the leader channel and leader streamers (200 μm and 100 μm, respectively).

Observing Three-Dimensional Structures of Streamer Discharge Channels

Three-dimensional structures of atmospheric and underwater streamer discharges are reconstructed using a newly developed 3-D observation method based on stereophotography. In this method, a streamer discharge is captured from three directions simultaneously. The advantage of the foregoing is the 3-D reconstruction with the least uncertainty of the 3-D coordinate and easy pairing of identical streamer channels between the two photographs taken from different directions. Reconstructed 3-D structures exhibit all characteristic morphological properties of streamer channels such as branching and zigzag-shaped paths.

Investigation of Three-Dimensional Characteristics of Underwater Streamer Discharges

We have developed a new three-dimensional (3D) observation method suitable for studying the structure of streamer discharges. Using this method we investigated the propagation of underwater streamer discharges generated in a nozzle-plate electrode system. As a result, the streamer branching angle was found to take a bell-shaped distribution having a mean value in the range of 65–75° with a standard deviation of about 20°. Moreover, we confirmed that the mean branching angle is a physical quantity that increases with water conductivity. The propagation velocity of the underwater streamer was estimated to be (2–3) ×104 m/s.

Atmospheric-pressure-plasma nitriding of titanium alloy

Atmospheric-pressure-plasma nitriding of titanium alloy Ti–6Al–4V has been achieved by using a pulsed-arc plasma jet with a N2/H2 gas mixture, where the plasma jet plume is sprayed onto the titanium surface under atmospheric pressure. We successfully formed a titanium nitride layer on the sample surface. Moreover, the diffusion layer was also formed, the hardness of which was increased from that of as-received titanium. The nitride layer growth was found to be diffusion-controlled, as in other conventional nitriding methods.

Improving Single-Chamber Solid Oxide Fuel Cell Performance by Plasma Treatment Using an Atmospheric-Pressure Helium Plasma Jet

An atmospheric-pressure helium plasma jet was used for the surface treatment of the electrodes in single-chamber solid oxide fuel cells (SC-SOFCs). The jet-type plasma source used in this study is suitable for the continuous and fine-area processing of materials, such as patterned electrodes. The basic plasma property was investigated by optical emission spectroscopy. Improvement in the performance of SC-SOFC was observed for the plasma-treated cell. From the scanning electron microscopy (SEM) observation, it was found that the surface morphology of the cell was largely changed. The increase in the area of the three-phase boundary among the electrode, electrolyte, and gas phase promoted electrochemical reactions. Under single-chamber operation condition at 850 °C, an open circuit voltage of 650 mV and a maximum power density of approximately 75 mW/cm2 were achieved for a coplanar-type cell.

Peculiar Relationship of Plume Brightness and Hard Layer Formation in Atmospheric-Pressure Plasma Jet Nitriding

Irradiating the plume of atmospheric-pressure pulsed-arc plasma jet with nitrogen/hydrogen mixture gas enables to harden steel surface by nitrogen diffusion into steel. An important parameter is the hydrogen mixture ratio. Images of plasma-jet plume and formed hard layer for several hydrogen mixture ratios are presented.