Tsuyohito Ito

Generation of micrometer-scale discharge in a supercritical fluid environment

The generation of micrometer-scale discharge in very high-pressure CO2 up to a supercritical fluid (SCF) environment using micrometer-gap (1, 2 μm) electrodes has been performed. A peculiar characteristic of breakdown voltage for CO2 is reported here. The results reveal an inflection near 2.5 MPa and a trough near the critical point. Near the critical point, the discharge can be generated with almost one fifth of the voltage required for a conventional gas, as estimated from Paschen’s law, which is probably caused by unstable clusters in SCF. This result reveals that an ionized state or plasma state in a SCF environment can be generated with very low voltage.

Application of microscale plasma to material processing

Abstract We generated microscale plasmas using novel electrodes fabricated by lithography and confirmed that Paschens law can be extended to gaps between electrodes as narrow as 5 μm. To develop the application of a microscale plasma to material processing, we performed plasma chemical vapor deposition (P-CVD) of carbon films with this microscale plasma using a CH 4 –H 2 gas system, for the first time. We deposited carbon films using various gas ratios of H 2 /CH 4 . Deposits were characterized by scanning electron microscopy (SEM), electron probe microscopy analysis (EPMA), Auger electron spectroscopy (AES), and Raman spectroscopy. Using Raman spectroscopy, it was found that the bonding nature of these carbon films changed from sp 2 to sp 3 with increasing H 2 /CH 4 ratio. Lastly, we proposed an integrated plasma processing apparatus on a substrate, namely a plasma chip.

Fabrication of spherical carbon via UHF inductively coupled microplasma CVD

We developed a CVD system employing inductively coupled micronplasma generated by applying UHF (UHF microplasma). This system can be operated easily in air at atmospheric pressure with a few tens of watts of UHF power and enables us to deposit material on a wide variety of substrates. Deposition of carbon from CH4/Ar gas on the substrate without additional heating at atmospheric pressure in ambient air resulted in the formation of spherical graphites on a micron-sized region, at a growth rate of about 2 µm min−1. In addition, carbon nano-onions of sub-30-nm diameters were formed in large quantity. Our results seem to suggest that this microplasma CVD system may be utilized in the near future for low-temperature preparation of crystallized nanostructures, leading to simplified fabrication of the nano-scale device.

Rapid formation of electric field profiles in repetitively pulsed high-voltage high-pressure nanosecond discharges

Rapid formation of electric field profiles has been observed directly for the first time in nanosecond narrow-gap parallel-plate discharges at near-atmospheric pressure. The plasmas examined here are of hydrogen, and the field measurement is based on coherent Raman scattering (CRS) by hydrogen molecules. Combined with the observation of spatio-temporal light emission profiles by a high speed camera, it has been found that the rapid formation of a high-voltage thin cathode sheath is accompanied by fast propagation of an ionization front from a region near the anode. Unlike well-known parallel-plate discharges at low pressure, the discharge formation process at high pressure is almost entirely driven by electron dynamics as ions and neutral species are nearly immobile during the rapid process.

Thermoelectron-enhanced micrometer-scale plasma generation

The use of thermoelectrons allowed easy generation of a micrometer-scale very high-frequency plasma (thermoelectron-enhanced micrometer-scale plasma) in a fine capillary, with an inner diameter of approximately 120 μm. A tungsten wire, whose diameter was 25 μm, inserted into the region was used to provide thermoelectrons. Approximately 5 W of power was sufficient to sustain this plasma. The plasma was very stable and the discharge continued for at least 60 min.

Decrease of breakdown voltages for micrometer-scale gap electrodes for carbon dioxide near the critical point: Temperature and pressure dependences

We performed measurements of breakdown voltages as a function of environmental pressure with 1-μm-gap tungsten electrodes for high-pressure carbon dioxide up to supercritical conditions at different temperatures (305.65, 308.15, and 313.15 K). Breakdown voltage curves exhibit an inflection at around 3 MPa and a drastic decrease near the critical point. The location of the drastic decrease shifts to the high-pressure range and the sharpness and depth decrease with increased temperature. The breakdown voltage in pressure environments higher than that at the inflection point was analyzed systematically using the Townsend theory and density fluctuations. Moreover, comparison with breakdown voltage measurements by 10-μm-gap electrodes indicates that one factor inducing the inflection and the decrease might be electron attachment to existing clusters in dense carbon dioxide.

Electric field measurement in an atmospheric or higher pressure gas by coherent Raman scattering of nitrogen

The feasibility of electric field measurement based on field-induced coherent Raman scattering is demonstrated for the first time in a nitrogen containing gas at atmospheric or higher pressure, including open air. The technique is especially useful for the determination of temporal and spatial profiles of the electric field in air-based microdischarges, where nitrogen is abundant. In our current experimental setup, the minimum detectable field strength in open air is about 100?V?mm?1, which is sufficiently small compared with the average field present in typical microdischarges. No further knowledge of other gas/plasma parameters such as the nitrogen density is required.

Pressure effect on ZnO nanoparticles prepared via laser ablation in water

ZnO nanoparticles were prepared via laser ablation of metallic Zn in neat deionized water at pressures up to ∼31 MPa and at constant ablation time, fluence, and wavelength. The high-pressure products were compared with those prepared at 1 atm, and the effect of water pressure on the product size and photoluminescent properties was studied. The results indicate that the use of pressure permits to control the particle size, the position of their exciton emission peak, and the intensity of their visible emission. While smaller and more homogeneous in size ZnO particles were produced at elevated pressures, their UV emission peak blue-shifted and green emission was enhanced. At pressures ∼22 MPa, a discontinuity in the improvement of the product green emission was observed, which should be related to the appearance of supercritical water in the ablated zone.

Micrometer-scale discharge in high-pressure H2O and Xe environments including supercritical fluid

We generated micrometer-scale discharge in high-pressure H2O and Xe up to supercritical conditions. In our previous paper, we reported the existence of two peculiar features in the breakdown voltages under high-pressure CO2. The first one was the downward shift at the right-hand side of Paschen’s curve above about 2.5MPa, and the second one was the drastic decrease in the breakdown voltages near the critical point. We have experimentally confirmed that these features are also observed in H2O and Xe, even though there are some differences among these materials. Our theoretical fitting involving a density fluctuation term FD agrees well with the experimental results, especially for Xe. We suppose that these unique features are brought about by decreases in the electron-to-particle cross section σ, ionization potential φi, and secondary electron coefficient γ′ and changes in the discharge space.

Carbon and copper nanostructured materials syntheses by plasma discharge in a supercritical fluid environment

This first report on applications of discharges (or discharge plasma) in a supercritical carbon dioxide environment describes simple generation of nanostructured materials such as carbon nanotubes, nanopolyhedra, and copper nanostructured materials. Carbon nanostructured materials are synthesized by supercritical carbon dioxide also as a raw material. Regarding copper nanostructured materials synthesis, comparison with a gaseous environment indicates one merit of generating discharges in a supercritical carbon dioxide environment with high-dissolving power. Without highly elevated environmental temperature, this process may be combined with merits offered by supercritical fluids.