Shinobu Mukasa

Microwave plasma in hydrocarbon liquids

The generation of microwave plasma in liquid with vapor bubbles has been achieved and will soon be applied to high-speed chemical vapor deposition. Vapor bubbles are induced from an electrode by heating. The deposition rate of diamondlike carbon films depends on the pressure and the power of the microwave supply. Polycrystalline silicon carbide is synthesized on a silicon substrate in a mixture of n-dodecane and silicone oil. The dispersion of water droplets in liquid creates many pores on the silicon carbide films. The synthesis of carbon nanotubes can be achieved in liquid benzene.

Radio frequency plasma in water

We generate a radio frequency (RF) plasma in water at an atmospheric pressure by applying an RF power of 13.56 MHz from an electrode. The plasma is in a bubble formed in water. On the basis of hydrogen spectral lines under the assumption of thermal equilibrium, the temperature of the plasma is estimated to be 4000–4500 K. Spectroscopic measurements show that hydrogen and oxygen are excited in the plasma. The plasma is also obtained in tap water or NaCl solution with a high conductivity. In the solution, sodium spectral lines are observed. Colored water containing methylene blue is exposed to the plasma. The absorbence spectra of the colored water before and after exposure to the plasma suggest the decomposition of organic matter due to chemical reactions involving active species, such as OH-radicals.

Discharge Characteristics of Microwave and High-Frequency In-Liquid Plasma in Water

The plasma in water is generated by applying high-frequency (HF) irradiation of 27.12 MHz or microwave (MW) radiation of 2.45 GHz from an electrode. The electrode is heated by joule heating by the HF or MW irradiation, and vapor bubbles are generated simultaneously. The plasma is then ignited inside the bubbles on the electrode. The glow discharge plasma can be maintained in spite of atmospheric pressure due to the cooling effect of the liquid itself. The electron temperature of the plasma generated by the 27.12 MHz radiation is higher than that generated by the 2.45 GHz radiation.

Production of hydrogen in a conventional microwave oven

Hydrogen is produced by generating in-liquid plasma in a conventional microwave oven. A receiving antenna unit consisting of seven copper rods is placed at the bottom of the reactor furnace in the microwave oven. 2.45 GHz microwave in-liquid plasma can be generated at the tips of the electrodes in the microwave oven. When the n-dodecane is decomposed by plasma, 74% pure hydrogen gas can be achieved with this device. The hydrogen generation efficiency for a 750 W magnetron output is estimated to be approximately 56% of that of the electrolysis of water. Also, in this process up to 4 mg/s of solid carbon can be produced at the same time. The present process enables simultaneous production of hydrogen gas and the carbide in the hydrocarbon liquid.

Temperature distributions of radio-frequency plasma in water by spectroscopic analysis

Distributions of emission intensity from radicals, electron temperature, and rotational temperature at a radio frequency of 27.12 MHz plasma in water are clarified by detailed spectroscopy measurement. Through this investigation, the following were observed. The points of maximum emission intensity of Hα, Hβ, O (777 nm), and O (845 nm) are almost the same, while that of OH shifts upward. The electron temperature decreases, while the rotational temperature increases with pressure. The distribution of the electron temperature changes at a threshold pressure, which is concerned with a change in the electron discharge mechanism. The self-bias of the electrode changes from a negative to positive at a threshold pressure. The point of the maximum rotational temperature of OH radicals shifts to approximately 1 mm above that for the maximum intensity of OH emission.

Optimization and analysis of shape of coaxial electrode for microwave plasma in water

The effect of the shape of the electrode to generate 2.45 GHz microwave plasma in pure water is examined. Three variations of a common coaxial electrode are proposed, and compared according to the power required for plasma ignition and the position of plasma ignition in pure water at 6 kPa using a high-speed camera. These coaxial electrodes are calculated using three-dimensional finite-difference time-domain method calculations. The superior shape of coaxial electrode is found to be one with a flat plane on the tip of the inner electrode and dielectric substance located below the tip of the outer electrode. The position of the plasma ignition is related to the shape of the coaxial electrode. By solving the heat-conduction equation of water around the coaxial electrode taking into account the absorption of the microwave energy, the position of the plasma ignition is found to be not where electric field is the largest, but rather where temperature is maximized.

27.12 MHz plasma generation in supercritical carbon dioxide

An experiment was conducted for generating high-frequency plasma in supercritical carbon dioxide; it is expected to have the potential for applications in various types of practical processes. It was successfully generated at 6−20 MPa using electrodes mounted in a supercritical cell with a gap of 1 mm. Emission spectra were then measured to investigate the physical properties of supercritical carbon dioxide plasma. The results indicated that while the emission spectra for carbon dioxide and carbon monoxide could be mainly obtained at a low pressure, the emission spectra for atomic oxygen could be obtained in the supercritical state, which increased with the pressure. The temperature of the plasma in supercritical state was estimated to be approximately 6000−7000 K on the assumption of local thermodynamic equilibrium and the calculation results of thermal equilibrium composition in this state showed the increase of atomic oxygen by the decomposition of CO2.

A supercritical carbon dioxide plasma process for preparing tungsten oxide nanowires

A supercritical carbon dioxide (CO(2)) plasma process for fabricating one-dimensional tungsten oxide nanowires coated with amorphous carbon is presented. High-frequency plasma was generated in supercritical carbon dioxide at 20 MPa by using tungsten electrodes mounted in a supercritical cell, and subsequently an organic solvent was introduced with supercritical carbon dioxide into the plasma. Electron microscopy and Raman spectroscopy investigations of the deposited materials showed the production of tungsten oxide nanowires with or without an outer layer. The nanowires with an outer layer exhibited a coaxial structure with an outer concentric layer of amorphous carbon and an inner layer of tungsten oxide with a thickness and diameter of 20-30 and 10-20 nm, respectively.

Degradation of methylene blue by radio frequency plasmas in water under ultraviolet irradiation.

The degradation of methylene blue by radio frequency (RF) plasmas in water under ultraviolet (UV) irradiation was studied experimentally. When the methylene blue solution was exposed to RF plasma, UV irradiation from a mercury vapor lamp enhanced degradation significantly. A lamp without power supply also enhanced degradation since weak UV light was emitted weakly from the lamp due to the excitation of mercury vapor by stray RF power. Such an enhancement is explained by the fact that after hydrogen peroxide is produced via the recombination process of OH radicals around the plasma, OH radicals reproduced from hydrogen peroxide via the photolysis process degrade methylene blue.

Characteristics of in-liquid plasma in water under higher pressure than atmospheric pressure

The excitation temperature, electron density, temperature of OH, and behavior of bubbles generated by a 27.12 MHz in-liquid plasma are investigated in water under pressures ranging from 0.1 to 0.4 MPa. The excitation temperature decreases as the pressure increases and, conversely, the temperature of OH and the electron density increase. Since the plasma can be generated stably even under high-pressure conditions and the liquid provides a cooling effect, the electrode is not damaged by the heat. The bubbles generated from the tip of the electrode have a fixed relationship between their diameter and departure frequency. The in-liquid plasma can be stably generated even under high pressures and it maintains a high superheated state of a few thousand K. A boiling phenomenon in the in-liquid plasma uses the plasma itself as a heat source.