Atsushi Nezu

Formation mechanism of electrically conductive nanoparticles by induction thermal plasmas

Abstract The purpose of this paper is to prepare electrically conductive nanoparticles such as borides, mixture of nitrides and borides by induction thermal plasmas. Another purpose is to correlate the prepared particle composition with thermodynamic parameters. For BNM (M=Ti, Cr, V) system that has low Gibbs free energy of nitridation and boridation, boride and nitride nanoparticles were prepared. While BNM (M=Co, Fe, Mo) system that has high Gibbs free energy of nitridation, boride nanoparticles were mainly prepared. For BNM (M=Ta, Nb) system that has much higher nucleation temperature of metal than that of boron, nanoparticles of nitride with small fraction of boride were prepared, because these systems have different nucleation mechanism. The composition of prepared nanoparticles depends on the Gibbs free energy as well as the nucleation temperature.

Thermal plasma treatment of waste ion-exchange resins doped with metals

Abstract The amount of the waste ion-exchange resins are increasing, therefore, the effective method to reduce the volume of the waste resins, is required. The purpose of this paper is to reduce the weight and volume of the resins by reactive thermal plasmas, and to investigate the reaction mechanism of the resins doped with Co and Cs. In this experiment, cation exchange resins doped with Co and Cs were used as sample resins. The resins were treated by thermal plasmas under atmospheric pressure. The weight was reduced to approximately 51% after 60 min treatment without oxygen injection. At 18 l/min of oxygen flow rate, the weight was reduced to approximately 91%. Small difference in the weight reduction was found among the undoped resins and Co and Cs doped resins. Thermal plasma treatment provides the effective and rapid reduction in weight and the volume of resins. The treatment has good advantages for the stabilization of the doped metals.

Actinometry Measurement of Dissociation Degrees of Nitrogen and Oxygen in N2–O2 Microwave Discharge Plasma

The dissociation degrees of N2 and O2 are examined in a nitrogen–oxygen mixed microwave discharge plasma in a cylindrical quartz tube of 26 mm inner diameter with a discharge pressure of 0.5–1.0 Torr and a microwave power of 600 W by the actinometry method. We measured the electron temperature and density with a Langmuir double probe, while the vibrational and rotational temperatures of the first and second positive bands of N2 were measured by optical emission spectroscopy. Even when the line intensity of atomic nitrogen was weak and partly coincided with the high-intensity band spectrum of the first positive system due to its small dissociation degree, the actinometry method was found to be feasible when the first positive band spectrum, calculated as a function of the rotational and vibrational temperatures, was subtracted from that observed experimentally. It was found that the dissociation degrees of both N2 and O2 increase with the molar ratio of nitrogen in the mixed N2–O2 discharge gas for the same total discharge pressure. The experimental results are discussed by comparison with a simple numerical model based on chemical kinetics in the plasma. It was found that the dissociation of oxygen molecules is enhanced by the collision with excited nitrogen molecules, particularly those with metastable states, whereas that of nitrogen is suppressed by an admixture of oxygen molecules due to the chemical quenching processes of nitrogen atoms.

Ion Acceleration in Arc Jet Plasma Along Open Field Lines

We experimentally study plasma parameters including the ion acoustic Mach number and the flow direction of expanding helium plasma jet flowing along open field lines. It is experimentally found that the ion Mach number increases. We discuss the mechanism of the ion acceleration. Since the ions do not rotate in electromagnets, the Hall and the swirl acceleration do not occur. We investigate the experimental results based on quasi-1-D flow model including the aerodynamic effect and the electric field. Our model describes the ion acceleration. Therefore, it is concluded that the helium ions are accelerated both by the electric field and by the increasing cross-sectional area of the supersonic flowing channel, which is determined by the magnetic flux conservation.

Numerical Study on Acceleration and Deceleration Mechanism of Weakly Ionized Plasma Flowing Supersonically Through Open Field Line

We study supersonic flow of rarefied weakly ionized plasma at diverging magnetic field. While the flows of ions and neutral are treated by particle-based direct simulation Monte Carlo method, the electron is treated as a fluid, i.e., we carried out hybrid simulation. We calculate number density, velocity, temperature, and electric potential of charged particles and neutrals when the arc plasma flows out of the uniform magnetic field into lower pressure region in a steady state. We find the velocity increase just after passing the open magnetic field line, followed by deceleration due to collisions with residual molecules. We also find the temperature increase during the deceleration. In this acceleration/deceleration phenomena, the velocity difference between the neutrals and charged species are found, which also changes the electric potential. We discuss the mechanisms of potential formation along the plasma flow mainly by the pressure difference and the friction force between the charged particles and neutral species. The numerical results are, at least qualitatively, consistent with our previous experimental results.

Characteristics of Cold Argon Arc-Jet Plasma Flowing Along Open-Field-Line and the Effects of Collisions on Deceleration

We report basic characteristics of a cold supersonic argon arc-jet plasma accelerated along open-field-line. The arc jet is generated by arc discharge, and ejected through an anode-nozzle into a rarefied gas wind tunnel with a uniform longitudinal magnetic field. The plasma further expands at the end of magnets, which we refer to as a magnetic-nozzle, where we examine variation in plasma parameters using a para–perp type Mach probe and a four-tip type Mach probe. Our experiments show that plasma potential drops at the magnetic-nozzle area and that the ion flow velocity increases up to about Mach 3, followed by deceleration into subsonic transition. To discuss the mechanism of acceleration and deceleration, we assume a 1-D quasi-neutral flow, and constructed a model to describe the deceleration because of friction between ions and neutral particles at the end of electromagnets, where charged particles are accelerated but neutrals are not. We find reasonable agreement of the experimental results with our modeling of the acceleration and deceleration, although a few issues are left for us to improve.

Spectroscopic determination of vibrational and rotational temperatures of NO molecules in N2–O2 mixture microwave discharge

By using an optical emission spectroscopic measurement method, the vibrational and rotational temperatures of nitrogen monoxide molecules are obtained for N2–O2 mixture microwave discharge plasma. We experimentally observed the γ-band (195–340 nm) spectrum of nitrogen monoxide. We also developed a method of theoretical calculation for its vibrational and rotational spectrum, with which we can fit the experimental results theoretically, so that the vibrational and rotational temperatures can be determined. Some experiments are carried out to examine relaxation process of excited states of the N2–O2 gas mixture plasma together with its N2 ratio dependence. We discussed the dominant molecular processes and excitation kinetics for the formation of NO A 2Σ+ state under different discharge conditions, particularly on the N2–O2 mixture ratio.

Excited State Distributions of Hydrogen Atoms in the Microwave Discharge Hydrogen Plasma and the Effect of Electron Energy Probabilistic Function

To understand the essentiality of the electron energy distribution function in a low-pressure discharge plasma, an experimental study is carried out on the diagnostics of microwave discharge hydrogen plasma with its discharge pressure ~1 torr in a cylindrical quartz tube. The electron kinetic temperature and density are measured by a Langmuir double probe. Number densities of electronically excited states of hydrogen atoms are experimentally examined by an optical emission spectroscopic (OES) measurement of line intensities of the Balmer series. The rotational and vibrational temperatures are observed for the Fulcher-α band spectrum of hydrogen molecule to understand the approximate value to the neutral gas temperature. The number density of the ground state of hydrogen atom is also experimentally estimated from the actinometry measurement. The electron energy probabilistic function (EEPF) is numerically calculated as a solution to the Boltzmann equation. Number densities of excited hydrogen atoms are calculated with the collisional-radiative (CR) model with experimentally measured data as input parameters. It is found that the population densities of excited states of hydrogen atoms become about one order or much larger than those determined by OES measurement if we assume Maxwellian EEPF. The CR model with the EEPF as a solution to the Boltzmann equation theoretically reproduce the experimentally measured values very well.

Kinetic model and spectroscopic measurement of NO (A, B, C) states in low-pressure N2–O2 microwave discharge

A self-consistent kinetic model is developed to study the atomic and molecular processes in the microwave discharge plasma of N2–O2 mixtures. We focus on the NO A 2Σ+, B 2Π, and C 2Π states in the mixture discharge. We find good agreement between the calculated and experimental NO A 2Σ+ densities. On the other hand, the radiation bands from the NO B 2Π and C 2Π states are observed only when the oxygen partial pressure is less than 3%. We discuss the de-excitation processes for the NO B 2Π and C 2Π states in this low-pressure plasma. We also propose that the de-excitation processes involve collision with O2 X for these levels, which can explain the observed spectral disappearance.

Excitation Kinetics of Oxygen O(1D) State in Low-Pressure Oxygen Plasma and the Effect of Electron Energy Distribution Function

Abstract We develop numerical model to discuss the number density and excitation kinetics of O(1D) state in low-pressure discharge oxygen plasma. The governing equations are the Boltzmann equation to describe the electron energy distribution function and the rate equations of the relevant excited states. We have calculated them back and forth until self-consistent solution is obtained. When the rate coefficient of the electron impact dissociation of O2 to O(3P) and O(1D) atoms is assumed to be functions of electron temperature Te with Maxwellian electron energy distribution, the obtained results are found to be inappropriate. When the rate coefficients are written as functions of electron energy distribution function, satisfactory results are obtained as number density distribution of excited states. We also experimentally examined and confirmed the validity of the model by actinometry measurement for number density of the O(3P) state. It is found that we should consider the electron energy distribution function in describing the excitation kinetics of O(1D) state in low-pressure oxygen plasma.