Tadahiro Sakuta

Investigation on plasma-quenching efficiency of various gases using the inductively coupled thermal plasma technique: effect of various gas injection on Ar thermal ICP

Application of the inductively coupled thermal plasma (ICTP) technique was proposed for investigating plasma-quenching efficiency of various gases including the arc-quenching medium of SF6. The ICTP enables us to study fundamentally the effect of gas injection on thermal plasma without any impurities because it has no electrode. Seven kinds of gases including CO2, SF6 and environmentally benign gases (N2, O2, air, He and H2) were injected into Ar ICTP. Spectroscopic observation was carried out in order to investigate a change in the excited state of Ar atoms due to addition of these gases. Radial distributions of the radiation intensity of Ar spectral lines and the temperature of Ar ICTP were estimated. It was found that 10% CO2 addition causes a remarkable decline of the radiation intensity and temperature at any radial position, similarly to 2% SF6 addition. Two-dimensional local thermal equilibrium modelling for Ar ICTP also revealed that CO2 causes temperature decay more than the other gases of N2, O2, air, He and H2 except SF6.

Chemically non-equilibrium modelling of N2 thermal ICP at atmospheric pressure using reaction kinetics

A two-dimensional hydrodynamic model for an N2 inductively coupled thermal plasma (ICTP or thermal ICP) at atmospheric pressure was developed using reaction rates without the chemical equilibrium (CE) assumption. Particle composition distribution in the N2 ICTP was derived by solving the mass conservation equations for each of the particles, considering diffusion, convection and production terms. The electrical conductivity, mass density and diffusion coefficient were calculated at each of the calculation steps with the derived particle composition distributions. Using this model, the influence of gas flow rate on chemical composition distribution was investigated. The dependence of mass flow of N atom on gas flow rate was obtained. From the result, a large deviation from CE in the distribution of the particle composition was found, especially near the wall of the ICTP.

Experimentally diagnosed transient behavior of pulse modulated inductively coupled thermal plasma

An argon/hydrogen (argon: 89% molar) pulse-modulated radio-frequency inductively coupled thermal plasma was generated at adequate power level (active plasma power up to about 15 kW) and has features such as control of thermal flux in time domain and introduction of nonequilibrium phenomenon. These characteristic features have ample attraction for thermal plasma materials processing. A solid state power source, which supplies the electric power at a nominal frequency of 1 MHz with high matching efficiency of 90%, was used as the pulsing generator to generate the induction plasma. Experimental measurements were carried out to analyze the transient responses of ArI (at 751 nm wavelength) line and current intensities for the imposed pulsing signal. Results were obtained for pressure range from 200 to 760 Torr, with a power on time in the range from 2 to 20 ms while the power off time ranged from 5 to 12 ms. The plate power level was varied from 11 to 17 kW though the results presented in the present article a...

Controlled generation of pulse-modulated RF plasmas for materials processing

Pulse-modulated radio frequency (RF) inductively coupled plasmas for materials processing applications were generated using a voltage-control-type power source, and the operating ranges for controlled generation were extended to sufficiently low shimmer power levels. When the plasma was generated at atmospheric pressure and a high power level of 17 kW, the low power level typically went down to about 4 kW for an Ar–H2 plasma and 3 kW for an Ar–N2 plasma, levels too low to sustain the continuous plasmas. The overshoot and undershoot at the beginnings of power change were reduced considerably by using an exponential voltage control signal. Spectroscopic measurements of the radiation intensity of the Ar atomic spectral line (751.5 nm) showed that the plasma temperatures varied with time and that the characteristic times of the plasmas depended on the operating conditions and the position in the plasma generator. The characteristic times in the discharge zone may be largely determined by the competition among ionization, recombination and convection in the pulsed plasmas. The characteristic times estimated using an electron transportation model are reasonably in line with those determined from measured emission intensities. The difference between the plasma properties at the higher and lower power levels was large enough to give rise to the nonequilibrium states at the instances of pulse-on and pulse-off, and to the increase in the concentration of chemically active radical species. This offers a unique physico-chemical condition for materials processing. The ranges of controlled generation were determined for the Ar–H2 and Ar–N2 plasmas at pressures from 27 to 101 kPa.

Synthesis of fullerenes from carbon powder by using high power induction thermal plasma

Radio frequency (rf) inductively coupled thermal plasma (ICTP) was used to fabricate fullerenes (C60,C70, etc.) by direct evaporation of carbon powder injected into the plasma. Spectroscopic observation of the plasma was made for molecular band spectra of C2 and atomic lines of C. The formation of fullerenes C60 and C70 as well as higher fullerenes were checked and recognized by high performance liquid chromatography (HPLC) and time-of-flight mass spectrometer (TFMS). The suitable conditions for the synthesis of fullerenes within the experimental conditions adopted were 10-kPa plasma pressure, with a considerably higher flow rate of approximately 150 l/min for mixed-gas condition of Ar, He and CO2, with carbon powder of average diameter 20 μm. The results showed that the productivity of fullerenes has a relation to the intensity of C2 molecular and C atomic spectra from the induction plasma. Mixing of Si with C particles has a kind of role in enhancing the synthesis rate of fullerenes C60, as well as the higher order fullerenes.

Effects of plasma diameter and operating frequency on dynamic behaviour of induction thermal plasma

A numerical approach is presented for the analysis of the dynamic behaviour of the induction thermal plasma under step-like change of the sustaining magnetic field. The time-dependent energy conservation equation which includes thermal conduction, radiation and convection loss terms was solved numerically in conjunction with the continuity equation and Maxwells electromagnetic field equations. A one-dimensional model with radial direction is adopted for the first approximation to analyse the transient aspects of the radial distributions of the temperature, the penetrating electromagnetic fields and the input and losses. Calculations were carried out for an Ar-induction thermal plasma with a diameter from 2 to 7 cm, operated at a frequency of 3 MHz and a pressure of 760 Torr. Special attention was given to the time constant which is a measure of the time necessary for the plasma to converge to a new steady state. The response time of the induction thermal plasma was a few milliseconds and had a strong dependence upon the diameter of the plasma across which the mass and energy diffusions take place under the off-balancing condition between the input and the loss. The transient aspect of a high-power induction plasma operated with a low frequency of 300 kHz is also discussed.

Effect of ambient gas and pressure on fullerene synthesis in induction thermal plasma

Abstract Fabrication of fullerenes (C 60 , C 70 , etc.) by direct evaporation of C–Si mixed powder using radio frequency inductively coupled thermal plasma were made to find a suitable gas kind and pressure for fullerene synthesis. The results showed that: (1) 150 Torr lower pressure and He/Ar mixed gas are more suitable for fullerene synthesis than higher pressure and pure Ar gas, and (2) it is important for fullerenes synthesis that the radiation intensity of C 2 and C spectra is stronger at 10 mm below coil end, i.e. high-temperature plasma torch region, and it becomes very weak at 150 mm below coil end, i.e. inside the reactor chamber. Furthermore, numerical simulation was carried out to derive the temperature and gas flow fields in the plasma torch and reactor chamber. The calculation results indicate that He mixing into Ar gas can increase the temperature-gradient at the center axis of the plasma torch, and that pressure has a distinct effect on plasma axial velocity distribution.

On the determination of the multi-temperature SF6 plasma composition

We have calculated the multi-temperature SF6 plasma composition by two different methods. One method is based on the mass action law whereas the other method uses a kinetic chemical reaction scheme. The first approach is furthermore split up in two different calculations. One is based on the multi-temperature Saha equation and the other is based on the excitation temperature of the involved species. With respect to the electron density we conclude that the multi-temperature Saha equation results in appreciable higher number densities compared to both the kinetic scheme and the excitation-temperature-based calculations. The difference increases with decreasing gas temperature and increasing electron temperature (i.e. with increasing degree of kinetic non-equilibrium). The kinetic scheme and the excitation-temperature-based calculation yield comparable results for higher gas temperatures. In this region the excitation-temperature-based method is preferable over the kinetic method. However, for the dielectric case (i.e. lower gas temperature) we find different results at higher electron temperatures. The excitation-temperature-based calculation method requires additional modelling at this point.

Nonthermal and nonequilibrium effects in high-power pulsed ICP and application to surface modification of materials*

Newly developed pulse-modulated high-power inductively coupled plasma (ICP) is expected to offer the unique physicochemical condition, such as the increased concentration of chemically reactive species, as well as the appropriate heat flux for materials processing. Two kinds of oxide materials, titanium and zinc oxide, were placed at the downstream of ArH2 ICP and irradiated in the plasma of continuous (CN) and pulse-modulated (PM) modes. The CN-ICP irradiation at the position close to the plasma tail gave rise to the thermal reduction of oxides. In the PM-ICP irradiation, the degree of thermal reduction depended on the lower power level during pulse-off time, as well as the total electric power. Irradiation in PM-ICP led to the increased formation of oxygen vacancies in titanium dioxide. In the case of zinc oxide, the UV emission efficiency was improved by PM-ICP irradiation, while the green emission became predominant by CN-ICP irradiation at the appropriate position. Induced effects in the two oxides by PM-ICP would be related to the high concentration of hydrogen radicals in the plasma.

Transport and thermodynamic properties of SF/sub 6/ gas contaminated by PTFE reinforced with Al/sub 2/O/sub 3/ and BN particles

The computational approach in which time-dependent balance equations of mass, momentum, and energy are solved numerically is becoming an important technique for analyzing electric arcs in a gas circuit breaker (GCB) or gas-insulated switchgear (GIS). In this paper, the transport and thermodynamic properties of SF/sub 6/ gas necessary for this approach as basic data are calculated under multimixed condition by PTFE(-C/sub 2/F/sub 4/-) reinforced with alumina (-Al/sub 2/ O/sub 3/-) or BN particles. Calculations are carried out for a wide range of temperatures from 1500 to 30000 K, of pressures from 0.1 to 0.4 MPa, and of concentration ratios from 0 to 50%. The results show that the change of electron density is significant for alumina-reinforced PTFE, but insignificant for BN-reinforced PTFE contamination. Henceforth, the electrical conductivity varies only for alumina-reinforced PTFE contamination. The thermal conductivity, however, changes distinctly by mixing alumina-reinforced PTFE as well as by mixing BN-reinforced PTFE. Up to three characteristic peaks (T<3600 K), the thermal conductivity decreases, but above this temperature, augmented thermal conductivity is noticed until 7000 K for alumina and until 12000 K for BN. All thermodynamic properties and viscosity vary only at a higher level of contamination, at or above 10% admixture ratio.