Pulse-modulated Induction Thermal Plasma Research Articles

Numerical study on the evaporation process of feedstock powder under transient states in pulse-modulated induction thermal plasmas for nanoparticle synthesis

A numerical model was developed on the evaporation process of intermittently fed feedstock into pulse-modulated induction thermal plasma (PMITP). This experimental method combining the PMITP with intermittent feedstock feeding has been developed for the synthesis of large amounts of nanoparticles. To treat this method, a full transient numerical model for the PMITP was combined with transient equations of particle behaviour with different time steps. Transient two-way interactions from particle evaporation to the PMITP were also taken into account at each time step as source terms in mass, momentum and energy conservations. The temperature, gas flow field and mass fraction of feedstock vapor were obtained in the transient state in the PMITP with the motion of feedstock particles. The calculation results were compared with the experimental results for the Ar I line and feedstock atomic line. They were found to be in fair agreement even in the transient state.

High rate synthesis of graphene-encapsulated silicon nanoparticles using pulse-modulated induction thermal plasmas with intermittent feedstock feeding

Graphite-coated silicon nanoparticles were synthesized using pulse-modulated induction thermal plasma (PMITP) with a time-controlled feedstock feeding (TCFF) method at high production rates. The PMITP+TCFF method is a high production method of nanoparticles with a high feedstock feeding rate of several g min−1. Micro-sized silicon powder as feedstock was injected synchronously into Ar/H2 PMITP to be evaporated completely. Argon/CH4 gas mixture was introduced as a quenching gas and a carbon source from three different downstream portions for comparison. As a result, graphene-encapsulated Si nanoparticles were successfully synthesized with Ar/CH4 gas mixture injection from adequate downstream port.

Numerical thermofluid simulation on tandem type of induction thermal plasmas with and without current modulation in a lower coil

This paper describes numerical calculations on tandem-type induction thermal plasmas with and without current modulation of the lower coil. The tandem type of induction thermal plasma (tandem-ITP) has been developed with two independent rf coils in one plasma torch for stable operation and spatial control of temperature field in ITP. Furthermore, the coil current modulation for tandem-ITP, i.e. tandem type of pulse-modulated induction thermal plasma (tandem-PMITP), is expected to offer a temporo-spatially varying temperature field in the plasma torch. First, as described in this paper, a numerical thermofluid model was developed for tandem-ITP/PMITP with an upper coil and a lower coil to investigate unique temperature and gas flow fields. Secondly, the influence of gap length between the upper coil and the lower coil was studied in the tandem-ITP under steady state conditions. Results show that a shorter gap length between two coils of the tandem-ITP is favourable for materials processing because it suppresses unfavourable radial gas flow to the wall in the plasma torch. Thirdly, transient calculation was conducted for the tandem-PMITP with current modulation in the lower coil to investigate effects of an off-time modulation parameter on the temperature field. Results showed that coil current modulation provides unique temporo-spatially varying temperature and gas flow fields. Finally, calculation results for a tandem-PMITP were compared with those for a single-coil PMITP. Comparison revealed that tandem-PMITP has higher robustness in modulated induction thermal plasma by maintaining a higher-temperature region than that of the single-coil MITP. The tandem-PMITP can provide more controlled temperature and gas flow fields.

High-rate synthesis of Si nanowires using modulated induction thermal plasmas

Using 20 kW Ar–H2 pulse-modulated induction thermal plasma (PMITP) with time-controlled feeding of feedstock (TCFF), numerous Si nanowires were synthesized rapidly at 1,000 mg h−1 without the intentional addition of catalysts. The PMITP + TCFF method is our original method for nanomaterial synthesis. The PMITP periodically provides a unique field including higher-temperature plasma during “on-time” and a lower-temperature plasma during “off-time”. For rapid and efficient evaporation, metal-grade Si powder feedstock was intermittently injected synchronously into the generated Ar–H2 PMITP. The synthesized products were analyzed using various analytical techniques. The synthesized products were Si nanowires 10–30 nm in diameter with a SiOx surface layer.

Two-dimensional spectroscopic observation of a pulse-modulated induction thermal plasma torch for nanopowder synthesis

The two-dimensional distributions of spectral radiation intensities in the plasma torch were observed for the pulse modulated induction thermal plasmas (PMITP) with continuous or intermittent feedstock feeding for TiO2 nanopowder synthesis. For this observation, an imaging spectrophotometer with a high speed video camera were adopted. The evaporation of feedstock Ti powder, the formation of TiO and TiO transportation were investigated from the observation results of a Ti atomic spectral line and TiO molecule spectra as well as those of Ar and O atomic lines. An interpretation was suggested from the observation results for Ti feedstock evaporation and TiO formation in nanoparticle synthesis using a PMITP with intermittent feedstock feeding.

A method for large-scale synthesis of Al-doped TiO2 nanopowder using pulse-modulated induction thermal plasmas with time-controlled feedstock feeding

A unique method of large-scale synthesis of Al-doped TiO2 nanopowder was developed, using 20 kW Ar–O2 pulse-modulated induction thermal plasmas (PMITP) with time-controlled feedstock feeding (TCFF). This PMITP–TCFF method is characterized by intermittent feedstock powder feeding synchronized with modulation of the power of the PMITP. The method enables heavy-load feeding of raw material powder to the thermal plasmas for complete evaporation. The nanopowder synthesized was analysed using different methods including field emission scanning electron microscopy, x-ray diffraction (XRD) spectroscopy, bright-field transmission electron microscopy (TEM), TEM/energy dispersive x-ray (EDX) mapping, x-ray photoelectron spectroscopy, and spectrophotometry. The results showed that Al-doped TiO2 nanopowder can be synthesized with mean diameters of 50–60 nm. The Al doping in TiO2 was confirmed from the constituent structure in the XRD spectra, the uniform presence of Al in the nanopowder in the TEM/EDX mapping, the chemical shift in the x-ray photoelectron spectra, and the absorption edge shift in the optical properties. The rate of production of the Al-doped TiO2 nanopowder was estimated as 400 g h−1.

Time Evolution in Radiation Intensities of C2 and H Spectra in Ar/CH4/H2 Pulse Modulated Induction Thermal Plasmas for Diamond Film Deposition

The present paper describes the first trial application of an Ar/CH4/H2 pulse modulated induction thermal plasma (PMITP) to polycrystalline diamond film deposition on a silicon substrate. The PMITP system has been originally developed by our group to control the heat flux and densities of chemical species in thermal plasma treatment. Polycrystalline diamond film was successfully deposited on a 25×25 mm2 Si(100) substrate using the PMITP. Spectroscopic observation was carried out to measure the time evolution in the spectral intensity from the PMITP irradiated to the Si substrate. We found that the C2 molecular spectra have strong intensities at 4–7 ms after rise-up of the coil current, while the H spectral line has much high intensity at 6–12 ms in a 15 ms modulation cycle. This implies that the PMITP produces repetitive irradiation from different chemical fields in one modulation cycle, which may effectively provide diamond film deposition.

A large amount synthesis of nanopowder using modulated induction thermal plasmas synchronized with intermittent feeding of raw materials

A large amount synthesis method for titanium dioxide (TiO2) nanopowder is proposed by direct evaporation of titanium powders using Ar-O2 pulse-modulated induction thermal plasma (PMITP). To realize a large amount synthesis of nanopowder, the PMITP method was combined with the intermittent and heavy load feeding of raw material powder, as well as the quenching gas injection. The intermittent powder feeding was synchronized with the modulation of the coil current sustaining the PMITP for complete evaporation of the injected powder. Synthesized particles by the developed method were analyzed by FE-SEM and XRD. Results indicated that the synthesized particles by the 20-kW PMITP with a heavy loading rate of 12.3 g min−1 had a similar particle size distribution with the mean diameter about 40 nm to those with light loading of 4.2 g min−1.

Influence of coil current modulation on TiO 2 nanoparticle synthesis using pulse-modulated induction thermal plasmas

This paper describes the relation between the size of synthesized nanoparticles and the temperature variation of the surrounding plasma in TiO 2 nanoparticle synthesis using pulse-modulated induction thermal plasmas (PMITP). The Ar–O 2 PMITP at 20 kW was used to obtain repetitional temperature fields at pressure of 200 Torr. The TiO 2 nanoparticles were synthesized by direct injection of Ti powder with mean diameter of 45 μm. Dependence of the nanoparticle size on modulation parameters such as the duty factor and the shimmer current level of the PMITP were investigated experimentally. Instantaneous temperature evolution was evaluated through spectroscopic observation. Experimental results show that the temperature decay rate in the PMITP reached 10 510 6 K/s, and that the mean diameter of synthesized particles decreased with the temperature decay rate.

Nanoparticle synthesis using high-powered pulse-modulated induction thermal plasma

Nanoparticle synthesis was performed using the high-powered pulse-modulated induction thermal plasma (PMITP) technique to study the effect of coil current modulation on synthesized nanoparticles. This is the first paper to present a summary of results of TiO2 nanoparticle synthesis using high-power Ar–O2 PMITP at 20 kW. The synthesized particles were analysed using field emission scanning electron microscopy and x-ray diffractometry. In addition, optical emission spectroscopy was used during nanoparticle synthesis experiments to measure TiO spectra and to determine the time-averaged vibrational and rotational temperatures of TiO in the reaction chamber. The results showed that the PMITP produced smaller nanoparticles and a narrower size distribution of particles. Moreover, PMITP provided a lower temperature region in the reaction chamber downstream of the plasma torch than such regions in non-modulated thermal plasmas.

Influence of on-time on increased number density of excited nitrogen atoms downstream of pulse-modulated induction thermal plasmas

The on-time is one operation parameter for pulse-modulated induction thermal plasmas (PMITPs). Its influence on the increased number density of excited nitrogen atoms flowing into a reaction chamber installed downstream of the Ar–N2 PMITP was investigated experimentally. The increased number density of excited nitrogen atoms downstream of the PMITP was found experimentally in our previous work (Tanaka et al 2006 Appl. Phys. Lett. 89 031501). The time evolution in the radiation intensity from the excited nitrogen atom was measured in Ar–N2 PMITP to obtain the on-time effect by changing the on-time in the wide range of 5–950 ms for different nitrogen gas flow rates. From these measurement results, an optimal on-time range was found for the increased density of the excited nitrogen atom by modulation of the coil current, which was equivalent to the averaged input power of 15–20 kW.

Control of Nitrogen Atomic Density and Enthalpy Flow Into Reaction Chamber in $\hbox{Ar/N}_{2}$ Pulse-Modulated Induction Thermal Plasmas

Simultaneous control of the nitrogen atomic density and the enthalpy flow onto the specimen installed downstream of the plasma torch was accomplished using Ar/N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> pulse-modulated induction thermal plasmas (PMITPs). Such simultaneous control was difficult to realize because the increasing input power into conventional nonmodulation thermal plasmas increases the number density of the nitrogen atoms, but it also increases the enthalpy flow onto the specimen. The behavior of the excited nitrogen atoms was measured through spectroscopic observation. The specimen's surface temperature was measured using a radiation thermometer. Then, the net enthalpy flow onto the specimen was estimated. Results showed that the modulation of coil current increases the time-averaged nitrogen atomic density and decreases the time-averaged enthalpy flow during the modulation cycle onto the specimen irradiated by the Ar/N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> PMITP. This result was confirmed by results from the developed 2-D two-temperature chemical nonequilibrium model of the Ar/N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> PMITP

Controlling the number of excited atoms flowing into the reaction chamber using pulse-modulated induction thermal plasmas at atmospheric pressure

Controlling the number of excited atoms into a reaction chamber was investigated in Ar pulse-modulated induction thermal plasma (PMITP) with molecular gases at atmospheric pressure. Such control is important in promoting the modification of substrate surfaces in thermal plasma processing. The coil current required for sustaining a PMITP was modulated by setting the firing angle of the metal-oxide-semiconductor field-effect transistor in the inverter power supply used as a high-frequency power source. The radiation intensities of the excited atomic lines were measured using the optical emission spectroscopy technique to estimate the number of excited atoms flowing into the reaction chamber. In particular, we found an increase in the number of excited Ar and H atoms flowing into the reaction chamber in Ar or Ar–H2 PMITPs as a result of the pulse modulation of the coil current.

Simultaneous control of numerical enhancement of N atoms and decrease in heat flux into reaction chamber using Ar–N2 pulse-modulated induction thermal plasmas

A numerical enhancement of nitrogen atoms and a simultaneous decrease in heat flux flowing into the reaction chamber were found using a high-power Ar–N2 pulse-modulated induction thermal plasma. Optical emission spectroscopy was carried out to estimate the relative number of excited nitrogen atoms flowing into the reaction chamber. The relative heat flux into the reaction chamber was evaluated from surface temperature measurement of a metal specimen installed at the downstream portion of the plasma torch. Results showed that decreasing the shimmer current level, which means the modulation degree of the coil current, increases the number of excited nitrogen atoms, while the heat flux can be reduced compared to a conventional steady state induction thermal plasma.

Time evolution in visible light emission from high-power Ar pulse-modulated induction thermal plasmas

Transient behavior of a 15-kW Ar pulse-modulated induction thermal plasma (PMITP) at a pressure of 31 kPa was observed using a high-speed video camera. The PMITP method is one of new techniques for controlling temperature and radical density in time-domain in high-power thermal plasmas. The adoption of a high-power metal-oxide-semiconductor field-effect transistor (MOSFET) inverter power supply makes it possible to sustain PMITPs with high power-conversion efficiency. The high-speed video pictures provide two-dimensional, time-dependent radiation intensity distributions from PMITPs, which are useful for understanding an unique transition behavior of PMITPs.

Dynamic responses of Ar–CO 2 and Ar–N 2 induction thermal plasmas in pulse amplitude modulation approach

Solving a two-dimensional LTE (Local Thermodynamic Equilibrium) code, simulation of Ar–CO 2 and Ar–N 2 PMITP (Pulse Modulated Induction Thermal Plasma) has been carried out. The active plasma power, fundamental frequency and gas flow rate are 27 kW, 0.45 MHz and 100/102.5 slpm, respectively. It has been found that the injection of excess dissociative molecular gases, shrinks the plasma and slows down the plasma response at the instant of off-to-on-pulsing transition, which results mainly from the molecular dissociation and ionization. Plasma response has been found faster at higher SCL (Shimmer Current Level) for the same duty factor during the on pulsing transition, and hardly affected by SCL and molecular gas addition during off pulsing transition. Pressure does not appreciably affect the response of Ar–CO 2 plasma, whereas response of Ar–N 2 plasma becomes a bit faster at increased pressure. Part of the simulated results is compared with the corresponding experimental results. The agreement is found in some cases, and the remaining discrepancies between the results are critically evaluated with regard to various possible reasons.

Stable operation region and dynamic behavior of Ar pulse‐modulated induction thermal plasma with different molecular gases

AbstractThe pulse‐modulated induction thermal plasma (PMITP) was established in Ar gas with additional gases such as H2, O2, N2, and CO2 to investigate the effects of various gas inclusions on the transient characteristics of thermal plasma. The dynamic response times of the thermal plasmas were estimated from the measured time‐varying radiation intensity of Ar spectral line. Furthermore, time evolution of the Ar excitation temperature was determined by the two‐line method. The results indicated that especially in Ar‐CO2 PMITP, decreasing the current level in a modulation cycle raised the on‐delay time which means a recovery time from a low‐temperature state to a high‐temperature one. In addition, the inclusion of CO2 in Ar PMITP caused lower temperatures just before the on‐operation compared with other gases used in the experiment. © 2003 Wiley Periodicals, Inc. Electr Eng Jpn, 143(4): 1–11, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.10144

Temperature control of Ar induction thermal plasma with diatomic molecular gases by pulse amplitude modulation of the coil current

A high-pressure inductively coupled thermal plasma (ICTP) system was developed; it can modulate the amplitude of the induction coil current periodically with a cycle of several hundred hertz by switching a metal-oxide semiconductor field effect transistor inverter power supply. This type of ICTP was named a `pulse-modulated induction thermal plasma (PMITP)'. The PMITP in different gases of Ar, Ar–H2, Ar–N2 and Ar–O2, was successfully generated at an input power of 30 kW under atmospheric pressure conditions. Shimmer current level (SCL), which is the ratio of lower to higher levels of coil current in modulation, can be reduced from 100% to 40% to sustain the Ar PMITP. The influence of SCL and diatomic molecular gas-inclusion on the PMITP behaviour was investigated. Temperature variation of PMITP was evaluated by the two-line method. The minimum value of the temperature measured during a modulation cycle proved to decrease drastically with a reduction of SCL by over 1000 K, especially with inclusion of the diatomic molecular gas, with the maximum temperature value almost unchanged. The inherent response time of the PMITP was found to have a magnitude of several milliseconds; it increases with decreasing SCL and with the inclusion of a diatomic molecular gas.

Extinguishing phenomenon and critical discharge boundaries of argon and molecular-gas-seeded argon pulse-modulated induction thermal plasmas

A stable discharge region of pulse modulated induction thermal plasma at atmospheric pressure has been determined in terms of the duty factor (DF) and the shimmer current level (SCL) both experimentally and numerically. The experiment was performed with a MOSFET-employed inverter-feed pulse modulated power supply unit having a maximum power of up to 50 kW. The on-time of the pulsing signal was fixed at 10 ms while the off-time was varied to determine the critical/minimum DF up to which plasma discharge sustained, for a fixed SCL. In this way, for different SCLs, critical/minimum DFs were determined for different gas combinations. The gas flow was composed of 100 lpm argon and 2.5 lpm H2, N2 or CO2 as secondary gas through the sheath channel only. Steady state coil currents for Ar, Ar–H2, Ar–N2 and Ar–CO2 were 244 A, 263 A, 296 A and 296 A, respectively. In the numerical part, a two-dimensional local thermodynamic equilibrium (LTE) code was used to predict the discharge boundary for the same operating conditions as those of the experiment. Both experimental and simulated results reveal that injection of dissociative molecular gases greatly shrinks the stable discharge region, which is attributed to the lowering of temperature and particle density. It was found that the stable discharge region predicted numerically was smaller than that of the experimental one. This implies that a non-LTE plasma has a discharge zone wider than that of an LTE plasma.

A comparative study of transient characteristics of argon and hydrogenated-argon pulse-modulated induction thermal plasma

Solving a time-dependent two-dimensional local thermodynamic equilibrium (LTE) model simulation of Ar and Ar-H/sub 2/ atmospheric pressure, a high-power RF-induction thermal plasma was performed. The effects of shimmer current level (SCL) in pulse-modulated mode and hydrogen concentrations on different flow fields were predicted. The radiation intensities of Ar I (751 nm) for different SCL were calculated from the temperature fields. For the same operating conditions as simulation, plasma was successfully generated in pulse-modulated mode and spectroscopic measurements were carried out to investigate the effects of SCL upon temporal plasma properties. Response times (rising, falling, on-delay, and off-delay time) of temporal radiation intensity were crosschecked for both experimental and simulated ones. The rising time increased gradually with the decrease of SCL, though the falling time remained almost unchanged with SCL. For example, for Ar plasma at 86%, 79%, 72%, 65%, 50%, and 40% SCL the rising times were 2.7, 3.0, 3.4, 3.4, 3.6, and 3.8 ms, respectively. And for Ar-H/sub 2/ plasma (2.4% H/sub 2/), at 87%, 77%, 72%, 63%, 55%, and 45% SCL, rising times were 2.5, 3.0, 3.0, 3.4, 3.7, 3.9, and 4.0 ms, respectively. Hydrogen inclusion slowed down the plasma response during the off-to-on pulsing transition at lower SCL and constricted the plasma axially. Finally, part of the simulated results was compared with experimental determinations and acceptable agreements were found. The discrepancies, in few cases, explicated mainly that the LTE assumption did not prevail in pulse-modulated plasma, especially around the on-pulse transition.