Three-dimensional numerical simulation of heating and melting behaviors of cerium oxide powders in radio frequency thermal plasma

The present study aims at the numerical simulations of the melting process of cerium oxide particles in RF thermal plasma.The physical model and the calculating method were described firstly;the interaction between cerium oxide particles and plasma was analyzed;specific attention was given to the effects of particle initial size,injection velocity on the particle melting and trajectory in plasma.The influence of the temperature field and velocity field distribution of the plasma around the particle trajectory on the melting effect is analyzed,and the relationship between the heat absorption efficiency of the particles and the particle size reduction process is further determined.It is also found that there exists an optimal particle initial injection velocity which led to a more concentrated final particle size distribution and a more significant reduction of particle size.The results could provide effective guidance for understanding the plasma spheroidization process of uranium dioxide and cerium dioxide powder particles.

Spheroidization of molybdenum powder by radio frequency thermal plasma

To control the morphology and particle size of dense spherical molybdenum powder prepared by radio frequency（RF） plasma from irregular molybdenum powder as a precursor, plasma process parameters were optimized in this paper. The effects of the carrier gas flow rate and molybdenum powder feeding rate on the shape and size of the final products were studied. The molybdenum powder morphology was examined using high-resolution scanning electron microscopy. The powder phases were analyzed by X-ray diffraction. The tap density and apparent density of the molybdenum powder were investigated using a Hall flow meter and a Scott volumeter. The optimal process parameters for the spherical molybdenum powder preparation are 50 g/min powder feeding rate and 0.6 m^3/h carrier gas rate. In addition, pure spherical molybdenum powder can be obtained from irregular powder, and the tap density is enhanced after plasma processing. The average size is reduced from 72 to 62 μm, and the tap density is increased from 2.7 to 6.2 g/cm^3. Therefore, RF plasma is a promising method for the preparation of high-density and high-purity spherical powders.

Methane conversion in the presence of oxygen under low-temperature radio frequency plasma

Methane conversion in the presence of oxygen under low-temperature radio frequency （RF） plasma was investigated.The experiment results indicated that the following four factors,i.e.,discharge voltage,discharge area,O2/CH4 molar ratio and total gas flowrate,affected remarkably the reaction performance.The optimum reaction conditions of methane conversion in the presence of O2 under RF plasma are as follows：discharge voltage 1050 V,discharge area 989.1mm 2,O2/CH4 molar ratio 1/10 and total gas flowrate 200 ml/min.A methane conversion of 91% could be reached under the optimum conditions.Oxygen is good for the breaking of C-H bonds and also acts as a sort of thinner.According to the low-temperature plasma characteristics,the macroscopic kinetics model of methane conversion in the presence of O2 under radio frequency plasma was studied.

Experimental Investigation on Electromagnetic Attenuation by Low Pressure Radio-Frequency Plasma for Cavity Structure

This paper reports on an experiment designed to test electromagnetic（EM）attenuation by radio-frequency（RF）plasma for cavity structures.A plasma reactor,in the shape of a hollow cylinder,filled with argon gas at low pressure,driven by a RF power source,was produced by wave-transmitting material.The detailed attenuations of EM waves were investigated under different conditions：the incident frequency is 1-4 GHz,the RF power supply is 13.56 MHz and1.6（-3） k W,and the argon pressure is 75-200 Pa.The experimental results indicate that 5-15 d B return loss can be obtained.From a first estimation,the electron density in the experiment is approximately（1.5-2.2）×1016m（-3）and the collision frequency is about 11（-3）0 GHz.The return loss of EM waves was calculated using a finite-difference time-domain（FDTD）method and it was found that it has a similar development with measurement.It can be confirmed that RF plasma is useful in the stealth of cavity structures such as jet-engine inlet.

Surface modification of silicone rubber by CF4 radio frequency capacitively coupled plasma for improvement of flashover

The flashover performance of insulating materials plays an important role in the development of high-voltage insulation systems.In this paper,silicone rubber(SIR)is modified by CF4 radio frequency capacitively coupled plasma(CCP)for the improvement of surface insulation performance.The discharge mode and active particles of CCP are diagnosed by the digital single-lens reflex and the spectrometer.Scanning electron microscopy and x-ray photoelectron spectroscopy are used for the surface physicochemical properties of samples,while the surface charge dissipation,charge accumulation measurement,and flashover test are applied for the surface electrical characteristics.Experimental results show that the fluorocarbon groups can be grafted and the surface roughness increases after plasma treatment.Besides,the surface charge dissipation is decelerated and the positive charge accumulation is inhibited obviously for the treated samples.Furthermore,the surface flashover voltage can be increased by 26.67%after 10 min of treatment.It is considered that strong electron affinity of C–F and increased surface roughness can contribute to deepening surface traps,which not only inhibits the development of secondary electron emission avalanche but also alleviates the surface charge accumulation and finally improves the surface flashover voltage of SIR.

One-step phenol production from a water–toluene mixture using radio frequency in-liquid plasma

The objectives of this research were to understand the process of converting toluene into phenol in a one-step process directly from a water–toluene mixture using the plasma in-liquid method.Experiments were conducted using 27.12 MHz radio frequency（RF） in-liquid plasma to decompose a solution of 30% toluene. Based on the experimental results as evaluated using gas chromatography–mass spectrometry（GC–MS）, along with additional analysis by the Gaussian calculation, density functional theory（DFT） hybrid exchange–correlational functional（B3LYP）and 6-311 G basis, the phenol generated from toluene was quantified including any by-products.In the experiment, it was found that OH radicals from water molecules produced using RF inliquid plasma play a significant role in the chemical reaction with toluene. The experimental results suggest that phenol can be directly produced from a water–toluene mixture. The maximum phenol yields were 0.0013% and 0.0038% for irradiation times of 30s and 60s,respectively, at 120 W.

Wave field structure and power coupling features of blue-core helicon plasma driven by various antenna geometries and frequencies

This paper deals with wave propagation and power coupling in blue-core helicon plasma driven by various antennas and frequencies.It is found that compared to non-blue-core mode,for blue-core mode,the wave can propagate in the core region,and it decays sharply outside the core.The power absorption is lower and steeper in radius for blue-core mode.Regarding the effects of antenna geometry for blue-core mode,it shows that half helix antenna yields the strongest wave field and power absorption,while loop antenna yields the lowest.Moreover,near axis,for antennas with m=+1,the wave field increases with axial distance.In the core region,the wave number approaches to a saturation value at much lower frequency for non-blue-core mode compared to blue-core mode.The total loading resistance is much lower for blue-core mode.These findings are valuable to understanding the physics of blue-core helicon discharge and optimizing the experimental performance of blue-core helicon plasma sources for applications such as space propulsion and material treatment.

Measurement of electronegativity during the E to H mode transition in a radio frequency inductively coupled Ar/O2 plasma

This paper presents the evolution of the electronegativity with the applied power during the E to H mode transition in a radio frequency(rf)inductively coupled plasma(ICP)in a mixture of Ar and O2.The densities of the negative ion and the electron,as well as their ratio,i.e.,the electronegativity,are measured as a function of the applied power by laser photo-detachment combined with a microwave resonance probe,under different pressures and O2 contents.Meanwhile,the optical emission intensities at Ar 750.4 nm and O 844.6 nm are monitored via a spectrograph.It was found that by increasing the applied power,the electron density and the optical emission intensity show a similar trench,i.e.,they increase abruptly at a threshold power,suggesting that the E to H mode transition occurs.With the increase of the pressure,the negative ion density presents opposite trends in the E-mode and the H-mode,which is related to the difference of the electron density and energy for the two modes.The emission intensities of Ar 750.4 nm and O 844.6 nm monotonously decrease with increasing the pressure or the O2 content,indicating that the density of high-energy electrons,which can excite atoms,is monotonically decreased.This leads to an increase of the negative ion density in the H-mode with increasing the pressure.Besides,as the applied power is increased,the electronegativity shows an abrupt drop during the E-to H-mode transition.

Radio Frequency Underwater Discharge Operation and Its Application to Congo Red Degradation

Radio frequency （RF） underwater discharge operation was performed for different liquid conductivities driven by different frequencies ranging from 13.56 MHz to 60 MHz, and its application to organic degradation was investigated. The RF underwater discharge was observed to be generated within the bubble at electrode surface formed by RF and plasma heating. It was shown that the sizes of the bubbles and plasmas increased as the driving frequency and the input power went up. The breakdown voltage decreased rapidly with the increase of the water conductivity and driving frequency. Comparative experiments of the UV-VIS absorbance spectra of Congo Red solution before and after discharge suggested effective degradation of the organic dye due to the active species generated during the discharge, such as .OH, .O, .H, etc. revealed by optical emission spectroscopy. The results show that higher exciting frequency and lower conduc- tivity of the solution are more effective for organic degradation. With the combination of Fourier Transform Infrared Spectroscopy （FT-IR） and Liquid Chromatography-Mass Spectrometry （LC- MS） data, one possible degradation process was proposed and the main conceivable components and structures of the products were also presented.

Effect of Plasma Spheroidization Process on the Microstructure and Crystallographic Phases of Silica, Alumina and Nickel Particles

During the plasma spheroidization process powders undergo different changes in their microstructures and crystal phases. In this paper, simple calculation of heat transfer between the plasma and a suspended particle was performed based on three hypotheses for the purpose of guiding experiments. Experimental investigation of the crystal phases and microstructural changes during the plasma processing was made using silica, alumina and nickel powders as starting materials. It has been revealed from the experimental results that these materials undergo different changes in crystal phases and microstructures, and these changes are essentially determined by the structures, properties and aggregate states of the starting materials.

Plasma preparation and low-temperature sintering of spherical TiC –Fe composite powder

A spherical Fe matrix composite powder containing a high volume fraction （82vo1%） of fine TiC reinforcement was produced using a novel process combining in situ synthesis and plasma techniques. The composite powder exhibited good sphericity and a dense structure, and the fine sub-micron TiC particles were homogeneously distributed in the α-Fe matrix. A TiC-Fe cermet was prepared from the as-prepared spherical composite powder using powder metallurgy at a low sintering temperature; the product exhibited a hardness of HRA 88.5 and a flexural strength of 1360 MPa. The grain size of the fine-grained TiC and special surface structure of the spherical powder played the key roles in the fabrication process.

Effect of the feeding rate on microstructure and properties of plasma spheroidized GH4169 powder

The growing interest in additive manufacturing of GH4169 alloy was accompanied by the demand on spherical GH4169 powders with high performance.The powder particles were treated by radio frequency plasma with the different feeding rates.The microstructure and morphology,the particle size distribution of as-treated powders were studied by scanning electron microscopy and laser particle size analysis.It was demonstrated that GH4169 powders with extremely fine followability were obtained by radio frequency plasma spheroidization technology.With the same plasma parameters,the spheroidization efficiency of the particles varied with the feeding rates.When the rate of the powder feeding rates was too small,the excessive absorption of the heating by the powders caused vaporization,then the collection decreased.When the feeding rates was too large,the powder particles were insufficiently absorbed,resulting in defects in the powders.The microstructure of the as-treated spherical particles was mainly cell crystals,columnar crystals,and even microcrystals.Under the suitable plasma parameters,the resulting powders haved a slightly increased average particle size,excellent spheroidization,surface smoothness,followability,and bulk ratio.

Sterilization of Turmeric by Atmospheric Pressure Dielectric Barrier Discharge Plasma

In this study atmospheric pressure dielectric barrier discharge （DBD） plasma has been employed for sterilizing dry turmeric powders. A 6 kV, 6 kHz frequency generator was used to generate plasma with Ar, Ar/Ou, He, and He/O2 gases between the 5 mm gap of two quartz covered electrodes. The complete sterilization time of samples due to plasma treatment was measured. The most important contaminant of turmeric is bacillus subtilis. The results show that the shortest sterilization time of 15 rain is achieved by exposing the samples to Ar/O2 plasma. Survival curves of samples are exponential functions of time and the addition of oxygen to plasma leads to a significant increase of the absolute value of time constant of the curves. Magnitudes of protein and DNA in treated samples were increased to a similar value for all samples. Taste, color, and solubility of samples were not changed after the plasma treatment.

Effect of driving frequency on electron heating in capacitively coupled RF argon glow discharges at low pressure

A one-dimensional（1D） fluid model on capacitively coupled radio frequency（RF） argon glow discharge between parallel-plates electrodes at low pressure is established to test the effect of the driving frequency on electron heating. The model is solved numerically by a finite difference method. The numerical results show that the discharge process may be divided into three stages： the growing rapidly stage, the growing slowly stage, and the steady stage. In the steady stage,the maximal electron density increases as the driving frequency increases. The results show that the discharge region has three parts： the powered electrode sheath region, the bulk plasma region and the grounded electrode sheath region. In the growing rapidly stage（at 18 μs）, the results of the cycle-averaged electric field, electron temperature, electron density, and electric potentials for the driving frequencies of 3.39, 6.78, 13.56, and 27.12 MHz are compared, respectively. Furthermore,the results of cycle-averaged electron pressure cooling, electron ohmic heating, electron heating, and electron energy loss for the driving frequencies of 3.39, 6.78, 13.56, and 27.12 MHz are discussed, respectively. It is also found that the effect of the cycle-averaged electron pressure cooling on the electrons is to ＂cool＂ the electrons; the effect of the electron ohmic heating on the electrons is always to ＂heat＂ the electrons; the effect of the cycle-averaged electron ohmic heating on the electrons is stronger than the effect of the cycle-averaged electron pressure cooling on the electrons in the discharge region except in the regions near the electrodes. Therefore, the effect of the cycle-averaged electron heating on the electrons is to ＂heat＂ the electrons in the discharge region except in the regions near the electrodes. However, in the regions near the electrodes, the effect of the cycle-averaged electron heating on the electron is to ＂cool＂ the electrons. Finally, the space distributions of the electron pressure cooling the electron ohmic heating and the electron heating at 1/4 T, 2/4 T, 3/4 T, and 4/4 T in one RF-cycle are presented and compared.

Balmer-alpha and Balmer-beta Stark line intensity profiles for high-power hydrogen inductively coupled plasmas

We compare Balmer-alpha （Ha） and Balmer-beta （Hβ） emissions from high-power （1.0-6.0 kW） hydrogen inductively coupled plasmas （ICPs）, and propose region Ⅰ （0.0-2.0 kW）, region Ⅱ （2.0-5.0 kW）, and region Ⅲ （5.0-6.0 kW）. In region Ⅰ, both Ha emission intensity （la） and Hβ emission intensity （1β） increase with radio frequency （RF） power, which is explained by the corona model and Boltzmann＇s law, etc. However, in region II, la almost remains constant while 1β rapidly achieves its maximum value. In region Ⅲ, 1α slightly increases with RF power, while 1β decreases with RF power, which deviates significantly from the theoretical explanation for the Ha and Hβ emissions in region I. It is suggested that two strong electric fields are generated in high-power （2.0-6.0 kW） hydrogen ICPs： one is due to the external electric field of high-power RF discharge, and the other one is due to the micro electric field of the ions and electrons around the exited state hydrogen atoms in ICPs. Therefore, the strong Stark effect can play an important role in explaining the experimental results.

Synthesis of Single-Walled Carbon Nanotubes by Induction Thermal Plasma

The production of high quality single-walled carbon nanotubes(SWCNTs)on a bulk scale has been an issue of considerable interest.Recently,it has been demonstrated that high quality SWCNTs can be continuously synthesized on large scale by using induction thermal plasma technology.In this process,the high energy density of the thermal plasma is employed to generate dense vapor-phase precursors for the synthesis of SWCNTs.With the current reactor system,a carbon soot product which contains approximately 40 wt%of SWCNTs can be continuously synthesized at the high production rate of~100 g/h.In this article,our recent research efforts to achieve major advances in this technology are presented.Firstly,the processing parameters involved are examined systematically in order to evaluate their individual inuences on the SWCNT synthesis.Based on these results,the appropriate operating conditions of the induction thermal plasma process for an effective synthesis of SWCNTs are discussed.A characterization study has also been performed on the SWCNTs produced under the optimum processing conditions.Finally,a mathematical model of the process currently under development is described.The model will help us to better understand the synthesis of SWCNTs in the induction plasma process.