Atmospheric Microwave Plasma Torch Research Articles

Improving the flowability of fine WC-12Co powder using atmospheric pressure nitrogen microwave plasma torch treatment

WC-Co powders are mainly used in thermal spray coatings, which improve corrosion and wear resistance as well as the regeneration capacity of damaged components. The use of fine powders in the thermal spray coating process is essential to form a dense coating layer. However, fine powders possess poor flowability due to their morphology and van der Waals forces between particles. These characteristics limit their application in thermal spray coatings. In this study, a nitrogen microwave plasma torch was applied to WC-12Co powder (D50 = 20 μm) to improve the flowability of the powder. The rough surface of the particle was melted and changed to a smooth, spherical surface. As a result, the flowability of the plasma-treated powder improved significantly from 0 to 4.02 g/s. Thus, a microwave plasma torch improves the flowability of fine WC-12Co powders and is suitable for continuous treatment and mass production of plasma spray powders.

Determination of 915-MHz Atmospheric Pressure Air Microwave Plasma Torch (MPT) Parameters

The gas temperature, electron temperature, and electron density are critical parameters that affect microwave plasma chemistry processes in industrial applications. Using optical emission spectroscopy (OES), the gas temperature and electron density of a 915-MHz atmospheric pressure air microwave plasma torch (MPT) were investigated at various absorbed power levels and radial positions. In addition, the variation in the electron temperatures of 915-MHz atmospheric pressure argon microwave plasma filaments was analyzed as a function of the absorbed power. The experimental results showed that: 1) the absorbed power had little effect on the radial gas temperature or electron density; 2) the skin effect was present in the 915-MHz atmospheric pressure air MPT, where it causing caused hollowing in the center of the radial gas temperature distribution; 3) the electron density reached approximately <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\times10$ </tex-math></inline-formula> 13 cm−3 and was almost insensitive to variations in the absorbed power in the center of the 915-MHz atmospheric pressure air MPT; and 4) the upper limit of the 915-MHz atmospheric pressure microwave plasma filament electron temperature increased from approximately 1 to approximately 3 eV as the absorbed power increased from 2.0 to 6.3 kW.

Microwave plasma torches for solid waste treatment and vitrification.

Conventional thermal plasma technology used in the treatment of solid waste has a high demand for power and a high rate of heat loss during solid waste treatment. We developed a novel approach for treating and vitrifying solid waste with a low-power microwave plasma torch (MPT). Based on theoretical thermodynamic equilibrium calculations, we studied the melting temperature of the residual ash and achieved vitrification of the residual ash with an MPT by adding specified ratios of discarded glass scraps. Thermocouples and a gas analyzer were used to characterize the temperature variations and gas concentrations in the plasma treatment chambers, respectively. An organic elemental analyzer and X-ray fluorescence (XRF) analyses were used to determine the chemical proportions of the solid waste residues. The morphologies of the residues and vitreous material were analyzed using scanning electron microscopy (SEM). The results showed that the microwave plasma treatment process converted 96 wt.% of the solid wastes into nonpolluting gases, leaving a residue of pure carbon and inorganic powder. Through theoretical calculations and experiments, atmospheric MPTs with power levels less than 10kW were identified as realistic means for treating and vitrifying solid wastes.

On the gas-phase graphene nanosheet synthesis in atmospheric microwave plasma torch: Upscaling potential and graphene nanosheet‑copper nanocomposite oxidation resistance

Efficient gas-phase synthesis of few-layer graphene nanosheets (GNS) is based on the controlled formation of the high-temperature environment and the reaction pathway of gas-phase species formed by the decomposition of organic precursors. Such a process results in the formation of high-quality carbon nanomaterial and hydrogen while the concentration of other gaseous by-products is minimized. In this work, the main factors affecting the efficiency of such processes in the TIAGO microwave plasma torch were investigated using detailed material analysis and mass spectrometry of the gas-phase products during the synthesis process. The results showed a limiting effect of increasing the microwave power (MW) on both the product yield as well as material quality, as shown by Raman and x-Ray photoelectron spectroscopy. The change in the reaction pathway increased the formation of C2H4, resulting in the upper limit of the achievable nanopowder yield. The prepared material showed a decrease in its high oxidation resistance, with increasing the delivered MW power as determined by thermogravimetry analysis. This behavior was related to the formation of GNS-Cu nanoparticles composite due to the presence of copper nanoparticles originating from erosion of the electrode of the TIAGO torch during the synthesis process at high MW powers.

The effect of plasma discharge on methane diffusion combustion in air assisted by an atmospheric pressure microwave plasma torch

An atmospheric pressure air microwave plasma torch is employed to assist methane diffusion combustions using a combination of a combustor and burner. Experimentally, the effect of the air microwave plasma on combustion is investigated with respect to the flame morphology and the variation of gas components in the exhaust with the fuel equivalence ratio φ or the methane flow rate by comparing plasma-assisted combustion (PAC) and natural combustion (NC) without plasma application. The combustion degree of CH4 in PACs is found to be much enhanced in rich fuel combustion than in NC in both types of burners, which is measured by Fourier transformation infrared spectrometer (FTIR). In PACs, with the use of an air microwave plasma torch, the radicals originating from excitation, ionization, and dissociation of N2 and O2 and the high gas temperature induced in the plasma discharge play an important role in assisting the combustion.

Experimental investigation of the methane combustion assisted by atmospheric pressure oxygen microwave plasma torch

The atmospheric-pressure oxygen microwave plasma is employed to enhance the methane combustion in the jet-diffusion combustor. The plasma-assisted combustion (PAC) is compared with natural combustion without plasma application in terms of the flame morphology, flame temperature, and combustion efficiency. It is found experimentally that the oxygen plasma assistance in combustion is prominent in lean oxygen condition, and the active species generated in the microwave plasma torch contribute to the combustion process more dominantly than the flame temperature change caused by heating of oxygen microwave plasma. The combustion degree of CH4 in PAC is much more enhanced in lean oxygen combustion, and the exhaust gas in combustion is effectively controlled with the use of the oxygen microwave plasma torch.

Mechanisms of gas temperature variation of the atmospheric microwave plasma torch

The gas temperature is a key parameter that affects the process of microwave plasma chemistry in industrial applications. Based on the molecular emission spectrometry of the A2Σ+→X2Πr electronic system of OH radicals, the gas temperature of the atmospheric air microwave plasma core at different absorbed microwave power levels, gas flow rates, gas humidities, and volume fractions of CO2 in air was analyzed. In the experiment, the absorbed microwave power, gas flow rate, gas humidity, and volume ratio of CO2 in air was varied from 560 to 1750 W, 10 to 24 l min−1, 30% to 95%, and 0% to 40%, respectively. Moreover, the axial gas temperature distribution of the plasma torch was measured. The experimental results showed that (i) the plasma gas temperature mainly ranged from 4000 to 7000 K, (ii) the plasma gas temperature rose with increasing absorbed microwave power but was hardly affected by the feeding gas flow rate, (iii) the plasma gas temperature decreased by ∼400 K for every 20% increase in the fraction of CO2 in air and decreased with increasing gas humidity, and (iv) the plasma torch gas temperature decreased along the axial direction. Due to the lack of a prevailing microwave discharge theory, an in-depth analysis of the mechanisms of gas temperature variation was performed based on the heat balance equation.

Design of a fully automatic microwave plasma torch system.

Several advantages are associated with an atmospheric microwave plasma torch, including low cost, easy processing, and high density of electrons and reactive species. This paper develops a compact microwave plasma torch (CMPT) operating at 2.45 GHz with an ultrabandwidth of 30 MHz. The structure and equivalent circuit of CMPT is established and analyzed based on theory of transmission line. The simulation results indicate that the resonant frequency will drift downward with the presence of plasma. In order to keep the better coupling between the microwave and plasma, a new design scheme of a fully automatic microwave plasma system is proposed. The effects of different microwave powers on the CMPT discharge are carried out on this system. Finally, in order to demonstrate the performance of the CMPT, an experiment to modify the surface of polymethyl methacrylate (PMMA) is conducted. It is found that the water contact angle is reduced from 85.4° to 32.3°, and the surface morphology characterized by atomic force microscope indicates significant improvement of the surface wettability of PMMA, which proves that the CMPT system is feasible and available.

Synthesis of SiOx Nano-Powders Using a Microwave Plasma Torch at Atmospheric Pressure

The silicon oxide nano-powders (SiOx-NPs) were obtained in an atmospheric microwave plasma torch using a gas-phase silicon tetrachloride (SiCl4) with N2 and H2. The gas-phase SiCl4 was injected with H2 gas into the microwave plasma torch generated by N2 and air swirl gas, and then the dark brown powders were deposited on the inner wall of a quartz tube. The sample was analyzed by an X-ray photoelectron spectroscopy (XPS), a scanning electron microscope (SEM), an energy dispersive spectrometer (EDS), and an X-ray diffraction (XRD). The average size and oxidation x values of synthesized SiOx-NPs were approximately 230 nm and 0.91, respectively. Furthermore, the volumetric charge capacity is 1127 mAh/g and has 89.2% retention after 100 cycles.

A portable optical emission spectroscopy-cavity ringdown spectroscopy dual-mode plasma spectrometer for measurements of environmentally important trace heavy metals: Initial test with elemental Hg

A portable optical emission spectroscopy-cavity ringdown spectroscopy (OES-CRDS) dual-mode plasma spectrometer is described. A compact, low-power, atmospheric argon microwave plasma torch (MPT) is utilized as the emission source when the spectrometer is operating in the OES mode. The same MPT serves as the atomization source for ringdown measurements in the CRDS mode. Initial demonstration of the instrument is carried out by observing OES of multiple elements including mercury (Hg) in the OES mode and by measuring absolute concentrations of Hg in the metastable state 6s6p (3)P(0) in the CRDS mode, in which a palm-size diode laser operating at a single wavelength 405 nm is incorporated in the spectrometer as the light source. In the OES mode, the detection limit for Hg is determined to be 44 parts per 10(9) (ppb). A strong radiation trapping effect on emission measurements of Hg at 254 nm is observed when the Hg solution concentration is higher than 50 parts per 10(6) (ppm). The radiation trapping effect suggests that two different transition lines of Hg at 253.65 nm and 365.01 nm be selected for emission measurements in lower (<50 ppm) and higher concentration ranges (>50 ppm), respectively. In the CRDS mode, the detection limit of Hg in the metastable state 6s6p (3)P(0) is achieved to be 2.24 parts per 10(12) (ppt) when the plasma is operating at 150 W with sample gas flow rate of 480 mL min(-1); the detection limit corresponds to 50 ppm in Hg sample solution. Advantage of this novel spectrometer has two-fold, it has a large measurement dynamic range, from a few ppt to hundreds ppm and the CRDS mode can serve as calibration for the OES mode as well as high sensitivity measurements. Measurements of seven other elements, As, Cd, Mn, Ni, P, Pb, and Sr, using the OES mode are also carried out with detection limits of 1100, 33, 30, 144, 576, 94, and 2 ppb, respectively. Matrix effect in the presence of other elements on Hg measurements has been found to increase the detection limit to 131 ppb. These elements in lower concentrations can also be measured in the CRDS mode when a compact laser source is available to be integrated into the spectrometer in the future. This exploratory study demonstrates a new instrument platform using an OES-CRDS dual-mode technique for potential field applications.

Electron impact excitation-cavity ringdown absorption spectrometry of elemental mercury at 405 nm

We report a new method of measuring elemental mercury (Hg) at the 405 nm transition line, which corresponds to the optical transition from the metastable state 5d106s6p 3P0 to the upper state 5d106s7s 3S1. Observation of the absorption spectra and measurements of the absolute number density of Hg in the metastable state are achieved by a stepwise electron impact excitation-cavity ringdown absorption technique, in which the population of Hg atoms in the metastable state is realized through electron impact excitation by the energetic electrons (< 10 eV) generated in an atmospheric argon microwave plasma torch and the detection of Hg is achieved via cavity ringdown spectroscopy (CRDS). The major difference between this method and the previously reported detection of Hg at 254 nm using the plasma-CRDS technique is that the plasma in this method not only serves as an atomization source to generate Hg atoms from the Hg-contained compounds injected in the plasma, but also functions as an electron impact excitation source. The merit in terms of analytical instrumentation is that this new method introduces an alternative way to measure Hg using a palm-size 405 nm laser source. This can potentially lead to a portable mercury ringdown spectrometer without constraints of importability of 254 nm laser sources. Compared with the 254 nm line, the 405 nm line has no plasma-associated spectroscopic interferences, such as absorption of the OH rovibrational lines. One analytical limitation of the detection of Hg at 405 nm is the low detection sensitivity achieved in this exploratory study, 50 μg ml−1 in aqueous sample solutions, compared with the previously reported detection sensitivity of 9.1 ng ml−1 using the plasma-CRDS technique at 254 nm. Given an improved plasma excitation source-CRDS system, theoretical detection limits at 405 nm are estimated to be 50 ng ml−1 and 1.2 ppbv in aqueous samples and in gaseous samples, respectively. These sensitivities are still desirable in many applications when real-time, portable, and in-line analysis all become a major concern.

Investigation of Variation Power and Additive Gas Effect on the $\hbox{SF}_{6}$ Destruction Using Atmospheric Microwave Plasma Torch

An atmospheric microwave plasma torch for the decomposition of SF6 accompanying additive gases (O2 , compressed air) was investigated experimentally in this paper. Applied microwave power and additive gas species as effective parameters on destruction and removal efficiency (DRE) were considered. It was approved that compressed air is a more efficient additive gas for the removal of SF6 compared to oxygen. Also, DRE is highly dependent on the variation of forward power. Highest efficiencies while using oxygen and compressed air as additive gases have been 98.4% and 99.1% (in 1100 W), respectively. In some case, the N2 was also used with SF6, resulting in a maximum efficiency (in 1100 W) of 99.4%. This paper approves that a microwave plasma torch sustained in atmospheric pressure can almost completely remove the SF6 in semiconductor industry.

Cu Nanoparticles From Evaporation of Cu Granule in a Microwave Torch Plasma at Atmospheric Pressure

The copper (Cu) nanoparticles (Cu-NPs) were obtained directly by evaporation of Cu granules in an atmospheric microwave plasma torch flame with a possibility for direct continuous preparation and mass production of Cu-NPs. The microwave plasma generated in a mixture of nitrogen (N2) and argon (Ar) gases evaporates Cu granules and produces Cu-NPs. This template- and catalyst-free process may be promising for the practical production and application of Cu-NPs.

Characterization of Microwave Plasma Torch for Decontamination

AbstractAn atmospheric low‐temperature microwave plasma torch has been developed and applied to disinfection. The size of the plasma output is relatively large (35 mm in diameter). Ar gas at a flow rate of 2.2 slm and 85 W microwave power are used. Plasma discharges are produced between the tip of each electrode and the inner surface of the cylinder. When an Escherichia coli culture is placed for 2 min at 20 mm below the torch, where the gas temperature is sufficiently cool, the bacteria are almost completely killed within a 40 mm diameter circle. The UV radiation is one of the major players responsible for killing bacteria, others being the reactive species and the charged particles.magnified image

Production of Nanocrystalline TiO2 Powder by a Microwave Plasma-Torch and Its Characterization

This work shows that nanocrystalline titanium dioxide (TiO2) particles are directly synthesized in an atmospheric microwave plasma-torch, using gas-phase titanium tetrachloride (TiCl4), O2, and H2 as reactants. The as-synthesized samples were characterized by employing scanning electron microscope (SEM), transmission electron microscope (TEM), and X-ray diffractometer (XRD). The ratio of anatase and rutile depended strongly on the flow rate of H2 injected into the microwave plasma-torch, showing the maximum anatase fraction of 94% at the flow rate of 30 sccm H2 for the range of O2 flow rate.

Characterization of Carbon Powder Produced by a Microwave-Plasma Torch at Atmospheric Pressure

Carbon powder synthesized by an atmospheric microwave-plasma torch is characterized in terms of the mixture ratio of argon to nitrogen in torch gas. Methane as a source of carbon atoms was directly injected into the microwave-plasma torch. The samples deposited on the inner wall of a discharge tube were analyzed by the X-ray diffraction, field-emission scanning electron microscope, field-emission transmission electron microscope, and Fourier transform infrared (FTIR). The synthesized samples showed thornbush-shaped, fingerprintlike, and onionlike structures. It was identified from FTIR data that the samples contain functional groups of very weak C-N, C=N, and CequivN

Synthesis and characterization of nano-sized nitride particles by using an atmospheric microwave plasma technique

Metal nitride nanoparticles, such as titanium nitride (TiN), vanadium nitride (VN), and silicon nitride (Si 3N 4), were directly synthesized via decomposition of gas-phase starting materials by an atmospheric microwave plasma torch. The reactants in gas-phase are fragmented to atoms or molecules and then the fragmented species create particles in micro or nano size. X-ray diffraction, field-emission scanning electron microscope, and field-emission transmission electron microscopy have been employed to investigate powder diffractions, surface morphologies, particle sizes, etc. The particle sizes of as-produced samples are approximately 10–25 nm, revealing well-ordered individual layers with high crystallinity.

Synthesis of oxide nanoparticles via microwave plasma decomposition of initial materials

Vanadium pentoxide (V 2O 5) and zinc oxide (ZnO) powders were synthesized by making use of an atmospheric microwave plasma torch. Zn granules for ZnO and vanadium oxytrichloride (VOCl 3) in liquid for V 2O 5 were used as initial materials. A sample for V 2O 5 consisted of nano- and micro-structured particles aligned with the plasma reactor structure. A sample for ZnO was typical nanorods with 100–200 nm in length. The V 2O 5 and ZnO samples were characterized by XRD, SEM, and TEM.

ZnO Nanorods Synthesized by Self-Catalytic Method of Metal in Atmospheric Microwave Plasma Torch Flame

Zinc oxide (ZnO) nanorods synthesized by an explosive oxidation reaction in a microwave plasma torch flame were presented. The structural characteristics and morphology of the as-synthesized ZnO nanorods were investigated by X-ray diffraction (XRD) analysis, field-emission scanning electron microscopy (FESEM), field-emission transmission electron microscopy (FETEM), and photoluminescence (PL) spectroscopy. It was revealed by FETEM that most of the ZnO nanorods were pure and had an average diameter of 80 nm and a length of up to 300 nm, and some ZnO nanorods had a hexagonal tip, tripod-like shape with a length of up to 1 µm. PL spectra show only strong UV emission at approximately 380 nm without any defect-related emissions, indicating good crystalline ZnO. Also, a possible mechanism of the growth of the ZnO nanorods is discussed.

ZnO nanocrystals synthesized by evaporation of Zn in microwave plasma torch in terms of mixture ratio of N2 to O2

Zinc oxide (ZnO) nanocrystals synthesized by an atmospheric microwave plasma torch are characterized in terms of the mixture ratio of N2 and O2 in the torch gas. Zinc (Zn) granules as a source material placed on the plasma reactor were evaporated and oxidized to ZnO immediately as soon as Zn granules were in contact with the plasma torch flame. The samples obtained from the different gas mixtures were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM), x-ray photoelectron spectroscopy (XPS), and photoluminescence (PL). As the N2 content in the gas mixture increased to 93.6%, the aggregates or clusters of ZnO crystals were synthesized without nanorod-like and multipod-like structures. Especially, ZnO crystals synthesized in the N2 content of 55.6% mainly revealed the tripod and tetrapod. Also, the visible to ultraviolet (UV) intensity ratio in the PL analysis was increased with increasing of the N2 content, revealing that the aggregated ZnO crystals have the high concentration of oxygen vacancies.