Atmospheric Microwave Plasma Research Articles

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.

Synthesis of MgO nanopowder in atmospheric microwave plasma torch

MgO nanopowders were synthesized directly in torch flame of an oxygen microwave plasma using Mg granules as a source material. The plasma torch vaporized Mg granules immediately and Mg vapors were explosively oxidized producing MgO powders. It was observed from the FETEM images that size of the MgO particles is approximately 10–50 nm, showing various particle shapes such as hexahedrons, plates, and rectangles. Also, the surface area and pore distribution properties of as-produced sample were analyzed by a nitrogen adsorption apparatus. This synthetic method may be suitable for a direct continuous preparation and mass production of MgO nanoparticles.

Surface modification of polymers by atmospheric microwave plasma and UV from low pressure mercury lamp

Surface modification of polymers by atmospheric microwave plasma and UV from low pressure mercury lamp

Reduction of perfluorinated compound emissions using atmospheric pressure microwave plasmas: Mechanisms and energy efficiency

Abstract The abatement of perfluorinated compound (PFC) gases is investigated using atmospheric pressure microwave-surface-wave plasmas. These PFCs are diluted in nitrogen gas with concentration ranging from 5000 to 10 000 ppmv. The abatement mechanisms of SF6 and CF4 are examined, and conversion schemes are presented in the case where oxygen is added to the gas mixture. The PFC fragments are oxidized, forming acid-like by-products that are finally trapped irreversibly using a humidified soda lime scrubber. Gas-phase analysis was performed using Fourier transform infrared (FTIR) spectroscopy. The abatement efficiency is found to increase with increasing absorbed microwave power and gas residence time. The energy efficiency of the abatement process is shown to increase, with PFC concentrations in the gas mixture up to 10 000 ppmv. A complete conversion of SF6 is achieved for energy densities ranging from 700 to 1200 J/cm3 for concentrations ranging from 5000 to 10 000 ppmv. Lowering the microwave excitation frequency and using swirling flow are shown to reduce the energy cost per abated molecule.

Thermal inequilibrium of atmospheric helium microwave plasma produced by an axial injection torch

The population density of several excited states has been obtained spectroscopically in a helium plasma sustained by a torch device at atmospheric pressure as a function of the radius in the plasma for different conditions of microwave power and plasma gas flow. The ground-state atom density is determined from the gas temperature, which is deduced from the rotational temperature of the molecular nitrogen ions. The population distribution is fitted to the theoretical results of a collisional-radiative model that includes particle transport. A large deviation of the measured populations is found from the theoretical populations for local thermodynamic equilibrium. The plasma at any radial position is far from local thermodynamic equilibrium; the equilibrium deviation parameter of the ground state is larger than 10 000. The equilibrium deviation parameters of the measured excited-state populations obey the theoretical pk−6 exponential law.

Gas-phase synthesis of nitrogen-doped TiO2 nanorods by microwave plasma torch at atmospheric pressure

Nitrogen (N)-doped titanium dioxide (TiO2) nanorods were directly synthesized via decomposition of gas-phase titanium tetrachloride (TiCl4) by an atmospheric microwave plasma torch. X-ray diffraction, field-emission scanning electron microscope, field-emission transmission electron microscope, and electron-energy-loss spectroscopy (EELS) have been employed to investigate fraction of the anatase and rutile phases, diameter and length, and chemical composition of the nanorods, respectively. The diameters of the nanorods are approximately 30–80 nm and the length is several micrometers. EELS data show that incorporation of N into the O site of TiO2 nanorods was enhanced in N2 gas by the microwave plasma torch. Also, a growth model of the rods was proposed on the basis of vapor-liquid-solid mechanism.

Production of Nanocrystalline Titanium Nitride Powder by Atmospheric Microwave Plasma Torch in Hydrogen/Nitrogen Gas

Nanocrystalline titanium nitride (TiN) particles were directly synthesized in an atmospheric microwave plasma torch, using gas‐phase titanium tetrachloride (TiCl4), with N2 and H2 as reactants. The gas‐phase TiCl4 was injected with H2 gas into the microwave plasma‐torch flame generated by N2 swirl gas, and then the dark blue powders were deposited on the inner wall of the quartz tube. The synthesized powder samples were analyzed by scanning electron microscopy (SEM), SEM‐EDX, X‐ray diffractometer (XRD), transmission electron microscopy, and FT‐Raman. The average crystallite sizes of TiN measured from XRD patterns decreased from 15.5 to 11.6 nm, as the flow rate of H2 injected into the microwave plasma torch increased from 0.03 to 0.3 liters per minute.

Band gap narrowing of TiO 2 by nitrogen doping in atmospheric microwave plasma

Nitrogen (N)-doped titanium dioxide (TiO 2) particles of nanometric size are directly synthesized in an atmospheric microwave plasma-torch using gas-phase titanium tetrachloride (TiCl 4). The absorption edge of the TiO 2 − x N x powders shifts from 365 to 500 nm in UV–visible spectrum, as the content x of nitrogen in titanium dioxide powders increases from x = zero. From the results of the XRD, XPS, and UV–visible data, incorporation of N into the O site of TiO 2 was enhanced in a mixture of N 2 and O 2 by an atmospheric microwave plasma torch, contributing to the band gap narrowing.

Comparative study of plasma-induced and wet-chemical cleaning of synthetic fibers

To obtain strongly adherent coatings on polymeric fibers, like metallizations, it is important that the fiber surfaces must be clean. Therefore, five different cleaning methods were compared in order to remove manufacturing residuals (sizes) on PET multifilament fibers, namely chemical cleaning by Soxhlet extraction and in a home-built cleaning bath, as well as etching in atmospheric and low pressure microwave and RF plasmas. Different criteria, such as efficiency, flexibility, ecology and industrial applicability are considered. In terms of these criteria, the RF plasma process performs the best. The continuous fiber treatment in a low pressure He/O2 discharge enables the complete removal of 4.4 wt.% size at a velocity of 10 m/min and a power of 50 W. Although atmospheric and low pressure MW plasmas show higher efficiencies (etching rates), their application is limited due to high heat loads yielding degradation of the PET fibers before cleaning is completed. On the contrary, both chemical methods evaluated are producing large volumes of waste and a nanolayer of size remains on the fiber surfaces.

Cavity ringdown measurements of mercury and its hyperfine structures at 254 nm in an atmospheric microwave plasma: spectral interference and analytical performance

The plasma-cavity ringdown spectroscopic (Plasma-CRDS) technique has been demonstrated as a powerful tool for elemental and isotopic measurements in recent studies. This work reports the first application of plasma-CRDS to measurements of elemental mercury and its stable isotopes at the 254 nm transition under atmospheric conditions. A microwave-induced plasma (MIP) operating at 80–100 W is used to generate Hg atoms from standard HgCl2 solutions diluted by 2% nitric acid solvent. It is found that a background absorption, attributed to the overlap of two broadened rovibrational transitions R21(21) and P1(15) of the OH A-X (3-0) band located at 253.65 nm, generates significant spectral interference with the absorption peak of Hg at 254 nm. With an optimized operating condition, including plasma powers, gas flow rates, and laser beam positions in the plasma, the detection sensitivity of Hg is determined to be 9.1 ng ml−1 in aqueous solution, equivalently 221 pptv in the gas phase; this detection limit is approximately 2-fold higher than the theoretical detection limit, 126 pptv, which was estimated by using the parameters of the instrument system and the calculated absorption cross-section, 2.64 × 10−14 cm2 atom−1, of the transition under atmospheric plasma conditions. High-resolution spectral scans show a clear contour of the stable isotopes of the 254 nm transition. The technical challenges encountered and the potential for further development of the Hg analyzer using the MIP-CRDS technique are discussed.

An atmospheric microwave plasma jet for ceramic material processing

Ceramic fibres are used in fibre reinforced polymer composites and cermets to improve their mechanical, thermal and chemical properties. This paper describes a microwave plasma system that is being developed to produce these fibres from a spinning ceramic rod. The system operates at 2.45GHz and uses a waveguide-based applicator to generate a microwave plasma jet (MPJ) at atmospheric pressure. The plasma is formed outside the cavity due to the strong electric field, generated by the microwave power, which occurs between the gas nozzle and an aperture in the applicator. The MPJ system has produced plasmas using a variety of gases and gas mixtures, including argon, helium, oxygen and nitrogen. This paper describes how the properties of the plasma jet can be tailored to suit different tasks, by adjusting the various parameters such as the type of the gas, the power, the nozzle aperture size and the flow rate, and how the speed of the spinning ceramic rod can be controlled to vary the size of the particles produced. Results obtained using quartz glass and alumina are reported.

Moving atmospheric microwave plasma for surface and volume treatment

A new type of atmospheric pressure microwave sustained plasma is introduced for surface and volume treatment. The special characteristic of this device is the self-moving behaviour of the plasma and its ability to move through the inside of tubular dielectric structures. These features are especially suitable for treatments of large and heat sensitive objects as could be demonstrated, e.g. by treatment of inner surfaces of plastic bottles. The plasma source consists of a magnetron of typical power of 2–6 kW, a circulator and a special waveguide for plasma operation including a termination by a movable shortcut and an ignition post. The device can be operated horizontal or vertical. The velocity of the plasma can be changed by microwave power and circulation gas flow. The presented work is focused on the first parameter studies performed on a plasma applicator designed for inner tube treatments.

Abatement of CF4 by atmospheric-pressure microwave plasma torch

An atmospheric microwave plasma torch is presented for post-pump destruction of perfluorocompound gases (PFCs), which are used widely in the semiconductor industry and are emitted with nitrogen gas for vacuum pump purges. Discharges of the microwave plasma torch are well suited for abatement of PFC contaminants discharged at a typical flow rate. The abatement was carried out using oxygen or air as additive gases. Analytical results are systematically compared to quadrupole mass spectroscopy and Fourier transform infrared (FTIR) data in the laboratory. Destruction and removal efficiency of more than 99% in FTIR data was achieved for carbon tetrafluoride.

Atmospheric microwave plasma jet for material processing

We have designed a low-cost and reliable 2.45-GHz waveguide-based applicator to generate a microwave plasma jet (MPJ) at atmospheric pressure. The MPJ system consists of a 1-6 kW magnetron power supply, a circulator, a water-cooled matched load and the applicator. The applicator includes a tuning section, which is required to reduce the reflected power and the nozzle section. The plasma is formed by the interaction of the high electrical field, generated by the microwave power, between the waveguide aperture and the gas nozzle. A variety of gasses have been used to produce the plasma including argon, helium, and nitrogen. A 2-kW 2.45-GHz MPJ constructed using a rectangular waveguide WG9A (WR340) has been investigated. An MPJ has been used for material processing applications including cutting, welding, glass vitrification, and quartz/ceramic processing. This paper discusses the design parameters and the potential of the MPJ for industrial applications and how the jet can be tailored to suit different tasks, by adjusting the various parameters such as the type of gas, the flow rate, the input power, and the nozzle design.

Characterization of an argon microwave plasma torch coupled to a Mattauch–Herzog geometry mass spectrometer

An atmospheric pressure argon microwave plasma torch (MPT) ionization source was coupled to a Mattauch–Herzog +mass spectrometer (MHMS) for elemental analysis. The operating conditions of the Ar MPT-MHMS were optimized for signal level, peak shape and resolution. The background spectrum was characterized, with most peaks arising from species that contain argon and nitrogen. Detection limits were in the tens to hundreds of pptr range while sensitivities are in the 106 counts s−1 ppm−1 regime for most elements. The major weakness of the Ar MPT-MHMS was isotope-ratio accuracy and precision.

Electronic excitation temperature profiles in an air microwave plasma torch

A 0.9- to 1.5-kW 2.45-GHz atmospheric pressure air microwave plasma torch has been operated efficiently with less than 1% reflected power. The plasma is sustained in a 28-mm internal diameter fused quartz tube, which penetrates perpendicularly through the wide walls of a tapered and shorted WR-284 (72/spl times/17-mm cross section) waveguide. A study has been made of the effects of power and airflow on the electronic excitation temperature, T/sub exc/. Abel inversion of radial profile chord averaged Fe I emission lines in the 370-377-nm range have been used to obtain localized profile measurements of T/sub exc/ inside the waveguide excitation region. In general, temperature profiles peak on axis with no evidence of a skin effect in the large diameter (10-mm full width at half maximum emission intensity) plasmas. A maximum central T/sub exc/ of 6550 K/spl plusmn/350 K is observed at an airflow rate of 28 Ipm. When maintaining a constant flow rate of 14 Ipm, a 55% increase in microwave power from 0.9 to 1.4 kW causes a /spl sim/100% increase in plasma volume without any noticeable effect on the central T/sub exc/ value. At a constant microwave power of 1.4 kW, an increase in total flow rate from 11 to 28 Ipm decreases the volume of the plasma by /spl sim/25% and increases the central T/sub exc/ by /spl sim/13%. The axially peaked temperature profiles are consistent with an electron density of /spl sim/10/sup 13/ cm/sup -3/.

Accurate and sensitive metals emissions monitoring with an atmospheric microwave-plasma having a real-time span calibration

Both high sensitivity and good measurement accuracy are required in a metals continuous emissions monitor in order for it to be acceptable for compliance monitoring. An atmospheric plasma sustained by microwaves with an attached source of a calibrated trace metals aerosol has been shown to be capable of achieving both of these requirements. The microwave plasma is continuous and operates in undiluted stack exhaust for atomic emission spectroscopy of trace metals. The plasma is sustained in a shorted waveguide that is attached to the stack by a short sample line (<50 cm). It is powered at 1.5 kW, at a frequency of 2.45 GHz. An undiluted stack slipstream is isokinetically drawn into the plasma by a suction pump at a nominal flow of 14 l/min. The pneumatic nebulizer attached to the sample line can momentarily, on command, inject a known concentration of metals solution providing a real-time span calibration. The system was tested on the exhaust stack of the rotary kiln incinerator simulator facility at the Environmental Protection Agency (EPA) National Risk Management Laboratory in Research Triangle Park. Three hazardous metals were monitored, lead, chromium, and beryllium. These measurements were referenced to EPA Method-29. A total of 20 spiked stack exhaust tests were carried out. Ten one-hour tests at high concentration (40–60 μg/actual m 3) and ten one and half-hour tests at low concentration (10–15 μg/actual m 3). The microwave plasma monitor achieved relative accuracies of approximately 20% for lead and beryllium and 40% for chromium with a threshold detection capability of < 3 μg/actual m 3 for a time response of ∼1 min. The relative accuracy deviation from the EPA Method-29 was found to be mostly systematic. This suggests the possibility of using a site-specific calibration to bring the microwave plasma into compliance with EPA's goal of 20% relative accuracy to the reference method. Laboratory work is continuing to add mercury, arsenic, and cadmium to the monitored metals. Mercury and arsenic present a particular challenge to achieving high detection sensitivity in undiluted stack exhaust because the plasma is less efficient in exciting Hg and because UV absorption interferes with As detection. This is a problem that is also in common with air ICP and in situ spark plasma methods for continuous emissions monitoring of metals.

Spectroscopic diagnostics of an atmospheric microwave plasma for monitoring metals pollution

A 1.5 kW, 2.45 GHz microwave sustained plasma in a flowing sample of stack exhaust gas has been shown in recent Department of Energy/Environmental Protection Agency testing to be an accurate monitor of trace hazardous metals (in the ppb range) such as lead, chromium, and beryllium. Optical-ultraviolet spectroscopic studies are being carried out to better understand this particular atmospheric pressure plasma torch. An experimental device has been modified with extra visible access for radial and axial profile measurements. Diffraction limited fiber optic plasma views and high-resolution grating spectrometer instrumentation for molecular rotational and atomic excitation temperature (Trot,Texc) measurements are being used for plasma studies. Initial chord averaged measurements have been made of the N2+(0,0) first negative system for Trot and Fe I levels for Texc in nitrogen and air plasmas, respectively.

An atmospheric pressure waveguide-fed microwave plasma torch: the TIA design

We report theoretical and experimental investigations of an atmospheric pressure, microwave plasma torch with axial gas injection (torche a injection axiale design). It is a waveguide-based structure comprising both waveguide and coaxial elements serving the purpose of wavemode conversion and impedance-matching. The device includes features common to various waveguide-fed torches disclosed previously by other authors, but not yet modelled. Our paper provides a simple equivalent circuit description of the torch operation that accounts for its impedance-matching, power transfer to plasma and tuning characteristics, as verified experimentally. From the outcome of the model and using our experimental results, we introduce new features in the torch design that enable one to optimize its performance. We also examine ways of simplifying its structure and operation.