Microwave Plasma Torch Research Articles

Study of graphene layer growth on dielectric substrate in microwave plasma torch at atmospheric pressure

The initial stage of graphene layer deposition on silicon oxide substrate by ethanol decomposition in dual-channel microwave plasma torch at atmospheric pressure was studied in dependence on precursor flow rate and delivered microwave power. Depending on ethanol flow rate and substrate temperature, horizontally or vertically aligned graphene nanosheets with various density could be prepared directly on dielectric substrate. In the regime with high microwave power, above 400 W, mixture of amorphous carbon particles and graphene sheets was deposited on the substrate. Prepared layers were analyzed by scanning electron microscopy (SEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The microwave plasma diagnostics was carried out using optical emission spectroscopy (OES). The sample analysis showed increasing density of horizontally aligned carbon nanosheets with increasing ethanol flow rate and their delamination and transition into vertically aligned graphene sheets with increasing substrate temperature. The Raman spectroscopy analysis of layers showed presence of D (1345 cm−1), G (1585 cm−1) and 2D (2685 cm−1) peaks with 2D/G ratio of 1.59 and full width at half maximum (FWHM) of 2D peak was 42 cm−1, corresponding to few layer graphene structure. In case of amorphous nanoparticles deposition, the D* peak at 1210 cm−1 and D** at 1500 sccm−1 was observed in Raman spectra with D/G ratio of 1.19 and C1s XPS spectra of carbon contained 20.4 at.% of sp3 carbon phase in comparison to 8.3 at.% in case of graphene nanosheets layer. High D/G ratio, up to 3.5, and low intensity 2D band was characteristic for vertically aligned graphene nanosheets layers. The possibility to influence density and size of graphene nanosheets on substrate represents promising alternative for future deposition of graphene on arbitrary substrate.

Experimental investigation on improving the efficiency of power coupling from the incident microwave to the discharge in a plasma torch

A high efficiency of power coupling from the incident microwave to the discharge is a general demand for large-scale and high-power applications of microwave plasma torches. This paper aims to experimentally explore the possibility of improving the power coupling efficiency by choosing a proper glass tube in a microwave plasma torch with a metallic enclosure, rather than using additional tuning measures such as a movable short-circuited ending plunger or a three-stub tuner. The power coupling efficiencies for the glass tubes with different wall thicknesses were experimentally measured under different incident microwave powers and different pressures. The results showed that the power coupling efficiency was improved with the increase in the glass tube wall thickness in a wide range of operating conditions for the microwave plasma torch with specific dimensions. An efficiency higher than 85% can be achieved once the wall thickness of the glass tube is in a particular range. This indicates that external tuning measures can be reduced for the microwave plasma torch with a proper glass tube, which helps to save the cost of tuning devices and enhance the flexibility in different working environments.

Spheroidization of Iron Powder in Microwave and Hybrid Plasma Torches

Spheroidization of Iron Powder in Microwave and Hybrid Plasma Torches

Combustion of aluminum powder with steam entrained in a helium plasma using a 2.45 GHz microwave plasma torch

Ignition and combustion of ~20 µm diameter aluminum powder was achieved with a helium–steam-based 2.45 GHz microwave plasma torch as a proof of concept due to interest in utilizing aluminum powder as a fuel source for power generation given the material’s high energy density. Optical emission spectroscopy of the plasma in air, with steam-entrainment, and with aluminum particle injection was obtained in order to observe the radical species present. The combustion products were analyzed with x-ray diffraction (XRD) to determine the post-combustion composition and compared with published XRD data involving aluminum-water combustion. The results revealed that a microwave plasma torch in a steam environment is capable of igniting aluminum particles and sustains combustion between aluminum powder and radicals from water molecules. The torch design results in a mixing delay between the aluminum and steam radical reactants prior to sustained combustion. Enhanced mixing will be pursued with an improved injector design.

Rapid analysis of tetracycline in honey by microwave plasma torch mass spectrometry with ablation samples

A rapid and sensitive mass spectrometric analysis method with minimal or no prior sample pretreatment was developed for the direct detection of trace amounts of tetracycline in honey products using a microwave plasma torch combined with mass spectrometry.

Portable photochemical vapor generation-microwave plasma optical emission spectrometer

A low power microwave plasma torch as an excitation source was combined with a photochemical vapor generator (PVG) and a miniaturized charge-coupled device to construct a portable optical emission spectrometer.

Thomson scattering versus modeling of the microwave plasma torch: a long standing discrepancy almost solved

This paper resolves a long standing discrepancy between theoretical modeling of atmospheric microwave plasma jets and their diagnostics by Thomson scattering. The discrepancy is found to be created by the filamentary behavior of the plasma.

Spheroidization of iron powder in a microwave and hybrid plasma torches

The process of porous iron powder spheroidization in microwave discharge and combined microwave and DC discharge modes in nitrogen and helium plasma was studied with powder particle sizes ranging from 45 to 85 μm. The powder was obtained by air spraying and subsequent hydrogen annealing. Plasma spraying produced hollow spheroidized particles with a wall thickness from 1 to 10 μm. The share of spheroidized powder particles in their total volume was determined. It was found that microwave power rising from 1.5 to 5 kW leads to a linear increase in the spheroidization degree of iron powder particles. When working in the hybrid plasmatron mode, microwave radiation conditions are combined with a DC discharge and make it possible to increase the plasma temperature. When the ratio of microwave and DC discharge power is 1 : 1, virtually 100 % iron powder spheroidization is obtained. The metallographic study of spheroidized particles showed that their final size differs from the initial one by about 10 times. It was found that iron powder oxidation occurs regardless of the spheroidization mode. This is due to the insufficient purification degree of plasma gases. The structure of particle surfaces when using nitrogen or helium as a plasma gas is different. Experiments showed that the use of helium is more preferable, since the particles have only a slight roughness in comparison with the particle structure during nitrogen spheroidization.

Dynamic mode optimization for the deposition of homogeneous TiO2 thin film by atmospheric pressure PECVD using a microwave plasma torch

An atmospheric pressure plasma-enhanced chemical vapor deposition process using a microwave plasma torch has been used for titania thin film synthesis. A dynamic deposition mode was set up to cover a square centimeter surface with a nanostructured TiO2 film. The process parameters were studied and optimized to control the coating crystallinity and morphology and limit the formation of powder in the plasma phase. Contrary to static deposition, the substrate movement promotes a film growth by particles agglomeration in the reference conditions, leading to a cauliflower-like morphology. Then, the precursor proportion in the plasma appears to be determinant in the TiO2 film microstructure. For a precursor flow rate beyond 0.2 slpm, the titania nanoparticles formation in the gas phase is promoted and the thin film is growing by particles agglomeration, leading to a columnar cauliflower-like morphology. At a flow rate of 0.2 slpm, the growth by surface reaction is promoted and the TiO2 film is columnar, where each column is an anatase crystal. After the optimization of the substrate holder movement, it was possible to deposit this last microstructure homogeneously on a square centimeter surface.

Synthesis of ZnO Nanomaterials Using Low-Cost Compressed Air as Microwave Plasma Gas at Atmospheric Pressure.

Zinc oxide (ZnO) nanomaterials were efficiently synthesized using a microwave plasma torch system at atmospheric pressure. The Zn powder was passed through a microwave plasma region, in which it melted and vaporized. Tetrapod-type ZnO nanomaterials with a diameter of 29.8 ± 8.0 nm were synthesized using a high-purity O2/N2 mixed gas. In particular, ZnO nanowires with a diameter of 109.5 ± 8.0 nm and a length of 5–6 μm were produced using an inexpensive compressed air as a microwave plasma gas. It was confirmed that the nanowires synthesized using the compressed air showed higher light absorption in the visible region than the tetrapod-type ZnO. In addition, the redshifts in the absorption peak and photoluminescence peak were observed from 370.6 to 375.2 nm and 380 to 390 nm, respectively. The obtained results can be explained by the change of energy levels due to the defects in the ZnO nanowires such as vacancies and interstitials of Zn and oxygen. Finally, we can conclude that cost-effective compressed air is appropriate not only for the synthesis of ZnO nanowire, but also the enlargement of optical absorption and emission range.

Atmospheric-Pressure Microwave Plasma Torch for CVD Technology of Diamond Synthesis

An electrodeless microwave jet plasma source is considered, and its various applications in the technology of chemical vapor deposition of diamond films and dimension increasing of small diamond single crystals synthesized at high pressures and temperatures are discussed. The plasma jet is ignited in an atmospheric-pressure gas (argon) flow with hydrogen and methane additives. The operation of the microwave jet reactor is described, and the plasma characteristics measured using emission spectroscopy are presented. The brightly glowing atmospheric-pressure plasma jet is ignited and stably burns at a microwave power of ≤1 kW supplied from a microwave oven magnetron. The specific microwave power density absorbed by the compact plasma jet (≤104 W/cm3) is comparable with that absorbed by a dc arc. The growth rate of the polycrystalline diamond layer amounts to 40 µm/h. The process of film deposition on the substrate can be controlled by scanning the substrate surface with the jet.

On the interplay between plasma discharge instability and formation of free-standing graphene nanosheets in a dual-channel microwave plasma torch at atmospheric pressure

The influence of the interplay between central (QC) and secondary (QS) channel gas flow, as well as delivered microwave power (PMW), during graphene nanosheet synthesis in a dual-channel electrode configuration of a microwave plasma torch at atmospheric pressure by ethanol decomposition was investigated. In the dual-channel configuration, plasma discharge can be sustained, even at high flow rates of ethanol, due to the separation of argon working and carrier gas. The plasma discharge instability was mainly influenced by an increase in the central channel flow, and a minor influence of secondary channel flow was also observed. With respect to the dependence on experimental conditions, the synthesized nanopowder consisted of amorphous carbon and nanocrystalline diamond nanoparticles, defective carbon nanosheets or few-layer graphene nanosheets. The synthesized nanosheets are rectangular in shape with a lateral size of several hundreds of nanometres and a few graphene layers thick, as shown by electron microscopy. Raman and x-ray photoelectron spectroscopy analysis of the synthesized nanosheets showed a good degree of graphitization, low oxygen content and increasing quality of graphene nanosheets with increasing microwave power. The number of defects in the synthesized nanosheets could be decreased by elongation of the graphene nanosheet assembly zone. An increase in the C2/C emission line intensity ratio correlated with a decrease in the number of defects in the graphene nanosheet structure. The achieved conversion yield of ethanol into graphene nanosheets was 8.3%, without negatively affecting the nanosheet quality.

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.

Propagating modes of the travelling wave in a microwave plasma torch with metallic enclosure

For the discharges sustained by the travelling electromagnetic waves, the wave propagation characteristics are of great importance to discharge maintenance and stability. This study aims to investigate the propagating electromagnetic modes of the travelling wave in a cylindrical discharge tube bound by a metallic enclosure based on a microwave plasma torch. In certain particular circumstances, it is found that the cylindrical discharge tube with metallic enclosure is able to change from a one-conductor circular waveguide to a two-conductor-like coaxial waveguide. Such a change allows the travelling wave to propagate in the transverse electromagnetic mode, which has rarely been noticed before. Regarding this transition, a new criterion is proposed to determine the formation of the two-conductor-like waveguide structure. Existence conditions of different propagating modes of the travelling wave are further discussed with respect to different glass tube wall thicknesses and dielectric properties. The results indicate that it is possible to control the propagating modes of the travelling wave for different purposes by choosing a proper glass tube.

Numerical Investigation of the Gas Flow Effects on Surface Wave Propagation and Discharge Properties in a Microwave Plasma Torch

Gas flow plays an important role in determining the discharge properties in microwave plasma torches sustained by the propagating surface wave. This paper aims to investigate the gas flow effects on the plasma column length, the surface wave propagation, and the heavy species temperature in a surface wave plasma torch operated at atmospheric pressure. The steadystate discharges of pure argon gas at atmospheric pressure under different gas inflow rates in the plasma torch are characterized by an improved 2-D axisymmetric fluid model. The obtained results demonstrate that increasing the gas inflow rate is able to decrease the surface wave propagation distance, accelerating the contraction of the plasma column length. The discharge instability is found to be motivated by the disappearance of the sustaining surface wave at high gas inflow rates. Besides, a temperature drop of about 64 °C is observed at the temperature-maximum point of the glass tube with an increase in the gas inflow rate from 1 to 16 L · min -1 . Therefore, for the surface wave plasma torch operated with only the axial gas flow, increasing the gas inflow rate cannot solve the glass overheating problem.

Spray-inlet microwave plasma torch and low temperature plasma ionization for ambient mass spectrometry of agrochemicals

The unique ionization characteristics of plasma can be used in multimodal ion sources, which have expanded plasma-based ambient ionization mass spectrometry rapidly to accommodate a range of analytical applications.

Microwave plasma torch synthesis of Zn[sbnd]Al oxides as adsorbent and photocatalyst for organic compounds removal

Zn-Al based oxides were synthesized using a microwave plasma torch (MPT) in which the precursors were introduced into the reaction chamber either upstream or downstream of the inlet, respectively. ZnAl layered doubled oxide (LDO) samples with a layer-by-layer structure obtained from the downstream path exhibited high adsorption capacity for anionic species, i.e., acid red 27 (AR27) and methyl orange (MO) dyes through hydrogen bonding and electrostatic interaction. The adsorption behavior follows a pseudo-second-order model and the Langmuir isotherm, suggesting that chemisorption of adsorbate on the adsorbent surface plays a significant role. ZnAl oxide (ZAO) powders prepared from the upstream path show high crystallinity, enabling them to photocatalytically degrade methyl blue (MB) dye solution under UV and visible light irradiation. The crystallization mechanisms for ZAO and LDO prepared by the MPT method were also proposed in this study. In summary, the MPT process is a facile, rapid, scalable, and cost-effective method for synthesizing of ZnAl based materials for many practical applications.

Production of active species in an argon microwave plasma torch

In this paper, results from the computational investigation of active species production in an argon microwave plasma torch are presented. We address the main paths of species creation and quenching and focus on their spatial distributions. We show that with the flowing Ar/O2 the radical production is relatively small. The operation of the argon torch can be more effective with an Ar/O2/N2 or Ar/O2/N2/H2O mixture as a feed gas. It was also shown that for high applied voltages the gas temperature inside the microwave torch can be high enough to significantly disturb the flow field. This disturbance can induce depletion and rarefaction of some species near the electrode and influence their production.

Microwave Plasma Torch Generated in Argon for Small Berries Surface Treatment

Demand for food quality and extended freshness without the use of harmful chemicals has become a major topic over the last decade. New technologies are using UV light, strong electric field, ozone and other reactive agents to decontaminate food surfaces. The low-power non-equilibrium (cold) atmospheric pressure operating plasmas effectively combines all the qualities mentioned above and thus, due to their synergetic influence, promising results in fruit surface decontamination can be obtained. The present paper focuses on the applicability of the recently developed microwave surface wave sustained plasma torch for the treatment of selected small fruit. Optical emission spectroscopy is used for the determination of plasma active particles (radicals, UV light) and plasma parameters during the fruit treatment. The infrared camera images confirm low and fully applicable heating of the treated surface that ensures no fruit quality changes. The detailed study shows that the efficiency of the microbial decontamination of selected fruits naturally contaminated by microorganisms is strongly dependent on the fruit surface shape. The decontamination of the rough strawberry surface seems inefficient using the current configuration, but for smooth berries promising results were obtained. Finally, antioxidant activity measurements demonstrate no changes due to plasma treatment. The results confirm that the MW surface wave sustained discharge is applicable to fruit surface decontamination.

Effects of carbon coating on LiNi0.5Mn1.5O4 cathode material for lithium ion batteries using an atmospheric microwave plasma torch

A novel carbon coating method on LiNi0.5Mn1.5O4 (LNMO) cathode active material powders for lithium-ion batteries is reported. Carbon coated LNMO (LNMO/C) cathode is prepared by an atmospheric 2.45 GHz microwave plasma torch system using an acetylene gas as carbon precursor. This method is one-step process. Therefore, its method is not only an easy control, short manufacturing time, and low-cost operation, but also gives similar results on LNMO/C produced by other carbon coating methods. The morphology and structure of prepared samples are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) analysis, and elemental analyzer (EA). Electrochemical properties of LNMO/C are tested in lithium half coin cells. The results show that the carbon coating on LNMO could greatly enhance the rate capability and cycle performance especially at high temperature compared to bare LNMO. At 10 C rate with increased temperature, the discharge capacity is 0.15 mAhg−1 for bare LNMO, while 104 mAhg−1 for LNMO/C. Also, the capacity retention of LNMO/C sample is 97% at 0.5 C rate after 50 cycles at elevated temperature. The simple carbon coating method used in this work may easily provide a surface modification approach to other materials for commercialization.