Microwave Plasma Reactor Research Articles

The lateral outward growth of single-crystal diamonds by two different structures of microwave plasma reactor

On semi-open holders, the lateral outward growth of single-crystal diamond was performed using two different structures of resonance cavity. Both reactors can be used for laterally enlarging the top surface of SCD without polycrystalline diamond rim.

CVD-synthesis of detector quality diamond for radiation hardness detectors of ionizing radiation

An advanced microwave plasma reactor ARDIS 300 was used to synthesize homoepitaxial structures of monocrystal diamond films at Project Center ITER. High-quality epitaxial diamond films were grown on boron-doped monocrystal diamond substrates using microwave plasma-assisted chemical vapor deposition from methane-hydrogen mixture. Structural and impurity perfection of diamond films were characterized by Raman spectroscopy, photoluminescence, and optical absorption. Prototypes of radiation detectors were created on the basis of grown diamond films with thickness 70-80 μm,. The p-type substrate with boron concentration ~100 ppm served as an electrical contact. Detectors were irradiated by 5.5 MeV particles and 14.7 MeV neutrons, corresponding pulse height spectra were measured and detector sensitivities were determined. Charge collection efficiency for synthesized diamond was shown to achieve 94% and 91% when ~ 4 V/μm electric field applied. Keywords: diamond films, epitaxy, diamond detector

Синтез CVD-алмаза детекторного качества для радиационно-стойких детекторов ионизирующего излучения

An advanced microwave plasma reactor ARDIS 300 was used to synthesize homoepitaxial structures of monocrystal diamond films at Project Center ITER. High-quality epitaxial diamond films were grown on boron-doped monocrystal diamond substrates using microwave plasma-assisted chemical vapor deposition from methane-hydrogen mixture. Structural and impurity perfection of diamond films were characterized by Raman spectroscopy, photoluminescence, and optical absorption. Prototypes of radiation detectors were created on the basis of grown diamond films with thickness 70-80 μm. The p-type substrate with boron concentration ~100 ppm served as an electrical contact. Detectors were irradiated by 5.5 MeV α-particles and 14.7 MeV neutrons, corresponding pulse height spectra were measured and detector sensitivities were determined. Charge collection efficiency for synthesized diamond was shown to achieve 94% and 91% when ~4 V/m electric field applied.

CFD Modeling of a Lab-Scale Microwave Plasma Reactor for Waste-to-Energy Applications: A Review

Rapidly increasing solid waste generation and energy demand are two critical issues of the current century. Plasma gasification, a type of waste-to-energy (WtE) technology, has the potential to produce clean energy from waste and safely destroy hazardous waste. Among plasma gasification technologies, microwave (MW)-driven plasma offers numerous potential advantages to be scaled as a leading WtE technology if its processes are well understood and optimized. This paper reviews studies on modeling experimental microwave-induced plasma gasification systems. The system characterization requires developing mathematical models to describe the multiphysics phenomena within the reactor. The injection of plasma-forming gases and carrier gases, the rate of the waste stream, and the operational power heavily influence the initiation of various chemical reactions that produce syngas. The type and kinetics of the chemical reactions taking place are primarily influenced by either the turbulence or temperature. Navier–Stokes equations are used to describe the mass, momentum, and energy transfer, and the k-epsilon model is often used to describe the turbulence within the reactor. Computational fluid dynamics software offers the ability to solve these multiphysics mathematical models efficiently and accurately.

Phase-sensitive detection of gas-borne Si nanoparticles via line-of-sight UV/VIS attenuation.

The distinct optical properties of solid and liquid silicon nanoparticles are exploited to determine the distribution of gas-borne solid and liquid particles in situ using line-of-sight attenuation measurements carried out across a microwave plasma reactor operated at 100 mbar. The ratio between liquid and solid particles detected downstream of the plasma varied with measurement location, microwave power, and flow rate. Temperatures of the liquid particles were pyrometrically-inferred using a spectroscopic model based on Drude theory. The phase-sensitive measurement supports the understanding of nanoparticle formation and interaction and thus the overall gas-phase synthesis process.

Design of an edge tapered 915 MHz/TM021 microwave plasma reactor by numerical analysis

Microwave plasma reactor is often of particular interest due to their wide range of industrial applications, especially in high-end manufacturing. However, there exist problems of unstable plasma and small discharge area in conventional microwave reactors at 2.45 GHz. An edge tapered 915 MHz/TM021 microwave reactor evolved from reactors at 2.45 GHz is presented to improve the strength, area, and uniformity of microwave field for plasma applications. An electromagnetic model of a microwave reactor with the new coupling antenna composed of a tapered platform and a coaxial inner conductor is simulated. The simulation results show that the electric field strength in the new reactor with a size optimized coupling structure is more concentrated compared to similar reactors. A 2D axisymmetric hydrogen plasma discharge model based on self-consistent fluid simulation of this new reactor is presented. The obtained results show that this kind of model can lead to a better understanding of the physical processes occurring in these kinds of microwave reactors.

Determination of tadalafil in pharmaceutical samples by vertically oriented multi-walled carbon nanotube electrochemical sensing device

Abstract In this work, an electrochemical sensor based on vertically oriented carbon nanotubes (voMWCNT), electrically wired via connection upon a copper plate, for the detection of tadalafil (TD) is reported. The voMWCNT is easily prepared via microwave plasma reactor with no additional post-treatment. The voMWCNT was characterized by scanning and transmission electron microscopy in addition to electrochemical characterization with cyclic voltammetry. Once TD is of easy access and used in the treatment of erectile dysfunction and related diseases, the quality control of this drug is of high importance. Using linear sweep voltammetry, the detection of TD was found to be possible from 0.85 to 8.9 μmol L−1, with an R2 of 0.992 with a limit of detection found to correspond to 78 nmol L−1. The quantification of TD in different pharmaceutical drugs, as well as the device stability, indicates that the voMWCNT has potential for the routine sensing of TD.

Economic and environmental evaluation of flux agents in the vitrification of resin waste: A SWOT analysis

Flux agents play an important role in the pyrolysis treatment of vitrifying hazardous wastes. Among these is plasma jets, a cost-less flux agent derived from shell powder which can be used to create vitrification. It is a promising option to be applied in the vitrification of elements and to remove the VOCs of hazardous waste, namely, resin from PCB scrap in an atmospheric-pressure microwave plasma reactor. In this study, a laboratory scale experiment was conducted. The experiment was performed in the pyrolysis of resin which was added with flux agents. The economic evaluation of the flux agents, and the circular economy concept of the final residue derived from the plasma pyrolysis was then analyzed post treatment. To test the strength and weakness of the experiment, the SWOT analysis was performed. The outcome helped in the understanding of the cost-less flux agent used in the pyrolysis treatment of hazardous waste. Results showed that fusing shell powder in resin was better for improving the removal efficiency of VOCs, such as benzene and toluene as well as toxic metals than compared to other flux agents such as limestone and quartz sand. Moreover, the final residue of resin was found to fulfil the concept of circular economy where it could be reused as an absorbent of methyl blue, thereby indicating good absorption performance, from 1 ppm–100 ppm. The twelve strategies that were derived from the SWOT analysis could be used as information outlining the current internal and external condition for the development and application of shell powder. Shell powder, as a cost-less flux agent, has the potential for enhancing waste management and circular economy when used in the pyrolysis treatment of future hazardous wastes.

Towards large area CVD diamond disks for Brewster-angle windows

Abstract In the frame of the EUROfusion Work Package Heating and Current Drive (WP HCD) of the Power Plant Physics and Technology (PPPT) program, CVD diamond disk Brewster-angle windows for gyrotron operation at multi-megawatt RF power levels and long pulses are under development. These windows allow for frequency step-tuneable operation. The Brewster-angle of 67.2° for diamond leads to an elliptical connection of the disk to the copper waveguides (WGs), requiring an advanced joining process. For proper transmission of the RF power, the disk consists of low loss CVD diamond of optical grade. The current target for the WG aperture of DEMO is 63.5 mm. It allows for an RF power transmission of 2 MW, but it requires a disk diameter of 180 mm for the 67.2° angle. In addition, a thickness of approximately 2 mm is needed to achieve the proper mechanical stability. State of the art microwave plasma reactors are not capable of growing disks of such size. The maximum available diameter of a polycrystalline CVD diamond disk suited to microwave applications is currently 140 mm. Thus, the industrial partner Diamond Materials GmbH (Freiburg, Germany) is doing extensive diamond growth experiments. A first of its kind, 180 mm thermal grade, crack-free, diamond disk was produced in the microwave plasma reactor with an average unpolished thickness of about 2 mm. First loss tangent measurements have been also performed. This paper describes the steps and the first results of this non-straightforward path, a challenging new field for diamond manufacturers and a major breakthrough for future frequency step-tuneable operation.

Effect of shell powder on removal of metals and volatile organic compounds (VOCs) from resin in an atmospheric-pressure microwave plasma reactor

Resin has been widely used for thermosetting printed circuit boards (PCBs) and is a key part of e-waste from scrap PCBs. It requires appropriate treatment because of its harmful elements (metals and metalloids) and organic compounds that are toxic to human health and the environment. The purpose of this study is to eliminate volatile organic compounds (VOCs) and elements (metals and metalloids) in resin via the use of powdered snail shell (Babylonia formosae) in an atmospheric-pressure microwave plasma reactor. Shell powder plays a significant role in the destruction of benzene and toluene with removal efficiency 98.8 % and 100 %, respectively, compared to quartz sand with removal efficiency 44.9 %. A high ratio of shell powder increases the inertization of metals and metalloids by more than 96 %. The crystalline structures of these materials are dominated by calcite formations (CaCO3), confirming the elimination of metals and metalloids. Raman spectroscopy shows that the shell powder vitrifies these elements. The use of shell powder is thus recommended to degrade hazardous substances and to vitrify elements from resin in plasma pyrolysis.

Hydrogen production from dry spirulina algae with downstream feeding in microwave plasma reactor assisted under atmospheric pressure

Advancement in the field of biomass and bioenergy has pulled in researchers to produce biofuels derived from naturally accessible feedstock by adopting diverse innovative approaches in biomass pretreatment. In this study, pyrolysis of micro algae with downstream processing of biomass from the resonator cavity under atmospheric pressure was conducted and hydrogen production produced under different microwave power was analyzed. With the increase in microwave power, the hydrogen gas yield accounted to be as twice as that at lower power, where the concentration of produced hydrogen accounts for about 30.80%, 33.20%, and 37.58% at microwave power levels of 800, 900 and 1000 W. It was also observed that no methane was produced in this study since most of the methane produced from microwave plasma conversion has reacted with CO2 and produced CO and H2, hence dropped in the concentration of CO2 with decline in power intensity.

Computer simulation of NOx removal from diesel engine off-gases under electron beam and wet scrubber system

NOx emission from diesel engine off-gases in marine transportation sector is still an environmental issue to be solved in present day. Electron beam (EB) followed by a seawater wet scrubber method to removal NOx has been experimentally studied (Chmielewski et al., 2018). In this work numerical simulation of NOx reduction using EB followed by a water scrubber system has been carried out. The first stage was to calculate NOx removal versus dose under EB irradiation. Computer code “KINETIC” and Gear method were used, the model has been improved based our previous work (Zwolinska E. et al., 2015) and verified by the experimental results. The calculation results of NOx removal efficiency was 4.9% at 10.9 kGy dose under EB irradiation, which was a little higher than experimental value (3.5%). Computer program MATLAB was used to study NOx absorption in water. The NOx removal efficiency was increased by 17.5% at 10.9 kGy dose for inlet concentration of NOx being 1333 ppm after EB-water absorption process. At the end, NOx reduction in the simulated marine off-gas using microwave plasma was simulated using computer program Chemical Workbench. Less than 20% NOx was removed from the off-gases in microwave plasma reactor even at 9000 kGy dose for the inlet concentration of NOx being 1500 ppm. EB is more energy efficient than microwave plasma for NOx removal.

Hydrogen and Methane Production from Styrofoam Waste Using an Atmospheric-pressure Microwave Plasma Reactor

ABSTRACT Polystyrene foam (PSF), has been used widely in oyster farming in Taiwan and is responsible for generate approximately 120,000 – 200,000 pieces floating PSF waste every year, this causes some issues related to its treatment, including, clogging during incineration, lack of economic benefits, and carcinogenic, and non-biodegradable components, and it also a menace to the seashore due to becoming debris. In this study, PSF waste was treated via an atmospheric-pressure microwave plasma reactor to obtain methane (CH4) and hydrogen (H2). The using of nitrogen as the carrier gas and higher microwave power (1200 W) show greater results as compared to argon in the production of H2 at concentrations of 4739 ppm, while the results for CH4 were less than 300 ppm. Higher amounts of PSF waste (0.2 g) indicate the production of CH4 and H2 at concentrations of 19,657 ppm and 440 ppm, respectively. The results of SEM-EDX and XRD testing confirmed the transformation of the PSF structure and a reduction of carbon (C) in the final residue. The application of atmospheric-pressure microwave plasma not only treats PSF waste but produces H2 and CH4 that recycles PSF waste as energy gases. This research contributed to an understanding of how plasma technology can be used to treat solid waste and produce valuable gases.

Carbon dioxide conversion in an atmospheric pressure microwave plasma reactor: Improving efficiencies by enhancing afterglow quenching

Microwave plasma is effective for decomposing CO2, which is a major contributor to global warming. Atmospheric pressure microwave plasma is of particular interest due to its vacuum-free configuration. Here, we investigated the CO2 conversion performance of a microwave plasma system operating at atmospheric pressure. CO2 conversion efficiency and energy efficiency were measured for various CO2 flow rates and input power levels. Lower CO2 flow and higher power yielded an enhanced CO2 conversion efficiency and lower energy efficiency. The afterglow temperature in the reactor was also measured. At a specific energy input (SEI) of 7.25 eV/molecule, a temperature of 1342 K was observed 12 cm below the gas injection nozzle. To improve the efficiencies of the plasma system, we propose an afterglow quenching method with enhanced heat transfer characteristics. This was achieved using a water-cooled quenching rod inserted into the plasma reactor to cool the afterglow gas stream, thus limiting the recombination reaction of CO to form CO2. At a SEI of 7.22 eV/molecule, CO2 conversion increased from 30.1% to 36.1% with the quenching rod; at 2.46 eV/molecule, the increment of CO2 conversion efficiency was 29 %. An energy efficiency of 28.9 % was achieved using an SEI of 1.19 eV/molecule. By tracking down the heat loss to cooling water flowing through the applicator, exhaust port, and quenching rod, we determined the energy distribution from the plasma plume to the exit gas stream. The quenching rod increased the chemical enthalpy of the product gas by reducing CO recombination.

Self-assembled nano-silicon/graphite hybrid embedded in a conductive polyaniline matrix for the performance enhancement of industrial applicable lithium-ion battery anodes

Nano-structured silicon-based composite materials have generated significant excitement for use as anode materials in high-performance Li-ion batteries. For making these materials commercially applicable, a high Coulombic efficiency at the first cycle must be achieved. Additionally, scalable synthesis routes need to be developed to provide access to practically-relevant material quantities. In this work, we propose a strategy for the production of Si/graphite/polyaniline (Si/graphite/PANI) composites that addresses both above mentioned challenges. Si nanoparticles were produced in a pilot-plant-scale microwave-plasma reactor using monosilane (SiH4) as precursor. This process enables the formation of high-purity Si nanoparticles with controllable crystal sizes at a production rate of 45 g/h. Si/graphite hybrids are fabricated through self-assembly by electrostatic attraction. The Si/graphite/PANI nanocomposite is then prepared by in situ polymerization of aniline monomer in the presence of the Si/graphite hybrid. With this approach, ~40 g of Si/graphite/PANI composite per batch can be produced at lab scale. The scalability of the underlying processes enables the use for commercial products. The nanocomposite shows favorable characteristics inherited from its three components: Si nanoparticles provide high capacity, graphite acts as an electrical conductor and gives a high Coulombic efficiency, and the polyaniline coating further enhances the electrical conductivity and protects the entire structure. A very good Coulombic efficiency of 86.2% at the initial cycle is recorded for this nanocomposite material. Galvanostatic charge/discharge tests demonstrate that this material can deliver a discharge capacity of 2000 mAh/g with a very good capacity retention of 76% after 500 cycles at a discharge rate of 0.5C (1.25 A/g). The capacity is 870 mAh/g measured at 5C (12.5 A/g).

Numerical optimization of hydrogen microwave plasma reactor for diamond film deposition

We present the results of optimization of microwave plasma generation in a prototypical diamond film deposition device by means of numerical simulation. The modification of the device was done by changing the shape of microwave resonant chamber, where the plasma generation occurs, in a way to focus the discharge near the outlet nozzle. The best results were obtained for a configuration with an addition of a conducting rod located on the axis of the chamber. This modified configuration provided a two orders of magnitude increase in electron number density near the nozzle exit and about 10 time increase (up to 5%) in dissociation degree of hydrogen molecules.

Microwave Plasma-Enhanced and Microwave Heated Chemical Reactions

Microwave plasma technology is revolutionizing reaction engineering fields such as methane dry reforming, chemical synthesis, biomass conversion, and waste treatment. Microwave generated plasma offers sustainable, cleaner and efficient operations compared to conventional methods. Microwave plasma reactions are more efficient when integrated with catalysts. In this article, a thorough categorization and comparison of microwave plasma-assisted catalytic reactions are presented, while highlighting their contribution to an energy efficient and sustainable future in chemical processing. An introduction on commercial applications of microwave plasma technology is also presented to emphasize its advantages in modern industries. Microwave irradiation can be used as a source of heat or plasma. The addition of heterogeneous catalyst to either microwave heated or microwave enhanced plasma systems can lead to complex pathways in reaction systems. A final section in this article is dedicated to comprehend this complexity in chemical reactions occurring in microwave heated and microwave plasma-enhanced catalytic systems.

Plug flow reactor model of the plasma chemical conversion of CO2

A 1D plug flow model suitable for describing the CO2 conversion into CO in microwave plasma reactors is proposed. The model is applied together with the Antwerp data set for the CO2 reaction kinetics to calculate parameter scans for a realistic experimental set up. The energy re-distribution pathways in the model calculations are analyzed. The analysis shows that despite the input power being initially deposited mainly into vibrational states the fast vibrational-translational (VT) transfer leads to dissociation of CO2 predominantly via the thermal quenching mechanism. Solutions with mitigated VT-losses can be obtained by increasing the specific input power–power per unit volume. In this regime the energy efficiency starts to be constrained by reverse processes.

Sewage Sludge-Derived Producer Gas Valorization with the Use of Atmospheric Microwave Plasma

Atmospheric microwave plasma was applied to the processing of the partially cleaned producer gas obtained from sewage sludge gasification. The plasma processing resulted in residual tar compounds conversion and changes in the gas composition. During the tests with a different gas flow rates and microwave power inputs, liquid and gaseous samples were collected to evaluate the plasma reactor’s performance. The conversion efficiency ranged from 19 to 100% and it depended on the specific energy input (SEI), gas flow rate, initial tar concentration, and the nature of the tars compounds. Generally, it was shown that the conversion rate increased with the SEI and that the aliphatic, cyclic and substituted compounds were converted much easier than benzene. Moreover, applying plasma led to the production of heavier aromatics (i.e. naphthalene, indene, acenaphthylene) but the rise in their concentration was significantly smaller than the amount of converted compounds. The gas composition changes revealed in the increase of H2 and CO concentration that was an effect of hydrocarbons and CO2 conversion. Additionally, it was indicated that the microwave plasma reactor’s performance was noticeably worse than in the case of the laboratory test with a simulated producer gas. This was mainly attributed to differences in the reactors’ geometry, lower hydrogen concentration and the presence of inorganic deposit on the reactor’s walls that might have inhibited microwaves transfer. In general, the microwave plasma technology seems promising in the context of cleaning and upgrading the producer gas, however, further optimization research is necessary to make it more reliable and less energy consuming.Graphic

Enhanced deposition rate of polycrystalline CVD diamond at high microwave power densities

We report on growth of polycrystalline diamond (PCD) by chemical vapor deposition (CVD) in a microwave plasma reactor under high absorbed microwave power density (MWPD) and high pressures, in conditions more typical for single crystal diamond epitaxy. The growth rates as high as 30–36 μm/h have been achieved in CH4-H2 mixtures at pressure of 320 Torr and MWPD of ~700 W/cm3 even at relatively low (3%) CH4 concentration, using PCD substrates with diameter of 5–20 mm and different textures. The structure and quality of the produced PCD thick films were assessed with SEM, Raman and X-ray diffraction. The plasma shapes and spatial profiles of species (excited atomic hydrogen and C2 dimer) were characterized with spatially resolved optical emission spectroscopy (OES) at moderate and high MWPD. The maximum rotational gas temperature Tg significantly enhanced to 3700 K in the latter case (675 W/cm3) compared with Tg ≈ 3100 K for lower pressure and MWPD (100 Torr, 160 W/cm3), being in agreement with the obtained high growth rates at high pressures. Analysis of numerous literature data on PCD growth rate vs substrate size in MPCVD reactors reveals a clear trend of enhanced growth rate with diminishing of the substrate dimension, our findings further confirming this tendency indirectly related to a change in absorbed power density.