Microwave Plasma Process Research Articles

Enhancing Graphitic Carbon Precursors from Coal Pyrolysis: A Comparative Analysis of Microwave Plasma and Conventional Thermal Upgradation Methods.

This study investigates and compares the efficacy of conventional thermal pyrolysis and microwave (MW) plasma pyrolysis in upgrading coal-derived precursors. Coal samples presenting a range of ranks were pyrolyzed under various reactive and nonreactive atmospheres using a pyroprobe, with the pyrolyzates analyzed by gas chromatography-mass spectrometry (GC-MS). Comparative MW plasma tests were conducted using a modified countertop MW unit, with condensed products similarly analyzed by GC-MS. A predominant coal devolatilization product-benzene was selected for analyzing the reactive MW plasma upgradation. Results demonstrate that conventional thermal pyrolysis lacks effectiveness in upgrading the precursors. To gain insight into the underlying reasons, chemical kinetic simulations were conducted. Oppositely, reactive MW plasma pyrolysis demonstrated remarkable precursor upgradation. These condensed MW plasma pyrolysis products were then subjected to a carbonization and graphitization heat-treatment with a comprehensive graphitic quality assessment conducted using X-ray diffraction and transmission electron microscopy. After graphitization, the MW plasma-upgraded precursor produced a carbon with a crystallite size several times greater than that of the initial benzene. By MW plasma processing, the poorly graphitizable benzene precursor was transformed into a highly graphitizable precursor comparable to coal tar pitch. The underlying reasons for this significant improvement were investigated by analyzing the compositional changes in the precursor under various reactive environments.

Effective technology for processing industrial volatile organic compounds by the atmospheric pressure microwave plasma torch

In this study, we investigated the abatement of volatile organic compounds (VOCs) by the atmospheric pressure microwave plasma torch (AMPT). To study the treatment efficiency of AMPT, we used the toluene and water-based varnish to simulate VOCs, respectively. By measuring the compounds and contents of the mixture gas before/after the microwave plasma process, we have calculated the treatment efficiency of AMPT. The experiment results show that the treatment efficiency of AMPT for toluene with a concentration of 17.32×104 ​ppm is up to 60 ​g/kWh with the removal rate of 86%. For the volatile compounds of water-based varnish, the removal efficiency is up to 97.99%. We have demonstrated the higher potential for VOCs removal of the AMPT process.

Ambient Carbon-Neutral Ammonia Generation via a Cyclic Microwave Plasma Process.

A novel reactor methodology was developed for chemical looping ammonia synthesis processes using microwave plasma for pre-activation of the stable dinitrogen molecule before reaching the catalyst surface. Microwave plasma-enhanced reactions benefit from higher production of activated species, modularity, quick startup, and lower voltage input than competing plasma-catalysis technologies. Simple, economical, and environmentally benign metallic iron catalysts were used in a cyclical atmospheric pressure synthesis of ammonia. Rates of up to 420.9 μmol min-1 g-1 were observed under mild nitriding conditions. Reaction studies showed that both surface-mediated and bulk-mediated reaction domains were found to exist depending on the time under plasma treatment. The associated density functional theory (DFT) calculations indicated that a higher temperature promoted more nitrogen species in the bulk of iron catalysts but the equilibrium limited the nitrogen converion to ammonia, and vice versa. Generation of vibrationally active N2 and, N2+ ions is associated with lower bulk nitridation temperatures and increased nitrogen contents versus thermal-only systems. Additionally, the kinetics of other transition metal chemical looping ammonia synthesis catalysts (Mn and CoMo) were evaluated by high-resolution time-on-stream kinetic analysis and optical plasma characterization. This study sheds new light on phenomena arising in transient nitrogen storage, kinetics, effect of plasma treatment, apparent activation energies, and rate-limiting reaction steps.

Use of a Microwave Plasma Process at Atmospheric Pressure for Bacterial Inactivation without Thermal Effects

Use of a Microwave Plasma Process at Atmospheric Pressure for Bacterial Inactivation without Thermal Effects

Ultralow‐k Amorphous Boron Nitride Based on Hexagonal Ring Stacking Framework for 300 mm Silicon Technology Platform

AbstractThe implementation of ultralow dielectric constant (k value ≈ 2) materials to reduce signal propagation delay in advanced electronic devices represents a critical challenge in next generations of microelectronics technologies. The introduction of well‐stacked and low polarity molecules that do not compromise film density may lead to improvements and desirable material engineering, as conventional porous SiOx derivatives exhibit detrimental degradation of thermo‐mechanical properties when their k values are further scaled down. This work presents a systematic engineering approach for controlling ultralow‐k amorphous boron nitride (aBN) deposition on 300 mm Si platforms. The results indicate that aBN grown from borazine precursor exhibits ultralow dielectric constant ≈2, high density, excellent mechanical strength, and extended thermodynamic stability. Unintentional boron ion doping during plasma dissociation that may induce artificial reductions of k value on n‐type substrates is alleviated by employing a remote microwave plasma process. Moreover, the adoption of low growth rate processes for ultralow‐k aBN deposition is found to be critical to provide for the superior mechanical strength and high density, and is attributed to the formation of hexagonal ring stacking frameworks. These results pave the way and offer engineering solutions for new ultralow‐k material introduction into future semiconductor manufacturing applications.

Facile sensitizing of PbSe film for near-infrared photodetector by microwave plasma processing

High quality PbSe film was first fabricated by a thermal evaporation method, and then the effect of plasma sensitization on the PbSe film was systemically investigated. Typical detectivity and significant photosensitivity are achieved in the PbSe-based photodetector, reaching maximum values of 7.6 × 109 cm⋅Hz1/2/W and 1.723 A/W, respectively. Compared with thermal annealing, plasma sensitization makes the sensitization easier and significantly improves the performance.

Memory Packaging Challenges for a Growing Market

Abstract Increasing demands for storage and high-performance memory in applications such as artificial intelligence (AI), machine learning and processing storage are driving long-term growth in the market. This paper will highlight the key challenges encountered in packaging next-generation memory devices and discuss some of the technological developments required to address them while considering performance, cost, and yield. An evaluation is highlighted of a remote microwave plasma process to remove damage caused by laser ablation during the wafer dicing process that shows an improvement in die strength using this process.

Self-Perpetuating Carbon Foam Microwave Plasma Conversion of Hydrocarbon Wastes into Useful Fuels and Chemicals.

White wastes (unseparated plastics, face masks, textiles, etc.) pose a serious challenge to sustainable human development and the ecosystem and have recently been exacerbated due to the surge in plastic usage and medical wastes from COVID-19. Current recycling methods such as chemical recycling, mechanical recycling, and incineration require either pre-sorting and washing or releasing CO2. In this work, a carbon foam microwave plasma process is developed, utilizing plasma discharge to generate surface temperatures exceeding ∼3000 K in a N2 atmosphere, to convert unsorted white wastes into gases (H2, CO, C2H4, C3H6, CH4, etc.) and small amounts of inorganic minerals and solid carbon, which can be buried as artificial "coal". This process is self-perpetuating, as the new solid carbon asperities grafted onto the foam's surface actually increase the plasma discharge efficiency over time. This process has been characterized by in situ optical probes and infrared sensors and optimized to handle most of the forms of white waste without the need for pre-sorting or washing. Thermal measurement and modeling show that in a flowing reactor, the device can achieve locally extremely high temperatures, but the container wall will still be cold and can be made with cheap materials, and thus, a miniaturized waste incinerator is possible that also takes advantage of intermittent renewable electricity.

Microwave plasma and rapid thermal processing of indium-tin oxide thin films for enhancing their performance as transparent electrodes

Indium-tin oxide (ITO) is widely used as a transparent electrode for optoelectronic devices given its large transparency and high conductivity. However, the particular properties of this material greatly depend on the overall fabrication process. We report on the effect of four different postfabrication processes on ITO thin films grown by electron beam evaporation. More specifically, the effect on the overall properties of the evaporated ITO thin films of microwave plasma annealing, rapid thermal processing, and the two processes combined were analyzed. In particular, the morphological, chemical, optical, and electrical properties of the annealed ITO thin films were studied. The experimental results show that the ITO thin films can be turned from opaque to transparent, and their conductivity can be improved by one order of magnitude depending on the particular postfabrication process.

Plasma Polymerized Allylamine-The Unique Cell-Attractive Nanolayer for Dental Implant Materials.

Biomaterials should be bioactive in stimulating the surrounding tissue to accelerate the ingrowth of permanent implants. Chemical and topographical features of the biomaterial surface affect cell physiology at the interface. A frequently asked question is whether the chemistry or the topography dominates the cell-material interaction. Recently, we demonstrated that a plasma-chemical modification using allylamine as a precursor was able to boost not only cell attachment and cell migration, but also intracellular signaling in vital cells. This microwave plasma process generated a homogenous nanolayer with randomly distributed, positively charged amino groups. In contrast, the surface of the human osteoblast is negatively charged at −15 mV due to its hyaluronan coat. As a consequence, we assumed that positive charges at the material surface—provoking electrostatic interaction forces—are attractive for the first cell encounter. This plasma-chemical nanocoating can be used for several biomaterials in orthopedic and dental implantology like titanium, titanium alloys, calcium phosphate scaffolds, and polylactide fiber meshes produced by electrospinning. In this regard, we wanted to ascertain whether plasma polymerized allylamine (PPAAm) is also suitable for increasing the attractiveness of a ceramic surface for dental implants using Yttria-stabilized tetragonal zirconia.

Structural and optical properties of nanoparticulate Y2O3:Eu2O3

Nanoparticulate Y2 O3 :Eu2 O3 with a small, uniform particle size and a well-defined composition was synthesized using a low temperature microwave plasma process. The structural evolution and the luminescence properties were studied in different states of annealing and Eu2 O3 addition using X-ray diffraction, transmission electron microscopy, and UV-photoluminescence spectroscopy. As synthesized, the samples were amorphous and showed only weak luminescence. Subsequent annealing steps from 500°C to 800°C lead to the formation and growth of cubic Y2 O3 :Eu2 O3 nanocrystals (5–20 nm) and a concomitant strong increase of the luminescence yield already at small grain sizes in the range of 10 nm. No self-quenching effects were observed up to 11 mol% Eu2 O3 .

Characterisation of titanium oxide layers using Raman spectroscopy and optical profilometry: Influence of oxide properties

Abstract This study evaluates the use of a combination of Raman spectroscopy and optical profilometry as a surface characterisation technique for the examination of oxide layers grown on titanium metal substrates. The titanium oxide layers with thickness of up to 8 µm, were obtained using a low-pressure oxygen microwave plasma treatment of the titanium metal substrate. The effect of the microwave plasma processing conditions (input power, pressure and treatment time) on the Raman bandwidth, intensity and peak position was evaluated. Also, the effect of these processing conditions on the surface roughness parameters (Sa, Sdq, Ssk and Sku) of the oxide layers was investigated. Analysis of the peak positions of Eg and A1g modes indicated that the effects of input power and chamber pressure was to induce a shift towards the lower frequency with increasing input power and pressure (1–2 kPa). The intensity of the Raman bands was found to be dependent on the morphology and surface chemistry of the oxide layer. The intensity of Raman band (A1g), was found to be particularly influenced by the average surface roughness (Sa) and the crystallite size. Exponential and polynomial relations were found to correlate with these properties. A two-latent variable Partial Least Squares Regression model developed on Raman spectral data could predict surface roughness with a coefficient of determination (R2) of approx. 0.87 when applied to the testing of an independent set of titanium oxide test coatings.

Formation of SiOx shell on Si nanoparticles and its effects on electrochemical properties as a Li-ion battery's anode

Crystalline Si core–amorphous SiOx shell nanoparticles were synthesized from SiCl4 using an atmospheric microwave plasma process and their microstructures were investigated. It was found that the amorphous SiOx shell thickness was determined by varying the applied microwave power. The increase of applied power directly increased the length of the flame at the end of the plasma. Through observation of the microstructures with varying lengths of the flame, it was found that a highly crystalline Si phase was formed in the plasma and amorphous SiOx phase was formed in the flame. The longer flame resulted in the thicker SiOx shell around the Si core. The electrochemical properties of the varying SiOx shell thicknesses were also investigated in that the synthesized Si-SiOx nanoparticles were used as anode materials in a lithium-ion battery (LIB). Thin shells such as native oxide did not influence the electrochemical behaviour of the Si core, whilst some shells were too thick to allow Li-ions to react with the Si core. Using Si-SiOx nanoparticles with the optimum SiOx:Si ratio of 18:82, the following electrochemical properties were obtained: a first reversible capacity of 932 mAh/g, an initial columbic efficiency (ICE) of 52.5%, and a capacity retention of 83.7% at the 100th cycle.

Combined Microwave and Laser Heating for Glazing of 8Y–ZrO2 and 8Y–ZrO2/ZrSiO4–Composites

Sintered porous yttria‐stabilized zirconia and zirconia composite ceramics with zirconium silicate are surface glazed by Laser‐Assisted Microwave Plasma Processing (LAMPP). Suitable process parameters for surface glazing are determined for those ceramics. The plasma process is monitored by means of pyrometry and optical emission spectroscopy. In order to prove the quality of the surface glazing and to characterize hot corrosion resistance, tests with molten vanadium pentoxide are performed. After 4 h of exposure, the penetration depth of the molten salt is investigated as a function of ceramic composition and pre‐treatment by glazing. Upon hot corrosion testing of glazed and non‐glazed ceramics, the molten vanadium pentoxide reacts selectively with yttrium oxide, forming yttrium vanadate, and causes crack formation in the zirconia ceramics due to transition to monoclinic zirconia. The results for LAMPP‐glazed ceramics show, that a surficial melting phase is achieved because process temperatures exceed 3000 °C. Hence, a dense, crack‐free and hardness‐enhanced surface layer achieves a better resistance to hot corrosion as compared to non‐glazed ceramics. Due to LAMPP‐glazing, the vanadium ingress is reduced from 33 to 7 μm for yttria‐stabilized zirconia and from 104 to 17 μm for zirconia composite ceramic. Reactions and microstructural changes taking place upon LAMP‐Processing are discussed.

Single grain analysis on a nanoscale in ZrO2:Al2O3 nano-composites by means of high-resolution scanning transmission electron Microscopy

Nano-particulate powders of Al2O3:ZrO2 composites and the pure constituents were prepared by microwave plasma process synthesis and studied by high resolution scanning transmission electron microscopy. The ZrO2:Al2O3 nanocomposite samples showed a structure of randomly arranged, crystalline grains of both ZrO2 and Al2O3 with a size in the range of 3–5 nm. For each constituent, both cubic and monoclinic grains were observed. Similarly prepared, single phase Al2O3 nanoparticles were found to be completely amorphous. In the case of ZrO2, the crystallite size and the fraction of the monoclinic ZrO2 phase is noticeably reduced in the ZrO2:Al2O3 nanocomposite in comparison to single phase samples. The observation of ultra-small crystalline Al2O3 in the composite samples, is tentatively ascribed to a modification of the interface energy by the contact of Al2O3 and ZrO2 as well as a possible uptake of ZrO2 into the Al2O3 grains from the gas phase during synthesis.

Effect of hydrogen on the microstructure and electrochemical properties of Si nanoparticles synthesized by microwave plasma

We synthesized silicon (Si) nanoparticles using an atmospheric microwave plasma process, and investigated the effects of hydrogen (H2) injection on their microstructure during the synthesis. Two nozzles were applied to inject H2 (swirling and rectilinear H2). Our microstructural analysis indicated that the amount and method of H2 injection were critical for completion of the reaction from silicon tetrachloride (SiCl4) to Si, as well as to obtain highly crystalline Si nanoparticles. The swirling H2 was especially critical due to its formation of vortex flow, which allowed relatively long residence time of the H-ions in plasma. The Si nanoparticles synthesized by the atmospheric plasma process had core-shell structures that consisted of crystalline Si cores with amorphous SiOx shells of 5–15 nm thickness. We also investigated the feasibility of the synthesized Si nanoparticles as anode materials in a lithium-ion battery (LIB). For the core-shell structured Si nanoparticles, we obtained the first reversible capacity of 1204 mAhg−1, and a capacity retention of 82.2% at the 50th cycle.

Potential of Redox Flow Batteries and Hydrogen as Integrated Storage for Decentralized Energy Systems

The requirement for low-cost access to energy storage technologies is increasing with the continued growth of renewable energy. The growth of the hydrogen economy is also expected to emerge to improve energy security and meet the growing pressures of environmental requirements. Hydrogen and redox flow batteries (RFB) have promising energy storage characteristics that can allow increased penetration of renewable energy and reduction in grid energy. The strong synergy between natural gas and hydrogen anticipates that new efficient methods of hydrogen production such as microwave plasma processing of natural gas might have a leading role. Additionally, the improvements in the carbon allotropes properties and their cost are expected to influence large-scale energy storage system costs. A technical and economic comparison of vanadium and all-iron RFB with hydrogen will be explored on an individual and integrated basis. The findings show hydrogen’s capability for bulk energy storage and highlights the benefits of integrated storage. This proves that integration of various storage systems can play an important role for the future development of the decentralized renewable energy systems.

Design for approaching Cicada-wing reflectance in low- and high-index biomimetic nanostructures.

Natural nanostructures in low refractive index Cicada wings demonstrate ≤ 1% reflectance over the visible spectrum. We provide design parameters for Cicada-wing-inspired nanotip arrays as efficient light harvesters over a 300-1000 nm spectrum and up to 60° angle of incidence in both low-index, such as silica and indium tin oxide, and high-index, such as silicon and germanium, photovoltaic materials. Biomimicry of the Cicada wing design, demonstrating gradient index, onto these material surfaces, either by real electron cyclotron resonance microwave plasma processing or by modeling, was carried out to achieve a target reflectance of ∼ 1%. Design parameters of spacing/wavelength and length/spacing fitted into a finite difference time domain model could simulate the experimental reflectance values observed in real silicon and germanium or in model silica and indium tin oxide nanotip arrays. A theoretical mapping of the length/spacing and spacing/wavelength space over varied refractive index materials predicts that lengths of ∼ 1.5 μm and spacings of ∼ 200 nm in high-index and lengths of ∼ 200-600 nm and spacings of ∼ 100-400 nm in low-index materials would exhibit ≤ 1% target reflectance and ∼ 99% optical absorption over the entire UV-vis region and angle of incidence up to 60°.

Identification of deep trap energies and influences of oxygen plasma ashing on semiconductor carrier lifetime

We have performed an analytical study of the effects of oxygen plasma ashing processes in semiconductor device fabrication and its impact on minority carrier lifetime in high voltage semiconductor devices. Our work includes a critical background study of life time killing mechanisms by deep traps imparted into the semiconductor by barrel plasma ashing. The Elymat technique provides the opportunity to measure lifetime and diffusion length of minority carriers and surface photo voltage (SPV) measurement was used to analyse influences of process parameters such as photoresist, time budget and positioning in the process chamber. It was shown that in microwave plasma processes the diffusion length changes severely with tempering at 200 °C, whereas RF-plasma processes show a significant process time-dependence. Batch tools in general suffer from a strong first wafer effect which could be correlated with the static electrical parameters of the semiconductor devices. The trap identities were detected by using deep level transient spectroscopy and the chemical species of the traps has been proven by inductive coupled plasma mass spectrometry. The deep-bandgap trap energies are reliable fingerprints of the chosen process parameters such as process time and of resist-influences. By microwave plasma processes intrinsic Fe and FeB-complex levels were identified and a good agreement with the SPV-measurement and electrical device characteristic was shown. RF-plasma processes impart levels attributed to Pt levels and an additional level, which could be identified as a trap level probably forming a complex of Pt and H.

Microwave-plasma synthesis of nano-sized silicon carbide at atmospheric pressure

A microwave plasma process operating at atmospheric pressure was developed for the synthesis of SiC nanoparticles. The process utilizes methyltrichlorosilane (MTS) as precursor, acting as both carbon and silicon source, along with an additional hydrogen feed to ensure a fully reducing reaction environment. In addition, argon served as carrier gas. The parameters studied were the H2:MTS molar ratio and the total enthalpy, in the range 0–10 and 70–220MJ/kg respectively. The particles size distribution ranged from 15 to 140nm as determined by SEM and TEM micrographs. It was found that an increase in enthalpy and a higher H2:MTS ratio resulted in smaller SiC particle sizes. The adhesion of particles was a common occurrence during the process, resulting in larger agglomerate sizes.