Hydrogen Plasma Research Articles

Dielectric barrier discharge plasma for catalytic-free palm oil hydrogenation using glycerol as hydrogen donor for further production of hydrogenated fatty acid methyl ester (H-FAME)

Catalytic-free parallel-plate-type dielectric barrier discharge (DBD) plasma was successfully employed to hydrogenate palm oil using glycerol as hydrogen donor in place of hydrogen gas for further production of hydrogenated fatty acid methyl ester (H-FAME) with enhanced oxidation stability. Most suitable reaction condition was 1:5 oil to glycerol molar ratio, 1 cm discharge gap size, 0.5 L/min He gas flow rate, atmospheric pressure, ambient temperature (rising to about 100 °C due to plasma heating), and 500 rpm stirring rate for 2 h at 130 W input power. Findings revealed that plasma could extract hydrogen radicals from glycerol, and they successfully reacted with CC bonds. Hydrogenated palm oil was subsequently converted into hydrogenated FAME (H-FAME) via conventional base-catalyzed transesterification. Higher saturation degree of H-FAME compared to FAME produced directly from palm oil was indicated by reduction of iodine value to 52.2 from 59.2. Oxidation stability of H-FAME was significantly enhanced from 17.7 to 23.1 h while cloud point slightly changed from 14.0 to 14.8 °C. Plasma reactive species generated during reaction and possible mechanisms of glycerol decomposition and plasma hydrogenation were presented. Developed catalytic-free plasma hydrogenation process is highly environmentally friendly and presents a way to add more value to waste glycerol and provide other potential hydrogen sources.

Impact of Iron Ore Pre-Reduction Degree on the Hydrogen Plasma Smelting Reduction Process

To counteract the rising greenhouse gas emissions, mainly CO2, the European steel industry needs to restructure the current process route for steel production. Globally, the blast furnace and the subsequent basic oxygen furnace are used in 73% of crude steel production, with a CO2 footprint of roughly 1.8 t CO2 per ton of produced steel. Hydrogen Plasma Smelting Reduction (HPSR) utilizes excited hydrogen states with the highest reduction potentials to combine the simultaneous reduction and smelting of iron ore fines. Due to the wide range of iron ore grades available worldwide, a series of hydrogen plasma experiments were conducted to determine how pre-reduced iron ore and iron-containing residues affect reduction behavior, hydrogen consumption, overall process time, and metal phase microstructure. It was discovered that, during the pre-melting phase under pure argon, wet ore increased electrode consumption and hematite achieved higher reduction levels, due to thermal decomposition. The reduction of magnetite ore yielded the highest reduction rate and subsequent hydrogen conversion rates. Both hematite and magnetite exhibited high utilization rates at first, but hematite underwent a kinetic change at a reduction degree of 80–85%, causing the reduction rate to decrease. In comparison to fluidized bed technology, it is possible to use magnetite directly, and the final phase of the reduction can move along more quickly due to higher temperatures, which reduces the overall process time and raises the average hydrogen utilization. A combination of both technologies can be considered advantageous for exhaust gas recycling.

In-situ non-destructive removal of tin particles by low-energy plasma for imitation of EUV optical mirrors self-cleaning

In this study, the feasibility was explored of realizing in-situ and non-destructive self-cleaning of the multilayers mirror (MLM) by utilizing hydrogen plasma at low input power (<10 W). The hydrogen plasma generated by the electrode at low power was comprehensively diagnosed using the Langmuir probe and the retarding field ion energy analyzer (RFEA). Meanwhile, the ion concentration ratios and atomic density in hydrogen plasma were calculated by constructing a zero-dimensional plasma chemical model. These experimental results show that the plasma distribution at 1 W is more uniform than that at 10 W, and the peak ion energy at 1 W is lower than 22 eV. Cleaning rates of 0.3–0.75 nm min−1 are achieved to remove discontinuous tin particles at different powers. According to the experiment and calculation results, the chemical etching of ions (breaking the Sn–Sn bond) occurred at 1 W. The etching yield is 0.23–0.87 Sn/ion for 10–40 eV. The analysis of surface morphology and cross-sectional morphology shows that the removal of tin particles is realized by top-down cleaning with hydrogen plasma at low power. This study is of great significance for understanding the detailed mechanism of MLM self-discharge cleaning tin pollutants at low power. It has made a comprehensive calculation and diagnosis of plasma parameters.

Effects of the Electrodes’ Shape and Graphite Quality on the Arc Stability During Hydrogen Plasma Smelting Reduction of Iron Ores

Reducing greenhouse gases (GHG), especially CO2, is necessary to counteract climate change. The European steel industry currently corresponds to 5.7% of the total EU emissions and must therefore minimize their GHG fractions in the future. One of the most promising technologies to eliminate CO2 emissions while directly reducing iron ore to steel in a single step is the hydrogen plasma smelting reduction. The stability of the plasma arc, which is determined by the properties and geometry of the graphite electrode, has a substantial impact on the process’ economic feasibility. To study the arc stability concerning the graphite quality, tip geometry, and electrode gap, a series of experiments is conducted. The results are evaluated to create stability maps and fields to identify stable process parameters. The geometry of the graphite cathode shows the primary influence on arc stability. Tips with a flat end (standard version) offering the most unstable and a machined step on the graphite cathode providing the most stable conditions. However, an additional coating to prevent side arcing leads to the deterioration of the arc. The two graphite grades tested, with different maximum grain sizes and price classes, show no great relevance to the stability of the arc.

Characterization of the hydrogen plasma in the RF negative ion source by optical emission spectroscopy

Optical emission spectroscopy (OES) is an important noninvasive diagnostic tool for providing insight into the plasmas of the negative ion sources. The plasma spectroscopic characterization of the RF driven negative ion source at Huazhong University of Science and Technology (HUST) is studied with the delivered power from 6 kW to 16 kW. The gas temperature, electron temperature, electron density and the density ratio of atomic hydrogen to molecular hydrogen are extracted from OES. The gas temperature is estimated from the Fulcher band of H2. A global thermal model is developed to investigate the gas heating mechanisms in hydrogen plasmas. The gas temperature calculated by the model using the electron density and electron temperature from OES as input parameters is in good agreement with that from the experiment. The thermal model shows that the dissociation of molecular hydrogen by electron impact is the dominant source of gas heating and that approximately 11% ∼12% of the delivered power is dissipated in gas heating at 0.3 Pa filling pressure. Moreover, the electron temperature, electron density and the ratio of atomic to molecular density are obtained from the absolute intensity of Balmer lines and Fulcher band by using collisional radiative (CR) models of H and H2. The kinetics of the excited states of H are also discussed.

Preparation of diamond on GaN using microwave plasma chemical vapor deposition with double-substrate structure

Combining diamond with GaN can significantly improve the heat dissipation performance of GaN-based devices. However, how to avoid the destructive damage to the GaN epi-layer caused by high-temperature hydrogen plasma during the diamond growth is still a problem. This study employed a Si transition layer and double-substrate structure microwave plasma chemical vapor deposition (MPCVD) to prepare diamond film on GaN epi-layer. The effects of double-substrate structure on the diamond growth were studied. The microwave plasma parameters of both single-substrate structure and double-substrate structure MPCVD diagnosed by emission spectra were comparatively investigated. It has been found that the microwave plasma energy of double-substrate structure MPCVD is relatively more concentrated and has higher radicals activity, which is beneficial to the diamond growth. The impacts of the Si transition layer on the diamond growth were also investigated. It demonstrates that the Si transition layer can effectively protect the GaN epi-layer from being etched by hydrogen plasma and improve the diamond growth. The relationship between the thickness of the Si transition layer and the diamond growth and the relationship between diamond film thickness and adhesion has been studied in detail.

Tuning electrical properties in Ga2O3 polymorphs induced with ion beams

Ion beam fabrication of metastable polymorphs of Ga2O3, assisted by the controllable accumulation of the disorder in the lattice, is an interesting alternative to conventional deposition techniques. However, the adjustability of the electrical properties in such films is unexplored. In this work, we investigated two strategies for tuning the electron concentration in the ion beam created metastable κ-polymorph: adding silicon donors by ion implantation and adding hydrogen via plasma treatments. Importantly, all heat treatments were limited to ≤600 °C, set by the thermal stability of the ion beam fabricated polymorph. Under these conditions, silicon doping did not change the high resistive state caused by the iron acceptors in the initial wafer and residual defects accumulated upon the implants. Conversely, treating samples in a hydrogen plasma converted the ion beam fabricated κ-polymorph to n-type, with a net donor density in the low 1012 cm−3 range and dominating deep traps near 0.6 eV below the conduction band. The mechanism explaining this n-type conductivity change may be due to hydrogen forming shallow donor complexes with gallium vacancies and/or possibly passivating a fraction of the iron acceptors responsible for the high resistivity in the initial wafers.

Reducing tin droplet ejection from capillary porous structures under hydrogen plasma exposure in Magnum-PSI

Liquid metal based divertors could be a more robust alternative to a solid tungsten design for DEMO. The liquid is confined in a sponge-like tungsten layer, called a capillary porous structure (CPS). It has been found previously that under certain conditions, many tin droplets eject from a CPS when it is brought into contact with a hydrogen plasma. These would present a contamination issue for the plasma core. Stability analysis suggests that droplet ejection can be suppressed by reduction of the pore size. To test this, stainless-steel CPS targets with pore size ranging from 0.5–100µm filled with tin were exposed to identical loading conditions. This was done in the linear plasma device Magnum-PSI, capable of reaching divertor relevant plasma conditions. Furthermore, the influence of the CPS manufacturing techniques is considered by comparing the performance of a 3D printed, a mesh felts and a sintered CPS, all made from tungsten. Each target was surrounded by four witness plates, which were analysed post-mortem for Sn content by Rutherford backscattering. During plasma exposure, tin droplets were observed using a fast visible camera and plasma light emission via survey optical emission spectroscopy. The results imply that Sn erosion can be reduced by a factor of 50 when reducing the pore size. Moreover, it highlights the importance of avoiding overfilling of CPS targets with Sn.

Prompt core confinement improvement across the L–H transition in DIII-D: Profile stiffness, turbulence dynamics, and isotope effect

We elaborate on the nature of the prompt core confinement improvement observed at the L–H transition in DIII-D, which is a long-standing issue unsolved for more than two decades and can impact future fusion reactor performance. Dynamic transport analysis suggests the essential role of the profile stiffness for understanding the mechanism of the prompt core confinement improvement. Beam emission spectroscopy shows that transport reduction at the core cannot be explained only by the ion scale turbulence density fluctuation suppression. Properties of nonlocal confinement improvement across the L–H transition are experimentally assessed in hydrogen (H) and deuterium (D) plasmas. Prompt core confinement improvement is found to be more rapid in the lighter hydrogen isotope.

Initial Nucleation Approach for Large Grain-Size and High-Quality Microcrystalline Silicon Active Layer in Thin Film Transistors

High mobility and stability are critical factors for thin film transistor (TFT) device quality. These parameters are directly dependent on the crystalline structure of the active layer materials. In this paper, the early nucleation approach was performed for increasing the crystalline grain size of microcrystalline silicon (μc-Si:H) active layer for TFT device quality. The crystalline nucleation is delicately regulated in an intense hydrogen plasma environment using the plasma enhanced chemical vapor deposition (PECVD). When compared to μc-Si:H deposition without the nucleation approach, the crystalline volume factor of μc-Si:H increased from 60% to over 80% by using the nucleation technique. The nucleation increases the crystalline grain size by five orders of magnitude. Furthermore, the surface roughness of μc-Si:H is decreased from 13.7 nm to 7.1 nm. A forming-gas post-annealing treatment (≤400 °C) is used to minimize defect density. With a low microstructural factor, thermal-treated film quality improves dramatically. Nucleation approach is to be a simple and efficient for producing high-quality TFT devices.

Effect of hydrogen plasma treatment of nanodiamond on the tribological properties of polytetrafluoroethylene-based nanocomposite coating

ABSTRACT Carbon atom recombination and etching of the surface of nanodiamond (ND) by hydrogen plasma and their effects on the dispersion and the tribological properties of a polytetrafluoroethylene (PTFE) nanocomposite coating were investigated. The surface-modified ND samples subjected to plasma treatment were characterized using FESEM, HRTEM, Raman spectroscopy, FT-IR spectroscopy and XPS. The results showed that the plasma treatment facilitates etching and recombination of the ND surface and modifies the crystallinity of the graphitic amorphous carbon. The tribological properties of the PTFE-based nanocomposite coatings prepared with the modified ND depended on the content of sp 3 -bonded carbon on the surface and the crystallinity of the ND. Hydrogen plasma treatment caused a significant improvement in the dispersion and tribological properties of the PTFE/ND nanocomposite coating due to surface modification of the ND.

The Contribution of The Joule-Thomson Effect To Solar Coronal Heatin

Two of the three gases that display isenthalpic Joule-Thomson (JT) warming under laboratory conditions are hydrogen and helium, the main constituents of the solar plasma, as the inversion point at which expansion results in heating is only ~51 K for helium and ~193 K for hydrogen, but the temperatures that are attained by this route are at most a few hundred K. Increases in ion temperature by several orders of magnitude are claimed for hydrogen plasmas subject to expansion into a vacuum; modest increases are reported for the short lived tests of this effect that have been carried out in space in the wakes of artificial satellites and of the Moon.

Low-Temperature H2/D2 Plasma–W Material Interaction and W Dust Production for Fusion-Related Studies

In this paper, results concerning hydrogen and deuterium plasma (RF, 13.56 MHz) interactions with tungsten surfaces, were reported. We used the Hollow-Cathode (HC) configuration for plasma–tungsten surface interaction experiments, along with the collection of tungsten dust, at different distances. Further on, the plasma-exposed tungsten surfaces and the collected dust were morphologically analyzed by contact profilometry, scanning electron microscopy, and energy dispersive spectroscopy measurements, along with chemical investigations by the X-ray photoelectron spectroscopy technique. The results showed that exposing the tungsten surfaces to the hydrogen plasma induces surface erosion phenomena along with the formation of dust and interconnected W structures. Herein, the mean ejected material volume was ~1.1 × 105 µm3. Deuterium plasma facilitated the formation of blisters at the surface level. For this case, the mean ejected material volume was ~3.3 × 104 µm3. For both plasma types, tungsten dust within nano- and micrometer sizes could be collected. The current study offers a perspective of lab-scaled plasma systems, which are capable of producing tungsten fusion-like surfaces and dust.

A study of hydrogen plasma-induced charging effect in EUV lithography systems

In the extreme ultraviolet lithography system, EUV-induced hydrogen plasma charging effect is observed by in situ embedded micro-detector array. The 4k-pixel on-wafer array can detect and store the distributions of H2 plasma in each in-pixel floating gate for non-destructive off-line read. The local uniformity of H2 plasma intensity extracted by the threshold voltages on an array and its distributions across a wafer by the average bit cell current of MDAs provide insights into the detailed conditions inside advanced EUV lithography chambers.

A three-dimensional numerical study on the effect of geometric asymmetry on arcjet thruster performance

In an arcjet thruster, the cathode and constrictor degrade with time, and the electrical arc discharge may become unsymmetrical. In this work, a three-dimensional numerical model of a hydrogen plasma arcjet is developed and validated to study the effect of unsymmetrical electric arc discharge on thruster performance. The unsymmetrical arc discharge is realized by introducing a radial shift of the cathode so that the cathode tip offset is 80 μm (25% of the constrictor radius). Simulations are conducted for both axially centered cathode (coaxial) and off-centered cathode (non-coaxial) configurations with identical propellant flow rates and input current. Simulations show asymmetrical arc discharge in the non-coaxial cathode configuration, resulting in azimuthally asymmetric Joule heating, species concentrations, and velocity field. This asymmetry continues as the plasma expands in the divergent section of the nozzle. Temperature, species concentrations, and axial velocity exhibit asymmetric radial distribution at the nozzle exit. The computed Joule heating was found to reduce with cathode shift, and consequently, the thrust and specific impulse of the thruster was decreased by about 6.6%. In the case of the non-coaxial cathode, geometric asymmetry also induces a small side thrust.

Self-Controlled Flashing Nuclear Fusion in Stationary Magnetized Low-Temperature Plasma

The prerequisites and mechanism for the implementation of efficient pulsed (flashing) nuclear fusion in a low-temperature hydrogen plasma with a temperature of 10 to 20 eV in a constant magnetic field are considered. It is shown for the first time that the natural very frequent alternation of the processes of ionization of atoms and recombination of ions leads to the synchronous formation of coherent correlated states of hydrogen nuclei and its isotopes. The formation of such states leads to the generation of very large fluctuations of kinetic energy (up to 10 to 100 keV) at the initial stage of each ionization event, which exists for most of the lifetime of the ionized state before ion recombination. It is shown that the relatively long duration of the existence of these fluctuations and their very large amplitude are sufficient for efficient nuclear fusion in such a magnetized low-temperature plasma.

Effect of Strain Rates on Mechanical Properties of a Duplex Stainless-Steel Sheet Charged in Hydrogen Plasma

Tensile properties of a JIS SUS329J4L duplex stainless steel with and without hydrogen plasma charging at various strain rates ranging from 1.38 × 10−7 to 1.38 × 10−3 s−1 in ambient atmosphere were investigated. Fracture surface was evaluated by scanning electron microscope (SEM) and the trapping state of hydrogen was assessed by thermal desorption spectroscopy (TDS). It was confirmed that ultimate tensile strength and elongation to failure were decreased with increasing charging time for testing at 1.38 × 10−3 s−1, while the ultimate tensile strength was affected less markedly by hydrogen charging than the elongation to failure. The extent of hydrogen embrittlement took a maximum at a strain rate of 1.38 × 10−6 s−1, where the proportion of smooth area (area without dimples) in relation to the total fracture area reached a maximum. This conformity between the extent of the degradation and the area without dimple was in accord with that reported in the authors’ previous paper using electrolytical hydrogen charging. On the other hand, the strain rate where the extent of degradation became maximum for the current tests was two magnitudes lower than that in the electrolytically charged test piece. It was assumed that the trapping site for plasma-charged hydrogen is more stable than that for electrolytically charged hydrogen. This was confirmed by TDS results that the desorption spectrum of the 24 h plasma-charged specimen had a shoulder at about 200°C and the peak temperature significantly higher than that of the 24 h electrolytically charged specimen.

Applications and mechanisms of anisotropic two-step Si3N4 etching with hydrogen plasma conditioning

The ability to precisely form Si3N4 spacers is critical to the success of dynamic random-access memory and NAND (NOT AND) flash memory technology development. In this study, we investigated the mechanisms and process windows of an innovative two-step nitride (Si3N4) etch consisting of H2 plasma processing in an inductively coupled plasma chamber followed by either buffered oxide etch (BOE, a mix of NH4F and HF solution) wet clean or in situ NF3 plasma etch. We obtained layer–by-layer removal with each layer’s removal capable of more than 10 nm. We revealed that H can penetrate more than 20 nm deep into the nitride film to transform pristine Si3N4 into SiON after air exposure, which can be subsequently removed by BOE wet clean. The H2 and BOE steps do not need to run back-to-back; the modified SiON layer is stable enough to sustain elevated temperature processing and can be removed by BOE later down-the-line integration. We also demonstrated that using NF3 plasma can have highly selective etch of nitride over oxide due to the incubation time difference between these two types of films. It takes much longer time to initiate the chemical reaction for oxide compared with nitride. Critically, the role of H2 is not the key for high selectivity; instead, it provides an etch directionality and shortens the incubation time for both nitride and oxide.

Remote Plasma Physics Research and Teaching by Example of Turbulence Study at the University-Scale Tokamak GOLEM

The university-scale tokamak GOLEM provides a unique opportunity to perform remote thermonuclear experiments [V. Svoboda, J. Fusion Energy, Vol. 38, Part 2, p. 253 (2019)]. Undergraduate plasma physics students from three universities—Moscow Institute of Physics and Technology (National Research University), RUDN University, and National Research Nuclear University MEPhI—carried out joint remote experiments to train in tokamak operation and to study topics relevant for mainstream fusion research such as plasma start-up, comparison of hydrogen versus helium plasma characteristics, electrostatic and electromagnetic turbulence, long-range correlations, etc. New observations of the long-range correlations between low-frequency (<50 kHz) quasi-coherent electrostatic and magnetic oscillations identified as m = 2 mode with several techniques were done, as well as of the broadband (<250 kHz) magnetic oscillations resolved in frequency and wave vector in helium and hydrogen plasmas. The presence of broadband electrostatic and broadband magnetic turbulence has also been established at the plasma edge.

The impact of hydrogen plasma on the structure and morphology of tin and lead micrometer sized particles

The stability of micrometer sized particles in hydrogen plasma is essential for extreme ultraviolet lithography, the ITER fusion program and the application of hydrogen plasma etching. We experimentally investigated the morphological evolution of tin (Sn), lead (Pb), and lead (II) oxide (PbO) micrometer sized particles on a surface that is exposed to a low pressure hydrogen plasma. Post exposure particle cross sections obtained by a scanning electron microscope accompanied by a focused ion beam demonstrated a significant influence of hydrogen plasma exposure on both the surface and the bulk material of the particles. Chemical sputtering at the surface and accumulation of pressurized hydrogen bubbles in cavities in the bulk material are the main drivers of the morphological changes. These mechanisms may influence the adhesion of particles to the surface through the introduction of asperities, increase of contact spot area, or fragmentation after the accumulation of mechanical stress.