Temperature Hydrogen Plasma Research Articles

The development of a zero-dimensional collisional-radiative model for interpreting plasma emission in low temperature divertor plasmas in tokamaks

Collisional-radiative models are commonly used to analyse atomic and molecular processes in low temperature plasmas by determining the distribution functions of excited states as functions of various plasma parameters. This paper outlines the improvements to a zero-dimensional collisional-radiative model, developed at Keio University, for purposes relevant to the analysis of emission measurements in low temperature hydrogen plasmas. The extension of this 0D model centres on the inclusion of additional molecular species and reactions, a calculation of emission intensity in order to directly compare with experimental work, and the addition of a simple wall model to allow for a deeper understanding of recycling of atoms and molecules in low temperature plasmas close to plasma facing components in fusion machines. Results from the improved model are then compared with both an existing CR model for benchmarking and experimental emission spectroscopy data from an inductively-coupled plasma device at the University of Liverpool. These results show how the developments to the model have increased the relevance to experimental plasmas, such as those in the power exhaust regions of fusion machines, with the ratio of the Hβ/Hα line intensities outputted from the improved CR model mirroring more closely the emission measured experimentally in the ICP device.

Low temperature hydrogen plasma permeation in palladium and its alloys for fuel recycling in fusion systems

Superpermeation of hydrogen isotopes through metal foils is a critical component for efficient fuel recycling in fusion power systems. In that context hydrogen permeation through foils of palladium and its alloys with silver and copper was studied at low temperature (60 - 200°C) under plasma exposure. These alloys differ significantly in both bulk and surface properties, and comparisons can provide mechanistic insights. Permeation was observed only during plasma operation, confirming the negligible contribution of molecular hydrogen to the observed flux. As-received foils required surface treatment to achieve top performance. For Pd and Pd75Ag25 an oxidation treatment increased permeation an order of magnitude, but proved unstable as this desirable surface was reduced under hydrogen plasma exposure. In contrast, an Ar plasma cleaning step provided both high and stable flux. As-received FCC phase Pd60Cu40 foils required annealing to transform it into the high permeability BCC phase that delivered top performance. All foils displayed similar temperature dependence with flux declining with temperature, suggesting that the primary rate-limiting step is absorption of superthermal hydrogen. Among these foils the hydrogen flux through PdCu was 3 - 5X greater than that of Pd or PdAg, which were similar. The superiority of BCC PdCu is attributed to its superior hydrogen desorption kinetics. Using PdCu 100% permeation of supplied hydrogen was achieved, and the flux saturated with increasing plasma power at values >10−2 mol H·m−2·s−1, and under these conditions permeation rates are equivalent with or without the membrane present. The fluxes achieved are the highest reported to date at these conditions, and the results highlight the important roles of both surface and bulk properties.

Experimental study of charging of dust grains in the presence of energetic electrons

The role of hot electrons in the charging of dust grains is investigated in a two temperature hydrogen plasma. A variety of dust particles are introduced into the system and secondary electron emission (SEE) from each of the dust types has been reported. A cylindrical Langmuir probe is used for determining the plasma parameters and a Faraday cup is connected to an electrometer in order to measure the dust current. The electrometer readings confirm the electron emission from the dust and SEE is observed from the tungsten dust in a low-pressure experimental plasma device for the first time.

Work function performance of a C12A7 electride surface exposed to low pressure low temperature hydrogen plasmas

The calcium aluminate electride [Ca24Al28O64]4+(e−)4 (C12A7:e−) is chemically and thermally stable in ambient atmosphere, offers strong electron donating capabilities and exhibits an intrinsic work function of 2.4 eV. Thus, it is an attractive material for electron emitter and low work function applications. For an implementation in negative hydrogen ion sources, where the electride could serve as a converter surface for the H− production, compatibility with moderate vacuum conditions and exposure to low pressure low temperature hydrogen plasmas must be ensured. Hence, the stability and work function performance of a polycrystalline C12A7:e− sample are investigated under ion source relevant vacuum (∼10−6 mbar) and plasma conditions (electron temperature ∼2 eV, electron density ∼1016m−3). The work function is measured via the photoelectric effect, showing that long-term hydrogen plasma exposure results in a steady-state surface work function of 2.75±0.10 eV. Since the electride is very sensitive to surface contaminations, an increase of the surface work function occurs as soon as the plasma is switched off. Vacuum heat treatment of the degraded C12A7:e− surface after plasma activation leads to a work function minimum of 2.9 eV for temperatures in the range of 670–770 K. Exceeding a temperature of ∼770 K leads to an increase of the C12A7:e− surface work function. The sample is stable during long-term hydrogen plasma exposure (up to 10 h total plasma-on time tested) and no plasma-induced erosion is observed.

Effect of a low pressure low temperature hydrogen plasma on the work function of europium

Europium is a ductile rare-earth element and has the lowest function among lanthanides (2.5 eV). Due to its low work function reachable without an activation procedure, Eu is a potential electron emitter. For its application to negative hydrogen ion sources for fusion, the work function of an Eu sample exposed to a low pressure low temperature hydrogen plasma is investigated here. First, a work function of 2.6±0.1 eV is achieved in the present vacuum conditions (∼10−6 mbar) by heating the sample up to 350°C. A short hydrogen plasma pulse of 1 min allows us to achieve the same low work function; however, a longer plasma exposure leads to an increase of the work function, which must be counteracted by heating the sample up to at least 400°C to maintain the low work function during plasma. Furthermore, the exposure to hydrogen plasma affects the reactivity of Eu to oxidation when it is exposed to air, showing evidence of a faster oxidation in the areas previously in contact with the plasma. In conclusion, due to the strong reaction to hydrogen plasma and the rapid oxidation in air, Eu is not suitable for application in negative ion sources for fusion.

Room temperature plasma hydrogenation – An effective way to suppress defects in ZnO nanorods

The densely packed, perpendicularly oriented ZnO nanorods were hydrothermally grown in dark and under UV irradiation on fused silica glass substrates seeded by ZnO nanocrystals. The exciton-related UV photoluminescence observed at room temperature at 380 nm increased significantly after a room temperature hydrogen plasma treatment in a negatively self-biased capacitive coupled radio frequency reactor whereas the defect-related yellow photoluminescence was noticeably reduced together with the F+ electron paramagnetic resonance (EPR) signal g = 1.96. Thus, the F+ EPR signals are surface type defects which amount is governed by the surface plasma treatment. The correlation between PL and EPR signals observed in ZnO nanorods exposed to the same treatment relates some optically active defects to paramagnetic centers.

Effect of plasma power on reduction of printable graphene oxide thin films on flexible substrates

Room temperature hydrogen plasma treatment on solution processed 300 nm graphene oxide (GO) films on flexible indium tin oxide (ITO) coated polyethylene terephthalate (PET) substrates has been performed by varying the plasma power between 20 W and 60 W at a constant exposure time of 30 min with a view to examining the effect of plasma power on reduction of GO. X-ray powder diffraction (XRD) and Raman spectroscopic studies show that high energy hydrogen species generated in the plasma assist fast exfoliation of the oxygenated functional groups present in the GO samples. Significant decrease in the optical band gap is observed from 4.1 eV for untreated samples to 0.5 eV for 60 W plasma treated samples. The conductivity of the films treated with 60 W plasma power is estimated to be six orders of magnitude greater than untreated GO films and this enhancement of conductivity on plasma reduction has been interpreted in terms of UV-visible absorption spectra and density functional based first principle computational calculations. Plasma reduction of GO/ITO/PET structures can be used for efficiently tuning the electrical and optical properties of reduced graphene oxide (rGO) for flexible electronics applications.

Hydrogen loss and its improved retention in hydrogen plasma treated a-SiN :H films: ERDA study with 100 MeV Ag7+ ions

Hydrogen loss from a-SiNx:H films under irradiation with 100 MeV Ag7+ ions using elastic recoil detection analysis (ERDA) experiments is reported. The results are explained under the basic assumptions of the molecular recombination model. The ERDA hydrogen counts are composed of two distinct hydrogen desorption processes, limited by rapid molecular diffusion in the initial stages of irradiation, and as the fluence progresses a slow process limited by diffusion of atomic hydrogen takes over. Which of the aforesaid processes dominates, is determined by the continuously evolving Hydrogen concentration within the films. The ERDA measurements were also carried out for films treated with low temperature (300 degrees centigrade) hydrogen plasma annealing (HPA). The HPA treated films show an improved diffusion of atomic hydrogen, resulting from healing of weak bonds and passivation of dangling bonds. Further, upon HPA, films also show evolution of hydrogen with significantly higher counts, at advanced fluences, relative to the as-deposited films. These results indicate the potential of HPA towards improved H retention in a-SiNx:H films. The study distinguishes clearly the presence of two diffusion processes in a-SiNx:H whose diffusion rates differ by an order of magnitude, with hydrogen radicals not being able to diffuse beyond ~1 nm from the point of their creation. The results are very relevant for the passivation applications of a-SiNx:H

Electrical power generation with PEM FC using hydrogen supply through Mg-Ni reaction with water

Hydrogen is considered an energy vector of the future because of its potential use for clean energy generation. Portable electronic devices can be powered when hydrogen is supplied to fuel cells. In order to avoid massive equipment for hydrogen storage, direct hydrogen production can be achieved on-site during the reaction between metals/metal alloys/metal hydrides and water. Magnesium hydride offers great perspective for widespread applications as its weight yield of hydrogen reaches 6.4% according to the reaction with water and it can even increase to 15.2% if water produced in the fuel cell is used in the reaction again. In the present work, Mg powder with the content of Ni was synthesized under low temperature hydrogen plasma conditions changing the DC magnetron current from 0.5 to 1 A. As pure Mg powder was immersed into hydrogen plasma, the simultaneous hydrogenation process was ensured. Nickel was chosen as a catalyst capable to influence the growth of hydride. The process of electric power generation was investigated when reaction between modified Mg powder and water was applied to laboratory-built equipment consisting of a reactor for hydrogen production, gas dryer before H2 introduction to the fuel cell, fuel cell, load and energy meter. Solutions of acetic acid and sodium chloride were used as promoters during powder-water reactions. The characterisation of predicted magnesium hydride powder was done using scanning electron microscopy, electron dispersive spectroscopy and X-ray diffraction. XRD analysis showed only Mg, MgO and Ni peaks indicating that hydrogen generation during powder-water reaction was evoked because of microgalvanic corrosion at Mg-Ni intersections.

Stability of AlGaN/GaN heterostructures after hydrogen plasma treatment

We report on the investigation of low temperature (300°C) hydrogen plasma treatment influence on the AlGaN/GaN heterostructures. This issue was raised in the frame of study on processes related to hybrid integration of diamond with GaN-based devices. At the same time, the capabilities of thin SiNx covering were investigated. The samples were exposed to low pressure hydrogen plasma ignited in the linear plasma system at low temperature. We analyze the surface morphology of samples by scanning electron microscopy while microstructural changes down to AlGaN/GaN interface were studied using secondary ion mass spectrometry. The sheet resistance, monitored using circular transmission line measurements, increases more than 3.5 times after 60min treatment. The basic transport properties of the fabricated circular high electron mobility transistors after H2 plasma treatment were analyzed. The sheet resistance increasing was attributed to the decrease of effective mobility. Whilst, the observed Schottky barrier lowering indicates necessity of gate contact protection.

Plasma heating power dissipation in low temperature hydrogen plasmas

A theoretical framework for power dissipation in low temperature plasmas in corona equilibrium is developed. The framework is based on fundamental conservation laws and reaction cross sections and is only weakly sensitive to plasma parameters, e.g., electron temperature and density. The theory is applied to low temperature atomic and molecular hydrogen laboratory plasmas for which the plasma heating power dissipation to photon emission, ionization, and chemical potential is calculated. The calculated photon emission is compared to recent experimental results.

Residual gas entering high density hydrogen plasma: rarefaction due to rapid heating

The interaction of background molecular hydrogen with magnetized (0.4 T) high density (1–5 × 1020 m−3) low temperature (∼3 eV) hydrogen plasma was inferred from the Fulcher band emission in the linear plasma generator Pilot-PSI. In the plasma center, vibrational temperatures reached 1 eV. Rotational temperatures obtained from the Q(v = 1) branch were systematically ∼0.1 eV lower than the Q(v = 0) branch temperatures, which were in the range of 0.4–0.8 eV, typically 60% of the translational temperature (determined from the width of the same spectral lines). The latter is attributed to preferential excitation of translational degrees of freedom in collisions with ions on the timescale of their in-plasma residence time. Doppler shifts revealed co-rotation of the molecules with the plasma at an angular velocity an order of magnitude lower, confirming that the Fulcher emission connects to background molecules. A simple model estimated a factor of 90 rarefaction of the molecular density at the center of the plasma column compared to the residual gas density. Temperature and density information was combined to conclude that ion-conversion molecular assisted recombination dominates plasma recombination at a rate of 1 × 10−15 m3 s−1. The observations illustrate the general significance of rapid molecule heating in high density hydrogen plasma for estimating molecular processes and how this affects Fulcher spectroscopy.

Reduction of Cobalt Oxide (Co3O4) by Low Temperature Hydrogen Plasma

The paper reports on reduction of cobalt oxide (Co3O4) to cobalt metal by low temperature hydrogen plasma. Hydrogen flow rate was varied between 1.167 × 10−6 to 2.5 × 10−6 m3 s−1 at a fixed microwave power of 750 W which gave rise to increase in pressure from 1.866 × 103 to 2.933 × 103 Pa and increase in temperature of the plasma-cobalt oxide interface from 823 to 1,039 K. Reduction of cobalt oxide to cobalt occurred in steps: Co3O4 → CoO → Co. The chemical reaction leading to reduction of cobalt oxide to cobalt is found to be first order with respect to hydrogen flow rate. The activation energy for the first stage of reduction i.e. Co3O4 to CoO is found to be 26.2 kJ/mol and that for the second stage reduction of CoO to Co metal is 13.3 kJ/mol.

Direct reduction of iron in low temperature hydrogen plasma

This paper describes the effects of various parameters on the reduction of hematite in the presence of microwave assisted non-thermal hydrogen plasma. The parameters include microwave power, hydrogen flowrate, pressure, microwave power density and temperature. It has been shown that hydrogen flowrate, pressure and microwave power are interrelated to effect the microwave power density that controls the plasma temperature. The experimental conditions encounter three temperatures: surrounding the sample, associated with the plasma and at the plasma/substrate interface. It has been deduced that the third one is the most effective in determining the rate of the reaction, and in the present case, activation energy of 20 kJ mol−1 is reported.

Hydrogen plasma induced modification of photoluminescence from a-SiNx:H thin films

Low temperature (250–350 °C) hydrogen plasma annealing (HPA) treatments have been performed on amorphous hydrogenated silicon nitride (a-SiNx:H) thin films having a range of compositions and subsequent modification of photoluminescence (PL) is investigated. The PL spectral shape and peak positions for the as deposited films could be tuned with composition and excitation energies. HPA induced modification of PL of these films is found to depend on the N/Si ratio (x). Upon HPA, the PL spectra show an emergence of a red emission band for x ≤ 1, whereas an overall increase of intensity without change in the spectral shape is observed for x > 1. The emission observed in the Si rich films is attributed to nanoscale a-Si:H inclusions. The enhancement is maximum for off-stoichiometric films (x ∼ 1) and decreases as the compositions of a-Si (x = 0) and a-Si3N4 (x = 1.33) are approached, implying high density of non-radiative defects around x = 1. The diffusion of hydrogen in these films is also analyzed by Elastic Recoil Detection Analysis technique.

Sensitivity of encapsulated diamond-protein transistor renewed by low temperature hydrogen plasma

Sensitivity of encapsulated diamond-protein transistor renewed by low temperature hydrogen plasma

Carbon migration during methane injection experiments under ITER divertor-relevant conditions

Abstract Methane was injected into a high density low temperature hydrogen plasma beam ( n e ⩽ 10 21 m −3 , T e ∼ 1 eV) in Pilot-PSI to study carbon transport and provide a benchmark case for modeling. Gas injection was performed through molybdenum samples that acted as collectors of redeposits. The CH A–X band emission plumes are localized near the injection hole with a typical width of ∼3 mm and a decay length of ∼1.5 mm. The geometry of the emission profile is not influenced by the electron temperature. The redeposition efficiency and codeposit structure depend on the surface temperature. Surface temperatures below 500 K promote growth of a-C:H films across the surface and lead to redeposition of up to 38% of injected carbon. Above 800 K the redeposition efficiency decreases strongly and carbon microparticles are found near the injection hole.

Removal of carbon deposited film and hydrogen retention control by low temperature H–C–N reactive plasmas

Abstract Control of tritium retention and its removal from the first wall of future fusion devices are the most crucial issues for safety and effective use of the fuel. Nitrogen injection into edge plasmas has been considered and tested as an effective method for suppression of carbon dust growth and reduction of hydrogen isotope inventory. In this paper we have investigated scavenger effects of nitrogen injected into H2/CH4 plasmas using a small helical device where low density (ne ∼ 1016 m−3) and low temperature (Te = 5–10 eV) hydrogen plasmas are generated in steady state condition like remote plasmas in fusion devices. It is shown from the comparison of the carbon film deposition and particle growth between those with and without N2 injection that the chemical erosion effects of nitrogen gas on the carbon film and particle growth strongly depends on the surface temperature. With increasing the surface temperature higher than ∼400 K, the nitrogen chemical erosion significantly works to reduce the hydrocarbon deposition.

Preliminary investigation into direct reduction of iron in low temperature hydrogen plasma

There has been an increasing interest in reduction of iron ore by hydrogen. This study deals with the reduction of haematite in a microwave assisted non-thermal hydrogen plasma. The plasma is composed of excited hydrogen molecules, hydrogen atoms, and ionic hydrogen among other gaseous species. The reduction in hydrogen plasma occurred even at temperatures as low as 573 K. In contrast, the same could not be achieved by merely introducing hydrogen gas to the reducing environment without creating the plasma. It is only ∼1073 K that the extent of reduction by gaseous means is comparable to that of reduction by hydrogen plasma. Based on the experiments, as well as the data available from literature, it was deduced that the reduction of haematite at a low temperature in hydrogen plasma could have been due to the contribution of vibrationally excited hydrogen molecules to the reduction process.

Reversible band gap tuning of metal oxide films using hydrogen and oxygen plasmas

We report an approach to the reversible tuning of the band gaps of metal oxide (MO) films. ZnO and CuO, synthesized by hydrothermal methods, were treated with hydrogen and oxygen plasmas. From UV–visible transmittance spectra, we have found that the optical band gaps of MO films blue-shifted with hydrogen plasma treatment, but red-shifted with oxygen plasma treatment. By alternating the treatment sequences of hydrogen and oxygen plasmas, the MO optical band gap values can be reversibly fine-tuned with the tunable ranges of 80 and 550meV for ZnO and CuO, respectively. The mechanism for reversible tuning of optical band gaps is proposed based on the results of optical emission, X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy characterization. Compared to conventional metal ion doping and high temperature annealing methods, the use of room temperature hydrogen and oxygen plasmas for tuning band gaps is more environmentally friendly.