Hydrogen Dissociation Degree Research Articles

Hydrogen dissociation degree in the Grimm-type glow discharge measured by optical emission spectroscopy

Hydrogen dissociation degree in the Grimm-type glow discharge measured by optical emission spectroscopy

Effect of pressure on the hydrogen dissociation degree in a hot tube

The effect of pressure decrease on the degree of dissociation of hydrogen flowing inside a hot tube was studied. The experimental data were analysed using the previously developed thermal model of the reactor. The heat exchange of gas with the tube wall was estimated based on the direct Monte Carlo simulation. It was shown that decreasing the pressure from 20 mm Hg down to 1 mm Hg leads to a slight increase in the degree of hydrogen dissociation, from 11% up to 13%.

H2 dissociation in Ar–H2 arc discharge of moderate pressure

The line intensity ratio technique for estimation of hydrogen-to-argon density ratio and hydrogen molecule dissociation degree in arc discharge in Ar–H2 mixtures at moderate pressures (2–3 Torr) is analyzed. Intensity ratios of three hydrogen lines of Balmer series (Hα (656.3 nm), Hβ (486.3 nm) and Hγ (434.0 nm)) to ArI 750.4 nm (2p1 → 1 s2) and ArI 811.5 nm (2p9 → 1s5) lines were examined. Effect of Ar fraction in Ar–H2 mixtures on the diagnostic technique and molecular hydrogen dissociation degree in the large-volume high-density moderate-pressure Ar–H2 arc was studied. Using ArI 811 nm line for the intensity ratio diagnostics in the arc discharge is not valid due to great role of the excitation from 1s5 in production of Ar 2p9 level. It is shown that to obtain the correct H density value it is necessary to account quenching of H* excited states by Ar atoms and H2 molecules. Pair of Hα and ArI 750 nm lines is the best choice for determining atomic hydrogen density using HI-to-ArI line intensity ratio technique because of the upper excited state of Hα (n = 3) is less subjected to quenching by Ar atoms and H2 molecules as compared with higher H* levels (n = 4, 5). It is shown increasing Ar fraction in Ar–H2 mixtures results in growth of the molecular hydrogen dissociation degree.

Hydrogen Dissociation in Rarefied Gas Flow Through a Wire Obstacle

The direct simulation Monte Carlo method was used to study plane–parallel flow of hydrogen through an obstacle formed by a series of parallel infinite wires. Particular attention was paid to the influence of the geometric parameters of the wire obstacle, the degree of rarefaction, and the flow velocity on the degree of hydrogen dissociation due to heterogeneous reactions on the wire surface.

Analysis of flows by deposition of diamond-like structures

The Direct Simulation Monte Carlo (DSMC) method is used to simulate the hydrogen–methane mixtures flowing through a heated cylindrical tungsten tube and expanding into a low-pressure chamber in the substrate holder direction. The DSMC method takes into account heterogeneous reactions in the tube and on the substrate surface. The results of DSMC simulation are used for the chemical kinetics calculations, i.e., axial distributions of species concentrations in various H/C mixtures. The effects of various parameters (reactor configuration, flow rate, initial concentration of methane in the mixture with hydrogen, and pressure in the chamber) on species fluxes to the substrate, the degree of hydrogen dissociation, the degree of methane decomposition, and further conversion of CxHy components up to atomic carbon C are numerically studied. The developed method provides a possibility of solving similar problems for nonequilibrium flows.

Heterogeneous physical and chemical processes in a rarefied-gas flow in channels

A flow with physical and chemical reactions on hot surfaces is investigated. On the basis of physical experiments, determining the hydrogen-dissociation degree in rarefied gas and calculation of the flow by the method of direct simulation Monte Carlo (DSMC), it is possible to specify certain unknown constants of interaction of molecules and atoms with a tungsten surface. By the example of the hydrogen flow in a hightemperature tungsten cylindrical channel, the role of dissociation, sorption, and recombination processes is shown in a wide range of flow regimes from free-molecular to continuum.

Studying a low-pressure microwave coaxial discharge in hydrogen using a mixed 2D/3D fluid model

This work presents a numerical model of hydrogen plasma in a microwave coaxial discharge at low pressure (25–250 Pa). The model is a mixed two-dimensional (2D)/three-dimensional (3D) model in that it combines three-dimensional geometry for the electromagnetic field and two-dimensional geometry for the transport equations. The model is validated against experimental results available in the literature and, where possible, simulations of comparable discharges. The model shows reasonable agreement in the relevant pressure range. A parametric study with respect to pressure is carried out and it is observed that the plasma contracts towards the quartz tube with increasing pressure. Increasing the pressure also influences the abundance of H+ ions but on the other hand it has little impact on hydrogen dissociation degree and electron temperature. Furthermore, the uniformity of the plasma above the substrate holder is analyzed. It is observed that at pressures over 150 Pa, the plasma gets non-uniform in the direction parallel to the plasma lines. Finally, the uniformity of particle and energy fluxes to the substrate holder are analyzed. Knowing the fluxes is especially useful for the material applications of the device.

Effect of argon addition on plasma parameters and dust charging in hydrogen plasma

Experimental results on effect of adding argon gas to hydrogen plasma in a multi-cusp dusty plasma device are reported. Addition of argon modifies plasma density, electron temperature, degree of hydrogen dissociation, dust current as well as dust charge. From the dust charging profile, it is observed that the dust current and dust charge decrease significantly up to 40% addition of argon flow rate in hydrogen plasma. But beyond 40% of argon flow rate, the changes in dust current and dust charge are insignificant. Results show that the addition of argon to hydrogen plasma in a dusty plasma device can be used as a tool to control the dust charging in a low pressure dusty plasma.

Hydrogen Degree of Dissociation in a Low Pressure Tandem Plasma Source

ABSTRACT Hydrogen dissociation degree in an inductively-driven tandem plasma source, operating at low pressure, is measured by applying optical actinometry method with an argon gas as actinometer. The gas temperature, a parameter in the investigations, is obtained from the rotational temperature, analyzing the intensity distribution of the Fulcher-α band. Several actinometric pairs are used for examining the dissociation degree and its pressure dependence. The influence−on the results−of the difference in the excitation cross sections for argon, commonly accepted, is analyzed. Two suitable actinometric pairs are proposed, one of which can also be applied for fast monitoring of the dissociation degree.

The role of inert gas in MW-enhanced plasmas for the deposition of nanocrystalline diamond thin films

Nanocrystalline diamond thin films have been deposited using microwave plasma enhanced deposition with gas mixtures of composition H 2/CH 4/X, where X was one of the inert gases He, Ne, Ar and Kr and typically constituted > 90% of the total gas flow. The diamond films obtained with each gas mixture deposited at approximately the same rate (0.15–0.5 µm h − 1 ), and all showed similar morphologies and average grain sizes, despite very obvious differences in the appearance and gas temperatures of the respective plasmas. These plasmas were probed by optical emission and cavity ring-down spectroscopy, and results from companion 2D chemical kinetic modelling of the Ar/H 2/CH 4 and He/H 2/CH 4 plasma were used to guide interpretation of the experimental observations. We conclude that the inert gas, though acting primarily as a buffer, also has significant effects on the thermal conduction of the gas mixtures, the electron temperature and electron energy distribution, and thereby changes the main channels of ionization and input power absorption. As a result, inert gas dilution elevates the electron and gas temperatures, enhances the hydrogen dissociation degree and affects the H/C mixture composition and deposition mechanisms.

Computer simulations of argon–hydrogen Grimm-type glow discharges

Computer simulations have been performed to describe the effect of small admixtures of hydrogen to an argon glow discharge in the Grimm-type configuration. The two-dimensional density profiles of the various plasma species (i.e., electrons, Ar+, ArH+, H+, H2+ and H3+ ions, H atoms and H2 molecules, Ar metastable atoms and sputtered Cu atoms) are presented for 1% H2 added to the argon glow discharge, and the effect of different H2 additions (varying between 0.1 and 10%) on the species densities, the hydrogen dissociation degree, and the sputtering process, are investigated. Finally, the relative contributions of various production and loss processes for the different plasma species are calculated.

Electron beam induced fluorescence measurements of the degree of hydrogen dissociation in hydrogen plasmas

The degree of dissociation of hydrogen in a hydrogen plasma has been measured using electron beam induced fluorescence. A 20 kV, 1 mA electron beam excites both the ground state H atom and H2 molecule into atomic hydrogen in an excited state. From the resulting fluorescence the degree of dissociation of hydrogen is determined. In addition, the absolute atomic hydrogen density can be determined if the gas temperature and pressure are known without any additional calibration. To check the consistency of the method the fluorescence from the first four Balmer transitions is measured. It is demonstrated that this technique can be applied in hydrogen and argon–hydrogen plasmas with a pressure of up to 1 mbar and 0.2 mbar, respectively.

Global Model of Plasma Chemistry in a High Density Argon/Hydrogen Discharge

A mathematical Model is developed to evaluate the influence of operating parameters on hydrogen dissociation and the nature of the major species present in Ar/H2 plasmas. The Boltzmann equation is solved to obtain the electron energy distributions. Mass and energy conservation equations are then solved simultaneously to calculate the species density and electron temperature as a function of pressure, power, and feed composition. Good agreement is found between the model results and experimental measurements. The primary effect of argon addition to hydrogen plasma is to increase the electron density, which in turn enhances the degree of hydrogen dissociation. It is also found that Ar/H2 plasma retains the basic properties of hydrogen plasma even for feed gases containing more than 70% argon. That is, values of electron temperature and H-atom density are similar, and the dominant ionic species is H3+ ion.

Diagnostics of the magnetized low-pressure hydrogen plasma jet: Molecular regime

Optical emission and absorption spectroscopy and double Langmuir probe diagnostics have been applied to measure the plasma parameters of an expanding magnetized hydrogen plasma jet. The rotational temperature of the excited state H2(d2Πu) has been determined by analyzing the intensity distribution of the spectral lines of the Fulcher-α system of H2. The gas temperature in the plasma, which is twice the value of the rotational temperature is equal to ≂ 520 K. Several clear indications of presence of the ‘‘hot’’ electrons have been observed in the plasma: (1) Langmuir probe measurements (Te≂1.4 eV), (2) appearance of the Fulcher-α system of H2 (excitation potential ΔE=13.87 eV), (3) low rotational temperature (T*rot≂260 K) of the excited H2(d3Πu) molecules, (4) local excitation in the plasma of Ar I(ΔE=15.45 eV), and Ar II(ΔE=19.68 eV) spectral lines, (5) local excitation in the plasma of He I(ΔE=23.07 eV and ΔE=24.04 eV) spectral lines. Optical actinometry has been applied to measure the absolute density of hydrogen atoms and hydrogen dissociation degree in the plasma. The measured absolute density of hydrogen atoms are in the (1–1.4)×1020 m−3 range, and the corresponding dissociation degree of the hydrogen plasma is in the range of 8%–13%.

Spectroscopic measurement of atomic hydrogen level populations and hydrogen dissociation degree in expanding cascaded arc plasmas

Optical absorption spectroscopy has been applied to measure the absolute population densities of the first excited levels of atomic hydrogen H*(n=2) and argon Ar*(4s) in an expanding cascaded arc plasma in hydrogen-argon mixture. It is demonstrated that the method allows us to determine both H*(n=2) and Ar*(4s) absolute density radial profiles for H2 admixtures in Ar ranging from 0.7% to 10% with good accuracy. The measured H*(n=2) densities are in the 1014–1016 m−3 range, and Ar*(4s) densities are in the range of 1015–1018 m−3. It has been shown, that the density of hydrogen excited atoms H*(n=2) serves as an indicator of the presence of argon ions and hydrogen molecules in the expanding plasma. A kinetic model is used to understand evolution of H*(n=2) density in the expansion, and to estimate the total atomic hydrogen population density and hydrogen dissociation degree in sub- and supersonic regions of the plasma.