Thermodynamic and Transport Properties of Two-temperature Oxygen Plasmas

Thermodynamic and transport properties of two-temperature oxygen plasmas are presented. Variation of species densities, mass densities, specific heat, enthalpy, viscosity, thermal conductivity, collision frequency and electrical conductivity as a function of temperature, pressure and different degree of temperature non-equilibrium are computed. Reactional, electronic and heavy particle components of the total thermal conductivity are discussed. To meet practical needs of fluid-dynamic simulations, temperatures included in the computation range from 300 K to 45,000 K, the ratio of electron temperature (T _e) to the heavy particle temperature (T _h) ranges from 1 to 30 and the pressure ranges from 0.1 to 7 atmospheres. Results obtained for thermodynamic equilibrium (T _e = T _h) under atmospheric pressure are compared with published results obtained for similar conditions. Observed overall agreement is reasonable. Slight deviations in some properties may be attributed to the values used for collision integral data and for the two temperature formulations used. An approach for computing properties under chemical non-equilibrium and associated deviations from two-temperature results under similar conditions are discussed.     

Determination of the Dominant Species and Reactions in Non-equilibrium CO<Subscript>2</Subscript> Thermal Plasmas with a Two-Temperature Chemical Kinetic Model

It has become increasingly clear that deviations from local thermodynamic equilibrium occur in thermal plasmas. This paper is devoted to investigating the non-equilibrium characteristics of CO₂ thermal plasmas, which have wide application in industry. A two-temperature chemical kinetic model with a comprehensive chemical system is developed to calculate the non-equilibrium characteristics of CO₂ thermal plasmas for a wide temperature range, from 12,000 to 500 K, at atmospheric pressure. The non-equilibrium results are compared to the equilibrium composition obtained by Gibbs free energy minimization, and significant deviations are found at lower temperatures. Based on the dependence of molar fractions on temperature, the dominant species are determined in three temperature ranges. The dominant reactions are then obtained by considering their contribution to the generation and loss of the dominant species. Using the dominant species and reactions, the full model is simplified into three simpler models and the accuracy of the simplified models is evaluated. It is shown that this approach greatly reduces the number of species and reactions considered, while showing good agreement with the full model, with a root-mean-square error of no more than 4 %. Thus, the complicated physicochemical processes in non-equilibrium CO₂ thermal plasmas can be characterized by relatively few species and reactions. It is suggested that the two-temperature chemical kinetic model developed in this paper can be applied to the full range of pressures that occur in arc welding, arc quenching and other industrial applications. In addition, the simplified methods can be applied in multi-dimensional models to reduce the chemical complexity and computing time while capturing the main physicochemical processes in non-equilibrium CO₂ thermal plasmas.     

Heat transfer from a rarefied plasma flow to a metallic or nonmetallic particle

Heat transfer from a plasma flow to a metallic or nonmetallic spherical particle is studied in this paper for the extreme case of free-molecule flow regime. Analytical expressions are derived for the heat flux due to, respectively, atoms, ions, and electrons and for the floating potential on the sphere exposed to a two-temperature plasma flow. It has been shown that the local or average heat flux density over the whole sphere is independent of the sphere radius and approximately in direct proportion to the gas pressure. The presence of a macroscopic relative velocity between the plasma and the sphere causes substantially nonuniform distributions of the local heat flux and enhances the total heat flux to the sphere. The heat flux is also enhanced by the gas ionization. Appreciable difference between metallic and nonmetallic spheres is found in the distributions along the oncoming flow direction of the floating potential and of the local heat flux densities due to ions and electrons. The total heat flux to the whole sphere is, however, almost the same for these different spheres. For a fixed value of the electron temperature, the heat flux decreases with increasing temperature ratio T_e/T_h.     

Two-Temperature Two-Dimensional Non Chemical Equilibrium Modeling of Ar–CO<Subscript>2</Subscript>–H<Subscript>2</Subscript> Induction Thermal Plasmas at Atmospheric Pressure

Here the authors developed a two-dimensional two-temperature chemical non-equilibrium (2T-NCE) model of Ar–CO₂–H₂ inductively coupled thermal plasmas (ICTP) around atmospheric pressure (760 torr). Assuming 22 different particles in this model and by solving mass conservation equations for each particle, considering diffusion, convection and net production terms resulting from 198 chemical reactions, chemical non-equilibrium effects were taken into account. Species density of each particle or simply particle composition was also derived from the mass conservation equation of each one taking the non chemical equilibrium effect into account. Transport and thermodynamic properties of Ar–CO₂–H₂ thermal plasmas were self-consistently calculated using the first order approximation of the Chapman–Enskog method at each iteration point implementing the local particle composition and temperature. Calculations at reduced pressure (500 and 300 torr) were also done to investigate the effect of pressure on non-equilibrium condition. Results obtained by the present model were compared with results from one temperature chemical equilibrium (1T-CE) model, one-temperature chemically non equilibrium (1T-NCE) model and finally with 2T-NCE model of Ar–N₂–H₂ plasmas. Investigation shows that consideration of non-chemical equilibrium causes the plasma volume radially wider than CE model due to particle diffusion. At low pressure with same input power, presence of diffusion is relatively stronger than at high pressure. Comparison of present reactive model with non-reactive Ar–N₂–H₂ plasmas shows that maximum temperature reaches higher in reactive C–H–O molecular system than non-reactive plasmas due to extra contribution of reaction heat.     

The use of different plasma gases (Ar, N 2 and air) for CMP-OES: figures of merit and temperature measurements

A comparative study of a 600 W capacitively coupled microwave plasma (CMP) operated with different plasma gases (Ar, N₂ and air) with respect to the achieved detection limits for Fe, Cr, Zn, Ca and Mg have been carried out. Radially and axially resolved rotational temperatures (T_rot), excitation temperatures (T_exc) and electron number densities (n_e) of these plasmas have been determined using OH (T_rot), Fe (T_exc) and Mg (n_e) as thermometric species. The influence of different gas flow rates on T_rot, T_exc and n_e, and of Li as an easily ionized element on T_exc has been investigated.     

Comparison of the Plasma Temperature and Electron Number Density of the Pulsed Electrolyte Cathode Atmospheric Pressure Discharge and the Direct Current Solution Cathode Glow Discharge

A novel-pulsed electrolyte cathode atmospheric pressure discharge (pulsed-ECAD) plasma source driven by an alternating current (AC) power supply coupled with a high-voltage diode was generated under normal atmospheric pressure between a metal electrode and a small-sized flowing liquid cathode. The spatial distributions of the excitation, vibrational, and rotational plasma temperatures of the pulsed-ECAD were investigated. The electron excitation temperature of H T_exc(H), vibrational temperature of N₂ T_vib(N₂), and rotational temperature of OH T_rot(OH) were from 4900?±?36 to 6800?±?108 K, from 4600?±?86 to 5800?±?100 K, and from 1050?±?20 to 1140?±?10 K, respectively. The temperature characteristics of the dc solution cathode glow discharge (dc-SCGD) were also studied for the comparison with the pulsed-ECAD. The effects of operating parameters, including the discharge voltage and discharge frequency, on the plasma temperatures were investigated. The electron number densities determined in the discharge system and dc-SCGD were 3.8–18.9?×?10¹⁴?cm^–3 and 2.6?×?10¹⁴ to 17.2?×?10¹⁴?cm^–3, respectively.     

Progress in the non-equilibrium vibrational kinetics of hydrogen in magnetic multicusp H− ion sources

The non-equilibrium vibrational kinetics of H₂ in multicusp magnetic discharges has been studied by improving a previous model developed by our groups. In particular, a complete set of V-T (vibrational translation) rates involving H-H₂(v) collisions, calculated by using a three-dimensional dynamics approach, has been inserted into our self-consistent model for better representing the corresponding relaxation. Different experimental situations are simulated with special emphasis on the temporal scales necessary for the different distributions (electron energy and vibrational distributions) to reach stationary values. Finally, a comparison between theoretical and experimental quantities such as vibrational temperature, electron temperature, electron number density and concentration of negative ions (H⁻) shows a satisfactory agreement, thus indicating the basic correctness of our model.     

Nonequilibrium effects in thermal plasma chemistry

Numerical calculations have been performed to assess the potential significance of nonequilibrium effects on chemical reactivity in thermal plasmas The calculations consider situations in which the electron temperature and/or the electron density are elevated above their equilibrium values corresponding to the local gas temperature. Such nonequilibrium may occur in the plasma torch itself or could be purposefully imposed by a controlled hybrid discharge in a downstream reactor region so as to augment reactivity over a longer residence time. The calculations account for finite ionization/recombination rates of atomic and molecular species, electron-impact dissociation, dissociative recombination, dissociative attachment, and predissociation effects, as well as thermal reactions between neutral chemical species. As an example of the possible nonequilibrium enhancement of molecular decomposition, initial consideration has focused on the dissociation rates of diatomic species where heavy particle reaction rates and cross sections can be reasonably estimated. The results show that for O₂ or H₂ in argon at moderate temperatures, electron-temperature elevation can give rise to a notable enhancement of the dissociation rate, in comparison with the equilibrium case. Depending on the situation, it is found that either relatively energetic electron-impact dissociation or dissociative attachment (for O₂) can dominate the enhanced dissociation rate—which can be more than a factor of 2 greater than in the absence of a discharge. Similar effects would be expected for the decomposition of more complicated molecules.     

Numerical Modeling of an Ar–H<Subscript>2</Subscript> Radio-Frequency Plasma Reactor under Thermal and Chemical Nonequilibrium Conditions

The species densities and the thermal and chemical nonequilibrium phenomena in an Ar–H₂ radio frequency inductively coupled plasma reactor used for hydrogenation of materials have been investigated through numerical simulation. The mathematical model consists of a two-temperature fluid dynamics model and a chemical kinetics model that takes into account the effect of local chemical nonequilibrium. Computations are carried out for the rf plasma running at 11.7 kW and 27 kPa for different Ar–H₂ mixtures and for pure argon. Predicted results for the electron and heavy-species temperatures, the species densities, as well as the degree of thermal and chemical nonequilibrium, are presented in detail. It is found that the electron and hydrogen atom densities in the reactor and in the near-wall region of the torch are strongly altered by nonequilibrium effects. The hydrogen atom density remains high in the reactor zone, and peaks in a region that has been found to be attractive for material processing. Deviations from thermal and chemical equilibrium are greatly reduced by the addition of hydrogen to an argon plasma.     

On the electron temperatures and densities in plasmas produced by the “torche à injection axiale”

The electron temperature and the electron density of plasmas created by the “Torche à Injection Axiale” (TIA) are determined using Thomson scattering. In the plasma with helium as the main gas, temperatures of around 25 000 K and densities of between 0.64 and 5.1 × 10²⁰m⁻³ are found. In an argon plasma the electron temperature is lower and the electron density is higher: 17 000 K and around 10²¹ m⁻³ respectively. From these results it can be established that the ionisation rates of both plasmas are much larger than the recombination rates, which means that the plasmas are far from Saha equilibrium. However, deviations from a Maxwell electron energy distribution function, as reported for the “Microwave Plasma Torch” (MPT), are not found in the TIA. The excitation and ionisation power of the TIA appears to be stronger than that of the MPT.     

Using the van der Waals broadening of spectral atomic lines to measure the gas temperature of an argon–helium microwave plasma at atmospheric pressure

The applications of plasmas generated with gas mixtures have become increasingly common in different scientific and technological fields. In order to understand the advantages of these discharges, for instance in chemical analysis, it is necessary to know the gas temperature (T_g, kinetic energy of the heavy particles) since it has a great influence on the atomization reactions of the molecules located in the discharge, along with the dependence of the reaction rate on this parameter. The ro-vibrational emission spectra of the molecular species are usually used to measure the gas temperature of a discharge at atmospheric pressure although under some experimental conditions, these are difficult to detect. In such cases, the gas temperature can be determined from the van der Waals broadening of the emitted atomic spectral lines related to this parameter. The method proposed is based on the van der Waals broadening taking into account two perturbers.     

Nonequilibrium vibrational populations and dissociation rates of oxygen in electrical discharges

Nonequilibrium vibrational distributions and dissociation rates of molecular oxygen in both electrical and thermal conditions have been calculated by solving a system of master equations including V-V (vibration-vibration), V-T (vibration-translation) and e-V (electron-vibration) energy exchanges. The dissociation constant under thermal conditions (i.e. without electrons) follows an Arrhenius law with an activation energy of 120 kcal/mole, while the corresponding rates under electrical conditions (5000 ? T_e ? 15000 K, 300 ? T_g ? 1000 K, 10¹¹ ? n_e ? 10¹² cm^?3,5 ? p ? 20 torr) increase with decreasing gas (T_g) and electron (T_e) temperatures and pressure (p) and with increasing electron density (n_e). These results are explained on the basis of the different interplay of V-V and V-T energy exchanges and are rationalized by means of simplified models proposed in the literature. The accuracy of the present results is discussed paying particular attention to the dependence of V-V and V-T rate coefficients on the vibrational quantum number. A comparison of the calculated dissociation rates with the corresponding ones obtained by the direct electron impact mechanism shows that the present mechanism prevails at low electron and gas temperatures. Finally a comparison is shown between theoretical and experimental dissociation rates under electrical and thermal conditions.     

Electron production and loss processes in a spectrochemical inductively coupled argon plasma

The behavior of inductively coupled plasmas for spectroscopic purposes has been studied extensively in the past. However, many questions about production and loss of electrons, which have a major effect on this behavior, are unanswered. Power interruption is a powerful diagnostic method to study such processes. This paper presents time resolved Thomson scattering measurements of the electron density n_e and temperature T_e in an inductively coupled argon plasma during and after power interruption. In the center of the plasma the measured temporal development of n_e and T_e can be attributed to ambipolar diffusion, three-particle recombination and ionization. However, at the edge of the plasma an additional electron loss process must be involved. In addition, the high electron temperature during power interruption indicates the presence of an electron heating mechanism. The energy gain by recombination processes is shown to be insufficient to explain this electron heating. These discrepancies may be explained by the formation and destruction of molecular argon ions, which can be present in significant quantities.     

Fundamental properties characterizing low-pressure microwave-induced plasmas as excitation sources for spectroanalytical chemistry

Experimental measurements of the spectroscopic temperature and the electron temperature in low-pressure rare gas plasmas sustained by a microwave generator operating at 2450 MHz have revealed divergent values. These measurements have been interpreted on the basis of a radiative recombination model originally proposed by Schlüter. The importance of Penning ionization by metastable rare gas atoms in the excitation of foreign atoms has been discussed in terms of this model.On the basis of the radiative recombination model for these plasmas, the parameters of analytical importance are the concentration and energy of electrons in a high energy electron group, the concentration and energy of electrons in a low energy electron group, and the concentration of metastable rare gas atoms. Measurements of the spectroscopic temperature of an argon plasma have revealed that the energy of electrons in the low energy electron group is not greatly affected by applied microwave power and pressure over the range from 1–25 torr. The energy of electrons in the high energy electron group is not greatly affected by pressure and applied microwave power over the range studied, but has been shown to depend on the ionization potential of the plasma gas. The total electron concentration is not greatly affected by gas pressure for low applied powers, but varies with applied power, particularly at low pressures. The concentration of metastable argon atoms has been shown to depend on both the applied power and pressure. Studies of the excitation of mercury by these plasmas have led to results which are consistent with the radiative recombination model.     

Modeling of an inductively coupled plasma at reduced pressures

Plasma sintering experiments in this laboratory at reduced pressures revealed efficient heating of the ceramic sample due to recombination of dissociated and/or ionized species on the surface. For establishing a model for this plasma sintering process, it is necessary to first consider the plasma itself. Therefore, a suitable model for an RF inductively coupled plasma has been developed considering reduced pressures. As the pressure decreases, the electron density also decreases at a fixed electron temperature, causing substantial deviations from chemical equilibrium. Due to the poor collisional coupling between electrons and heavy particles at reduced pressures, large deviations from kinetic equilibrium have also to be expected. The model is based on a rotationally symmetric plasma contained in a quartz tube. The power level ranges from 1.5 to 3 kW and the operating pressure is varied from 1 to 0.01 atm. Both deviations from chemical and kinetic equilibrium are included in this model. Thermodynamic and transport properties for two-temperature plasmas are used for this modeling work. The results indicate that for pressures below 0.1 atm, there is a strong ambipolar flux of charge carriers to the confining walls, leading to significant variations of the temperature across the tube. The electron temperature increases rapidly as the pressure decreases, whereas the heavy-particle temperature decreases.     

Studies on Gas Purification by a Pulsed Microwave Discharge at 2.46 GHz in Mixtures of N2/NO/O2 at Atmospheric Pressure

Pulsed microwave discharges operated at atmospheric pressure in gas mixtures containing N₂, O₂, and NO are investigated experimentally and theoretically for various gas mixture constituents and operating conditions with respect to the ability of exhaust gas purification. The rotational gas temperature and the vibrational temperature of N₂ are derived from CARS measurements. The composition of the exhaust gas after treatment is monitored using FTIR spectroscopy. The processes of the chemical, electronic, and vibrational kinetics are described by a model that has been developed to calculate the species densities. The results obtained show that in N₂/NO gas mixtures an overall reduction of NO_x takes place. In the case of N₂/O₂/NO gas mixtures, no net reduction of NO_x is achieved for a pulsed microwave power below 3600 W, a pulse length of 50 s, and a typical repetition frequency of 2 kHz.     

An approximate quantitative analysis of non-equilibrium plasma transport for high density plasmas

For the evaluation of transport in high density plasmas numerical models have been developed in which simultaneously the conservation laws for mass, momentum and energy are solved. For high density plasmas, which are not too far from equilibrium the commonly used thermodynamic quantities are, electron temperature T_e, electron density n_e, heavy particle temperature and neutral density (or pressure). In this contribution an alternative formulation is described in which the plasma state is described by electron density ne and total pressure p and two non-equilibrium parameters: the deviation from Saha equilibrium of the neutral ground state (δb₁ = n₁/n₁ ^saha−1) and the deviation from thermal equilibrium between electrons and heavy particles δΘ = 1−T_h/T_e. The latter two parameters are zero in local thermodynamic equilibrium.

The advantage of this formulation is, that the transport coefficients and radiative properties can be reformulated as function of mainly n_e (at constant pressure), as the influences of non zero δb₁ and δΘ are small or can be explicitly given. As a result a simpler approximate formulation of the transport problem can be obtained. As an example the procedure is illustrated for atmospheric argon plasmas and for one aspect a comparison is made with work from e.g. E. Pfender.

     

Some considerations on the microwave electrodeless discharge: Plasma diagnostics in argon,helium or nitrogen atmospheres

Plasma diagnostics of several microwave plasmas are determined by making electrical (with double floating probes) and optical measurements in pure Ar, He or N₂ plasmas, and also in Ar plasmas containing various metals, i.e. Cs, Tl or Zn; plasma parameters, such as, electric field (E), electron (j_e) and ion (j_i) current densities, electron density (n_e), electron temperature (T_e) electron conductivity (σ_e), ion density (n_i), electron mean free path (λ_e) electron (μ_e) and ion (μ_i) mobilities and electron [(v_e)_drift] and ion [(v_i)_drift] volocities are either directly measured or calculated. The reversal temperature (T_r) of excited (0.96 eV lower level) thallium atoms is measured, and the steady-state conditions of the plasma are analyzed by the energy balance equation. The experimental measurements indicate that the electric field strength E decreases as the space charge decreases (ionization extent) increases. Although the plasma appears to be under steady-state conditions, it is not under local thermodynamic equilibrium conditions, i.e. T_e >T_r. In addition, the measurements indicate that there is a deficiency of electrons in the plasma (n_e < n_i), probably due to electron affinity processes; and the plasma has a small positive space charge.     

Measurement of Spatial Distributions of Electron Density and Electron Temperature in Direct Current Glow Discharge by Double Langmuir Probes

The spatial distributions of electron temperature and density in a dc glow discharge that is created by a pair of planar electrodes were obtained by using double Langmuir probes. The contribution of double Langmuir probes measurement is to provide a relatively quantitative tool to identify the electron distribution behavior. Electrons gain energy from the imposed electric field, and electron temperature (T_e) rises very sharply from the cathode to the leading edge of the negative glow where T_e reaches the maximum. In this region, the number of electrons (N_e) is relatively small and does not increase much. The accelerated electrons lose energy by ionizing gas atoms, and T_e decreases rapidly from the trailing edge of the negative glow and extends to the anode. N_e was observed to increase from the cathode to the anode, which is due to the electron impact ionization and electron movement. The electron density was observed to increase with increasing discharge voltage while the electron temperature remained approximately. At 800 V and 50 mTorr argon glow discharge, Te ranged from 15 to 52 eV and Ne ranged from 6.3×10⁶/cm³ to 3.1×10⁸/cm³ in the DC glow discharge, and T_e and N_e were dependent on the axial position.     

Plasma chemistry in laser ablation processes

Based on the results of quantitative spectroscopic diagnostics (LIF in combination with time resolved emission spectroscopy) chemical dynamics in laser-produced plasmas of metallic (Ti, Al,), and graphite samples have been examined. The Nd-YAG (1064 nm, 10 ns, 100 mJ) and excimer XeCl (308 nm, 10 ns, 10 mJ) lasers were employed for ablation. The main attention was focused on the elucidation of a role of oxide and dimer formation in controlling spatio-temporal distributions of different species in the ablation plume. The results of the spatial and temporal analysis of a laser-produced plasma in air indicates the existence of diatomic oxides in the ablation plume both in the ground and excited states, which are formed from reactions between ablated metal atoms and oxygen. The efficiency of the oxidation reaction depends on the intensity and spot diameter of the ablation laser beam. The maximal concentration of TiO molecules are estimated to be of 1×10¹⁴ cm⁻³ at the time of 10 μs after the start of the ablation pulse. A comparison of spatial–temporal distributions of Ti atoms and excited TiO molecules allow us to find a correlation in their change, which proves that electronically excited Ti oxides are most probably formed from oxidation of atoms in the ground and low lying metastable states. The spectroscopic characterization of pulsed laser ablation carbon plasma has also been performed. The time–space distributions as well as the high vibrational temperature of C₂ molecules indicate that the dominant mechanism for production of C₂ is the atomic carbon recombination.