Xavier Duten

Investigation of chemical kinetics and energy transfer in a pulsed microwave H2/CH4 plasma

We present a modelling study of pulsed H2/CH4 microwave plasmas obtained under moderate pressure discharge conditions in a tubular quartz reactor. The transport in the reactor was described using a Nusselt model for a radially quasi-homogeneous plasma. The thermal behaviour of the plasma was modelled by distinguishing a single heavy species energy mode and the electron translation mode. The chemistry was described using a 30 species-130 reaction model. The time variations of the electron energy distribution function, the species concentrations and the gas temperature were determined by solving the coupled set of the electron Boltzmann equation, species kinetics equations and a total energy equation. Some of the results obtained from the present model were compared to measurements previously carried out on the plasmas considered. Good agreement was obtained for the time variations of the gas temperature, the relative concentration of the H-atom and the intensities of the Hα and the argon 750 nm emission lines. The effect of the duty cycle on the time-averaged composition and temperatures of the discharge was also studied. Results showed that moderate pressure H2/CH4 pulsed discharges obtained at duty cycles of less than 20% show different behaviour than those obtained at higher duty cycles. In particular, while the plasma reaches the permanent periodic regime in less than 2 pulse-periods, i.e. 60 ms, for duty cycle values of less than 20%, long-time-scale density variations of hydrocarbon species, ions and electrons are obtained when this parameter is greater than 20%. The model was also used to determine if the use of a pulsed regime may bring some improvements in plasma-assisted diamond deposition processes. For this purpose we analysed the variation with duty cycle of the time-averaged populations of the H-atom and CH3 that represent the key species for diamond deposition. Results showed that pulsed discharges with small duty cycle, of typically less than 20%, lead to a substantial enhancement of the time-averaged dissociation yield. On the other hand, the CH3 concentration exhibits a strong decrease with the duty cycle. The methyl concentration in the investigated pulsed discharge is generally smaller than in continuous wave discharges obtained in the same reactor. These results indicate that short-pulse discharges would favour the formation of films with higher Raman quality, while long duty cycle pulsed discharges would enable deposition at higher growth rates.

Atomic oxygen surface loss probability on silica in microwave plasmas studied by a pulsed induced fluorescence technique

The aim of this paper is to determine the atomic oxygen surface loss probability on silica under microwave plasma conditions around 133 Pa (1 Torr). A pulsed induced fluorescence technique where a main long pulse creates the plasma and a shorter one re-excites atoms in the time post-discharge was used. The method and its validity under the present experimental conditions are discussed at large. The oxygen surface loss probability on silica is found to be around 3% under plasma conditions, while it is estimated to be two orders of magnitude lower for a surface not submitted to the plasma.

On the hydrocarbon chemistry in a H2 surface wave discharge containing methane

Tunable infrared diode laser absorption spectroscopy has been used to detect the methyl radical and four stable molecules, CH4, C2H2, C2H4 and C2H6, in a H2 surface wave discharge (f = 2.45 GHz and power density ≈10-50 W cm-3) containing up to 10% methane under different flows (Φ = 22-385 sccm) and pressures (p = 0.1-4 Torr). The degree of dissociation of the methane precursor varied between 20% and 85% and the methyl radical concentration was found to be in the range of 1012 molecules cm-3. The methyl radical concentration and the concentrations of the stable C-2 hydrocarbons C2H2, C2H4, C2H6, produced in the plasma, increased with an increasing amount of added CH4 as well as with increasing pressure. For the first time, fragmentation rates of methane (RF(CH4) = 1×1015-2.5×1016 molecules J-1) and conversion rates to the measured C-2 hydrocarbons (RC(C2Hy): 5×1013-3×1015 molecules J-1) could be estimated with dependence on the flow and pressure in a surface wave discharge. The influence of diffusion and convection on the spatial distribution of the hydrocarbon concentration in the discharge tube was considered by a simple model.

Overview of the different aspects in modelling moderate pressure H2 and H2/CH4 microwave discharges

This paper deals with the modelling of moderate pressure H2 and H2/CH4 microwave plasmas used for diamond deposition. The first part of the paper reports on the development of a detailed collisional radiative model for moderate pressure pure H2 plasma. Several effects related to electron kinetics, H2 vibrational kinetics and H2 and H excited states kinetics and their effect on the overall dissociation and ionization kinetics are analysed with the model developed. This allows us to propose a reduced model which can be used for the self-consistent modelling of H2 discharges or as the modelling of the more complex H2/CH4 plasmas. The second part of the paper deals with the two-dimensional self consistent modelling of H2 plasmas obtained in a microwave cavity coupling system. The procedure used for coupling the plasma transport equations to the electromagnetic field equations is first discussed and some self-consistent simulation results with respect to the power deposition and plasma temperature and density distributions in the investigated plasma device are given. The last part of the work addresses the one-dimensional transport modelling of H2/CH4 plasmas used for diamond deposition. The spatial variations of neutral and charged hydrocarbon species densities on the reactor axis are presented and the main phenomena and processes that govern these variations are discussed.

Rotational temperature measurements of excited and ground states of C2(d 3Πg−a 3Πu) transition in a H2/CH4 915 MHz microwave pulsed plasma

The rotational temperature of a low energy (0.09 eV), C2 Swan band state (a 3Πu), obtained by white light absorption, is compared to the rotational temperatures of three electronic excited states [C2(d 3Πg),CH(A2Δ) and CN(B 2Σ+)] in a high power, H2/CH4 microwave plasma used for diamond deposition. All temperatures are measured at 50 mbar, and both continuous (as a function of microwave power) and pulsed (as a function of time after the pulse) modes of operation are investigated. The rotational temperature of C2’s excited state is found to be higher than that of the low energy state (assumed equal to the gas temperature), indicating that the excitation of C2 is to a large extent the result of chemical reactions (chemiluminescence) rather than electronic excitation. The rotational temperatures of CH(A 2Δ) and CN(B 2Σ+) excited states are also higher than that for C2’s low energy state temperature.

Acetaldehyde removal using an atmospheric non-thermal plasma combined with a packed bed: Role of the adsorption process

This work is an attempt in order to help towards understanding the influence of the adsorption process on the removal of a VOC (acetaldehyde, CH3CHO) using cyclic non thermal plasma (NTP) combined with a packed-bed of a catalyst support, α-Al2O3. In the first part, the results obtained by placing the saturated alumina pellets inside the plasma discharge zone are discussed, in terms of acetaldehyde removal, CO and CO2 production. In the second part, adsorption of CH3CHO, CO, CO2 and O3 was carried out, from single and multicomponent mixtures of the different compounds. The results showed that (i) the adsorption capacities followed the order CH3CHO≫  CO2>CO; (ii) O3 was decomposed on the alumina surface; (iii) CO oxidation occurred on the surface when O3 was present. In the third part, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to follow the alumina surface during acetaldehyde adsorption. DRIFTS measurements demonstrated that besides the bands of molecularly adsorbed acetaldehyde, several absorptions appeared on the spectra showing the intermediate surface transformation of acetaldehyde already at 300K. Finally, the relationship between the adsorption results and the NTP combined with a packed-bed process is discussed.

Time-resolved measurements of the gas temperature in a H2/CH4 medium pressure microwave 915 MHz pulsed plasma

Experimental measurements of the time variation of gas temperature in a non-equilibrium hydrogen-methane microwave plasma operating at medium pressure and under pulsed mode are presented. The hydrogen plasma is created using a 915 MHz microwave generator at an input microwave power up to 17 kW and the gas temperature is determined from the rotational distribution of the G 1Σg+ excited state of H2(T(G)). In continuous discharge, the rotational temperature T(G) of H2(G 1Σg+) is found to be nearly equal to the gas temperature determined from rotational distribution of the a3Πu metastable state of C2 molecule obtained from absorption spectroscopy. The time evolution of H2(G 1Σg+) rotational distribution in pulsed discharges reaches a Boltzmann equilibrium only 100 µs after the beginning of the pulse. Variations of the T(G) in-pulse value as a function of pressure, volumetric power density and gas velocity is studied.

Spatial and temporal evolutions of ozone in a nanosecond pulse corona discharge at atmospheric pressure

This paper deals with the experimental determination of the spatial and temporal evolutions of the ozone concentration in an atmospheric pressure pulsed plasma, working in the nanosecond regime. We observed that ozone was produced in the localized region of the streamer. The ozone transport requires a characteristic time well above the millisecond. The numerical modelling of the streamer expansion confirms that the hydrodynamic expansion of the filamentary discharge region during the streamer propagation does not lead to a significant transport of atomic oxygen and ozone. It appears therefore that only diffusional transport can take place, which requires a characteristic time of the order of 50 ms.

A Simplified Global Model to Describe the Oxidation of Acetylene Under Nanosecond Pulsed Discharges in a Complex Corona Reactor

The objective of this paper is to analyse the oxidation of acetylene under nanosecond pulsed N2/O2 discharges generated in a complex multi-pin-to-plane (MPP) corona reactor in the frame of Yan’s generic chemical kinetic model. We made use of the results obtained from the detailed kinetic model published previously (Redolfi et al. in Plasma Chem Plasma Process 29(3):173–195, 2009) in order to propose a global reactor models based on Yan’s generic chemical model and taking into account the non-homogeneous and non-stationary character of the discharges. This enables us expressing the energy cost in terms of physical and kinetic parameters of the discharge. We checked the model validity by comparing predicted and measured energy cost-values for acetylene in MPP reactor. The methodology presented may be adapted to predict the energy cost in other complex corona reactor provided the model parameters are determined experimentally.

Influence of applied voltage and electrical conductivity on underwater pin-to-pin pulsed discharge

A parametric study of an underwater pulsed plasma discharge in pin-to-pin electrode configuration has been performed. The influence of two parameters has been reported, the water conductivity (from 50 to 500 µS/cm) and the applied voltage (from 6 to 16 kV). Two complementary diagnostics, time resolved refractive index-based techniques and electrical measurements have been performed in order to study the discharge propagation and breakdown phenomena in water according to the two parameters. A single high voltage of duration between 100 µS and 1 ms is applied between two 100 µm diameter platinum tips separated by 2 mm and immersed in the aqueous solution. This work, which provides valuable complementary results of paper [1], is of great interest to better understand the mechanisms of initiation and propagation of pin-to-pin discharge in water. For low conductivity (from 50 to 100 µS/cm) results have confirmed two regimes of discharge (cathode and anode) and the increase of the applied voltage first makes the breakdown more achievable and then favors the apparition of the anode regime. For 500 µS/cm results have highlighted cathode regime for low applied voltage but a mixed regime (anode and cathode) for high applied voltage.