Ramasamy Pothiraja

An ac discharge is ignited with the frequency of 170 kHz at the tip of a sharpened electrode in He–N2 gas mixture under atmospheric pressure. Plasma parameters (electron density and reduced electric field) are determined using phase-resolved optical emission spectroscopy. An absolutely calibrated ICCD camera with an appropriate filter is used for the time- and space-resolved measurement of N2(C–B,0–2) as well as (B–X,0–0) emissions. From the temporal and spatial distributions of these emission bands, time- and space-resolved plasma parameters are determined. Limits of time and space resolutions of this diagnostic method are discussed.

An argon/nitrogen (0.999/0.001) filamentary pulsed discharge operated at atmospheric pressure in a quartz tube is characterized using voltage–current measurements, microphotography, optical emission spectroscopy (OES) and absorption spectroscopy. Nitrogen is applied as a sensor gas for the purpose of OES diagnostic. The density of argon metastable atoms Ar(3P2) is determined using tunable diode laser absorption spectroscopy (TDLAS). Using a plasma chemical model the measured OES data are applied for the characterization of the plasma conditions. Between intense positive pulses the discharge current oscillates with a damped amplitude. It is established that an electric current flows in this discharge not only through a thin plasma filament that is observed in the discharge image but also through the whole cross section of the quartz tube. A diffuse plasma fills the quartz tube during a time between intense current pulses. Ionization waves are propagating in this plasma between the spike and the grounded area of the tube producing thin plasma channels. The diameter of these channels increases during the pause between the propagation of ionization waves probably because of thermal expansion and diffusion. Inside the channels electron densities of ∼2 × 1013 cm−3, argon metastable densities ∼1014 cm−3 and a reduced electric field about 10 Td are determined.

A hard hydrocarbon film is deposited on the inner surface of glass tubes using a filamentary discharge at atmospheric pressure in Ar–C₂H₂–H₂ and Ar–CH₄ mixtures. Under similar conditions, a soft film is deposited with a high deposition rate in an Ar–C₂H₂ mixture. These differences in film hardness and deposition rate are interpreted on the basis of carbon and hydrogen elemental composition in the plasma. The deposition rate is varied along the axis of the tubes in the Ar–C₂H₂–H₂ plasma. This can be controlled by controlling the substrate (tube) temperature. Chemical erosion of the deposited film by hydrogen atoms is the probable reason for this effect. The plasma conditions (gas temperature, electron distribution function and electron density) are characterized by applying optical emission spectroscopy (OES), microphotography and numerical simulation for all three gas mixtures. The density of hydrogen atoms in the inter-electrode region of the tube is determined by applying OES in all gas mixtures. The rates of precursor molecule excitation and follow-up plasma-chemical reactions are calculated on the basis of the determined plasma parameters. Correlations between plasma conditions and film properties are discussed.

Uniform amorphous carbon film is deposited on the inner surface of quartz tubes having an inner diameter of 6 mm and an outer diameter of 8 mm. A pulsed filamentary plasma source is used for the deposition. Long plasma filaments (∼140 mm) are generated inside the tube in argon with methane admixture. FTIR–ATR, XRD, scanning electron microscope, laser scanning microscope and XPS analyses give the conclusion that deposited film is amorphous composed of non-hydrogenated sp² carbon and hydrogenated sp³ carbon. Plasma is characterized using optical emission spectroscopy, voltage–current measurement, microphotography and numerical simulation. On the basis of observed plasma parameters, the kinetics of the film deposition process is discussed.

A microplasma jet has been constructed for chemical vapour deposition on the inner surface of narrow long tubes and is tested on a quartz tube with the inner diameter of 6 mm and the outer diameter of 8 mm. A long plasma filament (>100 mm) is generated inside the tube in argon with methane admixture. Depending on the quantity of admixture in argon, the negative as well as the positive corona discharges are ignited. Both modes of plasma are characterized using optical emission spectroscopy. For this purpose, nitrogen is admixed with argon and its emission is used for the determination of gas temperature, electron velocity distribution function and electron density at various places along the axis of the tube. The formation of active species, such as C, C₂ and CH during the dissociation of methane, is observed through their characteristic emission. The rate of methane dissociation is determined at various places along the axis of the tube during both modes of discharge. Since the chemical kinetics of the film growth process is different, different kinds of films are obtained during different modes of discharge.