Andrei V. Pipa

Optical emission from atmospheric pressure micro-jet operating with pure argon (argon μAPPJ) flow has been detected with a moderate resolution spectrometer. Large broadening of the several argon (Ar) lines has been observed in the near infrared spectral region. This effect was attributed to resonance broadening of the s2 (Paschen notation) level in...

Measurements of current and voltage are the basic diagnostics for electrical discharges. However, in the case of dielectric barrier discharges (DBDs), the measured current and voltage waveforms are influenced by the discharge reactor geometry, and thus, interpretation of measured quantities is required to determine the discharge properties. This co...

Dielectric barrier discharges (DBDs) are characterized by electrical measurements interpreted on the basis of equivalent circuits. The concept of the simplest equivalent circuit of volume DBDs is expanded to surface DBDs (SDBDs); this was completed by taking into account the discharge expansion on the dielectric-gas interface. It is assumed, that t...

The work is devoted to the development of laser absorption spectroscopy (LAS) of plasma-assisted processes for application under industrial conditions. The interpretation of the LAS measurements was revised by taking into consideration the temperature gradient along the absorption path, which is unavoidable in a reactor for thermochemical treatment...

The density evolution of HCN, NH3, H2O and CO molecules over time was monitored by laser absorption spectroscopy (LAS) in a low pressure DC pulsed discharge in N2-H2 gas mixtures with addition of CH4 or O2. The discharge was maintained in an industrial-scale, active screen plasma nitrocarburizing (ASPNC) reactor with a steel active screen (AS). The...

Recent technological development of utilizing an active screen made of solid carbon for plasma-assisted thermochemical diffusion treatments opens up new possibilities for control over the in situ generated treatment environment to guarantee reproducible treatment conditions and material responses. Until now, the investigations of active-screen plas...

A planar volume dielectric barrier discharge (DBD) in pure carbon dioxide (CO2) for the formation of carbon monoxide (CO) is examined by combined electrical and CO density measurements. The influence of the type of electrode, the barrier material, the barrier thickness, and the discharge gap on the plasma power and the CO formation is analyzed syst...

Developing the fundamentals for the electrical diagnostics of surface dielectric barrier discharges (SDBDs) is of enormous importance for several applications, for example, flow control and gas cleaning. The main challenge is to account for the discharge expansion along the dielectric surface. Typically, a linear expansion with the amplitude of the...

Plasma nitrocarburizing by means of active screen technology using an active screen made of carbon fiber-reinforced carbon was carried out by varying the power at the active screen and using oxygen-containing fresh gas components (O2, CO2) in the N2:H2 plasma using the example of the quenched and tempered steel AISI 4140 (42CrMo4). The investigatio...

Avoiding ice accumulation on aerodynamic components is of enormous importance to flight safety. Novel approaches utilizing surface dielectric barrier discharges (SDBDs) are expected to be more efficient and effective than conventional solutions for preventing ice accretion on aerodynamic components. In this work, the realization of SDBDs based on t...

Low-temperature plasma nitrocarburizing treatments are applied to improve the surface properties of austenitic stainless steels by forming an expanded austenite layer without impairing the excellent corrosion resistance of the steel. Here, low-temperature active screen plasma nitrocarburizing (ASPNC) was investigated in an industrial-scale cold-wal...

In a distributed antenna array reactor, microwave H2-CH4-CO2 plasmas with admixture of N2 used for the low-temperature deposition of nanocrystalline diamond (NCD) films are studied by in situ infrared laser absorption spectroscopy (LAS) and optical emission spectroscopy techniques. The experiments are carried out in order to analyze the dependence...

Surface dielectric barrier discharges (SDBDs) were investigated in order to explore the combined effects of the barrier thickness and the microstructure of the exposed electrode on the ion wind generation. Actuators with straight and structured high voltage electrodes with characteristic sizes of 200 and 250 μm and dielectric thicknesses of 0.5, 1...

Low temperature plasma nitrocarburizing processes are widely used surface treatment techniques to improve the surface hardness and wear resistance of stainless steels without loss of their excellent corrosion resistance. In the present study, plasma nitrocarburizing based on the active screen technology was applied in an industrial-scale cold wall...

The active screen plasma nitrocarburizing technology is an improvement of conventional plasma nitrocarburizing by providing a homogeneous temperature distribution within the workload and reducing soot formation. In this study, an industrial-scale active screen (AS) made of carbon-fibre-reinforced carbon serves as the cathode as well as the carbon s...

Silane (SiH4) plasmas are widely used for the deposition of hydrogenated amorphous silicon (a-Si:H) films. Nevertheless, the chemical processes governing film deposition are still incompletely understood. Moreover, there is still no general method available to determine the absolute concentration of the silyl radical (SiH3), which is the accepted c...

As derived by Max Planck in 1903 from dispersion theory, Beer's law has a fundamental limitation. The concentration dependence of absorbance can deviate from linearity, even in the absence of any interactions or instrumental nonlinearities. Integrated absorbance, not peak absorbance, depends linearly on concentration. The numerical integration of t...

A microwave torch in nitrogen has been designed for the evaporation of aluminium (Al) wires potentially to be used for the synthesis of aluminium nitride (AlN). The torch, created on the tip of a metallic nozzle, is compatible with vacuum conditions and can be operated in continuous mode up to atmospheric pressure. Al wires to be evaporated are fed...

Differences in the liquid chemistry due to different ground electrode materials (titanium, stainless steel) were compared for corona discharges in water. The plasma was generated by applying positive high voltage pulses that are characterized by short rise times of about 20 ns, a peak voltage of 80 kV and pulse lengths of about 150–160 ns (FWHM). P...

Spectroscopic investigations of free-burning vacuum arcs in diffuse mode with CuCr electrodes are presented. The experimental conditions of the investigated arc correspond to the typical system for vacuum circuit breakers. Spectra of six species Cu I, Cu II, Cu III, Cr I, Cr II, and Cr III have been analyzed in the wavelength range 350–810 nm. The...

The present article is focused on a tungsten inert gas (TIG) welding process in two operation regimes, a normal and a constricted one. A computational model is applied for simulations of the liquid weld pool and the heat-affected zone of the workpiece coupled with a simulation of the welding arc. Welding experiments are used for model validation. T...

Experimental investigations of the impact of resonant laser absorption by a tungsten inert gas welding arc in argon are presented. The intensity increase of the arc’s radiation between the laser entrance height and the anode are observed, as well as the variation of arc voltage due to the presence of the laser beam. High-speed camera recordings fro...

Magneto-hydrodynamic simulations of the arc and fluid simulations of the weld pool can be beneficial in the analysis and further development of arc welding processes and welding machines. However, the appropriate coupling of arc and weld pool simulations needs further improvement. The tungsten inert gas (TIG) welding process is investigated by simu...

This work presents modelling results concerning a tungsten inert gas (TIG) welding arc. The model provides a consistent description of the free burning arc, the arc attachment and the electrodes. Thermal and chemical nonequilibrium is considered in the whole arc area, and a detailed model of the cathode space-charge sheath is included. The mechanis...

Microwave (MW) H2 plasmas with small admixtures of CH4 and/or B2H6 used for doped diamond deposition have been studied by infrared absorption and optical emission spectroscopy (OES) techniques. The experiments were carried out in order to analyze the dependence of the plasma chemistry on several parameters in the MW discharges, such as power, press...

The binary collision-based two dimensional SDTrimSP-2D model has been used to simulate the erosion process of a Cu target and its influence on the operational limit of a planar DC magnetron nanocluster source. The density of free metal atoms in the aggregation region influences the cluster formation and cluster intensity during the target lifetime....

The paper reports on an experimental investigation of the ion velocity distribution function (IVDF) for mass-selected copper cluster ions with a diameter in the range 5–10 nm. The cluster ion beam is produced by a dc magnetron-based gas aggregation source. The measurements are carried out by means of a quadrupole mass filter combined with a basic r...

A dielectric barrier discharge (DBD) of coaxial geometry has been investigated. The discharge cell was filled by 100 mbar of argon and driven by positive square voltage pulses with a rise time of 20 ns and 75 ns. The internal discharge characteristics such as the discharge current, the gas gap voltage, the instantaneous power and energy have been d...

We present an alternative approach to grow thick GaN single crystalline layers from Ga vapour, transported by an N_2 carrier gas flow, and from nitrogen, excited very near the substrate by 2.45 GHz microwaves at pressure in the 200–800 mbar range. Crystal growth requires high stability of process parameters, especially of plasma operation. A major...

The present paper reports on target erosion as one of the crucial parameters influencing the cluster size distribution. Size distributions of nanosized Cu clusters produced by a DC magnetron sputtering source during the lifetime of several different targets were monitored using a quadrupole mass filter. It is indicated that, during the target lifet...

The concept of the simplest equivalent circuit for a dielectric barrier discharge (DBD) is critically reviewed. It is shown that the approach is consistent with experimental data measured either in large-scale sinusoidal-voltage driven or miniature pulse-voltage driven DBDs. An expression for the charge transferred through the gas gap q(t) is obtai...

The determination of electrical parameters (such as instantaneous power, transferred charge, and gas gap voltage) in dielectric barrier discharge (DBD) reactors relies on estimates of key capacitance values. In the classic large-scale sinusoidal-voltage driven DBD, also known as silent or ozonizer discharge, capacitance values can be determined fro...

A pulsed microwave (MW) plasma was maintained in a waveguide with an ignition pin. Images of the MW discharge were recorded during ignition in ambient air with millisecond time resolution and correlated with measurements of absorbed MW power. This work is focused on the quantification of ignition stability and observation of the plasma structure. A...

The production of NO radicals by an atmospheric pressure plasma jet has been investigated by means of absorption spectroscopy in the mid-infrared region (IR) and optical emission spectroscopy (OES) in the ultraviolet (UV) part of the spectrum. The plasma jet investigated here operates in argon with air admixtures up to 1%. The study shows that OES...

A pulsed dielectric barrier discharge (DBD) has been investigated in a wide range of experimental conditions with the purpose of optimization for XeCl excimer radiation. For that the following operation parameters had been considered: four different lamps of coaxial geometry with gas gaps in the range of 1.3 - 6.5 mm; gas mixtures of xenon and chlo...

Limitations in ammonia-based vapour growth of bulk GaN require the search for a replacement of ammonia as a precursor of reactive nitrogen. We propose the implementation of a plasma-activated nitrogen source instead. In this contribution we present the current development status of a long-term stable plasma source for reactive nitrogen supply as we...

A dielectric barrier discharge (DBD) of coaxial geometry has been examined as source of intensive, short-lived UV radiation. A binary gas mixture consisting of 98% Xe + 2% Cl2 and a ternary composition of 96% Ne + 6% Xe + 0.2% HCl were investigated in the pressure range between 10 and 750 mbar. The discharge was excited by unipolar high voltage squ...

The mid-infrared (IR) absorption spectrum of the exhaust gas of an atmospheric pressure plasma jet operated in Ar with the admixture of 0.1% of air has been monitored in the spectral region from 4000 to 750 cm^-1. The absorption features of CO₂, CO, NO, NO₂, N₂O, HNO₂, and HNO₃ were i...

Tunable diode laser absorption spectroscopy (TDLAS) has been applied to measure the absolute production rate of NO molecules in the gas phase of an atmospheric pressure plasma jet (APPJ) operating at rf (13.56 MHz) in argon with small (up to 1%) admixtures of air. The resulting NO production rates were found to be in the range (0.1–80) × 10−3 sccm...

The ultraviolet (UV) emission from a fast-pulsed dielectric barrier discharge (DBD) in a He-air gas mixture has been investigated for a wide range of operating conditions (discharge gap: 1 cm, pressure: 100 - 750 Torr, voltage: 2.5 - 8 kV, sawtooth pulse: decay time ~ 10 ns, frequency: 1.5 - 23 kHz, He-air mixture with 3%-30% air). The short-pulse...

form given only. Atmospheric plasma jets (APPJ) are attractive plasma sources for biomedical applications due to opportunity of local treatment of complicated surfaces without thermal damage of treated materials. Nitric oxide (NO) is one of the chemically active molecules, which are interesting for biomedical applications and generated by APPJ. NO...

The number densities of Ar 3s^23p^54s levels have been measured by means of two methods: a self-absorption method with a mirror behind the discharge vessel and an absorption method with a high-frequency electrodeless discharge lamp. Concentrations ranging from 6 × 107–4 × 109 cm−3 were obtained for the Ar 3s23p54s levels, depending on the argon con...

The method of spectroscopic determination of the degree of dissociation of hydrogen in non-equilibrium plasma, proposed in our previous paper, was verified under various plasma conditions. Three different gas discharges have been used, namely: (i) a DC-arc discharge, (ii) an RF discharge, f = 200 kHz and (iii) an MW discharge, f = 2.45 GHz, under p...

A new spectroscopic method of determination of the degree of dissociation of hydrogen is proposed. The method is based on measurements of the relative intensities of two atomic lines of the Balmer series, Hα and Hβ, and of one molecular line, the (2-2)Q1 line of the Fulcher-α system. These intensities are related to two plasma parameters: (i) the r...

Recent studies of phenomena in molecular non-equilibrium microwave plasmas, which have been performed within the framework of the DFG-Sonderforschungsbereich 198, project A12, are presented in this review article. Special attention is devoted to the properties and kinetics of transient molecular species. The experimental methods used were optical e...

Zugl.: Greifswald, University, Diss., 2004. The results of the present work show an opportunity of pure spectroscopic detection of the degree of dissociation of hydrogen in low temperature plasmas and indicate an insufficient knowledge about elementary processes for plasma diagnostic.

Spectroscopic studies of microwave discharges in H 2 –Ar–B 2 H 6 gas mixtures (f = 2.45 GHz, P = 1.2–3.5 kW, p = 1–8 mbar) have been performed to improve the possibilities of diagnostics of non-equilibrium, low-pressure plasmas containing boron and hydrogen. For this purpose, UV–VIS optical emission spectroscopy and infrared absorption spectroscopy...

Many P- and R-branch lines of the fundamental bands of the (12)C(14)N and (13)C(14)N radicals in their ground ((2)Sigma(+)) electronic state have been measured at high resolution by diode laser absorption spectroscopy. The radicals were generated in a 2.45 GHz planar microwave plasma in methane with varying proportions of N(2) and H(2). From a fit...

The possibility of a correct account of the fine structure was shown for two limiting cases observed in beam and plasma experiments. A significant difference was found in the emission cross sections and the rate coefficients of direct and dissociative excitation of the H α and H β lines by electron impact in two limiting cases (e.g., this differenc...