Overview of Atmospheric Pressure Discharges Producing Nonthermal Plasma

Recently, much attention has been paid to gas discharges producing nonthermal plasma because of many potential benefits in industrial applications. Historically, past work focused on Dielectric Barrier (silent) Discharges (DBD) and pulse-periodical corona discharges. Recently, a number of new different discharge techniques succeeded in producing stable gas discharge at atmospheric pressure. Among these are repetitively pulsed glow discharge; continuous glow discharge in a gas flow; hollow-cathode atmospheric pressure discharge; RF and microwave (MW) discharges. Several new variants of the DBD have been demonstrated over a rather wide range of frequencies. All these forms of gas discharge are characterized by a strong nonequilibrium plasma state. We attempt to classify these discharges with respect to their properties, and an overview of possible applications is made. Conditions for the existence of homogenous and filamentary forms of each of the discharge types are discussed.     

Langmuir probe measurements of axial variation of plasma parameters in 27.1 MHz rf oxygen planar discharges

Langmuir probe studies have been performed on rf (27.1 MHz) discharges in O₂ under planar reactor conditions to determine the axial variation of the plasma parameters (positive ion density, electron temperature, and dc space potential) as a function of pressure (20–220 Pa) and power (10–150 W) or current (0.1–2 A). By monitoring the second derivative of the I–V probe characteristics, the suppression of the rf component in the probe circuit can be optimized. Referring to this problem, numerical studies provide relations for the determination of the residual rf component as well as of the dc component of the plasma potential at incomplete rf compensation. The positive ion density is obtained from the ion saturation currents. Here the effect of collisions between ions and neutral particles within the probe sheath (for p> 100 Pa) is considered. The electron energy distribution function is found to be of the Maxwellian type for all discharge conditions investigated here. If the pressure and the power exceed critical values, the axial charge carrier distribution is characterized by a valley formation in the bulk plasma center. This fact demonstrates that secondary electron emission due to ion impact on the electrode surfaces and following ionization by these electrons near the sheaths in front of the electrodes are significant processes for sustaining the discharge. At low pressures (p60 Pa) the dc plasma potential was found to be identical with the half-peak maintaining voltage of the discharge, in agreement with the model idea of a symmetric rf planar discharge where the rf voltage drop across the bulk plasma can be neglected. For growing pressure, however, the plasma system moves gradually toward a situation where the V-I characteristics of the discharge are significantly controlled by processes in the bulk plasma. This transition depends on the current density.     

Optical emission spectroscopy and actinometry in CCl4-Cl2 radiofrequency discharges

Radiofrequency discharges fed with CCl₄-Cl₂ mixtures have been studied in the pressure range 0.3 to 0.6 torr by means of emission spectroscopic actinometry with Ar, He, and N₂ as actinometers. Two different reactors, a parallel plate and a capacitively coupled tubular one, have been utilized for this study to obtain information for a large range of electron energy distributions. Analysis of the experimental results demonstrates the following: the utilization of actinometry and its range of validity, the importance of electron attachment to CCl_x species during the plasma decomposition process, and the effects of the presence of chlorine and “glowpolymer” in the discharge medium.     

Design and Operating Characteristics of a Simple and Reliable DBD Reactor for Use with Atmospheric Air

The paper reports on the construction and operating characteristics of a planar dielectric barrier discharge (DBD) plasma generator. The generator was powered from a commercial frequency inverter at 400 Hz through a high voltage transformer. It could be operated up to a specific energy density (power per gas flow) of 20 Wh/m³. The corresponding power density was about 0.5 W per cubic centimeter of discharge volume. Special emphasis was given to a simple and reliable construction, which was easy to assemble and is based on a new, nonexpensive barrier material with excellent electrical, mechanical, and thermal properties. The modular reactor design allows simple plasma power scale-up. The reactor works with undried ambient air without additional cooling. In the range up to 10 Wh/m³ the ozone generation from ambient air was directly proportional to the energy density at a rate of 60 g O₃ per kWh or 30 ppm/Wh/m³. Thus the generator can serve as an effective source for chemically active radicals in plasma gas cleaning applications.     

Hydrogen atom yield in RF and microwave hydrogen discharges

The hydrogen atom yield in pure-H₂ RF and microwave-sustained discharges is investigated both theoretically and experimentally. A particle balance model is developed that provides the concentrations of the H, H₂, H⁺, H ₂ ⁺ , and H ₃ ⁺ species. It is also shown that an approximate solution of this model is adequate for calculating the concentration of H atoms (required, for instance, in diamond film deposition) in the 0.1–10 torr range. Next, the validity of the actinometry technique applied to the determination of the H-atom density in pure-H₂ discharges is examined. Using this diagnostic, it is observed that the H-atom concentration decreases when the vessel wall temperature increases, owing to the increased efficiency of atomic hydrogen recombination on the wall. To overcome this effect, the discharge tube wall is cooled off with dimethyl polysiloxane, a low-loss dielectric liquid. It improves significantly the H-atom concentration at 2450 MHz provided the pressure is typically below a few torr and the power density is not too high.     

Low Cost Compact Nanosecond Pulsed Plasma System for Environmental and Biomedical Applications

Nanosecond pulsed non-thermal atmospheric-pressure plasmas are promising for numerous applications including air and water purification, ozone synthesis, surface sterilization, material processing, and biomedical care. However, the high cost of the nanosecond pulsed power sources has hindered adaptation of the plasma-based technologies for clinical and industrial use. This paper presents a low cost (<100US$) nanosecond pulsed plasma system that consists of a Cockcroft–Walton high voltage charging circuit, a compact nanosecond pulse generator using a spark gap as switch, and a plasma reactor. The nanosecond pulse power source requires only a 12 V DC input, hence is battery operable. Through the optimization of the experimental parameters, pulses with a peak voltage >10 kV, a 3 ns rise time (10 to 90 %), and a 10 ns pulse duration (full width at half maximum) at a pulse repetition rate of up to 500 Hz were achieved in the present study. It has been successfully tested to power three different plasma reactors to form pulsed corona discharges, dielectric barrier discharges, and sliding discharges. The energy efficiency of such a nanosecond pulsed sliding discharge system was assessed in the context of ozone synthesis using air or oxygen as the feed gas, and was found comparable to a previously reported non-thermal plasma system that used commercial high voltage pulsed power sources. This study demonstrated that this low-cost nanosecond pulsed power source can prove to be an energy efficient and simple supply to drive various non-thermal atmospheric-pressure plasma reactors for environmental, medical and other applications.     

NH3 Plasma Treatments of PET for Enhancing Aluminum Adhesion

A NH₃ plasma process has been studied for enhancing the adhesion of aluminum coatings on polyethyleneterephtalate (PET) films. According to our peel strength results, NH₃ plasmas increase markedly the adhesion of aluminum on PET compared to O₂ discharges, with a much shorter treatment time. A tentative model of nonhindered growth of Al-coating based on the Lewis basic character of the functionalities grafted by NH₃ plasma is proposed for Al-polymer interactions, and for explaining the various steps in the process. The effects of power input and treatment time on the polymer surface chemistry and on the metal-polymer peel strength have been evaluated. Treatment times as short as 0.1 s at 100 W proved to be the best conditions in NH₃ plasmas, for a significant increase in Al/PET adhesion, while longer treatments have a detrimental effect. This may explain why most authors have not discovered the benefits of NH₃ plasmas for improving the adhesion of metals on PET, and have preferred O₂ or air treatments. The relative basicity of PET grafted with N-containing functionalities has been measured by means of X-ray Photoelectron Spectroscopy (XPS) analysis of samples exposed to vapors of trichloromethane, a Lewis acid molecular probe. The Al/PET adhesion was evaluated by means of a 180° Peel Test.     

Scaling of Shielded Sliding Discharges for Environmental Applications

Shielded sliding discharges are nanosecond streamer discharges which develop along a dielectric between metal foil electrodes, with one of the foils extended over the entire rear of the dielectric layer. The electrode configuration not only allowed rearranging discharges in parallel due to the decoupling effect of the metal layer, but also to modify the electric field distribution in such a way that components normal to the surface are enhanced, leading to an increased energy density in the discharge plasma. By varying the electrode gap, the applied voltage, and the repetition rate, it is shown that by keeping the average electric field constant, the discharge voltage can be reduced from tens of kV to values on the order of a few kV, but only at the expense of a reduced energy density of the plasma. Varying the repetition rate from 20 to 500 Hz resulted in a slightly reduced energy per pulse, likely caused by residual charges on the dielectric surface. Measurements of the NO conversion to NO₂ and ozone synthesis in dry air showed that the conversion is only dependent on the energy density of the discharge plasma. Although reducing the pulse voltage from the tens of kV range to that of few kV, and possibly even lower, causes a reduction in energy density, this loss can be compensated for by increasing the electrode gap area. This and the possibility to form discharge arrays allows generating large volume discharge reactors for environmental applications, at modest pulsed voltages.     

CARS Diagnostic and Modeling of a Dielectric Barrier Discharge

Dielectric barrier discharges (DBD) with planar- and knife-shaped electrodes are operated in N₂O₂NO mixtures under a pressure of 20 and 98 kPa. They are excited by means of consecutive unipolar or bipolar high-voltage pulse packages of 10 kV at a pulse repetition rate of 1 and 2 kHz. The rotational and vibrational excitation of N ₂ molecules and the reduction of nitric oxide (NO) in the discharge have been investigated using coherent anti-Stokes Raman scattering (CARS) technique. Rotational (gas) temperatures near the room temperature and vibrational temperatures of about 800 K at atmospheric pressure and 1400 K at a pressure of 20 kPa are observed. Therefore, chemical reactions of NO with vibrationally excited N ₂ are probably insignificant. One-dimensional kinetic models are developed that balance 35 chemical reactions between 10 species and deliver equations for the population density of excited vibrational levels of N ₂ together with a solution of the Boltzmann equation for the electrons. A good agreement between measured vibrational temperatures of N ₂ , the concentration of NO, and calculated data is achieved. Modeling of the plasma discharge verifies that a DBD operated with a N₂NO mixture reduces the NO content, the simultaneous presence of O ₂ , already 1%, is enough to prevent the NO reduction.     

Diagnostics and Application of the High Frequency Plasma Pencil

The so called high frequency plasma pencil is a source of highly active environment (electrons, ions, reactive radicals, excited atoms, and molecules) which can be generated at atmospheric, reduced or increased pressure, preserving a good control of the performance. The discharge can be either unipolar or bipolar. The properties of unipolar atmospheric pressure discharges are discussed on the basis of simple theoretical considerations and electric probe measurements. The plasma pencil discharge is studied by optical emission spectroscopy in the gas phase at atmospheric pressure as well as immersed in liquid using argon as a working gas. From the emission spectra the electron, vibrational and rotational temperatures are calculated for various distances from the plasma pencil electrode. Several technological applications like restoration of archaeological glass and metal artifacts, fragmentation of molecules for microelectrophoresis and plasma polymerization are summarized. An advantage of the plasma pencil is that it can be easily operated and controlled.     

Energy efficiency of NO removal by pulsed corona discharges

Pulsed positive corona discharges are used to remove NO from the flue gas of a methane burner. At low power input this leads to an increase in NO₂, which shows that the process is oxidative. Removal efficiency is greatest when discharges are produced with high-voltage pulses, which are shorter in duration than the time required by the primary streamers to cross the discharge gap, in combination with a dc bias. Other important parameters are input power density and residence time. The best result obtained so far is an energy consumption of 20 eV per NO molecule removed, at 50% deNOx i.e., a removal of 150 ppm NO_x, using a residence time of 15 s and an input power density, of 3.5 Wh/Nm³. [Wh/Nm³ stands for watt-hour per normal cubic meter, i.e., at normal conditions (273 K and 1 bar). This implies that 1 Nm³ contains 2.505 10²⁵ molecules.] There appears to be room for improvement by the addition of gaseous and particulate chemicals or the use of multiple corona treatment. It is argued front comparison between results from models and experiments that the direct production of OH by the discharge is only the initiation of the cleaning process.     

Characteristics of vinyl iodide microwave plasma etching of GaAs/AlGaAs and InP/InGaAs heterostructures

Vinyl iodide (C₂H₃I) microwave discharges with additions of H₂ and Ar are found to provide faster etch rates than conventional CH₄/H₂/Ar discharges for InP, InGaAs, GaAs, and AlGaAs. This is a result of the relatively high volatilities of indium, gallium, and aluminum iodide species. The etched features are smooth and anisotropic over a wide range of do self-biases (–150 to –350 V), process pressures (1–20mTorr), and microwave powers (150–500 W). The polymer that forms on the mask during the plasma exposure can be readily removed in O₂ discharges. Electron spectroscopy for chemical analysis (ESCA) showed that the etched surfaces are slightly deficient in the group V elements under most conditions, but changes to the optical properties of the semiconductors are minimal. No defects are visible by transmission electron microscopy (TEM) in GaAs or InP samples etched at dc biases –250 V.     

Tunable Diode Laser Absorption Studies of Hydrocarbons in RF Plasmas Containing Hexamethyldisiloxane

Tunable infrared diode laser absorption spectroscopy has been used to detect the methyl radical and three stable molecules, CH₄, C₂H₂ and C₂H₆, in radio frequency plasmas (f=13.56 MHz) containing hexamethyldisiloxane (HMDSO). The methyl radical concentration and the concentration of the stable hydrocarbons, produced in the plasma, have been measured in pure HMDSO discharges and with admixtures of Ar, while discharge power (P=20–200 W), total gas pressure (p=0.08–0.6 mbar), gas mixture and total gas flow rate (=1–10 sccm) were varied. The methyl radical concentration was found to be in the range of 10¹³ molecules cm^-3, while methane and ethane are the dominant hydrocarbons with concentrations of 10¹⁴–10¹⁵ mol cm^-3. Conversion rates to the measured stable hydrocarbons (RC(C_xH_y): 2×10¹²–2×10¹⁶ molecules J^-1 s^-1) could be estimated in dependence on power, flow, mixture and pressure. Under the used experimental conditions a maximum deposition rate of polymer layers of about 400 nm min^-1 has been found.     

By-product formation in spark breakdown of SF6/O2 mixtures

The yields of SOF₄, SO₂F₂, SOF₂, and SO₂ have been measured as a function of O₂ content in SF₆/O₂ mixtures, following spark discharges. All experiments were made at a spark energy of 8.7 J/spark, a total pressure of 133 kPa, and for O₂ additions of 0, 1, 2, 5, 10, and 20% to SF₆. Even for the case of no added O₂, trace amounts of O₂ and H₂O result in the formation of the above by-products. However, addition of O₂ significantly increases the yields of SOF₄ and SO₂F₂, while SOF₂ is only slightly affected. The net yields for SOF₄ and SO₂F₂ formation range from 0.18×10^–9 and 0.64×10^–10 mol·J^–1, respectively, at 1% O₂ content to 10.45×10^–9 and 7.15×10^–10 mol·J^–1, respectively, at 20% O₂ content. The mechanism for SOF₄ production appears to involve SF₄, an important initial product of SF₆, as a precursor. Comparison of the SOF₄ and SO₂F₂ yield from spark discharges (arc and corona) shows that the yields from other discharges (arc and corona) shows that the yields can vary by at least three orders of magnitude, depending on the type of discharge and on other discharge parameters.     

Optical diagnostic studies of trimethyl phosphine-containing radio-frequency discharges

We have investigated the decomposition of P(CH₃)₃ in 13-MHz, radio-frequency discharges in He and H₂. Relative concentrations of atomic phosphorus were estimated actinometrically by measuring P emission intensities, relative to Ar, while quantitative, relative P₂ number densities were measured by ArF excimer laser-induced fluorescence. A radiative lifetime of 10.5±1.0 ns was measured for the v=11 level of the C¹ _u ⁺ state, resonantly excited by the 193 nm laser. Additions of P(CH₃)₃ to He discharges reduce the electron energy distribution as sensed by the quenching of optical emission from He and trace amounts of added Ar, and thereby causes the overall process P(CH₃)₃P+3CH₃ to occur with maximum efficiency at very low (0.5%) P(CH₃)₃ additions. H₂ discharges are 10 times less efficient at producing P and P₂, due to the lower electron energies, and possibly to hydrogenation reactions. P₂ production in both He and H₂ discharges increases linearly with pressure up to at least 10 torr. Under optimum conditions, plasma decomposition of P(CH₃)₃ (and by analogy PH₃) promises to be an attractive source of phosphorus for growth of phosporus-containing thin films.     

Surface Functionalization at Atmospheric Pressure by DBD-Based Pulsed Plasma Polymerization

Dielectric barrier discharge (DBD)-based plasma deposition at atmospheric pressure, using glycidyl methacrylate (GMA, methacrylic acid-[2,3-epoxy-propyl ester]) as a prototype precursor was investigated in order to evaluate the applicability of dielectric barrier discharges to obtain plasma polymers with a high degree of structural retention of the starting precursor. Using pulsed excitation of the discharge, up to about 90% of the epoxy groups of GMA can be retained in plasma polymers obtainableat deposition rates in the order of 3–5nm/s. Preliminary investigations of the mechanism of film formation under pulsed plasma conditions indicate that the reaction of intact monomer molecules withsurface radicals generated during the pulses play a prominent role.     

Ozone production and de-NO x modeling in dry air electrical discharges

Numerical simulations are performed to assess the possibility of using different kinds of electrical discharges operating in dry air under normal conditions as ozone generators. The ozone production and NO_x destruction efficiency are investigated as a function of discharge parameters such as the reduced electric field, the energy deposition, and the electron density. The specific energy deposition becomes a key parameter to compare the different classes of ozonators. During the operative phase the electric discharge produces an appreciable amount of nitrogen oxides. Taking into consideration both the active phase of flue gas processing and the post-discharge phase it is possible to achieve a good NO_x-removal efficiency even for initial concentrations as high as 1000 ppm. The main processes and characteristic times are discussed.     

Characterization of Microwave-Induced Electric Discharge Phenomena in Metal–Solvent Mixtures

Electric discharge phenomena in metal–solvent mixtures are investigated utilizing a high field density, sealed-vessel, single-mode 2.45 GHz microwave reactor with a built-in camera. Particular emphasis is placed on studying the discharges exhibited by different metals (Mg, Zn, Cu, Fe, Ni) of varying particle sizes and morphologies in organic solvents (e.g., benzene) at different electric field strengths. Discharge phenomena for diamagnetic and paramagnetic metals (Mg, Zn, Cu) depend strongly on the size of the used particles. With small particles, short-lived corona discharges are observed that do not lead to a complete breakdown. Under high microwave power conditions or with large particles, however, bright sparks and arcs are experienced, often accompanied by solvent decomposition and formation of considerable amounts of graphitized material. Small ferromagnetic Fe and Ni powders (<40 μm) are heated very rapidly in benzene suspensions and start to glow in the microwave field, whereas larger particles exhibit extremely strong discharges. Electric discharges were also observed when Cu metal or other conductive materials such as silicon carbide were exposed to the microwave field in the absence of a solvent in an argon or nitrogen atmosphere.     

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.     

DC plasma etching of silicon by sulfur hexafluoride. Mass spectrometric study of the discharge products

Polycrystalline silicon wafers were etched in dc discharges of SF₆. SF_x species were extracted from the discharges and measured with a mass spectrometer. A systematic procedure was used to measure the SF _x ⁺ signals such that they are indicators of events in the discharge close to the sample undergoing etching. The picture that emerges is remarkably simple and shows the relative stability of several SF_x species including SF₆, SF₄, SF₂, and SF which are shown to be extracted from the discharge both in the presence and absence of the silicon sample. When silicon is being etched on the cathode of the discharge cell, the only significant additional products are SiF₄ and S₂F₂. A comparison of blank and sample data for opposite substrate polarities shows that there is only a small cation-assisted etching effect and suggests that ions do not play an important role in the etching of silicon by SF₆ discharges.