Electrical,Thermal and Optical Diagnostics of an Atmospheric Plasma Jet System

Plasma diagnostics of atmospheric plasmas is a key tool in helping to understand processing performance issues. This paper presents an electrical, optical and thermographic imaging study of the PlasmaStream atmospheric plasma jet system. The system was found to exhibit three operating modes; one constricted/localized plasma and two extended volume plasmas. At low power and helium flows the plasma is localized at the electrodes and has the electrical properties of a corona/filamentary discharge with electrical chaotic temporal structure. With increasing discharge power and helium flow the plasma expands into the volume of the tube, becoming regular and homogeneous in appearance. Emission spectra show evidence of atomic oxygen, nitric oxide and the hydroxyl radical production. Plasma activated gas temperature deduced from the rotational temperature of nitrogen molecules was found to be of order of 400 K: whereas thermographic imaging of the quartz tube yielded surface temperatures between 319 and 347 K.     

Emission characteristics of mixed gas plasmas in low-pressure glow discharges

Glow discharge optical emission spectrometry (GD-OES) with mixed plasma gases is reviewed. The major topic is the effect of type and content of gases added to an argon plasma on the emission characteristics as well as the excitation processes. Emphasis is placed on argon–helium, argon–oxygen, and argon–nitrogen mixed gas plasmas. Results for non-argon-matrix plasmas, such as neon–helium and nitrogen–helium mixtures, are also presented. Apart from the GD-OES, glow discharge mass spectrometry and furnace atomization plasma emission spectrometry with mixed plasma gases are also discussed.     

An Optical Emission Study on Expanding Low-Temperature Cascade Arc Plasmas

The optical emission spectra from expanding low-temperature cascade arc plasmas were studied. The objective of this study was to examine the distinctive features of low-temperature cascade arc plasmas in comparison with a radio frequency (RF) plasma source. The principal results obtained in this study were: (1) in an expanding cascade arc plasma jet, active heavy particles (mainly excited argon or helium neutral species under our operating conditions), rather than electrons, are responsible for the excitation of reactive species when a reactive gas is injected into the plasma jet, (2) the excitation of reactive species was found to be controlled by the electronic energy levels of these excited argon or helium neutrals, (3) changing the operating parameters affected only the emission intensities of excited species, and no effect on the emission nature of plasmas was observed.     

Effect of operation parameters on the sputtering and emission processes in radiofrequency glow discharge. A comparison with the direct-current mode

In this paper, a comparison of the direct current (DC) and radiofrequency (RF) operating modes in glow discharge optical emission spectrometry (GD-OES) is carried out using the same discharge chamber, based on the Marcus design, powering alternatively with DC or RF energy. The effect of discharge pressure, the DC bias voltages and the delivered power divided by the DC-bias voltage on the sputtering rates, emission intensities and emission yields achieved for conducting materials was investigated in order to characterize both discharge types. Our results show that the effect of plasma variables on sputtering rates and emission yields using a DC-GD based on the chamber described by Marcus, can be considered to follow trends similar to those of the well-known DC-Grimm source. However, if the effect of plasma variables are compared for a DC-GD and a RF-GD, both generated by the same source as designed by Marcus, the behaviour of the DC and RF operations of the source proved to have some differences. Thus, at a fixed delivered power, the sputtering rate in the DC-GD decreases noticeably with pressure while the reverse effect is observed in the RF-GD. Moreover, under selected operating conditions, using the tin emission line (Sn I 380.102 nm), lower sputtering rates and higher emission yields were observed for the RF-GD than for the DC-GD source. Extension of known theoretical expressions and concepts from analytical DC-GD to RF-GD-OES work appears rather involved and is not yet possible.     

Evaluation of a radio frequency plasma for oxygen-selective detection in capillary gas chromatography

A radio frequency plasma detector for capillary GC has been modified for oxygen-selective detection. Purification of the plasma gas and purging of both ends of the discharge region with helium were crucial to minimizing oxygen background emission from impurities in the plasma. With a pure helium plasma, eluting hydrocarbons released oxygen from the discharge region resulting in interfering signals on the oxygen channel. These interfering signals were efficiently reduced by using a methane-doped (0. 15%) low power RF plasma (15 W) sustained in a high make-up flow (150 mL/min). With this plasma, a 10³:1 oxygen-to-carbon selectivity and a 100 pg oxygen/s detection limit were obtained. The detector was linear over three orders of magnitude. The detection system has been used to screen for oxygenated compounds in two environmental samples.     

Selective excitation of singly-ionized silver emission lines by Grimm glow discharge plasmas using several different plasma gases

The relative intensities of silver emission lines from Grimm glow discharge plasmas were investigated in the wavelength range from 160 to 600 nm when using different plasma gases. It was characteristic of the plasma excitation that the spectral patterns were strongly dependent on the nature of the plasma gas employed. Intense emission lines of silver ion were observed when argonhelium mixed gases were employed as the plasma gas. Selective excitation of the ionic lines could be principally attributed to the charge transfer collisions between silver atoms and helium ions.     

Deposition of polyacrylic acid films on PDMS substrate in dielectric barrier corona discharge at atmospheric pressure

This paper reports on deposition of acrylic acid films polymerized by an efficient and cost‐effective technique of dielectric barrier corona discharge at atmospheric pressure. The liquid acrylic acid was vaporized and carried by argon gas into plasma to deposit polyacrylic acid films on polydimethylsiloxane substrate. A nonthermal corona discharge was generated in a pyrex flask using a steel tube‐to‐plate asymmetric electrode configuration. The plasma was excited using an in‐house developed power supply operating with continuous wave signals of 10‐kHz frequency. The emission spectra of plasma species were recorded to know their contribution during deposition process. The deposited surfaces were characterized using contact angle measurements, atomic force microscopy, Fourier transform infrared spectroscopy, X‐ray photoelectron spectroscopy and film thickness measurements. A maximum film growth rate of 363 nm/min was achieved under optimal condition of discharge. The results suggest that this plasma technique is capable of depositing organic coatings with a high concentration of carboxylic functional groups that could be potentially used for biomedical and microfluidic applications.     

Non-thermal features of atmospheric-pressure argon and helium microwave-induced plasmas observed by laser-light Thomson scattering and Rayleigh scattering

Laser-light Thomson scattering and Rayleigh scattering have been measured from a microwave-induced plasma sustained at atmospheric pressure, using both argon and helium as a support gas. The measurements were performed at several spatial positions in each plasma, and at forward microwave power levels of 350 W for argon, and at 350 W and 100 W for helium. It was found from these measurements that both argon and helium plasmas deviate substantially from local thermodynamic equilibrium (LTE), Measured electron temperatures range from 13 000–21 500 K, whereas gas temperatures are generally lower by a factor of 2 to 10, depending on the support gas and the spatial position in the discharge. At the same forward microwave power, the electron temperature of the helium plasma is about 3500–7000 K higher than that of the argon plasma. Yet, the argon plasma has a higher electron number density than the helium plasma. Electron number densities in both argon and helium plasmas are roughly two to three orders of magnitude lower than what LTE would predict, based on the measured electron temperatures and the Saha Equation. Even more interestingly, signals in the far-wing portion of the Thomson-scattering spectrum were found to be significantly higher than are predicted by a fitted Maxwellian curve, indicating that there exists an over-population of high-energy electrons. It is concluded that, compared to the inductively coupled plasma, the microwave-induced plasma is highly non-thermal and remains in an ionizing mode in the analytical zone.     

Comparative study on emission characteristics of d.c.- and r.f.-powered Grimm glow discharge plasmas. Use of argon spectral lines

Equivalent discharge conditions, where the intensities of the analyte are almost the same between the d.c. and the r.f. power modes, have been investigated in Grimm glow discharge emission spectrometry. The two plasmas have similar emission and sputtering characteristics, enabling the conditions to be found easily. Various emission lines of argon ion are commonly observed from the argon discharges regardless of the power modes. A method to determine the equivalent discharge conditions is suggested, based on intensity analysis of the argon ionic lines.     

Characterization of High Voltage Cold Atmospheric Plasma Generation in Sealed Packages as a Function of Container Material and Fill Gas

While numerous experiments have demonstrated the efficacy of high voltage cold atmospheric pressure plasmas (HVCAPs) in sealed packages for microbial inactivation, the influence of the package on the emitted species measured during HVCAP discharge is poorly understood. This study elucidates the impact of the package on plasma generation in sealed packages for four separate gases (ambient air, commercial grade compressed air, a helium/air mixture, and nitrogen) placed in commercially available transparent plastic containers and bags representative of the materials used in the food industry. The container and bag individually reduced emission signal intensity by an average of 63 and 45%, respectively, across the measured wavelengths of 200–1100 nm, demonstrating that they acted as broadband absorbers. Neither the container nor bag caused additional emission lines to appear, indicating no significant effect on the types of species generated. Considering the minimum applied voltage necessary to induce a discharge, the power dissipated by the nitrogen and ambient air plasma generated at 72 ± 3.7 kV RMS were comparable to the compressed dry air discharge generated at 80 ± 3.7 kV RMS. The helium discharge at 37 ± 3.7 kV RMS absorbed approximately 92% more power than these gases. Rotational temperatures ranged from 285 K for helium to 479 K for compressed air. These results indicate that the package impacts the intensity distribution but not the presence of the most dominant peaks, although further studies are required to elucidate the impact on less intense peaks.     

Solvent-induced porosity in ultrathin amine plasma polymer coatings

Plasma polymers deposited from n-heptylamine onto silicon wafers have been found to form a porous microstructure when immersed in water and other solvents, with pores of dimensions and densities that vary considerably between coatings deposited under different plasma conditions. This solvent-induced pore formation was found to correlate with the observed percentage of extractable material. With low radio frequency (rf) power inputs, the resultant softer coatings possess considerably more extractable material than coatings deposited at higher applied power levels. The porosity is thus proposed to result from the formation of voids created by the extraction of soluble low-molecular-weight polymeric material, which produces shrinkage stress that the coating, firmly attached to the substrate, cannot relieve by macroscopic contraction. The microscopic contraction of plasma polymer volume creates voids that appear to span the entire film thickness. The effect of aging plasma polymers in air was also investigated. For films deposited at low power it led to reduced extraction of soluble material and different pore morphology, whereas for films deposited at higher rf power levels, the extracted amounts and pore formation were the same for aged coatings. It was also found that the density of surface amine groups was lower for films deposited under the two lowest power settings, in contrast to the commonly held belief that the use of minimal applied rf power aids retention of functional groups. These porous plasma polymer coatings with surface groups suitable for further interfacial chemical immobilization reactions may be useful for various membrane and biotechnology applications.     

New power source from fractional quantum energy levels of atomic hydrogen that surpasses internal combustion

Extreme ultraviolet (EUV) spectroscopy was recorded on microwave discharges of helium with 2% hydrogen. Novel emission lines were observed with energies of q·13.6 eV where q=1,2,3,4,6,7,8,9, or 11 or these lines inelastically scattered by helium atoms wherein 21.2 eV was absorbed in the excitation of He (1s²) to He (1s¹2p¹). These lines were identified as hydrogen transitions to electronic energy levels below the ‘ground’ state corresponding to fractional quantum numbers. Significant line broadening corresponding to an average hydrogen atom temperature of 33–38 eV was observed for helium–hydrogen discharge plasmas; whereas pure hydrogen showed no excessive broadening corresponding to an average hydrogen atom temperature of ≈3 eV. Since a significant increase in H temperature was observed with helium–hydrogen discharge plasmas, and energetic hydrino lines were observed at short wavelengths in the corresponding microwave plasmas that required a very significant reaction rate due to low photon detection efficiency in this region, the power balance was measured on the helium–hydrogen microwave plasmas. With a microwave input power of 30 W, the thermal output power was measured to be at least 300 W corresponding to a reactor temperature rise from room temperature to 900 °C within 90 s, a power density of 30 MW/m³, and an energy balance of about −4×10⁵ kJ/mol H₂ compared to the enthalpy of combustion of hydrogen of −241.8 kJ/mol H₂.     

Operation modes of the helium dielectric barrier discharge for soft ionization

Among different applications of dielectric barrier discharge (DBD) plasma, the soft ionization ability is certainly one of the most interesting. In this paper the helium plasma jet, produced by a capillary DBD, penetrating in the ambient atmosphere, has been spectroscopically investigated in dependence on applied voltage and helium flow. It was found that the change of the applied voltage leads to different discharge modes. Based on the measurements of the emission spectra of atomic He and N₂⁺ and N₂ molecules in the capillary and in the plasma jet with high spatial resolution, it can be assessed in which mode, i.e. under which conditions the plasma jet is expected to be most effective for soft ionization of molecules.     

Deposition of Polyacrylic Acid Films by Means of an Atmospheric Pressure Dielectric Barrier Discharge

The present work describes the plasma polymerisation of acrylic acid at atmospheric pressure. The influence of two operating parameters (monomer concentration and discharge power) on the properties of the deposited films is investigated. Results show that at a monomer concentration of 2.5 ppm and a discharge power of 9.5 W, the monomer is only slightly fragmented leading to a high amount of carboxylic acid groups on the deposited films. In contrast, when monomer concentration is decreased or discharge power increased, the incidence of monomer fragmentation processes is higher, leading to a lower amount of carboxylic acid groups on the films. This behaviour can be explained by a higher energy amount available per monomer molecule at low monomer concentrations and high discharge powers and a higher flux of positive ions attacking the surface at high discharge powers. Taking into account these results, it can be concluded that the deposition parameters should be carefully selected in order to preserve the stability of the monomer and thus obtain coatings with high carboxylic acid densities.     

Two Atmospheric-pressure Plasma Sources for Polymer Surface Modification

Two atmospheric-pressure plasma sources were studied. One was a helium plasma generated by a RF discharge, and the other was an air plasma generated by a dielectric barrier discharge (DBD). The two plasma sources were characterized on electron density, emission spectrum, and ozone density. The modification of polyethyleneterephthalate (PET) surfaces by the two plasmas was investigated. PET strips were exposed to the plasma at the exit of the plasma source. Water contact angles were measured for surfaces modified with different processing parameters. The change in contact angles was monitored as a function of time. Modification mechanisms were also investigated.     

Structural,optical and secondary electron emission properties of diamond like carbon thin films deposited by pulsed-DC plasma CVD technique

Plasma enhanced chemical vapor deposition (PECVD) technique using pulsed-DC power supply was used to fabricate diamond like carbon (DLC) films at deposition rates as high as 110 nm/min. The DLC films deposited by pulsed-DC and DC based power supplies under different gas flow ratios were studied for their suitability as dielectric layer coatings in plasma display panels (PDPs). The effect of deposition parameters on the properties of the DLC films were studied using Fourier transform infra-red spectroscopy (FTIR) and spectroscopic ellipsometry (SE). FTIR reveals that higher hydrogen dilution in gas mixture leads to higher sp³ content. SE studies in wide spectral range were analyzed using Tauc-Lorentz model dielectric function. A rise in the extracted refractive index was seen on increasing the H₂ content in the feed gas, thus resulting in optically denser films. Secondary electron emission coefficient (γ) was measured in the films deposited by the DC and pulsed-DC based PECVD. Firing voltage in the DLC samples was found to have very low variation in the operating pressure range used in commercial PDPs, suggesting possibility of enhanced long term reliability of DLC coatings in future PDP applications.     

Investigations on microstructure,composition and properties of PECVD TiNx-coatings

Nearly stoichiometric TiN_x-coatings have been deposited on different substrates using a gaseous reaction mixture of TiCl₄, N₂, H₂ and Ar in a pulsed d.c. plasma discharge. The influence of substrate temperature, plasma power density, argon partial pressure and type of substrate on chlorine content, texture and microstructure of the coatings has been investigated keeping constant the other parameters of the plasma enhanced chemical vapor deposition (PECVD) process. Microstructure has been characterized by scanning electron microscopy (SEM) fractographs of the coatings and by determination of texture. The chlorine content quantitatively determined from energy-dispersive X-ray spectra (EDX) using a chlorine containing mineral as standard decreases on an increase of substrate temperature or plasma power density. Texture changes from 〈200〉 to 〈111〉 and random and microstructure changes from columnar growth to granular for decreasing substrate temperature as well as decreasing plasma power density. Argon partial pressure does not affect the microstructure but the texture. The properties of the coatings are independent of type of substrate for higher plasma power densities. Oxygen present at the surface of the substrate stimulates the development of a texture at low plasma power densities.     

气相色谱-常压微波等离子体发射光谱系统操作参数的考察及最优化

本文对影响GC-MIP检测性能的操作参数(包括等离子体观测位置,微波功率和He支持气流速)进行了最优化研究。结果表明,对C、H、D、O、S、P、F、Cl、Br和I元素来说,每个元素都有各自的一套最佳操作参数。此外,还对记忆效应进行了讨论。     

Emission characteristics of Grimm-style glow discharge plasmas with helium matrix plasma gas containing small amounts of nitrogen

Glow discharge plasmas with helium–(0–16%) nitrogen mixed gas were investigated as an excitation source in optical emission spectrometry. The addition increases the sputtering rate as well as the discharge current, because nitrogen molecular ions, which act as primary ions for the cathode sputtering, are produced through Penning-type ionization collisions between helium metastables and nitrogen molecules. The intensity of a silver atomic line, Ag I 338.29 nm, is monotonically elevated along with the nitrogen partial pressure added. However, the intensities of silver ionic lines, such as Ag II 243.78 nm and Ag II 224.36 nm, gave different dependence from the intensity of the atomic line: Their intensities had maximum values at a nitrogen pressure of 30 Pa when the helium pressure and the discharge voltage were kept at 2000 Pa and 1300 V. This effect is principally because the excitations of these ionic lines are caused by collisions of the second kind with helium excited species such as helium metastables and helium ion, which are quenched through collisions with nitrogen molecules added to the helium plasma. The sputtering rate could be controlled by adding small amounts of nitrogen to the helium plasma, whereas the cathode sputtering hardly occurs in the pure helium plasma.     

Emission characteristics of Grimm-style glow discharge plasmas with helium matrix plasma gas containing small amounts of nitrogen

Glow discharge plasmas with helium–(0–16%) nitrogen mixed gas were investigated as an excitation source in optical emission spectrometry. The addition increases the sputtering rate as well as the discharge current, because nitrogen molecular ions, which act as primary ions for the cathode sputtering, are produced through Penning-type ionization collisions between helium metastables and nitrogen molecules. The intensity of a silver atomic line, Ag I 338.29 nm, is monotonically elevated along with the nitrogen partial pressure added. However, the intensities of silver ionic lines, such as Ag II 243.78 nm and Ag II 224.36 nm, gave different dependence from the intensity of the atomic line: Their intensities had maximum values at a nitrogen pressure of 30 Pa when the helium pressure and the discharge voltage were kept at 2000 Pa and 1300 V. This effect is principally because the excitations of these ionic lines are caused by collisions of the second kind with helium excited species such as helium metastables and helium ion, which are quenched through collisions with nitrogen molecules added to the helium plasma. The sputtering rate could be controlled by adding small amounts of nitrogen to the helium plasma, whereas the cathode sputtering hardly occurs in the pure helium plasma.