Volume 77, Issue 2

Laser spectroscopic electric field measurements have the potential to become a versatile tool for the diagnostics of low-temperature plasmas. From the spatially and temporally resolved field distribution in the sheath close to electrodes or surfaces in general, a broad range of important plasma parameters can be inferred directly: electron temperature; ion density distribution; displacement-, ion-, electron-diffusion current density; and the sheath potential. Indirectly, the electron and ion energy distribution functions and information on the ion dynamics in the sheath can also be obtained. Finally, measurements in the quasi-neutral bulk can also reveal even the plasma density distribution with high spatial and temporal resolution. The basic concepts for analysis of the field data are introduced and demonstrated by examples in hydrogen discharges.

A 3D model for the simulation of inductively coupled plasma torches (ICPTs) working at atmospheric pressure is presented, using a customized version of the computational fluid dynamics (CFD) commercial code FLUENT ® . The induction coil is taken into account in its actual 3D shape, showing the effects on the plasma discharge of removing the axisymmetric hypothesis of simplification, which characterizes 2D approaches. Steady flow and energy equations are solved for optically thin argon plasmas under the assumptions of local thermodynamic equilibrium (LTE) and laminar flow. The electromagnetic field equations are solved on an extended grid in the vector potential form. In order to evaluate the importance of various 3D effects on calculated plasma temperature and velocity fields, comparisons of our new results with the ones obtainable from 2D models and from an improved 2D model that includes 3D coil effects are presented. 3D results are shown for torches working under various geometric and operating conditions and with different coil shapes, including conventional helicoidal, as well as planar, elliptical, and double-stage configurations.

Nontransferred direct current (dc) laminar plasma jets of pure argon were generated at atmospheric pressure, with a generator having an interelectrode insert. Associated with the experimental investigation, similarity theory was adopted to examine the arc voltage characteristics, thermal efficiency, and jet length change of the laminar plasma. Jet flow temperature and velocity were evaluated by various methods. The jet shows good stability, reproducibility, and regular flow field change as functions of generating parameters. Applications of laminar plasma jets for ZrO 2 ceramics spray coating and remelt strengthening of metal surface were attempted. The results indicated favorite process efficiency and controllability of the laminar plasma heating.

The effect of various process parameters on the deposition rate of microcrystalline hydrogenated silicon is presented. The various pathways leading to high deposition rates involve the optimization of a combination of parameters, while maintaining the crystalline character of the film and avoiding the presence of particles. The deposition rate increases in conditions that enhance and at the same time “cool down” the electron population. Such conditions are: moderately higher frequencies, higher pressures, and the presence of larger molecules (like disilane) even in small quantities. There is an optimum pressure, determined by primary dissociation, for a certain silane fraction. In addition, silane fraction and power density must also increase up to the limit of transition to amorphous growth and before attachment becomes too important. In all of these cases, there is a need for optimizing the distance of the deposition substrate to the source of radical generation, especially at higher pressures, to further increase the already important contribution of higher silicon radicals. It is shown that similar deposition rates can be obtained via radical fluxes with very different compositions. In every case, there is a need for sufficient H atom fluxes to ensure crystalline growth.

H-assisted plasma chemical vapor deposition (HAPCVD) realizes control of deposition profile of Cu in trenches. The key to the control is ion irradiation to surfaces. With increasing the flux and energy of ions, the profile changes from conformal to subconformal and then to an anisotropic one, for which Cu material is filled from the bottom of the trench without deposition on the sidewall. H 3 + and ArH + are identified as the major ionic species which contribute to the control, and hence the deposition profile also depends on a ratio R  = H 2 /(Ar + H 2 ).

Nanostructured polytetrafluoroethylene (PTFE)-like thin films can be deposited, in certain experimental conditions, by modulated discharges fed with tetrafluoroethylene (TFE). These coatings are characterized by a unique morphology consisting of highly twisted micron-long ribbons, which leads to an extremely high water repellency of the surface. In the present work, the diagnostics of the plasma phase is presented, coupled with that of the coating, in order to understand the film growth mechanism in different discharge regimes. When the duty cycle (DC) is increased in modulated C 2 F 4 plasmas, the monomer depletion increases, too, and many recombination reactions take place at progressively higher rates, resulting in the formation of CF 4 , C 2 F 6 , C 3 F 6 , C 3 F 8 , and C 4 F 10 ; the formation of powders in the homogeneous phase, however, was never evidenced. The modulation of C 2 F 4 plasmas strongly affects the morphology of the resulting coating, as revealed by atomic force microscopy (AFM), ranging from bumpy to ribbon-like structures. The latter, moreover, are found to be more PTFE-like with respect to the remaining part of the film. In the last part of the paper, a deposition mechanism is proposed, where low radical densities in the plasma and surface migration of the precursors are the keys for the growth of ribbon-like structures.

Low-pressure plasmas offer a unique possibility of confinement, control, and fine tailoring of particle properties. Hence, dusty plasmas have grown into a vast field, and new applications of plasma-processed dust particles are emerging. During the deposition of thin amorphous films onto melamine formaldehyde (MF) microparticles in a C 2 H 2 plasma, the generation of nanosized carbon particles was also studied. The size distribution of those particles is quite uniform. In another experiment, the stability of luminophore grains could be improved by coating with protective Al 2 O 3 films that are deposited by a plasma-enhanced chemical vapor deposition (PECVD) process using a metal-organic precursor gas. Coating of SiO 2 microparticles with thin metal layers by magnetron sputtering is also described. Especially the interaction of the microsized grains confined in a radio frequency (rf) plasma with the dc magnetron discharge during deposition was investigated. The observations emphasize that the interaction between magnetron plasma and injected microdisperse powder particles can also be used as a diagnostic tool for the characterization of magnetron sputter sources.

Many kinds of both metallic and ceramic powder particles were plasma-sprayed onto the mirror-polished metallic substrate surface, and the effect of both substrate temperature and ambient pressure on the flattening behavior of the particle was systematically investigated. In the flattening behavior of the sprayed particle onto the substrate surface, critical conditions were recognized both in the substrate temperature and ambient pressure. That is, the flattening behavior changed transitionally on that critical temperature and pressure range, respectively. We defined and introduced a transition temperature, T t , and transition pressure, P t , respectively, for those critical conditions. The role of the related factors (such as solidification of the bottom surface of the splat, desorption of adsorbates on the substrate surface and wetting at interface) on the transition behavior in the flattening was clarified from several points of views. The fact that the dependence both of transition temperature and transition pressure on the sprayed particle material had a similar tendency indicated that the wetting of the substrate by the molten particles seemed to be a domination in the flattening. A three-dimensional transition map by combining both transition temperature and transition pressure dependence was proposed as a controlling principle of the thermal spray process.

Advanced coatings deposited by plasma spraying are used in a large variety of industrial applications. The sprayed coatings are employed typically in industry to protect parts from severe operating conditions or to produce surfaces with specific functions. Applications are found in many industrial sectors such as aerospace, automobile, energy generation, and biomedical implants. Coatings are built by the successive deposition of molten or partially molten particles that flatten and solidify upon contact on the substrate, forming lamellae. The coating properties are intimately linked to the properties of these lamellae, which in turn depend on in-flight particle properties as well as substrate temperature during spraying. Consequently, the development of diagnostic tools for monitoring and controlling these spray parameters will help provide the necessary information to study the coating formation process, optimize the coating properties, and, eventually, control the spray process in production. In this paper, a review of some recent developments of optical diagnostic techniques applied to monitor plasma-sprayed particles is presented. In the first part of the paper, two different sensing techniques for in-flight particle measurement are described. First, time-resolved diagnostics on individual particles is described. This technique is used to study the instabilities of the particle characteristics associated with the plasma fluctuations. Secondly, a technique adapted for use in an industrial production environment for measuring the particle jet characteristics as an ensemble is presented. In the second part of the paper, the use of an optical system to study the influence of the substrate temperature on the flattening and solidification of sprayed particles impacting on a flat substrate is described. The last part of this paper describes the optimization of nanostructured coatings based on a tight control of the temperature and velocity of the plasma-sprayed particles.

This work deals with the potentialities of mercury-free mixtures for the efficient operation of luminous signs used for publicity lighting. The experimental study of rare-gas-based, cold hollow cathode, low-pressure luminescent discharges is reported. Electrical and spectroscopic measurements are presented with a special emphasis on the radiation properties of the positive column. The best mixtures for phosphor excitation are described in terms of composition and pressure together with their sensitivity to potential fluctuations of the operating conditions during sign lifetime. Aging studies are presented, showing evidence of a strong trade-off between the output power and the sign lifetime. The selective trapping of the heavier gas present in the gas mixture is presented and proven to be at present the main reason for the sign lifetime limitation. The limitation of commercial (sinusoidal or square waveform) ballasts for the excitation of such mercury-free discharge is presented. Finally, preliminary measurements, performed using a lab-developed short rise time and low duty-cycle driver, are reported, showing a significant increase of the illuminance and efficiency (up to a factor three) of mercury-free signs as compared to the values obtained with conventional excitation.

This paper is devoted to the presentation of our actual knowledge in plasma spraying. It presents successively: the parameters controlling the impact of the molten particles onto the substrate and resulting splat formation followed by splats layering and coating formation; the engineering of nano- or finely structured coatings with different possible routes; and the actual possibilities for the on-line control of the spray process.

The dielectric barrier discharge (DBD) has a number of industrial applications and has been a subject of research for many years. Many studies have been carried out to understand the underlying DBD physics. Despite the fact that much progress has been made, some important issues are still far from being clear. In this work, we summarize the basics of DBD physics and introduce innovative concepts of discharge behavior that were discovered recently.

Rules are pointed out to protect the magnetochemist from pitfalls in both measurement and interpretation of magnetic data. Carefully chosen magnetic field strengths during magnetic susceptibility measurements guarantee the recording of genuine data. With the help of examples, the effect of too strong applied fields is demonstrated producing magnetic saturation and, for example, quenching of weak ferro- or antiferromagnetic spin–spin couplings. In consequence, the data run the risk of being misinterpreted unless model susceptibility equations are applied that take the field dependence of χ m into consideration. Recommendations are given for the presentation of experimental and theoretical data. The limited applicability of the most overworked formula in paramagnetism, the Curie–Weiss law χ m = C /( T – θ ), is clearly presented (magnetically condensed systems, pure spin magnetism). While rough and ready susceptibility formulae are applicable to specific 3d and 4f systems, the complex situation for the remaining d and f centers, including actinides, demands computer programs which consider simultaneously interelectronic repulsion, ligand field potential, spin-orbit coupling, interatomic exchange interactions, and applied magnetic field.