Laser Doppler anemometry under plasma conditions. Part I. Measurements in a d.c. plasma jet

A study is presented on the use of laser Doppler anemometry (LDA) techniques for the measurement of the gas and particle velocities under plasma conditions. Experimental data is presented for a d.c. plasma jet in which alumina particles are injected under different operating conditions. The results reveal that the plasma velocity at the exit of the jet is of the order of 200–300 m/s. The intensity of turbulence is as high as 30 to 40% in the free shear layer and the particle velocity distribution is shown to be asymmetric, with particle dispersion in the plane of injection considerably more important than that in the perpendicular direction. The average particle velocity depends on the composition of the plasma gas, the torch current, and power.     

Parameters controlling the generation and properties of plasma sprayed zirconia coatings

D.C. plasma jets temperature and velocity distributions as well as the arc root fluctuations at the anode were studied for Ar-H₂ (25 vol%) plasma forming gases. The parameters were the arc current up to 700 A, the total gas flow rate up to 100 slm, and the nozzle diameter which was varied from 6 to 10 mm. The trajectories of partially stabilized zirconia particles into the jet were studied by a 2D laser imaging technique and two fast (100 ns) two color pyrometers. The results have revealed the difficulty to inject small particles into the plasma flow since most were found to by-pass the jet rather than penetrate it. The results also show the broad trajectory distribution within the jet and the influence of the arc root fluctuations on the mean particle trajectory distribution within the jet. Beside the measurements of the particle surface temperature and velocity distributions in flight, the particle flattening and the cooling of the resulting splats were studied statistically for single particles all over the spray cone. Such studies have emphasized the drastic influence of the substrates or previously deposited layers temperature on the contact between them and the splats. At 200–300°C this contact is excellent (cooling rates of the order of 100 K/μs for 1 μm thick splats) and it results in a columnar growth within the splats and the layered splats of a bead (up to 500 layered splats). This growth can be observed through passes provided the bead surface temperature has not cooled too much (a few tens of K) before the next bead covers it. A/C values up to 60 MPa were achieved with PSZ coatings. The effect of impact velocity of the particles, of substrate preheating temperature, of relative movments torch to substrate, of substrate oxidation on A/C values and splat formation were also studied.     

Effects of externally-through-internally-mixed soot inclusions within clouds and precipitation on global climate

This paper examines the incremental global climate response of black carbon (BC), the main component of soot, due to absorption and scattering by BC inclusions within cloud and precipitation particles. Modeled soot is emitted as an externally mixed aerosol particle. It evolves to an internal mixture through condensation, hydration, dissolution, dissociation, crystallization, aqueous chemistry, coagulation, and cloud processing. Size-resolved cloud liquid and ice particles grow by condensation onto size-resolved soot and other particles. Cloud particles grow to precipitation by coagulation and the Bergeron process. Cloud and precipitation particles also undergo freezing, melting, evaporation, sublimation, and coagulation with interstitial aerosol particles. Soot, which is tracked in cloud and precipitation particles of all sizes, is removed by rainout, washout, sedimentation, and dry deposition. Two methods of treating the optics of BC in size-resolved cloud liquid, ice and graupel are compared: the core-shell approximation (CSA) and the iterative dynamic effective medium approximation (DEMA). The 10-year global near-surface incremental temperature response due to fossil fuel (ff), biofuel (bf), and biomass burning (bb) BC within clouds with the DEMA was slightly stronger than that with the CSA, but both enhancements were <+0.05 K. The ff+bf portion may be approximately 60% of the total, suggesting that BC inclusions within clouds may enhance the near-surface temperature response of ff+bf soot due to all processes (estimated as approximately 0.27 K), by <10%, strengthening the possible climate impact of BC. BC cloud absorption was also found to increase water vapor, decrease precipitation, and decrease cloud fraction. The increase in water vapor at the expense of precipitation contributed to warming in addition to that of the cloud BC absorption itself. Aerosol-hydrometeor coagulation followed by hydrometeor evaporation may have caused almost twice the BC internal mixing as aerosol-aerosol coagulation.     

Numerical simulation of laser-induced breakdown spectroscopy: Modeling of aerosol analysis with finite diffusion and vaporization effects

The application of laser-induced breakdown spectroscopy (LIBS) to aerosol systems has been shown to provide quantitative analysis of particle-derived species; however, the exact nature of the plasma/particle interactions remains to be fully understood. Although the plasma/particle interaction may be idealized within a framework of instantaneous vaporization and analyte diffusion throughout the plasma volume, experimental evidence suggests that these processes actually occur on finite time scales relative to the plasma decay times at which measurements are frequently taken. In the present work, a numerical simulation of the temperature and species concentration fields of a plasma containing a single particle, including dissociation and diffusion on semi-empirical finite time scales, is developed. Using these results, the intensity of analyte emission is calculated as a function of time, and the standard ion/neutral ratios typical of aerosol-derived LIBS signals are calculated. Furthermore, the ratio of ion/neutral ratios for two different species was used to assess the temperature homogeneity of the particle-derived analytes in comparison to the overall plasma temperature field. From this numerical study, it is shown that the finite time scale of evaporation and diffusion of aerosol material results in a non-uniform spatial distribution in concentration. This results, in turn, in temperature and free electron density gradients within the plasma, leading to variation between the local conditions surrounding aerosol mass and the bulk conditions of the plasma as a whole.     

Powder Loading Effects of Yttria-Stabilized Zirconia in Atmospheric dc Plasma Spraying

Powder loading effects have been reexamined for various yttria-stabilized zirconia powders under atmospheric dc plasma spraying. A laser illumination method was utilized to observe powder injection into the plasma jet, while single particle and ensemble methods to measure particle state parameters. Statistical temperature distributions of in-flight particles suggested a rapid increase in the number of semi-molten particles above a certain powder loading rate. Despite drops in the particle temperature and velocity due to the powder loading effect, the deposition efficiency tends to have increased in some cases. Reliability of the single particle and ensemble methods has also been examined at various powder feed rates. Particle temperature measurement by the ensemble method at low powder feed rates could cause a significant error, which may affect powder injection optimization. Particle plume trajectory was not affected as much by the powder loading, which hence had only a limited effect on the particle diagnostics.     

Plasma Spray Process Operating Parameters Optimization Based on Artificial Intelligence

During plasma spray process, many intrinsic operating parameters allow tailoring in-flight particle characteristics (temperature and velocity) by controlling the plasma jet properties, thus affecting the final coating characteristics. Among them, plasma flow mass enthalpy, flow thermal conductivity, momentum density, etc. result from the selection of extrinsic operating parameters such as the plasma torch nozzle geometry, the composition and flow rate of plasma forming gases, the arc current intensity, beside the coupled relationships between those operating parameters make difficult in a full prediction of their effects on coating properties. Moreover, temporal fluctuations (anode wear for example) require “real time” corrections to maintain particle characteristic to targeted values. An expert system is built to optimize and control some of the main extrinsic operating parameters. This expert system includes two parts: (1) an artificial neural network (ANN) which predicts an extrinsic operating window and (2) a fuzzy logic controller (FLC) to control it. The paper details the general architecture of the system, discusses its limits and the typical characteristic times. The result shows that ANN can predict the characteristics of particles in-flight from coating porosity within maximal error 3 and 2 % in temperature and velocity respectively. And ANN also can predict the operating parameters from in-flight particle characteristics with maximal error 2.34, 4.80 and 8.66 % in current intensity, argon flow rate, and hydrogen flow rate respectively.     

Numerical Prediction of the Noise Emission from a Turbulent Argon Thermal-Plasma Jet Issuing into Ambient Air

This paper attempts to predict the noise emission characteristics of a turbulent argon thermal-plasma jet issuing into ambient air. The flow, temperature and concentration fields and turbulence characteristics of the turbulent plasma jet are computed at first, and then the noise emission from the plasma jet to a sideline far-field observer is calculated using the approach proposed by Fortuné and Gervais (AIAA J. 37(1999)1055) for predicting the noise emission from a turbulent, hot but non-ionized, air jet after some modification. The diffusion of ambient air into the turbulent argon plasma jet is handled using the turbulence-enhanced combined–diffusion-coefficient method. Velocity fluctuation correlations (aerodynamic noise source) in the plasma jet are calculated still using the K-ɛ two-equation turbulence model, but the temperature-velocity fluctuation correlations (entropic noise source) within the jet are calculated by solving a second-order turbulent Reynolds heat-flux transport equation in order to better deal with the contribution of temperature fluctuation to the noise emission. It is shown that among the contributions of aerodynamic noise source, entropic noise source and their mixed effect, the entropic noise source (i.e. the temperature-velocity fluctuation correlations) is dominant for the noise emission from the turbulent plasma jet to the sideline observer. The noise intensity increases with increasing plasma jet temperature or velocity. The predicted noise frequency spectrum characteristics and noise intensity levels are shown to be reasonably consistent with available experimental data.     

Influence of velocity and surface temperature of alumina particles on the properties of plasma sprayed coatings

In this paper are described the main characteristics of the plasma spraying process of alumina deposits, i.e., the temperature and flow field of the plasma jets obtained with the classical spraying torches, the injection of the particles into the plasma jet, the particle surface temperature and velocities in the plasma (measured for calibrated alumina particles), and the coating generation. The measurements on the alumina particles are compared with the predictions of a mathematical model. The experimental and computed particle velocities are in rather good agreement. However, this is not the case for the particle surface temperature. Possible reasons for the discrepancy are proposed (influence of the carrier gas, thermophoretic forces, and poor penetration of the particles into the plasma core even for an injection velocity twice that of the optimal calculated one, as shown by recent measurements). Finally the correlations between the particle velocities and surface temperature, and the properties of the alumina coating (porosity, crystal structure, mechanical properties) are studied.     

Excitation Temperature and Constituent Concentration Profiles of the Plasma Jet Under Plasma Spray-PVD Conditions

Plasma spray-physical vapor deposition (PS-PVD) is a promising technology to produce columnar structured thermal barrier coatings with excellent cyclic lifetime. The characteristics of plasma jets generated by standard plasma gases in the PS-PVD process, argon and helium, have been studied by optical emission spectroscopy. Abel inversion was introduced to reconstruct the spatial characteristics. In the central area of the plasma jet, the ionization of argon was found to be one of the reasons for low emission of atomic argon. Another reason could be the demixing so that helium prevails around the central axis of the plasma jet. The excitation temperature of argon was calculated by the Boltzmann plot method. Its values decreased from the center to the edge of the plasma jet. Applying the same method, a spurious high excitation temperature of helium was obtained, which could be caused by the strong deviation from local thermal equilibrium of helium. The addition of hydrogen into plasma gases leads to a lower excitation temperature, however a higher substrate temperature due to the high thermal conductivity induced by the dissociation of hydrogen. A loading effect is exerted by the feedstock powder on the plasma jet, which was found to reduce the average excitation temperature considerably by more than 700 K in the Ar/He jet.     

Particle Injection in Direct Current Air Plasma Spray: Salient Observations and Optimization Strategies

External injection of high-melting point low thermal conductivity ceramics orthogonal to a typical direct current thermal plasma jet plays a vital role in determining the in-flight state of the particles and the process downstream. The interactions between low density ceramic particles and high temperature plasma jet is quite complex, which influences the spray process and associated deposition. Detailed in-flight particle diagnostics as well as spray stream visualization have significantly enhanced our capability to diagnose and control the process. In this paper we present some salient observations on the role of key variables on particle injection. A number of experiments were conducted using a 7MB torch (Sulzer Metco, Westbury, NY) with both Ar–H₂ and N₂–H₂ plasma gases, where the carrier gas flow to inject Yttria Stabilized Zirconia (YSZ) was varied systematically and the resulting in-flight particle state was captured using an array of particle and spray stream sensors arranged in a 3D set-up. A notable observation is the existence of a “sweet-spot” in the plasma jet where the particle temperatures and velocities achieved a maximum. This sweet-spot can be characterized by the plume position (location of centroid of the spray stream) rather than carrier gas flow rate and is independent of primary gas flows and other process/material conditions. This result suggests a possible approach to optimize particle injection independent of plasma-forming-torch-parameters. Controlling particle injection at this sweet-spot has shown to benefit the overall process efficiency (in terms of melting) and process reliability (both in-flight measurement and coating build-up) with concomitant application benefits.     

Effects of charge and size on condensation of supersaturated water vapor on nanoparticles of SiO2

The effects of size and charge on the condensation of a supersaturated water vapor on monodisperse nanoparticles of SiO(2) were investigated in a flow cloud chamber. The dependences of the critical supersaturation S(cr) on particle size at diameters of 10, 12, and 15 nm as well as on charge and charge polarity are determined experimentally. A novel electrospray aerosol generator was developed to generate a high concentration of SiO(2) nanoparticles of less than 10 nm by electrospraying silicon tetraethoxide (STE) ethanol solution followed by the thermal decomposition of STE. The effects of liquid flow rate, liquid concentration, flow rate of carrier gas, and liquid conductivity on the particle size distribution and concentration were examined. For charged particles, the nucleation occurs at a critical supersaturation S(cr) lower than that on neutral particles, and the charge effect fades away as particle size increases. The charge effect is stronger than the theoretical predictions. In addition, a sign preference is detected, i.e., water vapor condenses more readily on negatively charged particle, a trend consistent with those observed on ions. However, both effects of charge and charge polarity on S(cr) are stronger than that predicted by Volmer's theory for ion-induced nucleation.     

The stage of nonisothermal nucleation of supercritical particles of a new phase under nonstationary conditions of particle diffusion growth and heat transfer to a medium

An analytical theory has been formulated for the stage of nonisothermal nucleation of supercritical particles in a metastable medium with instantaneously generated initial supersaturation. The theory takes into account the nonuniformities of metastable substance concentration and temperature, which result from the nonstationary diffusion of the substance to growing particles and the nonstationary transfer of the heat of the phase transition from the particles to the medium. The formulated theory extends the approach based on the concept of excluded volume that has recently been used in the theory of the stage of nucleation under isothermal conditions. This approach implies that the nucleation intensity of new particles is suppressed in spherical diffusion regions with certain sizes that surround previously nucleated supercritical particles and remaining unchanged in the rest of the medium. It has been shown that, when self-similar solutions are used for nonstationary equations of substance diffusion to particles and heat transfer from the particles, the ratio between the excluded volume and the particle volume is independent of particle size, thereby enabling one to analytically solve the integral equation for the excluded volume throughout a system as a time function at the stage of nucleation. The main characteristics of the phase transition have been found for the end of the stage of nucleation. Comparison has been carried out with the characteristics obtained in terms of the isothermal and nonisothermal nucleation theory upon uniform vapor consumption and heat dissipation (the mean-field approximation of vapor supersaturation and temperature).     

Application of fluorescence correlation spectroscopy: a study of flocculation of rigid rod-like biopolymer (schizophyllan) and colloidal particles

The flocculation between the rod-like biopolymer Schizophyllan and two types of colloidal particles (latex with diameter 40 nm and alumina with diameter 60 nm) has been investigated by means of fluorescence correlation spectroscopy (FCS). The concentration ratio of Schizophyllan/particle q was varied in the range 0.1 approximately 20. Under conditions of pH about 5.7, 1 mmol.L(-1) NaCl, and room temperature (22+/-0.5 degrees C), the particles are strongly charged (alumina particles positively charged, latex negatively), while Schizophyllan is neutral. We observed that Schizophyllan chains flocculate with both types of particles, which suggests that the charge neutralization does not play a decisive role in these interactions. The ratio of fluorescence intensity of one floc over that of one particle, Q(f)/Q(p), and the corresponding hydrodynamic radius (r(h)) of the flocs have been measured. For a Schizophyllan-latex system, Q(f)/Q(p) reached a maximum value of 5 for q=3 indicating that the flocs contained five particles on average. The corresponding value of r(h) was r(h)=455 nm. The flocculation kinetic of latex particles with Schizophyllan was too fast to be measurable by FCS. For the Schizophyllan-alumina system, Q(f)/Q(p) was stable at about 1 in the whole studied range of q but r(h) increased with q suggesting that many Schizophyllan chains are adsorbed on individual particles. The flocculation kinetic of this system was studied by FCS and the obtained results were compatible with those of photon correlation spectroscopy.     

Thermal oxidation of 6 nm aerosolized silicon nanoparticles: size and surface chemistry changes

The earliest stages of thermal oxidation of 6 nm diameter silicon nanoparticles by molecular oxygen are examined using a tandem differential mobility analysis (TDMA) apparatus, Fourier-transform infrared (FTIR) spectroscopy, time-of-flight secondary ion mass spectroscopy (ToF-SIMS), and X-ray photoelectron spectroscopy (XPS). Particles are synthesized in and then extracted from a nonthermal RF plasma operating at approximately 20 Torr into the atmospheric pressure TDMA apparatus. The TDMA apparatus was used to measure oxidation-induced size changes over a broad range of temperature settings and N2-O2 carrier gas composition. Surface chemistry changes are evaluated in situ with an FTIR spectrometer and a hybrid flow-through cell, and ex situ with ToF-SIMS and XPS. Particle size measurements show that, at temperatures less than approximately 500 degrees C, particles shrink regardless of the carrier gas oxygen concentration, while FTIR and ToF-SIMS spectra demonstrate a loss of hydrogen from the particles and minimal oxide formation. At higher temperatures, FTIR and XPS spectra indicate that an oxide forms which tends toward, but does not fully reach, stoichiometric SiO2 with increasing temperature. Between 500 and 800 degrees C, size measurements show a small increase in particle diameter with increasing carrier gas oxygen content and temperature. Above 800 degrees C, particle growth rapidly reaches a plateau while FTIR and XPS spectra change little. ToF-SIMS signals associated with O-Si species also show an increase in intensity at 800 degrees C.     

A model for ultrafine powder production in a plasma reactor

A model is proposed for the analysis of the production of ultrafine particles in thermal plasma reactors. The model initially solves the fluid flow, temperature, and concentration fields using a classical control volume approach. The nucleation and growth of ultra fine particles are then solved along each streamline. The evolution of the particle distribution is described by a statistical approach, using the first moments of the distribution as the dependent variables. Brownian coalescence is considered in the free molecular regime. In the discussion, the model is used to demonstrate the effects of some important parameters, such as the initial concentration of metal vapor, its radial distribution, and the radial injection of a cooling gas, on the particle size distribution.     

In-flight particle diagnostics in induction plasma processing

A laser Doppler anemometer combined with a particle-emission spectrometer, are used for the study of the induction plasma spraying process. For this, the effects of chamber pressure, spray distance and torch nozzle design on the particle surface temperature and velocity as well as the fraction of hot particles included in the stream of processed material, were investigated. A comparison between the velocity measurements by laser Doppler anemometry (LDA) and by the particle time-of-flight technique is presented in order to emphasize the deference between the velocity of the hot particles, and that of the total particle population, cold and hot. The influence of the individual particle mass on particle entrainment in the plasma jet from the ambient atmosphere in the vacuum chamber is discussed.     

Electrization of nanocrystals in flows of supersaturated vapors

It is found that nano- and microcrystals of a number of compounds grown in ion-free vapor of high supersaturation acquire electric charge. The electrophoretic mobility of these crystals changes, as is demonstrated by the data concerning aerosols formed in cooling the vapor above boiling melts of a number of substances (boiling temperature, 500?C1800 K) placed into a 300 K carrier gas. Nano- and microparticles assembled into aggregates are found in flows of the cooled vapor. It is found that the application of an electric field with an intensity of 150?C1000 V/cm induces the movement of particles toward the negatively charged electrode at a rate of 5?C15 cm/s, the speed of their movement being related linearly to the field intensity and corresponding to the presence of ??10³ charge carriers per every particle. It is established that the experimental data are in accordance with the assumption that the growth of each particle results in the decomposition of vapor molecules adsorbed by the particle into ions with the nonequivalent liberation of cations and anions into vapor; i.e., an adsorption-ionization-desorption route for the charging of growing particles is discovered.     

A spectroscopic study of mercury vapor adsorption on gold nanoparticle films

We have modified the surfaces of glass and Si(100) with 3-aminopropyltrimethoxy silane, a fourth generation amine-terminated poly(amidoamine) dendrimer, and poly(diallydimethyl ammonium chloride) to facilitate adsorption onto colloidal gold particles (average diameter 3, 5, 12, and 22 nm). UV-vis absorption spectroscopy and atomic force microscopy monitored the adsorption process, which is governed by particle diffusion to the surface. The differences in adsorption to the three adhesion layers as a function of pH are discussed. Mercury vapor was exposed to the gold particle films and quantified by X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy. The surface plasmon oscillation of 5-, 12-, and 22-nm particles blue-shifts after exposure to parts-per-million levels of mercury vapor in air. Particle films prepared from the 3-nm gold particles develop a broad peak centered near 530 nm after exposure to mercury vapor. The results demonstrate a novel "litmus" film for mercury vapor.     

Surface Modification of Poly-ε-Caprolactone with an Atmospheric Pressure Plasma Jet

In this work, poly-ε-caprolactone samples are modified by an atmospheric pressure plasma jet in pure argon and argon/water vapour mixtures. In a first part of the paper, the chemical species present in the plasma jet are identified by optical emission spectroscopy and it was found that plasmas generated in argon/0.05 % water vapour mixtures show the highest emission intensity of OH (A–X) at 308 nm. In a subsequent section, plasma jet surface treatments in argon and argon/water vapour mixtures have been investigated using contact angle measurements and X-ray photoelectron spectroscopy. The polymer samples modified with the plasma jet show a significant decrease in water contact angle due to the incorporation of oxygen-containing groups, such as C–O, C=O and O–C=O. The most efficient oxygen inclusion was however found when 0.05 % of water vapour is added to the argon feeding gas, which correlates with the highest intensity of OH (X) radicals. By optimizing the OH (X) radical yield in the plasma jet, the highest polymer modification efficiency can thus be obtained.