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.     

Infrared dielectric mirrors based on thin film multilayers of polystyrene and polyvinylpyrrolidone

Thin film polymer multilayers were prepared by spin coating alternating layers of polystyrene and polyvinylpyrrolidone. Samples with 10, 20, 30, 40, and 50 layers were prepared with individual layer thickness values in the range 223–508 nm. These samples were measured using a Fourier transform infrared spectrometer and were found to display narrow photonic band gaps (～ 0.04 to 0.06 μm wide) in their spectral response over the wavelength range 1.6 ? 2.6 μm. The position of the photonic band gaps was controlled by varying the thickness of the individual layers within the multilayer structures. This was achieved by varying the spin speed used during the deposition of the polymer layers. The peak reflectance of the multilayers was controlled by varying the number of layers within the multilayer samples giving values in the range 20–80% (corresponding to transmittances of 80–20%). Calculated transmittance spectra were also obtained using an optical transfer matrix method. These calculated spectra were shown to be in good agreement with the experimental data obtained. These experiments demonstrate a facile approach to the production of low cost dielectric mirrors that have tailored photonic properties over a range of wavelengths that are currently important for applications in fibre optic based telecommunications. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Optimization of the Thermal Radiation Extinction of Silicon Carbide in a Silica Powder Matrix

In order to optimize the infrared extinction of a SiC-powder in a silica powder matrix, Mie scattering calculations for spherical SiC-particles have been performed. A single oscillator-model was applied to calculate the optical constants of SiC. Taking into account the particle size distribution of a commercially available SiC-powder, its wavelength dependent extinction coefficient was calculated. The result is in very good agreement with the extinction spectrum of the powder derived by infrared optically measurements. Mie scattering theory also was used to find the optimum mean SiC-particle diameter of a mixture of 20% SiC-powder and 80% silica powder. This revised version was published online in July 2006 with corrections to the Cover Date.     

Calculation of the transport and relaxation properties of methane. I. Shear viscosity, viscomagnetic effects, and self-diffusion

Transport properties of pure methane gas have been calculated in the rigid-rotor approximation using the recently proposed intermolecular potential energy hypersurface [R. Hellmann et al., J. Chem. Phys. 128, 214303 (2008)] and the classical-trajectory method. Results are reported in the dilute-gas limit for shear viscosity, viscomagnetic coefficients, and self-diffusion in the temperature range of 80-1500 K. Compared with the best measurements, the calculated viscosity values are about 0.5% too high at room temperature, although the temperature dependence of the calculated values is in very good agreement with experiment between 210 and 390 K. For the shear viscosity, the calculations indicate that the corrections in the second-order approximation and those due to the angular-momentum polarization are small, less than 0.7%, in the temperature range considered. The very good agreement of the calculated values with the experimental viscosity data suggests that the rigid-rotor approximation should be very reasonable for the three properties considered. In general, the agreement for the other measured properties is within the experimental error.     

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.     

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.     

Effect of particle charging on momentum and heat transfer from rarefied plasma flow

The features of interaction of a spherical metallic particle with a rarefied thermal plasma flow due to the presence o charges-electrons and ions in the gaseous phase-are considered. Analytical expressions describing charge, momentum, and energy exchange between the plasma and the particle für the cases of strong and weak Debye screening are obtained. It is illustrated that the efficiency of particle heating in the plasma considerably grows as compared with a hot molecular gas due to participation of electrons and ions in file transfer processes.     

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.     

Particle dynamics and particle heat and mass transfer in thermal plasmas. Part II. Particle heat and mass transfer in thermal plasmas

This paper is concerned with a review of heat and mass transfer between thermal plasmas and particulate matter. In this situation various effects which are not present in ordinary heat and mass transfer have to be considered, including unsteady conditions, modified convective heat transfer due to strongly varying plasma properties, radiation, internal conduction, particle shape, vaporization and evaporation, noncontinuum conditions, and particle charging. The results indicate that (i) convective heat transfer coefficients have to be modified due to strongly varying plasma properties; (ii) vaporization, defined as a mass transfer process corresponding to particle surface temperatures below the boiling point, describes a different particle heating history than that of the evaporation process which, however, is not a critical control mechanism for interphase mass transfer of particles injected into thermal plasmas; (iii) particle heat transfer under noncontinuum conditions is governed by individual contributions from the species in the plasma (electrons, ions, neutral species) and by particle charging effects.     

Fluctuation Flows in Random Fibrous Media and Their Influence on the Efficiency of Aerosol Particle Deposition

The distribution function of random velocities in the fibrous medium of parallel fibers and the generating functional of correlation functions in a 3D fibrous medium are calculated. It is shown that the velocity fluctuations are Gaussian in the rarefied medium at a small packing density. The influence of 3D fluctuation hydrodynamic gas flows on the efficiency of aerosol particle deposition is studied.     

Transport phenomena in bubble column reactors I: Flow pattern

A new model has been developed for the liquid phase flow pattern in bubble columns. This model satisfies the mass, momentum and energy balances for the gas and liquid phases. Further, it takes into account the variations in the flow pattern due to changes in the physical properties of the liquid phase. The model can be used for batch as well as continuous (co-current and counter-current) modes of operation. A satisfactory agreement is shown between model predictions and experimental observations. A simplified procedure is suggested for quick estimation of the liquid phase velocity profile. The proposed model is expected to be useful for the rational prediction of transfer coefficients for momentum, mass and heat. The prediction and measurement of the pressure drop is discussed in the companion paper.     

Improvement of plasma spraying efficiency and coating quality

A commercial torch has been modified to introduce an additional anti-vortex and shroud gas flow to counter the detrimental effects brought about by the vortex plasma gas flow which is used to stabilize the cathode arc attachment and to increase the anode life. Deposition efficiency and coating quality are used as criteria to judge the modified versus the nonmodified torch. High-speed videography and computerized image analysis systems are used to determine the particle trajectories, velocities, and the plasma jet geometry. The results show that the additional anti-vortex and shroud gas flow to the torch can keep the particles closer to the torch axis and reduce the amount of entrainment of cold air into the plasma jet. The consequence is that deposition efficiency and coating quality are substantially improved.     

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.     

On the friction coefficient of straight-chain aggregates

A methodology to calculate the friction coefficient of an aggregate in the continuum regime is proposed. The friction coefficient and the monomer shielding factors, aggregate-average or individual, are related to the molecule-aggregate collision rate that is obtained from the molecular diffusion equation with an absorbing boundary condition on the aggregate surface. Calculated friction coefficients of straight chains are in very good agreement with previous results, suggesting that the friction coefficients may be accurately calculated from the product of the collision rate and an average momentum transfer, the latter being independent of aggregate morphology. Langevin-dynamics simulations show that the diffusive motion of straight-chain aggregates may be described either by a monomer-dependent or an aggregate-average random force, if the shielding factors are appropriately chosen.     

Atmospheric Plasma Spraying Evolution Since the Sixties Through Modeling,Measurements and Sensors

This paper presents, through examples, the evolutions of atmospheric plasma spraying since the sixties. The drastic improvement of the spray conditions and coatings reproducibility during more than 50 years was linked both to researches in laboratories and developments of spray equipment’s (plasma torches, computerized control panels, robots to spray coatings on complex parts, sensors working in the harsh environment of spray booths…). This evolution is illustrated through the following topics: (1) plasma forming gas thermodynamic and transport properties either at local thermodynamic equilibrium or more recently at two temperatures; (2) evolution of plasma spray torches since the nineties; (3) plasma jet and in-flight particle measurements with laboratory equipment’s and then sensors in spray booths; (4) plasma jets and torches modeling as well as heat and momentum transfer to particles; (5) splats formation and layering.     

Modelling particle formation and growth in a plasma synthesis reactor

A model for particle nucleation and growth in a thermal plasma reactor is discussed. A nondimensional form of the aerosol general dynamic equation is derived under a set of simplifying assumptions which are appropriate to plasma powder synthesis, and the resulting set of equations is solved numerically. The results are converted to dimensional form for the case of iron powder, for which experimental data are available, and for silicon carbide. Calculated particle sizes increase significantly with increasing reactant concentrations and with decreasing cooling rate, although the influence of cooling rate is mainly a residence time effect.     

An overall mechanism for the deposition of plasma polymers from methane in a low-pressure argon plasma jet

An overall mechanism for plasma polymer deposition from a methane-seeded argon plasma jet was established from experimental measurements and a simplified model of reaction kinetics within the plasma jet. Total mass deposition rates were obtained at various substrate positions and methane flow rates. Methane consumption was estimated from residual gas analysis. The influence of substrate coolant temperature on deposition rate was evaluated. The model was based on particle densities, jet temperature, and jet velocity data published previously, and reaction rate constants from the literature were used. No adjustable parameters were employed in this model. Experimental results for total deposition rate and methane consumption were in good agreement with model predictions. The overall deposition mechanism consists of three steps: Penning ionization of methane by excited argon neutrals, followed by dissociative recombination of CN _x ⁺ to yield CH, followed by incorporation of CH into the growing film upon impact. Contributions of species other than CH to the total deposition rate are minor, and adsorption is not a prerequisite for incorporation into the growing film.     

Comparison of calculated and experimental flow velocities upstream from the sampling cone of an inductively coupled plasma mass spectrometer

Experimentally-determined flow velocities in the 6 mm upstream from the sampling cone of an inductively coupled plasma mass spectrometer were compared with velocities determined from a computer simulation of the flow and those calculated from a modified hemispherical sink model. The measured values and those from the simulation agreed within experimental error, but differed from the values calculated from the modified hemispherical sink model by as much as 30%. An empirical alternative to the modified hemispherical sink model is presented that allows for accurate calculation of flow properties upstream from the sampling cone under a range of plasma conditions.     

The self-preserving size distribution theory. II. Comparison with experimental results for Si and Si3N4 aerosols

The gas to particle synthesis route is a relatively clean and efficient manner for the production of high-quality ceramic powders. These powders can be subsequently sintered in any wanted shape. The modeling of these production systems is difficult because several mechanisms occur in parallel. From theoretical considerations it can be determined, however, that coagulation and sintering are dominant mechanisms as far as shape and size of the particles are considered. In part I of this article an extensive theoretical analysis was given on the self-preserving size distribution theory for power law particles. In this second part, cumulative particle size distributions of silicon and silicon nitride agglomerates, produced in a laser reactor, were determined from TEM pictures and compared to the distributions calculated from this self-preserving theory for power law particles. The calculated distributions were in fair agreement with the measured results, especially at the high end of the distributions. Calculated and measured particle growth rates were also in fair agreement. Using the self-preserving theory an analysis was made on the distribution of annealed silicon agglomerates, of interest in applications to nanoparticle technology.