A Proposed Process Control Chart for DC Plasma Spraying Process. Part II. Experimental Verification for Spraying Alumina

The role of particle injection velocity in influencing the nature of alumina coatings obtained by plasma spraying was studied. Previously reported process chart obtained by computational fluid dynamics (CFD) study on the particle states of alumina with respect to particle injection velocity and size was verified experimentally. For this purpose, alumina particles of three different size ranges with a mean size of 25, 40, and 76 m were subjected to different injection velocities. The coating obtained was analyzed for cross-sectional microstructure and thickness by optical microscopy. In addition, the role of particle injection velocity and size in influencing the coating-deposition efficiency was studied. The experimental results agreed well with the CFD results, which had indicated the dependence of particle trajectory in the plasma plume on the particle injection velocity and size leading to the changes in the extent of melting. While a higher coating thickness and deposition efficiency was obtained with 25-m particles, with further increase in particle size, a reverse trend was observed. This was attributed to the changes in heat-transfer characteristics of the particles with size, which governed the coating buildup and deposition efficiency.     

Particle velocity measurements in induction plasma spraying

Laser Dopple anemometry (LDA) measurements of the particle velocity are carried out during an induction plasma spraying operation. The velocity of nickel alloy particles, or molten droplets, at the exit of an induction plasma torch prior to impact on the substrate is shown to vary with the plasma and powder injection conditions. Plasma spraying under soft vacuum (150–450 Torr) gives rise to substantially higher particle velocities (40–60 m/sec) compared to those attained at atmospheric pressure (10–20 m/sec).     

Temperature Distribution in a Pilot Plasma Tundish: Comparison Between Plasma Torch and Graphite Electrode Systems

Arc, bath, and refractory wall temperatures are measured in a pilot transferred-arc plasma furnace by atomic emission spectroscopy (AES) and multiwavelength pyrometry. Argon plasma torch and graphite electrode with nitrogen as plasma gas are both examined and compared using the steel bath as anode. With argon, a two-slope characteristic curve is measured for arc temperature, which ranges from 9000 to 25,000 K. Another trend is observed with nitrogen for temperatures in the range 8000–12,000 K. In this latter case, the bath temperature is very sensitive to arc length: more than 100 K increase results in arc length rise from 50 to 150 mm. Experimental data shows the variation of heat transfer efficiency between the two configurations, which is supported by results about surface emissivity in the spectral domain 1–15 m.     

Experimental and Theoretical Study of the Impact of Alumina Droplets on Cold and Hot Substrates

A complex experimental set-up was built to study the impact of liquid alumina droplets on different substrates (stainless steel 304L, sintered alumina, carbon–carbon) kept at temperatures up to 2100 K. The impact behavior: rebound, deposition, splashing, spattering was systematically studied as well as the resulting splat shapes. The set-up consists in a controlled atmosphere chamber where molten alumina particles with diameters between 10 and 90 m, are produced by a d.c. plasma torch, substrates being heated by a second d.c. plasma torch. In such conditions, it was possible to achieve particle temperatures between 2300 and 4200 K with velocities in the range 50 to 300 m/s. The particle behavior at impact was characterized by the Sommerfeld parameter K (K=We^1/2 Re^1/4 We and Re being respectively the Weber and Reynolds numbers of impacting particles). It was possible to vary K between 3 and 1300. Low K values were obtained by tilting the substrate up to 60°. The parameters of a single particle at impact were measured: its velocity v_p and diameter d_p by Phase Doppler Anemometry (v_p=5%, d_p=10%) and its temperature T_p by fast (100 ns) two color pyrometry (T_p=15%). The particle impact was visualized by a fast camera coupled to a microscope (exposure delay time 50 ns . . .100 ms) with complex synchronization and light intensity problems. To solve the latter, the impacting particle had to be illuminated with a 2 W c.w. Ar⁺ laser at 488 nm. Unfortunately, the controlled atmosphere chamber did not allow to change the substrate after each particle impact. Starting from a smooth surface for the first impact, due to the successively deposited splats, rapidly droplets impacted on a rough surface (Ra5 m). For splats collected on a hot alumina substrate (2100 K), where flattening is completed before solidification starts (case similar to that of ethanol droplets on cold copper) deposition occurs for K between 4 and 90 while splashing occurs for K as low as 30. These results are slightly different from those related to the ethanol droplet for which deposition occurs for 357.7. This could be due to the precision of measured values and the rough surface. For splats collected in spraying conditions splashing is always the rule K values up to 1400) especially on rough surfaces. However the particle impact velocity and temperature, the substrate temperature and tilting plays an important role on the resulting splat diameters, distortion and elongation rates. The question which is still pending is which quantity of splashed material is incorporated within the constructing coating and how does it affect its thermophysical properties.     

Experimental Testing of a Curvilinear Gas Shroud Nozzle for Improved Plasma Spraying

The performance of a new gas shroud nozzle attachment for plasma spraying was tested experimentally using particle diagnostics, flow visualization, and coating characterization techniques. A nozzle attachment with curvilinear inner walls was tested and compared with a commercially available conical nozzle. Particle temperatures were measured with a high-speed ratio pyrometer and particle velocities were measured with an intensified camera and a two-laser illumination system. Flow visualization was performed by seeding the surrounding air with smoke. Particle temperatures at the spraying distance were 300 K higher with the curvilinear insert. The plasma jet was narrower but the particle velocity distribution at the spraying distance was unchanged. Higher temperatures and improved particle melting with the curvilinear insert resulted in a reduction in coating porosity (from 7.0 to 7.2 to 4.5–5.1%) and an increase in coating adhesion strength (from 27.2 to 42 MPa). Shrouding as injected through a circular slot around the nozzle exit was also seen to provide better protection than gas injected with the standard sixteen-port configuration.     

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.     

A Proposed Process Control Chart for DC Plasma Spraying Process

A process control chart is proposed for DC plasma spraying process based on the in-flight simulation of the injected states of the particles determined by computational fluid dynamics analysis (via FLUENT V4.3). The chart consists of five regions, i.e., the unmelted, melted, vaporized, escaped, and rebounded, which represent the various states of the particles at impact on the substrate. The X and Y axes of the chart are particle entry conditions, i.e., diameter (ranging from 20 to 100 m) and injection velocity (between 10 to 50 m/s), respectively. The regions indicate the fate of the particle on impact. A grid-array of (14×11) entry conditions is simulated in developing the chart. The proposed chart is aimed at providing a general guideline for plasma spraying process in achieving a thoroughly melted particle on arrival at the substrate to be coated.     

Some Mechanisms of SiO2 Micro-tubes Formation in Plasma Jet

A chamotte rod is transformed in vapors through chemical reactions, at temperatures exceeding 5000 K. SiO₂ micro-tubes formation is determined by a low vapor concentration and a stable vapor flow, along the streamlines of the plasma jet, and, respectively, by the difusion processes, in non-stationary regime, inside the liquid membrane. A mass of SiO₂ vapor, in the range of 0.5×10^–8 kg–10×10^–8 kg, allows one to obtain micro-tubes with the outer diameter between 6.2 and 28.8 m and the inner diameter between 3.8 and 12.2 m.     

In-Flight Spheroidization of Alumina Powders in Ar–H<Subscript>2</Subscript> and Ar–N<Subscript>2</Subscript> Induction Plasmas

In-flight spheroidization of alumina powders in Ar–H₂ (H₂–7.6%, vol/vol) and Ar–N₂ (N₂–13.0%, vol/vol) RF induction plasmas was investigated numerically and experimentally. The mathematical model for the plasma flows incorporates the k– turbulence model, and that for particles is the Particle-Source-in-Cell (PSI-Cell) model. Experimental results demonstrate that spheroidized alumina particles are produced in both Ar–H₂ and Ar–N₂ RF plasmas, with different particle size distributions and crystal phases. Agreement between the predicted and measured particle size distributions is satisfactory under high particle feed rate conditions, while the results obtained for the Ar–H₂ plasma are better than those for the Ar–N₂ plasma. The discrepancy occurring in low feed rate conditions suggests that particle evaporation is an important factor affecting the plasma–particle heat transfer.     

Preparation of spherical,mixed SiO2/TiO2 particles by the sol technique

Preparation of silica, titania and mixed silica/titania particles has been studied. The region for formation of monodisperse SiO₂ particles in the phase diagram tetraethyl orthosilicate (TEOS)-ethanol-H₂O was studied as a function of NH₃ concentration at room temperature. Titania particles could be prepared at lowered temperatures and concentration of ammonia up to 0.01 M. The size of SiO₂ particles was 0.03–1 m whereas TiO₂ particles were size range 0.5–0.8 m. Mixed SiO₂/TiO₂ particles were prepared from prehydrolyzed TEOS/EtOH solutions by adding tetraethyl orthotitanate (TEOT). This was accomplished at 3°C and slightly alkaline solutions. The final particle size of the mixed particles was about 0.3 m.     

Ozone Production in the Positive DC Corona Discharge: Model and Comparison to Experiments

A numerical model of ozone generation in clean, dry air by positive DC corona discharges from a thin wire is presented. The model combines the physical processes in the corona discharge with the chemistry of ozone formation and destruction in the air stream. The distributions of ozone and nitrogen oxides are obtained in the neighborhood of the corona discharge wire. The model is validated with previously published experimental data. About 80% of the ozone produced is attributed to the presence of excited nitrogen and oxygen molecules. A parametric study reveals the effects of linear current density (0.1–100 A/cm of wire), wire radius (10–1000 m), temperature (300–800 K) and air velocity (0.05–2 m/s) on the production of ozone. The rate of ozone production increases with increasing current and wire size and decreases with increasing temperature. The air velocity affects the distribution of ozone, but does not affect the rate of production.     

Model of the Negative DC Corona Plasma: Comparison to the Positive DC Corona Plasma

A numerical model of the negative DC corona plasma along a thin wire in dry air is presented. The electron number density and electric field are determined from solution of the one-dimensional coupled continuity equations of charge carriers and Maxwell's equation. The electron kinetic energy distribution is determined from the spatially homogeneous Boltzmann equation. A parametric study is conducted to examine the effects of linear current density (0.1–100 A per cm of wire length), wire radius (10–1000 m), and air temperature (293–800 K) on the distribution of electrons and the Townsend second ionization coefficient. The results are compared to those previously determined for the positive corona discharge. In the negative corona, energetic electrons are present beyond the ionization boundary and the number of electrons is an order of magnitude greater than in the positive corona. The number of electrons increases with increasing gas temperature. The electron energy distribution does not depend on discharge polarity.     

The Particle In-Flight Characteristics in Plasma Spraying Process Measured by Phase Doppler Anemometry (PDA)

Detailed measurements of particle in-flight characteristics have been carried out using a PDA system for benchmarking as well as to provide further information to aid the development of simulation models. The parameters studied included four conditions of primary gas flow rate and carrier gas flow rate. The particle velocities, diameters, and the corresponding volume flux at different locations were obtained. Due to the one port particle injection arrangement, it was noted that particles in general sprayed with an angle deviated from the nozzle axis Z_n, to the opposite side of the powder feeder port. The particles would also deviate from the spraying cone axis with a divergence angle (). The deviation and divergence angles were examined under different plasma spraying conditions. The measurement data rates at different cross-sectional planes were also obtained so as to compare the results derived from the volume flux measurement and the actual coating on a substrate at the equivalent standoff distance. It was found that the spraying area obtained from the measurement-data-rate increased with downstream distance and a linear relationship between spraying area and distance was also established. Comparing the integrated results, it was noted that the spraying areas derived from the measurement data rate were close to the actual spraying areas obtained from the coordinate measurement machine (CMM) results.     

Ultrafine BaPb1-xBixO3 powders prepared by the spray-ICP technique

Ultrafine powders of a trnary oxide system, Ba–Pb–Bi–O, were prepared by spraying aqueous mixed solutions of Ba(NO₃)₂, Pb(NO₃)₂, and Bi(NO₃)₃ into an argon inductively coupled plasma of ultrahigh temperature above 5000 K (the spray-ICP technique). Phases of the powders were largely dependent on the powder collectors enclosing the tail flame and its successive gas flow. In the water-cooled collector, mixtures of amorphous and crystalline materials were formed. In the collector where the gas flow was spontaneously maintained at about 550°C by ICP itself, ultrafine BaPb_1–xBi_xO₃ (BPBO) 10–40 nm in particle diameter was obtained. The BPBO thus obtained had a few wt.% of water and carbonate. They were lost by heat treatment up to 550°C, and a single-phase BPBO was formed. The as-prepared BPBO (x=0.25) showed no superconducting transition down to 1.5 K, but the one having a particle diameter of 1 m formed by heating the as-prepared BPBO up to 1000°C had a superconducting transition temperature at 11.3 K.     

HPLC Determination and Pharmacokinetic Study of Valdecoxib in Human Plasma

A sensitive, simple, and accurate high-performance liquid chromatographic method has been developed for determination of valdecoxib and the internal standard rofecoxib in human plasma. Protein was precipitated from plasma samples by addition of perchloric acid (HClO₄); the drug was then extracted with diethyl ether. Separation was performed on a Cosmosil C₁₈ column (150 mm × 4.6 mm i.d., 5 m particles) with ammonium acetate buffer-acetonitrile, 60:40 (v/v), containing 0.1% TEA, pH 6.5, as mobile phase. Detection and quantification were performed by UV-visible detection at 239 nm. Detection and quantification limits were 3 and 5 ng mL^–1, respectively. The linear concentration range for valdecoxib was 5–400 ng mL^–1. The validated RP HPLC method was used for determination of the pharmacokinetic data for the drug in humans.     

Laser Doppler anemometry under plasma conditions. Part II. Measurements in an inductively coupled r.f. plasma

Laser Doppler anemometry is used for the measurements of the plasma and particle velocity profiles in the coil region of an inductively coupled r.f. plasma. Results are reported for a 50 mm i.d. induction plasma torch operated at atmospheric pressure with argon as the plasma gas. The oscillator frequency is 3 MHz and the plate power is varied between 4.6 and 10.5 kW. Plasma velocity measurements are obtained using a fine carbon powder as a tracer. Measurements are also given for larger silicon particles ( ) centrally injected into the discharge under different operating conditions.Nomenclature d _p particle diameter - P ₀ plasma power - Q ₁ powder carrier gas flow rate - Q ₂ plasma gas flow rate - Q ₃ sheath gas flow rate - r distance in the radial direction - V axial plasma velocity - V _p axial particle velocity - Z distance in the axial direction - standard deviation     

Preparation and Properties of Nb-Coated Barium Titanate Composite Particles by Surface Modification with Nb Alkoxide in Hydrophobic Solvent

Chemical processing for the preparation of Nb-coated barium titanate composite particles was investigated using surface modification technology, hydrolyzing Nb ethoxide on the surface of barium titanate particles dispersed in hydrophobic solvent.It was confirmed from the measurements of specific surface area and zeta potential as well as SEM, TEM and EDX observations of the resulting composite particles that the original barium titanate particles were coated uniformly with hydrolysis product of Nb ethoxide.Barium titanates coated with 1 wt% of Nb as oxide were well sintered at 1200–1300°C. The dielectric constants of the sintered barium titanates showed flattened temperature dependence, but it depended upon the average particle size of original barium titanate. The sintered bodies of Nb-coated barium titanate powders with average particle size of 0.2 m gave dielectric constants of 2000–3000 and those of barium titanate with average particle size of 0.5 m showed dielectric constants of 3000–4000 at room temperature.The microstructure of the sintered barium titanate coated with Nb oxide consisted of grains of about 1 m, smaller than those of sintered original barium titanate.     

Cathode erosion in inert gases: The importance of electrode contamination

Experimental results are presented for electrode erosion on copper electrodes in magnetically rotated arcs in argon and helium. Measurements were also made of the arc voltage and velocity. The effects due to the contamination of the electrode surface by either a native contaminant layer (copper oxide and carbon traces) or the continuous injection of very small amounts of various diatomic gases (nitrogen, oxygen, chlorine, and carbon monoxide) into the inert plasma gases were determined. The erosion rates for pure argon were significantly higher than those for pure helium (13.5 g/C for argon and 1 g/C for helium) and with both gases, very high arc velocities were measured initially (>60 m/s for argon and >160 m/s for helium) when a natural contaminant layer was still present on the cathode. The removal of this layer resulted in lower velocities (2m/s for argon and 20m/s for helium) and higher erosion rates. The removal of the layer was much faster with argon, due possibly to higher electrode surface current densities for argon arcs.     

Drag on a metallic or nonmetallic particle exposed to a rarefied plasma flow

Drag force on a metallic or nonmetallic spherical particle exposed to a plasma flow is studied for the extreme case of a free-molecule regime. Analytical expressions are derived for the drag components due to, respectively, atoms, ions, and electrons and for the total drag on the whole sphere due to all the gas species. It has been shown that the drag is proportional to the square of the particle radius or the drag coefficient is independent of the particle radius. At low gas temperatures with a negligible degree of ionization, the drag is caused mainly by atoms and could be predicted by using the well-known drag expression given in ordinary-temperature rarefied gas dynamics. On the other hand, the drag is caused mainly by ions at high plasma temperatures with a great degree of ionization. The contribution of electrons to the total drag is always negligible. Ignoring gas ionization at high plasma temperatures would overestimate the particle drag. There is a little difference between metallic and nonmetallic spheres in their total drag forces, with a slightly higher value for a metallic sphere at high plasma temperatures, but usually such a small difference could be neglected in engineering calculations. The drag increases rapidly with increasing gas pressure or oncoming speed ratio. For a two-temperature plasma, the drag increases at low electron temperatures but decreases at high electron temperatures with the increase in the electron/heavy-particle temperature ratio.Nomenclature C _d Drag coefficient - e Elementary charge - f _D,F _D Local and total drag (N/m ² andN) - f ^– Velocity distribution function for incident gas particles - f ⁺ Velocity distribution function for reflected gas particles - k Boltzmann's constant - m Gas particle mass (kg) - n Number density of gas species (m ^–3) - P ^–,P ⁺ Surface pressure due to incident and reflected gas particles - R ₀ Sphere radius (m) - S Speed ratio,S _j=U/(2kT _j/m_j)^1/2 - T _e,T _h Electron and heavy-particle (atom, ion) temperature - T _w Wall temperature - U Oncoming plasma flow velocity - v _x, v_y, v_z Velocity components of gas particles in thex, y, andz directions (m/sec) - v Thermal motion speed of gas particles,v _j =(8kT _j /m _j )^1/2 - v _ze Smallestv _z of electrons which could reach the sphere surface,v _ze=(2e/m _e)^1/2 (m/sec) - v _zw Value ofv _z of ions or electrons as arriving at the sphere surface (m/sec) - Center angle - Gas density (kg/m³) - Shear stress (N/m²) - Absolute value of the floating potential (V) - , Local and total particle fluxes incident to the surface - a Atoms - e Electrons - h Heavy particles - i Ions - j jth gas species - m Metallic sphere - mn Nonmetallic sphere A preliminary version of this paper was presented at the Eighth International Symposium on Plasma Chemistry held in Tokyo, September 1987.     

Composition and properties of freeze-dried products of nicotinic acid withβ-cyclodextrin and heptakis (2,6-0-dimethyl)-β-cyclodextrin

The purpose of the study was to examine the formation of inclusion compounds in the freeze-dried products obtained from aqueous solutions of nicotinic acid and -cyclodextrin (-CD), or heptakis (2,6-0-dimethyl)--cyclodextrin (DIMEB). The molar ratios used were 1:1 and 2:1. In addition two freezing temperatures (–40 and –196°C) and different secondary drying temperatures (+50 and +80°C) were used. Freeze-dried products with -CD obtained after low temperature freezing are of the same crystallographic structure as seen in a pure inclusion compound prepared by coprecipitation. Amorphous products were formed after fast freezing. The molar ratios of included nicotinic acid in the freeze-dried products vary — dependent on the preparation conditions — between 0.75:1 and 1:1. A factorial design proves that the included drug amount can be increased by enhancement of the amount of nicotinic acid used, by faster freezing, and by the combination of these two factors. The proof of inclusion formation was given by a combination of X-ray diffractography, differential scanning calorimetry, thermogravimetry and thermofractography.The freeze-dried preparations obtained with DIMEB were amorphous mixtures of the two components. No proof for inclusion of the nicotinic acid in the cyclodextrin cavity could be given. Higher (–40°C) or lower (–196°C) freezing temperatures and the running of the secondary drying process could not influence these results. The very low stability constant of the complex and steric reasons are responsible for this behavior.