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.     

Plasma spraying of alumina: Plasma and particle flow fields

A comprehensive experimental examination of the interaction between a subsonic thermal plasma jet and injected alumine, particles is presented. Measurements of plasma velocity, temperature, and entrained air were obtained from an enthalpy probe and mass spectrometer combination. A diffusive separation, or demixing, of the Ar and He plasma gas was observed. Centerline plasma velocities and temperatures ranged from 1501500 m/s and 2000 to 14,000 K, respectively, in the region between the torch and a typical substrate location of 90 mm. Measurements of particle size, velocity, tempearture and local number density were obtained from a combination laser particle sizing system, Laser doppler velocimeter (LDV), and two-color pyrometer. Centerline temperatures and velocities for the nominally 30 m particles used were 2400–2800 K and 150–200 m/s, respectively.     

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.     

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.     

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.     

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.     

Numerical Modeling of an Impinging and Compressible Dusted Plasma Jet Controlled by a Magnetic Field

The present study has been conducted to clarify the magnetic control characteristics of a particle-laden compressible plasma jet impinging on a substrate for the improvement of a low-pressure plasma spraying process and its controllable optimization. The plasma jet is described by an Eulerian approach and each injected particle is described by a Lagrangian approach, respectively, taking into account the compressible effect, variable transport properties, and plasma–particle interactions, coupled with the Maxwell's equations. The effects of the location of an applied rf electromagnetic field and of the injected particle size on the plasma jet characteristics and the particle behavior are clarified by numerical simulation. It is concluded that the particle dispersion is effectively influenced and, furthermore, the plasma temperature and the particle temperature can be strongly controlled by applying the rf electromagnetic field to the nozzle. The reasonable agreement of particle velocity between calculation and experiment is obtained.     

Influence of nonishothermicity on drag coefficient of a spherical Al particle in a plasma

Experimental results of drag coefficient measurements of spherical Al particles with diameter 100–300 m in an electric-arc argon plasma for Re ynolds numbers 0.4-10, Mach numbers 0.05 in a plasmatron channel with an intersection cavity for an arc current of 75–190A, gas flow rate 0.2–2.75g/.sec, and channel radius 0.75–1.5cm are reported. Plasma flow characteristics obtained using MHD equations are used for treatment of experimental data.     

Sensitive Determination of Felodipine in Human and Dog Plasma by Use of Liquid–Liquid Extraction and LC–ESI–MS–MS

Summary A sensitive and selective liquid chromatographic method coupled with electrospray ionization tandem mass spectrometry (LC–ESI–MS–MS) has been developed for quantification of felodipine in human and dog plasma. Compounds were separated on a 2.0 mm × 150 mm, 5.0 m particle, C₈ column with 1 m m ammonium acetate–acetonitrile, 20:80, pH 6.0, as mobile phase at a flow rate of 200 L min^–1. Nifedipine was used as internal standard. Plasma samples were extracted with diethyl ether, the centrifuged upper layer was evaporated, the residue was reconstituted with mobile phase, and the reconstituted samples were injected. The analytical column lasted for at least 1000 injections. By use of multiple reaction monitoring (MRM) mode in MS–MS felodipine and nifedipine were detected without severe interference from the human or dog plasma matrix. Felodipine produced a protonated precursor ion ([M + H]⁺) at m/z 384 and a corresponding product ion at m/z 338. And internal standard (nifedipine) produced a protonated precursor ion ([M + H]⁺) at m/z 347 and a corresponding product ion at m/z 315. Detection of felodipine in human and dog plasma was accurate and precise, with a limit of quantification of 0.05 ng mL^–1. The method has been successfully applied to preliminary pharmacokinetic study of felodipine in human and dog plasma.     

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.     

High-Throughput Analysis of Sofalcone in Human Plasma by Use of Automated 96-Well Protein Precipitation and LC-MS-MS

A sensitive and selective method for the determination of sofalcone in human plasma was established by use of protein precipitation and liquid chromatography-tandem mass spectrometry. Plasma samples were transferred into 96-well plate using an automated sample handling system and spiked with 10 L of 2 g mL^–1 internal standard solution (d₃-sofalcone). 0.5 mL of acetonitrile was added to the 96-well plate and the plasma samples were then vortexed for 30 sec. After centrifugation, the supernatant was transferred into another 96-well plate and completely evaporated at 40 °C under a stream of nitrogen. The dry residue was reconstituted with mobile phase. All sample transfer and protein precipitation was automated through the application of both the PerkinElmer MultiPROBE II HT and TOMTEC Quadra 96 workstation. The limit of quantitation of sofalcone was 2 ng mL^–1 using a sample volume of 0.2 mL for the analysis. The reproducibility of the method was evaluated by analyzing five samples at nine quality control (QC) levels over the nominal concentration range from 2 ng mL^–1 to 1000 ng mL^–1. Validation of the method showed that the assay has good precision and accuracy. Sofalcone and internal standard produced a protonated precursor ion ([M+H]⁺) at m/z 451 and 454, and both gave a corresponding product ion at m/z 315. The high sample throughput of the method has been successfully applied to a pharmacokinetic study of sofalcone in human plasma.     

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.     

Investigation of concentration waves in the Fe(dipy) 3 2+ -NaBrO3-CH2(COOH)2-H2SO4 reaction

The concentration waves in the Belousov-Zhabotinskii reaction with the participation of the iron (II, III) complexes with 2,2-dipyridine have been discussed, and the concentration region for their existence has been determined. The dependence of the parameters of the autonomous waves (their velocity, amplitude, length, and period) on the initial concentrations of the reactants has been established experimentally. It has been shown that the dependence of the period of the waves on the initial bromate concentration is described by the equation T(c)= 1.1 [NaBrO₃]^–2.1. The dependence of the velocity of the waves on the initial bromate concentration is accurately described by the equations v (mm/sec)=0.210[NaBrO₃]^1/2 and v (mm/sec)=0.205[NaBrO₃]^1/2, respectively, for waves with maximal and minimal lengths (periods). It has been concluded that pacemaker centers which generate waves with short periods (and lengths) appear under the influence of heterogeneities in the solution. The influence of heterogeneities on the parameters of the waves with minimal lengths makes them less suitable for the study of the concentration dependence of the parameters.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 6, pp. 669–676, November–December, 1985.     

A Computational Examination of the Sources of Statistical Variance in Particle Parameters During Thermal Plasma Spraying

Computational modeling is used to systematically examine many of the sources of statistical variance in particle parameters during thermal plasma spraying. Using the computer program LAVA, a steady-state plasma jet typical of a commercial torch at normal operating conditions, is first developed. Then, assuming a single particle composition(ZrO₂) and injection location, real world complexity (e.g., turbulent dispersion, particle size and density, injection velocity, and direction) is introduced ``one phenomenon at a time to distinguish and characterize its effect and enable comparisons of separate effects. Calculations are also performed wherein all phenomena are considered simultaneously to enable further comparisons. Both nonswirling and swirling plasma flow fields are considered. Investigating each phenomenon separately provides valuable insight into particle behavior. For the typical plasma jet and injection conditions considered, particle dispersion in the injection direction is mostsignificantly affected by (in order of decreasing importance): particle size distribution, injection velocity distribution, turbulence, and injection direction distribution or particle density distribution. Only the distribution of injection directions and turbulence affect dispersion normal to the injection direction and are of similar magnitude in this study. With regards to particle velocity and temperature, particle size is clearly the dominant effect.     

Yttria-stabilized zirconia films deposited by plasma spraying and sputtering

Thin films of yttria-stabilized zirconia (YSZ) deposited by sputtering and plasma spraying have been analysed as solid electrolyte for oxygen gas sensors and solid oxide fuel cells. Different substrates have been considered in order to provide good adhesion, dense electrolyte films, and mechanical and thermal stability. By optimization of the sputtering parameters, a Pt/YSZ/Pt assembly of 1.5 µm thickness and with an electrical resistance of 500 M at room temperature has been obtained. The plasma-sprayed films (150 µm thickness) have shown oxygen ion conductivity with a reproducible sensitivity on oxygen partial pressure starting at 400 °C and an activation energy of ~1 eV. Lanthanum strontium manganate (350 µm thickness) was sprayed as cathode material.Presented at the OSSEP Workshop Ionic and Mixed Conductors: Methods and Processes, Aveiro, Portugal, 10–12 April 2003     

AES investigations of plasma sprayed Al2O3 coatings on Ni

Summary The system of plasma sprayed Al₂O₃ on Ni substrates is investigated by means of AES/depth profiling. The influence of two process parameters — preoxidation procedure and spraying temperature — is examined. Rupture between substrate and ceramic layer occurs between a residual — or, in the case of excessive preoxidation, a superfluous — NiO layer on Ni, the thickness of the former depending on preoxidation conditions and the Al₂O₃ layer, the back side of which being partially covered with NiO. The thickness of this NiO layer increases up to about 1 m with the thickness of the initial NiO layer on the substrate, until this layer is about 1.3 m thick, and remains constant thereafter. The same dependence is observed for the width (0.1–1 m range) of the mixed oxide interface between the sprayed Al₂O₃ layer and the NiO layer below. These results represent the chemical contribution to adherence. Contrary to excessive preoxidation, an increase of the spraying temperature from 300°C to 500°C effects broader interfaces.This poster was awarded the First Prize in Poster Section C by the Deutscher Arbeitskreis für Spektroskopie (DASp)     

The use of a gold-containing membrane for ion-sensitive electrodes and their application in analysing systems. I

Summary Pellets of a material, resulting from the precipitation of silver salts (AgCl, AgBr, AgI and Ag₂S) onto finely divided gold (particle size less than 3 m) showed a selective ion-sensitive behaviour for halogens, silver and sulphide, resp. Silver iodide on gold turned out to be sensitive for cyanide as well.Electrodes with this type of membrane combined a low redox sensitivity with good mechanical strength and chemical resistance and with a low membrane impedance (1–500 kOhm mm^–1 as a maximum). Settling times vary in the range of 0.1–10 sec. A linear Nernstian response was shown in the ranges 10^–1×10^–5 M (chloride), 10^–1 –10^–6 M (bromide, iodide, cyanide) and 10^–1–10^–7 M (silver, sulphide). In order to obtain good cleaning facilities for heavy duty applications (e. g. measurements in slurries) the membrane pellet was mounted in the electrode body in a way that its cylindrical side contacted the solution to be measured or monitored.

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Verwendung einer goldhaltigen Membran für ionensensitive Elektroden und ihre Anwendung in analytischen Systemen. IHerstellung und Charakteristik ionensensitiver Elektroden

Zusammenfassung Tabletten eines Materials, das durch Fällung von Silbersalzen (AgCl, AgBr, AgJ und Ag₂S) auf fein-pulverisiertem Gold (Teilchengröße weniger als 3 m) entstand, wurden zur Fertigung selektiver ionen-sensitiver Elektroden für Halogene, Silber und Sulfid eingesetzt. Silberjodid auf Gold war auch für Cyanid empfindlich. Diese Elektroden vereinigen eine niedrige Redox-Empfindlichkeit mit einer guten mechanischen und chemischen Stabilität sowie einer niedrigen Elektrodenimpedanz (Höchstwert: 1–500 kOhm mm^–1). Der Signalwert bildet sich in 0,1–10 sec. Das Elektrodensignal ist proportional der Ionenaktivität gemäß dem Nernstschen Gesetz im Bereich von 10^–1–3·10^–5 M (Chlorid), 10^–1–10^–6 M (Bromid, Jodid, Cyanid) und 10^–1–10^–7 M (Silber, Sulfid). Die Tablette ist im Elektrodengehäuse so montiert, daß nur die cylindrische Außenseite von der zu messenden Flüssigkeit berührt wird. Die Reinigung der Elektrode im anspruchsvollen Dauerbetrieb (z. B. kontinuierliche Messungen in Festkörper enthaltenden Abwässern) wird durch diese Anordnung erheblich vereinfacht.

     

Mixing study of the induction plasma reactor: Part I. Axial injection mode

A systematic study of the gas-mixing pattern in an induction plasma reactor under atmospheric and low pressure conditions is reported. Different reactor configurations were investigated in which nitrogen is injected as an auxiliary gas either axially into an Ar/H₂, discharge in the center of the induction coil region, or radially through multiple orifices, into the plasma jet at tire exit nozzle of the torch. Concentration mapping in the mixing zone was carried out, using a VG-Microniass-PC 300 D mass spectrometer at plasma power levels and reactor pressures, in the range of 13–24 k 6V and 35–93 kPa, respectively. Comparison of these results with cold-flow measurements underlined the substantial difference in the mixing pattern in each of these two cases. A considerably faster mixing of the gases is noted under cold flow conditions compared to that in the presence of the discharge. The results are discussed from the viewpoint of their use for optimum reactor design applied to tire vapor-phase synthesis of ultrafine ceramic powders, using induction plasma technology.     

The Treatment of Water-Based Toxic Waste Using Induction Plasma Technology

A study of the treatment of liquid wastes in a radio frequency (rf) induction plasma reactor is reported. Ethylene glycol was used as a surrogate for the waste because of safety considerations. Thermodynamic analyses demonstrated complete and safe decomposition at the conditions studied. The solution was injected axially into the center of an argon–oxygen plasma operated at a plate power of 50 kW to study blast atomization and operating conditions. A factorial analysis revealed, at a confidence level of 0.99, that both reduction of pressure and liquid flow rate increase the destruction and removal efficiency (DRE) and that a higher plate power increased DRE. The study also revealed that poor atomization was responsible for the reduction of the DRE by 10–15% (to 80–85%) and that 94% of the exothermic energy of the reaction was available for further use. The specific energy requirement (SER) of the process was estimated at 8.33 kWh/kg of solute. This value can be expected to drop significantly with scale-up of the process.     

Modeling of Oxidation During Plasma Spraying of Iron Particles

The plasma-sprayed metallic powder deposits contain usually some amounts of oxides originating from a gas–liquid reaction after melting the particles flying in the plasma plume, and a gas–solid reaction after the particle impact and solidification. In the present paper, a model of the in-flight oxidation of molten metallic paricles is described. Only the growth of a continuous oxide layer on the molten particle surface is assumed, whereas melt convection and oxide nodule formation inside the particles are not taken into consideration. The model is applied to spraying of pure iron powder by a water-stabilized plasma gun. After the presumed flight time, the calculated and measured values of the oxide thickness are of the same order of magnitude.