Modeling of a counterflow plasma reactor

Modeling of a counterflow plasma reactor is presented, using liquid injection for the synthesis of fine particles. An experimental reactor has been developed in this laboratory, and feasibility has been demonstrated for synthesizing advanced ceramic powders. The flow field calculations show two major recirculating regions which are of importance for increasing the particles' residence time inside of the reactor. In addition, the temperature within these recirculation zones remains relatively uniform. For simulation, water droplet trajectories have been calculated for droplets produced by an injection probe. It is shown that the droplets in a size range below 50 m in diameter will follow the streamlines and evaporate completely within a short traveling distance. This finding suggests that this reactor configuration provides a favorable environment for the synthesis of fine particles using liquid precursors.     

The effect of gas injection on a thermal argon plasma flow in a water-cooled tube

The effect of gas injection on an atmospheric thermal argon plasma flow in a water-cooled tube was investigated experimentally and numerically. The injection gas is argon, helium, or nitrogen. The static pressure with helium injection increases greatly because of its high thermal conductivity, while little increase occurs for nitrogen injection because of the dissociation. The increasing rate of the static pressure depends on the ratio of the momentum term to the viscosity term. The heat flux to the tube wall with gas injection changes less than that without injection. The numerical results showed variations similar to the experimental ones.     

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.     

Effects of metal salts on the thermal decomposition of edta-gel precursors for ferroelectric ceramic powders

Raw chemicals such as metal nitrates and chlorides were found to affect the thermal decomposition behaviour of EDTA-gel precursors used for the production of ceramic powders. Fine, homogeneous ceramic powders were produced from nitrate solutions while chlorides gave segregated phases. In studies on the production of lead zirconate titanate (PZT) using chlorides, the segregation and loss of lead was observed and shown to be caused by the formation and evaporation of PbCl₂. Thermal analysis (DTA/TG) quantitatively proved the suggested reaction mechanism for this phase segregation. Crystallization of the desired perovskite phase of lead zirconate titanate (PZT) and barium titanate (BT) initiated at temperatures as low as 250°C in the nitrate-EDTA precursors. Water of crystallization and formation of BaCO₃ in the barium titanate precursor were suggested to account for differences in the observed decompositional behaviours of the BT and PZT precursors.     

Effect of quenching conditions on particle formation and growth in thermal plasma synthesis of fine powders

The vapor-phase synthesis of ultrafine powders in reactive thermal plasma systems is studied. A mathematical model is developed to determine the effect of quenching conditions on the size characteristics of powders produced. The particle nucleation is considered to be due to both condensation of product vapor and surface reaction between adsorbed reactant species. The particle growth is considered to be exclusively due to further condensation of product vapor. Numerical predictions on powder formation are explored through a case study for the synthesis of zinc oxide powders from zinc vapor and oxygen carried in argon gas. The results of the present srudy indicate that the size characteristics of plasma-produced powders can be significantly enhanced by gradual, regulated quenching, as opposed to the rapid quenching conventionally used in the past. The results further indicate that distribution of the quench gas along the reactor provides an effective means to accomplish the much desired control over the powder properties.     

Thermal plasma synthesis of nanotitania and its characterization

Titanium dioxide (TiO₂) nanoparticles were prepared by the thermal plasma synthesis; which give a highly crystalline product. Their morphological studies are carried out by using techniques like SEM, EDAX, TEM and SEAD. Crystal size was calculated by XRD using Scherrer equation; which is observed at two current amperes; at 80 A size ranges between 25 and 30 nm and at 120 A size ranges between 30 and 42 nm. Composition analysis was done by TEM–EDAX, FTIR and Fast Fourier Transform techniques. The FTIR peaks clearly show that synthesized TiO₂ nanoparticles are in anatase phase; this phase is generally preferred because of its high photocatalytic activity, since it has a more negative conduction band edge potential (higher potential energy of photogenerated electrons), high specific area, nontoxic, photochemically stable and relatively in expensive.     

Mixing study of the induction plasma reactor: Part II. Radial injection mode

A study was undertaken on the mixing pattern in am induction plasma torch and reactor system. The results presented in this part of the paper relate to the radial injection mode, in which an auxiliary gas is injected into the main plasma stream through a set of 2, 4, or 8 injection ports located in the torch nozzle at the level of the torch exit flange. A much faster mixing of the gases occurred in this mode compared to the axial injection mode investigated in Part I of this paper. As in the case of axial injection, the present study demonstrates that gas mixing, in the presence of the discharge, is considerably more difficult than under ambient temperature conditions. Lower turbulence levels exist in the plasma reactor. due to the considerably higher viscosity of the gases under plasma conditions. Results obtained with a three-point injection flange, in which the injection ports were oriented at 45° to the torch and reactor axes toward the upstream .side, mere most interesting since they achieved essentially the same degree of mixing as was obtained with the radial injection ports without the need to locate the injection ports at the exit nozzle of the plasma torch. This arrangement provides for added flexibility in reactor design.     

Homogeneous nucleation of particles from the vapor phase in thermal plasma synthesis

Particle nucleation and growth are simulated for iron vapor in a thermal plasma reactor with an assumed one-dimensional flow field and decoupled chemistry and aerosol dynamics. Including both evaporation and coagulation terms in the set of cluster-balance rate equations, a sharply defined homogeneous nucleation event is calculated. Following nucleation the vapor phase is rapidly depleted by condensation, and thereafter particle growth occurs purely by Browntan coagulation. The size and number of nucleated particles are found to be affected strongly by the cooling rate and by the initial monomer concentration. An explanation is presented in terms of the response time of the aerosol to changing thermodynamic conditions.This work appears in abbreviated from in the proceedings of the International Symposium on Combustion and Plasma Synthesis of High Temperature Materials, San Francisco, Oct. 24–26, 1988, to be published asCombustion and Plasma Synthesis of Hig Temperature Materials, Z. A. Munir and J. B. Holt (eds.), VCH, New York (in press).     

Dual-purpose gas chromatographic injection device for pressurized liquid and gas injection

A dual-purpose gas chromatographic injection device, capable of injecting pressurized liquid sample of up to 5000 psig and gas sample with a volume as high as 5000 μL, has been successfully developed and implemented. The injection device is synergized by the effectiveness of a classical flash vaporization of a syringe injection and the reliability of a proven rotary valve. Depending on the matrix involved, this injection device employs either a commercially available four-port internal valve for liquid sampling or a six-port external valve for gas sampling, a modified removable needle used in standard liquid syringe, and an auxiliary flow stream that can be either mechanical or electronic flow controlled for solute transfer. For pressurized liquid, the device was found suitable of up to nC₁₆ hydrocarbon with no observable carry-over despite the injection device was operating at ambient temperature. A relative standard deviation of less than 2% (n = 20) was obtained for hydrocarbon compounds ranging from nC₈ to nC₁₆. For gas injection, the device performed well even under difficult chromatographic conditions such as with a low column inlet pressure of less than 1 psig. A relative standard deviation of less than 0.5% (n = 10) was obtained for reactive sulfur compounds such as alkyl mercaptans. The device can be operated manually or automated with pneumatic or electrical actuator, is platform neutral, and can be moved amongst instruments without hardware modification as well as implemented for on-line or in situ applications. In this paper, the utility of the device was also demonstrated with selected GC applications of industrial significance.     

Determination of procaine and tetracaine in plasma samples by micellar liquid chromatography and direct injection of sample

Summary A liquid chromatographic procedure is proposed for the determination of procaine and tetracaine in plasma samples with direct injection. The method uses a Spherisorb octadecylsilane ODS-2 C₁₈ analytical column and a micellar mobile phase containing 0.15 M sodium dodecyl sulphate, 0.5% triethylamine at pH 2.5 and 10% propanol. The UV detection was carried out at 300 nm. Plasma sample preparation required only adequate dilution with the mobile phase before injection into the chromatographic system. The proposed method allows the determination of procaine and tetracaine in plasma at therapeutic levels.     

Diamond Deposition on Substrates with Different Geometries in a Thermal Plasma Reactor

The effects of process parameters on diamond film deposition have been considered in an atmospheric-pressure dc thermal plasma jet reactor. Two different precursor injection systems have been evaluated, counterflow and side injection. The precursor flow rate using ethanol has been found to strongly affect crystal size as well as orientation of crystal growth planes. Further, crystal size on sharp edges has been found to be up to five times larger than on planar surfaces. The effects of substrate geometry on the morphology and area of deposited diamond have been investigated as well. The results of this study show that dc thermal plasma jets can provide high diamond deposition rates, for example on wires and drills, although crystal size and film thickness show substantial variation.     

Direct injection HPLC method for the determination of selected phenothiazines in plasma using a Hisep column

A direct plasma injection HPLC method has been developed for the determination of selected phenothiazines (promethazine, promazine, chlorpromazine) using a Hisep column. The method is easy to perform and requires 20 microL of a filtered plasma sample. The chromatographic run time is less than 11 min using a mobile phase of 15:85 v/v acetonitrile-0.18 m ammonium acetate pH 5.0 and UV detection at 254 nm. The method is linear in the concentration range 0.1-25 microg mL(-1) (r > 0.99, n = 6) for each analyte with RSD less than 6%. Interday and intraday variability were found to be < or =14%. The limits of detection and quantitation were 0.1 (S/N > 3) and 0.25 microg mL(-1) (S/N > 10), respectively, for each of the three phenothiazines. We can also apply this method to separate three other phenothiazines (ethopromazine, trifluoroperazine, prochlorperazine), although it lacks the selectivity to determine the concentration of all six drugs concurrently. The separation is feasible using these drugs in certain combinations.     

The kinetics of Al-Si spinel phase crystallization from calcined kaolin

The crystallization of Al-Si spinel from medium ordered kaolin with high content of kaolinite was investigated using the differential thermal analysis (DTA). The apparent activation energy of the process was evaluated from the dependence of exothermic peak of crystallization on heating rate. Within the applied interval of heating rate (1-40 K min⁻¹) the temperature of peak maximum increases from initial value of 1220.5 K in about 54.2 K. The apparent activation energy of the process 856±2 kJ mol⁻¹was calculated using the Kissinger equation. The growth morphology of Al-Si spinel crystal was evaluated from the Avrami parameter. The average value of morphology parameter determined within the observed interval of heating rate is 3.08±0.03. This value indicates that crystallization mechanism of Al-Si spinel phase proceeds by bulk nucleation of the new phase with constant number of nuclei and that the three-dimensional growth of crystals is controlled by the reaction rate on the phases interface.     

High-Performance liquid chromatographic determination of dibromomannitol in plasma

Summary Myelobromol 1,6-dibromo-1,6-dideoxy-D-mannitol (dibromomannitol, DBM) is a bifunctional alkylating agent that has been in clinical use since 1963. It is currently included at high dose in preconditioning regimen for bone marrow transplantation (BMT) and is a main-stay of treatment for polycythaemia vera. A high-performance liquid chromatographic method was developed for the determination of DBM in the plasma. The basis of the assay is a derivatization with sodium-diethyldithiocarbamate at 42°C in the presence of 1-bromo-1-deoxy-3,6-anhydro-galactitol as internal standard (IS). The analysis was carried out on a 250×4mm Hypersil 5 CPS column equipped with a 20×4 mm Hypersil 10 CPS precolumn. The eluent consisted of heptane:isopropyl-alcohol: glacial acetic acid=600:76:80 w/w. The flow rate was 1.2 mL min⁻¹. UV detection was performed at 254 nm. The calibration graph was linear in the concentration range of 2.5–260 μM of DBM in plasma. The limit of detection was 1.0 μM. The precision and accuracy of the method was between the good laboratory practice (GLP) required limits. Presented at: Balaton Symposium on High-Performance Separation Methods, Siófok, Hungary, September 3–5, 1997     

Ionic liquid assisted synthesis of S-heterocycles

Some organic solvents are highly toxic, flammable, and even explosive. In particular, high vapor pressures and toxicity of certain volatile organic solvents may cause significant environmental problems. Therefore, alternative solvents or media with tunable and versatile solvation properties for conducting chemical reactions and materials synthesis have been actively sought. Ionic liquids have numerous applications not only as environmentally benign reaction media, but also as catalysts and reagents. Due to the increase of environmental consciousness in chemical research and industry, the challenge for a sustainable environment calls for clean procedures that avoid the use of harmful organic solvents. Due to the special properties of ILs (ionic liquids) such as wide liquid range, good solvating ability, negligible vapor pressure, non-inflammability, non-volatility, environment friendly medium, high thermal stability, good stability in air and moisture, easy recycling and rate promoters etc. they are used in organic synthesis. Therefore, ionic liquids have attracted the attention of chemists and act as catalyst and reaction medium in organic reactions with high activity. Highly efficient methods are explored for the preparation of S-heterocycles with the application of ILs as catalyst and reaction medium.     

Direct injection of plasma into column liquid chromatographic systems

Summary Since the late 1970’s several techniques involving the direct injection of biological fluids, like plasma and serum have been published. Most of the systems presented utilize a precolumn for the reception of the biological fluids, trace enrichment and preliminary clean-up before the analyte fraction is transferred to the separation column. The most common solid phase for the precolumn is silanized silica, but many other types have been used. One was especially designed for the purpose of direct injections of biological fluids: the Internal Surface Reversed Phase support. The main problem with direct injections is the development of column instabilities due to the denaturation and precipitation of plasma proteins on the support. It has, however, been possible to design systems which can handle the injection of 25ml total plasma volume without the need to exchange precolumn. The most promising techniques in this respect are the so called “pre-column” venting plug technique and the use of micellar mobile phases. Losses of analytes may be a problem when protein-binding extends 95%. However, remedies are available, such as the use of more hydrophobic solid phases, the addition of competitive binders and pH adjustments of injected solutions.     

Development and validation of a direct plasma injection LC-MS method for YM087 and YM440 and metabolites in human plasma

Summary A fully automated, direct plasma-injection technique using an LC coupled to a quadrupole mass spectrometric detector with an on-line, sample clean-up procedure for two new pharmaceutical products, YM087 and YM440, has been developed. Plasma samples containing YM440 were mixed with internal standard or plasma containing YM087 and were injected into a chromatographic system based on two coupled columns. An extraction column packed with alkyl-diol-silica (ADS) was used for online sample cleanup. Using a back-flush technique analytes were subsequently transferred to an analytical column, for separation. Detection was based on tandem mass spectrometry either in electronspray or in APCI mode. Despite the relatively small volumes of plasma injected, reasonably low limits of quantitation were achieved. Validation of both assays was performed using guidelines concerning method validation. All parameters studied were within acceptance criteria. The methods were successfully applied to clinical pharmacokinetic studies.     

Diamond synthesis by DC thermal plasma CVD at 1 atm

Diamond crystals and films have been success full y synthesized by DC thermal plasma jet CVD at a pressure of I atrn. A novel triple torch plasma reactor has been used to generate a convergent plasma volume to entrain the participating gases. Three coalescing plasma jets produces! by this reactor direct the dissociated and ionized gaseous species onto ( 100) silicon wafer substrates where the diamond grows. In a typical 10-min run, depending on the method of .substrate preparation, either microcrystals with sizes up to 8 m or continuous films with thicknesses of 1–2 m have been obtained. X-ray diffraction, scanning electron microscopy, and Raman spectroscopy have been used for the characterization of the crystals and of the films.     

Modeling of RF plasma torch with a metallic tube inserted for reactant injection

Flow, temperature, and electromagnetic (EM) fields in a radio-frequency thermal plasma torch designed for the preparation of superconducting powders or films have been analysed by using a new two-dimensional modeling approach with the electric field intensity as the fundamental EM field variable. The insertion of a stainless steel injection tube into the torch leads to large induction currents in this tube. Although such large induction currents cause pronounced changes of the EM fields near the injection tube, flow and temperature fields are little affected. There exists only one large toroidal vortex in the upper part of the present torch, while the maximum temperature appears at an off-axis location within the coil region.     

Thermal plasma synthesis of transition metal nitrides and alloys

Applications of arc plasma processing to high-temperature chemistry of Group V nitrides and Si and Ge alloys are studied. The transition metal nitrides -VN, -NbN, and -TaN are directly synthesized in a dc argon-nitrogen plasma from powders of the metals. A large excess of N₂ is required to form stoichiometric -VN, while the Nb and Ta can only be synthesized with a substoichiometric N content. In a dc argon plasma the alloys V₃Si, VSi₂, NbSi₂, NbGe₂, Cr₃Si, and Mo₃Si are obtained from powder mixtures of the corresponding elements. The compounds are identified by x-ray diffraction patterns and particle shape and size are studied by electron microscopy.