DC Thermal Plasma Research Articles

Ultrafine carbon black produced by pyrolysis of polyethylene using a novel DC-thermal plasma process

Ultrafine carbon black was synthesized successfully by decomposition of polymer using a novel DC-thermal plasma process. Polyethylene was used as a carbon source. The carbon black is produced in plasma stream as spherical nanoparticles with a high degree of turbostratic organization. This was confirmed by field emission scanning electron microscopy (FESEM) and field emission transmission electron microscopy (FETEM). A high-ordered graphite structure was proven by powder X-ray diffraction (XRD), Raman spectroscopy, and high-resolution transmission electron microscopy (HRTEM). The Brunauer–Emmett–Teller (BET) results revealed a high specific surface area and pore volume of the carbon product.

Thermal plasma assisted rapid sintering of MgO-doped Al–Zr oxide composites

MgO-doped Al–Zr oxide composites of Zr 0.95Al 1.05Mg x O with x = 0, 0.01, …, 0.05, 0.75, 1 were sintered to high density by both dc thermal plasma assisted rapid sintering as well as conventional sintering. It was observed that the above mentioned MgO-doped Al–Zr oxide composites were sintered to high density end products by the thermal plasma heating technique within a short sintering time (a few minutes) as compared to a conventional long range resistive heating schedule (a few tens of hours). Moreover, the densities of plasma sintered samples were found to be more than that of conventional sintered samples. It was also noticed that there exists a critical limiting point of MgO doping concentration ( x = 0.05 in Zr 0.95Al 1.05Mg x O) for maximum physical densification in both conventional as well as plasma sintered samples. Sintered specimens were analyzed by XPS, XRD and EPMA studies. XRD, XPS and EPMA studies revealed the presence of a high density phase, Mg 2Zr 5O 12 prominently in x = 0.05 sintered sample which resulted in maximum densification of the sample. Thus, MgO doping in small amounts promotes physical densification.

Study of field emission of multiwalled C nanotubes synthesized by arc discharge

Carbon nanotubes (CNTs) were synthesized by a DC thermal plasma method. Afteroptimizing the synthesis parameters such as the pressure, current etc, the synthesizedproducts, and the anodic deposit in particular, were characterized by scanning electron andtransmission electron microscopy. The morphology of the product was ascertained to bemultiwalled carbon nanotubes in a large ratio. The diameters of these nanotubes were ofthe order of 30–50 nm with lengths extending up to a micron and sometimes even 2 or3 µm.The field emission properties of these nanotubes were then studied. The CNTs were depositedon a metal stub which acted as the cathode. Care was taken to ensure complete coverage ofthe stub to remove any possibility of emission from the metallic stub itself. The emissionstudies were performed in a stainless steel chamber under a dynamic vacuum in the range of10−8 Torr. The field emitted current was detected using a phosphorus coated indium tin oxide (ITO)glass. The phosphorus coating also helped in imaging the tips of the nanotubes. This wascrucial in estimating accurately the emitting area and thus the field enhancement factor. TheI–V curves for the field emission were recorded for various distances between the electrodes. Theresults are shown here.

Synthesis and characterization of iron aluminide nanoparticles by DC thermal plasma jet

Intermetallic iron aluminide alloy nanoparticles were synthesized by non-transferred DC plasma spray torch at atmospheric pressure. These powder particles were characterized by vibrating sample magnetometer (VSM), X-ray diffraction (XRD), environmental scanning electron microscopy (ESEM), laser scattering particle size distribution analyzer, energy dispersive X-ray spectra (EDX) and thermo gravimetric-differential thermal analysis (TG/DTA) studies and the results are discussed. Commercially available iron and aluminium powders in the size range between 40 and 100 μm were ball milled with suitable ratio (Fe-85% and Al-15%) by weight to form a iron–aluminium blende; this was used as feed stock to synthesize iron aluminide nanoparticles. These nanoparticles particles were spherical in shape and black in colour, with the size range between 30 and 70 nm observed by ESEM. It is seen that the powders processed at higher power levels (15.3 kW) have more number of single spherical nanoparticles than powders processed at lower power levels (9.8 kW). The formation of iron aluminide nanoparticles is confirmed by XRD analysis and an exothermic reaction by DTA studies.

Plasma spheroidization of iron powders in a non-transferred DC thermal plasma jet

In this paper, the results of plasma spheroidization of iron powders using a DC non-transferred plasma spray torch are presented. The morphology of the processed powders was characterized through scanning electron microscopy (SEM) and optical microscopy (OM). The percentages of spheroidized powders were calculated by the shape factors such as the Irregularity Parameter (IP) and Roundness (RN). A maximum of 83% of spheroidization can be achieved. The spheroidization results are compared with the theoretical estimation and they are found to be in good agreement. The phase composition of the spheroidized powder was analyzed by XRD. The effect of plasma jet temperature and plasma gas flow rate on spheroidization is discussed. At low plasma gas flow rates and at high plasma jet temperatures, the percentage of spheroidization is high.

ZnO nanopowders fabricated by dc thermal plasma synthesis

Ultrafine ZnO nanopowders were successfully synthesized by the dc thermal plasma process with a high production rate of 1.2 kg/h. The combination of plasma was found to affect the nanoparticles morphology. Both X-ray diffraction and Raman spectrum analysis confirmed a high quality wurtzite structure of the ZnO nanopowders. Photoluminescence study exhibited strong ultraviolet emission corresponding to the near band edge emission in the ZnO nanopowders. Random laser action observed in the ZnO nanostructures by optical pumping demonstrated high optical and crystal quality of the ZnO nanopowders fabricated by the dc thermal plasma synthesis.

Effect of powder loading on the excitation temperature of a plasma jet in DC thermal plasma spray torch

Abstract A DC non-transferred mode plasma spray torch was fabricated for plasma spheroidization. The effect of powder-carrier gas and powder loading on the temperature of the plasma jet generated by the torch has been studied. The experiment was done at different input power levels; the temperature of the jet was within 5000–7000 K argon was used as plasma gas and powder-carrier gas. Nickel powder particles in the size range from 40 to 100 μm were processed. The temperature of the jet was estimated after flowing powder-carrier gas only into the plasma jet and with powder-carrier gas feeding powder into the flame. On introduction of powder-carrier gas and powder loading the temperature of the jet was found to decrease appreciably down to 11%. The temperature of the plasma jet was estimated using the Atomic Boltzmann plot method.

In-flight formation and characterization of nickel aluminide powders in a dc thermal plasma jet

The strength of some intermetallics increases with temperature instead of exhibiting a decrease: thus, they are ideally suited for high temperature applications. The formation of intermetallic compound during the spray process leads to metallurgical bonding due to the high exothermicity of the formation reaction. In this paper formation of nickel aluminide in a thermal plasma jet from ball-milled nickel–aluminium powders is reported. Commercially available nickel and aluminium powders were mixed in the appropriate amount and injected in to a thermal plasma jet produced by an atmospheric plasma torch. During in flight the formation of nickel aluminide takes place in plasma jet. Powders were collected at two different collection distances (40 and 80 cm) for three power levels (10, 15 and 20 kW), different plasma gas flow rates (15 and 20 lpm) and different powder feed rate (7 and 14 g/min). The powders are characterized using SEM, optical micrograph, TG/DTA and XRD analysis. The effect of plasma parameters on the formation have been studied and reported. The formation is better at higher power levels, low powder feed rate, low plasma gas flow rate and longer collection distances.

Dc thermal plasma synthesis and properties of zinc oxide nanorods

In this research, a dc thermal plasma reactor was used to produce ZnO nanorods with a diameter of about 30nm and a length of 100–200nm. In the photocatalytic study, visible light absorption of the ZnO nanorods was achieved by doping up to a few thousands ppm of nitrogen. The nitrogen-doped ZnO nanorods under the visible light radiation exhibited excellent antimicrobial ability. UV-visible spectroscopy of the N-doped ZnO nanorods annealed in a reductive atmosphere revealed a strong absorption of near-IR light starting at around 1μm, which is attributed to the effect of plasmon resonance. Room-temperature photoluminescence spectroscopy of the N-doped ZnO nanorods showed an UV emission peak at 380nm, a green emission peak at 520nm, and a weak near-IR emission peak at 760nm. The UV emission was assigned to the near band-edge emission, while the green and the near-IR emissions corresponded to the deep-level emission from different defects. In addition, we found that photoluminescent characteristic of the ZnO nanorods depends strongly on the synthesis and annealing atmospheres. Finally, discrete UV lasing modes were observed in the random-packed ZnO nanorods at room temperature. This may be attributed to recurrent light scattering that provides coherent feedback for lasing.

The dc thermal plasma synthesis of ZnO nanoparticles for visible-light photocatalyst

It is known that ZnO is an n-typed semiconductor with band-gap energy of ∼3.2 eV and thus only absorbs UV light with the wavelength ≤385 nm. However, as far as photocatalytic efficiency and practical applications are concerned, it is desirable that ZnO absorbs not only UV but also visible light. In the present study, a novel dc thermal plasma reactor was used to synthesize ZnO nano-particles. We found that the kind of plasma affects the nanoparticle morphology and N 2 plasma favors formation of the spherical nanoparticles. Visible light absorption of the ZnO nano-particles was achieved by doping up to a few thousands ppm of nitrogen into the material. The effect of doping concentration and particle morphology on photocatalytic characteristics of the ZnO nanoparticles will be described.

DC thermal plasma CVD synthesis of coatings from liquid single source SiBCN and SiCNTi precursors

DC plasma jet CVD is applied to synthesize coatings from liquid state single source precursors on metallic substrates. For SiBCN coating synthesis hexamethylcyclotrisilazanoboranes with Si/B ratio 3:1, 3:2 and 3:3 are used. SiCNTi coatings are produced with tris(dimethylamino)- and tris(diethylamino)hexamethylcyclotrisilazanotitanium. Attempts to deposit SiCNZr coatings by use of tris(diethylamino)hexamethylcyclotrisilazanozironium are not successful due to too high precursor viscosity. Vacuum plasma sprayed nickel, molybdenum and copper interlayers prove to permit effective suppression of nitride and boride reaction zone formation, which is observed during direct SiBCN coating deposition on mild steel substrates. By use of adapted process parameters coatings with columnar or dense morphology and amorphous or nanocrystalline structure can be synthesized. Deposition rates exceeding 2000 μm/h are achieved. By relative movement between DC plasma torch and substrate surface the coated area is enlarged. Simultaneous improvement of coating density and homogeneity is observed.

Raman spectroscopy determination of phases within thermal sprayed hydroxyapatite splats and subsequent in vitro dissolution examination

Phase compositions at various locations within individual thermal sprayed hydroxyapatite (HA) splats were qualitatively determined using Raman spectroscopy analysis and, confirmed through in vitro dissolution exercise. The purpose of the present study was to verify the locations of different phases within a HA splat deposited by thermal spray techniques. The splats were prepared through dc thermal plasma and high velocity oxy-fuel (HVOF) spray processes. Raman spectroscopy studies were performed to determine the locations of various phases that exist within individual splats, and the results were verified with in vitro dissolution tests. Results showed that the Raman spectroscopy is a feasible technique for qualitative determination of the phases within a single HA splat. It was revealed that the thermal decomposition of HA into tricalcium phosphate (TCP), amorphous calcium phosphate (ACP), tetracalcium phosphate (TTCP), CaO and other phases, occurred mainly within the melted portion of the sprayed particle during the thermal spray process. The in vitro tests of the individual HA splats in simulated body fluid established a reasonable correlation between localized dissolution behavior and the phases revealed by the Raman spectroscopy plots. This study demonstrates the feasibility of employing the Raman spectroscopy technique to provide phase distribution information within a single HA splat. Henceforth, a better understanding and control of optimizing the properties of a bulk HA coating through controlling the spray process could be possible.

Characterization of La0.8Sr0.2MnO3 Produced by a Reactive DC Thermal Plasma Spray System

An atmospheric pressure, reactive DC plasma spray system was used to evaluate a process for depositing porous, complex oxide thin films. A mixture of La3O3, SrCO3, and MnCo3 was used to produce a porous cathode layer for potential application in planar solid oxide fuel cells. The coated lanthanum strontium manganite (LSMO) layer made from the mixture was compared to ones generated using a pre-reacted LMSO powder made by solid-state reaction. The results showed that the crystallization of the reactive-spray formed coating layer on the zirconia substrate was higher than that of the coating layer on the mild steel from the pre-reacted LMSO powder. It is both a simpler process and gave better crystallization. The controlled porous coating layers with open pore size of less than 1 μm were successfully produced in a reactive DC plasma spray system from the mixture of raw materials.

Direct Production of Porous Cathode Material (La<SUB>1-<I>x</I></SUB>Sr<SUB><I>x</I></SUB>MnO<SUB>3</SUB>) Using a Reactive DC Thermal Plasma Spray System

The cathode material (La0.8Sr0.2MnO3) in solid oxide fuel cells (SOFCs) was plasma sprayed on mild steel in a reactive DC thermal plasma spray process. This high-speed process of depositing thin films for the components of SOFCs was examined experimentally. The results showed that a coating layer of La0.8Sr0.2MnO3 with a particular porosity could be obtained directly using both prereacted La0.8Sr0.2MnO3 and mixed raw materials (La2O3, SrCO3, and MnO or MnCO3) as feed materials with or without a pore former. The heat treatment of the plasma coating material at 1073 K (800 °C) for 3 h significantly enhanced the desired crystallization of La0.8Sr0.2MnO3 in the coated material.

Preparation of ultra-fine alumina powders by D. C. plasma jet

Ultra-fine alumina was prepared by DC thermal plasma. The influence of reactor configuration, input power and cooling conditions was investigated on properties such as particle diameter and phase composition. The powders synthesized without reaction tube were observed to be of two forms. One was metastable nano-sized powder with an average diameter of 30 nm due to rapid quenching of the vapor phase. The other form was larger-sized powder with an average diameter of 200 nm, and it was considered to be due to quenching of small liquid droplets, which had not fully vaporized in short residence time. With a reacting tube, the powder collected at the reaction tube was dominated by α-phase Al2O3, whereas the powder collected at the quenching chamber was mainly composed of intermediate γ- and δ-Al2O3. Input power and injection position of oxygen did not significantly affect the powder characteristics. To increase the conversion, aluminum powder should be vaporized completely. Most of the powders synthesized appeared fluffy spherical white powders with particle diameter of below 150 nm.

Theoretical Investigations on Sprayed Particle-Plasma Interactions to Optimize Processing Parameters in D.C. Thermal Plasma Spraying

A numerical model is proposed to simulate the behavior of particles injected into a dc spraying plasma in conjunction with the code that calculates the plasma jet characteristics. The sprayed particle-plasma interaction, particle loading effect and the particle internal heat transfer including phase transformations are considered together in the model to present a realistic prediction for the characteristic features of plasma spraying. The temperature and velocity fields in a plasma jet expanding region outside the nozzle are found by a pre-developed code based on the arc-gas interactions inside the nozzle depending on the operating conditions of the torch. The influence of particle injection on the plasma jet characteristics is treated as the source in the fluid equations governing the plasma jet in semi-three dimensional calculations caused by non-axisymmetric injection of powders. The calculated velocity profiles of powder particles along the axial distance are compared with those obtained from experiments, and the determination of optimal spraying conditions are discussed from the results calculated from the present model. This model gives more realistic information for plasma spraying than earlier works which have not included the loading effects and non-axisymmetric injection of powders in dc thermal plasma spraying. The theoretical predictions of the sprayed particle characteristics using this model can be helpful for determining preliminary range and optimal values of operating parameters for plasma spray processes.

Decomposition of Chlorofluorocarbon Using DC Thermal Plasma. Utilization of water as an additive reactant.

The decomposition of dichlorodifluoromethane was carried out using the thermal argon plasma generated by DC arc discharge, where water was added to react with CCl2F2. Complete decompositions of CCl2F2 were observed in all experimental conditions and the product gases consisted of CO2 and CO in regard to carbon, which agreed with the results obtained by numerical calculations. The numerical analyses indicated the following results. Only hydrogen fluoride was formed from fluorine, while HCl and Cl were produced from chlorine. The formation of Cl2 was suppressed by the addition of H2, the amount of which was reduced when the ratio of H2O to CCl2F2 was increased. The suppression of Cl formation was also observed in the experiments.

Rapid synthesis of diamond by counter-flow liquid injection into an atmospheric pressure plasma jet

Diamond films with high growth rates of up to 1 mm/h have been deposited at atmospheric pressure in a counter-flow liquid injection DC thermal plasma reactor. A variety of liquid organics has been used as precursors to deposit diamond on different substrates. Diamond films have been obtained in all cases. Scanning electron microscopy reveals that the diamond films are well-faceted. X-ray diffraction and Raman spectroscopy have confirmed the phase purity of the diamond films. Exhaust gas samples from some diamond synthesis processes have been analyzed to determine the destruction and removal efficiencies for the input liquid precursors which have been also considered as simulants of toxic wastes.

Process study of thermal plasma chemical vapor deposition of diamond, part I: Substrate material, temperature, and methane concentration

Effects of process parameters on diamond film synthesis in DC thermal plasma jet reactors are discussed including substrate material, methane concentration and substrate temperature. Diamond has been deposited on silicon, molybdenum, tungsten, tantalum, copper, nickel, titanium, and stainless steel. The adhesion of diamond film to the substrate is greatly affected by the type of substrate used. It has been found that the methane concentration strongly affects the grain size of the diamond films. Increased methane concentrations result in smaller grain sizes due to the increased number of secondary nucleations on the existing facets of diamond crystals. Substrate temperature has a strong effect on the morphology of diamond films. With increasing substrate temperature, the predominant orientation of the crystal growth planes changes from the (111) to the (100) planes. Studies of the variation of the film quality across the substrate due to the nonuniformity of thermal plasma jets indicate that microcrystalline graphite formation starts at the corners and edges of diamond crystals when the conditions become unfavorable for diamond deposition.

Characterization of thermal plasma CVD diamond coatings and the intermediate SiC phase

Diamond films have been successfully deposited by DC thermal plasma jet CVD at a rate of 40 μm/h under atmospheric and subatmospheric pressures. Transmission electron microscopy (TEM) has been used for the characterization of the diamond films and the intermediate phase. The orientation and the distribution of β-SiC at the interface between the diamond and silicon substrate have been observed using selected-area electron diffraction with the associated dark-field images. X-ray diffraction, scanning electron microscopy, and Raman spectroscopy are used for the characterization of the produced diamond films. Potential applications of selected-area channeling patterns are discussed for investigating the correlations between the growth direction and the crystalline perfection.