Dc Torch Research Articles

Effect of H2 addition on the preparation of ZrO2 powder from zircon (ZrSiO4) using a plasma torch

In this study, the plasma torch is utilized for the preparation of highly pure ZrO2 from zircon (ZrSiO4). The incorporation of H2 into the discharge gas Ar improves the plasma physical parameters and serves as a reducing agent to promote the irreversibility of the pyrolysis reaction of ZrSiO4. ZrSiO4 powder is fed internally into a DC arc plasma torch through a powder feeder device. The DC torch generates thermal plasma that can fully heat all the ZrSiO4 within a short period for pyrolysis, extraction, and separation, eventually producing high-purity ZrO2. According to the results, the addition of H2 to the working gas increases the material temperature in the flame by increasing the plasma temperature, thereby promoting the pyrolysis of ZrSiO4. The temperature of the material is 2500–2600 °C, which is higher than the boiling point of the pyrolysis product SiO2 while maintaining the high melting points of ZrSiO4 and ZrO2. Furthermore, H2 plasma acts as a reducing agent in the pyrolysis reaction, where it combines with the O of the product to prevent the reversibility of the pyrolysis reaction of ZrSiO4 and retains only ZrO2.

Hydroxyapatite Coatings on Polymers Using a Custom Low-Energy Plasma Spray System

The use of polymers in modern orthopedics and tissue engineering represents a big opportunity to overcome the current challenges facing metallic implant technologies, such as the release of toxic metallic ions and particles, as well as the lack of isoelastic property. Furthermore, it is widely sought that these kinds of medical devices support bone ingrowth and enhance osseointegration. Therefore, implants can be coated with bioactive materials such as hydroxyapatite (HAp) by means of atmospheric plasma spray (APS), which is the most widely used for coating implants with materials of high capacity for osseointegration. So far, APS has been primarily applied to metallic substrates, and only a few studies are available on polymeric and composite materials. Consequently, the aim of this paper was to apply HAp coatings on polyetheretherketone (PEEK) substrates—a polymer commonly used for orthopedics implants—using a custom designed low-energy plasma spray (LEPS) system equipped with a built in-house dc torch. Due to the lower enthalpy in the LEPS system, it is expected that it will not degrade the shape or surface condition of the polymers during the process, and still achieve enough energy to properly melt the HAp powder particles. Observation of the as-coated surfaces by scanning electron microscopy shows that a successful deposition with very good coverage was achieved on the PEEK substrates.

Suspension and precursor solution plasma spraying by means of synchronous injection in a pulsed arc plasma

Research has led to the development of nanostructured coatings by suspension plasma spraying (SPS) or solution precursor plasma spraying (SPPS). Both techniques open a new way to applications in the field of microelectronic devices. However, a better control of plasma/material interactions is necessary. Mono-electrode DC plasma torches indeed generate strongly fluctuating plasma that modifies the thermal and dynamic transfers to the injected suspension droplet, resulting in an inhomogeneous treatment of the latter. This directly influences the texture and microstructure of deposits and subsequently their properties. Efforts to understand the origins of these instabilities have been made and have led to propose a new approach, i.e. an alternative to instabilities attenuations: the reinforcement and modulation of the instabilities. The adjustment of process parameters has allowed obtaining a pulsed laminar plasma and a modulation of its properties. This device is synchronized with an ink-jet print head to reproduce the same conditions of plasma/material interaction for each injected droplet. A home-made DC torch was made. His power of 1kW is well below the standard torches (~30kW for commercial torches) but is well adapted to the flow rate that can be delivered by the print head. This low powered home-made DC torch is used and operates with pure nitrogen as plasma forming gas. Aluminum nitrate aqueous solutions and TiO2 suspensions are injected. The objectives of this work are firstly to characterize this pulsed plasma and its interactions with droplets to facilitate the understanding of heat and dynamics transfers. They are analyzed by time-resolved imaging and optical emission spectroscopy. Secondly, coatings are characterized by SEM, TEM and DRX analysis.

Thick tungsten coating on ferritic–martensitic steel applied with a vacuum plasma spray coating method

The characteristics of thick tungsten layers coated onto ferritic–martensitic (FM) steel blocks using a vacuum plasma spray system were investigated. Because of the large differences in thermal properties between tungsten and FM steel, it is difficult to coat a thick tungsten layer onto FM steel without delamination or detachment of the coated layer from the substrate. To avoid detachment of the tungsten layer from the FM steel surface, the optimization of the coating process was studied in this work. The tungsten layer having a 3.7mm thickness was coated using 43kW DC torch power at 16kPa of chamber pressure with a mixture of argon, hydrogen, and helium. The adhesion between the coating layer and the FM steel was enhanced by inserting pre-heating process before coating process, which increased the penetration of molten tungsten droplets into the FM substrate. The hardness of the coating layer was about 380–400HV, which was 80 to 95% of the hardness levels for commercial bulk tungsten monoblocks, and the porosity, purity, and morphology properties of the tungsten coating layer were also discussed. To evaluate the homogeneity of the layer bonding between the FM steel and the tungsten coating layer, an ultrasonic test was also performed.

High Speed Video Camera and Electrical Signal Analyses of Arcs Behavior in a 3-Phase AC Arc Plasma Torch

A 3-phase AC plasma torch has been developed and aims at overcoming some limits of the classical DC torches in terms of efficiency, cost and reliability. However, the arc behavior in 3-phase plasma torch remains poorly explored. This paper is dedicated to the high speed video camera at 100,000 frames per second and electrical signal analyses of arcs behavior in a 3-phase AC arc plasma torch. First, a reference case at 150 A, in nitrogen as working gas, has been deeply analyzed. Afterwards, a parametric study based on current and inter-electrode gap has been carried out. Results show that only one arc can exist at a given time and arcs rotate by switching from a pair of electrodes to another one, following the maximal electrical gap potential. However, a particular “abnormal” arc behavior was sometimes observed. Indeed, the arc motion within the inter-electrode gap increases the heat exchange and stabilizes the 3-phase discharge whereas the system is unbalanced when the arc is in the periphery. The analysis highlights that the arc motion is strongly influenced by the electrode jet velocity and repulsive Lorentz forces. The parametric study shows that the current increases both jet velocity and arc discharge stability. Elsewhere, the increase of the inter-electrode gap can also stabilizes the electrical 3-phase arc discharge. Furthermore, the correlation between arc motion and current waveform is highlighted. This work is likely to open the way toward a better understanding of 3-phase discharges in the perspective of their further optimization.

Ac-driven plasma torch with a well-type cathode working in air and CO2

This work is devoted to the investigation of the discharge characteristics of an ac arc plasma torch with a well-type cathode working in air and CO2. The electrical characteristics, plasma jet image and optical emission profile are investigated by varying the discharge power and the gas flow rate. The dynamic behaviour of arc voltage and current is characterized by smooth-shaped waveforms, regularly iterated with a driving frequency. The moving of arc spot positions is inferred from the measured waveforms, and compared with that observed in dc torches. A simple lifetime test is performed to verify the speculation that the arc spots in ac operation move continuously over a wide region with a very high speed. The result shows that the electrode lifetime can be enhanced by combining the vortex motion of arc gas with the ac driving.

Suspension Plasma Spraying of Intermediate Temperature SOFC Components Using an Axial Injection DC Torch

Intermediate temperature SOFC components, such as dense, nanostructured SDC electrolytes (samarium doped ceria) and porous anode sublayers were fabricated by suspension plasma spraying using an axial feed dc plasma torch. The liquid carrier employed in this approach allowed for controlled injection of much finer particles than in conventional thermal spraying, leading to thin coatings with a refined microstructure. Dense, thin (<10(m) and non-fractured electrolytes were created. Various processing routes for SOFC half-cells, using tape-cased, plasmasprayed and suspension-sprayed anodes, were explored. Loss of integrity and non-continuous coverage of the anode constituted the principal difficulties in the subsequent electrolyte deposition. The role of suspension feedstock particle size is discussed. Amongst various schemes investigated, a processing route that employs sequential suspension plasma spraying steps for both the electrolyte and the anode, using relatively large primary particles in the feedstock, constituted the most promising approach.

Plasma gasification of wood and production of gas with low content of tar

The gasification of biomass is a very smart way of its utilization. Tar content in the produced gas always plays an important role because high tar content disables some potentialities of the further treatment of the produced gas. There is a strong effort to produce gas with low tar content from this point of view. High concentrations of hydrogen and carbon monoxide also advance possibilities of the usage of the produced gas. Experiments were carried out in a reactor for plasma gasification equipped with hybrid gas-water stabilized DC torch. They confirmed that the reactor is absolutely suitable for this purpose mainly due to extremely low flow of plasma composed of argon, hydrogen and oxygen and its high inner temperature. The measurements of the tar content, using wood as a treated material, were based on SPE method. The determination of benzene and toluene in a gas phase revealed really low concentrations of tar, mostly below 10 mg/Nm3.

Pyrolysis of waste using a hybrid argon–water stabilized torch

An experimental plasmachemical reactor equipped with the novel IPP-ASCR hybrid gas–water stabilized DC torch (160 kW) has recently been started at IPP Prague for the innovative and environmentally friendly plasma treatment of waste streams with a view to their sustainable energetic and chemical valorization and to a reduction of the emission of greenhouse gases. Since the process energy is provided by direct heat transfer from plasma, gases of widely varying chemical composition may be used. The use of electrical energy also reduces the gas flows and requirements for exhaust-gas treatment, and offers control over the chemistry. Pyrolysis of biomass was experimentally studied using wood chips as a model substance. Syngas with a high content of hydrogen and CO was produced. The influence of adding CO 2 for increase of oxygen content in the reactor was investigated.

Investigation of plasma sprayed coatings formation by visualization of droplet impact and splashing on a smooth substrate

In plasma spraying, individual droplet behavior at impact with the substrate is the fundamental element in understanding of coating microstructure. A new experimental setup with two fast shutter cameras (exposure time: 100 ns...1 ms) provides visualization at impact of a single particle, plasma sprayed with a direct current (dc) torch. Images of plasma-sprayed particles at impact on a smooth substrate are presented. The resultant splashing phenomena can be resolved into two parts: the initial impact and particle flattening response. Matter ejection due to the impact is independent of the substrate temperature but this is not the case for flattening splashing. Image correlation with recorded test parameters signals, transistor-transistor logic (TTL) pulse for cameras triggering, pyrometer signal, enable the events chronology, which is essential for the understanding the complete phenomena, to be studied.

Study of the thermal plasma etching at atmospheric pressure on silica rods

Etching of SiO2 rods has been obtained with a dc torch with argon as the process gas in an air environment at atmospheric pressure; the high temperature of the plasma jet causes vaporization of the exposed area. The apparatus and torch operative parameters have been set up to obtain a depth etch rate of up to 0.6 mm min−1 corresponding to 0.826 g min−1.An enthalpy probe has been employed to monitor the plasma conditions before the thermal plasma etching process and from the experimental etch rate a surface rod temperature of Tsur = 2057 K has been derived. Etching has been obtained with uniformity over the entire exposed area with peak to peak differences below 1%.The plasma to rod heat transfer has been simulated using a commercial CFD code Fluent©. The model consists of a non-steady two-dimensional simulation for a compressible turbulent fluid, with an adapted grid calculation. Boundary conditions have been set out using the enthalpy probe plasma jet map. In particular for the flow, the continuity equation for chemical species, the species transport equations (Fick's Law), the momentum conservation equations, the energy equation and the turbulent kinetic energy have been solved. In the solid regions the convective energy transfer due to the rotational motion of the rod, the conduction and radiation energies have been considered. The heat transfer between the fluid and the silica walls (continuity temperature between the fluid and the solid) accounts for the energy loss due to surface radiation (qr) and vaporization (qv) obtained from experimental data.A rod surface temperature of 2057 K obtained from experimental etch rate data agrees well with the calculated temperature obtained using Fluent© (Tsim = 2010 K).

Synthesis of Si–C–N coatings by thermal Plasmajet chemical vapour deposition applying liquid precursors

AbstractMaterials in the system Si–C–N feature excellent properties for wear protection applications, even at elevated temperatures, and an excellent thermal shock resistance. As these materials have no melting point, coatings have to be manufactured via a synthesis. Conventional chemical vapour deposition (CVD) processes have the disadvantage of low deposition rates. Thermal Plasmajet CVD processes with liquid feedstock feature the highest deposition rates among the gas‐phase synthesis processes. Single and triple DC torches and HF torches with supersonic nozzles have successfully been applied to produce Si–C(–N) coatings on different steel, aluminium, titanium and copper alloys, as well as on graphite. Besides chlorosilanes, hexamethyldisiloxane, tetramethyldisiloxane and hexamethyldisilazane have been used as liquid single precursors. Deposition rates up to 1500 μm h−1 have been achieved. The coatings show cauliflower, columnar or dense morphology and an amorphous or nanocrystalline structure. The formation of both α‐ and β‐Si3N4 has been verified by X‐ray diffraction. The application of chlorosilanes always results in chlorine‐containing coatings. The chlorine causes severe corrosion in the interface to mild carbon steel substrates. The processes are compared taking into account their characteristics concerning the injection modes, gas temperature and velocity profiles determined by enthalpy probe measurements. The process conditions are correlated to the coating microstructure and the adhesion to the substrates and guidelines for the optimum production of ­Si–C–N coatings by Plasmajet CVD are deduced. Emission spectroscopy is used to determine the mechanisms of the coating formation. Full dissociation of the liquid feedstock in the plasma jet has been verified. Copyright © 2001 John Wiley & Sons, Ltd.

Determination of plasma velocity from light fluctuations in a dc plasma torch

The fluctuations generated by arc root motion have been used to determine the velocity in a plasma jet. The phase angle relations resulting from time variations of light intensity profiles taken simultaneously across different sections of the plasma have been used to determine the velocity for different operating conditions of the dc torch. The experimental results have been compared with the theoretical predictions of a 2D MHD model of the dc plasma torch. The experimental and theoretical values appear to agree with a maximum deviation in the 5-10% range.

A Thermodynamic Analysis of Nonequilibrium Argon Plasma Torches

The nonequilibrium process of argon plasma torches is analyzed theoretically. Thermodynamic diagrams of different degrees of ionization are developed to aid in understanding and analyzing the transition from chemical equilibrium to frozen flow in dc plasma torch operations. A thermodynamic model is developed to describe the nonequilibrium processes in a dc argon plasma torch. In the model the ionization process is approximated as a constant-pressure heating process, with little deviation from the equilibrium state upon completion of heating. If the plasma flow is frozen shortly after heating, the entropy increase is small during the transition from equilibrium to frozen flow. In this case the frozen flow will have nearly the same composition and entropy as the flow at the heating section exit. For singly ionized argon plasmas in the entropy range relevant to dc torch operation, the frozen flow solutions on the affinity–pressure diagram are found to be insensitive to entropy change. Therefore the present model predicts that argon plasmas generated at different power levels will have almost identical affinity at the torch exit for the same operating pressure. This prediction agrees with experimental observations except for very low torch power levels.

DC Plasma Synthesis of Aluminum Nitride Ceramic Powders

A novel Triple DC Torch Plasma Reactor (TTPR) has been developed for the plasma synthesis of fine ceramic powders. The reactor consists of three identical plasma torches. The plasma jets from these torches form a converging plasma volume into which the starting powder is fed. Thermodynamic equilibrium calculations have been performed for predicting the behavior of an aluminum-nitrogen system. Aluminum nitride has been synthesized by using the TTPR operating in the non-transferred mode. Product characterizations indicate that single hexagonal phase AIN ceramic powder has been obtained.