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Poorly controlled heat and momentum transfers between plasma and material, plasma instabilities are some of the difficulties encountered in suspension plasma spraying. The improvement of this method is usually attempted by means of the reduction of arc fluctuations. This paper presents a new approach to the injection of reactive material in an arc...
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... plasma spraying method, the plasma jet is produced by the torch, in this work, supplied by direct current source, presented in Fig. 1. The cathode tip ignites the gas, fed to the torch (gas input in Fig. 1), to form the plasma, which flows through the extended anode nozzle and emerges out at a high velocity through the exit of the torch. The previous studies (Ref 5, 15, 16) demonstrated that plasma torch, even powered by dc regulated source, generates highly ...
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... plasma spraying method, the plasma jet is produced by the torch, in this work, supplied by direct current source, presented in Fig. 1. The cathode tip ignites the gas, fed to the torch (gas input in Fig. 1), to form the plasma, which flows through the extended anode nozzle and emerges out at a high velocity through the exit of the torch. The previous studies (Ref 5, 15, 16) demonstrated that plasma torch, even powered by dc regulated source, generates highly fluctuated plasma jet. The ''stick and slip'' motion of the arc at the nozzle ...
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... c g and P g are, respectively, the isentropic coefficient of the cold gas and the mean pressure in cathode cavity; q p is the plasma density; S, L c , and V g are, successively, the cross section area, the length of the nozzle channel, and the volume of the cathode cavity, as shown in Fig. 1. We present the possibility of coupling Helmholtz and restrike modes of instabilities to obtain pulsed laminar arc plasma jet ( Ref 17, 18). In this case, the arc root at the nozzle wall experiences a very regular back and forth movement with highly repeatable rearcing leading to the emission of thermal plasma balls at the resonance ...
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... that time, the vaporization-seeding process does not concern this droplet but the injected one period earlier and it gives the plasma ball at the right of Fig. 9(b). Figure 10(a)-(d) present time-resolved imaging of the dynamic interaction between the plasma jet and the droplets. These pictures are obtained with a low-magnifi- cation objective for the different time delays over one period. ...
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... left in the jet after solvent vaporization, but their individual images are strongly oversized by diffraction, scattering, or resolution due to pixel size. The velocities of the center of mass of plasma balls can also be measured by image analysis and vary between 30 and 50 m/s (±10 m/s) depending on the injection delay s j , j = 1-4, as shown in Fig. 10. Several hundreds of pictures similar to those of Fig. 10 were recorded and they display the same features provided they are triggered with the same s j . These pictures clearly demonstrate that the trajectories and related thermal history of injected materials depend on the moment when droplets penetrate into the plasma. In case of s ...
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... images are strongly oversized by diffraction, scattering, or resolution due to pixel size. The velocities of the center of mass of plasma balls can also be measured by image analysis and vary between 30 and 50 m/s (±10 m/s) depending on the injection delay s j , j = 1-4, as shown in Fig. 10. Several hundreds of pictures similar to those of Fig. 10 were recorded and they display the same features provided they are triggered with the same s j . These pictures clearly demonstrate that the trajectories and related thermal history of injected materials depend on the moment when droplets penetrate into the plasma. In case of s 1 = 0 ls, the most significant part of materials travels ...
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... in vortices emis- sion. Moreover, this phenomenon requires more studies and it is the subject of future research. In case of s 3 = 480 ls, a low material dispersion is observed corre- sponding to the longest transport distance. At last, the case of s 4 = 620 ls, because it is almost the 700 ls period, resembles the first case s 1 = 0. The picture (Fig. 11) is recorded at low magnification to increase the observation area and with a time aperture of the camera of 60 ls. The interferential filter is removed so that the picture integrates the traces of incandescent solid particle whose speed is estimated in the range 30- 40 m/s. The light reflection on the front part of the torch and on ...
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The uncontrolled arc plasma instabilities are one of the difficulties encountered in in suspension plasma spraying. The improvement of this process is usually attempted by means of the reduction of arc fluctuations. The following paper presents a new approach to overcome these arc instabilities. The principle is to produce a pulsed laminar plasma j...
Citations
... Amplifying arc instabilities in a controlled fashion has been proposed as an alternative method to control heat and momentum transfer provided a pulsed and synchronized injection of materials is associated with it [18][19][20][21][22][23]. In the case of a DC torch operating in self-sustained pulsed mode at frequency f H [20][21][22], it has been shown that when the frequency of modulation of the arc current f 0 approaches the Helmholtz frequency f H , the stability of the dynamics of the arc reattachment is improved [16]. ...
... Amplifying arc instabilities in a controlled fashion has been proposed as an alternative method to control heat and momentum transfer provided a pulsed and synchronized injection of materials is associated with it [18][19][20][21][22][23]. In the case of a DC torch operating in self-sustained pulsed mode at frequency f H [20][21][22], it has been shown that when the frequency of modulation of the arc current f 0 approaches the Helmholtz frequency f H , the stability of the dynamics of the arc reattachment is improved [16]. ...
... First, it is interesting to observe that the arc voltage fluctuates at the same frequency as the arc current. In previous studies [20][21][22], a DC current was imposed (without amplitude modulation) at a lower current (15 A) and saw-tooth shape signals for voltage were obtained. They were characterized by a linear or quasi-exponential increase in voltage from about 40 V up to 110 V. ...
Plasma spraying of liquid feedstock is a relevant process for the deposition of finely structured ceramic coatings. The control of arc instabilities is a challenging purpose to obtain the desired material properties so that efforts are devoted to develop segmented direct current torches that intend to fix the length of the arc. An alternative method consists in promoting the arc oscillations and in associating a pulsed liquid injection with the torch. This paper presents a study of the influence of the amplitude modulation of a direct current on the arc dynamics confined in a plasma spray torch. The dependence of the electrical features of the torch on the current amplitude modulation is presented. Plasma speed and temperature are also measured.
Time-resolved imaging of the arc attachment permits to correlate its motion, and especially the arc reattachment on the anode surface, with arc current and voltage. Measurements are compared with a simplified model providing the time-dependence of the plasma properties. It is shown that the current modulation favors the arc restrike by modulating the arc radius but also that the electric field strength of the arc column should be modified during current variations.
... New torch designs (cascaded arc) are used to limit the amplitude of the arc root motion [18] and they are associated with mechanical or atomization injection systems which produce continuous liquid jet. Recently, an alternative approach has been proposed that relies on the amplification of arc electric instabilities to form a pulsed arc jet [19][20][21][22][23][24]. The basic idea is to modulate plasma properties and to synchronize the liquid injection with a drop-on-demand printhead to control the moment of liquid injection in order to improve the material treatment. ...
... Previous studies have shown that the coupling of the Helmholtz and restrike modes in DC arc plasma torches can lead to a resonance mode called ''Mosquito mode'' [19][20][21][22][23][24][25] which generates a self-sustained pulsed arc jet with periodic arc voltage oscillations around 1.4 kHz at atmospheric pressure. Recent works have proved that the stability of this mode is improved by the modulation of the arc current [25]. ...
In the field of plasma spray technologies, new processes were developed in order to obtain coatings with nanostructured architectures. Among them, solution precursor plasma spraying is a promising technique. However, to be industrially viable, efforts must be made to understand and control the steps of the process. In alternative to instabilities attenuations, the time-modulation of current-voltage characteristics of a low-powered arc plasma jet was proposed, resulting in time-modulation of plasma properties. This modulation was synchronized with a drop-on-demand printhead for injection to improve the control of plasma/material interaction. An aluminum nitrate ink was formulated to be compatible with a drop-on-demand printhead dispenser. Plasma composition was calculated by the Gibbs free energy minimization method. Moreover, in-flight diagnostics coupled with coating characterization were carried out. Results showed that the controlled injection system leads to the formation of vapors of aluminum oxide that leaded to the deposition of nanostructured cauliflowers-like structures. Amorphous alumina coatings with inclusions of γ-alumina microsized particles were obtained and a description of main in-flight mechanisms was proposed.
... As has been highlighted above, in the plasma jet generated by dc torch the oscillations modes, restrike and Helmholtz, have been identified. The following paper shows the possibility of coupling these two modes to obtain pulsed laminar arc plasma jet [8,9]. It is possible to obtain by a special torch characterized by a larger cathode cavity (V g =17.8 cm 3 ). ...
The uncontrolled arc plasma instabilities are one of the difficulties encountered in in suspension plasma spraying. The improvement of this process is usually attempted by means of the reduction of arc fluctuations. The following paper presents a new approach to overcome these arc instabilities. The principle is to produce a pulsed laminar plasma jet combined with phased injection of liquid droplets. This is achieved by the particular design of the plasma torch which allows coupling two modes of the plasma oscillations, restrike and Helmholtz. The plasma produced in a new mode is laminar and pulsed, characterized by a modulated enthalpy, what permits to make the synchronization with the suspension injection. The droplets are injected using a piezoelectric device, based on drop-on-demand method, triggered by the voltage signal. The results are evaluated by time-resolved imaging technique and Time- Resolved Optical Emission Spectroscopy.
This work investigates the dynamic behavior of an arc column in a DC thermal plasma torch using a three-dimensional transient model. The model assumes an axial magnetic field of constant magnitude inside the plasma torch domain which mimics a magnetic field that is generated either by a solenoid or a permanent magnet encircling the anode. Under the influence of a strong external magnetic field, the plasma arc swirls inside the torch with one end of the arc sweeping the inner surface of the anode while the other end pivoted at the cathode tip. The dynamic variation of the electric arc and its structure is analyzed by simulating a special case with a step-change in magnetic field strength. The transient simulation traces the formation of a helical arc structure as soon as the magnetic field is imposed. The arc column is seen to be axially constricted and radially diffused in the presence of a strong magnetic field. The impact of the magnetic field is also reflected in the arc attachment gap, which is the axial distance between cathode and anode attachments. The influence of magnetic field strength, arc current and gas flow rate on arc rotational frequency, voltage drop, and temperature distribution are analyzed.
Due to their favorable characteristics, e.g. high temperature, low operating cost and environmental requirements, direct current thermal plasma jets have been widely used. Correspondingly, in order to meet the different requirements of the diverse applications, different thermal plasma torches have been invented and their characteristics have been widely studied. The characteristics of a thermal plasma torch could be decided by its construction and working parameters. Though the influences of some factors on the characteristics of different thermal plasma torches have been experimentally and theoretically studied, there is little study on the influences of different gas flow rates on the characteristics of plasma torch working with two gas injection channels. Therefore, an optimized thermal plasma torch working with two gas injection channels has been proposed and its characteristics have been experimentally studied and discussed in this paper. The design considerations of the proposed plasma torch are discussed firstly. Then the experimental methods and results are introduced and discussed. The experimental results showed that the first gas flow rate nearing the cathode has little influences on the characteristics of the proposed thermal plasma torch while the second gas flow rate nearing the anode influences its characteristics obviously.
In the field of plasma spray technologies, new processes are developing to obtain coatings with nanostructured architectures. Difficulties of understanding and controlling the process originate from the continuous injection of a liquid material and the power instabilities of the current torches which strongly affect the heat and momentum transfers to the nanometric particles. This paper reports an original method to make TiO2 coatings by suspension plasma spraying. A direct current (DC) power supply applying time-modulated current amplitude to a custom DC torch is used to generate at low power (1.5 kW) a pulsed laminar plasma jet with periodic oscillations of its properties. To make best use of this pulsed mode, a synchronization device was developed. It allows triggering from the arc voltage an inkjet nozzle to deliver at a precise moment a single droplet to improve the control of plasma/material interaction. An ink of TiO2 anatase solid particles is formulated to be compatible with a drop-on-demand printhead dispenser. In-flight diagnostic is made by optical emission spectroscopy and a fast shutter camera. TiO2 coatings are characterized by scanning electron microscopy, x-ray diffraction and transmission electron microscopy. Results show that homogeneous TiO2 coatings of nanostructured cauliflowers shapes are obtained thanks to the controlled injection system. A competition between nucleation mechanism and liquid particles deposition are also observed. These deposits correspond to a mixture of anatase and rutile phases.
Plasma spray is one of the most versatile and established techniques for the deposition of thick coatings that provide functional surfaces to protect or improve the performance of the substrate material. However, a greater understanding of plasma spray torch operation will result in improved control of process and coating properties and in the development of novel plasma spray processes and applications. The operation of plasma torches is controlled by coupled dynamic, thermal, chemical, electromagnetic, and acoustic phenomena that take place at different time and space scales. Computational modeling makes it possible to gain important insight into torch characteristics that are not practically accessible to experimental observations, such as the dynamics of the arc inside the plasma torch. This article describes the current main issues in carrying out plasma spray torch numerical simulations at a high level of fidelity. These issues encompass the use of non-chemical and non-thermodynamic equilibrium models, incorporation of electrodes with sheath models in the computational domain, and resolution of rapid transient events, including the so-called arc reattachment process. Practical considerations regarding model implementation are also discussed, particularly the need for the model to naturally reproduce the observed torch operation modes in terms of voltage and pressure fluctuations.
Direct current plasma torches for plasma spraying applications generate electric arc instabilities. The resulting fluctuations of input electrical power hamper a proper control of heat and momentum transfers to materials for coating deposition. This paper gives an overview of major issues about arc instabilities in conventional dc plasma torches. Evidences of arc fluctuations and their consequences on plasma properties and on material treatments are illustrated. Driving forces applied to the arc creating its motion are described and emphasis is put on the restrike mode that depends on the arc reattachment and the boundary layer properties around the arc column. Besides the arc root shown as a key region of instability, the Helmholtz oscillation is also described and accounts for the whole plasma torch domain that can generate pressure fluctuations coupled with voltage ones.
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 1 kW is well below the standard torches (~ 30 kW 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.
