Characterization of supersonic low pressure plasma jets

Low Pressure Plasma Spraying (LPPS) processes use a DC plasma jet expanding at low pressure for fast deposition of dense coatings in a controlled atmosphere. The LPPS technology is widely used industrially in particular in the aeronautics and medical industries among others. Unlike atmospheric pressure plasma jets, which have been extensively studied experimentally and theoretically, the interest in low pressure DC plasma jets only occurred recently. However, the process development has been mainly based on empirical methods and the basics of the physical mechanisms that govern them still remain to be investigated. Further improvement of the processes requires, in particular, the knowledge of physical properties of the plasma jet such as the temperature, flow velocity and plasma density. Low pressure plasma jets present unconventional properties such as low collisionality, large dimensions and supersonic flow. Therefore specific diagnostics have to be adapted to these conditions. In this study, argon plasma jets at pressures between 2 and 100 mbar are investigated. Imaging has been used to allow a qualitative description of the plasma jet topology for different pressures and torch parameters. Low pressure plasma jets are most of the time supersonic, compressible and in an aerodynamic non-equilibrium, which results in visible successive compression and expansion zones corresponding to a variation of the local pressure, temperature and density. Imaging, combined with pressure measurements inside the plasma torch, has evidenced three different types of flow regimes with respect to the chamber pressure. For chamber pressures below 45 mbar, the flow is under-expanded and is characterized by an exit pressure higher than the chamber pressure. For pressures above 45 mbar, the plasma jet is over-expanded, in this case the exit pressure is lower than the chamber pressure. When the exit pressure is equal to the chamber pressure, the plasma jet is in the so-called design pressure regime. A diagnostic tool, extensively applied on atmospheric plasma jets, the enthalpy probe system, has been modified in order to allow gas sampling from the plasma jet at low pressures. A shock wave appears in front of the probe when it is immersed in a supersonic plasma jet, making the interpretation of enthalpy measurements more difficult.The free-stream properties, like the Mach number, temperature and free-stream enthalpy have to be inferred from stagnation measurements. Two interpretations of enthalpy probe measurements are described in this study. The first method uses the energy conservation equation and LTE assumptions with calorically perfect gas and neglecting the aerodynamic non-equilibrium, whereas the second method, uses a complementary measurement of the static pressure just after the shock using a specially developed tool: the Post Shock Static Pressure Probe (PSSPP). This allows the use of the conservation equations to determine the free-stream properties of the plasma jet without the assumption of calorically perfect gas and aerodynamic non-equilibrium. Determination of the free-stream enthalpy, Mach number and temperature were possible on over-expanded jets for pressures higher than 40 mbar. At 100 mbar with torch parameters of 400 A and 40 SLPM argon flow, the temperature of the plasma jet reaches 10000 K and the velocity is about 3000 m/s on axis. Measurement of plasma jet properties such as the Mach number, electron density and temperature, were performed using double Langmuir probes and Mach probes. In particular, under-expanded jets are studied in detail by performing complete mappings of plasma jet properties at 10 and 2 mbar chamber pressure. These results show that the measured physical properties are consistent with the jet flow phenomenology such as the presence of periodic expansion and compression zones, the effect of the pressure and the location of the shocks. It is shown in particular that for highly under-expanded jets at 2 mbar, the Mach number reaches 2.8 in the first expansion zone followed by a strong drop to subsonic flow revealing the presence of a Mach reflection. The flow is accelerated further and a periodic structure of compression/expansion cells is observed until the local static pressure is in equilibrium with the surrounding pressure. Another diagnostic often used in plasma spraying is optical emission spectroscopy (OES) which is non-intrusive and gives information about the plasma excited species.However, the determination of the excitation temperature, obtained by the Boltzmann plot method, relies on the assumption of local thermodynamic equilibrium (LTE), which is no longer satisfied at low working pressures. The result of the deviation from LTE is that the heavy particle, electron and excitation temperatures are different. In this study, Boltzmann plots have been used to evaluate the deviation from LTE as a function of the working pressure and the location in the plasma jet. It has been shown that the plasma jet is closer to LTE in the compression zones and close to the axis. Measurements of spectral line broadening due to the Stark effect allowed to determine the electron density for under-expanded jets and give results similar than with electrostatic probe measurements.On the other hand, excitation temperatures are systematically lower than the electron temperature for the same plasma conditions. For a 10 mbar plasma jet, the excitation temperature of argon is between 0.73 and 0.78 eV whereas the electron temperature is between 0.7 and 1.2 eV. This shows that at low pressure the plasma jets are not in LTE.These results contribute to the understanding of the supersonic plasma jet behavior at low pressure and can be used to quantify the deviation from local thermodynamic equilibrium (LTE). The extensive mapping of the measured physical properties of the jet will also serve as input for modeling.

Process monitoring of alumina nanoparticle synthesis by inductively coupled RF thermal plasma

Jong-Won Shin

Since the first inductively coupled plasma (ICP) torch was developed in the 60's, RF thermal plasmas have found a variety of applications in material processing such as crystal growth, thermal spray coatings, spheroidization and vaporization of refractory materials. In recent decades, inductively coupled thermal plasmas have been used for the synthesis of high purity nanoparticles, thank to their remarkable advantages such as high energy density, variable operating pressures and low product contamination. The formation of nano-sized particles in thermal plasmas particularly using solid refractory precursor is a complex process. Hereby the injected precursor powders are heated by the high temperature of the thermal plasma. The heated powders are vaporized and decomposed into vapor species. During the following cooling, the vapor species are condensed to nano-sized particles. In this synthesis process, characterized by evaporation-condensation, the properties of the synthesized particles such as particle size, size distribution, phases and morphology are affected by various process parameters. Gas pressure, flow rates of the different torch gases, size of the precursor powders and powder loading in the plasma are considered as the influencing process parameters. In order to optimize and control the process for tailor-made nanoparticle synthesis, it is necessary to understand the effects of the process parameters on plasma properties and on the particle-plasma interactions resulting in final properties of the synthesized powders. In the present work, firstly, the enthalpy probe technique, a well-established plasma diagnostic tool for high pressure thermal plasma, has been applied to characterize the inductively coupled Ar-H2 thermal RF plasma used for alumina nanoparticle synthesis at different operating conditions. The plasma enthalpy has been determined, and the plasma temperature under a given gas pressure could subsequently be calculated assuming local thermodynamic equilibrium (LTE), once the gas composition has been determined by mass spectrometry. The gas velocity of the plasma jet could also be obtained by using the Bernoulli equation and stagnation pressure. Secondly, the influence of flow rates of central gas and precursor feed rates on the Al2O3 precursor evaporation has been investigated by process monitoring. Optical emission spectroscopy (OES) and laser light extinction (LE) measurements have been carried out to monitor in-situ the injection of the alumina precursors and to obtain information on the ongoing precursor evaporation. The emission line intensities of the aluminum vapor were used to determine the dependence of process parameters on precursor evaporation. Furthermore, the laser light extinction was used to calculate the number density of non plasma-treated powders, from which the number fraction of evaporated powders could be deduced. The size and morphology of the synthesized particles have been characterized ex-situ. Finally, the influence of quenching conditions on the condensation and formation of the nano-sized alumina particle has been studied by process analysis of the gas phase reactions of alumina. Optical emission spectroscopic was performed at different axial positions to monitor the gas phase reactions of the vaporized particles. Axial profiles of the emission line intensities of the main vapor species of alumina, Al(g) and AlO(g), were compared with the results of the enthalpy probe measurements and the thermal decomposition reactions of alumina found in literature. The results allow explanation of the alumina reactions in the thermal plasma as well as gives indications for the optimal axial position of the quenching gas injection. In addition to the determination of the optimal quenching position, the influence of the flow rates of quenching gas on the nanoparticle formation has been investigated. The synthesis of the submicron alumina using vapor-condensation principle predominantly leads to various thermodynamically metastable crystal structures called transition alumina. Therefore, the occurrence of different transition aluminas is discussed with regards to the flow rates of the quenching gas. The results presented in this thesis show that the synthesis process of the nanoparticle can be described on the basis of experimental results and basic information on the powder materials. The explanation of the influence of process parameters on powder evaporation and nanoparticle formation shows the usefulness of the in-situ plasma process monitoring, and the large potential for optimizing and controlling the nanopowder synthesis process in an inductively coupled RF thermal plasma.

EPFL2006

Inclusion of aerodynamic non-equilibrium effects in supersonic plasma jet enthalpy probe measurements

Malko Gindrat, Christoph Hollenstein

Low pressure plasma spraying (LPPS) is a thermal spraying technique that has found a niche for low oxidation products. It uses a low pressure environment (i.e:, chamber pressure between 2 and 90 kPa) and yields supersonic plasma jets. The enthalpy probe technique is a common measurement method in plasmas. However LPPS jets are difficult to diagnose as their supersonic nature forces the apparition of a shock wave in front of any measuring device inserted in the jet. Incomplete or erroneous assumptions are usually invoked to overcome the difficulties associated with this shock wave and carry out the LPPS jet diagnosis from enthalpy probe measurements. In this work, a new device is designed to gain access to an additional physical quantity, which is needed to assess the aerodynamic non-equilibrium state of the jet. It is combined with enthalpy probe measurements, and the resulting set of experimental data is used with a numerical procedure based on gas dynamics theory, yielding free-stream supersonic plasma jet values from the measurements behind the induced shock wave. The results agree well with the phenomenology of supersonic jets in aerodynamic nonequilibrium. However this new method is restricted by the local thermodynamic equilibrium assumption, which is directly linked with the pressure and temperature conditions of the plasma jet.

2005

Process monitoring of alumina nanoparticle synthesis by inductively coupled RF thermal plasma

Jong-Won Shin

EPFL2006