Gilberto Petraconi

Study of SF6 and SF6/O2 plasmas in a hollow cathode reactive ion etching reactor using Langmuir probe and optical emission spectroscopy techniques

In this work, electrical and optical studies of SF6 and SF6/O2 plasmas generated in a hollow cathode reactive ion etching reactor were performed using the Langmuir probe and optical emission spectroscopy techniques, respectively. We carried out an investigation aimed at understanding the influence of radio-frequency power, gas pressure and O2 gas mixing ratio on plasma parameters, namely electron temperature, electron density and electronegativity, and also atomic fluorine density. The results indicate an increase of up to one order of magnitude in electron density and atomic fluorine in the overall gas volume when compared with a conventional reactive ion etching plasma generated under the same operation conditions.

Formation of electrostatic double-layers and electron-holes in a low pressure mercury plasma column

Experimental studies of the formation of electrostatic double layers (DLs) and electron-holes (e-holes) are reported. The measurements were performed in the positive column of a mercury arc discharge operating in the low-pressure range of (2.0–14.0) × 10−2 Pa with current density in the range of (3.0–8.0) × 103 A m−2. Stable and unstable modes of the discharge were identified as the current was gradually increased, keeping constant the vapour pressure. The discharge remains stable until a critical current from which a slight increase of the current leads to an unstable regime characterized by high discharge impedance and strong oscillations. This mode ceased after a DL was formed in the plasma column. To induce the DL formation and to transport it smoothly along the discharge column, a low intensity B-field (7–10) × 10−3 T produced by a movable single coil was used. The B-field locally increases the electron current density and makes the DL form at the centre of the magnetic constriction where it remained at rest. Electrostatic potential structures compatible with ordinary DLs and multiple-layers could be formed in the plasma column by dealing with the combined effects of the operational parameters of the discharge. It is noticeable that a pure e-hole, which is a symmetric triple-layer having a bell shape potential profile, could easily be formed by means of this experimental technique. A partial kinetic description, based on the space charge structure derived from an experimental e-hole, is presented in order to infer the charged particle populations that could contribute to the space charge of the e-hole. Evidence is shown that strong e-hole formation might be driven by an ion beam, therefore it could not be formed in isolation since its formation requires a nearby ion accelerating potential structure. Probe measurements of the plasma properties, at various radial positions of the stable positive column, are also presented. In the stable mode, prior to current limitation, the probe data reveal a substantial radial decrease of the electron drift velocity. This result calls for a review of the free fall theories of low pressure plasma columns to take into account this non-uniformity of the electron drift velocity.

Comparison of the Ablation Mechanism of C/C-SiC Composite under Atmospheric and Low Pressure

Comparisons of heating tests at atmospheric pressure and low pressure by using a thermal plasma torch were performed. A constant heat flux on the sample surface was applied in the study of the oxidation mechanism of C/C-SiC composite, used in thermal protection systems. The SEM and EDS analysis show an intensive glassification at the surface, which are strongly depend on the oxygen partial pressure and the sample surface temperature. For vacuum conditions, at maximum surface temperature of 1450 °C and the oxygen partial pressure of about 66 Pa, a uniform passivation layer of SiO2 is formed. At atmospheric pressure, under an oxygen partial pressure of 2.1×104 Pa, the maximum surface temperature is 400 °C higher than obtained in vacuum, reaching levels of 1850°C. Under these conditions, the protective oxide layer is partially volatilized with time, increasing the specific mass loss rate by a sublimation of the composite, directly exposed to the plasma jet. This effect is alike to what occurs in the process of transition from passive to active oxidation of SiC.

Study of the CF 4 capacitive plasma chemistry through mass spectrometry technique and global model

In this work the chemistry of CF4 capacitive plasma is studied. For this, experimental measurements were made by mass spectrometry technique which allowed the analysis of neutral species generated during the fragmentation of the source gas by the electrical gas discharge. Additionally, we use global model simulations in order to complement the experimental results, allowing to discern the main chemical processes occurring in the CF4 plasma. The global model developed here considers the main chemical reactions in CF4 plasma: momentum transfer, vibrational, ionization, dissociation, electron attachment and loss, recombination between charged and neutral species in the gas phase and the reactor walls.

Electron energy distribution function measurements in a low pressure expanding jet plasma

In this paper, we report on the existence of two electron temperature populations in the low pressure expanding dc plasma jet through the investigation of the spatial evolution of EEDF measured by a single Langmuir probe system. It was observed that for argon the plasma jet is Maxwellian type in all discharge axes. However, when an electronegative gas was inserted/replaced the EEDF turns “two-temperature” Maxwellian type or is deformed, increasing the high energy electron population. Finally, the effect of gas discharge power shows different behaviors when an electropositive gas was replaced by an electronegative gas.

DEGRADATION OF CARBON-BASED MATERIALS UNDER ABLATIVE CONDITIONS PRODUCED BY A HIGH ENTHALPY PLASMA JET. doi: 10.5028/jatm.2010.02013340

A stationary experiment was performed to study the degradation of carbon-based materials by immersion in a plasma jet. In the experiment, graphite and C/C composite were chosen as the target materials, and the reactive plasma jet was generated by an air plasma torch. For macroscopic study of the material degradation, the sample’s mass losses were measured as function of the exposure time under various temperatures on the sample surface. A microscopic analysis was then carried out for the study of microscopic aspects of the erosion of material surface. These experiments showed that the mass loss per unit area is approximately proportional to the exposure time and strongly depends on the temperature of the material surface. The mass erosion rate of graphite was appreciably higher than the C/C composite. The ablation rate in the carbon matrix region in C/C composite was also noticeably higher than that in the fiber region. In addition, the latter varied according to the orientation of fibers relatively to the flow direction. These tests indicated an excellent ablation resistance of the C/C composite, thus being a reliable material for rocket nozzles and heat shielding elements of the protection systems of hypersonic apparatuses from aerodynamic heating.