Laurent Therese

The concept of plasma cleaning in glow discharge spectrometry

A plasma cleaning procedure to improve elemental depth profiling of shallow layered materials by glow discharge spectrometry is proposed. The procedure is based on two approaches applied prior to depth profiling, either individually or sequentially. The first approach employs a plasma generated at low power, i.e. a “soft” plasma, for removal of contaminants adsorbed on the surface of the target material. In the second approach, sacrificial material is sputtered under normal conditions, e.g. those used for depth profiling, to clean the inner surface of the anode of the glow discharge source. It is demonstrated that plasma cleaning in glow discharge optical emission spectrometry and glow discharge time-of-flight mass spectrometry improves significantly the spectrum of the target material, particularly at the commencement of sputtering due to stabilisation of the plasma as a result of removal of contaminants. Furthermore, modelling and validation studies confirmed that the soft plasma cleaning does not sputter the target material.

Silica nanofilled varnish designed for electrical insulation of low voltage inverter-fed motors

The resistance to partial discharges of pure, microfilled and nanofilled Polyetherimide varnish for low voltage motors fed by inverters is presented. Micro and nano-sized Silica particles (1.5 % wt) are used as fillers. Experimental results of lifetime versus temperature (-55 to 180°C), frequency (5 to 15 kHz) and level of the applied square voltage (+/- 1 kV to +/- 3 kV) clearly demonstrate that nanofillers are more efficient than microfillers. The lifetime of nanofilled polyetherimide varnish is found to be up to thirty times higher than the standard one. Crucial properties for a varnish such as its electrical conductivity, its dc and ac dielectric strengths and its bonding strength are measured. Due to the presence of aggregates having a micrometric size, the microfilled varnish exhibits the worst characteristics, except for the ac dielectric strength which remains unchanged. These aggregates act as bridges for charge carriers, increasing the conduction current and reducing the resulting dc dielectric breakdown field. They also reduce the mechanical properties of the varnish.

Investigations on dielectric properties of enameled wires with nanofilled varnish for rotating machines fed by inverters

The dielectric and mechanical properties of standard (PEI), nanoscale-filled and microscale-filled varnishes are detailed and analyzed in this communication. Fumed silica (10nm and 400nm) have been tested as a filler. A particular attention has been paid to Partial Discharge Inception Voltage, Lifetime under pulse voltage and bonding strength which are crucial for varnish to be used in rotating machines fed by inverters.

Improvement of the analytical performance in RF-GD-OES for non-conductive materials by means of thin conductive layer deposition and the presence of a magnetic field

Radiofrequency glow discharge coupled to optical emission spectrometry (RF-GD-OES) is a well-known analytical technique for bulk, surface and depth profiling and can be applied in the direct analysis of conductors, semiconductors and non-conductors, however for the latter case limits still exist. The problem is related to the low power deposited in the plasma due to a voltage drop developing inside the material. The voltage transfer coefficient, defined as the ratio between the peak voltage at the front and at the back of the sample. This depends on the sample capacitance, which itself is dependant on the material surface, thickness and permittivity. In order to improve the analysis of such non-conductive materials, thin conductive top layers are deposited on both sides of the sample which increases their voltage transfer coefficient. The aim of this work is to study the influence of these thin layers on the optical and electrical signals measured for the samples with varying thickness and diameter. Additionally, the influence of applying a magnetic field during the GD analysis has been evaluated as an attractive option in order to obtain higher sputtering rates, together with better ionisation and excitation efficiencies and as a consequence give improved emission intensities.

DC conduction current, dielectric strength and electroluminescence of standard, nanofilled and microfilled polyetherimide varnishes

Results of DC conduction, breakdown strength and electroluminescence of standard (PEI), nanoscale-fllled and microscale-fllled varnishes are described and analyzed in this communication. Silica (10nm and 400nm) has been used as a filler to increase the lifetime of such varnishes submitted to a partial discharge activity (varnish to be used in rotating machines fed by inverters). Experimental results have shown that the dielectric properties of the nanofilled varnish, which exhibits the higher PD lifetime under a PD stress, are not highly affected by the fillers. On the contrary, the microfilled varnish properties are strongly modified.

Characterization of an asymmetric DBD plasma jet source at atmospheric pressure

Non-thermal atmospheric pressure plasma jets or plasma plumes have recently been largely investigated in applications like materials processing, biomedical treatment and analytical chemistry. In this paper He-discharges in an asymmetrical Dielectric Barrier Discharge (DBD) have been experimentally studied with an Intensified Charge Coupled Device (ICCD) camera, Optical Emission Spectroscopy (OES) and Time-Of-Flight Mass Spectrometry (TOF-MS) measurements. The development of the plasma plume, its apparent length and the spatial distribution of the excited and ionized species along the plasma plume have been characterized as a function of the applied voltage and the gas flow rate. A TOF-MS spectrometer has been used to characterize ionized species (positive and negative ions) produced by the plasma alone and the plasma with addition of ethanol, both as a function of the distance between the plasma plume and the vacuum interface of the spectrometer.

Improve the luminous efficacy of dielectric barrier discharge xenon lamp

Xenon excimer dielectric barrier discharge is a good candidate for free mercury lightning. The studied device1 is consisting of two glass plates separated by a constant gas gap. The glass thickness is 4 mm and the gap 2 mm. A transparent conducting material (electrode) has been deposited on both external sides (plane to plane electrodes) and white phosphors2 on both internal sides of the dielectric. The lamp is filled with Neon-xenon gas mixture and operates in a pressure range of 100-400 torr. In a previous work1, for sinusoidal and pulsed excitations, we have shown the influence of the applied voltage (amplitude and frequency) on the consuming power, the light emission and mostly on the homogeneity of the discharge (modeling and experimental works).

Spatial emission behaviour of dielectric barrier discharge lamp driven by sinusoidal or pulsed voltage excitation

Xenon excimer dielectric barrier discharge is a good candidate for free mercury lamp. The studied device is consisting of two glass plates separated by a constant gas gap. The glass thickness is 4 mm and the gap 2 mm. A transparent conducting material (electrode) has been deposited on both external sides and phosphors (white emitting powders) on both internal sides of the dielectric. The lamp is filled with rare gas mixture and operates in a pressure range of 100-400 torr. A sinusoidal or a pulsed excitation voltage can be used up to 3000 V in a frequency range of 10-50 kHz. In a previous work for sinusoidal excitation, we shown the influence of the applied voltage (amplitude and frequency) on the consuming power, the light emission and mostly on the non-homogeneity of the discharge. Using a 2 dimensional model developed in our laboratory, the effects of the applied voltage (amplitude and frequency) and the pressure will be studied. Particularly on the distance between the streamers when the discharge is not homogeneous. In this work, for pulsed excitation, we will present some results concerning a Ne-Xe 50% mixture for three different pressures and we will discuss the influence of the applied voltage (waveforms, amplitude and frequency) on the consuming power, the light emission (ICCD and luminance) and mostly on the spatial emission of the discharge. We will also compare experimental results between sinusoidal and pulsed excitations.

Heating power at the substrate, electron temperature, and electron density in 2.45 GHz low-pressure microwave plasma

To control the temperature during a plasma treatment, an understanding of the link between the plasma parameters and the fundamental process responsible for the heating is required. In this work, the power supplied by the plasma onto the surface of a glass substrate is measured using the calorimetric method. It has been shown that the powers deposited by ions and electrons, and their recombination at the surface are the main contributions to the heating power. Each contribution is estimated according to the theory commonly used in the literature. Using the corona balance, the Modified Boltzmann Plot (MBP) is employed to determine the electron temperature. A correlation between the power deposited by the plasma and the results of the MBP has been established. This correlation has been used to estimate the electron number density independent of the Langmuir probe in considered conditions.

Comparison of plasma sources for surface treatment applications: Radiofrequency inductively coupled and surface-wave microwave plasma sources

Summary form only given. This work was conducted within the framework of PAUD (Plasma Airborne molecular contamination Ultra Desorption), a collaborative program funded by OSEO and certified by French global competitive clusters Minalogic and Trimatec. The main object of interest is to develop new technology bricks for next generation molecular decontamination systems. As part of our research effort in evaluating the potential of plasma treatment as a solution, the performance of different plasma sources was evaluated.Here, we will show the electronic density and temperature radial profiles, as well as plasma emission at different locations obtained with both plasma sources. The results will be presented as a function of sources injected power (up to 2 kW), argon gas flow (10 ~ 300 sccm) and pressure (5 10-3 ~5 10-1 mbar).