T. Leblanc

Contribution to the assessment of the power balance at the electrodes of an electric arc in air

The aim of this paper is to improve our knowledge concerning the power balance at the electrode surfaces in the case of an electric arc of short duration with a small electrode gap burning in air at atmospheric pressure.With this aim in view, we propose a simple method using the experimental results obtained through the observation of the tracks left by the arc and a numerical simulation of the thermal phenomena occurring in the electrode during the arc heating.This method, based on the analysis of the compatibility between experimental results and modelling results, allows us to assess a zone of possible values for the main parameters of the arc root (power and surface power density brought by the arc to the electrodes). A simple usual volt equivalent approach of the power balance is proposed. Calculations and experiments have been conducted for several copper anodes and cathodes. The values for the volt equivalent at the anode are found in the range 9?12.6?V, for the cathode 5.4?9?V. The values for the surface power density are found to be near 6.5 ? 109?W?m?2 at the cathode and 6 ? 109?W?m?2 for the anode.

Development of laser ablation multi-collector inductively coupled plasma mass spectrometry for boron isotopic measurement in marine biocarbonates: new improvements and application to a modern Porites coral†

RATIONALE Laser Ablation coupled to Multi-Collector Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICPMS) is a powerful tool for the high-precision measurement of the isotopic ratios of many elements in geological samples, with the isotope ratio ((11) B/(10) B) of boron being used as an indicator of the pH of oceanic waters. Most geological samples or standards are polished and ablation occurs on flat surfaces. However, the shape and the irregularities of marine biocarbonates (e.g., corals, foraminifera) can make precise isotopic measurements of boron difficult. Even after polishing, the porosity properties and the presence of holes or micro-fractures affect the signal and the isotopic ratio when ablating the material, especially in raster mode. METHODS The effect of porosity and of the crater itself on the (11) B signal and the isotopic ratio acquired by LA-MC-ICPMS in both raster and spot mode was studied. Characterization of the craters was then performed with an optical profilometer to determine their shapes and depths. Surface state effects were examined by analyzing the isotopic fractionation of boron in silicate (NIST-SRM 612 and 610 standards) and in carbonate (corals). RESULTS Surface irregularities led to a considerable loss of signal when the crater depth exceeded 20 µm. The stability and precision were degraded when ablation occurred in a deep cavity. The effect of laser focusing and of blank correction was also highlighted and our observations indicate that the accuracy of the boron isotopic ratio does not depend on the shape of the surface. After validation of the analytical protocol for boron isotopes, a raster application on a Porites coral, which grew for 18 months in an aquarium after field sampling, was carried out. CONCLUSIONS This original LA-MC-ICPMS study revealed a well-marked boron isotope ratio temporal variability, probably related to growth rate and density changes, irrespective of the pH of the surrounding seawater. Copyright

High intensity contact opening under DC voltage

This work concerns the study of contact opening under DC voltage in the case of high intensity current (a few hundred amperes). The application fields are mainly automotive and aeronautical systems. An experimental device has been designed in order to study the opening of contacts under various conditions. The influence of several parameters on contact opening is studied. These are the following: the opening velocity, the current intensity through the closed contact, the nature of the contact materials (Cu, Ag, Ag-12 wt% SnO2), and the value of the inductance in the electrical circuit. The measured parameters characterizing the opening phase are: the arc duration, the contact resistance, the values of the electrode gap when the arc extinguishes, the arc energy and the electrode erosion. The structure and the behavior of the arc during the opening phase are investigated with the help of a high speed cinematography device. The study of the erosion is achieved with the help of a 3D profiler enabling the measurement of mass transfer and, for instance, of the amount of welded or evaporated metal. The use of a 2D numerical model of the thermal phenomena, induced by the arc at the contact opening, makes it possible to get interesting information concerning the power balance at the contact electrodes during the opening.

High-Intensity Contact Opening Under 36-V DC Voltage Application for Mild Hybrid Vehicles

This paper concerns the study of contact opening under dc voltage in the case of high-intensity current (a few hundred amperes). The application fields are mainly automotive and aeronautical systems. An experimental device has been designed in order to study the opening of contacts under various conditions. The influence of several parameters on contact opening is studied. These are the following: the opening velocity, the current intensity through the closed contact, the nature of the contact materials (Ag, Ag-12wt%SnO2), the value of the inductance in the electrical circuit. The measured parameters characterizing the opening phase are as follows: the arc duration, the value of the electrode gap when the arc extinguishes, the arc energy, the maximal value of the contact voltage during the opening phase, and the electrode erosion. The study of the erosion is achieved with the help of a 3-D profllometer enabling the measurement of mass transfer and, for instance, of the amount of molten or evaporated metal.

Temperature Measurement of Tungsten Electrode Surface at Electric Arc Extinction in Air–Power Flux Estimation

This paper deals with temperature measurements of the surface of tungsten electrodes submitted to electric arcs in nonstationary regimes in air at atmospheric pressure and with the estimation of the power and power density brought by electric arcs to the electrodes. Temperature measurements are proposed for tungsten anodes and cathodes for current intensity of ~30 A and duration of 3 ms. The temperature of the electrode surface at the instant the arc extinguishes is estimated in both electrode polarities. These experimental results are used in a model to estimate the power and the power density brought to the tungsten electrode surface by electric arcs. We found that the power surface density brought by electric arcs to the electrodes was in the range 1. × 109-1.5 × 109 W/m2 in the case of anodes and in the range 1.3×109-1.9×109 W/m2 in the case of cathodes. It has also been found that reactions of tungsten oxidation had to be considered to explain the high values obtained for the power brought to the electrodes.

Parametric Study of the Current–Voltage Characteristics of a 100-mbar DC Discharge in Argon: From the Diffuse Glow Discharge to the Arc Regime

The aim of this paper is to investigate the glow discharge in its diffuse and filamentary regime, and to study its transition to arc in argon at 100 mbar. The structure of the discharge is observed in correlation with its electrical signals in both static and in dynamic modes. Glow discharges were identified with a positive column that can be fully diffuse, fully filamentary, or a mix of the two with the filamentary part of the column always attached to the anode. Spontaneous transitions between glow discharges and arcs have also been observed and their dynamics are studied. Using high-speed imaging, the transition between glow and arc discharge was identified as the occurrence of two distinct phenomena: a propagation mechanism in the positive column and a constriction of the cathode root. An evaluation of the duration of each phenomenon as a function of electrode gap and current intensity was obtained. The duration of the constriction of the cathode root is found to be on the order of tens of nanoseconds and the duration of the propagation mechanism in the range 50–550 ns.

Experimental assessment of the surface temperature of copper electrodes submitted to an electric arc in air at atmospheric pressure

This paper concerns the assessment of the surface temperature of copper electrodes submitted to an electric arc in a non stationary regime in air. An infrared camera is used to measure the decrease of the temperature surface just after a controlled and very fast arc extinction. In the first part, the experimental method is described. In the second part, results are presented for 60–70 A with an electric arc duration in the range 3–4 ms. The temperature decrease after the arc extinction allows to reach an assessment of the surface temperature just at the arc switching off. In the present experimental conditions the mean temperatures reached for copper cathodes and anodes are in the range 750–850°C.

An original method to assess the surface power density brought by an electric arc of short duration, and short electrode gap to the electrodes-case of copper electrodes

In this paper, an original method is proposed in order to obtain an interval of possible values for the surface power density brought to the electrodes by an electric arc of short duration and short electrode gap. Results for copper electrodes are presented in the case of an electric arc in air at atmospheric pressure.