Jérôme Rossignol

Numerical modelling of thermal ablation phenomena due to a cathodic spot

A numerical simulation of the ablation problem for the cathode spot that is based on the enthalpy formulation is presented and solved with a finite-element method using the Euler explicit scheme. Vaporization latent heat and ablation phenomena constitute the main difficulties of the cathodic surface erosion. We deduce characteristic information related to the cathode spot such as the energy repartition in three phases and the ablation length for different energy fluxes.

The modelling of the cathode sheath of an electrical arc in vacuum

This paper presents a simple model of the fragment in the cathode electrical arc root taking into account the physical phenomena occuring on the cathode surface and the sheath. The goal is the obtainment of characteristics values of the heat flux, the electrons, and atoms density in the sheath. Computation is carried out on a one-dimensional model with a coupling between the equation obtained in the sheath and an enthalpy model of the cathode to describe the temperature evolution. In the modelling, we introduce a friction zone above the sheath edge to characterize the heavy particle interactions. Numerical simulation shows that the ionic friction phenomenon deriving from ion–atom collision regulates the heat flux lightening the surface, and the crucial necessity to obtain a good evaluation of the cross section of the charge exchange.

Microstrip Spiral Resonator For Microwave-Based Gas Sensing

This paper presents the design of a new microstrip gas sensor based on a trapezoidal spiral resonator covered by a titanium dioxide nanoparticle layer. Reflection and transmission coefficient measurements between 1 and 8GHz revealed multiple resonances that can be used in the context of gas detection. The sensor response upon ammonia adsorption has been evaluated on the whole frequency span with a simple spectral analysis. Kinetic parameters such as response time, reversibility, and measurement stability have been determined with a monofrequential temporal analysis. The sensor showed at each resonance a good sensitivity to ammonia for concentrations between 50 and 300ppm, with a response time less than 1 min. The maximal response was reported at 7.76GHz with a reflection coefficient variation of 0.45dB after the injection of 300ppm of ammonia, which is 3.5 times higher than previously reported.

Metal Oxide Nanoparticles Obtained by Microwave Synthesis and Application in Gas Sensing by Microwave Transduction

In literature, many papers describe the applications of semiconductor as sensitive material in sensor field. The gas sensor using tin oxide requires a strictly controlled high operating temperature in order to detect both reducing and oxidizing gases. The semiconductor nanoparticles, with their high specific surface area, increase the gas sensing performance. The originality of this work is to valorize the nanoparticle of metal oxide like SnO2, TiO2 obtained by microwave thermohydrolysis synthesis, using a gas sensing microwave transduction. The present synthesis is to prepare metal oxide nanocrystalline powder with a high surface area by microwave-induced thermohydrolysis. We propose to study the influence of the metal oxide nanoparticle, as a sensitive layer, in gas sensing measurement. The pollutant is added into an argon flow (dynamic regim). This work highlights a specific sensor response to each ammonia concentration at room temperature. It shows a quasi-linear relationship between the set of points of the real part of the response and the ammonia concentration. The authors are currently working on these issues as well as the interaction mechanism between adsorbed gas molecules and metal oxide films.

Damage in Composite Material: A Microwave Detection

In the field of the transport, the increase of the security rule recommends to a periodic control of the structure to detect damage due to mechanical loadings. Now, current materials, used in the case of transport applications, are the composite materials. The methods, to control these materials or composite structures, need to be low cost, non-destructive, in-situ and swiftness as far as possible. The scientific literature reports many methods to control the damage in composite materials and structures. However the above requirements and the adaptation to composite materials reduce the number of methods that can be used. Currently, the adapted methods are based on infrared thermography, acoustical emission, EMIR (ElectroMagnetic InfraRed) or microwave imagery. We present an innovative non-destructive method of detecting damages in composite materials. The method is based on the observation and analysis of the modification in dielectric material resulting from damage. The originality of this method is that the diagnostic is obtained by using a microstrip resonator at microwave frequencies. The feasibility of the method is demonstrated by the detection of a fibre break into an unidirectional composite submitted to a flexural loading. The fibre break is the damage to detect. The perspective of this work is to develop a quantification and a localization of damages.

Theoretical elements about cathode arc root

In this presentation the authors show how it is possible to solve numerically the equation describing the heat flux in the metallic electrode situated under the cathode spot. The 1D model gives a good scale of sizes in time and length. The other phenomena that occur on the electrode surface are also presented and discussed.