Nucleation, solvation and boiling of helium excimer clusters

Helium excimers generated by a corona discharge were investigated in the gas and normal liquid phases of helium as a function of temperature and pressure between 3.8 and 5.0 K and 0.2 and 5.6 bar. Intense fluorescence in the visible region showed the rotationally resolved $d^3\Sigma_u^+ \rightarrow b^3\Pi_g$ transition of He$_2^*$. With increasing pressure, the rotational lines merged into single features. The observed pressure dependence of linewidths, shapes and lineshifts established phases of coexistence and separation of excimer-helium mixtures, providing detailed insight into nucleation, solvation and boiling of He$_2^*$-He$_n$ clusters.Comment: 5 pages, 5 figure

Caractérisation de phénomènes physiques associés à l'ouverture et à la fermeture dans un relais MEMS.

Cette thèse s'inscrit dans la continuité des études menées pour améliorer la fiabilité des relais MEMS ohmiques et comprendre les mécanismes de dégradation se produisant au niveau du contact électrique aux échelles micro et sub-micrométriques. Les deux premiers chapitres de ce manuscrit permettent d'établir l'état de l'art du domaine et de décrire les différentes techniques expérimentales utilisées afin de caractériser les mécanismes physiques se produisant lors de l'ouverture et la fermeture d'un relais MEMS sous courant. Le troisième chapitre étudie qualitativement et quantitativement le transfert de matière aux distances sub-micrométriques. L'utilisation d'un microscope à force atomique (AFM) permet d'identifier les paramètres clés, notamment la tension de contact à l'état ouvert et la vitesse de commutation. L'origine de ce transfert de matière est attribuée à des émissions de courant se produisant dans les derniers nanomètres avant la fermeture du contact. Un plasma métallique est également observé et caractérisé pendant les phases de commutations. Ces observations conduisent à l'élaboration d'un scénario permettant d'expliquer le transfert de matière à ces dimensions. Le quatrième chapitre se consacre en première partie à l'étude des rebonds lors de la fermeture du contact. On montre que des rebonds peuvent apparaître quelques s après la fermeture du contact au cours des cycles. Ceux-ci semblent être des indicateurs de la fin de vie du composant. D'autres rebonds, liés aux forces électrostatiques de contact, sont également mis en évidence lors de fermetures à faibles vitesses (qq nm/s). L'importance de ces forces est néanmoins du second ordre et ces derniers rebonds n'interviennent pas directement dans la phase de fermeture d'un relais MEMS. L'étude de la quantification de la résistance de contact lors de l'ouverture du contact constitue la deuxième partie de ce dernier chapitre. La nature quantique de ce phénomène est mise en évidence dans deux dispositifs : un interrupteur MEMS et à l'aide d'un AFM. Il est notamment montré que ce phénomène est seulement observable pour des courants inférieurs à 100 A. Finalement, l'ensemble de ces travaux mènent à différentes recommandations, détaillées en conclusion, nécessaires pour assurer le bon fonctionnement des relais MEMS.This thesis aims to improve the reliability of ohmic MEMS switches and focuses on the degradation mechanisms of the electrical contact at the micro and nano-scales. The two first chapters of the manuscript provide a state-of the-art of MEMS switches and describe the different experimental techniques used to characterize the physical phenomena involved in the opening and closure of a MEMS switch under current ( hot switching actuation ). The third chapter studies qualitatively and quantitatively the material transfer at sub micrometer scale. An Atomic Force Microscope (AFM) is used to identify the main parameters involved in this phenomenon such as the opening contact voltage and the closing velocity. The origin of the material transfer is attributed to field emission in the last tens of nanometers before the contact closure. A metallic plasma is also observed and characterized during switching operations. According to the different observations, a scenario is suggested to explain material transfer at such small dimensions. The fourth chapter deals with dynamic observation during switching operations. First, bounces can be detected after a few millions of operations, they usually appear a few s just after the first contact. Such bounces seem to be an early indicator of the lifetime of those devices. Other types of bounces related to the electrostatic contact force can be observed at very low closing velocity (a few nm/s). Nevertheless in a MEMS switch the closing and opening velocity is high enough to avoid such bounces. The second part of this chapter investigates the contact conductance quantization during the opening phase of a contact. We show that this phenomenon can be observed in a MEMS switch and with an AFM when the current is lower than 100 A. As a conclusion, several recommendations are provided to improve the reliability of MEMS switches.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF

Hydrodynamic parameters of air-bubble stimulated underwater spark discharges

Considerable amount of research work has been focused on investigation and optimization of strong acoustic waves generated by spark discharges in water. There are several methods to achieve and to stimulate underwater spark breakdowns, including free-discharges, wire-guided and gas-bubble stimulated discharges. In the present work, air bubbles are injected into water in order to achieve spark discharges in relatively long inter-electrode gaps. This paper reports on hydrodynamic and acoustic characteristics of spark discharges stimulated by air bubbles and presents the functional relationships between the hydrodynamic and electrical parameters of such discharges, including breakdown voltage, spark plasma resistance and energy available in the discharge. A hydrodynamic analytical model has been developed and used to calculate the acoustic efficiency of the underwater spark discharges

Modelling of positive streamers in SF6 gas under non-uniform electric field conditions : effect of electronegativity on streamer discharges

The use of SF6 in electrical insulation and fast-switching applications cannot be overemphasized. This is due to its excellent dielectric properties and high breakdown voltage, which are especially important for practical applications such as gas-insulated switchgears and pulsed power switches where pressurized SF6 is used. Breakdown in the gas occurs via streamer–leader transition; however, this transition is difficult to quantify numerically at atmospheric pressure because of the electronegativity of the gas. In the present work, streamer discharges in SF6 gas at pressures of 10 and 100 kPa were studied using a plasma fluid model implementation. Analysis of the electric field in the streamer body, streamer velocity, diameter, and the effect of the high electronegativity of the gas on streamer parameters are presented for positive polarity in a point-to-plane geometry. The streamers in SF6 for non-uniform background fields are compared to those in air, which have already been studied extensively in the literature

Field-time breakdown characteristics of air, N2, CO2, and SF6

The dielectric performance of gases in insulation systems used in high voltage power and pulsed power applications is a subject of intensive theoretical and experimental investigations. Transient breakdown processes in gases stressed with short, high-field impulses, have been studied for many decades. However, there are still significant gaps in the understanding of the main breakdown processes and mechanisms associated with fast transient breakdown processes in gases. This knowledge is important for the optimization of gaseous insulating systems and for the coordination of gaseous insulation in power and pulsed power apparatuses. This information is also required for the development of gas-filled components such as circuit breakers and plasma closing switches. This article is aimed at the analysis of the field-time breakdown characteristics of air, N2, CO2, and SF6, using kinetic and drift-diffusion approaches. The kinetic approach is based upon the avalanche-to-streamer transition criterion, while the fluid drift-diffusion model requires self-consistent numerical solution of the continuity equations for charged species, and the Poisson equation for the electric field. The time to breakdown as a function of the applied field was obtained for all investigated gases. The obtained analytical results agree well with the experimental data reported in the literature, which suggests that both approaches can be used for insulation coordination, and for the development of gas-insulated power and pulsed power systems and components

Impulsive discharges in water : acoustic and hydrodynamic parameters

Underwater spark discharges are used in multiple practical applications including plasma closing switches, water treatment, plasma channel drilling and mineral processing, waste recycling, treatment of metals and medical lithotripsy. Spark discharges in water have been studied for several decades, however, despite significant research efforts and progress in this area, further investigation into the efficiency of plasma-acoustic sources and their optimisation is required in order to expand their practical applications. This paper is aimed at investigation of the electrical and hydrodynamic parameters of underwater plasma-generated cavities, including plasma resistance, energy delivered into the plasma cavity, period of cavity oscillations and characteristics of pressure impulses. Different energy levels, breakdown voltages and gap distances were used in the present study to allow systematic analysis of these electrical and hydrodynamic parameters. Empirical scaling laws which link the maximum acoustic pressure and the period of cavity oscillations with the energisation parameters and the resistance of plasma have been obtained. These empirical functions can be used for optimisation of the plasma-acoustic sources and for tailoring their parameters for specific practical application

Application de la spectroscopie à l’étude des décharges électriques dans les milieux denses

National audienc

Application de la spectroscopie d'émission à l'étude des décharges électriques dans les milieux denses

International audienc