Caractérisation d’un plasma radiofréquence d’argon avec injection pulsée de gaz en vue d’une application au dépôt de couches minces nanocomposites.

Les matériaux (nano)composites font partie intégrante de l’industrie de l’aéronautique et de l’espace depuis plus de 50 ans. De nos jours, le concept de matériaux multifonctionnels combinant diverses propriétés pour réaliser des objectifs de performance multiples en un seul et unique système est devenu une exigence pour le développement de surfaces innovantes, et ce, pour une vaste gamme d’applications technologiques. Cependant, pour plusieurs applications, un des principaux verrous est l’obtention de revêtements formés de nanoparticules isolées (non-agrégées) et de petite taille (<10 nm) dispersées de manière contrôlée dans une matrice. Dans ce contexte, une nouvelle méthode de synthèse souple, verte, sécuritaire et industrialisable a récemment été proposée. Celle-ci repose sur un réacteur-injecteur de nanoparticules et permet de synthétiser des nanoparticules à partir de précurseurs organométalliques liquides juste avant de les injecter dans un réacteur de dépôt par plasma en limitant les phénomènes d’agglomération associés à la vaporisation de gouttelettes et en évitant les problèmes de toxicité éventuelle en lien avec la manipulation de nanoparticules avant le dépôt. Cependant, contrairement aux procédés de dépôt par plasma habituels qui s’effectuent la plupart du temps à pression constante, la conception du réacteur-injecteur de nanoparticules implique inévitablement une dynamique temporelle complexe associée à des variations assez brutales de la pression dans le réacteur à plasma. À l’évidence, ces variations temporelles de pressions vont se répercuter sur l’évolution temporelle des propriétés fondamentales du plasma telles que la densité et la température des électrons. Dans ce travail de maîtrise, nous avons eu recours à la spectroscopie optique d’émission couplée aux prédictions de modèles collisionnels-radiatifs pour déterminer des conditions opératoires du réacteur-injecteur dans un plasma rf d’argon minimisant ces variations d’une part, et permettant de mieux comprendre leurs implications sur la température des électrons, d’autre part. Ces travaux serviront ainsi d’effet levier à des études plus complexes en présence de précurseurs et de nanoparticules.(Nano) Composite materials have been an integral part of the aeronautics and space industry for more than 50 years. Nowadays, the concept of multifunctional materials combining various properties to achieve multiple performance objectives in a single system has become a prerogative in the development of innovative surfaces for a wide range of technological applications. However, for several applications, one of the main challenges is the production of coatings formed of isolated (non-aggregated) and small (<10 nm) nanoparticles dispersed in a controlled manner in a matrix. In this context, a new flexible, green, safe and scalable method of synthesis has recently been proposed. It is based on a reactor-injector of nanoparticles and can synthesize nanoparticles from liquid organometallic precursors just before injecting them into a plasma deposition reactor by limiting the agglomeration phenomena associated with the vaporization of droplets and by avoiding problems of possible toxicity related to the manipulation of nanoparticles before the deposit. However, unlike conventional plasma deposition processes, which are usually carried out at constant pressure, the design of the reactor-injector of nanoparticles inevitably involves a complex temporal dynamic associated with rather sudden changes in the pressure in the plasma reactor. Obviously, these temporal variations of pressures will affect the temporal evolution of the fundamental properties of the plasma such as the density and the temperature of the electrons. In this master work, we used optical emission spectroscopy coupled with the predictions of collisional-radiative models to determine operating conditions of the reactor-injector in an argon rf plasma minimizing these variations on the one hand, and to better understand their implications on the electron temperature, on the other hand. This work can thus be expected to serve as building blocks for more complex studies in the presence of precursors and nanoparticles

Electrical and optical characterization of a capacitively-coupled RF plasma with a pulsed argon gas injection

International audienceNanocomposite thin film deposition using a reactor-injector of nanoparticles implies a pulsed gas injection. In a plasma, this can affect the behavior of the downstream process. Here, the case of an asymmetric low-pressure RF plasma with a pulsed argon gas injection is analyzed by electrical and optical emission spectroscopy measurements. It is found that this injection mode can highly affect the plasma stability: both the electron temperature and density are modified during the rise and the decrease of the gas pressure. A new injection mode combining continuous and pulsed injections is proposed to obtain more stable conditions

Time-Resolved Analysis of the Electron Temperature in RF Magnetron Discharges with a Pulsed Gas Injection

International audiencePulsed gas injection in a plasma can affect many fundamentals, including electron heating and losses. The case of an asymmetric RF magnetron plasma with a pulsed argon injection is analyzed by optical emission spectroscopy of argon 2p-to-1s transitions coupled with collisional-radiative modeling. For a fully detailed population model of argon 2p levels accounting for direct and stepwise electron-impact excitation in optically thick conditions, a rapid decrease in the electron temperature, Te, is observed during each gas injection with the sudden pressure rise. The opposite trend, with unrealistic Te values before and after each pulse, is observed for analysis based on simple corona models, thus emphasizing the importance of stepwise excitation processes and radiation trapping. Time-resolved electron temperature variations are directly linked to the operating parameters of the pulsed gas injection, in particular the injection frequency. Based on the complete set of data, it is shown that the instantaneous electron temperature monotonously decreases with increasing pressure, with values consistent with those expected for plasmas in which charged species are produced by electron-impact ionization of ground state argon atoms and lost by diffusion and recombination on plasma reactor walls

Time-Limited Therapy with Belatacept in Kidney Transplant Recipients

International audienc

Evidence and circularity in multimodal discourse analysis

In the context of rapid theoretical development in multimodal discourse analysis, and of its growing interdisciplinary influence, it is crucial that those working in the field give due consideration to methodological rigour. The corpus-based approach described here offers a means of addressing some key methodological issues. Firstly, this approach provides a check on over- and under-interpretation and also reveals a more nuanced picture of data about specific genres than might be derived from even the closest observation of individual instances. Thus it helps avoid pitfalls associated with relying on hand-picked examples. Secondly, the semi-automated implementation of a multilayered annotation scheme, which separates the representation of layout from rhetorical structure, supports the empirical investigation of a variety of research questions, while minimizing the influence of the analyst on the data by delaying interpretation insofar as possible until it becomes unavoidable. This article illustrates the corpus-based approach through a contrastive case study of one very visual genre, product packaging, with data taken from two locales, Taiwan and the UK. In so doing, issues of the selection of texts for inclusion and corpus design are addressed and the principles and practicalities involved in data preparation are discussed. Consideration is also given to the types of question which such an approach enables us to explore. In addition, since the data analyzed here are drawn from different languages and cultures, the present study sheds light on some issues of interest from the perspective of localization. Finally, some benefits of the approach are suggested, among which not least is that a stronger basis for the critique of designs in turn supports identification of opportunities for their improvement. This is not possible when the analysis is itself circular