1,720,986 research outputs found
Experiments and modelling for glow discharge plasmas applied to niobium sputter deposition in superconducting radiofrequency cavities
Get PDFThin film coatings are ubiquitous in modern daily lives, from the semi-conductor industry to aesthetic applications, and rely on the modification of a given material to enhance its surface properties.
At CERN (European Organization for Nuclear Research), thin film coatings are used for several applications related to the accelerator technology field, such as electron cloud mitigation, continuous pumping or radio-frequency (RF) superconductivity.
This PhD thesis aims at studying the use of a Particle-in-Cell Monte Carlo/ Direct Simulation Monte Carlo (PICMC/DSMC) numerical code applied to the modelling of deposition processes.
The present study focuses on niobium thin film deposition on copper RF cavities of peculiar geometries and large sizes. Numerical simulations can help to understand and improve process parameters in existing systems, and guide the design of future coating systems such that thin ï¬ lm characteristics can be predicted and expensive times of Research and Development can be reduced.
In the first part of this thesis, critical parameters are identified to provide an accurate modelling of glow discharge plasmas commonly used in coating applications with the PICMC module of the numerical code. A dedicated experimental system is designed to enable both DC diode and DC magnetron operation with a coaxial cylindrical plasma source. Suitable ion-induced secondary electron emission yield and energy distribution are numerically assessed by matching simulated discharge voltages and currents with experimental ones. Simulated local plasma parameters, such as electron density and energy, are compared with experimental measurements obtained with a Langmuir probe.
In the second part, transport simulations of sputtered neutral niobium atoms performed with the DSMC method are validated by comparing simulated deposition rates on a substrate with experimental ones with a compact hollow cathode magnetron sputtering source.
At last, the validated ab initio methodology for coating process modelling, from plasma simulation and extraction of sputtering profiles to sputtered niobium atoms transport, is applied to typical coating systems used at CERN. The scalability of low discharge power simulation results to realistic powers is first demonstrated for a planar magnetron source in terms of cathode erosion and deposited thin film thickness profiles. Then, an elliptical RF cavity is used as a case study to apply the full methodology in a real substrate of complex shape. PICMC and DSMC simulation results are further extended towards thin film growth modelling, and simulated thin film morphology is compared with experimental Focused Ion Beam/Scanning Electron Microscopy imaging of the niobium layer.SP
Caractérisation de décharges à barrières diélectriques atmosphériques et sub-atmosphériques et application à la déposition de couches d'oxyde de silicium
Get PDFDielectric Barrier Discharges (DBD) have been used for more than a century, especially for ozone production. Research conducted within the last twenty years has investigated the discharge mechanisms involved and the different discharge regimes observable (filamentary, glow, Townsned and multi-peaks). These studies highlight the fundamental role of metastable species to establish and maintain a homogeneous discharge. These recent improvements in understanding the physics of DBD's open perspectives for new applications and new interests in atmospheric pressure surface treatment. Working at atmospheric pressure for silicon oxide deposition is of great interest : the possibility of continuous process, no vacuum component costs and maintenance, no loading/unloading time. However, in comparison with a classical plasma enhanced chemical vapor deposition (PECVD) process, the high pressure and thus the high gas density may result in a gas phase chemistry and a larger formation of dust particles. Exploring a new pressure range from 10 to 1000 mbar could be an alternative for this process. In the first part of this work the effect of the pressure on a DBD in non-reactive gases (helium, argon and nitrogen), then in neutral gas/oxygen mixture has been investigated with electrical measurements (discharge current and applied voltage), with high-speed imaging and with time-resolved optical emission spectroscopy. The second part of this work is dedicated to SiOx barrier coating characterization (FTIR-ATR, XPS, AFM and SEM) as a function of pressure in oxygen/hexamethyldisiloxane (HMDSO) gas mixture highly diluted in nitrogen. The exploration of discharge regimes as a function of pressure shows, in nitrogen, a progressive transition from Townsend to multi-peaks regime between 320 and 160 mbar. A detailed study of this regime in helium and nitrogen with high-speed imaging shows that each multi-peak corresponds to a new spatially homogeneous discharge. However, the discharge is not completely extinguished between each pulse and the remaining light emission reveals the metastable activity (excitation transfer or Penning effect). Paschen's curves obtained from electrical characterization of the discharge show an inversion (compared to standard cuves) of argon and helium cuves. This inversion shows the importance of metastable energies and capabilities to ionize almost all impurities, in the case of helium. This explains why in helium a breakdown under a lower electric field than in argon is possible. A detailed study of a glow discharge in helium as a function of pressure and impurities with time-resolved spectroscopy showed the metastables evolution within a discharge and the role of impurities in quenching or creation rate of metastables. This study also shows a 4 minutes time for thermal stabilization of the discharge (electrode heating and thermosdesorption). In helium and nitrogen, the very first microseconds of discharge are filamentary and change after 2-3 periods (∼ 200 µs) to the glow or Townsend regime respectively. Adding oxygen, an electronegative and metastable quencher gas, make the discharge change to filamentary when a proportion of more than 1500 ppm is added at atmospheric pressure. This rate is increased until 2 % in nitrogen at 350 mbar. The presence of a polymer substrate reduced this Townsend working domain due to the increase of impurities in the discharge caused by polymer etching. However, this process in pure nitrogen is very efficient for implanting nitrogen functional groups on the surface of polymer films. An incorporation of 23 % of nitrogen onto a PET surface has been reached. Regarding SiOx thin film deposition, adding HMDSO, even for ∼ 100 ppm make the discharge change to filamentary. A pressure below 40 mbar must be reached to obtain a multi-peak regime. This high pressure process is fast and deposition rate of 17 nm/s could be obtain at 500 mbar. FTIR, AFM and SEM characterisation of the coatings showed an inhomogeneous composition and structure of the layer between entrance and exit of the discharge along the gas flow. These conditions could explain that the best oxygen barrier obtained was 40.5 cm3/(m2· atm · day). Depending on the discharge parameters (frequency, residence time, power and pressure) the coatings are more or less organic. A progressive decarbonification of the layer due to progressive monomer depletion explains this behavior. A higher oxygen rate allows a better film composition homogeneity along the discharge, while a longer deposition time results in a rougher coating but has no effect on the layer composition. The particular geometry of the discharge cell (6 cm by 6 cm electrodes and 2 mm gap) with gas injection from one side, leads to a different chemistry along the gas flow. At the entrance, the coating is smooth (Ra ∼ 5 nm) and dense but organic whereas at the exit it is rough (Ra ∼ 15 nm) but has a quasi-stoechiometric composition. These differences are explained by heterogeneous reactions, comparable to PECVD process (surface chemistry) close to the gas input and a progressive transition to homogeneous reactions at the exit of the discharge (volume chemistry) which result in particle formation of nanometric size. Analysis of these particles by laser light scattering (LLS) shows a pressure threshold of 200 mbar with a constant gas mixture and flow within all the pressure range. At this pressure, the first detectable particles appear at the end of the discharge. From 200 to 1000 mbar, this threshold becomes closer and closer to the discharge entrance, but it always corresponds to a residence time of the gas in the discharge of around 30 ms. Thus this time corresponds to the characteristic formation time of detectable particles in the discharge. We also showed that this threshold varies linearly with the power injected in the discharge, the higher the power the faster the particles appear. Spatio-temporal LLS measurements show a cyclic (Τ ∼ 1-2 s) formation of particles. This behavior is linked to a rapid growth and a trapping of the particle in the discharge when they reach a 200 nm size. Then, when their size or density increases (∼ 240 nm) they are collected on the electrodes or expelled by the flow drag force which becomes preponderant in comparison with the electrostatic trapping force. Finally, they agglomerate at the exit of the discharge up to ∼ 300 nm size. Then a new cycle starts. This global approach of SiOx deposition process by DBD opens new perspectives of applications in a new pressure range and show the key parameters to be adjusted for an industrial application.CRPPSP
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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