324 research outputs found
Dielectric barrier discharges : a promising tool for the fabrication of anti-fogging coatings
La « vue floue » typique des surfaces embuĂ©es peut ĂȘtre extrĂȘmement frustrante. Des exemples tels que les lunettes qui sâembuent pendant lâactivitĂ© physique, la condensation qui se forme Ă lâintĂ©rieur des fenĂȘtres pendant lâhiver ou les miroirs qui se couvrent de buĂ©e pendant la douche le dĂ©montrent. En outre, la prĂ©sence de buĂ©e sur les surfaces cause des effets nĂ©fastes dans certains secteurs dâactivitĂ© comme lâindustrie automobile (pare-brise et rĂ©troviseurs), lâindustrie optique (objectifs, camĂ©ras, tĂ©lescopes et capteurs), lâindustrie solaire (modules photovoltaĂŻques), lâindustrie alimentaire (emballages dâaliments) et le secteur mĂ©dical (lunettes et endoscopes). Au cours de la derniĂšre dĂ©cennie, lâapplication de revĂȘtements (super)hydrophiles a suscitĂ© un intĂ©rĂȘt croissant, en raison de leur capacitĂ© dâattĂ©nuer les effets de la buĂ©e. Leur principe de fonctionnement repose sur lâutilisation de matĂ©riaux interagissant avec les gouttes dâeau pour en modifier leur morphologie, gĂ©nĂ©rant une couche mince dâeau sur la surface. Ainsi, la lumiĂšre incidente nâest pas dispersĂ©e et les effets de la buĂ©e sont amoindris. JusquâĂ prĂ©sent, la plupart des techniques de dĂ©pĂŽt explorĂ©es pour produire des revĂȘtements (super) hydrophiles sont inaccessibles Ă la production de masse en raison de leur nature multiĂ©tape. Pour cette raison, lâexploration de techniques adaptĂ©es Ă ce type de production, telles que les dĂ©charges Ă barriĂšre diĂ©lectrique Ă pression atmosphĂ©rique (AP-DBD), un type de procĂ©dĂ© de dĂ©pĂŽt chimique en phase vapeur assistĂ© par plasma (AP-PECVD), est cruciale afin dâĂ©largir lâutilisation des revĂȘtements antibuĂ©e au-delĂ du laboratoire. Dans un procĂ©dĂ© AP-PECVD contrĂŽlĂ© par des barriĂšres diĂ©lectriques (AP-DBD), certains prĂ©curseurs inorganiques ou organomĂ©talliques (e.g., TiCl4, TiN, SiH4, Si2O(CH3)2) sont introduits entre deux Ă©lectrodes parallĂšles avec un gaz vecteur (e.g., N2, Ar, He) Ă la pression atmosphĂ©rique, oĂč ils se fragmentent Ă la suite dâinteractions avec les espĂšces du plasma. Les fragments rĂ©sultants rĂ©agissent les uns avec les autres ou avec le substrat afin de produire les espĂšces rĂ©actives requises au dĂ©pĂŽt du revĂȘtement. Les caractĂ©ristiques structurelles et fonctionnelles des revĂȘtements PECVD (e.g., la rugositĂ© de surface, la biocompatibilitĂ©, les propriĂ©tĂ©s optiques et de mouillage) dĂ©pendent des certains paramĂštres de dĂ©pĂŽt, tels que la puissance dissipĂ©e dans la dĂ©charge, le type de dĂ©charge, la concentration de prĂ©curseurs et le dĂ©bit de gaz. La possibilitĂ© de se procurer des Ă©chantillons de verre dotĂ©s de la propriĂ©tĂ© antibuĂ©e via APPECVD a Ă©tĂ© dĂ©montrĂ©e dans cette thĂšse. En contrĂŽlant les paramĂštres de dĂ©pĂŽt, les revĂȘtements antibuĂ©e ont Ă©tĂ© prĂ©parĂ©s en utilisant du 1,3,5,7-tĂ©tramĂ©thylcyclotĂ©trasiloxane (Si4O4H4(CH3)4) et de lâoxyde nitreux (N2O) au moyen dâune DBD fonctionnant en N2 Ă la pression atmosphĂ©rique. Dans le cas des revĂȘtements fabriquĂ©s dans des conditions statiques (aucun mouvement entre lâĂ©chantillon de verre et les Ă©lectrodes), lâĂ©valuation quantitative de la rĂ©sistance Ă la buĂ©e (ASTM F 659-06) a rĂ©vĂ©lĂ© que les revĂȘtements obtenus avec un rapport [N2O]/[TMCTS] Âł 30 ou avec une puissance dissipĂ©e Âł 0,25 W cm-2 sont antibuĂ©e (transmittance > 80%) en raison de leur nature hydrophile. La quantitĂ© de prĂ©curseur et dâoxydant injectĂ©e dans la dĂ©charge, exprimĂ©e par la somme « [N2O] + [TMCTS] », nâagissait que peu sur la performance antibuĂ©e. En lâabsence de changements significatifs dans la rugositĂ© de surface (Rrms et Ra Ă©tant compris entre 3 et 6 nm), lâorigine de la performance antibuĂ©e a Ă©tĂ© attribuĂ©e Ă la chimie de surface. CouplĂ© aux rapports O/Si (rĂ©sultats XPS), un paramĂštre arbitraire, appelĂ© « rapport dâembuage » a Ă©tĂ© dĂ©fini en considĂ©rant les rĂ©sultats FTIR pour expliquer les performances antibuĂ©e observĂ©es. On a pu constater quâun rapport O/Si â„ 2,3 couplĂ© Ă un rapport dâembuage dans lâintervalle de 0-0,10, rĂ©sultant de la prĂ©sence de fonctionnalitĂ©s hydrophiles, telles que les groupes silanol, hydroxyle, carboxyle or ester Ă la surface Ă©taient nĂ©cessaires pour atteindre la propriĂ©tĂ© antibuĂ©e. Par ailleurs, les revĂȘtements prĂ©parĂ©s dans des conditions dynamiques utilisant trois autres prĂ©curseurs aux structures diffĂ©rentes quant Ă la prĂ©sence dâun cycle et au nombre de groupes Si-H et Si-CH3 (lâoctamĂ©thylcyclotĂ©trasiloxane, le 1,1,3,3-tĂ©tramĂ©thyldisiloxane et lâhexamĂ©thyldisiloxane) nâĂ©taient pas antibuĂ©e. Ce rĂ©sultat porte Ă croire que la structure cyclique du TMCTS et la forte rĂ©activitĂ© des liaisons Si-H est Ă lâorigine de la formation de ces fonctionnalitĂ©s hydrophiles et par consĂ©quent, Ă la performance antibuĂ©e observĂ©e dans les verres traitĂ©s en injectant du TMCTS dans la dĂ©charge plasma.Experience shows that the âblurred viewâ typical of fogged surfaces can be incredibly frustrating. Eyewear fogging up during physical activity, condensation forming on the inside of windows during the winter, or bathroom mirrors steaming up when taking a shower are some obvious examples. In addition to being upsetting, the fogging of surfaces has been reported to cause adverse effects on sectors of activity as diverse as the automotive industry (e.g., windshield glass and rearview mirrors), the optical industry (e.g., lenses, cameras, telescopes, and sensors), the solar industry (e.g., photovoltaic modules), the food industry (e.g., food packaging), and medicine (e.g., goggles and endoscopes). Over the last decade, interest has been growing in the application of hydrophilic and superhydrophilic coatings, as they can efficiently mitigate the effects of fogging by changing the morphology of fog drops. The working principle of a (super)hydrophilic surface is based on the use of materials producing a thin film of water on the solid surface on interaction with fog drops. As a result, incident light transmits without being scattered and the effects of fogging are minimized. Unfortunately, most of the deposition techniques used thus far for the fabrication of (super)hydrophilic coatings involves multiple steps, thus making their integration into mass production a challenging task. For this reason, the exploration of deposition techniques adapted for large-scale production is crucial to broaden the range of application of antifogging coatings beyond the laboratory. In this regard, numerous studies on the use of dielectric barriers in plasma enhanced chemical vapor deposition at atmospheric pressure (AP-PECVD) are strongly emerging to address this issue. In a typical AP-PECVD controlled by dielectric barriers, inorganic or organometallic precursors (e.g., TiCl4, TiN, SiH4, Si2O(CH3)2) are introduced between two parallel electrodes along with a carrier gas (e.g., N2, Ar, He) at atmospheric pressure where, on interaction with plasma species, undergo fragmentation. The resulting fragments can react with the substrate or with each other to produce short-lived species required for coating deposition. The structural and functional features of PECVD coatings (e.g., surface roughness, biocompatibility, wetting and optical properties) depend on several deposition parameters, including the power dissipated in the discharge, type of plasma discharge, precursor concentration, and the flow rate of gases. With this in mind, the feasibility of conferring fogging resistance to commercial glass samples via AP-PECVD has been demonstrated in this doctoral thesis. By appropriately controlling the deposition parameters, anti-fogging coatings were prepared using 1,3,5,7- tetramethylcyclotetrasiloxane (Si4O4H4(CH3)4) and nitrous oxide (N2O) by a dielectric barrier discharge operated in N2 at atmospheric pressure (AP-DBD). When coating deposition was conducted in static conditions, that is, with no relative movement between the glass sample and the electrodes, quantitative assessment of the fogging resistance (ASTM F 659-06 standard) revealed that coatings obtained under [N2O]/[TMCTS] ratios Âł 30 or under a dissipated power Âł 0.25 W cm-2 endowed glass samples with the anti-fogging property (transmittance > 80%), because of their hydrophilic nature. In terms of the [N2O] + [TMCTS] sum, the amount of TMCTS and N2O injected into the discharge did not appear to have a great impact on the anti-fogging performance. Indeed, as no significant changes in surface roughness were observed (Rrms and Ra were between 3 and 6 nm), the origin of the anti-fogging performance was attributed to the surface chemistry. To this end, an arbitrary parameter, called âfogging ratioâ, was defined considering FTIR results to account for, along with O/Si ratios (XPS results), the observed anti-fogging performance. Fogging ratios in the 0-0.10 range coupled with O/Si ratios Âł 2.3, resulting from the presence of hydrophilic functionalities, such as silanol (Si-OH), hydroxyl (C-OH) carboxyl (COOH), and ester (COOR) groups at the coating surface were necessary to attain the anti-fogging property. Interestingly, coatings prepared in dynamic conditions using three other precursors with different structures and different number of Si-H and Si-CH3 groups; namely, octamethylcyclotetrasiloxane (OMCTS), 1,1,3,3-tetramethyldisiloxane (TMDSO), and hexamethyldisiloxane (HMDSO) were not fogging-resistant. This result leads us to believe that the cyclic structure of TMCTS in conjunction with the high reactivity of Si-H bonds is behind the formation of the above-mentioned hydrophilic functionalities, and thus the antifogging performance of TMCTS-coated glasses
Water drop-surface interactions as the basis for the design of anti-fogging surfaces : theory, practice, and applications trends
Glass- and polymer-based materials have become essential in the fabrication of a multitude of elements, including eyeglasses, automobile windshields, bathroom mirrors, greenhouses, and food packages, which unfortunately mist up under typical operating conditions. Far from being an innocuous phenomenon, the formation of minute water drops on the surface is detrimental to their optical properties (e.g., light-transmitting capability) and, in many cases, results in esthetical, hygienic, and safety concerns. In this context, it is therefore not surprising that research in the field of fog-resistant surfaces is gaining in popularity, particularly in recent years, in view of the growing number of studies focusing on this topic. This review addresses the most relevant advances released thus far on anti-fogging surfaces, with a particular focus on coating deposition, surface micro/nanostructuring, and surface functionalization. A brief explanation of how surfaces fog up and the main issues of interest linked to fogging phenomenon, including common problems, anti-fogging strategies, and wetting states are first presented. Anti-fogging mechanisms are then discussed in terms of the morphology of water drops, continuing with a description of the main fabrication techniques toward anti-fogging property. This review concludes with the current and the future perspectives on the utility of anti-fogging surfaces for several applications and some remaining challenges in this field
Energy and entropy on irreversible chemical reaction-diffusion systems with asymptotic stability
In the framework of irreversible thermodynamics, we study autonomous systems
of reaction-diffusion equations to show how the entropy and free energy of an
open and irreversible reactor depend on concentrations. To do this, we find a
Lyapunov function that depends directly on the eigenvalues and eigenvectors of
the linearized problem valid for linear and asymptotically stable systems. By
suggesting the role of this Lyapunov function as internal free energy, we were
able to reflect the properties of the dynamical system directly into the second
law of thermodynamics making a redefinition of the chemical potentials. We
demonstrate the consistency of our hypotheses with basic thermodynamic
principles such as the proportionality between flows and forces proposed by
Onsager, the spectral decomposition of entropy production and the
Glansdorff-Prigogine evolution criterion of a thermodynamic system.Comment: 12 pages, 3 figure
Centroid-Based Clustering with ab-Divergences
Centroid-based clustering is a widely used technique within unsupervised learning
algorithms in many research fields. The success of any centroid-based clustering relies on the
choice of the similarity measure under use. In recent years, most studies focused on including several
divergence measures in the traditional hard k-means algorithm. In this article, we consider the
problem of centroid-based clustering using the family of ab-divergences, which is governed by two
parameters, a and b. We propose a new iterative algorithm, ab-k-means, giving closed-form solutions
for the computation of the sided centroids. The algorithm can be fine-tuned by means of this pair of
values, yielding a wide range of the most frequently used divergences. Moreover, it is guaranteed to
converge to local minima for a wide range of values of the pair (a, b). Our theoretical contribution
has been validated by several experiments performed with synthetic and real data and exploring the
(a, b) plane. The numerical results obtained confirm the quality of the algorithm and its suitability to
be used in several practical applications.MINECO TEC2017-82807-
A Joint Optimization Criterion for Blind DS-CDMA Detection
This paper addresses the problem of the blind detection of a desired user in an asynchronous DS-CDMA communications system
with multipath propagation channels. Starting from the inverse filter criterion introduced by Tugnait and Li in 2001, we propose
to tackle the problem in the context of the blind signal extraction methods for ICA. In order to improve the performance of the
detector, we present a criterion based on the joint optimization of several higher-order statistics of the outputs. An algorithm that
optimizes the proposed criterion is described, and its improved performance and robustness with respect to the near-far problem
are corroborated through simulations. Additionally, a simulation using measurements on a real software-radio platform at 5 GHz
has also been performed.Ministerio de Ciencia y tecnologĂa TEC2004-06451-C05-0
Efectos macroeconĂłmicos asociados a cambios en la tasa marginal impositiva
In this paper is analyzed the macroeconomic effects associated with increases in the capital tax rate and labor tax rate under a Neoclassical model with elastic labor supply and positive externalities due to government's spending. It is found that these policies reduce the production, the investment and the labor supply; however, the effects over the consumption are different. It is concluded that tax reforms based on labor income is more contractive that tax reforms based on capital income.Tasa marginal impositiva, modelos estocĂĄsticos, simulaciĂłn
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