arXiv subjects

Thierry Dufour

Publications and source records attributed to Thierry Dufour.

47 records · Page 3Linked to original sources

arXiv2016

LDPE Surface Modifications Induced by Atmospheric Plasma Torches with Linear and Showerhead Configurations

Low density polyethylene (LDPE) surfaces have been plasma modified to improve their nanostructural and wettability properties. These modifications can significantly improve the deposition of subsequent layers such as films with specific barrier properties. For this purpose, we compare the treatments induced by two atmospheric plasma torches with different configurations (showerhead vs. linear). The modifications of LDPE films in terms of chemical surface composition and surface morphology are evidenced by X-ray photoelectron spectro-scopy, water contact angles measurements, and atomic force microscopy. A comparison between the two post-discharge treatments is achieved for several torch-to-substrate distances (gaps), treatment times, and oxygen flow rates in terms of etching rate, roughening rate, diffusion of oxygen into the subsur-face and hydrophilicity. By correlating these results with the chemical composition of the post-discharges, we identify and compare the 'species which are responsible for the chemical surface functionalization, the surface roughening, and etching.

physics.plasm-ph↗

arXiv2016

In-depth diffusion of oxygen into LDPE exposed to an Ar--O2 atmospheric post-discharge: a complementary approach between AR-XPS and Tof-SIMS techniques

The in-depth oxygen diffusion into a low density polyethylene film is performed in the post-discharge of an atmospheric plasma torch, supplied in argon as carrier gas and with or without oxygen as reactive gas. The chemical and structural properties of the polymer surface and bulk are studied in terms of plasma parameters (treatment time, power, and reactive gas flow rate). A good correlation between XPS and Fourier transform infrared spectroscopy analyses is demonstrated. The penetration depth of oxygen into the bulk of the polymer is investigated by angle resolved-XPS and time-of-flight SIMS. It is shown that, depending on the plasma conditions, oxygen could penetrate up to 20--40 nm into the low density polyethylene during the atmospheric plasma treatment.

physics.plasm-ph↗

arXiv2016

Etching Processes of Polytetrafluoroethylene Surfaces Exposed to He and He-O2 Atmospheric Post-discharges

A comparative study of polytetrafluoroethylene (PTFE) surfaces treated by the post-discharge of He and He-O2 plasmas at atmospheric pressure is presented. The characterization of treated PTFE surfaces and the species involved in the surface modification are related. In pure He plasmas, no significant change of the surface has been observed by X-ray photoelectron spectroscopy (XPS), dynamic water contact angles (dWCA) and atomic force microscopy (AFM), in spite of important mass losses recorded. According to these observations, a layer-by-layer physical etching without any preferential orientation is proposed, where the highly energetic helium metastables are the main species responsible for the scission of --(CF2)n-- chains. In He--O 2 plasmas, as the density of helium metastables decreases as a function of the oxygen flow rate, the treatment leads to fewer species ejected from the PTFE surfaces (in agreement with mass loss measurements and the detection of fluorinated species onto aluminum foil). However, the dWCA and AFM measurements show an increase in the hydrophobicity and the roughness of the surface. The observed alveolar structures are assumed to be caused by an anisotropic etching where the oxygen atoms etch mainly the amorphous phase.

physics.plasm-ph↗

arXiv2016

Influence of ambient air on the flowing afterglow of an atmospheric pressure Ar/O2 radiofrequency plasma

The influence of ambient air on the flowing afterglow of an atmospheric pressure Ar/O2 radiofrequency plasma has been investigated experimentally. Spatially resolved mass spectrometry and laser induced fluorescence on OH radicals were used to estimate the intrusion of air in between the plasma torch and the substrate as a function of the torch-to-substrate separation distance. No air is detected, within the limits of measurement uncertainties, for separation distances smaller than 5 mm. For larger distances, the effect of ambient air can no longer be neglected, and radial gradients in the concentrations of species appear. The Ar 4p population, determined through absolute optical emission spectroscopy, is seen to decrease with separation distance, whereas a rise in emission from the N2(C--B) system is measured. The observed decay in Ar 4p and N2(C) populations for separation distances greater than 9mm is partly assigned to the increasing collisional quenching rate by N2 and O2 molecules from the entrained air. Absorption measurements also point to the formation of ozone at concentrations from 10^14 to 10^15 cm-3, depending both on the injected O2 flow rate and the torch-to-substrate separation distance.

physics.plasm-ph↗

arXiv2016

Low-density polyethylene films treated by an atmospheric Ar--O2 post-discharge: functionalization, etching, degradation and partial recovery of the native wettability state

To optimize the adhesion of layers presenting strong barrier properties on low-density polyethylene (LDPE) surfaces, we investigated the influence of argon and argon-oxygen atmospheric pressure post-discharges. This study was performed using x-ray photoelectron spectroscopy, atomic force microscopy, optical emission spectroscopy (OES) and dynamic water contact angle (WCA) measurements. After the plasma treatment, a slight increase in the roughness was emphasized, more particularly for the samples treated in a post-discharge supplied in oxygen. Measurements of the surface roughness and of the oxygen surface concentration suggested the competition of two processes playing a role on the surface hydrophilicity and occurring during the post-discharge treatment: the etching and the activation of the surface. The etching rate was estimated to about 2.7 nm.s-1 and 5.8 nm.s-1 for Ar and Ar-O2 post-discharges, respectively. The mechanisms underlying this etching were investigated through experiments, in which we discuss the influence of the O2 flow rate and the distance (gap) separating the plasma torch from the LDPE surface located downstream. O atoms and NO molecules (emitting in the UV range) detected by OES seem to be good candidates to explain the etching process. An ageing study is also presented to evidence the stability of the treated surfaces over 60 days. After 60 days of storage, we showed that whatever the O2 flow rate, the treated films registered a loss of their hydrophilic state since their WCA increased towards a common threshold of 80{\textdegree}. This 'hydrophobic recovery' effect was mostly attributed to the reorientation of induced polar chemical groups into the bulk of the material. Indeed, the relative concentrations of the carbonyl and carboxyl groups at the surface decreased with the storage time and seemed to reach a plateau after 30 days.

physics.plasm-ph↗

arXiv2016

Chemical mechanisms inducing a dc current measured in the flowing post-discharge of an RF He-O2 plasma torch

The post-discharge of an RF plasma torch supplied with helium and oxygen gases is characterized by mass spectrometry, optical emission spectroscopy and electrical measurements. We have proved the existence of a dc current in the post-discharge (1--20 A), attributed to the Penning ionization of atmospheric nitrogen and oxygenated species. The mechanisms ruling this dc current are investigated through experiments in which we discuss the influence of the O2 flow rate, the He flow rate and the distance separating the plasma torch from a material surface located downstream.

physics.plasm-ph↗

arXiv2016

The influence of power and frequency on the filamentary behavior of a flowing DBD-application to the splitting of CO2

In this experimental study, a flowing dielectric barrier discharge operating at atmospheric pressure is used for the splitting of CO2 into O2 and CO. The influence of the applied frequency and plasma power on the microdischarge properties is investigated to understand their role on the CO2 conversion. Electrical measurements are carried out to explain the conversion trends and to characterize the microdischarges through their number, their lifetime, their intensity and the induced electrical charge. Their influence on the gas and electrode temperatures is also evidenced through optical emission spectroscopy and infrared imaging. It is shown that, in our configuration, the conversion depends mostly on the charge delivered in the plasma and not on the effective plasma voltage when the applied power is modified. Similarly, at constant total current, a better conversion is observed at low frequencies, where a less filamentary discharge regime with a higher effective plasma voltage than that at a higher frequency is obtained.

physics.plasm-ph↗

arXiv2016

Integrated micro-plasmas in silicon operating in helium

Microplasma arrays operating in helium in a DC regime have been produced in silicon microre-actors. Cathode boundary layer (CBL) type microdevices were elaborated using clean room facilities and semiconductor processing techniques. Ignition of the micro-discharge arrays having either 50 or 100 m diameter cavities was studied. Two different structures (isotropically etched or anisotropically etched cavity) and various conditions (the two different voltage polarities, pressures etc.) were investigated. 100 microdis-charges of 50 m diameter could be ignited in parallel at 1000 torr. At high current, some parasitic and transient sparks appeared at the edge of the sample. When the polarization was reversed (cathode side corresponding the opened electrode), more current was needed to light all the microdischarges. A thermally affected zone around the hole on the anode side was obtained after operation.

physics.ins-det↗

arXiv2016

Experimental study and simulation of a micro-discharge with limited cathode area

We report in this paper simulation results and experimental measurements to characterize a micro-discharge generated in a single micro cavity device operating in helium. By spatially limiting the cathode surface area using a dielectric layer, we demonstrate the ability of the micro-discharge to work in a steady-state abnormal glow regime. The physical properties of this regime are discussed.

physics.plasm-ph↗

arXiv2016

RF impedance measurements of DC atmospheric micro-discharges

The available diagnostics for atmospheric micro-plasmas remain limited and relatively complex to implement; so we present a radio-frequency technique for diagnosing a key parameter here. The technique allows one to estimate the dependencies of the electron density by measuring the RF-impedance of the micro-plasma and analyzing it with an appropriate equivalent circuit. This technique is inexpensive, can be used in real time and gives reasonable results for argon and helium DC micro-plasmas in holes over a wide pressure range. The electron density increases linearly with current in the expected range consistent with normal glow discharge behavior.

physics.plasm-ph↗

arXiv2016

Synthesis and texturization processes of (super)-hydrophobic fluorinated surfaces by atmospheric plasma

The synthesis and texturization processes of fluorinated surfaces by means of atmospheric plasma are investigated and presented through an integrated study of both the plasma phase and the resulting material surface. Three methods enhancing the surface hydrophobicity up to the production of super-hydrophobic surfaces are evaluated: (i) the modification of a polytetrafluoroethylene (PTFE) surface, (ii) the plasma deposition of fluorinated coatings and (iii) the incorporation of nanoparticles into those fluorinated films. In all the approaches, the nature of the plasma gas appears to be a crucial parameter for the desired property. Although a higher etching of the PTFE surface can be obtained with a pure helium plasma, the texturization can only be created if O2 is added to the plasma, which simultaneously decreases the total etching. The deposition of CxFy films by a dielectric barrier discharge leads to hydrophobic coatings with water contact angles (WCAs) of 115{\textdegree}, but only the filamentary argon discharge induces higher WCAs. Finally, nanoparticles were deposited under the fluorinated layer to increase the surface roughness and therefore produce super-hydrophobic hybrid coatings characterized by the nonadherence of the water droplet at the surface.

physics.plasm-ph↗