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Immobilization of plasma filaments in a dielectric barrier discharge: fundamental study and application to the synthesis of patterned coatings

par Demaude, Annaelle
Président du jury Schweicher, Guillaume
Promoteur Reniers, François
Publication Non publié, 2024-02-09

Thèse de doctorat

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Résumé :

Dielectric barrier discharges (DBDs) have been used for ozone production since the 19th century. Beyond their historic application, their versatility extends to diverse fields, including plasma medicine and material surface modification. DBDs most commonly operate in the filamentary regime, characterized by short-lived microdischarges (often called filaments) igniting randomly in the inter-electrode space, which can cause inhomogeneities in surface treatment or film deposition. Although this makes them generally undesirable for such applications, filaments hold significant potential as micro-reactors for localized treatment or deposition if their ignition mechanism and chemistry can be understood and controlled. While considerable work exists on understanding DBD regimes and avoiding the filamentary mode, poor reporting has been made on deposition processes at the filament scale. In this thesis, we aimed to address this with a system of immobilized DBD filaments. The latter was achieved through local gap reduction via dielectric patterning. This approach facilitated local discharge analysis and allowed us to test the potential of these filaments for surface patterning applications. For this last point, propargyl methacrylate (PMA) was chosen as test-case precursor for thin film deposition. The study of the discharge focused on the dynamics and properties of individual filaments and of the overall discharge under varying operating parameters such as gap width, applied voltage, PMA feed rate, and dielectric pattern geometry. For this purpose, we performed high-speed camera imaging, electrical measurements, and optical emission spectroscopy (OES) imaging of a filament. Deposition with the immobilized filaments system from Ar/PMA mixture resulted in patterned coatings. The higher reactivity inside the filaments generated circular hill-like areas (spots), while a thinner film was also deposited between the spots. Spatially resolved characterizations using profilometry, X-ray photoelectron spectroscopy (XPS), infrared reflection absorption spectroscopy (IRRAS) and water contact angle (WCA) measurements revealed correlations between the properties of the coatings and the discharge characteristics.In addition to morphological contrast, chemical and wettability contrasts were observed under different conditions. Varying plasma parameters allowed tuning all these contrasts. For example, adjusting the power-to-PMA feed ratio primarily influenced the chemical composition of the films in a region-specific manner within the pattern, while changing the voltage waveform affected the morphological contrast and the preservation of PMA chemical structure. By leveraging these insights, area-selective deposition of spots only was achieved. This method could virtually be extended to any types of precursors and could open a new route for surface patterning.