Estudo dos mecanismos de interação de plasma de Ar e Ar-O2 com ácido esteárico

Tese (doutorado) - Universidade Federal de Santa Catarina, Centro Tecnológico, Programa de Pós-Graduação em Ciência e Engenharia de Materiais, Florianópolis, 2011Devido à sensibilidade das técnicas de tratamento superficial quanto à presença de contaminação, o emprego de um tratamento efetivo de limpeza, como o processo a plasma, tem papel fundamental no desempenho do produto final. Estudos que possibilitam identificar os mecanismos reacionais das espécies presentes em plasma com o contaminante permitem o aperfeiçoamento desta técnica. Neste trabalho foi estudada a interação de ácido esteárico (AE), com fórmula molecular C18H36O2, com as espécies presentes em plasma de Ar e Ar-O2. Foram realizados ensaios em pós-descarga de plasma de Ar-O2 gerado por uma fonte de micro-ondas, onde é possível estudar o efeito das espécies neutras (excitadas ou não) na degradação do AE. Também foram realizados ensaios em plasma de Ar e Ar-O2 gerado por uma descarga DC pulsada, onde é possível estudar o efeito conjunto das espécies neutras com as espécies carregadas (íons e elétrons) e fótons. Os resultados obtidos em pós-descarga mostram que o processo de decomposição é favorecido a baixa temperatura e que existe a necessidade de se controlar o tempo de relaxação térmica, sendo necessários tempos longos para relaxar a energia térmica depositada pelas reações químicas exotérmicas e endotérmicas e tempos curtos a temperaturas mais altas para facilitar a fragmentação do AE, conduzindo à formação de compostos voláteis. Outra forma de se obter uma decomposição eficaz do AE em pós-descarga é trabalhar com camadas finas, onde os processos de funcionalização e de ramificação ocorrem com menor intensidade. Já pelos resultados obtidos em uma descarga DC pulsada, observa-se que existe um sinergismo das espécies quimicamente ativas com as espécies carregadas e fótons. Tal sinergismo consiste na quebra de ligações do AE pelas espécies carregadas e fótons, e reação dos radicais formados com as espécies quimicamente ativas. Isto dificulta o restabelecimento das ligações quebradas e, também, a ramificação das cadeias carbônicas.Due to the sensitivity of surface treatment techniques for the presence of contamination, the use of an effective treatment for cleaning, as the plasma process, plays a fundamental role in the final product performance. Studies to identify possible mechanisms of reactive species in plasma with the contaminant enhancement capability of this technique. We studied the interaction of stearic acid (SA), with molecular formula C18H36O2, with the species present in plasma of Ar and Ar-O2. Treatments were conducted in post-discharge of Ar-O2 plasma generated by a microwave source, where it is possible to study the effect of neutral species (excited or not) on the degradation of SA. The treatments were, also, realized in Ar and Ar-O2 plasmas generated by a pulsed DC discharge, where the combined effect of the neutral species with the charged species (ions and electrons) and photons can be studied. The results obtained in post-discharge showed that the decomposition process is favored at low temperature and that there is a need to control the thermal relaxation time, requiring long times to relax the thermal energy deposited by the exothermic and endothermic reactions and short times at higher temperatures to facilitate the fragmentation of SA leading to formation of volatile compounds. Another way to obtain an efficient decomposition of the SA in post-discharge is working with thin layers, where the process of branching and functionalization occurs with less intensity. Since the results obtained in a pulsed DC discharge, it is observed that there is a synergism of chemically active species with charged species and photons. This synergism is the bond breaking of the SA by photons and charged species, and reaction of radicals formed with the chemically active species. This makes difficult the restoration of broken links and also the branching of carbon chains

Influence of neutral and charged species on the plasma degradation of the stearic acid

In this work, stearic acid (SA) was degraded in an Ar-O2and Ar-H2post-discharge environment created by a plasma reactor with a microwave source and in an Ar, Ar-H2and Ar-O2DC (Direct Current) discharge environment created in a cathode-anode confined system. The afterglow region is useful for understanding the role of the chemically active species (O, O2, H and H2). In contrast, the discharge region allows the observation of the effects of chemically active species, charged species (ions and electrons) and photons. The influence of these species on the grafting and etching of SA was evaluated by measuring the mass variation, mass variation rate and chemical composition. The results showed that when only chemically active oxygen species are present, the SA is preferentially grafted. However, when both photons and charged species are present, the SA is more efficiently etched. When the Ar-H2and Ar environments are utilized; the SA is not efficiently degraded

Dependence of E-H transition in argon ICP discharges for treatment of organic molecules

<div><p>ABSTRACT Plasma surface cleaning is an alternative process that aims at the fully removal of organic contaminants on several kinds of materials. Despite its advantages, there are still lacks on the comprehension of the complex relations between plasma-generated species and organic molecules during plasma cleaning. In the present work, a linear alkane (hexatriacontane - C36H74), used as a contaminant model, was exposed to an Argon radiofrequency (RF) inductively coupled plasma (ICP). The by-products from degradation were monitored by optical emission spectroscopy (OES) and residual gas analysis (RGA), to identify the influence of sample positioning on E and H discharge modes regions. The exposition to H-mode discharge resulted in more intense and profuse emissions of C-H and C2 systems. RGA results show similar byproducts from degradation in both modes; however, the intensity from treatment in H-mode is largely greater. It was also observed that plasma etching in H-mode is enough to melt the sample, while E-type discharge leaves the surface of the sample apparently unchanged.</p></div

Modification of stearic acid in Ar and Ar-O2 pulsed DC discharge

Stearic acid (CH3(CH2)16COOH) was treated with Ar and Ar-O2 (10%) pulsed DC discharges created by a cathode-anode confined system to elucidate the role of oxygen in plasma cleaning. The treatment time (5 to 120 minutes) and plasma gas mixture (Ar and Ar-O2) were varied, and the results showed that the mass variation of stearic acid after Ar-O2 plasma exposure was greater than that of pure Ar plasma treatment. Thus, compared to Ar*, active oxygen species (O and O2, in all states) enhance the etching process, regardless of their concentration. During the treatments, a liquid phase developed at the melting temperature of stearic acid, and differential thermal analyses showed that the formation of a liquid phase was associated with the breakage of bonds due to treatment with an Ar or Ar-O2 plasma. After treatment with Ar and Ar-O2 plasmas, the sample surface was significantly modified, especially when Ar-O2 was utilized. The role of oxygen in the treatment process is to break carbonaceous chains by forming oxidized products and/or to act as a barrier again ramification, which accelerates the etching of stearic acid