REPOSITORY > RESULTS

Abstract

In this dissertation, we studied heterogeneous surface recombination of neutral oxygen atoms. We described plasma as a reliable source of neutral oxygen atoms and the behavior of neutral atoms in low-pressure plasma sustained by high-frequency discharges. Neutral atom density in the flowing afterglow and the recombination coefficient on several surfaces were measured. The recombination is explained via two mechanisms: Eley-Rideal and Langmuir-Hinshelwood. We pointed out the factors influencing the recombination coefficient, such as the temperature of the surface facilitating the recombination process, the morphology of said surface, and pressure in the experimental system. We deemed calorimetry a suitable measurement method for measuring both the neutral atom density and the recombination coefficient, with a detailed explanation of the workings of a standard catalytic probe, while a laser fiber-optics catalytic probe and its drawbacks are mentioned. Examples of practical applications utilizing neutral atoms were provided for context. Recombination coefficients of oxygen atoms on different materials, as revealed from the literature, were compared, and conclusions were drawn from correlating factors, such as the increase of the recombination coefficient with the increasing surface temperature. A few hypotheses were established: the viability of the Šorli-Ročak method to determine the absolute number density of neutral oxygen atoms, the variable nature of the recombination coefficient, and the increase of the recombination coefficient with increasing temperature and surface roughness, as well as decreasing pressure. The Šorli-Ročak method was explained in detail and used in a low-pressure microwave discharge to successfully determine the absolute neutral oxygen atom density with three different materials: nickel, cobalt, and iron, confirming the first hypothesis. In the same system, the recombination coefficient of oxidized nickel was determined. We discovered that the recombination coefficient is not constant but increases with increasing surface temperature and decreasing pressure, confirming three hypotheses. An empirical formula was devised to describe the temperature and pressure dependence of the recombination coefficient. We carried out similar experiments with a catalytic probe with an oxidized cobalt tip, with similar results as for nickel, re-confirming the hypotheses. We found oxidized cobalt to have a stable recombination coefficient at elevated temperatures. The validity of the Šorli-Ročak method was again confirmed with the study of the penetration depth of oxygen atoms inside metallic tubes, with nickel, cobalt, and copper proven to be efficient recombinators. Carbon nanomaterials were examined as possible candidates for efficient recombination due to their high surface-to-mass ratio. We explained the synthesis of various carbon nanomaterials, and the importance of a carbon atom source was shown. The materials were examined using various measurement techniques. Lastly, the much higher recombination coefficient of carbon nanomaterial-covered oxidized cobalt samples was determined, confirming the last hypothesis.