Einfluss der Porenstruktur und der mineralogischen Zusammensetzung auf Mechanismus und Kinetik der Oxidation von nitridgebundenem Siliziumcarbid für den Einsatz in der thermischen Abfallverwertung

Silicon nitride bonded silicon carbide (NSiC) refractory materials are used in municipal waste treatment. In these applications, the materials undergo a destructive volume expansion caused by oxidation. Therefore, NSiC materials are being routinely tested for oxidation after manufacturing, using a long-term oxidation test. Testing is necessary because, based on material and pore characteristics in the as-manufactured state, the oxidation mechanism of NSiC and its kinetics cannot be predicted yet.

This study focussed on the determination of pore characteristics which influences the oxidation behaviour of NSiC. For the description of the oxidation behaviour the quality parameter of the oxidation test was changed from volume to mass gain. Hereby it was possible to describe the oxidation process from the beginning of the oxidation test. This new test routine allows to identify pore characteristics, which are responsible for the oxidation behaviour of the material.

It was shown that the oxidation of high porosity material samples (open porosity ≈ 30 – 40 %) depends on the specific pore surface area of the material, while the oxidation in medium porosity material samples (open porosity ≈ 22 – 27 %) is dominated by the proportion of the pore fraction greater than 0.3 µm. These results indicate that the reaction kinetics switch from a reaction-controlled mechanism in high porosity materials to a diffusion-controlled mechanism in medium porosity materials. Thorough analysis of two more classes of NSiC materials (low porosity laboratory materials with an open porosity < 20 % and industrially manufactured materials) showed that the oxidation kinetics of the low porosity laboratory materials is also being dominated by the proportion of the pore fraction in the materials that is larger than 0.3 µm. This suggests a diffusion-controlled mechanism in low porosity laboratory materials.

Industrially manufactured materials show a broad range in chemical composition and pore characteristics. The chemical composition was found to have only a minor effect on the oxidation kinetics, whereas the pore characteristics are dominating on the oxidation kinetics. This indicates that the oxidation mechanism of industrially manufactured NSiC materials is mainly diffusion driven.