Simeon Cavadias

CO2 valorization by means of Dielectric Barrier Discharge

As atmospheric pollution is causing several environmental problems it is incumbent to reduce the impact of pollution on the environment. One particular problem is the production of CO2 by many transport and industrial applications. Instead of stocking CO2 and instead of being a product, it can be used as a source. The case considered is the CO2 reformation of methane producing hydrogen and CO. It is an endothermic reaction, for which the activation barrier needs to be surpassed. This can be done efficiently by the method of Dielectric Barrier Discharge. The process relies on the collision of electrons, which are accelerated under an electrical field that is created in the discharge area. This leads to the formation of reactive species, which facilitate the abovementioned reaction. This study is performed using a Matlab program with the Reaction Engineering module in COMSOL (with an incorporated kinetic mechanism) in order to model the discharge phase. Then COMSOL (continuity and Navier-Stokes equations) is used to model the flow in the post-discharge phase. The results showed that both a 2D and 3D model can be used to model the chemical-plasma process. These methods need strongly reduced kinetic mechanism, which in some cases can cause loss of precision.

Barriers of Oxidation ans Ageing of Space Shuttle Material

During the atmospheric re-entry phase, the thermal protection of a spacecraft vehicle is submitted to air plasma. The transfer of the kinetic energy of the gases to the surface leads to an important heating of the nose cap and leading edges. The energy released from the recombination of oxygen atoms on the surface produces an additional amount of heat, increasing further the temperature. This excess of heat can damage the protection materials. This last mechanism leads to the oxidation of the surface and an accelerated ageing of the material. The oxidation produces a new surface layer of oxide such as SiO2 (Passive oxidation) and leads to the ablation of the material by the formation of CO, CO2, SiO and to the diffusion of oxygen in the bulk creating micro-cracks in the material (Active oxidation). Thus the composition of the surface and consequently the protection material properties are modified. The atmospheric re-entry conditions are created with a non equilibrium low pressure RF plasma chemical reactor. The ageing of the material and the diffusion of oxygen are studied by mass spectrometry analysis of 18O isotope for temperatures ranging from 300 – 1000 K. Silicon Carbide targets are covered with two different coatings, Chromium oxide, and Chromium oxide/ Silica. Oxygen diffusion in the material bulk for the targets was followed by Secondary Ion Mass Spectroscopy (SIMS).Mass Spectrometry shows the formation of CO and CO2 pointing out the process of ablation. This ablation is much lower in the case of polluted sample (with Chromium oxide). SIMS analyses show that Cr2O3 acts like a barrier in the oxidation of the material by limiting the diffusion of oxygen in the bulk.