Recombination coefficient of atomic oxygen on ceramic materials in a CO2 plasma flow for the simulation of a Martian entry

In order to design heat shields for space vehicles, materials must be characterized in simulation conditions close to those encountered in space environments. The most important conditions for simulating a Martian entry phase of space vehicles (high temperatures, low pressure CO2 plasma…) are achieved through the MESOX set-up associating a solar radiation concentrator and a microwave plasma generator. After the previous determination of the thermal flux of recombination transferred to the material (thermal approach, accommodation), a chemical approach was developed for evaluating the recombination coefficient of atomic oxygen and/or carbon monoxide by Optical Emission Spectroscopy. The gas temperature above the material, close to the surface where recombination occurs, was evaluated using the rotational temperature of a CO band in the Ångström system. The recombination coefficients of atomic oxygen with O and/or CO on surface were determined in function of temperature-up to 2200 K according to the material resistance-for three sintered ceramic materials: SiC, Si3N4 and Al2O3 that can be used in space. An Arrhenius fit is obtained for each material leading to the activation energy of the atomic oxygen recombination reaction in CO2 plasma and these results are compared to the ones previously obtained in air plasma (Earth entry).