R. Berjoan

Recombination coefficient of atomic oxygen on ceramic materials under earth re-entry conditions by optical emission spectroscopy

Abstract To develop heat shields for space vehicles, materials must be characterized in simulation conditions close to those in space environments. The most important conditions for simulating the Earth re-entry phase of space vehicles (high temperatures, low pressure air plasma,…) are achieved through the MESOX set-up associating a solar radiation concentrator and a microwave plasma generator. After determining the thermal recombination flux transferred to the material (thermal approach, accommodation), we developed a chemical approach for evaluating the recombination coefficient of atomic oxygen γO by Optical Emission Spectroscopy. We also measured the gas temperature above the material, close to the surface where recombination occurs, using the rotational temperature of nitrogen. The recombination coefficients of atomic oxygen were determined in function of the temperatures of two types of silica materials. Finally, the activation energy of the atomic oxygen recombination reaction was obtained for each material. Results showed that the microstructure of the topmost layer has an important effect and that the recombination coefficient of β-cristobalite is four times higher than that of quartz.

Structural characterization of amorphous SiCxNy chemical vapor deposited coatings

Chemical bonding and local order around the different atoms of thick amorphous SiCxNy deposits [0.03⩽x/(x+y)⩽0.67] prepared with chemical vapor deposition at 1000–1200 °C using TMS–NH3–H2 have been investigated using x-ray photoelectron spectroscopy (XPS), Raman spectrometry, Fourier transform infrared spectrometry (FT-IR), electron energy loss spectroscopy (EELS) and 29Si magic-angle spinning nuclear magnetic resonance spectrometry (MAS-NMR). XPS analyses have shown that the main bonds are Si–C, Si–N, and C–C, and have suggested the existence of C–N bonds. According to Raman analyses and complementary FT-IR absorption of thin films, the coatings are nonhydrogenated. Si, C and N atomic chemical environments are more complicated than in a mixture of pure Si3N4–SiC phases. The examination of the Si KL2,3L2,3 line shapes recorded by XPS have allowed one to state the existence of Si(C4−nNn) units. Mixed coordination shells around silicon have been confirmed by EELS analyses. Additionally, FT-IR reflection ana...

Physico-chemical behavior of carbon materials under high temperature and ion irradiation

Abstract Carbon/carbon (C/C) composites have been chosen for the conception of the thermal shield of the “Solar Probe” space mission. To understand their behavior under solar aggressions (high temperature and ion irradiation) and know the possible interactions with the shipped-in instruments, these materials are tested in a facility that allows to partially simulate the solar environment and to carry out in situ measurements. The mass loss rate of the materials is recorded and the composition and distribution of the ejected species are followed using an opened source mass spectrometer and a post-treatment XPS analysis of the condensed species on a gold-plated copper target is also performed. Besides this, the sputtering processes of graphite at high temperature and ion irradiation have been theoretically investigated in order to understand the main parameters that can influence the erosion of the material and predict accurately the physico-chemical behavior of the C/C composites.

An XPS and FTIR study of SO2 adsorption on SnO2 surfaces

Abstract The adsorption of SO2 on SnO2 surfaces obtained from hydrolysis of SnCl4 and NH3 was studied by X-ray Photoelectron Spectroscopy. Tin dioxide surfaces were treated with SO2 at temperatures ranging from 300 to 900°C. For this temperature range, XPS shows that SO2 is adsorbed on tin dioxide surfaces to form two sulphured species: SO32− for temperatures ranging from 300 to 500°C and SO42− for temperatures ranging from 300 to 900°C. With regards to these two chemical species, we were able to confirm their formation using a second analytical technique (FTIR). The formation of these species will be discussed in terms of a comparison between the results of the two techniques. The role played by OH groups in these interactions is also discussed in this article.

Surface investigation of plasma HMDSO membranes post-treated by CF4/Ar plasma

Abstract Fluorination treatment has been performed on polysiloxane membranes using a plasma glow discharge of a gases mixture CF4 and argon (plasma enhanced chemical vapor deposition). Atomic force microscopy, XPS analyses and contact angle measurements have been undertaken to explain the surface transformation and behavior, which strongly depend on the morphology, the composition and the hydrophilic/hydrophobic character of the plasma-polymerized initial membranes. Main result is that fluorination, which leads to hydrophobic membranes, has a more relevant effect on amorphous silica-like membranes than on polymer-like ones, according to their chemical composition whereas the plasma surface reaction induces a substantial microstructural modification of the polymer-like membrane, i.e. the grains and aggregates size and distribution.

Oxidation stages of aluminium nitride thin films obtained by plasma-enhanced chemical vapour deposition (PECVD)

Abstract Oriented (100) aluminium nitride thin films grown on silicon wafers (100) and other substrates, were deposited at 330°C by the metal-organic PECVD process. The oxidation behaviour in air of these films was studied at temperatures from 500°C to 1300°C by X-ray diffraction, scanning electron microscopy and Auger electron spectroscopy. Two textural changes occur: granular and porous textures at 900°C and 1100°C, respectively. These correspond to the amorphous Al 2 O 3 and the crystalline α-alumina formation. Infra-red absorption spectroscopy shows that the oxidation effectively starts at 600°C and reveals an oxynitride phase between the amorphous Al 2 O 3 coating which is formed, and the AlN remaining.

XPS and FTIR study of silicon oxynitride thin films

SiN x , SiO x and SiO x H y deposits containing various hydrogen concentrations were prepared in a plasma enhanced chemical vapour deposition (PECVD) reactor using SiH 4 , NH 3 and N 2 O as precursor gases. These deposits were made for anti-reflection coatings on polymer substracts. In this work, we present a study of the compositions and the chemical environments of silicon, oxygen and nitrogen in these films by using XPS, XAES and FTIR characterization methods. It is shown that the Sio x N y deposits are constituted by various silicon environments which can be described by the presence of Si(O x N y H z ) tetrahedra with x + y + z = 4. The amount of Si-H bonds increases in the deposits when the nitrogen concentration increases. Si-OH chemical bonds were detected for low nitrogen concentration.

High-temperature oxidation of sintered silicon carbide under pure CO2 at low pressure: active–passive transition

Abstract The study of the oxidation of SiC under CO 2 at high temperature and low pressure is achieved by means of theoretical and experimental determinations of the transition zone between the passive oxidation with formation of a protective silica layer and the active oxidation with major vaporization of SiO and etching of the SiC material. The theoretical determination is done using both models: a computer model “Solgasmix”, applied for a closed system at equilibrium and the other, our analytical model, which takes into account the mass transfer at the solid/gas interface. The comparison is done for these two models and then with the experimental results. XPS analyses are performed on “passive” and “active” samples to confirm the position of the transition line. Moreover, SEM micrographs complete the diagnostics.

Organic/inorganic thin films deposited from diethoxydimethylsilane by plasma enhanced chemical vapor deposition

Abstract Thin a-SiOxCyHz films were deposited from diethoxydimethylsilane in a radio-frequency plasma polymerization process. The plasma deposition and the film properties were characterized with respect to the deposition rate evaluated from scanning electron microscopy observations, the film refractive index determined by spectroscopic ellipsometry and the film density measured by X-ray reflectometry. The layer chemical structure was investigated by Fourier-transform infrared spectroscopy analysis, 29Si solid-state nuclear magnetic resonance technique, and X-ray photoelectron spectroscopy measurements. The film growth rate exhibits a typical evolution as a function of the process parameters and appears to be dependent on the nature of the substrate. We are able to correlate the structure of the films obtained with the important composite parameter V/Pp (RF voltage V over the monomer partial pressure Pp) characterizing the input energy per unit mass of monomer. Chemical analysis spectra show that the concentration of [(CH3)2–Si(–O)2] silicon environment increases with lowering the V/Pp ratio; at very low V/Pp values, plasma polymers are close to polydimethylsiloxane. By contrast, the thin layers contain a high [SiO4] concentration, characteristic of SiO2, when V/Pp increases. Moreover, increasing V/Pp results in higher C/Si and O/Si ratios while the films density and refractive index also rise.

29Si NMR and Si2p XPS correlation in polysiloxane membranes prepared by plasma enhanced chemical vapor deposition

Abstract Thin a-SiO x C y H z membranes were deposited from two different organosilicon precursors, a linear one the octamethyltrisiloxane (MDM) and a cyclic one the hexamethylcyclotrisiloxane (D3), in a low-frequency plasma polymerization process. The chemical structure of polysiloxane plasma materials was characterized using two different spectroscopic analyses: the X-ray Photoelectron Spectroscopy (XPS) and the 29 Si solid-state Nuclear Magnetic Resonance (NMR). Both techniques show that it is possible to synthesize a wide range of materials, more or less organic and close to silicone-type conventional polymers, depending on whether the energetic character of the plasma is low or high. The structural differences between plasma films synthesized from MDM (PP-MDM films) and plasma materials deposited from D3 (PP-D3 films) are all the more pronounced that the plasma conditions are soft due to the better preservation of the monomer chemical structure. The NMR analysis allows to display the presence of cyclic siloxane chains in PP-D3 layers whereas PP-MDM materials are exclusively composed of linear siloxane chains.