First in-flight synchrotron X-ray absorption and photoemission study of carbon soot nanoparticles.

Many studies have been conducted on the environmental impacts of combustion generated aerosols. Due to their complex composition and morphology, their chemical reactivity is not well understood and new developments of analysis methods are needed. We report the first demonstration of in-flight X-ray based characterizations of freshly emitted soot particles, which is of paramount importance for understanding the role of one of the main anthropogenic particulate contributors to global climate change. Soot particles, produced by a burner for several air-to-fuel ratios, were injected through an aerodynamic lens, focusing them to a region where they interacted with synchrotron radiation. X-ray photoelectron spectroscopy and carbon K-edge near-edge X-ray absorption spectroscopy were performed and compared to those obtained for supported samples. A good agreement is found between these samples, although slight oxidation is observed for supported samples. Our experiments demonstrate that NEXAFS characterization of supported samples provides relevant information on soot composition, with limited effects of contamination or ageing under ambient storage conditions. The highly surface sensitive XPS experiments of airborne soot indicate that the oxidation is different at the surface as compared to the bulk probed by NEXAFS. We also report changes in soot's work function obtained at different combustion conditions.

Influence of CH(4) partial pressure on the microstructure of sputter-deposited tungsten carbide thin films.

Tungsten carbide thin films have been prepared by reactive rf sputtering from a tungsten target in various Ar-CH(4) mixtures. The composition, structure, microstructure and chemical state of the films have been investigated by the complementary use of RBS, NRA, XRD, GIXRD, TEM and XPS analyses. These characteristics of the films were then correlated to their mechanical properties determined by hardness (H), Young's modulus (E(r)) and friction coefficient measurements. Under low CH(4) pressures, the formation of a mixture of nanocrystalline WC(1-x) and W(2)C phases has been observed. A pure WC(1-x) phase was observed in films having a composition close to W(1)C(0.9). With increasing CH(4) pressure, the amount of carbon in the films increases, leading to a progressive amorphization of tungsten carbide deposited layers. Nanocomposite films appeared to be formed, with WC(1-x) nanograins (<3 nm) dispersed in an amorphous carbon matrix. The film deposited at 30% of CH(4) exhibits a-C:H phase. The nature of the phases present in the films plays an important role on their mechanical properties, as shown by the wide domain of variation of the films' hardness (between 22 and 5.5 GPa) and the plastic deformation parameter H(3)/E(r)(2) (between 0.08 and 0.04).