Methodology for Ablation Investigations in the VKI Plasmatron Facility: Preliminary Results with a Carbon Fiber Preform

Following the current developments of a new class of low-density, carbon/resin composite ablators, new efforts were initiated at the VKI on ablation research to understand the complex material response under reentry conditions and to develop and validate new material response models. Promising experimental results were obtained by testing the low-density monolytic composite ablator (MonA) in the 1.2MW inductively heated VKI Plasmatron facility. The application of a high speed camera with short exposure times (2μs) enabled in-situ analysis of both (3D) surface recession and spallation and further made it possible to demonstrate the outgassing effects of pyrolizing ablators. A change in the surrounding gas phase was observed, which is likely due to outgassing products keeping away the hot surrounding plasma before burn-off in the boundary layer. Time-resolved emission spectroscopy helped to identify carbonic species and to capture thermo-chemical effects. This knowledge was then translated into the development of a testing methodology for charring, low-density ablators in order to investigate the material response in the reactive boundary layer. The successful application of emission spectroscopy encouraged the extension of the setup by two more emission spectrometers for not only temporal but also spatial observations. The extracted experimental data will be employed for comparison with model estimates enabling validation of a newly developed stagnation line formulation for ablation thermochemistry. It was further understood that a proper examination of tested samples has to be performed, especially of the subsurface char layer, which is subjected to ablation. Degradation of the carbon fibers can vary with pressure and surface temperature due to the changing diffusion mechanisms of oxygen that can weaken the internal structure, leading to spallation and mechanical failure. This necessitates ablation tests in combination with microscopic analysis tools (SEM/EDX) for sample examination at the carbon fiber length scale (~10μm). Such microscale characterization was recently started at the VKI: A low-density carbon fiber prefom (without phenolic impregnation) was tested in the Plasmatron facility at varying static pressures from 1.5-20kPa at a constant cold wall heat flux of 1MW/m2, resulting in surface temperatures of around 2000K. Surprisingly, it was found that recession and mass loss of the test specimen was highest at low static pressure (1.5kPa). Furthermore, high-speed-imaging as well as conventional photography revealed strong release of particles into the flow field, probably assignable to spallation. Micrographs showed that packages of glued fibers (fiber bundles) are embedded in between randomly oriented, individual fibers. It is therefore assumed that ablation of the individual fibers leads to detachment of such whole fiber bundles. It was further found that in an ablation environment of 10kPa ablation lead to an icicle shape on a top layer of 250μm of the fibers with constant thinning, whereas at low pressure (1.5kPa), the fibers showed strong oxidation degradation over their whole length (650μm). Computed diffusion coefficients of atomic oxygen in the boundary layer were more than ten times higher in the case of 1.5kPa compared to 20kPa. This, together with a much lower atomic oxygen concentration at 1.5kPa (decreasing the fiber’s reactivity) may allow oxygen to penetrate into the internal material structure. More investigation on both experimental and numerical level is required to confirm those trends. A comprehensive test campaign on a fully developed low-density ablator, ASTERM, is planned for spring 2012 at the VKI

Doped ordered mesoporous carbons as novel, selective electrocatalysts for the reduction of nitrobenzene to aniline

Ordered mesoporous carbons (OMCs) doped with nitrogen, phosphorus or boron were synthesised through a two-step nanocasting method and studied as electrocatalysts for the reduction of nitrobenzene to aniline in a half-cell setup. The nature of the dopant played a crucial role in the electrocatalytic performance of the doped OMCs, which was monitored by LSV with a rotating disk electrode setup. The incorporation of boron generated the electrocatalysts with the highest kinetic current density, whereas the incorporation of phosphorus led to the lowest overpotential. Doping with nitrogen led to intermediate behaviour in terms of onset potential and kinetic current density, but provided the highest selectivity towards aniline, thus resulting in the most promising electrocatalyst developed in this study. Density functional theory calculations allowed explaining the observed difference in the onset potentials between the various doped OMCs, and indicated that both graphiticN and pyrdinic N can generate active sites in the N-doped electrocatalyst. A chronoamperometric experiment over N-doped OMC performed at -0.75 V vs. Fc/Fc(+) in an acidic environment, resulted in a conversion of 61% with an overall selectivity of 87% to aniline. These are the highest activity and selectivity ever reported for an electrocatalyst for the reduction of nitrobenzene to aniline, making N-doped OMC a promising candidate for the electrochemical cogeneration of this industrially relevant product and electricity in a fuel cell setup

Electrochemical impedance spectroscopy beyond linearity and stationarity - a critical review

Electrochemical impedance spectroscopy (EIS) is a widely used experimental technique for characterising materials and electrode reactions by observing their frequency-dependent impedance. Classical EIS measurements require the electrochemical process to behave as a linear time-invariant system. However, electrochemical processes do not naturally satisfy this assumption: the relation between voltage and current is inherently nonlinear and evolves over time. Examples include the corrosion of metal substrates and the cycling of Li-ion batteries. As such, classical EIS only offers models linearised at specific operating points. During the last decade, solutions were developed for estimating nonlinear and time-varying impedances, contributing to more general models. In this paper, we review the concept of impedance beyond linearity and stationarity, and detail different methods to estimate this from measured current and voltage data, with emphasis on frequency domain approaches using multisine excitation. In addition to a mathematical discussion, we measure and provide examples demonstrating impedance estimation for a Li-ion battery, beyond linearity and stationarity, both while resting and while charging

Preface to the Special Issue in the honour of Claudine Buess-Herman on the occasion of her 65th anniversary

SCOPUS: ed.jinfo:eu-repo/semantics/publishe

New Insights in Nano-electrodeposition: An Electrochemical Aggregative Growth Mechanism

textcopyright Springer International Publishing Switzerland 2016. Supported nanostructures represent the cornerstone for numerous applications in different fields such as electrocatalysis (fuel cells) or electroanalysis (sensors). In contrast to other methods, electrochemical deposition allows the growth of the nanostructures directly on the final support, improving the electron pathway within the substrate, nanostructure, and electrolyte. However, despite the increasing number of publications in the field, the early stages of electrochemical nanocrystal formation are still under discussion. In this chapter, we first provide a survey on the traditional approaches to study the early stages of electrochemical nucleation and growth, together with the classical theories used to understand them. Next, we describe our most recent findings which have led to reformulate the Volmer-Weber island growth mechanism into an electrochemical aggregative growth mechanism which mimics the atomistic processes of the early stages of thin-film growth by considering nanoclusters of few nm as building blocks instead of single atoms. We prove that the early stages of nanoelectrodeposition are strongly affected by nanocluster selflimiting growth, surface diffusion, aggregation, and coalescence.info:eu-repo/semantics/publishe

Experimental and theoretical study of CVV Auger peaks of selected aluminium and carbon compounds

The Auger valence peak of Al2O3, Al and carbon compounds (graphite, fullerene, carbides) has been studied experimentally and theoretically. It is demonstrated, from a comparison of the experimental spectrum with the self-convolution of the valence band, that the Al CVV transition in Al2O3 is an intra-atomic transition. The behaviour of the C KVV Auger peak is shown to be intermediate between atomic and band-like transitions. The different experimental carbide peaks could be reconstructed by a self-convolution of the valence band and the introduction of a hole-hole repulsion term extracted from the Cini-Sawatzki equation. A non-expected behaviour of the satellite peak at 280 eV for tungsten and chromium carbide is shown and interpreted, with a comparison with the Ramaker approach. Copyright © 2004 John Wiley & Sons, Ltd.SCOPUS: cp.jFLWINinfo:eu-repo/semantics/publishe