Alberto Ortona

Wetting and contact-line effects for spherical and cylindrical droplets on graphene layers: a comparative molecular-dynamics investigation.

In molecular dynamics (MD) simulations, interactions between water molecules and graphitic surfaces are often modeled as a simple Lennard-Jones potential between oxygen and carbon atoms. A possible method for tuning this parameter consists of simulating a water nanodroplet on a flat graphitic surface, measuring the equilibrium contact angle, extrapolating it to the limit of a macroscopic droplet, and finally matching this quantity to experimental results. Considering recent evidence demonstrating that the contact angle of water on a graphitic plane is much higher than what was previously reported, we estimate the oxygen-carbon interaction for the recent SPC/Fw water model. Results indicate a value of about 0.2 kJ/mol, much lower than previous estimations. We then perform simulations of cylindrical water filaments on graphitic surfaces, in order to compare and correlate contact angles resulting from these two different systems. Results suggest that a modified Youngs equation does not describe the relation between contact angle and drop size in the case of extremely small systems and that contributions different from the one deriving from contact line tension should be taken into account.

SiC–SiCf CMC manufacturing by hybrid CVI–PIP techniques: process optimisation

SiC–SiCf ceramic matrix composites (CMC) are candidate structural material for fusion power reactor applications because of their favourable thermo-mechanical and low-activation properties. Among their different manufacturing techniques, present, the most employed ones are chemical vapour infiltration (CVI) and polymer infiltration and pyrolysis (PIP). These two techniques are based on the common principle of filling the porosity among the fibres with SiC resulting from precursor decomposition. CVI process deposits high purity crystalline SiC with good properties onto fibres whereas PIP leaves lower characteristic amorphous SiC with traces of oxygen between fibres. PIP, on the other hand, seems to be much more industrially effective than CVI. In the attempt to maximise the properties and reduce costs, some work has been done on the so called ‘hybrid techniques’ in which CVI and PIP are both employed. The work performed by ENEA and FN S.p.A. consists of a series of combined CVI–PIP process cycles and the subsequent product characterisation.

Aging of reticulated Si-SiC foams in porous burners

Abstract Abstract Si-SiC open cell foams with porosity >87% and high pore sizes (4-7 mm) are commonly employed as active zone in porous burners for heat radiation applications. In a porous burner, the solid porous body let the heat recirculate from the hot combustion products to the incoming reactants. The result is that the flame is confined within the foam, meaning high thermomechanical loadings on its constituent material. A set of commercial Si-SiC foams from the same production batch was aged with flat porous burners. Thermal cycles ramp-up, dwell and cooling, as well as burner set-up (power: 15 kW, fuel/air ratio: 1·5), were chosen based on previous experience. Before aging, each foam was first cut in bars ready for bending tests, reassembled into the burner foam configuration and operated. As produced and aged samples were physically, mechanically and chemically analysed and results compared.

High temperature oxidation of multilayered SiC processed by tape casting and sintering

The oxidation behaviour of a multilayer SiC ceramic was investigated at high temperatures. The ceramic samples were processed by tape casting of a slurry containing α-SiC powders, forming of green tubular components and sintering without pressure. The oxidation resistance of this ceramic material was investigated by temperature programmed oxidation (TPO); the gaseous oxidation products were analysed by mass spectrometry. Thermal gravimetric analyses (TGA) were also performed. Buckles, machined from the multilayer ceramic tubes, were submitted to long-term oxidation treatments carried out at 1200, 1400 and 1600 °C in air. The microstructure and the radial compressive strength of these specimens were compared before and after oxidation. Microstructure was investigated by SEM–EDS and X-ray diffraction. The multilayered SiC showed increased toughness with respect to conventional ceramics, owing to delamination phenomena occurring before failure. Oxidation mainly affected the material surface, where a thin layer of silica grew. Oxidation treatments did not suppress the energy adsorbing mechanism occurring during the fracture process.

Development of 2D and 3D Hi-Nicalon fibres/SiC matrix composites manufactured by a combined CVI–PIP route

SiCf/SiC composites with true 3D textures were manufactured in order to investigate the effects of the texture on thermal and mechanical properties. Hi-NicalonTM fibre preforms having a total fibre volume content of about 40% with 25 and 50% relative fibre content through the thickness were infiltrated using a mixed chemical vapour infiltration–polymer infiltration and pyrolysis technique. The issues and the main characteristics of the obtained 3D composites are presented and compared with the ones of 2D composites prepared employing the same materials and process.

Coarse-graining MARTINI model for molecular-dynamics simulations of the wetting properties of graphitic surfaces with non-ionic, long-chain, and T-shaped surfactants

We report on a molecular dynamics investigation of the wetting properties of graphitic surfaces by various solutions at concentrations 1-8 wt. % of commercially available non-ionic surfactants with long hydrophilic chains, linear or T-shaped. These are surfactants of length up to 160 Å. It turns out that molecular dynamics simulations of such systems ask for a number of solvent particles that can be reached without seriously compromising computational efficiency only by employing a coarse-grained model. The MARTINI force field with polarizable water offers a framework particularly suited for the parameterization of our systems. In general, its advantages over other coarse-grained models are the possibility to explore faster long time scales and the wider range of applicability. Although the accuracy is sometimes put under question, the results for the wetting properties by pure water are in good agreement with those for the corresponding atomistic systems and theoretical predictions. On the other hand, the bulk properties of various aqueous surfactant solutions indicate that the micellar formation process is too strong. For this reason, a typical experimental configuration is better approached by preparing the droplets with the surfactants arranged in the initial state in the vicinity of contact line. Cross-comparisons are possible and illuminating, but equilibrium contact angles as obtained from simulations overestimate the experimental results. Nevertheless, our findings can provide guidelines for the preliminary assessment and screening of surfactants. Most importantly, it is found that the wetting properties mainly depend on the length and apolarity of the hydrophobic tail, for linear surfactants, and the length of the hydrophilic headgroup for T-shaped surfactants. Moreover, the T-shaped topology appears to favor the adsorption of surfactants onto the graphitic surface and faster spreading.

Surface growth for molten silicon infiltration into carbon millimeter-sized channels: Lattice–Boltzmann simulations, experiments and models

The process of liquid silicon infiltration is investigated for channels with radii from

Surface Growth Effects On Reactive Capillary-Driven Flow: Lattice Boltzmann Investigation

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The influence of cell morphology on the effective thermal conductivity of reticulated ceramic foams

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Innovative Thermal Management Concepts and Material Solutions for Future Space Vehicles

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