Suchergebnisse

The process of liquid silicon infiltration is investigated for channels with radii from 0.25 to 0.75 [mm] drilled in compact carbon preforms. The advantage of this setup is that the study of the phenomenon results to be simplified. For comparison purposes, attempts are made in order to work out a framework for evaluating the accuracy of simulations. The approach relies on dimensionless numbers involving the properties of the surface reaction. It turns out that complex hydrodynamic behavior derived from second Newton law can be made consistent with Lattice-Boltzmann simulations. The experiments give clear evidence that the growth of silicon carbide proceeds in two different stages and basic mechanisms are highlighted. Lattice-Boltzmann simulations prove to be an effective tool for the description of the growing phase. Namely, essential experimental constraints can be implemented. As a result, the existing models are useful to gain more insight on the process of reactive infiltration into porous media in the first stage of penetration, i.e. up to pore closure because of surface growth. A way allowing to implement the resistance from chemical reaction in Darcy law is also proposed. ; The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement nº 280464, project "High-frequency ELectro-Magnetic technologies for advanced processing of ceramic matrix composites and graphite expansion" (HELM).

Abstract
The existence of a phase transition between two distinct liquid phases in single-component network-forming liquids (e.g. water, silica, silicon) has elicited considerable scientific interest. The challenge, both for experiments and simulations, is that the liquid–liquid phase transition (LLPT) occurs under deeply supercooled conditions, where crystallization occurs very rapidly. Thus, early evidence from numerical equation of state studies was challenged with the argument that slow spontaneous crystallization had been misinterpreted as evidence of a second liquid state. Rigorous free-energy calculations have subsequently confirmed the existence of a LLPT in some models of water, and exciting new experimental evidence has since supported these computational results. Similar results have so far not been found for silicon. Here, we present results from free-energy calculations performed for silicon modeled with the classical, empirical Stillinger-Weber–potential. Through a careful study employing state-of-the-art constrained simulation protocols and numerous checks for thermodynamic consistency, we find that there are two distinct metastable liquid states and a phase transition. Our results resolve a long-standing debate concerning the existence of a liquid–liquid transition in supercooled liquid silicon and address key questions regarding the nature of the phase transition and the associated critical point.

We demonstrate that an hydrogenated amorphous silicon (a:SiH)-liquid crystals hybrid device could be used for the recording of high resolution (0.8-2 µm) dynamic holograms. A maximum diffraction efficiency of 3.3% was obtained at low power (1.5 mW) He-Ne laser. The nonlinear refractive index change at 0.6 W/cm2 is n2~1x10−2 cm2 /W, although small compared to that obtained in dye-doped liquid crystal, is equal to the reported in pure liquid crystal although with much higher power density (~50 W/cm2 ). The device operates in the red to near-infrared part of the spectrum which makes it attractive due to its potential applications in telecommunications and military applications.

Der vorliegende Artikel beschreibt anhand von zwei Gefechtsberichten die Einsatz- und Kampfgrundsätze von Infiltrationskräften im Rahmen der Kleinkriegführung im Dschungel von El Salvador. Die Aktionen wurden von Angehörigen des Marinekorps durchgeführt. Sie waren mit den regionalen Gegebenheiten und dem Verhalten der im Einsatzraum lebenden Zivilbevölkerung vertraut. Von wesentlicher Bedeutung war auch eine detaillierte Geländekenntnis, wobei auf Luftbildaufnahmen zurückgegriffen wurde

High-temperature infiltration is an important process that is used to add strength to skeletal microstructures. In this study, particulate metal matrix composites (MMCs) are fabricated. MMCs are applied in a wide variety of industries, including military, aircraft, tooling and automotive. In this paper, various materials for infiltrating selective laser sintered (SLS) silicon carbide and titanium carbide preforms are considered based on fundamental knowledge of SLS and infiltration mechanics. Proposed infiltrant materials systems include an aluminum-silicon alloy infiltrant and a silicon carbide preform, ductile iron infiltrated into a titanium carbide preform, and commercially pure silicon infiltrated into a silicon carbide preform. The first two infiltrants are considered because they add ductility to the brittle silicon carbide or titanium carbide part, thus broadening the range of applications. They also will model a broader field of possible infiltrants, including magnesium and iron-based materials, (e.g., steel). Silicon is investigated because it adds strength to silicon carbide, is robust at high temperatures, and has a comparable coefficient of thermal expansion. Presented is a feasibility assessment of these systems based on infiltration theory. ; Mechanical Engineering

To design and optimize liquid-assisted processes such as reactive infiltration for fabricating refractory SiC/ZrSi2 composites, basic investigations on the interfacial phenomena occurring when liquid Si-based alloys are in contact with C and SiC substrates, are key steps. Indeed, targeted wettability studies may provide helpful indications for finding the suitable set of operating conditions to succeed the fabrication of composites via the reactive infiltration and for predicting the key influencing mechanisms. The wettability of glassy carbon (GC) by two different Si-rich Si-Zr alloys as a function of the Si content has been investigated by the sessile drop method at T = 1450°C. The more relevant results obtained in terms of equilibrium contact angle values, spreading kinetics, reactivity and developed interface microstructures are reported in the paper and compared with the behaviour previously observed in the Si-27Zr/GC system. The increase of Si-content only weakly affected the overall phenomena observed at the interface, which from the practical point of view means that even the Si-Zr alloys with higher Si-content, as respect to the eutectic alloy (Si-27Zr), could be potentially used as infiltrant materials. ; The work performed by DG, W.P, G.B. and A.K. was supported by National Science Center of Poland through POLONEZ project number UMO-2016/23/P/ST8/01916. This project is carried out under POLONEZ-3 program which has received funding from European Union's Horizon 2020 research and innovation program under Marie Skłodowska-Curie grant agreement. No 665778.

In designing advanced refractory composites for highly demanding applications, reactive infiltration of liquid Si-enriched Si-Zr alloys into C-or SiC-based preforms may be a viable cost-less manufacturing process. In such cases, in view to optimize liquid assisted processes such as reactive infiltration, fundamental investigations of the interfacial phenomena occurring when the liquid Si-Zr alloys are in contact with C and SiC substrates, are key steps. For this reason, aiming to ''mimic'' the conventional operating conditions imposed within a reactive infiltration process, the contact heating sessile drop method was applied to perform a basic study concerning the interaction phenomena taking place at the interface of Si-10% at Zr alloy/Glassy Carbon substrate under an Ar atmosphere. Specifically, the contact angle values as a function of time were measured in the temperature range of 1354e1500 C. The final contact angle values decreased slightly with an increase in temperature.Moreover, at T ¼ 1450 C, the contact angle increased over a larger time interval before reaching its final value. The kinetics of SiC crystal growth at the interface and the related processing parameters such as temperature and time were carefully analysed. The growth of SiC crystals and their packaging phenomenon are time and temperature-dependent phenomena. ; The NCN-National Science Center, Poland is greatly acknowledged for the financial support through the POLONEZ project number UMO-2016/23/P/ST8/01916. This project was carried out under POLONEZ-3 program which has received funding from European Union's Horizon 2020 research and innovation program under Marie Skłodowska-Curie grant agreement. No 665778.