Numerische Simulation der Faserbewegung beim Strangpressen kurzfaserverstärkter Glasschmelzen

Numerical Simulation of the Fibre-Motion during the Extrusion of Short-Fibre-Reinforced Glass-Melts Fibre-reinforced materials are characterized by an anisotropic behaviour of the mechanical properties, which is caused by the alignment of the embedded fibres. In the case of short-fibre-compounds this behaviour is strongly influenced by the mechanism of flow during the manufacturing process. Numerical simulation Methods are preferentially used to get informations about the orientation of the reinforced fibres at the end of the molding-process and to improve the properties of the compound. For that, a model is developed, which calculates the motion of the short-fibres in the area of flow, basing on a three-dimensional finite-element-computation. Thereby the interaction between the Particular fibres during the orientation process is considered by using an interaction coefficient. Examplified at the extrusion of short fibre reinforced glass-melts, the fibre orientation is determined at models with different geometries of the pressing tool and variable boundary conditions. This procedure allows to determine the influence of the process-parameters On the expected quality of the composite. The represented simulation-model can also be used for other molding- and extrusion-processes of fibre reinforced materials.     

Ermüdungsverhalten und Dauerfestigkeit von Graphit und Aluminium – infiltriertem Graphit

Fatigue behaviour and endurance limit of graphite and of aluminium‐infiltrated graphite Fatigue properties of polycrystalline, isotropic graphite FU2590 and of FU2590 infiltrated with AlSi7Mg (FU2590/AlSi7Mg) were investigated in reversed bending tests at 25 Hz at numbers of cycles below 10⁷ and in tension‐compression tests at 20 kHz below 10⁹ cycles. The open porosity of Graphite (10‐11 Vol.‐%) was infiltrated with the aluminium alloy using the squeeze casting infiltration method, which led to an increase of the bending strength by 50 %, increase of tensile strength by 30 % and increase of stiffness by 15 %. Fully reversed tension‐compression loading of FU2590 delivers a mean endurance limit at 10⁹ cycles at the normalized maximum stresses (i.e. maximum tension stress of a cycle divided by the static strength) of 0,65±0,03. Mean numbers of cycles to failure of 10⁴ were found in fully reversed bending tests at the normalized maximum stress of 0,78. The infiltrated material shows approximately 30 % higher cyclic strength in reversed bending tests, and the mean endurance limit under tension compression loading increases by 15 %. The increased endurance limit of the infiltrated material is caused by the increased stiffness. The increased toughness of graphite due to the infiltration with aluminium is of additional beneficial influence at the higher cyclic stresses investigated in reversed bending tests and in static tests.     

Inherent emf relaxation of electrochemical cells with nanocrystalline Ag electrodes

The open circuit voltage of the electrochemical cell Ag (nano)|solid silver electrolyte|Ag (macro) is found to be inherently unstable. Even under conditions which support the morphological stability of the arrangement of nanocrystalline silver, the particles grow significantly as soon as they function as electrodes; i.e. when they are in contact with a silver electrolyte and connected electronically at the same time. The process is shown to be due to electrochemical Ostwald ripening with the interfacial transfer of Ag⁺ through the Ag/electrolyte interface being the rate limiting step. Its activation energy is 0.01 eV. The decay is in good agreement with modelling results.     

A Micellar Route to Ordered Arrays of Magnetic Nanoparticles: From Size‐Selected Pure Cobalt Dots to Cobalt–Cobalt Oxide Core–Shell Systems

Starting with Co‐salt‐loaded inverse micelles, which form if the diblock copolymer polystyrene‐block‐poly(2‐vinylpyridine) is dissolved in a selective solvent like toluene and CoCl₂ is added to the solution, monomicellar arrays of such micelles exhibiting a significant hexagonal order can be prepared on top of various substrates with tailored intermicellar distances and structure heights. In order to remove the polymer matrix and to finally obtain arrays of pure Co nanoparticles, the micelles are first exposed to an oxygen plasma, followed by a treatment in a hydrogen plasma. Applying in‐situ X‐ray photoelectron spectroscopy, it is demonstrated that: 1) The oxygen plasma completely removes the polymer, though conserving the original order of the micellar array. Furthermore, the resulting nanoparticles are entirely oxidized with a chemical shift of the Co 2p_3/2 line pointing to the formation of Co₃O₄. 2) By the subsequent hydrogen plasma treatment the nanoparticles are fully reduced to metallic Co. 3) By exposing the pure Co nanoparticles for 100 s to various oxygen partial pressures pequation/tex2gif-inf-5.gif, a stepwise oxidation is observed with a still metallic Co core surrounded by an oxide shell. The data allow the extraction of the thickness of the oxide shell as a function of the total exposure to oxygen (pequation/tex2gif-inf-7.gif × time), thus giving the opportunity to control the ferromagnetic–antiferromagnetic composition of an exchange‐biased magnetic system.     

Sustainability: ecological, social, economic, technological, and systems perspectives

Sustainability is generally associated with a definition by the World Commission on Environment and Development, 1987: "... development that meets the needs of the present without compromising the ability of future generations to meet their own needs ..." However, there is no mathematical theory embodying these concepts, although one would be immensely valuable in humanity's efforts to manage the environment. The concept of sustainability applies to integrated systems comprising humans and the rest of nature; the structures and operation of the human component (society, economy, law, etc.) must be such that they reinforce the persistence of the structures and operation of the natural component (ecosystem trophic linkages, biodiversity, biogeochemical cycles, etc.). One of the challenges of sustainability research lies in linking measures of ecosystem functioning to the structure and operation of the associated social system. We review the nature of this complex system including its ecological, social, economic, and technological aspects, and propose an approach to assessing sustainability based on Information Theory that bridges the natural and human systems. These principles are then illustrated using a model system with an ecological food web linked to a rudimentary social system. This work is part of the efforts of a larger multidisciplinary group at the U.S. Environmental Protection Agency's National Risk Management Research Laboratory.