Reaktionstechnik bei industriellen Hochdruckverfahren

Reaction engineering in industrial high-pressure processes . The essential task of industrial chemistry is to transform cheap raw materials into high-quality materials and substances economically. From the very beginning, the chemical industry has used the energy sources coal, oil, and gas as the starting point for synthesis. These substances are introduced into the product line through high-pressure reactions. Joint developments in catalytic chemistry reaction kinetics, and plant design have permitted construction and operation of high-capacity, high-pressure plants at remote sites, where coal, oil, and gas are won. Industrial high-pressure chemistry is thus capable of transforming in situ raw materials that could be used otherwise into high-value products worth transporting to energy-poor regions and whose distribution is secured. The second part of this paper considers the technical limits of pumps, compressors, and high-pressure reactors. Approximate estimates of investment costs as a function of the pressure as process parameter are given for a high-pressure process step. Aspects to be taken into consideration during design, construction, and installation of factory-built high-pressure reactors are considered in condensed form in the last part.     

Comparing hydrogen testing methods for wrought aluminum

In this study on selected wrought aluminum alloys, several commonly used methods for determining hydrogen were compared and verified with regard to accuracy, reproducibility, and process-related limits. The two direct methods studied—the AlScan and CHAPEL (continuous hydrogn analysis by pressure evaluation in liquids) techniques—are able to quantitatively determine hydrogen content directly in the melt. In testing, both showed outstanding agreement with regard to accuracy and reproducibility. Two analytical methods were also studied—vacuum solid extraction and carrier fusion extraction. Both the direct and analytical methods yielded satisfactory agreement among the results. Also studied was an indication method, reduced-pressure testing, which is a quick and simple method suitable for the qualitatively evaluating hydrogen content. Here, however, the relationship between the density index and the hydrogen content of the melt is influenced by numerous factors.     

The influence of natural reinforcement fibres on insulation values of earth plaster for straw bale buildings

This work aimed to measure the thermal conductivity of some natural plaster materials that could be used for straw bale buildings. Thermal conductivity is very important to determine the insulation value and other thermal parameters for natural plaster materials. Plaster materials consisted of soil, sand and straw. Straw is used as a reinforcement fibre for plaster. Three types of fibres were used such as wheat straw, barley straw and wood shavings. The results indicated that the thermal conductivity of all materials decreased with increasing straw fibre content and decreased with increasing sand content. The straw fibres have greater effect on the change of thermal conductivity than the effect of sand. The results also revealed that plaster reinforced by barley straw fibres has the highest values of thermal insulation.     

Direct determination of minority carrier diffusion lengths at axial GaAs nanowire p-n junctions

Axial GaAs nanowire p-n diodes, possibly one of the core elements of future nanowire solar cells and light emitters, were grown via the Au-assisted vapor-liquid-solid mode, contacted by electron beam lithography, and investigated using electron beam induced current measurements. The minority carrier diffusion lengths and dynamics of both, electrons and holes, were determined directly at the vicinity of the p-n junction. The generated photocurrent shows an exponential decay on both sides of the junction and the extracted diffusion lengths are about 1 order of magnitude lower compared to bulk material due to surface recombination. Moreover, the observed strong diameter-dependence is well in line with the surface-to-volume ratio of semiconductor nanowires. Estimating the surface recombination velocities clearly indicates a nonabrupt p-n junction, which is in essential agreement with the model of delayed dopant incorporation in the Au-assisted vapor-liquid-solid mechanism. Surface passivation using ammonium sulfide effectively reduces the surface recombination and thus leads to higher minority carrier diffusion lengths.     

Does C–S–H particle shape matter? A discussion of the paper ‘Modelling elasticity of a hydrating cement paste’, by Julien Sanahuja,Luc Dormieux and Gilles Chanvillard. CCR 37 (2007) 1427–1439

In their inspiring paper, Sanahuja et al. (CCR (37) 1427–1439) add a new dimension and degree of freedom for implementing the Materials Science Paradigm between microstructure and properties of cement-based materials, which is the particle aspect ratio of C–S–H elementary building block. The question addressed in this discussion is how far this departure from the perfect disordered morphology of C–S–H is truly relevant for cement-based materials.     

Thermo-Chemo-Mechanics of ASR Expansion in Concrete Structures

The critical nature of the alkali-silica reaction (ASR) on premature concrete deterioration requires the quantitative assessment, in time and space, of the chemomechanical impact of ASR expansion on the dimensional stability of concrete structures. In particular, the coupled problem of heat diffusion and ASR kinetics can be critical, as the ASR is a thermoactivated chemical reaction. The quantitative analysis of these coupled effects on both material and structural level is the main objective of this paper. Starting from the governing micromechanisms of ASR expansion, a chemoelastic model is developed that accounts for ASR kinetics and the swelling pressure exerted by the ASR reaction products on the skeleton. This chemoelastic model is a first-order engineering approach to capture timescale and magnitude of ASR expansion. It is shown that the realistic prediction of ASR structural effects requires the consideration of two timescales: (a) A latency time associated with the dissolution of reactive silica; and (2) a characteristic time associated with the ASR product formation. In addition, a dimensional analysis of the governing equations reveals that the ASR deterioration of “massive” concrete structures is driven by the simultaneous activation of heat diffusion and reaction kinetics within a surface layer defined by a characteristic ASR heat diffusion length. In turn, in “slender” structures, it is the simultaneous activation of moisture diffusion and ASR kinetics that drives the surface layer delamination. This is illustrated through finite-element case studies of ASR effects in structures of different dimensions: a concrete gravity dam and a bridge box girder.     

An undetected reason for severe fetal growth retardation

Cytologic examination of synovial fluid has been shown to be useful in the diagnosis of various joint diseases. Few pathologists are proficient in the interpretation of the occasional synovial fluids that most cytology laboratories receive. The majority of the aspirated synovial fluids are sent only for bacterial culture and valuable information from the cytologic examination is missed. We describe how pathologists should perform analysis of these fluids and what kind of information they can provide to the physician.     

Use of neural networks for fitting of FE probabilistic scaling model parameters

The probabilistic crack approach, based on the Monte Carlo method, was recently developed for finite element analysis of concrete cracking and related size effects. In this approach the heterogeneity of the material is taken into account by considering the material properties (tensile strength, Young modulus, etc.) to vary spatially following a normal distribution. N samples of the vector of random variables are generated from a specific probability density function, and the N samples corresponding to a simulation are functions of the mean value and of the standard deviation that define the Gauss density function. The problem is that these statistical moments are not known, a priori, for the characteristic volume of the finite elements used in the analysis. The paper proposes an inverse finite element analysis using neural networks for the determination of the statistical distribution parameters (e.g., for a normal distribution, the mean and the standard deviation) from a given response of the structure (for instance, an average load-displacement curve). From FE-analysis of 4-point bending beam tests, it is shown that the backanalysis technique developed in this paper is a powerful tool to determine the probabilistic distribution functions at the material level from structural tests for material volumes which are generally not accessible to direct testing. This revised version was published online in July 2006 with corrections to the Cover Date.     

Chemomechanics of concrete at finer scales

Concrete, like many other materials (whether man-made, geological or biological), is a highly heterogeneous material with heterogeneities that manifest themselves at multiple scales. As new experimental techniques such as nanoindentation have provided unprecedented access to micro-mechanical properties of materials, it becomes possible to identify the mechanical effects of chemical reactions at the micro-scale, where the reactions occur, and trace these micro-chemo-mechanical effects through upscaling techniques to the macro-scale. The focus of this paper is to review recent developments of a microchemomechanics theory which ultimately shall make it possible to capture chemomechanical deterioration processes at the scale where physical chemistry meets mechanics. This is illustrated through application of the theory to early-age concrete and calcium leaching, and an outlook to biologically mediated deterioration processes in solid materials is given.

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Résumé Le béton comme beaucoup d'autres matériaux, soit artificiels, géologiques ou biologiques, est un matériau très hétérogène, dont les hétérogénéités se manifestent à de multiples échelles. Comme des techniques expérimentales nouvelles, telle la nano-indentation, ont donné un accès non-précédent aux propriétés micromécaniques des matériaux, il est possible d'identifier les effets mécaniques des réactions chimiques à l'échelle microscopique, où les réactions ont lieu, et tracer ces effets au travers des méthodes de changement d'échelle vers l'échelle macroscopique. Cet article fait le point sur le développement d'une modélisation micro-chimicomécanique qui a comme but de modéliser la détérioration chimico-mécanique à partir de l'échelle physico-chimique. Ces développements sont illustrés au travers des applications au béton au jeune age et à la lixiviation des bétons. Enfin, l'extension de cette modélisation aux processus de détérioration bio-chimique des matériaux est mise en perspective.

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Editorial note Prof. Franz-Josef Ulm presented a lecture of this paper at the 2002 RILEM Annual Meeting in Madrid, Spain, when he was awarded the 2002 Robert L'Hermite Medal in recognition of his work in the field of durability mechanics. He is a RILEM Senior Member and Associate Editor forConcrete Science and Engineering.