Polymer derived ceramic materials from Si,B and MoSiB filler-loaded perhydropolysilazane precursor for oxidation protection

Silica-based coating systems were developed using polymer derived ceramics (PDCs) technology. Ceramic composites on the base of a SiO₂ and SiNO matrix and homogeneously distributed Mo₅SiB₂, SiB₆, Si and B fillers were manufactured. The coating systems have low porosity and provide a high oxidation resistance up to 100 h at 800 °C and 1100 °C in air. The influence of temperature and atmosphere of pyrolysis on the polymer precursor, the volume fraction of filler materials on the chemical composition of compacts as well as their high-temperature oxidation protection was investigated.     

Impacts of situational factors on process attribute uses for food purchases

Consumer buying decisions for food reflect considerations about food production.However, consumers’ interest in process-related product characteristics does not always translate into buying intentions. The present study investigates how situational factors affect the use of process-related considerations when consumers select food products. A conjoint study provides estimated part worth utilities for product alternatives that differ on five product attributes (including four process-related factors) across two products (bread and sports drink) that differ on perceived naturalness. The investigation of the utilities of the process-related attributes features both an internal (priming of environmental values/value centrality) and an external (time pressure) situational factor. The results indicate that the importance of process-related attributes is product specific and also depends on situational factors.     

Influence of the geometry on the pressure distribution of granular heaps

We investigate the effect of the geometry of granular heaps on the pressure distribution. For given pressure distributions under cones we compute the pressure distribution under wedges using linear superposition. For cones with a pressure minimum, the pressure minimum for the corresponding wedge vanishes. Comparisons with experimental data gives good qualitative aggreement, but the total pressure is overestimated.     

Confocal fluorescence microscopy in alumina-based ceramics: Where does the signal come from?

Confocal Cr³⁺ fluorescence microscopy is an ideal technique for investigating residual stresses in alumina-based ceramics. Due to their transparency, however, it is important to understand where the collected signal comes from by characterising the probe response function (PRF). Here, a PRF is proposed that captures all the relevant physical effects, including a newly identified consequence of scattering by pores and grain boundaries. The new PRF describes the response of a range of alumina-based ceramics to depth scanning in a high resolution confocal fluorescence microscope in a manner that balances physical significance with the accuracy of empirical fitting. The results showed that measurements could be made deep within single crystals of sapphire and ruby, although refraction degraded the depth resolution from about 3 μm at the surface to 25 μm at a depth of 500 μm. Scattering and absorption limited the depth to which polycrystalline alumina could be probed to ～15 μm. This was further reduced to ～4 μm for an alumina–10 vol.% SiC nanocomposite. However, the absorption increased the accuracy of near surface measurements in these materials by preventing contamination from subsurface fluorescence.     

An FE-analysis of anisotropic creep damage and deformation in the single crystal SRR99 under multiaxial loads

Multidimensional stress–strain and damage analyses of engineering structural components with the help of numerical simulations are of great interest. These can only be done by using adequate material models and suitable numerical methods. Bertram and Olschewski (Computational modelling of anisotropic materials under creep conditions, Math. Modelling Sci. Comp. 5 (1995) 100–109; Anisotropic creep modeling of the single crystal superalloy SRR99, J. Comp. Mater. Sci. 5 (1996) 12–16), proposed a three-dimensional creep model for single crystals. An anisotropic creep damage model for single crystals was also suggested by Qi and Bertram (W. Qi, A. Bertram, Anisotropic creep damage modeling of single crystal superalloys, Tech. Mech. 17 (1997) 313–322; W. Qi, Modellierung der Kriechschadigung einkristalliner Superlegierungen in Hochtemperaturbereich, Ph.D. dissertation, Technical University Berlin, VDI Verlag, Düsseldorf, 1998; W. Qi, A. Bertram, Damage modeling of the single crystal superalloy SRR99 under monotonous creep, Comput. Mater. Sci. 13 (1998) 132–141). The coupled model has been used to predict the creep deformation and the lifetime of the single crystal SRR99 under uniaxial creep loads at 760°C. The purpose of this work is the application of the coupled model to the simulation of multiaxial creep behavior and damage development, and its dependence upon non-proportional loading paths of SRR99 at 760°C.     

Closed and approximate analytical solutions for rectangular Mindlin plates

Summary Basing on the Nádai-Lévy and the Vlasov-Kantorovich methods closed and approximate analytical solutions of Mindlin's plate equations in the case of rectangular plates are discussed. For elastic, homogeneous and isotropic plates three unknowns of the governing two-dimensional boundary value problem are formulated as series of products of functions depending on a single coordinate. Specifying the functions for one of the in-plane coordinate directions the governing partial differential equations for a special type of boundary conditions and the principle of virtual displacements for the general case yield a set of ordinary differential equations. The analytical solution of these equations provides expressions for functions depending on the other in-plane coordinate. For plates with simply supported edges for one of the coordinate directions and for arbitrary homogeneous boundary conditions for the other one the Nádai-Lévy method provides a closed or exact solution in the sense that the infinite series for displacements and stress resultants can be truncated to obtain any desired accuracy. In the general case of nonsimply supported edges the iterative Vlasov-Kantorovich method yields an approximate analytical solution. Both methods are nonsensitive to a reduction of the thickness with respect to accuracy and represent the boundary layer solutions in terms of exponential functions. Applications to rectangular plates with various types of boundary conditions are presented.     

On the interdependence between shuffle and crossing-over operations

 We investigate the link between the crossing-over operation on languages as a formal model of the linkage and recombination of genes in chromosomes, and the shuffle operation on languages. The obtained results indicate a strong connection between the two operations. Received: August 31, 1995/January 24, 1996     

Betriebliche Umweltinformationssysteme (BUIS) – eine Literaturanalyse

Eingegangen am 23. 08. 1996, in überarbeiteter Form am 26.03.1997     

Bulk and Surface Properties of a VPO Catalyst Used in an Electrochemical Membrane Reactor: Conductivity-, XRD-, TPO- and XPS-study

In the first part of this work, the electrical conductivity of vanadium phosphorous oxide (VPO) catalyst was investigated by means of the 2-probe EIS method. The VPO showed an extremely low conductivity at low oxygen partial pressure, which is the prevailing condition in the anodic compartment in an electrochemical membrane reactor (EMR). In the second part of this study, fresh as well as VPO catalyst already used in an EMR were characterised with XRD, XPS and temperature programmed oxidation (TPO). The XRD measurements revealed an unchanged bulk phase structure after operation in the EMR. Significant differences in the average oxidation states of vanadium in the catalyst layer in the EMR were determined via XPS, where the catalyst surface facing the electrolyte membrane was more oxidised than the surface facing the anodic gas compartment. The lowered uptake and release of oxygen was observed in TPO experiments for the catalyst used in the EMR.     

Single component and competitive adsorption of propane, carbon dioxide and butane on Vycor glass

Equilibrium of gas phase adsorption on Vycor glass has been investigated. Adsorption isotherms for propane, carbon dioxide and butane as pure gases, binary mixtures and ternary mixtures were determined experimentally as a function of temperature using a volumetric method. The single-component isotherms were described with the Langmuir and Freundlich equations. Additionally, a second order isotherm based on statistical thermodynamics and an isotherm equation based on vacancy solution theory taking into account real phase behavior were used for fitting single-component equilibrium data. In order to describe the measured partial isotherms for binary mixtures, at first simple extensions of the single-component isotherm models were used, i.e., the conventional competitive Langmuir model and a multi-Freundlich equation based on the ideal adsorbed solution theory (IAS). Since these two simple isotherm models failed to represent the unusual competitive behavior observed, three model extensions using additional mixture parameters were applied, i.e., two modified multi-Langmuir equations based on: (a) statistical thermodynamics and (b) vacancy solution theory and a modified multi-Freundlich IAS model correcting spreading pressure uncertainties. These three model equations were found to be capable to describe the observed behavior better. Finally, the measured partial adsorption equilibrium data of the ternary system were correlated based on the extended equations using the determined additional binary parameters. The results obtained reveal the difficulty to predict accurately multi-component adsorption equilibria.