Editorial: Die Dokumentarische Methode in der kindheitspädagogischen Forschung

Das Editorial stellt die Kontur und die Beiträge des Themenhefts zu den dokumentarischen Methoden des Fallarchiv Kindheitspädagogische Forschung vor

CHARACTERIZATION OF THE MICROSTRUCTURE OF YAG CERAMICS VIA STEREOLOGY-BASED IMAGE ANALYSIS

The microstructure of transparent YAG ceramics is investigated by stereology-based microscopic image analysis using SEM and FE-SEM micrographs. Interface densities, mean curvature integral densities and the related grain size measures (mean chord length and Jeffries size) have been determined with relative errors of 9-12 % for interface densities and mean curvature integral densities and 6-9 % for the corresponding grain size measures. A comparison of the two grain size measures confi rmed an excellent linear correlation between the Jeffries size and the mean chord length, with a mean-chordlength-to-Jeffries-size ratio of 0.928 +/- 0.086. The overall range of average grain sizes is approx. 11-34 μm. It has been found that the sintering time has a signifi cant influence on the grain size, especially for YAG ceramics without Yb doping. When the sintering time is increased by a factor 8 (from 2 h to 16 h) the grain size increases by more than 200 % in undoped YAG ceramics, whereas the grain growth is much weaker in Yb-doped YAG ceramics (grain growth only 50-70 % for YAG ceramics with 5-10 at.% Yb). Thus it can be concluded that the Yb dopant acts as a grain growth inhibitor in YAG ceramics, at least for suffi ciently long sintering times (8 h and more). The infl uence of the sintering additive (tetraethyl orthosilicate TEOS) content on the grain size is negligible in the concentration range tested (0.3-0.5 wt.%)

EFFECTIVE ELASTIC CONSTANTS OF PLAGIOCLASE FELDSPAR AGGREGATES IN DEPENDENCE OF THE ANORTHITE CONTENT - A CONCISE REVIEW

Recent work on the elastic constants of plagioclase feldspars is reviewed. Based on the 21 elastic constants (stiffnesses) reported in the literature for triclinic plagioclase monocrystals of different composition, the effective elastic constants of dense, single-phase, polycrystalline plagioclase aggregates with isotropic microstructure are calculated via standard VoigtReuss-Hill averaging. Master curves show the good agreement of the constants obtained via the two approaches (experiment and simulation) and recall the fact that values reported in the older literature have been underestimated. Fit relations are given for the calculation of Young modulus, shear modulus, bulk modulus, Poisson ratio and density in dependence of the composition. These fit relations may serve as handy tools for obtaining relevant input information for the calculation of effective elastic constants of multiphase ceramics produced with Ca and Na containing raw materials

1. Editorial

Das Editorial stellt die Kontur und die Beiträge des Themenhefts zu den dokumentarischen Methoden des Fallarchiv Kindheitspädagogische Forschung vor. DOI 10.18442/06

A GENERALIZED CLASS OF TRANSFORMATION MATRICES FOR THE RECONSTRUCTION OF SPHERE SIZE DISTRIBUTIONS FROM SECTION CIRCLE SIZE DISTRIBUTIONS

A generalized formulation of transformation matrices is given for the reconstruction of sphere diameter distributions from their section circle diameter distributions. This generalized formulation is based on a weight shift parameter that can be adjusted from 0 to 1. It includes the well-known Saltykov and Cruz-Orive transformations as special cases (for parameter values of 0 and 0.5, respectively). The physical meaning of this generalization is explained (showing, among others, that the Woodhead transformation should be bounded by the Saltykov transformation on the one side and by our transformation from the other) and its numerical performance is investigated. In particular, it is shown that our generalized transformation is numerically highly unstable, i.e. introduces numerical artefacts (oscillations or even unphysical negative sphere frequencies) into the reconstruction, and can lead to completely wrong results when a critical value of the parameter (usually in the range 0.7-0.9, depending on the type of distribution) is exceeded. It is shown that this numerical instability is an intrinsic feature of these transformations that depends not only on the weight shift parameter value and is affected both by the type and the position of the distribution. It occurs in a natural way also for the Cruz-Orive and other transformations with finite weight shift parameter values and is not just caused by inadequate input data (e.g. as a consequence of an insufficient number of objects counted), as commonly assumed. Finally it is shown that an even more general class of transformation matrices can be defined that includes, in addition to the aformentioned transformations, also the Wicksell transformation

Alumina ceramics prepared with new pore-forming agents

Porous ceramics have a wide range of applications at all length scales, ranging from fi ltration membranes and catalyst supports to biomaterials (scaffolds for bone ingrowths) and thermally or acoustically insulating bulk materials or coating layers. Organic pore-forming agents (PFAs) of biological origin can be used to control porosity, pore size and pore shape. This work concerns the characterization and testing of several less common pore-forming agents (lycopodium, coffee, fl our and semolina, poppy seed), which are of potential interest from the viewpoint of size, shape or availability. The performance of these new PFAs is compared to that of starch, which has become a rather popular PFA for ceramics during the last decade. The PFAs investigated in this work are in the size range from 5 μm (rice starch) to approximately 1 mm (poppy seed), all with more or less isometric shape. The burnout behavior of PFAs is studied by thermal analysis, i.e. thermogravimetry and differential thermal analysis. For the preparation of porous alumina ceramics from alumina suspensions containing PFAs traditional slip casting (into plaster molds) and starch consolidation casting (using metal molds) are used in this work. The resulting microstructures are investigated using optical microscopy, combined with image analysis, as well as other methods (Archimedes method of double-weighing in water, mercury intrusion porosimetry)

Editorial: Die Dokumentarische Methode in der kindheitspädagogischen Forschung

Das Editorial stellt die Kontur und die Beiträge des Themenhefts zu den dokumentarischen Methoden des Fallarchiv Kindheitspädagogische Forschung vor