Papers by Author: Eva Gregorová

Abstract: In this work we demonstrate the application of stereology-based image analysis for the characterization of highly porous cellular ceramics (alumina foams) prepared by biological foaming with yeast and subsequent drying (80-105 °C) and firing (1570 °C). It is shown that the ceramics prepared usually have total porosities in the range 78-84 % and that the porosities made up by large pores (volume fraction of foam bubbles) are usually in the range 58-75 %. Further it is shown that the mean chord length and the Jeffries size, i.e. pore size measures related to the interface density and the mean curvature integral density, respectively, are relatively close to each other (usually 0.8-1.4 and 0.8-1.2 mm) with a ratio close to unity (0.9-1.3) and that the mean surface-to-surface distance of pores gives a realistic picture of the average pore wall thickness (usually 0.46-0.69 mm). Using a special processing variant (excess ethanol addition) it is possible to obtain microstructures with lower porosity (total porosity 68-70 %, foam bubble volume fractions 50-56 %) and smaller pore size (approx. 0.5 mm). Absolute errors are calculated using normalized deviations corresponding to 95 % reliability in the Student distribution and the standard errors for the quantities in question (both observed and estimated). Relative errors are found to be below 12 % when the number of measurements is of order 400-1000.

Abstract: Micromechanical calculations and elasticity standard relations are used to predict the elastic properties of porous alumina, zirconia and kaolin-based ceramics, as well as the high-temperature Young moduli of alumina-zirconia and alumina-mullite composites. The predictions are compared with experimental results obtained via impulse excitation. It is found that the Young moduli of highly porous (cellular) alumina ceramics can be predicted via the Gibson-Ashby power-law relation, whereas for partially sintered kaolin-based ceramics our exponential relation, albeit better than the Gibson-Ashby relation, does not give a satisfactory prediction. However, once the Young moduli are known, the shear and bulk moduli can be reliably predicted in both cases, based on rough information on the Poisson ratio. The temperature dependence of the Youngs moduli of two-phase composites can be quite precisely predicted as soon as the master curves of the constituent phases and the type of porosity (convex, concave, or saddle-point) are known.

Abstract: Youngs moduli of talc-based ceramics from the system MgO-Al₂O₃-SiO₂ are measured for temperatures up to 1000 °C via impulse excitation. It is shown that, after pressing at 50 MPa and firing at 1280 °C, MgO-rich compositions exhibit higher porosity and lower Youngs moduli (approximately 2030 % lower than predicted via micromechanical relations). The Young moduli of materials with less MgO decrease with temperature, but those of MgO-rich ceramics increase with temperature and exhibit a large hysteresis between heating and cooling. Lower absolute values are mainly due to increased porosity, but the reason for the modulus increase with temperature and the hysteresis is the higher enstatite content in the MgO-rich compositions. For a special composition the Youngs moduli are more or less temperature-independent and without significant hysteresis effects, probably due to the low content of enstatite and the high content of sapphirine.

Abstract: This work deals with the preparation and characterization of macroporous alumina ceramics and permeable laminates with a stepwise (layerwise) porosity gradient in the range of approx. 20–50 %. Layered structures are made by sequential casting and draining of ceramic suspensions containing corn starch (median size approx. 14 micrometers), using both traditional slip casting (TSC) and starch consolidation casting (SCC). In both techniques starch acts as a poreformer, which is eliminated during firing. The influence of the alumina concentration and starch content in the suspension on the porosity, pore size and pore connectivity in the individual layers is studied. It is shown that differential shrinkage of the layers in the case of SCC, caused by the different starch content, may be avoided by controlling the alumina content. The distribution of pore throat diameters (cell window sizes) is determined by mercury porosimetry, whereas the distribution of pore cavity diameters (cell sizes) is measured by microscopic image analysis.

Abstract: An overview is given of the rheological behavior of biopolymers in aqueous suspensions and of their role in new ceramic shaping processes (starch consolidation casting and carrageenan gel casting). In particular, we give a state-of-the-art account of the viscometric behavior, measured via rotational viscometry (apparent viscosity, including its shear-rate and concentration dependence), and the viscoelastic properties characterized via oscillatory shear rheometry (storage modulus, loss modulus and phase angle, including their temperature dependence), of starch-water systems, starchcontaining alumina suspensions, carrageenan-water systems and carrageenan-containing zirconia suspensions.

Abstract: The cross-property relations between the elastic moduli and the thermal conductivity of porous ceramics are reviewed from the viewpoint of micromechanics (composite theory). Consequences of the rigorous Milton-Torquato and Gibiansky-Torquato relations (in the form of bounds, i.e. inequalities derived between bulk or shear moduli on the one hand and thermal conductivity on the other) are compared to various approximate relations (equalities) recently proposed between the tensile modulus (Young’s modulus) and thermal conductivity, among them the two new cross-property relations proposed by the authors. The relations are critically discussed and applied to the case of porous alumina, zirconia and alumina-zirconia composite ceramics.