Papers by Keyword: Cellular Ceramics

Abstract: Cellular ceramics were prepared by emulsification of aqueous alumina suspensions with liquid paraffin, using collagen as a shape stabilizer and a dispersant to adjust viscosity. Rheological measurements of emulsions and microstructural analysis of the sintered samples were performed. The results show that the concentration of collagen, dispersant content and stirring rate affect directly the emulsion viscosity which causes changes in porosity and average cell size.

Abstract: Magnesium aluminate (MgAl₂O₄) was synthesized via combustion reaction, using as fuel glycerine from biodiesel, for the production of cellular ceramics. A rheologically stable suspension was prepared in a ball mill and polyurethane sponges were impregnated. In order to define a firing cycle which would result in ceramic foams with high density ceramic walls (struts), synthesized and calcined powders were uniaxially pressed at 5.4 MPa and, after drying (110°C/120 min), fired at different temperatures (1450-1650°C) for 120 min. Thus, ceramic foams (fired at 1600°C/120 min) containing MgAl₂O₄ as the only formed crystalline phase with a porosity of 88 ± 0.5% and compression strength of 1.0 ± 0.3 MPa were obtained.

Abstract: Cellular ceramics are attracting material solutions for high temperature applications because of their outstanding properties. SiC cellular ceramics in particular withstand harsh environments at high temperatures for long operating times and are particularly resistant to thermal shock. Ceramic foams though, being random fragile structures, comprise properties which are rather scattered and difficult to engineer. This presentation shows how finite element analysis is used to study the effect of morphological features on ceramic foams in respect of their mechanical properties. Mean morphological parameters, obtained by X-ray computed tomography (XCT) on a commercially available SiSiC foam produced by the replica method, were used to generate a set of lattices in which one parameter was varied at a time. Starting from this approach, further work was then dedicated to optimize their properties. Polymeric lattices and foams, in which some characteristics were digitally modified learning from the optimization work were, produced by 3D printing and ceramized via the replica method. Both foams and lattices were then mechanically tested. Results show that some features such as strut shape and cell stretching affect the mechanical behavior of ceramic foams.

Abstract: Considered in this paper is a semi-infinite cellular ceramic solid containing an edge crack. The solid is subjected to a sudden cooling on its surface. The temperature field and associated thermal stress field for the uncracked solid are calculated. The stress for uncracked medium is used as the crack surface stress with opposite sign to formulate the mixed boundary value problem. The stress intensity factors as the function of crack length, time and relative density are calculated. It is found that the presence of porosity in the ceramic is generally beneficial to increasing the thermal shock strength of the ceramics if the failure is dominated by a pre-existing crack. The paper may be helpful for the design and manufacturing of advanced thermal shock resistive cellular ceramics.

Abstract: Cellular ceramics have been produced by several methods, giving a wide range of macrostructures and properties. This work describes a novel route to produce cellular ceramics based on the gel casting of emulsions consisting in an aqueous ceramic suspension containing water-soluble organic monomers and an emulsified insoluble liquid phase. The effects of solids loading and kerosene fraction on the rheological properties of emulsions were characterized. Samples with different kerosene additions (20, 30, 40 and 50 %vol.) were produced and their green and sintered densities were measured. The results have shown a good correlation between the volumetric fraction of kerosene into the emulsion and the porosity of samples. Diametrical compression was used to evaluate the strength of sintered samples, which varied with the porosity between 18 and 37 MPa. The cellular structure was analyzed by SEM and revealed isolated pores for samples with low porosity, which changed to an interconnected network of pores as the porosity increased.