The compressive mechanical characterization of highly porous ceramics is problematic due to sensitivity to stress concentrations and localized fractures.In this paper a review of the methods used by the scientific communityis done and their typical results, advantages, and shortcomings are discussed. Here, a new methodology that can address some of the problems associated with the current measurement procedures is presented. The proposed method is applied to highly porous barium titanate foams. This procedure producesmore consistent data thatcan be analyzed and interpreted objectively. We show that properties such as the crushing or collapse stress as well as the compressive modulus cannot be extracted correctly by the conventional methods. By contrast, the proposed method collects data that is more accurate, consistent with what is expected of porous ceramics and that yields smaller measurement to measurement variation.
Rare-earth doped ceria materials are amongst the top choices for use in electrolytes and composite electrodes in intermediate temperature solid oxide fuel cells. Trivalent acceptor dopants such as gadolinium, which mediate the ionic conductivity in ceria by creating oxygen vacancies, have a tendency to segregate at grain boundaries and triple points. This leads to formation of ionically resistive blocking grain boundaries and necessitates high operating temperatures to overcome this barrier. In an effort to improve the grain boundary conductivity, we studied the effect of a modified sintering cycle, where 10 mol% gadolinia doped ceria was sintered under a reducing atmosphere and subsequently reoxidized. A detailed analysis of the complex impedance, conductivity, and activation energy values was performed. The analysis shows that for samples processed thus, the ionic conductivity improves when compared with conventionally processed samples sintered in air. Equivalent circuit fitting shows that this improvement in conductivity is mainly due to a drop in the grain boundary resistance. Based on comparison of activation energy values for the conventionally processed vs. reduced-reoxidized samples, this drop can be attributed to a diminished blocking effect of defect-associates at the grain boundaries
Porous barium titanate has gained significant attention in recent years for their potential use in applications such as scaffolds for bone tissue engineering, stress sensors, gas sensors, and many others. However, there is very little control over the grain size of the material during the sintering processes specially to achieve little or no growth of the starting powders. Here, using the two‐step sintering method barium titanate foams were shown to be synthesized with controlled grain size of the struts without significant differences in the pore structure of the materials. In order to evaluate the applicability of two‐step sintering for a variety of processing methods, highly porous (>80% porosity) foams synthesized through the direct polyurethane foaming method were used to create conditions furthest from bulk where two‐step sintering has shown success. Two‐step sintering parameters were identified and the processing conditions were confirmed to not alter the mechanical properties of the samples due to expected residual stresses or thermal shock resulting from the rapid heating and cooling rates employed.
Silicon carbide (SiC) formed through pyrolysis of preceramic polymers loaded with SiC particles has gained significant attention for applications such as coatings, composite matrix modifications, and most importantly additive manufacturing. This work presents combined synchrotron XRD, Raman spectroscopy, scanning electron microscopy, nano-indentation, and Vickers indentation of pyrolysis bonded SiC to shed light on the changes of composition and mechanical properties of these materials. Characterization was performed on samples that were heat treated ranging from the synthesis 850 °C up to 1500 °C. Pre-treatments of the powders prior to pellet synthesis, such as heat treatment and etching using a hydrofluoric acid (HF), were investigated. It is shown that the degradation of mechanical properties when exposed to higher temperatures is due to the burnout of amorphous carbon clusters remnant of the pyrolysis process of the preceramic polymer. Furthermore, prior HF etching and removal of the native oxide layer of the powders showed improved density and hardness values in the final pellets. The average Vickers hardness of the control samples were 4.59 GPa and later 3.74 GPa when exposed to 1500 °C, while the samples synthesized using powders that were etched with HF had an average hardness value of 9.37 GPa and later 6.86 GPa when exposed to 1500 °C.
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