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A detailed study of a novel synthesis via colloidal sol-gel route for obtaining nanoparticulate Nb 2 O 5 was performed. Parameters such as temperature and H + :Nb 5+ and Nb 5+ :H 2 O molar ratios were controlled in order to determine the best conditions of synthesis. Moreover, particle size distribution, zeta potential, structure by X-ray diffraction, and the photocatalytic activity of the particulate sols were also evaluated. The obtained results indicate that the colloidal sol-gel synthesis is a good alternative for obtaining Nb 2 O 5 either as stable nanoparticulate sol or as a nanosized powder. Nb 2 O 5 amorphous nanoparticles with an average size of 20 nm were obtained by controlling the synthesis variables. The heat-treatment process allowed the formation of Nb 2 O 5 with orthorhombic structure that transforms at higher temperatures to monoclinic phase. The highest photocatalytic activity was observed under k = 365 nm, the smallest UV energy used in the experimental tests.

An increase in hardness with reducing grain sizes is commonly observed in oxide ceramics in particular for grain sizes below 100 nm. The inverse behavior, meaning a decrease in hardness below a critically small grain size, may also exist consistently with observations in metal alloys, but the causing mechanisms in ceramics are still under debate. Here we report direct thermodynamic data on grain boundary energies as a function of grain size that suggest that the inverse relation is intimately related to a size‐induced increase in the excess energies. Microcalorimetry combined with nano and microstructural analyses reveal an increase in grain boundary excess energy in yttria‐stabilized zirconia (10YSZ) when grain sizes are below 36 nm. The onset of the energy increase coincides with the observed decrease in Vickers indentation hardness. Since grain boundary energy is an excess energy related to boundary strength/stability, the results suggest that softening is driven by the activation of grain boundary mediated processes facilitated by the relatively weakened boundaries at the ultra‐fine nanoscale which ultimately induce the formation of an energy dissipating subsurface crack network during indentation.

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Using rheological parameters of ceramic suspensions, it is possible to taylor the structure of the ceramic foams produced by replica. This method consists in the impregnation of a polymeric flexible template (polyurethane foam) with a ceramic suspension (slurry) containing the appropriate additives, followed by burning out organic compounds and additives and sintering the ceramic structure. In this work, ceramic foams were produced by the replica method from Al2O3and 3% Y2O3-ZrO2. Rheological parameters of the ceramic suspensions were investigated to improve the mechanical performance of final structures. Different types and quantities of raw materials were combined in order to select the formulations for ceramic foams. The parameters that have a significant influence on the process are the binder type and the amount of solids. Significant changes on the hysteresis area of the suspensions resulted in a lower density of macrodefects in the material. Likewise, when the shear rate viscosity is enhanced, the thickness of the struts increased proportionally. Lastly, when the hysteresis area magnitude and the ceramic thickness increased, the material with higher uniformity was internally densified, and the stress concentration of the internal defects was smoothed

This paper deals with the rheological characterization of agar and foaming surfactant‐containing suspensions for obtaining stoichiometric cordierite samples with tailored open macroporosity and their characterization through density and microstructural studies. The influence of the processing parameters solid loading (20, 30, and 45 vol%), slip temperature (65°C, 45°C, and 40°C), and agar/surfactant ratio (10.2, 8.0, and 5.6) on the obtained bodies is discussed. Open Porosity (up to 76 vol%) and average cell size were found to be strongly dependent on solids loading.

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