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A novel methodology was developed that uniquely employs alternating current (AC) electric field to fabricate ice-templated ceramics. In the methodology, AC field was applied to an aqueous ceramic suspension, and then ice-templating was performed. Field application to an aqueous ceramic suspension resulted in a net motion of ceramic particles due to DC voltage generation. Suspension concentration increased near one of the electrodes, which was controllable through AC frequency and field duration. Density of the very bottommost regions in sintered materials increased with field duration, attributed to increased local suspension concentration. In sintered materials, the thickness of non-templated region increased with field duration, whereas templated microstructure turned increasingly dendritic with field duration, suggesting a strong influence of AC field on the growth characteristics of ice crystals. AC-field assisted materials exhibited remarkably enhanced compressive strength without any change in porosity. This work contributes to advancing the ice-templating technology using externally energized fields.

We investigated the effects of platelets and strain rate on the compressive response of ice-templated porous alumina, processed from ultrafine, equiaxed particles and from a mixture of ultrafine, equiaxed and large platelet particles. Results revealed a remarkable enhancement of the quasistatic and dynamic compressive mechanical properties of the templated ceramics in the presence of platelets. Specific compressive mechanical properties of the ice-templated alumina containing platelets are superior in comparison to those of the various metallic, syntactic and natural cellular solids. In the presence of platelets, compressive strength exhibited contrasting strain rate sensitivity between the elastic and inelastic stages of deformation.

We attempted to develop a phenomenological understanding of the evolution of the fracture events within fine-grained lamella walls of partially deformed ice-templated sintered Al 2 O 3 scaffolds and revealed the effects of the platelets on the crack propagation characteristics. Detailed microstructural analyses suggest that intergranular cracks within the walls evolved at two orientations, parallel and perpendicular to the loading direction. We proposed probable mechanisms including one based on the so-called wing-crack model to rationalize the observed crack orientations. We presented crack propagation behavior in lamella walls in presence of the platelets and discussed the effects in terms of the crack deflection mechanism.

This paper describes the effects of the grain-level anisotropy on the microstructure and uniaxial compressive response of the ice-templated alumina scaffolds. Highly porous (~ 80 vol.%) scaffolds were fabricated from alumina powders of equiaxed morphology as well as from powder mixtures containing equiaxed and small amount of platelet particles. Presence of the platelets (diameter 8 μm and thickness 400 nm) led to the formation of the interlamellae bridges, and significantly enhanced the stiffness and compressive strength of the scaffolds. Measured improvement of the mechanical response is rationalized based on the stiffening and strengthening effects exhibited by the intralamella and interlamella platelets.

The extent of dislocation-based plasticity in ceramics is limited and often accompanied by microcracking. This limitation has precluded the investigation of post-yield plastic response in ceramics. Using low-load nanoindentations and by tracking the hardness of selected grains before and after plastic deformation, the influence of microcracking and variability in microhardness due to grain orientation were minimized and the post-yield response of polycrystalline ZrB 2 ceramic was determined. The observed increase in nanoindentation hardness after plastically deforming isolated ZrB 2 grains indicates conclusively the existence of post-yield strain hardening.

Macroscopic manifestation of microscale plasticity at room temperature in the form of slip-lines has been elusive in brittle solids. Recently, such a phenomenon has been reported in ZrB2 ceramics and ZrB2–SiC composites, where slip-lines were detected in room-temperature deformation. Here we show that the slip-line spacing increases with grain size in these ceramics and their composites. Such behavior has been rationalized on the basis of geometrically necessary dislocations and the role of grain boundaries and interfaces.

Ceramics such as ZrB 2 and HfB 2 are potential candidates for ultra-high-temperature applications. Their electrical conductivity values are comparable to those of metals. Such unusual electrical properties arise from the presence of metallic bonds in their crystal structure. We argue that the metallicity in chemical bonding is also reflected in their room-temperature mechanical deformation, which was investigated through indentation-induced slip bands and the resulting dislocation activity. These observations were rationalized on the basis of metallic character of their dislocation core structure.