Effect of ceramic preform freeze-casting temperature and melt infiltration technique on the mechanical properties of a lamellar metal/ceramic composite

Elastic properties, compressive stress–strain behaviour and progressive damage evolution of poly-domain metal/ceramic composite samples fabricated by infiltration of Al12Si melt in freeze-cast alumina preforms are studied. Two different preform freezing temperatures were employed to vary the lamellae size while infiltration was carried out using two different techniques – squeeze-casting and die-casting. Due to the faster cooling kinetics at the lower freezing temperature, the lamellae size in the composites based on these preforms are finer and this results into higher compressive strength and stiffness of this composite along the freezing direction. Among the two techniques employed for melt infiltration, the very fast rate of pressure application in die-casting distorts the lamellar structure of the ceramic along the freezing direction. As a result, in die-cast composite samples, the strength and stiffness along the freezing direction are reduced significantly in comparison to the samples infiltrated by squeeze-casting. In-situ scanning electron microscopy under external compression was used to study the progressive damage mechanism in one poly-domain composite sample infiltrated by squeeze-casting. Transverse cracking of the high-angle ceramic lamellae is identified as the predominant damage mechanism.

Pore Architectures and Mechanical Properties of Porous α-SiAlON Ceramics Fabricated via Unidirectional Freeze Casting Based on Camphene-Templating

Porous α-SiAlON ceramics were fabricated using the camphene-based unidirectional freeze casting method, in which a gradient porous structure was formed as a result of the decreased solidification velocity in the freezing direction. Microstructure, porosity and pore size distribution of different parts of as-prepared samples were examined and compared, and correlated with their mechanical properties. For a given solid loading, the overall pore size and porosity of the top part were greater than those of the bottom part. Interestingly, despite its higher porosity, the flexural strength and fracture toughness of the top part were both higher than those of the bottom part, suggesting that apart from porosity, pore morphology and size affected mechanical properties of as-prepared porous α-SiAlON ceramics.

High piezoelectric sensitivity and hydrostatic figures of merit in unidirectional porous ferroelectric ceramics fabricated by freeze casting

High performance lead zirconate titanate (PZT) ceramics with aligned porosity for sensing applications were fabricated by an ice-templating method. To demonstrate the enhanced properties of these materials and their potential for sensor and hydrophone applications, the piezoelectric voltage constants (g33 and g31), hydrostatic parameters (dh, gh, -d33/d31, dh·gh and dh·gh/tanδ) and AC conductivity as a function of the porosity in directions both parallel and perpendicular to the freezing temperature gradient were studied. As the porosity level was increased, PZT poled parallel to the freezing direction exhibited the highest dh, -d33/d31 and figures of merit dh·gh, dh·gh/tanδ compared to the dense and PZT poled perpendicular to the freezing direction. The gh, g33 and g31 coefficients were highest for the PZT poled perpendicular to the freezing direction; the gh was 150% to 850% times higher than dense PZT, and was attributed to the high piezoelectric activity and reduced permittivity in this orientation. This work demonstrates that piezoelectric ceramics produced with aligned pores by freeze casting are a promising candidate for a range of sensor applications and the polarisation orientation relative to the freezing direction can be used to tailor the microstructure and optimise sensitivity for sensor and hydrostatic transducer applications.

Enhanced pyroelectric and piezoelectric properties of PZT with aligned porosity for energy harvesting applications

This paper demonstrates the significant benefits of exploiting highly aligned porosity in piezoelectric and pyroelectric materials for improved energy harvesting performance. Porous lead zirconate (PZT) ceramics with aligned pore channels and varying fractions of porosity were manufactured in a water-based suspension using freeze casting. The aligned porous PZT ceramics were characterized in detail for both piezoelectric and pyroelectric properties and their energy harvesting performance figures of merit were assessed parallel and perpendicular to the freezing direction. As a result of the introduction of porosity into the ceramic microstrucutre, high piezoelectric and pyroelectric harvesting figures of merits were achieved for porous freeze-cast PZT compared to dense PZT due to the reduced permittivity and volume specific heat capacity. Experimental results were compared to parallel and series analytical models with good agreement and the PZT with porosity aligned parallel to the freezing direction exhibited the highest piezoelectric and pyroelectric harvesting response; this was a result of the enhanced interconnectivity of the ferroelectric material along the poling direction and reduced fraction of unpoled material that leads to a higher polarization. A complete thermal energy harvesting system, composed of an parallel-aligned PZT harvester element and an AC/DC converter successfully demonstrated by charging a storage capacitor. The maximum energy density generated by the 60 vol.% porous parallel-connected PZT when subjected to thermal oscillations was 1653 μJ/cm3 respectively, which was 374% higher than that of the dense PZT with an energy density of 446 μJ/cm3. The results are of benefit for the design and manufacture of high performance porous pyroelectric and piezoelectric materials in devices for energy harvesting and sensor applications.

Fabrics of Icing-Mound and Pingo Ice in Permafrost

Crystallization histories of some ice layers in permafrost are inferred from crystal size, shape, dimensional and lattice orientation, and inclusion patterns. In an icing mound, formed by injection of water beneath frozen active-layer soil, early growth was rapid, indicated by copious small crystals and bubbles, followed by slower growth giving rise to crystals and bubbles elongate parallel to the freezing direction, c-axes were normal to crystal long axes. In a small pingo, bulk water existed temporarily at the freezing interface and freezing was unidirectional. In a larger pingo, variations in freezing rate were inferred. Later flow of ice modifies growth fabrics.

Fabrics of Icing-Mound and Pingo Ice in Permafrost

Crystallization histories of some ice layers in permafrost are inferred from crystal size, shape, dimensional and lattice orientation, and inclusion patterns. In an icing mound, formed by injection of water beneath frozen active-layer soil, early growth was rapid, indicated by copious small crystals and bubbles, followed by slower growth giving rise to crystals and bubbles elongate parallel to the freezing direction, c-axes were normal to crystal long axes. In a small pingo, bulk water existed temporarily at the freezing interface and freezing was unidirectional. In a larger pingo, variations in freezing rate were inferred. Later flow of ice modifies growth fabrics.