Adam J. Stevenson

MATERIALS SCIENCE : Toward Pore-Free Ceramics

Efforts are under way to create perfectly dense ceramics for use in applications ranging from lasers to health care.

Mechanical properties and failure behavior of unidirectional porous ceramics

We show that the honeycomb out-of-plane model derived by Gibson and Ashby can be applied to describe the compressive behavior of unidirectional porous materials. Ice-templating allowed us to process samples with accurate control over pore volume, size, and morphology. These samples allowed us to evaluate the effect of this microstructural variations on the compressive strength in a porosity range of 45–80%. The maximum strength of 286 MPa was achieved in the least porous ice-templated sample (P(%) = 49.9), with the smallest pore size (3 μm). We found that the out-of-plane model only holds when buckling is the dominant failure mode, as should be expected. Furthermore, we controlled total pore volume by adjusting solids loading and sintering temperature. This strategy allows us to independently control macroporosity and densification of walls, and the compressive strength of ice-templated materials is exclusively dependent on total pore volume.

Color center formation in vacuum sintered Nd3xY3−3xAl5O12 transparent ceramics

Color center formation was studied in vacuum sintered Nd3xY3−3xAl5O12 transparent ceramics. The primary color centers were F- and F+-centers as evidenced by optical absorption in the 250–400 nm wavelength range and the presence of an electron spin resonance (ESR) line at g=1.9977. Annealing in air at 1600 °C for 10 h reduced the number of color centers to below the detection limit of ESR. Color center formation is controlled by oxidation and reduction of Fe2+/3+ impurities.

Gas permeability of ice-templated, unidirectional porous ceramics

We investigate the gas flow behavior of unidirectional porous ceramics processed by ice-templating. The pore volume ranged between 54% and 72% and pore size between 2.9 m and 19.1 m. The maximum permeability ( m) was measured in samples with the highest total pore volume (72%) and pore size (19.1 m). However, we demonstrate that it is possible to achieve a similar permeability ( m) at 54% pore volume by modification of the pore shape. These results were compared with those reported and measured for isotropic porous materials processed by conventional techniques. In unidirectional porous materials tortuosity () is mainly controlled by pore size, unlike in isotropic porous structures where is linked to pore volume. Furthermore, we assessed the applicability of Ergun and capillary model in the prediction of permeability and we found that the capillary model accurately describes the gas flow behavior of unidirectional porous materials. Finally, we combined the permeability data obtained here with strength data for these materials to establish links between strength and permeability of ice-templated materials. Graphical Abstract

The effect of wall thickness distribution on mechanical reliability and strength in unidirectional porous ceramics

Macroporous ceramics exhibit an intrinsic strength variability caused by the random distribution of defects in their structure. However, the precise role of microstructural features, other than pore volume, on reliability is still unknown. Here, we analyze the applicability of the Weibull analysis to unidirectional macroporous yttria-stabilized-zirconia (YSZ) prepared by ice-templating. First, we performed crush tests on samples with controlled microstructural features with the loading direction parallel to the porosity. The compressive strength data were fitted using two different fitting techniques, ordinary least squares and Bayesian Markov Chain Monte Carlo, to evaluate whether Weibull statistics are an adequate descriptor of the strength distribution. The statistical descriptors indicated that the strength data are well described by the Weibull statistical approach, for both fitting methods used. Furthermore, we assess the effect of different microstructural features (volume, size, densification of the walls, and morphology) on Weibull modulus and strength. We found that the key microstructural parameter controlling reliability is wall thickness. In contrast, pore volume is the main parameter controlling the strength. The highest Weibull modulus () and mean strength (198.2 MPa) were obtained for the samples with the smallest and narrowest wall thickness distribution (3.1 m) and lower pore volume (54.5%). Graphical Abstract

Linking Rheology and Printability for Dense and Strong Ceramics by Direct Ink Writing

Direct ink writing of dense and strong ceramic objects remains an important open challenge. We develop a universal dimensionless criterion for printing such objects. Boehmite, an Al2O3 precursor, was used to assess the rheological properties leading to dense structures in ceramics manufactured by direct ink writing. Boehmite suspensions undergo time dependent gelation, thus providing a rheological laboratory of flow behaviours that can be correlated with printability requirements. We measured the evolution of rheological properties over several days and quantified the deformation of simple printed shapes at different aging times. We then identified the relevant physical parameters leading to printable suspensions. We defined a dimensionless number, Ξ, based on measured rheological properties, that predicts deformation of the printed object and determines the printability criterion. An important difference with this criterion is that Ξ necessarily accounts for capillary forces and gravitational slumping. We show that boehmite inks reach a printed shape fidelity > 90% when Ξ > 1, and that Al2O3 bars printed under these conditions can be sintered to 97% density, without printing defects, and have flexural strengths (500–600 MPa) competitive with commercial aluminas. Using Ξ, researchers can rationally design inks for printing dense materials by tailoring their rheological properties such that Ξ ≈ 1.

Corrigendum: Strong, tough and stiff bioinspired ceramics from brittle constituents

This corrects the article DOI: 10.1038/nmat3915.

Confocal Micro-Fluorescence and Raman Spectroscopy across Grain Boundaries in Transparent Nd 3+ :YAG Ceramic Laser Gain Media

Confocal micro-Raman and micro-fluorescence studies have been performed on unetched Nd3+:YAG transparent ceramic laser media. Evidence of Nd segregation at grain boundaries and the possibility of generating 3D spatial mapping across the sample are demonstrated.