Figure 1.
Influence of ZRA over the porous ice-templated structures.
SEM micrographs of ice-templated zirconia without (A) and with (B-D) zirconium acetate (18 g/L of Zr), perpendicular to the solidification direction. The pores dimensions (cross-section) are very homogeneous throughout the bulk of the samples as shown by the histogram (F). Micrographs taken along the solidification direction (E). Scale bars: A, B, D: 50 µm, C: 5 µm, E: 100 µm.
Figure 2.
Processing conditions for faceting and relationships to the ZRA ionic complexes.
(A) Relative fraction of the three types of ionic complexes adopted by ZRA in water as a function of pH (adapted after [10]). (B) Required pH and zirconium concentration for faceting. F: faceting, PF: partial faceting, NF: no faceting.
Figure 3.
In situ characterization of growth kinetics, morphologies, and zirconium distribution.
(A,B) In situ X-rays radiography of the growing ice crystals, rounded and dendritic without (A) and needle-like shaped with (B) ZRA. (C, D) Cross-sections from a tomography reconstruction, perpendicular to the growth direction: the ice crystals (green) are clearly facetted (white arrows) in presence of ZRA (D) and not facetted in their absence (C). KI (red) was used to image the boundaries of the crystals. (E) Solidification interface position in presence and in absence of ZRA; no influence of ZRA is observed. (F) EPMA map of zirconium distribution in an ice-templated alumina samples (8g/L Zr), after ice sublimation and sintering. No segregation of Zr at crystal interface can be observed. (G) SEM micrographs in BSE mode showing the composites structures obtained in an ice-templated alumina samples (18 g/L Zr). Nanoscopic zirconia grains are homogeneously distributed within the alumina matrix. Scale bars: B,C: 250 µm, D, E: 500 µm, F: 5 µm, G: 2 µm.
Figure 4.
Determination of the ice crystal orientation by XRD and correspondence with the faceted porosity.
(A) The formation of the main peaks of ice in the XRD pattern is followed in situ to detect the moment when the ice crystals reach the top of the sample and avoid the formation of ice crystals from the ambient moisture. (B) XRD patterns (Z: 8 mol.% yttria-stabilized zirconia, JCPDS 30-1468) I: ice Ih (JCPDS 74-1871), obtained in absence of ZRA, an orientation pattern can already be observed. In presence of ZRA (C), a complete extinction of all peaks but the (002) is observed. Faceting remains when the imposed growth velocity is lower, but crystals show a preferential growth in one direction (D). The (0001) plane is perpendicular to the growth direction (E), while the s1-s3 faces of the crystals corresponds to the (1(-1)00) and equivalent planes obtained with the six-fold symmetry of the hexagonal structure. Scale bar: 10 µm.