Model for the crystallization and sintering of unseeded and seeded boehmite gels

Porous Alumina Ceramics Prepared by the Hydrolysis-Assisted Solidification Method

“…For heterogeneous nucleation, lower temperatures are needed in comparison with the homogeneous nucleation of the yto a-alumina transformation. 16,17 As shown by Burtin et al, 18 this process starts at the neck regions, which means that in the case of HAS-prepared alumina ceramic specimens the transformation starts in the contact region between the primary alumina particles. The combined effect of retarded sintering, due to the nanostructured layer of transitional aluminas on the surface of the particles, and the change of morphology of this layer during the yto a-alumina transformation, must have an effect on the microstructure 19 and the mechanical properties of the partially sintered alumina ceramics formed by the HAS process in comparison with conventionally prepared, partially sintered alumina ceramics under the same sintering conditions.…”

“…In the presence of α‐alumina particles the phase transformation of θ‐ to α‐alumina takes place on the surface of these particles due to the heterogeneous nucleation of α‐alumina. For heterogeneous nucleation, lower temperatures are needed in comparison with the homogeneous nucleation of the θ‐ to α‐alumina transformation 16,17 . As shown by Burtin et al , 18 this process starts at the neck regions, which means that in the case of HAS‐prepared alumina ceramic specimens the transformation starts in the contact region between the primary alumina particles.…”

The fabrication of porous corundum spheres with core-shell structure for corundum-spinel castables

“…During the reconstructive transformation from θ-Al 2 O 3 to α-Al 2 O 3 , there is a specific volume reduction (from 28.6 to 25.6 cm 3 /mol Al 2 O 3 ) due to the increase of density (from 3.6 to 3.986 g/cm 3 Al 2 O 3 ) [13,22]. The low intrinsic nucleation density results in large space among single crystal α-Al 2 O 3 grains surrounded by continuous pore channels.…”

Preparation Method of Submicrometer‐Sized α‐Alumina by Surface Modification of γ‐Alumina with Alumina Sol

“…[1][2][3] Nanosized ␥-alumina powder can be prepared but nanosized ␣-alumina powder can hardly be prepared in an agglomerate-free state, because the ␣-alumina phase transformation occurs at a temperature of Ͼ1200°C, where sintering also occurs. 3,4 Thus, the smallest ␣-alumina powder that is commercially available now has a median particle size (d 50 ) of ϳ0.2 m. Research has continued in regard to control of the morphology and particle-size distribution of ␣-alumina powder by means of reducing the phase-transformation temperature for the 3 ␣ phase transformation, via the introduction of seeds or the use of controlled precipitation from an aluminum salt or alkoxide, hydrothermal processing, or mechanochemical processing. [3][4][5][6][7][8][9][10][11][12][13][14][15] One approach involves the introduction of nucleation sites, i.e., seeds.…”