Microcavity Formation in Alumina Using Ti Templates II: Mechanism and Kinetics
In part I of this work, it was found that titanium (Ti) wire encapsulated within mechanically milled alumina powder and sintered at 1350°C forms potentially useful microcavities due to the consolidation of Kirkendall porosity. Here a series of samples sintered at 1350°C in the range 0–24 h has shown the remarkable way in which these cavities form. The cavity has already started in samples quenched from the top of the heating ramp (0 min at 1350°C). It is surrounded by a diffusion zone ∼300 m in diameter, which does not change size throughout the firing process although the contents change markedly. The diffusion zone microstructure is initially complex with phase sequence TiO2/Al2O3/TiO2+Al2O3/Al2TiO5. Microstructure evolution may be summarized as outward growth of the cavity accompanied by inward growth of the Al2TiO5 resulting in a ∼190-m-diameter cavity surrounded by a 50-m-thick layer of Al2TiO5. The formation of the cavity and surrounding microstructure is discussed although some features, such as the nucleation of Al2TiO5 in the part of the diffusion zone furthest from the Ti source and the ring of Al2O3, which persists in between Ti-rich parts of the diffusion zone are still poorly understood.
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