Microcavity Formation in Alumina Using Ti Templates II: Mechanism and Kinetics

摘要

In part I of this work, it was found that titanium (Ti) wire encapsulated within mechanically milled alumina powder and sintered at 1350℃ forms potentially useful microcavities due to the consolidation of Kirkendall porosity. Here a series of samples sintered at 1350℃ 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℃). It is surrounded by a diffusion zone ～ 300 μm 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 TiO_2/ Al_2O_3/TiO_2+Al_2O_3/Al_2TiO_5. Microstructure evolution may be summarized as outward growth of the cavity accompanied by inward growth of the Al_2TiO_5 resulting in a ～ 190-μm-diameter cavity surrounded by a 50-μm-thick layer of Al_2TiO_5. The formation of the cavity and surrounding microstructure is discussed although some features, such as the nucleation of Al_2TiO_5 in the part of the diffusion zone furthest from the Ti source and the ring of Al_2O_3, which persists in between Ti-rich parts of the diffusion zone are still poorly understood.