It has been found that fully enclosed microcavities can be engineered within Al2O3 ceramics using 125 μm diameter Ti wires as templates. A high‐energy milling pretreatment of the alumina causes diffusion of the Ti into the surrounding alumina leaving all of the Kirkendall porosity consolidated into a central cavity. Control experiments using unmilled alumina confirm the necessity of the milling procedure and experiments with different milling media have excluded incidental doping by milling induced contamination as a primary driver of cavity formation. The internal microcavities produced here may lead to new applications in small scale instrumentation and implantable therapeutic devices.
In part I of this work, it was found that titanium (Ti) wire encapsulated within mechanically milled alumina powder and sintered at 13501C forms potentially useful microcavities due to the consolidation of Kirkendall porosity. Here a series of samples sintered at 13501C 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 13501C). It is surrounded by a diffusion zone B300 lm 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 2 O 3 /TiO 2 1Al 2 O 3 /Al 2 TiO 5 . Microstructure evolution may be summarized as outward growth of the cavity accompanied by inward growth of the Al 2 TiO 5 resulting in a B190-lm-diameter cavity surrounded by a 50-lm-thick layer of Al 2 TiO 5 . The formation of the cavity and surrounding microstructure is discussed although some features, such as the nucleation of Al 2 TiO 5 in the part of the diffusion zone furthest from the Ti source and the ring of Al 2 O 3 , which persists in between Ti-rich parts of the diffusion zone are still poorly understood.
The thermal stability of the relaxor ferroelectric Pb(Zn1/3Nb2/3)O3 (PZN) upon heating was studied using high‐temperature X‐ray diffraction (XRD) and scanning electron microscopy. It was found that single‐phase PZN is stable below 700°C, whereupon the first decomposition phase, the pyrochlore Pb1.83 Nb1.71Zn0.29O6.39 was observed. With the increasing temperature, the amount of pyrochlore increased until at 1100°C there is no PZN remaining. Pyrochlore formation was accompanied by the precipitation of ZnO above 800°C and after holding at 1100°C, the formation of Nb2ZnO6. Rietveld refinements based on the XRD patterns have allowed the relative phase proportions as well as the atomic site occupancies of the metal ions in PZN and the pyrochlore to be estimated. The Zn site occupancy within the perovskite PZN decreases upon heating in parallel with the pyrochlore formation. An elemental mass balance based on the XRD results shows that PZN decomposition is well under way before any Pb is lost from the sample. This indicates that Zn egress rather than Pb volatility is the determining factor in the instability of PZN, contrary to prior published work.
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