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The Wulff Shape of Alumina: IV. Ti4+-Doped Alumina (2005)

Kitayama, Mikito ; Glaeser, Andreas M.

Malden, USA : Blackwell Science Inc

Journal of the American Ceramic Society 88 (2005), S. 0

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ISSN: 1551-2916

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics

Notes: Controlled-geometry cavities, initially ≈20 μm × 20 μm × 0.5 μm, were introduced into 〈inlineGraphic alt="inline image" href="urn:x-wiley:00027820:JACE00604:JACE_00604_mu9" location="equation/JACE_00604_mu9.gif"/〉 -plane titanium-doped (≈210 ppma;≈500 wt. ppm) sapphire substrates using photolithographic methods, and subsequently internalized by diffusion bonding. The samples were annealed in air for prolonged periods at 1600° and 1800°C to convert the titanium to the 4+ state and to allow the pore shapes to adjust. Pores with an equivalent spherical radius of ≈3.6 μm reached a quasi-equilibrium shape within 160 h at 1600°C and within 48 h at 1800°C. The Wulff shape was determined using optical microscopy, scanning electron microscopy, and atomic force microscopy. The Wulff shape of Ti4+-doped alumina includes well-defined c(0001), 〈inlineGraphic alt="inline image" href="urn:x-wiley:00027820:JACE00604:JACE_00604_mu10" location="equation/JACE_00604_mu10.gif"/〉 , and 〈inlineGraphic alt="inline image" href="urn:x-wiley:00027820:JACE00604:JACE_00604_mu11" location="equation/JACE_00604_mu11.gif"/〉 facets and smoothly curved sections. The 〈inlineGraphic alt="inline image" href="urn:x-wiley:00027820:JACE00604:JACE_00604_mu12" location="equation/JACE_00604_mu12.gif"/〉 and a 〈inlineGraphic alt="inline image" href="urn:x-wiley:00027820:JACE00604:JACE_00604_mu13" location="equation/JACE_00604_mu13.gif"/〉 facets, which are components of the Wulff shape of undoped sapphire, are not discernable. In contrast to undoped alumina, for which the r-plane has the lowest energy, the c-plane is the lowest energy plane in Ti4+-doped alumina. The surface energy sequence of the stable c, r, and p surfaces differs from that in undoped alumina. The Wulff shape varies with temperature. Samples equilibrated at 1800°C were re-annealed at 1600°C. Pore shape changes were reversible, indicating that the observed pore shapes were close to the equilibrium (Wulff) shape.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1551-2916.2005.00604.x

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The Kinetics of Pore Shape Evolution in Alumina (1998)

Kitayama, Mikito ; Glaeser, Andreas M.

Springer

Journal of materials synthesis and processing 6 (1998), S. 161-167

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ISSN: 1573-4870

Keywords: Alumina ; surface ; kinetics ; diffusion ; SALK

Source: Springer Online Journal Archives 1860-2000

Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics

Notes: Abstract The kinetics of shape evolution of a completely faceted crystal/internal void by surface diffusion was modeled. Arrays of micron-sized cavities were generated in sapphire substrates with known surface orientations using microlithography and ion beam etching and converted to internal intragranular pores of nonequilibrium shape by diffusion bonding of the etched substrate to an identical-orientation unetched sapphire substrate. Pore shape evolution rates during high-temperature anneals were monitored and found to be highly sensitive to the orientation of the substrate surface. The observed evolution rates were compared with the predictions of the kinetic model using diffusivity values for alumina that span the range from the highest to the lowest diffusion constants reported in the literature. The comparison suggests that surface-attachment-limited kinetics (SALK) play a major role in surface mass transport on stable low-index planes of alumina.