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  • 1
    Electronic Resource
    Electronic Resource
    Liquid-Film-Assisted Formation of Alumina/Niobium Interfaces (2002)
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 85 (2002), S. 0 
    Details
    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: Alumina has been joined at 1400°C using niobium-based interlayers. Two different joining approaches were compared: solid-state diffusion bonding using a niobium foil as an interlayer, and liquid-film-assisted bonding using a multilayer copper/niobium/copper interlayer. In both cases, a 127-μm-thick niobium foil was used; ∼1.4-μm- or ∼3-μm-thick copper films flanked the niobium. Room-temperature four-point bend tests showed that the introduction of a copper film had a significant beneficial effect on the average strength and the strength distribution. Experiments using sapphire substrates indicated that during bonding the initially continuous copper film evolved into isolated copper-rich droplets/particles at the sapphire/interlayer interface, and extensive regions of direct bonding between sapphire and niobium. Film breakup appeared to initiate either at niobium grain boundary ridges or at asperities or irregularities on the niobium surface that caused localized contact with the sapphire.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.2002.tb00490.x
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  • 2
    Electronic Resource
    Electronic Resource
    The Wulff Shape of Alumina: III, Undoped Alumina (2002)
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 85 (2002), S. 0 
    Details
    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 were introduced into the m{10〈inlineGraphic alt="inline image" href="urn:x-wiley:00027820:JACE611:JACE_611_mu1" location="equation/JACE_611_mu1.gif"/〉0} plane of undoped sapphire substrates using photolithographic methods, and subsequently internalized by diffusion bonding the etched sapphire to an undoped high-purity polycrystalline alumina. Pore-boundary separation during growth of the sapphire seed into the polycrystal entrapped the pores within the single crystal. Pores with an equivalent spherical radius of ≈1 μm reached a quasi-equilibrium shape after prolonged anneals at 1600° and 1800°C. The introduction of mechanically induced surface defects accelerated pore shape equilibration. The Wulff shape of undoped alumina was determined by characterizing the shape and facet structure of these equilibrated internal pores using optical microscopy, scanning electron microscopy, and atomic force microscopy. The observed planes in the Wulff shape of undoped alumina, c(0001), r{〈inlineGraphic alt="inline image" href="urn:x-wiley:00027820:JACE611:JACE_611_mu1" location="equation/JACE_611_mu1.gif"/〉012}, s{1〈inlineGraphic alt="inline image" href="urn:x-wiley:00027820:JACE611:JACE_611_mu1" location="equation/JACE_611_mu1.gif"/〉01}, a{11〈inlineGraphic alt="inline image" href="urn:x-wiley:00027820:JACE611:JACE_611_mu2" location="equation/JACE_611_mu2.gif"/〉0}, and p{11〈inlineGraphic alt="inline image" href="urn:x-wiley:00027820:JACE611:JACE_611_mu2" location="equation/JACE_611_mu2.gif"/〉3} planes, were consistent with those reported by Choi et al.; however, a different energy sequence is inferred. The absence of the m-plane in the Wulff shape is consistent with other experimental studies, but inconsistent with those lattice simulations that predict the m-plane to be one of the lowest energy planes in pure alumina. A comparison of Wulff shapes at 1600° and 1800°C suggests that the surface energy of undoped alumina becomes more isotropic as temperature increases.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.2002.tb00140.x
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  • 3
    Electronic Resource
    Electronic Resource
    Orientation Effects on the High-Temperature Morphological Evolution of Pore Channels in Sapphire (2000)
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 83 (2000), S. 0 
    Details
    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: Arrays of semi-infinite and of controlled-aspect-ratio pore channels, both of controlled orientation, were introduced into undoped basal-plane sapphire substrates, using microfabrication techniques, ion-beam etching, and hot pressing. The breakup of these channels via Rayleigh instabilities during a series of annealings at a temperature of 1700°C was monitored. In all cases, the channels broke up with a characteristic wavelength (λ) that was much larger than that expected for a material with isotropic surface energy, which reflected stabilization effects that were due to anisotropy of the surface energy. The break-up wavelength also was very dependent on orientation: channels that were oriented along the [11¯00] and [112¯0] directions yielded the smallest and largest pore spacings (λ-values), respectively, which is in qualitative agreement with prior observations. The critical (minimum) aspect ratio for the breakup of finite-length channels into multiple pores also is dependent upon channel orientation, and the trend mirrors that observed for semi-infinite channels. The pattern of channel evolution suggested two-fold rotational symmetry within the basal plane, where, because of the nature of the experiment, apparent six-fold symmetry is expected. Several factors that may contribute to or cause an apparent or real loss of symmetry have been discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.2000.tb01550.x
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  • 4
    Electronic Resource
    Electronic Resource
    The Wulff Shape of Alumina: I, Modeling the Kinetics of Morphological Evolution (2000)
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 83 (2000), S. 0 
    Details
    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: The rate at which fully facetted nonequilibrium shaped particles and pores approach their equilibrium (Wulff) shape via surface diffusion was modeled, and calculations relevant to alumina were performed to guide experimental studies. The modeling focuses on 2-D features, and considers initial particle/pore shape, size, surface energy anisotropy, and temperature (surface diffusivity) as variables. The chemical potential differences driving the shape change are expressed in terms of facet-to-facet differences in weighted mean curvature. Two approaches to modeling the surface flux are taken. One linearizes the difference in the mean chemical potential of adjacent facets, and assumes the flux is proportional to this difference. The other approach treats the surface chemical potential as a continuous function of position, and relates the displacement rate of the surface to the divergence of the surface flux. When consistent values for the relevant materials parameters are used, the predictions of these two modeling approaches agree to within a factor of 1.5. As expected, the most important parameters affecting the evolution times are the cross-sectional area (volume in 3-D) and the temperature through its effect on the surface diffusivity. Pores of micrometer size are predicted to reach near-equilibrium shapes in reasonable times at temperatures as low as 1600°C. The detailed geometry of the initial nonequilibrium shape and the Wulff shape appear to have relatively minor effects on the times required to reach a near-equilibrium shape.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.2000.tb01591.x
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  • 5
    Electronic Resource
    Electronic Resource
    The Wulff Shape of Alumina: II, Experimental Measurements of Pore Shape Evolution Rates (2000)
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 83 (2000), S. 0 
    Details
    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: The rate at which a facetted tetragonal cavity of nonequilibrium shape approaches a cubic equilibrium (Wulff) shape via surface diffusion was modeled. The shape relaxation rate of a facetted “stretched cylinder” was also modeled. For the first geometry, only an approximate solution based on linearizing the mean potential difference between the source and sink facets was obtained. For the stretched cylinder, both an approximate and an exact solution can be obtained; the approximate solution underestimates the evolution rate by a factor of ∼2. To assess the applicability of the models, nonequilibrium shape pores of identical initial geometry (∼20 μm × 20 μm × 0.5 μm) were introduced into (0001), {10[Onemacr]2}, {1120}, and {100} surfaces of sapphire single crystals using microfabrication techniques, ion-beam etching, and hot pressing. The large (∼20 μm × 20 μm) faces of the pore are low-index surfaces whose nature is dictated by the wafer orientation. A series of anneals was performed at 1900°C, and the approach of the pore shape to an equilibrium shape was monitored. The kinetics of shape evolution are highly sensitive to the crystallographic orientation and stability of the low-index surface that dominates the initial pore shape. The measured variations of the pore aspect ratio were compared to those predicted by the kinetic model. The observations suggest that when the initial bounding surface is unstable, shape relaxation may be controlled by diffusion. However, surface-attachment-limited kinetics (SALK) appears to play a major role in determining the pore shape evolution rate in cases where the initial bounding surfaces have orientations that are part of the Wulff shape.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.2000.tb01592.x
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  • 6
    Electronic Resource
    Electronic Resource
    High-Temperature Morphological Evolution of Lithographically Introduced Cavities in Silicon Carbide (2001)
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 84 (2001), S. 0 
    Details
    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: Internal cavities of controlled geometry and crystallography were introduced in 6H silicon carbide single crystals by combining lithographic methods, ion-beam etching, and solid-state diffusion bonding. The morphologic evolution of these internal cavities (negative crystals) in response to anneals of up to 128 h duration at 1900°C was examined using optical microscopy. Surface energy anisotropy and faceting had a strong influence on the geometric and kinetic characteristics of evolution. Decomposition of {12〈inlineGraphic alt="inline image" href="urn:x-wiley:00027820:JACE921:JACE_921_mu2" location="equation/JACE_921_mu2.gif" extraInfo="missing"/〉10} cavity edges into {101〈inlineGraphic alt="inline image" href="urn:x-wiley:00027820:JACE921:JACE_921_mu1" location="equation/JACE_921_mu1.gif" extraInfo="missing"/〉x} facets was observed after 16 h anneals, indicating that {12〈inlineGraphic alt="inline image" href="urn:x-wiley:00027820:JACE921:JACE_921_mu2" location="equation/JACE_921_mu2.gif" extraInfo="missing"/〉10} faces are not components of the Wulff shape. The shape evolution kinetics of penny-shaped cavities were also investigated. Experimentally observed evolution rates decreased much more rapidly with those predicted by a model in which surface diffusion was assumed to be rate limiting. This suggested that the development of facets and the associated loss of ledges and terraces during the initial stages of evolution resulted in an evolution process limited by the nucleation rate of attachment/detachment sites (ledges) on the facets.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.2001.tb00769.x
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  • 7
    Electronic Resource
    Electronic Resource
    The Wulff Shape of Alumina: IV. Ti4+-Doped Alumina (2005)
    Malden, USA : Blackwell Science Inc
    Journal of the American Ceramic Society 88 (2005), S. 0 
    Details
    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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  • 8
    Electronic Resource
    Electronic Resource
    Preface (1998)
    Springer
    Journal of materials synthesis and processing 6 (1998), S. 157-157 
    Details
    ISSN: 1573-4870
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1022655717597
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  • 9
    Electronic Resource
    Electronic Resource
    The Kinetics of Pore Shape Evolution in Alumina (1998)
    Springer
    Journal of materials synthesis and processing 6 (1998), S. 161-167 
    Details
    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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1022665418505
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  • 10
    Electronic Resource
    Electronic Resource
    Preface (1998)
    Springer
    Journal of materials synthesis and processing 6 (1998), S. 369-369 
    Details
    ISSN: 1573-4870
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1021816619234
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