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  • 1
    Electronic Resource
    Electronic Resource
    Coarsening of Faceted Crystals (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: The influence of the nucleation energy barrier on the capillary-driven coarsening of faceted crystals that exchange material by diffusion is quantified. Our calculations are based on the assumption that the transport of material between particles must happen in series with the nucleation of partial layers on flat facets. Using a numerical model based on this idea, we simulate the time evolution of distributions of crystals that are made up of perfect faceted crystals (without step-producing defects), crystals containing step-producing defects, and mixtures of the two types. We find that the coarsening of a distribution containing only perfect faceted crystals is arrested at a size where the nucleation energy barrier becomes prohibitive. This critical size ranges from a few nanometers to several hundred nanometers, depending on material parameters and experimental conditions. When a small fraction of the crystals have step-producing defects (for these crystals the nucleation energy barrier vanishes), they can grow to large sizes at the expense of the perfect crystals and a bimodal grain size distribution is created. Based on these results, we hypothesize that when abnormal coarsening is observed in nature, it results from the presence of a small number of crystals with step-producing defects.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.2002.tb00149.x
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  • 2
    Electronic Resource
    Electronic Resource
    Structure Se nsitivity of Photochemical Oxidation and Reduction Reactions on SrTiO3 Surfaces (2003)
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 86 (2003), 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 photochemical reduction of Ag+ and oxidation of Pb2+ from aqueous solution by SrTiO3 leave insoluble reaction products (silver and PbO2, respectively) on the surface. Microscopic analysis has been used to relate the rates of these two reactions to the structure and orientation of SrTiO3 surfaces. The nonpolar (100) surface is the most reactive for silver reduction and the composition of the termination layer does not influence this reaction. On the polar (111) surface, the reduction and oxidation reactions occur on terraces with different terminations and opposite charges; this leads to a nonuniform distribution of reaction products. The polar (110) surface is the least reactive, and the majority of the reaction products are observed at steps along 〈100〉 directions where the more reactive {100} surfaces are exposed. The distribution of oxidation products found on (110) terraces is also influenced by the composition and charge of the surface termination. The results show that the photochemical reactivity of SrTiO3 is anisotropic and that, on polar surfaces, dipolar fields arising from charged surface domains influence the transport of photogenerated charge carriers and promote spatially selective oxidation and reduction reactions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.2003.tb03445.x
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  • 3
    Electronic Resource
    Electronic Resource
    Surface Energy Anisotropy of SrTiO3 at 1400°C in Air (2003)
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 86 (2003), 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: Geometric and crystallographic measurements of grain-boundary thermal grooves and surface faceting behavior as a function of orientation have been used to determine the surface energy anisotropy of SrTiO3 at 1400°C in air. Under these conditions, thermal grooves are formed by surface diffusion. The surface energy anisotropy was determined using the capillarity vector reconstruction method under the assumption that Herring's local equilibrium condition holds at the groove root. The results indicate that the (100) surface has the minimum energy. For surfaces inclined between 0° and 30° from (100), the energy increases with the inclination angle. Orientations inclined by more than 30° from (100) are all about 10% higher in energy and, within experimental uncertainty, energetically equivalent. A procedure for estimating the uncertainties in the reconstructed energies is also introduced. Taken together, the orientation dependence of the surface-facet formation and the measured energy anisotropy lead to the conclusion that the equilibrium crystal shape is dominated by {100}, but also includes {110} and {111} facets. Complex planes within about 15° of {100} and 5° of {110} are also part of the equilibrium shape.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.2003.tb03584.x
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  • 4
    Electronic Resource
    Electronic Resource
    Inhibition of Sintering and Surface Area Loss in Phosphorus-Doped Corundum Derived from Diaspore (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: The influence of magnesium, phosphorus, and iron additions on the low-temperature (≤1000°C) sintering of nanocrystalline α-Al2O3 derived from α-AlOOH has been investigated. α-AlOOH powder with a surface area of 50 m2/g yielded α-Al2O3 products with surface areas of 150 and 80 m2/g after dehydration at temperatures of 400° and 500°C, respectively. However, these products were prone to sintering at 〉600°C, and the surface area was reduced to 15 m2/g within only 1 h at 1000°C. Although magnesium and iron doping had no discernible effect, the presence of phosphorus inhibited sintering and surface-area loss significantly. Samples doped with 1%–2% phosphorus had surface areas of 〉31 m2/g after 100 h at 1000°C. Atomic force microscopy studies of α-Al2O3 pseudomorphs derived from α-AlOOH single crystals also demonstrated the inhibiting effect of phosphorus, as the rate of crack elimination was reduced on phosphorus-modified surfaces. The effects of the dopants are discussed with regard to their potential influence on α-Al2O3 surface energy and diffusivity.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.2002.tb00455.x
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  • 5
    Electronic Resource
    Electronic Resource
    Nucleation Energy Barriers for Volume-Conserving Shape Changes of Crystals with Nonequilibrium Morphologies (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: This paper quantifies the nucleation energy barrier that must be overcome if a faceted, defect-free particle with a nonequilibrium morphology is to change shape by intraparticle transport. Two types of crystals are considered: those whose equilibrium form is a truncated sphere and those whose equilibrium form is a cube. Numerical estimates show that, for a particle near equilibrium, the barrier becomes insurmountable for a facet larger than a few tens of nanometers. For nonequilibrium shapes where material must be transferred from faceted surfaces to uniformly curved surfaces to reach the equilibrium shape, the facets enlarge without a nucleation barrier (at a rate limited by diffusion or surface attachment kinetics) until they reach a fraction of their equilibrium size that is typically between 0.5 and 0.75. At this point, a significant barrier is encountered that, in the absence of step producing defects, prevents the particle from continuing toward equilibrium. For nonequilibrium shapes where material must be transferred to faceted surfaces from other parts of the crystal for it to reach the equilibrium shape, significant energy barriers for the nucleation of new layers persist even when the shape is far from equilibrium. Predictions from our model are compared to experimental observations reported by other researchers.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.2001.tb00965.x
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  • 6
    Electronic Resource
    Electronic Resource
    Distribution and Energies of Grain Boundaries in Magnesia as a Function of Five Degrees of Freedom (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: The multiplicity of distinct grain boundary configurations in polycrystals has made it difficult to determine the relative frequency with which each configuration is adopted. As a result, the physiochemical properties of each boundary and the influence of the distribution of boundaries on macroscopic materials properties are not well understood. Using a semiautomated system, we have measured all five macroscopically observable degrees of freedom of 4.1 × 106 boundary plane segments making up 5.2 × 106μm2 of grain boundary interface area in a magnesia polycrystal. Our observations demonstrate that not all grain boundary configurations occur with the same frequency and that the relative free energies of the different interfacial configurations influence the population distribution. Furthermore, the results indicate that relative grain boundary energies can be estimated based on the free surface energies.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.2002.tb00583.x
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  • 7
    Electronic Resource
    Electronic Resource
    Influence of Diaspore Seeding and Chloride Concentration on the Transformation of “Diasporic” Precursors to Corundum (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: “Diasporic” precursors derived from sols formed by the controlled hydrolysis of aluminum sec-butoxide in the presence of HCl yielded mixtures of corundum and amorphous alumina when calcined between 500° and 800°C. The fraction of corundum in the calcined products depended on the HCl/alkoxide ratio used during hydrolysis and was maximized at 64 wt% when the molar ratio was 1. Precursors formed from sols hydrolyzed in the presence of HNO3 rather than HCl yielded only amorphous products or transition aluminas when treated below 900°C. The corundum yield of the precursors was enhanced when they were seeded with diaspore (α-AlOOH) crystals. Precursors synthesized with an HCl/alkoxide ratio of 1 and seeded with 7.6 × 1016 diaspore seeds/(mol of Al2O3) were transformed to phase-pure corundum within 12 h at 700°C. Based on studies of the phase evolution of the precursors during calcining, it was concluded that the diaspore seeds promoted conversion by acting as corundum nuclei once they decomposed at ∼450°C.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.2001.tb00933.x
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  • 8
    Electronic Resource
    Electronic Resource
    Experimental Method for Determining Surface Energy Anisotropy and Its Application to Magnesia (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: An experimental technique has been developed to determine the surface energy anisotropy of crystalline solids. The technique is based on atomic force microscopy measurements, which are used to quantify the geometry of thermal grooves, and electron backscattered diffraction pattern measurements, which are used to specify crystallographic orientations. Observations are made at circumferential thermal grooves, where it is assumed that Herring's local equilibrium condition for a triple junction holds and that the grain-boundary energy is independent of its boundary plane. A truncated double Fourier series is used to approximate the surface energy, and the unknown coefficients of the series are determined by fitting the observations to the local equilibrium condition. The method, which should be applicable to most polycrystalline materials, has been tested on magnesia that has been thermally grooved at 1400°C in air. The maximum of the best-fit surface energy function is at (111) and the minimum is at (100). The relative surface energies of the low-index planes are γ110/γ100= 1.040 ± 0.008 and γ111/γ100= 1.072 ± 0.010.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.2000.tb01358.x
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  • 9
    Electronic Resource
    Electronic Resource
    Nucleation Barrier for Volume-Conserving Shape Changes of Faceted Crystals (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: This paper considers volume-conserving shape changes of small faceted particles or cavities; these changes are driven by surface-energy reduction. If these changes require normal motion of the facets (singular surfaces), and if the perimeter free energy σp (per unit area) of a nucleus of a new facet layer is comparable to a typical surface free energy (σ∼ 1 J/m2), the energy barrier is prohibitively large for facets larger than a limiting size of approximately a nanometer unless they are intersected by dislocations that provide a source of steps. In the absence of such dislocations, particles that contain these facets are immobilized in their initial shapes. If σp 〈 σ, the limiting facet size is increased by a factor of approximately σ/σp.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.2000.tb01173.x
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  • 10
    Electronic Resource
    Electronic Resource
    Conversion of Diaspore to Corundum: A New α-Alumina Transformation Sequence (1997)
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 80 (1997), 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: A new two-step transformation sequence has been identified for the conversion of diaspore (α-AlOOH) to corundum (alpha-Al2O3). The powder X-ray diffraction pattern of the intermediate phase, designated alpha'-Al2O3, has been indexed to a monoclinic (P2) cell, with a = 9.566(5) Å, b = 5.124(2) Å, c = 9.156(4) Å, and ß= 91.76(3)°. Nuclear magnetic resonance of alpha'-Al2O3 indicates that 15-20% of the aluminum is on tetrahedral sites, with the remainder in octahedral coordination. Controlled-atmosphere high-temperature X-ray diffraction shows that water accelerates the second transformation step, alpha'-Al2O3→〉 alpha-Al2O3, acting as a mineralizer for the conversion process.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.1997.tb03171.x
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