Polycrystalline Alumina (polycrystalline + alumina)

Distribution by Scientific Domains


Selected Abstracts


Mechanisms and Mechanics Governing the Indentation of Polycrystalline Alumina

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 9 2008
Zhensong Wei
A constitutive model for the inelastic deformation of polycrystalline alumina that accounts for both microcrack growth and plastic slip is implemented into a commercial finite element code. To establish its applicability, the code is used to simulate the deformation that occurs upon spherical and conical indentation. Inelastic zones and indentation pressures are predicted over a range of grain size and compared with measurements. The model replicates the mechanism transition from plasticity control at small grain sizes to micro-crack-control at large grain sizes. It also predicts an extensive micro-crack-dominated inelastic zone at large grain size that reduces the indentation pressures. [source]


Solubility of Magnesia in Polycrystalline Alumina at High Temperatures

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 2 2001
C. Greskovich
High-purity Al2O3 compacts were doped with 0,350 ppm (by weight) of MgO using a liquid immersion technique and equilibrated at temperatures between 1700 and 2000C under hydrogen. The solubility limits of MgO in Al2O3 at temperatures of 1720 and 1880C were very low, ,75 and 175 ppm, respectively. Variation of MgO solubility with temperature could be represented by the equation, ln Mg/Al = 3.80,2.63 104/T. The small MgO solubilities were understood by the high enthalpy (326 kJ/mol) of solution. The results of this study suggested that previous investigations on sintering and grain-growth mechanisms in MgO-doped Al2O3 were probably not done in single-phase Al2O3 solid solutions. However, the conclusions on sintering and grain-growth mechanisms in prior research work in MgO-doped A2O3 may be correct. The effects of SiO2 impurity and grain size on MgO solubility are discussed. Previous grain-growth experiments in MgO-doped Al2O3 are described that demonstrate the clearest evidence for grain-boundary mobility controlled by a solid-solution mechanism. [source]


A Screening Design Approach for the Understanding of Spark Plasma Sintering Parameters: A Case of Translucent Polycrystalline Undoped Alumina

INTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY, Issue 5 2010
Yann Aman
An experimental screening design was used to evaluate the effects of spark plasma sintering (SPS) parameters such as heating rate, sintering temperature, dwell duration, and green-shaping processing on the relative density, grain size, and the optical properties of polycrystalline alumina (PCA). It is shown that heating rate and sintering temperature are the most critical factors for the densification of PCA during SPS. Green-shaping processing could prevent grain growth at low SPS sintering temperatures. No predominant SPS parameters are observed on the optical properties. Hence, the optical properties of PCA are controlled by microstructural evolution during the SPS process. [source]


Mechanisms and Mechanics Governing the Indentation of Polycrystalline Alumina

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 9 2008
Zhensong Wei
A constitutive model for the inelastic deformation of polycrystalline alumina that accounts for both microcrack growth and plastic slip is implemented into a commercial finite element code. To establish its applicability, the code is used to simulate the deformation that occurs upon spherical and conical indentation. Inelastic zones and indentation pressures are predicted over a range of grain size and compared with measurements. The model replicates the mechanism transition from plasticity control at small grain sizes to micro-crack-control at large grain sizes. It also predicts an extensive micro-crack-dominated inelastic zone at large grain size that reduces the indentation pressures. [source]


Transparent Polycrystalline Alumina Ceramic with Sub-Micrometre Microstructure by Means of Electrophoretic Deposition

MATERIALWISSENSCHAFT UND WERKSTOFFTECHNIK, Issue 4 2006
A. Braun
Abstract The optical quality attainable in coarse-grained polycrystalline alumina is severely limited by grain-boundary scattering, which is inherent to non-cubic materials. The optical properties of sub-micrometre polycrystalline alumina are of growing interest triggered by the fact that a decrease in the grain sizes of the final sintered material yields an improvement in the optical quality while the scattering mechanism changes as the grain size becomes comparable with the wavelength of light. To achieve transparent alumina ceramics with a fine-grained microstructure, however, porosity and other defects must be avoided. This necessitates the optimization of processing and sintering procedures. Electrophoretic deposition (EPD) is a colloidal process in which ceramic bodies are directly shaped from a stable suspension by application of an electric field. Electrophoretic deposition enables the formation of homogeneous, uniform green microstructures with high density, which can be sintered to transparency. It is a simple and precise technique to synthesize not only monoliths, but also composites with complex geometries [1]. Alumina green bodies were deposited from stabilized aqueous suspensions with and without doping. Green alumina compacts were evaluated based on their pore size distribution and density. Densification behaviour was characterized by dilatometric studies conducted at constant heating rate. Samples were sintered at different temperatures with subsequent post-densification by hot isostatic pressing. Transparency was evaluated by means of spectroscopic measurements. The measured in-line transmission of the samples at 645 nm was more than 50,% and that is 58,% of the value of sapphire. The influence of dopings on transparency was investigated. The mechanical properties of the samples were tested. [source]