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Role of impurities, LiF, and processing on the sintering, microstructure, and optical properties of transparent polycrystalline magnesium aluminate (MgAl2O4) spinel, The
Rubat du Merac, Marc
Rubat du Merac, Marc
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2014
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2014
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Transparent polycrystalline magnesium aluminate (MgAl2O4) spinel has an exceptional combination of properties that is well-suited to fulfill demanding optical applications that few other materials can satisfy. However, spinel is inherently difficult to densify due to high defect formation energies, variable stoichiometry, and extreme sensitivity to powder and processing parameters. In addition, the LiF sintering additive typically required to impart transparency degrades optical and mechanical properties, precluding wider application. Furthermore, there remains a fundamental lack of understanding of the processing-structure-property relationships required to obtain high transparency and good mechanical properties. In this work, hot-press experiments were designed to determine the role of impurities and LiF and the key variables required to obtain transparent spinel. Hot-pressed compacts were characterized with electron microscopy, chemical spectroscopy, and spectrophotometry, and impurities present in parts-per-million in starting powders were found to cause restricted grain size and opacity. LiF addition was found to reduce the content of some impurities by one order of magnitude, counteract absorption, and impart transparency, but also to cause grain coarsening, grain-boundary embrittlement, and scatter. Thermal analysis and residual gas analysis of prepared powders in combination with thermodynamic modeling demonstrated for the first time the specific mechanism by which LiF acts as a cleanser. LiF reacts with impurities to form volatile fluorides, and the temperature at which pressure is applied during hot-pressing determines the extent to which compact-scale differential sintering either traps LiF and volatile fluorides or allows their removal, the latter enabling transparency. The main cause of absorption in hot-pressed spinel compacts was found to be carbon contamination from graphitic hot-press components and it could be completely eliminated with proper shielding. Hot-press experiments with Al2O3, lithium, and fluorine additives, along with thermodynamic simulations and spectrophotometry indicated that aluminum oxy-carbides were likely partly responsible for absorption and that lithium was likely responsible for counteracting absorption. Dilatometry was conducted to study the effect of processing variables, and for the first time of neutron irradiation of starting powders, on the sintering behavior of cold-pressed powder compacts. Green-body density, stoichiometry, and impurities had the greatest effect on densification, whereas powder neutron irradiation and reducing atmosphere had small, but non-negligible effects. Dilatometry in combination with sintering experiments demonstrated that LiF coarsens crystallites by vapor and likely surface transport. Although these mechanisms do not contribute to densification, when combined with high grain-boundary transport and pressure, enhanced densification takes place. Dilatometry also demonstrated that transparent compacts could be fabricated by pressureless sintering. Pressureless field-assisted sintering of spinel was also demonstrated for the first time, producing translucent compacts with fine grain size. Electrochemical impedance spectroscopy, secondary-ion mass spectroscopy, and Raman spectroscopy, in combination with characterization by electron microscopy, were used to relate the dielectric properties of hot-pressed spinel compacts to microstructure. This was the first study to characterize the conductivity of fully-dense, transparent polycrystalline spinel compacts. The higher conductivity of polycrystalline spinel compacts compared to single crystals was attributed to increased conductance from impurities at grain boundaries parallel to the applied field. The lower conductivity of compacts with LiF addition compared to those without LiF addition was attributed to larger grain size and lower impurity content. Compacts with LiF addition exhibited distinct bulk and grain boundary impedances and the uniform distribution of lithium in the bulk was postulated to lower bulk conductivity compared to compacts without LiF addition.
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