UC San Diego

Nanocrystalline oxides can have attractive properties such as high mechanical hardness, wear resistance, and high temperature plasticity. Fine (<500nm) grain and pore sizes are also beneficial for optical transparency and can help reduce dopant agglomeration beneficial for photoluminescence. However, most transparent polycrystalline oxides have large grains (>10μm) as a byproduct of their high-temperature fabrication conditions. Use of nanocrystalline powders as precursors and reactants seems an obvious choice for producing nanocrystalline oxides, but their use is complicated by high surface energies and reactivity with the environment. An improved understanding of the reaction and densification of nanocrystalline reactants is necessary to produce oxide ceramics of varying grain sizes, porosity ranges, and functionalities. Here, the reaction and densification kinetics of three important oxide ceramics, MgO, Al2O3¬, and Magnesium Aluminate Spinel (MgAl2O4) are studied. Investigations lead to novel method to produce polycrystalline MgAl2O4 via widely available metastable and nanocrystalline reactants using current-activated, pressure-assisted densification (CAPAD). First, a comprehensive processing and densification study was performed, exploring the effects of nanocrystallinity and moisture adsorption on nanocrystalline MgO - which may be used as an analogue for alternative hygroscopic oxide systems and is an integral part of synthesizing MgAl2O4 by reaction densification. Second, the effects of metastability and nanocrystallinity (MgO and Gamma-Al2O3, <30nm) on MgAl2O4 phase formation, densification, and transparency of CAPAD densified reactant powder was considered. Finally, transition metal doping of MgAl2O4 from reactant powder is explored. These results show metastable and nanocrystalline reactant powders combined with the flexibility of CAPAD allow for enhanced densification and reactivity. A wide array of microstructures, and new functionalities in polycrystalline MgAl2O4 are demonstrated. The metastable/nanocrystalline reactant powders facilitate comparably low temperatures for product formation and full density in MgAl2O4 – with up to 400°C reduction in the temperature for full phase formation and 300°C reduction for full density when compared to large-grained, stable reactant densification studies. The microstructural flexibility of this route is also highlighted with the production of both fully-dense transparent and robust nanoporous spinels, which conventionally arise from entirely different processing routes. Inclusion of transition metal dopants with reactant powders yields dense polycrystalline MgAl2O4, capable of tunability in the visible and previously unreported in published literature in polycrystalline form.