Magnetically active transition metal cation-substituted alumina.

Alumina (Al2O3) is one of the most widely used ceramic materials for innumerable applications, due to its unique combination of attractive physical and mechanical properties. These intrinsic properties are dictated by the numerous phases that Al2O3 forms and its related phase transformations. Transition metal (TM) cation dopants (iron (Fe), cobalt (Co), nickel (Ni) and manganese (Mn)), even in sparse amounts, have been shown to significantly affect the phase transformation and microstructural evolution of Al2O3. Small concentrations of TM cation dopants have successfully been incorporated to synthesize magnetically active Al2O3, while reducing the θ to α phase transformation temperature by 150 °C, and maintaining the outstanding mechanical properties. In addition, first-principle calculations based on density-functional theory with hybrid functional (HSE06) and the PBE+U methods have provided a mechanistic understanding of the formation energy and magnetism of the TM-doped α and θ phases of Al2O3. The results reveal a potential route for phase transition regulation and external magnetic field-induced texturing of Al2O3 ceramics.

Co-precipitation of rare-earth-doped Y2O3 and MgO nanocomposites for mid-infrared solid-state lasers.

Mid-infrared, solid-state laser materials face three main challenges: (1) need to dissipate heat generated in lasing; (2) luminescence quenching by multiphonon relaxation; and (3) trade-off in high thermal conductivity and small maximum phonon energy. We are tackling these challenges by synthesizing a ceramic nanocomposite in which multiple phases will be incorporated into the same structure. The undoped majority species, MgO, will be the main carrier of high thermal conductivity, and the minority species, Er:Y2O3, will have low maximum phonon energy. There is also an inherent challenge in attempting to make a translucent part from a mixture of two different materials with two different indexes of refraction. A simple, co-precipitation technique has been developed in which both components are synthesized in situ to obtain intimate mixing. These powders compare well to commercially available ceramics, including their erbium spectroscopy, even when mixed as a composite, and can be air-fired to ∼96% of theoretical density, yielding translucent parts. As the amount of Er:Y2O3 increases, the translucency decreases as the number of scattering sites start to coalesce into large patches. If the amount of Er:Y2O3 is sufficiently small and dispersed, the yttria grains will be pinned as individuals in a sea of MgO, leading to optimal translucency.