Bulk Alumina Glasses and Glass-Ceramics with Multiple Stabilizers

Alumina rare-earth oxide glasses have unique properties, including high chemical resistance, high hardness, and a high refractive index in the IR region, with potential applications in optical devices. Alumina (Al₂O₃) alone cannot be obtained as bulk glass, although it can act as a network former. Alumina can form glasses when alloyed with calcia, zirconia, and other rare-earth oxides. It has been observed that the glass-forming ability and short-range ordering are enhanced if the size difference of the constituent cations is large. Having excellent optical and mechanical properties, the development of alumina based glasses is hampered by their high melting points and the high cooling rates required to obtain the glassy structure. Several studies have been conducted to obtain alumina-based glasses using splat quenching, aerodynamic levitation, and flame spraying; limiting the size to thin films, ribbons, fibers, and small spheres. Obtaining bulk alumina glasses is challenging because of their limited thermal stability against crystallization. Chemical synthesis techniques, including co-precipitation, sol-gel, spray pyrolysis, and solution combustion synthesis (SCS), provide alternate routes to obtain glassy powders. In the present work glassy powders were obtained by utilizing SCS process, because it is simple, fast and cheap. Compositional space for Al₂O₃ based glasses was explored by varying the amounts of different stabilizers such as CaO, Gd₂O₃, La₂O₃, TiO₂, Y₂O₃, Yb₂O₃, ZnO, ZrO₂. The stabilizers were chosen based on their efficacy in stabilizing the glassy state. Compositions were designed by varying alumina content from 60-80 mol% and rest in four to eight stabilizers in equimolar ratio. The amorphous phase stability and crystallization sequence of the synthesized powders were investigated. A few compositions showing higher glass phase stability were selected for the densification studies. Sintering of calcined powders was carried out under pressureless, vacuum, and hot press sintering conditions. First time, extensive densification was observed under free sintering and vacuum sintering conditions without undergoing crystallization. This study opens up the opportunity to obtain bulk dense alumina glasses.<br/>Being metastable, a glassy phase always has the driving force for crystallization. Bulk dense glasses can act as precursors for nanostructured ceramics as the crystallization is carried out at low temperatures so that the nucleation rate is much greater than the growth rate. Crystallization of bulk alumina glasses into nanocrystalline oxides was carried out by suitable thermal treatment with and without applied pressure. X-ray line broadening and HRTEM images were used to find the sizes of nano-crystals. Microstructural characterization was carried out on these nanocrystalline glass-ceramics, in addition to basic mechanical property evaluation. Precession electron diffraction (PED) analysis was carried out on crystallized glasses, for phase mapping and crystallite orientation mapping.<br/>Structural modification taking place during consolidation were studied by ²⁷Al MAS NMR and X-ray absorption spectroscopy (XAS). ²⁷Al MAS NMR was utilized to probe the local structural units around Al cations and clearly indicated the presence of AlO_x units (x = 4, 5, 6) in the calcined powder. The relative abundances of different Al sites, isotropic chemical shift (δ_iso), and average quadrupolar coupling constant (&lt;|Q_C|&gt;) fitting parameters were extracted using a simple Czjzek model. Extended X-ray absorption fine structure (EXAFS) was used to probe the local structural changes around different stabilizing cations. EXAFS studies indicated that the local structure around the stabilizing ions remains unaffected during pressureless sintering. Characterization by NMR spectroscopy suggests that structural changes/relaxation during densification originate from the short-range collective rearrangement of AlO_x polyhedra.