Alumina (Al₂O₃) is regarded as network modi?er in silicate glasses but it cannot form glass readily. Alumina rare earth oxide glasses have unique properties which include high chemical resistance, high refractive index and high hardness. The glass can also be devitrified into nanocrystalline oxides. It is observed that glass formation and short-range ordering is enhanced if the size difference of constituent ions is large. Due to very high cooling rate and limited thermal stability, obtaining bulk amorphous alumina is challenging. In this work we have explored a compositional space for Al₂O₃ based glasses with different additives which include varying amounts of ZrO₂, Y₂O₃, TiO₂, Yb₂O₃, La₂O₃, Gd₂O₃, ZnO and CaO to enhance thermal stability of glassy powders against crystallization. Glass powders were synthesized by solution combustion technique with nitrate precursors using citric acid as fuel. Amorphous phase stability and crystallization behavior of synthesized powders were studied. X-ray line broadening and HRTEM images were used to find the sizes of nano-crystals and their evolution with thermal treatment was monitored. A few compositions with the glassy phase stable up to 950°C were obtained. These were selected for sintering studies.
Sintering of calcined powder was carried out in pressureless sintering, vacuum sintering and hot press sintering condition. First time it has been reported that alumina based glassy powder undergo extensive densification in free sintering without crystallization. This study opens up the opportunity to obtain bulk dense alumina glasses. Crystallization of bulk alumina glasses into nanocrystalline oxides was carried out by suitable thermal treatment with and without applied pressure. Microstructural characterization was carried out on these nanocrystalline glass ceramics, in addition to basic mechanical property evaluation. This study opens up the opportunity to obtain genuinely nanocrystalline bulk alumina-based glass ceramics.
Structural modification taking place during consolidation was studied by ²⁷Al MAS NMR and X-ray absorption spectroscopy. ²⁷Al MAS SSNMR was utilized to probe local structural units around Al cations and clearly indicates the presence of AlO_x units, (x = 4, 5, 6) in calcined powder. Relative abundance of different Al sites, isotropic chemical shift (δ_iso), quadrupolar coupling constant (Q_C) fitting parameters were extracted by using simple Czjzek model, which is used for fitting MAS NMR spectra of amorphous materials having quadrupolar nuclei. Extended X-ray Absorption Fine Structure (EXAFS) was used to probe local structural changes around different cations. EXAFS measurements were carried out at energy-scanning beam line (BL-9) at the INDUS-2 Synchrotron equipped with Si (111) double crystal monochromator, RRCAT, Indore, India. It is found from NMR studies that only the relative abundance AlO_x species changes during free sintering, comparing to the initial powder; while EXAFS studies shows that local structure of other cations remain essentially the same after sintering.