Synthesis of fast sinterable oxygen deficient nano α−alumina & its optical properties

Highlights: • Gel combustion of Al(NO₃)₃ in urea-formaldehyde resin base produces nano α-alumina (∼40 nm). • Absorption and photoluminescence behavior substantiates large oxygen deficiency. • Deconvolution of PL emission band into Gaussian sub components evidence active color centers. • Large defect concentration reduces band gap value (5.6 eV) far below than bulk sapphire (8.8 eV). • Defect driven solid state sintering induces fast densification under spark plasma process. Nanocrystalline α-alumina powder was prepared successfully following gel combustion process using for the first time the combination of aluminum nitrate as oxidant and urea-formaldehyde as fuel. The combustion reaction was explained through thermal analyses (differential thermal analysis, DTA & thermo-gravimetric analysis, TGA). The synthesized powder was characterized by X-ray diffraction (XRD), infrared spectroscopy (Fourier Transformed Infrared Spectroscopy, FTIR), electron microscopy (Field Emission Scanning Electron Microscopy, FESEM & Transmission Electron Microscopy, TEM), optical spectra (absorption spectroscopy & photoluminescence, PL), etc. As evidenced from XRD pattern the as- synthesized powder was found to be amorphous. Full crystallinity of the as synthesized powder was obtained upon calcination above around 1100 °C. Synthesized particles are nanoporous, held in loosely agglomerated mass of nano sized spherical forms with size ranging in general between 50 and 70 nm. The photo luminescent spectrum shows a broad emission band that was deconvoluted into Gaussian sub-components. The resolved emission peaks imply emission from oxygen vacancy related radiative color centers. With heat treatment temperature, the PL intensity diminishes due to reduced defect (oxygen deficiency) concentration in nano-crystals. The presence of large concentration of anion deficiency, related to defects, are reflected in the intense emission of PL spectra and in lowering the band gap value with respect to the bulk counterpart; both phenomena being explained in this text. An initial sintering trial under spark plasma sintering shows the powders are quickly responsive to densification facilitated by the presence of defects.