Crystallographic phase transformation of nanocrystalline ultrafine alumina

Ultrafine alumina was synthesized by a unique, cost-effective and simple precipitation method followed by calcination at different prominent temperatures at 750 °C, 950 °C and 1200 °C to investigate phase transformation behavior from aluminum hydroxide to α-alumina. Alpha (α), theta (θ) and gamma (γ) phases of alumina were identified by X-ray diffraction (XRD) analysis. Quantitative analysis of different phases was done by Rietveld Refinement in the whole powder pattern fitting (WPPF) method. The XRD pattern shows that initially at 110 °C bayerite phase of aluminum hydroxide is prominent. Where 97.0 % bayerite phase transferred to the γ phase, 95.0 % theta (θ) phase and 100.0 % alpha (α) phase at 750 °C, 950 °C and 1200 °C respectively. The bands around 600 cm⁻¹ are assigned to the stretching mode of the Al–O group of alumina revealed by Fourier transform infrared spectroscopy (FTIR). The hydrodynamic diameter and isoelectric point (IEP) were determined using the dynamic light scattering (DLS) method which was 282.3, 252.9 and 209.1 nm and 7.0, 4.7 and 5.0 for γ, θ and α alumina respectively. The thermal analysis by differential scanning calorimetry (DSC) and thermo-gravimetric analysis (TGA) shows that the final mass loss was 21.08 % observed over 250 °C–1150 °C due to the formation of alumina from aluminum hydroxide. The average particle size was around 42.0–49.0 nm and the atomic mass of elements aluminum (Al) = 52.89 % and oxygen (O) = 47.03 % which is almost similar to the theoretical value that is confirmed by transmission electron microscopy (TEM) coupled energy dispersive spectroscopy (EDS).