Synthesis, characterization, and Taguchi–Grey relational process parameter optimization of alumina ceramic nanoparticles

Controlling crystallite size and phase purity remains a persistent challenge in the sol–gel synthesis of alumina (Al₂O₃) nanoparticles, as prior studies have largely examined process parameters in isolation rather than through integrated multi-response optimization. This study aimed to identify the combination of precursor material, calcination temperature, and calcination time that minimizes both crystallite size and thermogravimetric mass loss in sol–gel-derived alumina synthesis. Nine samples were prepared using aluminum nitrate, aluminum sulfate, and aluminum chloride precursors via the Taguchi L9(3³) orthogonal array. Calcination was performed at 800–1200 °C for 3–6 h. Samples were characterized by X-ray diffraction (XRD) and thermogravimetric analysis (TGA). Grey Relational Analysis (GRA), along with the Taguchi signal-to-noise ratio, was used to simultaneously optimize both responses. XRD identified different alumina phases (α, θ, δ, and γ) with crystallite sizes ranging from 5.2 to 26.5 nm, while TGA mass loss ranged from 4.90% to 6.53%. ANOVA of the S/N ratios ranked calcination temperature as the dominant factor. The optimal combination of aluminum sulfate, 1200 °C, and 4 h was validated by a confirmation experiment that reproduced predominantly α-Al₂O₃ with a crystallite size of 5.3 nm against a predicted 5.2 nm. This work establishes an integrated Taguchi–GRA framework that jointly optimizes crystallite size and thermal stability for sol–gel alumina synthesis and provides a reproducible, statistically grounded route for producing nanocrystalline α-alumina for catalytic, ceramic-reinforcement, and protective-coating applications. Graphical Abstract