Effect of processing parameters on the development of anisotropic α-Al2O3 platelets during molten salt synthesis

It is of utmost importance to control the size and morphology of anisotropic α-Al₂O₃ platelets owing to its extensive usage in various applications, including most recent synthetic biomimetic abalone nacre-like composites. In the present article, a wide range of well-developed and well-dispersed α-Al₂O₃ platelets were produced with diameter and thickness ranging from 4 - 8 μm and 0.6–1 μm respectively, by taking γ-Al₂O₃ precursor and sodium sulfate as low melting flux. The effects of various processing parameters like calcination temperatures, molar ratios of Al/Na and soaking time on the phase formation and morphology development of the final platelets were investigated in rigor. The results indicated that high calcination temperature of 1200 °C and moderate soaking time (60 min) helped to develop α-Al₂O₃ platelets with hexagonal morphology and uniform sizes. The sulfate salts created favorable conditions for growth of anisotropic platy α-Al₂O₃ and Al/Na molar ratio has significant effect in controlling the aspect ratio of the platelets. The impact of deagglomeration treatment by the ultrasound irradiation on the calcined aggregates of platelets was also examined. The breakage of platelets clusters by fracture/erosion due to the interaction with imploding cavitation bubbles under ultrasound energy, reached maximum after 90 min of deagglomeration treatment. The detail phase transformation sequences from γ-Al₂O₃ to α-Al₂O₃ was explained by correlating thermal (DT-TGA) and X-ray analysis. The growth mechanism of the anisotropic α-Al₂O₃ particles in the presence of molten Na₂SO₄ liquid was also unraveled.