Effect of nano-TiO₂ and nano-SiO₂ addition on the morphological control of α-Al₂O₃ platelets via solid-state reaction

Controlling the morphology of α-Al₂O₃ plate-like powders is essential, asmulti-morphological α-Al₂O₃ platelets are needed in various applications. The α-Al₂O₃ platelets investigated in this work, were prepared via conventional solid-state reaction using gibbsite and pseudo-boehmite as the starting materials; nano-TiO₂, nano-SiO₂, silica gel, and the mixtures of nano-SiO₂ and nano-TiO₂ were used as morphology modifiers during the synthesis. The effect of the type and amount of modifiers used, on the morphology of the platelets was determined via scanning electron microscopy, X-ray diffraction, and measurements of the specific surface area. The results indicated that nano-TiO₂ resulted in small and thick platelets. In fact, nano-TiO₂ led to increased crystallinity and accelerated phase transformation to α-Al₂O₃, and additions of ≥2.00 wt% resulted in severe aggregation of the α-Al₂O₃ particles. Conversely, addition of nano-SiO₂ led to significant increases in the diameter and reductions in the thickness of the α-Al₂O₃ platelets; for equal additions, liquid silica gel, which has excellent mobility, was more effective in controlling the morphology than solid SiO₂. Additions of up to 1.00 wt% of the nano-SiO₂ resulted in an incomplete α phase transformation, owing to SiO₂ inhibition of the transformation to α-Al₂O₃. Moreover, simultaneous additions of nano-TiO₂ and nano-SiO₂ led to competing effects on the morphology of the α-Al₂O₃ particles, as evidenced by changes in the specific surface area.