Influence mechanism of halide additives on phase conversion, morphology, and purity of alumina powders prepared by solid-phase calcination method

High purity ultrafine alumina powders with different crystalline phases and morphologies were prepared by solid-phase sintering method. The influence mechanism of halides on the particle characteristics of alumina powders was studied. The results showed that NH 4 F additives induced the conversion of metastable alumina to α-alumina at high temperature, lowered the phase transformation temperature of α-alumina by about 300 °C, increased the α-alumina conversion rate by about 11.6%, reduced the sodium oxide content by about 72.3 wt%, and caused the alumina crystals to grow in a platelet shape manner. By comparison, NH 4 Cl additives had less effect on the transition temperature of α-alumina and the impurity content, and the particle morphology of alumina was a mixture of vermicular and platelet shapes. This study revealed the influence mechanism of two kinds of halides on the phase conversion, morphology evolution, and sodium removal of alumina powders from sintering mechanism and the theories of crystal structure, adsorption on the solid surface, and electronegativity of atoms.