Preparation and Properties of Micron Near-Spherical Alumina Powders from Hydratable Alumina with Ammonium Fluoroborate.

Micron-sized near-spherical α-Al2O3 powders are widely used as thermal fillers due to their high thermal conductivity, high packing density, good flowability, and low cost. During the high-temperature calcination, the resulting α-Al2O3 powders often exhibit an aggregated worm-like morphology owing to limitations in solid-state mass transfer. Researchers have employed various mineralizers to regulate the morphology of α-Al2O3 powders; however, the preparation of micron-sized highly spherical α-Al2O3 powders via solid-state calcination is still a great challenge. In this work, micron-sized near-spherical α-Al2O3 powders were synthesized through high-temperature calcination using hydratable alumina (ρ-Al2O3) as precursor with water-soluble mineralizer ammonium fluoroborate (NH4BF4). ρ-Al2O3 can undergo a hydration reaction with water to form AlO(OH) and Al(OH)3 intermediates, serving as an excellent precursor. With the addition of 0.1 wt% NH4BF4, the product exhibits an optimal near-spherical morphology. Excessive addition (>0.2 wt%), however, significantly promotes the transformation of α-Al2O3 from a near-spherical to a plate-like structure. Further studies reveal that the introduction of NH4BF4 not only modulates the crystal morphology but also effectively reduces the content of sodium impurities in the powder through a high-temperature volatilization mechanism, thereby enhancing the thermal conductivity of the powder. It is shown that the thermal conductivity of the micron-sized α-Al2O3/ epoxy resin composites reaches 1.329 ± 0.009 W/(m·K), which is 7.4 times that of pure epoxy resin.