Preparation and growth mechanism of the flower-like whiskers of γ-, θ-, and α-Al2O3 phases by homogeneous precipitation/calcination method

The different alumina whiskers phases are extensively used to reinforce high-performance composite materials. The understanding of the mutual relationship of the different phases is key for simulating Al2O3 whiskers containing systems and is of general interest for the metal oxide and ceramic communities. In this paper, we used homogeneous precipitation to synthesized flower-like boehmite whisker precursors from aluminum sulfate and urea. The synthesized precursors were calcined at 600, 900, and 1200 °C to obtain flower-like whiskers of γ-, θ-, and α-Al2O3 phases, respectively. The precursor boehmite crystals prepared in closed, semi-closed, and open reactor systems were qualitatively analyzed using SEM. The results showed that the usea hydrolysis, which serves as a source of OH− ions, is slow under closed-reactor conditions, which favors the growth of the boehmite crystal nuclei into whiskers. We adopted the L16 (43) orthogonal design of experiment to analyze the influence of reaction temperature, time, and the amount of urea (relative to the amount of Al2(SO4)3) in the closed system. Synthesis temperature of 160 °C and the processing time of 8 h yielded optimal boehmite flower-like whiskers. Physicochemical properties of the precursor and series of alumina phases were characterized using thermogravimetric/differential thermal analysis (TG/DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM) methods. Finally, the obtained results and the thorough analysis of the growth mechanism suggested the step-growth model of boehmite flower-like whiskers.