###### Abstract  
To perform an in-depth study of the crystal growth habits and phase changes of alumina and its precursors in reaction systems, this paper studied the effects of reactant type and addition order on the morphology of alumina using hydrothermal methods with different precipitants and aluminum sources as reactants. Research has shown that sodium bicarbonate and ammonium bicarbonate can be used as precipitants to prepare adhered spherical alumina and irregular short rod alumina, while potassium bicarbonate can be used as a precipitant to prepare hexagonal flake alumina. Using aluminum sulfate octahydrate, aluminum chloride hexahydrate, and aluminum nitrate, nine hydrates were prepared as aluminum sources, and agglomerated alumina, irregular short rod-shaped alumina, and fused alumina were obtained. The order of reactant addition affects the precursor phase of alumina, thereby affecting the microstructure of alumina after calcination, resulting in flake alumina with pores and short rod alumina. The results of this paper will provide theoretical guidance for the preparation of alumina with different micromorphologies. \(\alpha\)-Al\({}_{2}\)O\({}_{3}\); microscopic morphology; reactant types; addition sequence; ammonium aluminum carbonate hydroxide; Boehmite +
Footnote †: journal: Medical  
synthesized highly stable active alumina using the low-temperature solid-phase precursor method. The alumina, even after treatment at 1100 \({}^{\circ}\)C for 4 h, maintained the \(\gamma\)-phase, with a specific surface area of 92 m\({}^{2}\)/g and a pore volume of 0.68 cm\({}^{3}\)/g. [PERSON] et al. [12] prepared alumina powder using the solid-phase sintering method. Their study found that the addition of NH\({}_{4}\)F additive reduced the phase transition temperature of \(\alpha\)-alumina by about 300 \({}^{\circ}\)C, resulting in the crystallization of alumina crystals in a platelet growth manner. On the other hand, the addition of NH\({}_{4}\)Cl additive had a relatively minor effect on the transformation temperature and impurity content of \(\alpha\)-alumina, resulting in alumina particles with a mixture of worm-like and platelet-like morphologies. The gas-phase method has stringent requirements for airtight equipment during production. Owing to its high production cost and low yield, this method is still in the laboratory research stage and has not achieved mass production. [PERSON] et al. [13] introduced the application of ceramic composite materials prepared by electron beam physical vapor deposition (EB-PVD) in the aerospace industry. They discussed the surface coating of aerospace materials with alumina-zirconia composite coatings, characterized by an exceptionally high melting point of approximately 2700 \({}^{\circ}\)C, excellent mechanical properties, and stability under oxidative conditions. [PERSON] et al. [14] synthesized polycrystalline \(\alpha\)-Al\({}_{2}\)O\({}_{3}\) coatings via thermal activated chemical vapor deposition (CVD). Due to the advantageous combination of the chemical inertness, thermal stability, and high thermal hardness of alumina coatings, they are widely used as wear-resistant hard coatings to protect cutting tools in the metal cutting industry. Compared to the other two methods, the liquid-phase method has significant advantages in controlling the micromorphology of alumina. Among these methods, hydrothermal methods can be used to control the particle sizes and microstructures of products by changing the hydrothermal reaction conditions, such as the reactant type, temperature, concentration, and pH of the reaction system, and by adding crystallization control agents to obtain particles with good dispersion and uniform sizes [15; 16; 17]. Additionally, hydrothermal methods are characterized by their low cost, simple operation, and ease of industrial production. As a result, the use of these materials has gradually become common for producing high-purity ultrafine alumina.