## 3 Results and Discussion
### Influence of reactant Addition Order on the Precursor Morphology of Aluminum Oxide  
To further investigate the impact of the order of addition of the reactants on the precursor morphology of aluminum oxide, this study utilized hexahydrate aluminum chloride and ammonium bicarbonate as raw materials. This research focused on exploring the influence of the order of reactant addition on the precursor and microstructure of aluminum oxide. The test results are presented in Figures 11 and 12. Figure 11: SEM images of alumina precursors prepared by different agent addition sequences. (**a**) Aluminum chloride added dropwise to ammonium bicarbonate; (**b**) ammonium bicarbonate added dropwise to aluminum chloride. Figure 10: Nitrogen adsorption–desorption isotherms and BJH pore size distribution plots of alumina prepared with different aluminum sources. (**a**) Aluminum sulfate; (**b**) aluminum chloride; (**c**) aluminum nitrate. When aluminum chloride was added dropwise to ammonium bicarbonate, the precursor primarily consisted of boehmite, with an average crystal grain size of 26.69 nm. When ammonium bicarbonate was added dropwise to aluminum chloride, the precursor primarily consisted of sodium basic aluminum ammonium carbonate, with an average crystal grain size of 20.03 nm. The grain size of alumina precursors prepared in different addition orders is shown in Table 4.  
From Figure 13a, it can be observed that there are two peaks at 100 \({}^{\circ}\)C and 300 \({}^{\circ}\)C in the DTG curve, which are likely caused by the removal of excess water molecules with different binding energies from the surface and interlayers of boehmite. The sharp decrease in the TG curve at 431 \({}^{\circ}\)C is attributed to the transformation of boehmite into \(\gamma\)-alumina. As shown in Figure 13b, the DTG curve indicates that the mass change before 279 \({}^{\circ}\)C is due to the removal of adsorbed water and thermal decomposition, while the mass change after 279 \({}^{\circ}\)C is caused by the removal of crystalline water from the aluminum oxide precursor. According to the TG curves of aluminum aluminate carbonate and boehmite, it can be inferred that aluminum aluminate carbonate is more readily converted into \(\alpha\)-alumina compared to boehmite. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Addition Order** & **Lattice Parameters (Å)** & **Average Crystalline Size, D (nm)** \\ \hline (a) & \(\gamma\)-AlOOH (a = 3.7, b = 12.2, c = 2.8) & 26.69 \\ \hline (b) & NH\({}_{4}\)Al (OH)\({}_{2}\)CO\({}_{3}\)(a = 6.6, b = 11.9, c = 5.7) & 20.03 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Crystal grain size of aluminum oxide precursors prepared with different addition sequences. (a) Aluminum chloride added dropwise to ammonium bicarbonate; (b) Ammonium bicarbonate added dropwise to aluminum chloride. Figure 12: XRD patterns of alumina precursors prepared by different agent addition sequences. (a) Aluminum chloride added dropwise to ammonium bicarbonate; (b) ammonium bicarbonate added dropwise to aluminum chloride. From Table 1 and Figures 14 and 15, it can be observed that the precursor prepared by adding an aluminum chloride solution dropwise into an ammonium bicarbonate solution is an irregular rod-shaped sodium basic aluminum ammonium carbonate. On the other hand, the precursor primarily consists of hexagonal beohmite, with a small amount of short rod-shaped sodium basic aluminum ammonium carbonate when ammonium bicarbonate solution is added dropwise into the aluminum chloride solution. This phenomenon may be related to the initial concentration of ions in the solution. When aluminum chloride was added dropwise to the ammonium bicarbonate solution, amorphous aluminum hydroxide was initially formed. Due to the abundance of hydroxyl groups on the surface of amorphous aluminum hydroxide, a high-energy state is created on the surface. At this point in the reaction solution, the concentrations of carbonate ions and ammonium ions are much greater than that of aluminum ions.