## 3 Results and Discussion
### Results of X-ray and SEM Analysis
Figure 6: SEM images of boehmite obtained in oxidation experiments using 99.99% aluminum: (**a**) t = 10 h, T = 280 \({}^{\circ}\)C, steam, \(\eta\) = 96.73% (50 microns); (**b**) t = 10 h, T = 280 \({}^{\circ}\)C, steam, \(\eta\) = 96.73% (5 microns); (**c**) t = 10 h, T = 280 \({}^{\circ}\)C, liquid, \(\eta\) = 100.0% (5 microns); (**d**) t = 10 h, T = 280 \({}^{\circ}\)C, liquid, \(\eta\) = 100.0% (5 microns); (**e**) t = 4 h, T = 280 \({}^{\circ}\)C, steam, \(\eta\) = 68.44% (50 microns); (**f**) t = 4 h, T = 280 \({}^{\circ}\)C, steam, \(\eta\) = 68.44% (5 microns); (**g**) t = 10 h, T = 240 \({}^{\circ}\)C, steam, \(\eta\) = 24.38% (5 microns); (**h**) t = 10 h, T = 240 \({}^{\circ}\)C, steam, \(\eta\) = 24.38% (5 microns). The purity of aluminum is a critical factor in the hydrothermal oxidation process. The experiments previously outlined highlight the importance of aluminum purity for the degree of its conversion during oxidation. It was observed that aluminum with a higher chemical purity (99.99%) exhibited much more effective oxidation compared to aluminum with a lower purity (99.9% and 99.7%). The oxidation process results in the conversion of aluminum into a compound called boehmite (AlOOH), as represented by Equation (1). When using high-purity aluminum (99.99%), nearly complete conversion to boehmite was achieved at 280 \({}^{\circ}\)C in liquid water, whereas the conversion degree was significantly lower for aluminum of lower purity (99.9% and 99.7%), at less than 2%. One of the factors that can explain this phenomenon is the presence of impurities such as silicon (Si), iron (Fe), copper (Cu), and magnesium (Mg) in lower purity aluminum. The concentration of these impurities is significantly higher in 99.7% and 99.9% aluminum compared to that in 99.99% pure aluminum. For instance, Si and Fe were found to be approximately 7.5 times more concentrated in 99.9% aluminum, and around 50 times more concentrated in 99.7% aluminum, as compared to the concentrations in 99.99% pure aluminum. Similarly, Cu and Mg were found in significantly higher concentrations in lower purity aluminum. These impurities tend to accumulate near the surface of the aluminum granules and act as barriers to oxidation. They can hinder the reaction process by forming solid inclusions of boehmite on the surface, which in turn prevents the deep penetration of the chemical reaction into the granules. This could be due to the impurities forming passivation layers or themselves reacting in a manner that consumes reactants or forms phases that inhibit the oxidation of aluminum. Moreover, it was observed that the oxidation of aluminum in liquid water is more effective than in steam as the temperature increases beyond 220 \({}^{\circ}\)C. However, the purity of  
Figure 8: Photos of aluminum granules with a chemical purity of 99.9 and 99.7% after the oxidation experiments at 280 \({}^{\circ}\)C and 10 h, with the conversion degree (n) of less than 2%. Figure 7: SEM images of boehmite obtained in oxidation experiments using 99.99% aluminum: (a) holding time = 10 h, T = 280 \({}^{\circ}\)C, liquid, \(\eta\) = 100.0%; (b) holding time = 10 h, T = 280 \({}^{\circ}\)C, steam, \(\eta\) = 96.73%. the aluminum still played a significant role in this effectiveness. High purity aluminum showed a smoother and higher conversion degree in liquid compared to that in steam. In conclusion, the purity of aluminum is crucial for the hydrothermal oxidation process. Higher purity aluminum is more susceptible to complete or nearly complete conversion to boehmite, whereas lower purity aluminum, due to the impurities present, shows a significantly reduced conversion. These impurities appear to inhibit the oxidation reaction, especially at the surface of the aluminum granules.