## 3 Results and Discussion
### Results of X-ray and SEM Analysis  
The obtained samples of the solid product were analyzed for phase composition, and the results are shown in Figure 5.  
The presence of the main phase, AlOOH (boehmite) with a rhombic lattice structure, in both samples indicates that the hydrothermal oxidation reaction is successfully producing the expected product. Boehmite is a well-known aluminum oxide hydroxide phase and is typically produced when aluminum reacts with water under hydrothermal conditions. However, the presence of another phase, \(\chi\)-Al\({}_{2}\)O\({}_{3}\), in the samples is noteworthy. \(\chi\)-Al\({}_{2}\)O\({}_{3}\) is another phase of aluminum oxide that typically forms at high temperatures. The distinction in the prominence of \(\chi\)-Al\({}_{2}\)O\({}_{3}\) phase at 48 and 67 degrees (2 Theta) in the sample obtained with steam suggests that the conditions during the steam-based hydrothermal oxidation are conducive to the formation of this phase. This could be due to the higher energy state of water in steam form, which could facilitate the formation of \(\chi\)-Al\({}_{2}\)O\({}_{3}\) through a different pathway, or due to a change in the thermodynamics of the system. Furthermore, the presence of metallic aluminum inclusions in the samples obtained with steam indicates that the oxidation process is not complete. This might be attributed to the fact that steam, being in the gas phase, does not have as intimate contact with the aluminum surface, as does liquid water. Additionally, the higher kinetic energy of steam might lead to less effective mass transfer to the aluminum surface, which can hinder the oxidation process. This, in turn, leads to a product that is a mixture of different phases with a somewhat heterogeneous structure due to varying grain parameters. The difference in grain parameters between the phases could have significant implications for the mechanical and chemical properties of the resultant material. For example, different grain sizes and structures can affect the material's strength, hardness, porosity, and chemical reactivity. These properties are crucial in applications such as catalysis, structural materials, and energy storage. A series of SEM images was also obtained to study the microstructure of the oxidation product. Figure 6 shows SEM images of boehmite samples obtained in oxidation experiments with 99.99% aluminum. From the analysis of the images, it follows that an increase in the conversion degree of aluminum (\(\eta\)) leads to the formation of macropores (with a pore diameter of more than 50 nm) on the surface of the sample where the conversion degree (\(\eta\)) is greater than 95% (on a scale of 50 \(\mu\)m, Figure 6). The surface of the particles (e), (f) and (g), (h), where \(\eta<70\%\), is denser, without visible macropores. This is clearly seen in the image of the sample (h) 5 \(\mu\)m, as the observed surface is monolithic without crystalline inclusions; the degree of transformation (\(\eta\)) of this sample is 24.38%. Figure 7 shows photos of boehmite samples obtained by the oxidation of aluminum with a chemical purity of 99.99%. The sample in Figure 7a is oxidized in a liquid with the conversion degree of 100.0% and exhibits a white color characteristic of boehmite. The sample in Figure 7b is oxidized in steam with the conversion degree of 96.73%, has a dark gray color, and is bound with the presence of unoxidized aluminum. Aluminum granules with a chemical purity of 99.9 and 99.7% after the oxidation experiments at 280 \({}^{\circ}\)C and 10 h, where the conversion degree was less than 2%, are shown in Figure 8. The oxidation process took place on the surface, without deep penetration of the chemical reaction into the granules. Figure 5: The results of X-ray analysis of boehmite obtained by the oxidation of aluminum with a chemical purity of 99.99% at 280 \({}^{\circ}\)C and 10 h: A—in liquid; B—in steam.