# The Isothermal Oxidation of High-Purity Aluminum at High Temperature
## 3 Results and Discussion
### Scanning Electron Microscopy  
Samples of aluminum which had been oxidized at temperatures between 750 \({}^{\circ}\)C and 1550 \({}^{\circ}\)C were potted in epoxy resin and sectioned with a diamond saw to reveal cross-sections of the specimens. These were then imaged in a scanning electron microscope. Representative images depicting the alumina shell that formed upon oxidation for the various specimens are shown in Figure 6.  
Figure 4: TGA results foraluminum heated to 1350 \({}^{\circ}\)C or higher under argon, held at that temperature for 15 min, then exposed to air. The vertical dashed lines tie the respective TGA curves to the point on the temperature curve (dotted line) where the isothermal hold began. In this plot, the beginning of each experiment is taken as \(t=0\). The point at which air flow began is recognized by the sudden mass increase in each curve. Figure 5: Rates of mass loss duringaluminum evaporation taken from Figure 4 compared to the published vapor pressure of aluminum at similar temperatures. Thealuminum vapor pressure data was taken from [[http://www.knowledgedoor.com/2/elements_handbook/vapor_pressure.html](http://www.knowledgedoor.com/2/elements_handbook/vapor_pressure.html)]([http://www.knowledgedoor.com/2/elements_handbook/vapor_pressure.html](http://www.knowledgedoor.com/2/elements_handbook/vapor_pressure.html)) (accessed on 28 April 2017). The non-linear variation with oxidation temperature of alumina shell thickness is apparent, as is the change in shell morphology with temperature. Based upon the images, an estimate was made of the thickness of the alumina shell for each specimen, and the results are plotted in Figure 7. Additionally, in Figure 7 are the total mass increases upon oxidation recorded by TGA for the various specimens. The qualitative agreement between the two sets of data is quite good, despite the large uncertainties associated with estimating the thickness of such irregular shell structures. Figure 6: SEM images of cross sections of aluminum specimens after oxidation in air at the indicated temperature (\({}^{\circ}\)C). Note that all images are shown at the same magnification (scale bar = 5 \(\upmu\)m), except for that of the specimen oxidized at 1550 \({}^{\circ}\)C (scale bar = 20 \(\upmu\)m). Figure 7: Thicknesses of the alumina shells estimated from the images shown in Figure 6 (filled triangles), and total mass gain during oxidation for each specimen recorded by TGA (open circles). Arbitrary scaling has been applied to both y-axes to visually align the two data sets. Despite the fact that all of the specimens shown in Figure 6 were oxidized at a temperature above the melting point of aluminum, only those heated to 1050 \({}^{\circ}\)C or higher formed into spheroidal droplets. The specimens oxidized at 950 \({}^{\circ}\)C or below each retained the overall shape of the initial aluminum sheet, presumably due to the stabilizing nature of the native oxide layer which resists fragmentation at the lower temperatures. These samples came out of the TGA/DSC the same shape as they were beforehand (flat sheet), whereas the higher temperature treated samples all converted to spheroidal particles At the higher temperatures reorganization of the alumina shell does occur; this may be a function of increasing flexibility of the alumina with temperature, expansion of the molten aluminum core, increased vapor pressure of aluminum causing fragmentation of the shell, and/or recrystallization of the alumina at higher temperatures allowing facile restructuring of the shell architecture. Additionally, the morphology of the alumina shell changes phase within the temperature range investigated in this study. Between 700 \({}^{\circ}\)C and 1050 \({}^{\circ}\)C, the morphology of alumina exists in the polycrystalline \(\theta\) phase with a Face Centered Cubic (FCC) lattice structure. At temperatures above 1050 \({}^{\circ}\)C, alumina forms the \(\alpha\) phase and takes a Hexagonal Close Packed (HCP) lattice structure.