## 3 Results and Discussion  
### Surface Morphology Examination  
The thermal oxidation of the samples in the different oxidizing gas mixtures generated various oxide surface morphologies on the C84400 substrate. These different morphologies are as shown in Figure 2.  
From the observed morphologies, the shapes of the various oxides formed were irregular in nature, as the nucleation and growth process were affected by the thermal oxidation conditions. Long, fluffy oxide granules were formed after 1 h of thermal oxidation in the N\({}_{2}\)-0.75 wt.% O\({}_{2}\) gas mixture. At 2 h of thermal oxidation, the morphology of the oxidized surface layer showed flowery oxide features interspersed with spherically-shaped oxide granules. This morphology can be understood, since low  
Figure 2: Scanning electron microscope (SEM) images of thermally oxidized C84400 in different oxidizing atmospheres: (**a**,**b**,**c**) 1, 2, 3 h in N\({}_{2}\)-0.75 wt.% O\({}_{2}\); and (**d**,**e**,**f**) 1, 2 and 3 h in N\({}_{2}\)-5 wt.% O\({}_{2}\) gas mixtures, respectively, at 650 \({}^{\circ}\)C.  
oxygen partial pressure to a certain range is expected to decrease the oxide nucleation rate. Under such conditions and with fewer nucleation sites, due to the low amount of oxygen, the oxides grew laterally into these flowery forms. The measured width of the flowery oxides formed was an average of \(85\pm 9\) nm in thickness. However, more defined oxide granules were formed after the 3 h of the thermal oxidation process, with an average oxide granule size of \(110\pm 40\) nm. An appraisal of the chemical composition of these flowery oxides formed after 2 h of thermal oxidation in the N\({}_{2}\)-0.75 wt.% O\({}_{2}\) gas mixture by energy dispersive X-ray analysis showed that they consisted of zinc, lead, copper and oxygen in the weight percentage of 34.75 wt.%, 33.38 wt.%, 6.40 wt.% and 11.78 wt.%, respectively. This chemical composition showed that these flowery oxides were essentially formed from merged thin oxide flakes of zinc, copper and lead. The EDX spot analysis is shown in Figure 3.  
In the N\({}_{2}\)-5 wt.% O\({}_{2}\) gas mixture, the oxide granules on the surfaces of the samples were more consistent in form and size. The increase in the weight percentage of oxygen caused an increase in the number of nucleation sites, which resulted in a more regular and consistent morphology. No flowery oxide morphology was formed on the samples oxidized in the gas mixture with five weight percent of oxygen. The sizes of the oxide granules formed were consistent and comparable in the range of \(70\pm 22\) nm to \(130\pm 15\) nm, with the oxide granules progressively becoming smaller as the time for the thermal oxidation increased. The sizes of these oxide granules formed in the N\({}_{2}\)-O\({}_{2}\) gas mixtures are stated in Table 1. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multicolumn{1}{c}{**N\({}_{2}\)-O\({}_{2}\) gas**} & **Time** & **Average granule** & **Fractional surface** & **Estimated pore** & **Calculated** \\
**mixture** & **(h)** & **size, \(\mathbf{a}\) (nm)** & **area of oxide, \(f_{\infty}\)** & **size, D (\(\mathbf{\mu}\)m)** & **porosity, \(\varphi\)** \\ \hline N\({}_{2}\) 0.75 wt.\% & 1 & 750 \(\pm\) 50 & 0.74 & 0.19 \(\pm\) 0.05 & 0.06 \\ O\({}_{2}\) & 2 & 130 \(\pm\) 15 & 0.67 & - & - \\ O\({}_{2}\) & 3 & 110 \(\pm\) 40 & 0.58 & 0.09 \(\pm\) 0.03 & 0.27 \\  & 1 & 134 \(\pm\) 45 & 0.61 & 0.517 \(\pm\) 0.13 & 0.49 \\ N\({}_{2}\) 5 wt.\% O\({}_{2}\) & 2 & 101 \(\pm\) 43 & 0.32 & 0.587 \(\pm\) 0.10 & 0.58 \\  & 3 & 70 \(\pm\) 14 & 0.11 & 0.509 \(\pm\) 0.23 & 0.64 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The average oxide granule size, \(\alpha\); the fractional surface area, \(f_{\infty}\); the estimated pore size, \(D\); and the calculated oxide layer porosity, \(\varphi\).