## 3 Results and Discussion
In the HT-1 sample, the surface layer shown in Figure 2 (reaction layer + oxide layer) was intact with a sharp edge line, while a rough surface layer was observed in the HT-3 sample. In a high-magnification  
Figure 4: Thermal profile of the sintering process in the head-end of pyroprocessing. Figure 1.0 SEM and EDS results on the reaction layer of the HT-3 sample after oxidation. observation of the HT-3 sample, the outermost surface layer appears to have peeled off partially or mostly (indicated by an arrow in Figure 5b). However, the interfaces between the substrates and reaction layers were maintained as straight lines in all samples, suggesting that the thermal treatment hardly gives rise to additional oxygen diffusion into the substrates. In other words, oxidative attack during ramping was negligible in the Hf\({}_{6}\)Ta\({}_{2}\)O\({}_{17}\) superstructure sample. To clarify the cause of the different peeling behaviors among the samples with different Ta contents, the chemical composition was observed by SEM/EDS at the samples' surface regions. Figure 6 shows the SEM/EDS line scan results on the cross-sectional surfaces of the HT-1 and HT-3 samples after the thermal cycle. The average atomic ratios of the constituent elements were measured as 18.6 Hf-8.2 Ta-74.2O for the HT-1 sample and as 27.2 Hf-8.1 Ta-64.7O for the HT-3 sample in the vicinity of the surface. However, the measured region was different depending on whether or not the sample surface was conserved after the thermal cycle. The chemical composition could be examined at the outermost oxide layer only in the HT-1 sample with no severe peeling. In contrast, the EDS results for the HT-3 sample show the chemical composition near the interface between the substrate and the reaction layer. According to the position, the atomic ratios in the HT-1 and HT-3 samples are similar to those at Position 5 and at Position 2 in the EDS results shown in Figure 1.1 Therefore, no evidence for the cause of the surface peeling behavior was drawn from the EDS measurements. Meanwhile, the atomic ratio of Ta slightly increased in both samples compared to that in the sample before the thermal cycle. This is attributed to Ta diffusion from the substrate to the surface region during the thermal cycle at a high temperature of \(\sim\)1450 \({}^{\circ}\)C. The Ta diffusion would be more dynamic in the HT-3 sample than in the HT-1 sample because of the higher Ta content in the substrate region. Nevertheless, the outward diffusion of Ta and Hf may be insignificant in both samples considering the slight compositional changes in the samples before and after the thermal cycle. This secures chemical integrity, which is a benefit in the practical use of the material in a sintering crucible. Figure 1.2 SEM micrographs of the (**a**) HT-1 and (**b**) HT-3 samples after heat treatment in a 4% H\({}_{2}\)-Ar atmosphere. Figure 7 shows the XRD pattern of the residual powder collected from the HT-3 sample after the thermal cycle. The inset micrographs show changes in the appearance of the samples. Note that only a small amount of residual powder was formed from the HT-1 sample with little change of color. On the other hand, a larger amount of residual powder was observed in the HT-2 and HT-3 samples with significant color changes of the sample surface. In contrast to the HT-2 sample with partial color change, the HT-3 sample color was uniformly white. This color change behavior may be evidence of phase transformation from the original Hf\({}_{6}\)Ta\({}_{2}\)O\({}_{17}\) phase to other phases of brighter color. To examine the constituent phases, the residual powder was collected from the HT-3 sample and analyzed by XRD, as shown in Figure 1.3 The measured pattern reveals that the residual powder contained Hf\({}_{6}\)Ta\({}_{2}\)O\({}_{17}\) as a major constituent; that is, the superstructure still remained after the thermal cycle at 1450 \({}^{\circ}\)C. However, Hf and Ta were observed as separate phases distinguished by other peaks.