## 3 Uhs
### The Application of UHS in SOFCs  
In recent years, researchers have started to investigate the utilization of UHS in the field of SOFCs. However, as this new technique was just invented recently, the majority of ongoing research is predominantly focused on the sintering of electrolyte materials. [PERSON] et al. [44] proposed that the carbon felt showed significant heat dissipation during the UHS heating process, and the measured temperature on the carbon felt surface was 200-250 \({}^{\circ}\)C lower than the actual temperature of the sample. This resulted in significant heat  
Figure 13: Pictures of various ceramics sintered by the UHS technique in \(\sim\)10 s. (Reproduced from [43] with permission from the American Association for the Advancement of Science). loss and impeded the densification of the sample. [PERSON] et al. [127] addressed this problem by introducing a Thermally-insulated UHS (TI-UHS) process, as shown in Figure 14. This approach employed fiberboard insulation to limit the dissipation of heat in the carbon felt during the UHS sintering process. The TI-UHS method successfully produced a 99% dense and fine-grained 3 YSZ within 60 s at 35 A, resulting in a 40% reduction in energy consumption compared to the UHS method. [PERSON] et al. [128] synthesized 8 YSZ by UHS and conducted a comparison with CS and FS. The findings demonstrated that UHS, as compared to CS, could attain a greater density of 8 YSZ at the same sintering temperature and in a shorter duration. Additionally, the apparent activation energy for the densification of 8 YSZ under UHS was lower than that under CS and FS. [PERSON] et al. [129] presented a new technique called UH-SPS, which combines the UHS and SPS techniques. This method utilizes tungsten wire heating instead of carbon felt. Translucent YSZ ceramics were effectively sintered utilizing UH-SPS, without experiencing problems such as blackening or carbon thermal reduction, which are typically observed when processing samples at temperatures above 1800 \({}^{\circ}\)C with carbon felt. [PERSON] et al. [130] utilized UHS to sinter 3 YSZ nano-powder and prepared conventional sintered samples for comparison. Compared with conventionally sintered samples, 3 YSZ processed by UHS exhibited a fine-grained structure, with the grain size reduced by more than 60% at an equivalent density. However, the grain boundary electrochemical response and the hardness of samples were found to be similar between UHS and conventionally sintered samples. The results indicate that UHS can refine the microstructure of the sintered body to a certain extent, yet it does not impact the hardness and grain boundary electrochemical properties. In addition, in order to examine the influence of static electric fields on YSZ sintering, the authors conducted conventional sintering both with and without the application of an electric field (where no current passed through the sample). The findings revealed that the electric field had only a minor effect on the microstructure and properties during sintering. Figure 14: The schematic of **(a)** UHS setup and **(b)** TI-UHS setup. (Reproduced from [127] with permission from Elsevier). [PERSON] et al. [102] made an attempt to sinter GDC10 ceramics using UHS technology for the first time. Results revealed that modifying the heating profile of UHS to a multi-step gradient heating technique facilitated the efficient fabrication of GDC without any cracks or defects. The density achieved a level of 95%. In addition, a half-cell was prepared using GDC10 as the electrolyte and upper and lower LSF cathodes. The sintered electrolyte exhibited a high level of density, and no surface fractures were detected. Figure 15 illustrates a robust bonding between the electrode and the electrolyte interface with no evidence of delamination, indicating a significant potential of UHS for co-sintering multilayered materials in SOFCs. Besides the electrolyte, there is also research showing the potential application of UHS in other materials for SOFCs. As shown in Figure 16, [PERSON] et al.