## 2 Flash Sintering
### The Application of FS in SOFCs
#### 2.2.4 SOFC Multilayers  
One of the key challenges in manufacturing SOFCs is the requirement for sintering processes that can achieve both densified electrolyte and porous electrode structures simultaneously [94; 95]. Conventional co-sintering techniques often need high sintering temperatures, which promote densification but also lead to grain coarsening of the electrode structure [96; 97]. Additionally, when co-sintering, the presence of stress mismatches across layers can result in deformation and delamination [98; 99; 100]. Therefore, there is a need for co-sintering methods that can function at reduced temperatures and shorter time periods, while simultaneously taking into account stress relief throughout the sintering process [101; 102]. FS is a highly efficient process used to simultaneously sinter multiple layers of materials in SOFCs. The study conducted by [PERSON] et al. [103] employed the FS technique to co-sinter multilayered materials, including an anode (a composite of YSZ and NiO) and an electrolyte (cubic zirconia). The results indicated that utilizing the FS technique at sintering temperatures lower than 1000 \({}^{\circ}\)C enabled the fabrication of samples that achieved electrolyte densification and anode porosity simultaneously. Furthermore, the FS technique effectively reduced and eliminated the cracks and delamination that are commonly caused by internal stresses during the conventional multilayer co-sintering process in SOFCs [104]. [PERSON] et al. [105] introduced a novel sintering technique, the DC-assisted sintering technique (DC-AST), for fabricating NiO-YSZ! YSZ! LSM cells, as depicted in Figure 7. Previous studies have shown that 8 YSZ can undergo fast sintering within seconds when exposed to a DC electric field of 150 V/cm at a furnace temperature of 750 \({}^{\circ}\)C [72]. However, when subjected to such elevated voltage and significant power usage, instantaneous shrinkage occurred, leading to uncontrollable collapse. The researchers postulated that the densification of the electrolyte film should occur gradually, making DC-assisted sintering techniques preferable. These techniques involve controlled, incremental, and moderate electrical currents. This approach has the benefit of preventing uncontrollable collapse. The article describes the effective fabrication of an anode with a suitable microstructure and a dense YSZ layer utilizing DC-assisted sintering at reduced temperatures. As shown in Figure 8, the grain size of DC-AST samples was only 440 nm. The constructed single cell demonstrated exceptional electrochemical performance, achieving maximum power densities of 0.8, 1.1, and 1.4 W/cm\({}^{2}\) at temperatures of 650 \({}^{\circ}\)C, 700 \({}^{\circ}\)C, and 750 \({}^{\circ}\)C, respectively. The performance achieved through DC-assisted sintering surpassed that of conventional sintering, highlighting the potential effectiveness of DC-assisted sintering in the production of SOFCs. The study conducted by [PERSON] et al. [103] showed that multilayered materials consisting of a NiO-ZrO\({}_{2}\) anode and a ZrO\({}_{2}\) electrolyte layer could be flash-sintered in a few seconds at temperatures below a furnace temperature of 1000 \({}^{\circ}\)C. Figure 9 demonstrates that, unlike conventional sintering, there was a strong and seamless bonding between the anode and electrolyte layers, with no evidence of delamination. Under the effect of the electric field, the electrolyte and anode were connected in parallel, and the FS temperature  
Figure 8: Grain size with (**a**) DC-assisted sintering and (**b**) conventional sintering. (Reproduced from [105] with permission from Elsevier). Figure 7: Illustration of a DC-assisted sintering device. The current radiates from the central part of the electrolyte to the edge. (Reproduced from [105] with permission from Elsevier). of the multilayered material closely approached that of the anode layer. Moreover, the anode layer was responsible for the overall linear shrinkage rate of the multilayer.