## 2 Flash Sintering
### The Application of FS in SOFCs
#### 2.2.4 SOFC Multilayers
The FS process can be influenced by interactions between atoms and defects across different layers. Additionally, variations in the composition and structure of the multilayered material result in distinct electrical responses when subjected to an electric field. The FS behavior of multilayered materials cannot be simply predicted based on the behavior of individual layers. However, it is still highly effective in rapidly advancing the fabrication of SOFCs. One of the main obstacles in the development of SOFC applications is dealing with carbon deposition [106; 107]. The study conducted by [PERSON] et al. [108] utilized FS to fabricate anode-supported SOFC single cells with La\({}_{0.2}\)Sr\({}_{0.7}\)TiO\({}_{3}\) (LST), Ni and YSZ (LST-Ni/YSZ) gradient materials, which simultaneously possess coke resistance and high conductivity. As shown in Figure 10, the electrolyte layer, anode functional layer, cathode functional layer, and four anode support layers, A, B, C, and D (ASL-A, B, C, D), were securely fused together, preventing the deformation and delamination that is commonly experienced in conventional sintering methods. In contrast, with conventional sintering at a temperature of 1400 \({}^{\circ}\)C, the cell will deform toward the anode side because of the higher coefficient of thermal expansion of LST compared to NiO-YSZ. Prior to the electrolyte, the LST layer underwent densification, resulting in a porosity of less than 1%. Figure 10: SEM images of an LST-Ni/YSZ FGA-supported SOFC obtained by FS. (**a**) Cross-section of the anode and YSZ electrolyte, (**b**) magnification of the labeled part of (**a**). (Reproduced from [108] with permission from Elsevier). Figure 9: Micrographs of the cross sections of conventionally sintered and flash-sintered specimens of multilayers. (Reproduced from [103] with permission from John Wiley and Sons). [PERSON] et al. [109] utilized an AC electric field during the co-sintering process of a NiO-YSZ \(|\)YSZ \(|\)LSM single cell at a furnace temperature of 1200 \({}^{\circ}\)C. This led to a higher shrinkage rate of 26%, compared to the 15% achieved with conventional sintering. As shown in Figure 11, SEM images revealed that both the anode and cathode displayed a porous structure. The electrolyte is relatively dense, with some pores concentrated on the outer surface. FS resulted in a 63% reduction in the total electrical resistance of the single cell, as compared to conventional sintering. GDC and LSCF are considered the most favorable electrolyte and cathode materials for SOFCs operating at intermediate temperatures [91]. Nevertheless, LSCF exhibits a significantly higher conductivity compared to GDC, with a difference of several orders of magnitude [93]. In order to address this problem, [PERSON] et al. [110] employed FS to co-sinter LSCF/GDC bilayers. This research investigates the application of LSCF coatings of different lengths along the current direction, as shown in Figure 12. Specifically, only the LSCF in the GDC-LSCF-17 sample was electrically connected to the power source, while the LSCF in the other samples was detached from the power supply. The findings indicated that FS yielded tightly fused, dense GDC layers and porous LSCF layers, with a GDC density reaching 92.86% and an LSCF porosity of 52.26% observed in the GDC-LSCF-5 sample. The LSCF/GDC interface exhibited superior bonding and showed no signs of inadequate element diffusion or transverse cracks during the sintering process, in contrast to conventional sintering. This work successfully co-sintered multilayer ceramic materials with distinct conductivities using flash sintering. This approach offers a potential solution to address the issue of inconsistent sintering temperatures across different layers in multilayered materials for SOFCs. Figure 11: SEM image of a cross-section of a planar unitary three-layer cell after FS with 310 V/cm, 1 kHz, 1 A, for 5 min at 1200 \({}^{\circ}\)C. (Reproduced from [109] with permission from John Wiley and Sons).