## 2 Flash Sintering
### The Application of FS in SOFCs  
#### 2.2.1 Electrolyte  
In conventional sintering, electrolytes need to be exposed to high temperatures and kept for several hours to achieve a microstructure with closed pores [17]. These processes are not only costly in terms of energy and equipment expenses but also result in negative reactions with other components of the SOFC system [62]. Since [PERSON]'s first report on the FS phenomenon of 3 YSZ, where a DC field is applied for a few seconds at a furnace temperature of 850 \({}^{\circ}\)C, FS technology has been widely used for sintering SOFC electrolytes such as YSZ, gadolinium-doped ceria (GDC), BaZr\({}_{0.1}\)Ce\({}_{0.7}\)Y\({}_{0.1}\)Y\({}_{0.1}\)O\({}_{3}\) (BZCYYb), and La\({}_{0.8}\)Sr\({}_{0.2}\)Ga\({}_{0.8}\)Mg\({}_{0.2}\)O\({}_{3}\) (LSGM) [63; 64; 65; 66; 67; 68; 69; 70; 71]. [PERSON] et al. [72] conducted a comprehensive study on the FS of 8 YSZ, which has better conductivity compared to 3 YSZ. The study demonstrated that the closed-porosity microstructure of 8 YSZ could be achieved using FS at a reduced furnace temperature of 750 \({}^{\circ}\)C, which is 100 \({}^{\circ}\)C lower than the sintering temperature required for 3 YSZ. The FS process for 8 YSZ material was initiated at an electric field intensity of 30 V/cm, whereas 3 YSZ required 60 V/cm. FS, with its low temperature and shortened sintering time, is advantageous in the production of SOFCs. It not only cuts costs but also mitigates the negative chemical interactions between zirconia and other constituents. In addition to the rapid densification achieved through FS, researchers are also concerned about its impact on dielectric properties. [PERSON] et al. [73] compared the dielectric properties of flash-sintered and conventionally sintered 3 YSZ samples. An electric field of 55 V/cm was applied at 900 \({}^{\circ}\)C. Once the current density reached 125 mA/mm\({}^{2}\), the process switched to current control, which was maintained for 20-60 s. The results indicated that FS had only a slight impact on the sample's conductivity and dielectric constant, but it significantly affected its grain boundary properties. Specifically, FS led to a 30% reduction in grain boundary thickness compared to conventionally sintered samples and increased the concentration of defects near these interfaces. Based on these findings, the authors proposed the following mechanism: a large number of defects in the space charge region nucleated with the assistance of the electric field, which increased grain boundary conductivity and promoted the sintering process. Consequently, the authors concluded that the impact of FS on the dielectric properties of materials is primarily reflected in the reduction of grain boundary semi-blocking properties. Based on the previous research, [PERSON] et al. [74] compared the effects of varying electric field strengths on the electrical properties of flash-sintered 3 YSZ samples by measuring complex impedance spectra. The results, similar to those of conventional sintering, showed that FS leads to a decrease in grain boundary thickness. However, as electric field intensity increases, both the thickness and specific conductivity of grain boundaries increase. Specifically, when the current density is 110 mA/mm\({}^{2}\) and the applied electric field is increased to 300 V/cm, the grain boundary thickness of the flash-sintered sample approaches that of the conventionally sintered sample. Additionally, experiments revealed an increase in the concentration of oxygen vacancies within the grains. Consequently, the authors proposed an alternative explanation based on the space charge layer model. The thickness of this layer depends not only on the concentration of oxygen vacancies within the layer but also on the concentration of oxygen vacancies in the grains. Assuming that the distribution of oxygen vacancy concentration remains constant and is independent of the concentration level, the thickness of the layer is proportional to the difference in concentration between the grain and the layer.