## 2 Flash Sintering
### The Application of FS in SOFCs
#### 2.2.1 Electrolyte
In the work conducted by [PERSON] et al. [75], 8 YSZ samples were fabricated using current-restricted flash sintering (when the current reaches the set mark, the power supply switches from the voltage control stage to the current control stage). SEM images revealed that the microstructure exhibited a high level of density and a smaller grain size. Figure 3 displays the conductivity obtained from EIS analysis of the samples that were sintered at different isothermal furnace temperatures while being subjected to electric fields of 100 and 300 V/cm. Out of the samples, the one that was flash-sintered at a current density of 67 mA/mm\({}^{2}\), under an electric field of 300 V/cm and a furnace temperature of 565 \({}^{\circ}\)C, showed a conductivity of 0.056 S/cm at 850 \({}^{\circ}\)C.  
Aside from furnace temperature, the properties of flash-sintered materials are also affected by the intensity of the electric field. [PERSON] et al. [76] conducted a study on isothermal FS of 8 YSZ at various electric field strengths ranging from 50-350 V/cm, with a maximum current density of 200 mA/mm\({}^{2}\). They observed that the density of flash-sintered 8 YSZ was not influenced by an electric field. Nevertheless, when subjected to an electric field strength of 350 V/cm, the crystal structure of 8 YSZ underwent a phase change from cubic to tetragonal. FS improved the conductivity of 8 YSZ in comparison to CS. The elevated concentration of oxygen vacancies in the flash-sintered samples has enhanced the conductivity at the grain boundaries. The conductivity of grain boundaries is determined by the space charge layer. The rapid increase in the concentration of oxygen vacancies in the space charge layer of flash-sintered samples enhances the conductivity of grain boundaries. In addition, the authors also listed another possible explanation that the formation of Frenkel defects reduces the activation energy required for grain conductivity. When subjected to an electric field of 80 V/cm, flash-sintered 8 YSZ demonstrated a conductivity of 0.084 S/cm at a furnace temperature of 800 \({}^{\circ}\)C, while the conductivity of conventionally  
Figure 3: Arrhenius plot of ionic conductivities of the current-limiting flash-sintered 8 YSZ samples (**a**) under the electric field of 100 V/cm, (**b**) under the electric field of 300 V/cm. (Reproduced from [75] with permission from Elsevier). sintered samples was only 0.067 S/cm. The level of conductivity is suitable for its use as an SOFC electrolyte. [PERSON] et al. [77] have shown that strip-shaped 20 GDC specimens, which were compacted with a pressure of 470 Mpa, can be fully densified in just a few seconds at 545 \({}^{\circ}\)C using FS. They achieved this with an initial electric field strength of 20 V/cm, which is lower than the electric field strengths required for YSZ (30 V/cm for 8 YSZ and 60 V/cm for 3 YSZ) [72; 78]. The power dissipation is 10 mW/mm\({}^{3}\), which is the same as YSZ. As shown in Figure 4, as the magnitude of the external electric field increases, the grain size of GDC diminishes until it approximates the initial grain size of the raw material. The use of fine-grained GDC can enhance its mechanical performance as an SOFC electrolyte. The densification of electrolytes at low temperatures creates new possibilities and prospects for the advancement of nano-structured SOFC anode. [PERSON] et al. [79] applied FS to 10 GDC tapes prepared by tape-casting and studied the influence of sintering parameters on the microstructure of the samples. The results showed that the initial electric field value required for FS of 10 GDC is 100 V/cm, significantly higher than the critical electric field of 20 GDC, reported as only 20 V/cm in previous studies. The authors hypothesized that the reason for this discrepancy may be the increased vacancy concentration, which reduces the critical electric field needed for FS to occur. Moreover, while the applied electric field influences the temperature at which FS takes place, it does not affect the densified structure of the sample.