## 2 Flash Sintering
### The Application of FS in SOFCs
#### 2.2.1 Electrolyte
As the current density increases, so does the densification of the sample. For instance, under a constant electric field of 100 V/cm and with a current density of 220 mA/mm\({}^{2}\) applied, a sample with 90% density can be achieved. [PERSON] et al. [80] conducted FS experiments on 10 GDC under different atmospheres, namely air and Ar/5%H\({}_{2}\). The results indicated that FS altered the electrical properties of GDC; the process activated n-type electrical conductivity associated with partial electrochemical reduction in GDC, and these changes were found to be partially retained after FS. Consequently, the authors proposed that the partial electrochemical reduction of the oxide, together with Joule heating, constitutes the triggering mechanism for the FS of GDC. The application of SOFCs is limited by their operation at high temperatures, which highlights the importance of developing intermediate-temperature SOFCs as a significant trend [17; 81]. In recent years, proton-conducting SOFCs (H\({}^{+}\)-SOFCs) have received significant attention for their ability to achieve high efficiency, high theoretical voltage, and low activation energy for conduction at medium to low operating temperatures [82; 83]. BZCYYb, a potential electrolyte material for SOFCs, is known for its high proton conductivity and stability. However, achieving a relative density of 96% requires conventional sintering at 1550 \({}^{\circ}\)C for 15 h [71]. In a report by [PERSON] et al. [70], an improved DC sintering technique was proposed and applied to BZCYYb sintering, which involves limiting the current density to a certain value to adjust the sintering rate and extending the sintering time from seconds to an hour, as shown in Figure 5. When the electric field is 70 V/cm and the current density is 70 mA/mm\({}^{2}\), this technique allowed for sintering at a lower furnace temperature of 850 \({}^{\circ}\)C for only one hour, resulting in full densification. Additionally, by controlling the current density and holding time, samples with the desired grain size could be obtained. LSGM, although a promising electrolyte material for intermediate-temperature SOFCs, faces limitations in its sinterability, which prevents its wider application [84; 85]. [PERSON] et al. [69] employed current-restricted flash sintering to enhance the densification of LSGM, which restricts the electric current within a specific range and prolongs the duration of FS from a few seconds to several minutes or even hours. This process design mitigates concerns such as abrupt shrinkage, deformation, cracking, and other complications caused by current avalanches during flash sintering. At a current density of 90 mA/mm\({}^{2}\) and an electric field strength of 100 V/cm, LSGM underwent FS at a furnace temperature of 690 \({}^{\circ}\)C, achieving a relative density of 97.4%. The material obtained at a furnace temperature of 800 \({}^{\circ}\)C had a conductivity of 0.049 S/cm, similar to that achieved through conventional sintering at 1400 \({}^{\circ}\)C. However, even when switched from voltage control to current control,sudden shrinkage still occurred and the power density increased significantly. Building upon the foundation of current-restricted flash sintering, [PERSON] et al. (2018) proposed a stepwise current-limiting flash sintering technique (SCFS), which was successfully applied to the densification of LSGM. As illustrated in Figure 6, the current was initially set at 0.05 A and maintained for a duration of 100 s. Subsequently, it was increased to 0.1 A for a further 100 s, and this pattern continued until it reached the uppermost current limit of 1.3 A (the corresponding current density is 173 mA/mm\({}^{2}\)), which was maintained for 10 or 30 min. The findings demonstrated that LSGM samples, which were fabricated using a multi-step current-limiting FS technique, displayed enhanced density and a more homogeneous microstructure. Furthermore, their electrical conductivity was about equal to that of their conventionally sintered counterparts.