## 2 Flash Sintering
### The Application of FS in SOFCs
#### 2.2.5 Interconnect Coating  
Interconnectors are vital elements that facilitate the passage of electric current between the cathodes and anodes of neighboring cells in SOFCs. In order to inhibit the corrosion of metal interconnects in both oxidizing and reducing conditions, ceramic coatings are often applied as protective layers [111]. Manganese cobalt oxide (Co\({}_{2}\)MnO\({}_{4}\)) exhibits a conductivity of 60 S/cm at 800 \({}^{\circ}\)C. This high conductivity makes it a highly suitable option for applying as a coating on interconnectors in SOFCs [112]. Conventional sintering processes usually require extended heating at a temperature of 1300 \({}^{\circ}\)C for several hours in order to achieve complete densification of Co\({}_{2}\)MnO\({}_{4}\). However, [PERSON] et al. [113] successfully accomplished FS of Co\({}_{2}\)MnO\({}_{4}\) within seconds at a reduced furnace temperature of 325 \({}^{\circ}\)C by applying a DC field of 12.5 V/cm. To investigate the FS behavior of MnCo\({}_{2}\)O\({}_{4}\) spinel, [PERSON] et al. [114] conducted a study indicating that MnCo\({}_{2}\)O\({}_{4}\) can be rapidly sintered at temperatures between a furnace temperature of 120 \({}^{\circ}\)C and 150 \({}^{\circ}\)C using a voltage of 15.0 to 17.5 V/cm. This temperature range is significantly lower than the CS temperatures of over 1080 \({}^{\circ}\)C. Considering the microstructure, phase structure, and electrical conductivity, the author proposed that the mechanism of flash sintering in MnCo\({}_{2}\)O\({}_{4}\) spinel is the natural ion rearrangement that occurs during flash accompanied by an increase in electrical conductivity. The authors have also elaborated on the local temperature range required for the sintering densification of the sample. This information is vital for the practical use of protective coatings in metal interconnects for SOFCs.  
Compared to conventional sintering methods, FS offers numerous advantages in SOFC applications:  
1. It accelerates the sintering process and lowers the sintering temperature, enabling ceramics to be processed together with metals or other materials;
2. It suppresses grain growth and satisfies the required mechanical properties;
3. It is a more straightforward and cost-effective alternative to conventional sintering.  
However, the occurrence of hotspots and temperature gradients during the FS process poses a considerable challenge in attaining a consistent microstructure, hence impeding its industrial advancement to a great extent [115]. Consequently, FS has mostly been employed on small, rod-shaped samples in laboratory settings. It has been shown to be challenging to sinter larger samples with FS. In order to tackle the problem of uneven sintering, researchers have recently put forward other methods, such as incorporating small quantities of conductive phase, which have low activation energy, into the sample to increase the conductivity [116]. This helps to minimize temperature differences between the center and surface of the sample. Additional techniques include non-contact FS to avoid the need for direct contact between the electrode and sample, as well as multi-step current-limiting FS to regulate the flow of current and prevent the occurrence of localized high temperatures [117; 118; 119].  
Figure 12: The setup of FS with different samples: GDC, GDC-LSCF-17, GDC-LSCF-10, and GDC-LSCF-5. (Reproduced from [110] with permission from Elsevier).