## 4 Current Progress and Other Applications
UHS enables the rapid synthesis and sintering of a variety of materials, with sintering temperatures up to 3000 \({}^{\circ}\)C achieved in an exceedingly short time frame. This feature is particularly beneficial when integrated with 3D printing technology, providing innovative approaches for creating complex geometries and developing new materials. For instance, UHS has been applied to 3D-printed 3 YSZ components, specifically spiral-shaped disk components with a diameter of 10.1 mm and a thickness of 3.1 mm, for both thermal de-binding and densification processes [46]. This process is performed in a single step following the initial chemical degreasing phase. Additionally, researchers have proposed a novel Powder medium-based UHS approach (P-UHS) for heating spiral-shaped, honeycomb, and heat exchanger tube structure alumina samples, which are cylindrical in shape and have dimensions of \(\Phi\) 9 mm \(\times\) 9 mm. In the P-UHS setup, the sample is completely embedded in the graphite powder bed, the two electrodes are inserted into the graphite powder, and a DC power supply is connected. Similar to graphite felt, the graphite powder used in P-UHS effectively serves as both a heater and a thermal insulator [140]. It is demonstrated that UHS is indeed suitable for heating additively manufactured porous components. Meanwhile, FS significantly reduces sintering time and temperatures by applying electrical fields and currents, and it improves material properties via the control of electrical parameters. Current efforts are focused on deepening the understanding of their underlying fundamental mechanisms, accelerating the research and development of new materials, enhancing energy efficiency, and merging with other cutting-edge technologies. For instance, contactless FS using cold plasma represents a scalable and energy-efficient sintering method [141], while Water-assisted Flash Sintering (WAFS) opens new pathways for low-temperature sintering [142]. Flash-SPS technology represents a densification method that achieves high heating rates under pressure, resulting in samples with superior mechanical and dielectric properties, including thinner grain boundaries, compared to those prepared by SPS [143]. Combining UHS with Reactive Cold Sintering (RCS) presents an effective method for producing highly dense and fine-grained MgAl2O4 ceramics [144]. UHS, despite its relatively nascent development, is already finding applications across various fields. It is currently utilized in the production of SOFC, solid electrolytes [145; 146], polymer-derived ceramic nanocomposites (PDC-NC) [147], thermal barrier coating materials [41; 148; 149], thermoelectric nanomaterials [150], metal and alloy materials [151], biomaterials [152], and quartz glass [153], etc. FS, with its broad applicability, extends beyond SOFCs and solid electrolytes [154] to include semiconductor materials [55], metallic-like conductors such as borides, nitrides, and carbides of transition metals [113], armor [155; 156], biomaterials [157; 158], the joining of dissimilar materials [159; 160], and nuclear fuel [161; 162; 163]. Notably, in the nuclear fuel sector, preliminary experiments have demonstrated the capability of FS technology to produce high-density nuclear fuels at significantly lower temperatures than those required for conventional sintering processes [161]. Although these two techniques have already had a considerable research foundation and have been used in the preparation of various materials, the sintering mechanisms of FS and UHS remain incompletely understood due to their relatively short period of development. Furthermore, the technology has not yet achieved mass production standards due to an immature process flow, high costs, and the need for improved product yield. In light of the ongoing worldwide environmental crisis, FS/UHS technology plays a significant role in promoting the development of SOFCs. However, additional research and investigations are necessary to meet practical requirements.