**2. Theoretical aspects of spark plasma sintering**

SPS is a novel effective sintering technique, which has been used in the research and devel‐ opment of various kinds of materials. However, there is no uniform understanding in the sintering mechanism of SPS. In general, SPS is a pressure sintering method based on the simultaneous application of axial pressure and high temperature plasma momentarily

generated in the gaps between powder materials by electrical discharge at the beginning of ON-OFF DC pulse energizing [13]. In addition to have the Joule heating due to the electric current and plastic deformation produced by pressure, SPS also generates DC pulse voltage between the powder particles, and effectively makes use of the spontaneous heat generated by the discharge between powder particles, thus resulting in some special characteristics. Compared to the conventional sintering, SPS has two important characteristics [14, 15]: (1) SPS process can make high-energy pulse focus on the grain junction point, thus saving the energy; (2) A high energy, low voltage spark pulse current momentarily generates spark plasma and produces a high localized temperature from several to ten thousand ◦C between the particles and then resulting in optimum thermal diffusion and grain boundary migration, i.e. more material transfer can be intensified and thus high density ceramics can be obtained through spark plasma sintering with a low sintering temperature and a short sintering period. Fig. 1 shows the schematic of SPS furnace [16].

**Figure 1.** Schematic of SPS furnace [16].
