The SPS process is dynamic non-equilibrium processing whose phenomenon varies from early-stage, mid-stage to late-stage of sintering mechanism, along with reacted material characteristics. Although long years of research work concerning SPS mechanism, by many material researchers, the SPS effect, in other words, the effect of pulsed high current on the generation of spark plasma, peculiar properties in consolidated materials remains unclear [ 10 30 ]. There are two major theories on SPS mechanism in discussion. One is with effect of spark and plasma at initial stage, the other is no appearance of spark and plasma during entire SPS process. And, in both theories, it should be noted that metals and ceramics materials show different phenomena. However, based on the theory of with spark example, following is one of the most basic proposed ideas as a mechanism of SPS processing. The ON-OFF DC pulse energizing method generates; (1) spark plasma, (2) spark impact pressure, (3) Joule heating, and (4) an electrical field diffusion effect. In the SPS process, the powder particle surfaces are more easily purified and activated than in conventional electrical sintering processes, and material transfers at both the micro and macro levels are promoted, so a high-quality sintered compact is obtained at a lower temperature and in a shorter time than with conventional processes. Figure 8 illustrates a typical ON-OFF DC current path and how pulse current flows through powder particles inside the SPS sintering using conductive powder material, die, and punch assembly made of graphite material. Conventional electrical hot press processes use DC or commercial AC power, and the main factors promoting sintering in these processes are the Joule heat generated by the power supply (IR) or high-frequency induction heating elements and the plastic flow of materials caused by the hydraulically or mechanically driving pressure. The SPS process is an electrical sintering technique that applies an ON-OFF DC pulse voltage and high current from a special pulse generator to a powder of particles, and in addition to the factors prompting sintering described above, also effectively discharges between particles of powder to powder and/or graphite die wall surface occurring at the initial stage of the pulse energizing for sintering. When sparking occurred, even one or two sparks, a high-temperature field with sputtering phenomenon generated by spark plasma and spark impact pressure occurs. They work to eliminate adsorptive gases and oxide films and impurities existing on the surface of the powder particles. The action of the electro-magnetic field enhances high-speed diffusion due to the electro-migration effects of ions. The application of the pulse voltage induces various phenomena as shown in Figure 9 in terms of the effects of the ON-OFF DC pulse energizing method. In the case without sparking due to non-conductive material used or the pulse energizing conditions, it is considered that there is still the effect of ON-OFF DC pulse current energizing which provides enhancement of sinterability and densification rate on the material. The large pulsed energy generates an electro-magnetic field-effect such as an electro-migration and preferential orientation of crystalline [ 31 32 ].
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