During the last two decades, nanoma

During the last two decades, nanomaterials have aroused worldwide attention owing to their great scientific interest and gradual applications in industrial fields.1−7 A wide variety of applications in a broad range of fields has been realized, for example, high strength-ductility metals, high-efficiency catalysts, specific drug targeting and delivery, and low-temperature ductile ceramics.8−12 Specifically for applications in the field of ceramic science, it has been widely observed that utilizing nanoparticles, instead of coarse particles (generally around micrometer size), as starting material could reasonably reduce the sintering temperature,since the high surface-to-volume ratio of nanoparticles provides a strong driving force to promote the diffusion process, especially at elevated temperatures.13 However, such a sintering temperature reduction is limited to a reasonable range, and high temperature, generally >1000 °C, is still required to obtain dense ceramics. For example, in the case of conventional thermal sintering of BaTiO3, effective consolidation is generally accomplished at temperatures as high as 1300−1400 °C if coarse particles are adopted as starting powders;14 even though nanoparticles are frequently employed as raw powders, and the sintering process is assisted by advanced techniques, densesolids are normally achieved in the temperatures range 1000−1300 °C (Figure 1a).14−24 As is known, the densification and coarsening processes take place simultaneously and competewith each other during the sintering process, and a hightemperature process often yields a microstructure with overcoarsening grains, which is deleterious for the densification.25 Therefore, densification of ceramics has been one of the major subjects in materials science and the engineering community for the past several decades, and intense research around the world has led to a rapid development of numerous sintering techniques, such as two-step sintering (TSS), flashsintering (FS), high-pressure sintering (HPS), spark plasma sintering (SPS), microwave sintering (MVS), rate-controlled sintering (RCS), or a combination of some of these.22,26−30