The microstructures of the samples

The microstructures of the samples cold‐sintered at 280 and 350℃ for 1 hour after postannealing were studied, and the corresponding FE‐SEM images are shown in Figure 11A‐D. The microstructure of the cold‐sintered sample at 280℃ exhibits that the particles were well rearranged and are in contact with each other with an average grain size of 310 nm. On the other hand, the microstructure of the sample cold‐sintered at 350℃ indicates that the microstructure consists of well‐defined grains and grain boundaries. The average grain size for this sample is about 700 nm. In addition, FE‐SEM images suggest that a higher level of densification has been achieved for the sample cold‐sintered at 350℃ compared to the one at 280℃.