# Exploring Surface-Driven Mechanisms for Low-Temperature Sintering of Nanoscale Copper
## 3 Results and Discussion
### Sintering of Multiple Copper Nanoparticles
#### 3.1.1 Energy Changing During Sintering
However, once the sintering temperature reaches 600 K or higher, the energy difference before and after sintering increases to approximately three times that observed at temperatures below 500 K, and this value stabilizes. The sintering process is characterized by a reduction in surface energy, where particles come into contact with each other, and through structural evolution, the surface area is minimized to lower the surface energy. At lower sintering temperatures, the energy of the atoms within the nanoparticles is insufficient to allow the sintering system to adjust further after initial contact. When the sintering temperature reaches 600 K, rapid dynamic motion of atoms occurs on the surfaces of the nanoparticles in the nanofiller, enabling the system to adjust further. This allows the sintering system at 600 K to achieve effects comparable to those at 900-1000 K, with nearly identical levels of energy reduction before and after sintering. Figure 3b is the morphology and crystal phase distribution of the sintered samples obtained at sintering temperatures of 300 K, 600 K, and 900 K. At 300 K, the nanoparticles in the system retain their initial contact morphology even after sintering is complete. The pores between these particles have not closed or evolved, and the system is filled with numerous dislocations and slip planes. However, when the temperature reaches 600 K, most of the voids in the system are closed, leaving only a few small pores. This corresponds to the energy difference at a sintering temperature of 600 K, shown in Figure 3a, where the surfaces in the system have nearly been eliminated, resulting in a significant decrease in surface area and surface energy before and after sintering. By the time the sintering temperature reaches 900 K, there are almost no voids left in the system, with only a few slip planes and dislocations remaining. From an energy perspective, samples sintered at 600 K should exhibit a comparable quality level to those sintered at 900-1000 K. However, as illustrated in Figure 3b, the  
Figure 3: (**a**) The difference in total energy of the system before and after sintering at different sintering temperatures. (**b**) The morphology and crystal phase distribution of the sintered samples were obtained at sintering temperatures of 300 K, 600 K, and 900 K. The green represents the FCC phase, the red indicates the slipped FCC phase, and the white denotes the amorphous phase. structures corresponding to sintering temperatures of 600 K and 900 K reveal differing quality levels. The system at 600 K also contains a small number of defects or pores. This paper will subsequently investigate the structural evolution during the sintering process from a structural standpoint. Through the analysis of structural evolution, we will gain clearer insights into the sintering process of nano solder under the control of low-temperature sintering mechanisms and further explore the role of rapid dynamic surface states during the sintering process.