# Effect of Thermal Treatment on Nano- and Micro-Copper Particles for Jewelry Making
## 3 Results and Discussion
### Effect of Heating Temperature  
To investigate the influence of heating temperature on the physical properties of the nano- and micro-copper clays, both clays were heated under varying heating temperatures from 200 to 1000 \({}^{\circ}\)C, whereas the same heating rate and holding time were 10 \({}^{\circ}\)C/min and 30 min, respectively. Figures 2 and 3 show the microstructure of the fired clays at different heating temperatures. For the fired nano-copper clay (Figure 2), the copper particles started sintering at the neck region when heated to 200 \({}^{\circ}\)C (Figure 2a). Afterward, the dumbbell shape of the interconnected nanoparticles was formed, and the sintering process could be assured (Figure 2b). When the nanoparticles were heated to 600 \({}^{\circ}\)C, the aggregated copper left many pores, seemingly corresponding to the schematic representation in Figure 2e. At this stage, it can be assumed that the copper atoms diffused from the nanoparticles at the intersections between the differently oriented particles continued to deposit onto the neck region [7,9]. Furthermore, as the temperature increased to 700 \({}^{\circ}\)C, the pores significantly decreased in size and became more isolated (Figure 2f). Thus, it is difficult to distinguish the particle shape from the neck regions, which enter sintering at the final stage [9,10]. Above 800 \({}^{\circ}\)C, the grain growth continued to form a dense, porous polycrystalline copper (Figure 2g). Finally, the copper atoms moved from the convex surface on one side of the grain boundary to the concave surface on the other side, resulting in a homogeneous phase of sintered materials. However, the sintered particle features collapsed and reformed after the fired temperature approached the melting point of bulk copper metal at 1000 \({}^{\circ}\)C (Figure 2i). For micro-copper clay, the micro-particle was not sintered at 200 \({}^{\circ}\)C (Figure 3a). Comparing the micro- and nano-copper clay (Figures 2b and 3b), the micro-copper particles were connected to form a larger aggregated particle. The size of the pores significantly increased and became more isolated after the heating temperature reached \(>\)400 \({}^{\circ}\)C (Figure 3c). The fracture surfaces of the samples contained fewer dimples, yet many pores, including obtration due to fracture. The microcrack initiation tended to occur in areas containing pores, which rapidly propagated. After reaching the temperature of 700 \({}^{\circ}\)C (Figure 3f), the particle size and density correspondingly enlarged with a higher temperature; however, it also decreased the number of pores [10,11]. The fracture surface of the sintered samples showed the characteristics of the mixed fracture. Aside from the dimple fractures, intercrystalline  
Figure 1: The TG and DTA curves of nano- (**a**) and micro-copper clays (**b**) fired at different temperatures. and transcrystalline cleavage fractures were also presented. Those features have resulted in increasing strength [12; 13]. Comparing the results from Figure 2g,h and Figure 3g,h, the fracture surface of the sintered micro-copper clay indicated fewer pores and homogeneous tissue than nano-clay. It can be concluded that the particle size influenced the sintering densification process. The clay's microstructure became denser when the heating temperature was increased. Similar to nano-copper clay, the sintered particles of micro-copper clay collapsed and reformed, accompanied by several pores when the heating temperature approached 1000 \({}^{\circ}\)C (Figure 3i). Regarding heat temperature optimization, we concluded that micro-copper clay provided more sturdily and uniform densification than nano-copper clay compared to the same heating conditions. As a result, it could be affected by their physical properties, especially density and hardness. Figure 4 shows the relationship between the volume shrinkage, bulk density, and hardness of the fired clays at different heating temperatures.