# Effect of Thermal Treatment on Nano- and Micro-Copper Particles for Jewelry Making
## 3 Results and Discussion
### Effect of Holding Time  
The effect of holding time was investigated by controlling the same heating temperatures and heating rates at 800 and 10 \({}^{\circ}\)C/min, respectively. The holding times were varied for 0, 30, and 60 min. As illustrated in Figures 9 and 10, the extent of the holding time tends to improve the densification of both clays. In the case of the holding time at 0 min (Figures 9a and 10a), the micro- and nanoparticles were sintered and grew to a bigger micron size; however, complete densification did not occur. When the holding time was set to 60 min (Figures 9c and 10c), the SEM images of both nano- and micro-copper clays revealed grain growth in a uniformed micron structure with an even surface. Moreover, the obtained grain growth of micro-copper clay is smoother than nano-copper clay. Figure 11a shows the extent of holding times, which increase the volume shrinkage in both nano- and micro-copper clays. At 0 min of holding time, nano- and micro-copper clays revealed similar volume shrinkage at about 5%. At 30 and 60 min of holding time, the volume shrinkage of the nano-copper clay was higher than micro-copper clay. These results may be due to the influence of particle size on the volume change. The decomposition of the binders and densification phenomena led to an increase in the volume shrinkage percentage. A long holding time is unnecessary for the sintering process in jewelry fabrication because of high volume shrinkage [4], especially in the case of nano-copper clays. However, the hardness of the specimens must be considered in this application. Figure 9: SEM images of sintered nano-copper clay with different heating times at (**a**) 0 min, (**b**) 30 min, and (**c**) 60 min. Figure 11b reveals the bulk density of nano- and micro-copper clays fired at different holding times. The extent of holding time tends to improve the density of the finished product of nano- and micro-copper clays. At the holding time of 0 min, the nano-copper clays revealed very low bulk density compared to the micro-copper clays. After the  
Figure 11: (**a**) Volume shrinkage, (**b**) bulk density, and (**c**) hardness of sintered nano- and micro copper clay with different holding times. Figure 10: SEM images of sintered micro-copper clay with different heating times at (**a**) 0 min, (**b**) 30 min, and (**c**) 60 min. holding time increased to 30 min, the micro-copper clay showed an increase in density. The nano-copper clay revealed a slight decrease in the bulk density, likely due to the copper nanoparticles becoming necked and sintered after prolonging the holding time (Figure 9). Since the number of open surface pores reflected the decline in density [25; 26; 27], the density of the nano-copper clays was lower than that of the micro-clay when the holding time was extended to 30 min. After the holding time approached 60 min, the sintered copper microparticles of micro- and nano-copper clays were connected and obtained grain growth with a uniform micron structure and even surface. Although the bulk densities of both clays were almost equal at longer holding times, a longer holding time is necessary for the sintering process of jewelry fabrication by copper clay, especially for nano-copper clays. Therefore, in the case of jewelry fabrication, considering only volume shrinkage, density, and hardness of the final product is not enough. Considering the relation between holding time and hardness of nano- and micro-copper clays in Figure 11c, the hardness trend is similar to those of bulk density in Figure 11b. However, micro-copper clay has a higher hardness than nano-copper clay. Herein, it is well known that hardness relates to the packing density of particles in their densification process [4; 8]. Although the density of nano-copper clay is similar to micro-clay with an extended holding time of 60 min, the hardness of the micro-copper clay is higher than nano-copper clay at the same holding time of the firing process.