# Effect of Thermal Treatment on Nano- and Micro-Copper Particles for Jewelry Making
## 3 Results and Discussion
### Effect of Heating Temperature
Considering volume shrinkages at different heating temperatures (Figure 4a), dimensional changes occurred, resulting in volume shrinkage upon the complete removal of organic binders at about 200-600 \({}^{\circ}\)C. The volume shrinkage of the nano-copper clay was higher than the micro-copper clay at the same heating temperature because volume shrinkage alterations generally relate to the packing density of the metal particle during the densification process [13; 14; 15]. The results correspond well with the SEM images of nano- and micro-copper clays in Figures 2 and 3. Due to the high porosity between nanoparticles, the exceedingly small nano-copper particles drastically shirked to connect and provide higher volume shrinkage when the heating was performed. Therefore, the nano-copper clay has higher volume shrinkage than the micro-copper clay. As mentioned previously, volume shrinkage is related to material density. When considering the density of the clays (Figure 4b), the figures show the same increasing trend of bulk densities by increasing heating temperatures. At the initial stage of 200-600 \({}^{\circ}\)C, both clays display a slight change in their bulk density due to the decomposition of water and organic binders. Upon the complete removal of the binders, Figure 4 shows a dramatic change in density when the temperature reaches 600-900 \({}^{\circ}\)C. The bulk density increased, corresponding to the decreasing porosity of the fired clay due to sintering densification [16; 17]. In the 600-900 \({}^{\circ}\)C stage, micro-copper clay displays a higher bulk density than nano-copper clay because the packing density of the micro-copper clay during the densification process is greater than that of the nano-copper clay. SEM images at high firing temperatures verify these results in Figures 2 and 3. Moreover, the density of the micro-copper clay (about 8 g/cm\({}^{3}\)) is close to the density value of bulk copper material, 8.96 g/cm\({}^{3}\). At 1000 \({}^{\circ}\)C, the density of nano- and micro-copper clays decreased, corresponding to their SEM images in Figures 2i and 3i, due to the sintered particles of both clays collapsing and deforming, accompanied by several pores when the heating temperature approached the melting point of copper metal (1085 \({}^{\circ}\)C). Figure 4: (**a**) Volume shrinkage, (**b**) bulk density, and (**c**) hardness of sintered nano- and micro-copper clays with different heating temperatures. Hardness is an important parameter in fabricating jewelry. As shown in Figure 4c, the hardness of both fired clays increased according to the rising heating temperature. The dramatic hardness change was observed at 600-900 \({}^{\circ}\)C when the copper particles were sintered and approached a complete densification process after losing organic binders and water. Since the hardness is related to the packing density of copper particles in its densification process [18; 19], the results corresponded to the trend of bulk density at high fired temperatures (Figure 4b), as supported by SEM micrographs. The grain increased its size when the firing temperature increased. The fracture surface of sintered nano-clay exhibits open pores, leading to the relatively low porosity of the specimen at \(>\)800 \({}^{\circ}\)C. Consequently, the density and hardness of the nano-silver clay increased after 600 \({}^{\circ}\)C and was the highest at 900 \({}^{\circ}\)C.  
It was remarkable that the hardness of the fired micro-copper clay was higher than that of nano-copper clay. However, the nanomaterial should have a higher hardness value than the micro-copper material [20]. It was suggested that thermally treating nanophase samples in the as-produced condition might result in structural changes, such as densification, as seen in the hardness of microstructures via SEM images. Moreover, some of the literature suggests that it may be due to artifacts from the sample preparation or the heating conditions performed [5].