# Effect of Thermal Treatment on Nano- and Micro-Copper Particles for Jewelry Making
## 3 Results and Discussion
### Effect of Heating Rate  
To investigate the effects of heating rate, different heating rates, including 10 \({}^{\circ}\)C/min, 30 \({}^{\circ}\)C/min, and 60 \({}^{\circ}\)C/min, were performed at the same heating temperatures and holding times. The SEM images of nano-copper clay in the micro-features of fired specimens changed with the varying heating rates, as shown in Figure 6. When the heating rate increased from 10 to 30 \({}^{\circ}\)C/min, the microstructure decreased the densification with a few sintering necks. With another increase in the heating rate, up to 60 \({}^{\circ}\)C/min, the nano-copper particle sintered, and its effects became more pronounced on porosity and densification, indicating that the high heating rate could not provide sufficient time for improving the packing density and densification of the microstructure [13; 17]. Figure 5: The percentage by weight of sulfur (S) and oxygen (O) in the sintered nano- and micro-copper clay specimens determined by SEM-EDS. The SEM images of sintered micro-copper clay at different heating rates are shown in Figure 7. The microstructure shows the sintered particles and grain growth where densification was more pronounced at the lowest heating rate (10 \({}^{\circ}\)C /min). On the other hand, the results showed that a high heating rate of 60 \({}^{\circ}\)C/min could not use the uniformization of grain growth because the process is quick to employ any densification. Therefore, both sintered nano- and micro-copper clays firing under a high heating rate have higher porosity and poor particle packing than low heating rates. Comparing the SEM images of nano- and micro-copper clay at 10 \({}^{\circ}\)C/min (Figures 6a and 7a) and 30 \({}^{\circ}\)C/min (Figures 6b and 7b), there are a few different grain sizes in the sintered  
Figure 6: SEM images of sintered nano-copper clay with different heating rates at (**a**) 10 \({}^{\circ}\)C/min, (**b**) 30 \({}^{\circ}\)C/min, and (**c**) 60 \({}^{\circ}\)C/min. Figure 7: SEM images of sintered micro-copper clay with different heating rates at (**a**) 10 \({}^{\circ}\)C/min, (**b**) 30 \({}^{\circ}\)C/min, and (**c**) 60 \({}^{\circ}\)C/min. sample performing at these heating rates. On the other hand, the SEM images at the 60 \({}^{\circ}\)C/min heating rate showed dramatically different grain sizes (Figures 6c and 7c). As for the results, it can be confirmed that the heating rate has a profound effect on the final grain grown compared to the starting particle size of both nano- and micro-copper clays. Thus, complete densification can be achieved with a low heating rate. Figure 8a shows the volume shrinkage versus the heating rate operated on the sintered samples. Notably, the shrinkage rate depends on the copper clay's heating rate and particle size as they increase, resulting in decreased shrinkage [9,22,23]. Moreover, the lowest heating rate (10 \({}^{\circ}\)C/min) produced a higher shrinkage because the sample remained in the furnace and reached its isotherm temperature for a long time. The maximum shrinkage rate was observed at a low heating rate of 10 \({}^{\circ}\)C/min and low shrinkage with the increased heating rate. The samples sintered with heating rates of 10 and 30 \({}^{\circ}\)C/min yielded a relatively high density and hardness value compared with 60 \({}^{\circ}\)C/min, as shown in Figure 8b,c. By comparing the bulk density of sintered nano- and micro-copper clays fired at different heating rates (Figure 8b), slow heating rates induce a high density. The bulk density of both clays fired at 10 \({}^{\circ}\)C/min was slightly better than 30 and 60 \({}^{\circ}\)C/min. Our results are supported by SEM micrographs of the fired nano- (Figure 6) and micro-copper clays (Figure 7). When the heating rate increased at a constant temperature, the porosity increased with the increase in the heating rate due to the sintering densification process [24,25].