## 3 Results and Discussion
For example, the electrical resistivity approached its minimum value of 35-45 \(\upmu\Omega\cdot\)cm at a sintering temperature of about 650 \({}^{\circ}\)C.  
Figure 4 also shows that reductive sintering of oxidized copper nanoparticles begins to be noticeable at sintering temperatures above 450 \({}^{\circ}\)C. At these temperatures, the lines become conductive (Figure 4a), and their color changes from black to red-orange (Figure 4b). Moreover, the oxygen concentration decreases from 3.0 to 0.5 wt% when the temperature \(T_{\text{sint}}\) increases from 350 \({}^{\circ}\)C to 450 \({}^{\circ}\)C, respectively. For example, the oxygen concentration in the unsintered lines was 3-5 wt%. The oxygen concentration is totally removed only at sintering temperatures above 550 \({}^{\circ}\)C according to EDX analysis. Thus, it was experimentally found that a temperature of 450 \({}^{\circ}\)C becomes sufficient to activate the chemical reaction of copper oxide reduction, according to the following expression: Cu\({}_{\text{x}}\)O + H\({}_{2}\)(g) \(\rightarrow\) xCu + H\({}_{2}\)O(g). The high temperature of reductive sintering of 450 \({}^{\circ}\)C is probably associated with a rather thick 3-5 nm oxide shell on the surface of the synthesized nanoparticles (see Figure 2b), since copper oxides have a higher melting point than metallic copper [39]. High sintering temperatures can limit the use of thermosensitive polymer substrates such as polyimide, polyethylene naphthalene, and others. To explain the changes in electrical resistivity during reductive sintering, we additionally performed a microstructural analysis of the surface of lines sintered at different  
Figure 4: (**a**) Values of electrical resistivity and (**b**) corresponding optical images of printed lines on substrates of Al\({}_{2}\)O\({}_{3}\), BG, and Si, depending on the reduction sintering temperature of 350–650 \({}^{\circ}\)C.  
temperatures, the results of which are shown in Figure 5. Samples sintered at 350 \({}^{\circ}\)C were a typical fine-grained structure formed by a network of oxidized copper nanoparticles, see Figure 5a. Thereby, these samples had a high electrical resistivity, much more than 400 \(\upmu\Omega\cdot\)cm (see Figure 4a). Figure 5b shows that the reduction sintering of the lines began at a temperature of 450 \({}^{\circ}\)C as a result of the coarsening of grains and a decrease in the porosity of the microstructure. Further compaction of the microstructure accelerated with an increase in temperature to 550 \({}^{\circ}\)C, as shown in Figure 5c. This condensation of the microstructure resulted in a sharp decrease in the electrical resistivity from 275 to 100 \(\upmu\Omega\cdot\)cm, see Figure 4a. At the same time, with an increase in the sintering temperature to 650 \({}^{\circ}\)C, the growth of microstructure compaction became less noticeable, and the sintered lines reached the minimum electrical resistivity at the level of 35-45 \(\upmu\Omega\cdot\)cm. It should be noted that high sintering temperatures of 650 \({}^{\circ}\)C could also lead to cracking and partial peeling of lines due to differences in the coefficients of thermal expansion of the particle and substrate materials. In this connection, it was experimentally found that the recommended sintering temperature should be reduced to 550 \({}^{\circ}\)C. At this temperature, the sintered lines have good adhesive strength and acceptable electrical resistivity. From the results of elemental analysis (Figure 6a), measured by SEM, it was found that with an increase in the sintering temperature from 350 \({}^{\circ}\)C to 650 \({}^{\circ}\)C, there is a significant increase in the mass fraction of copper from 78.4 to 99.7 wt%, respectively. This increase is indicative of effective copper reduction with complete removal of oxides from sintered microstructures. Figure 6a shows that there is a zinc impurity in the samples.