## 3 Results and Discussion
Due to the oxidation of nanoparticles, the initial lines had a high resistance at room temperature of 25 \(\pm\) 5 \({}^{\circ}\)C equal to >300 M\(\Omega\), which is above the measurement limit by the multimeter used. In this case, the presence of zinc impurities in the samples is explained by the evaporation of brass holders during the synthesis of nanoparticles. Zinc impurity is also observed in platinum nanoparticles produced using a similar spark discharge generator with brass holders [36]. Figure 2: (**a**,**b**) TEM images of nanoparticles synthesized in a spark discharge by erosion of copper electrodes in an argon atmosphere with hydrogen (Ar 95% + H\({}_{2}\) 5%) and the corresponding (**c**) particle size distribution and (**d**) electron diffraction pattern. Based on the results of measurements of the temperature dependence of the resistance, it was determined that the initial lines of nanoparticles exhibit a semiconductor type of conductivity. Thus, the resistance of the samples decreased monotonically with an increase in their temperature in the range from room temperature to 300 \({}^{\circ}\)C (573 K) (Figure A1, Appendix A). This semiconducting character of conductivity indirectly indicates the presence of oxidized nanoparticles in the composition of printed lines, for example, CuO or Cu\({}_{2}\)O, which are typical metal oxide semiconductors [37]. Moreover, based on the results of gas-sensitive measurements, it was determined that unsintered lines demonstrate \(p\)-type conductivity, characteristic of CuO [37]. This type of conductivity is indirectly confirmed by an increase in the resistance of the unsintered line in the presence of a gas impurity (carbon monoxide and ammonia). It is known that the resistance increase in a concentrated gas environment occurs due to the formation of surface-active radicals exhibited by the dissociation of gas impurity molecules at heated oxide surfaces [10]. This leads to the formation of free electrons. The excessive free electrons appearing because of the surface reactions recombine with holes at \(p\)-type oxides reduces a free carrier concentration and thus increasing the resistance of the oxide [10]. Figure 3: Cross-sectional profiles of printed lines from nanoparticle formed on substrates of (**a**) alumina, (**b**) borosilicate glass, and (**c**) silicon, (**d**) typical energy-dispersive X-ray spectroscopy EDX-spectrum of presented lines measured by SEM ]SM-7001F. It is known that the presence of surface oxides can be reduced by sintering nanoparticles in a reducing atmosphere [38]. Thus, with the aim of producing conductive microstructures, reductive sintering of the formed lines was performed. Sintering was carried out in a tube furnace with the length and internal diameter of the chamber 560 and 30 mm, respectively, in a hydrogen atmosphere with a gas flow rate of about 50 sccm. The gas flow was controlled at the inlet to the tube furnace using a mass gas flow regulator. The heating rate of the samples was 5 \({}^{\circ}\)C/min with holding at a given temperature of 350-650 \({}^{\circ}\)C for 60 min. Sintering of the lines at the furnace temperature of \(\geq\)750 \({}^{\circ}\)C led to the destruction of the lines and an increase in the resistivity, respectively. Based on the results of these experiments, the effect of the sintering temperature on the electrical resistivity, morphology, and elemental composition of printed lines was determined. Figure 4 shows the change in the electrical resistivity of the lines \(\rho\) on various substrates depending on the sintering temperature \(T_{\text{sint}}\), as well as their corresponding optical images. As the sintering temperature increases, the electrical resistivity of the lines decreases noticeably, and then reaches saturation, see Figure 4a.