## 3 Results and Discussion  
In the process of \"dry\" AJP, the synthesis of nanoparticles was carried out continuously as a result of a spark discharge between the electrodes. Thus, copper-based nanoparticles were obtained in the process of evaporation-condensation of electrodes in a controlled gas atmosphere [30]. Figure 2a,b shows the corresponding transmission electron microscopy (TEM) images of nanoparticles synthesized in a spark discharge by erosion of copper electrodes in an argon-hydrogen atmosphere (Ar 95% + H\({}_{2}\) 5%). The figures show that the resulting nanoparticles are fractal-like agglomerates consisting of separate near-spherical primary nanoparticles [31]. The average size of primary nanoparticles is 10.8 \(\pm\) 4.9 nm, according to the analysis of TEM images, see Figure 2c. In this case, according to measurements on an aerosol spectrometer, the mode size of the agglomerates is 98 \(\pm\) 9 nm (Figure A2, Appendix A). These dimensions are typical for the synthesis of nanoparticles using a spark discharge [32]. Based on the results of the analysis of TEM images and the electron diffraction pattern, it was determined that the resulting nanoparticles have a crystalline structure with a small fraction of an amorphous oxide shell on the surface. The high-resolution TEM image (Figure 2b) shows that the thickness of the amorphous layer on the surface of the recently obtained nanoparticles is approximately 3-5 nm. The formation of an amorphous layer on the surface of nanoparticles can be associated with the process of oxidation of the samples during their exposure and storage in air. Earlier it was reported that copper nanoparticles synthesized by other gas-phase methods [33], also inevitably consist of an oxide shell layer that affects the conductivity of microstructures. The crystal structure of copper nanoparticles was determined by analyzing the electron diffraction pattern. As shown in Figure 2d, the electron diffraction pattern consists of rings corresponding to interplanar spacings of 2.06, 1.83, 1.26, and 1.09 A. These values are in good agreement with the values characteristic of the (111), (200), (220), and (311) planes of pure copper without any impurities with a cubic face-centered unit cell and space group Fm3m [34]. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Working** & **Target Gas** & **Concentration, ppm** & **Relative Humidity, \%** \\
**Temperature**, \({}^{\circ}\)C & CO & 8.5, 17 and 40 & 50 \\  & NH\({}_{3}\) & 7, 14, 28 and 40 & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The mode of measuring the gas-sensitive characteristics of the initial unsintered lines using the “Mikrogaz-F” gas mixing system. Figure 3a-c shows the cross-sectional profiles of printed lines of copper-based nanoparticles formed on alumina, borosilicate glass, and silicon substrates, respectively. This figure shows that the line profiles have a similar Gaussian shape, a width of 100-150 \(\upmu\)m and a height of 5-7 \(\upmu\)m, regardless of the substrate material and the degree of their roughness. In this case, the broadening effect of the deposited material does not significantly appear, as, for example, in the case of inkjet or microplotter printing [35], since in the process of \"dry\" AJP, the formation of lines is carried out due to the direct deposition of nanoparticles on the substrate without the use of inks and solvents. This feature can be a key advantage when forming functional lines on different substrates over traditional \"wet\" printing processes. As known, copper nanoparticles tend to oxidize easily under environmental conditions [33; 34]. Printed lines based on copper nanoparticles synthesized in a spark discharge are no exception. From the analysis of the EDX spectra, it was found that the initial un-sintered lines, stored in air for several days, have noticeable oxygen peaks at the level of 3-4 wt%, see Figure 3d.