## 3 Results and Discussion
The electrical resistivity of the lines made of spherical nanoparticles was minimal and amounted to 5 \(\upmu\Omega\cdot\)cm (Table 3), which is approximately three times higher than that of bulk copper. These printed lines also showed long-term stability up to 2 months of storage in the environment without any significant reduction in electrical resistivity. At the same time, the achieved values of resistances of printed lines are comparable with competing methods based on the use of nano-ink. For example, at work [33] it was reported that arc discharge inkjet nano-ink had a resistivity of 5.4 \(\pm\) 0.6 \(\upmu\Omega\cdot\)cm at a sintering temperature of 300 \({}^{\circ}\)C. Thus, it has been experimentally shown that the method of \"dry\" aerosol jet printing followed by sintering can be used to create conductive copper lines from spherical nanoparticles with excellent electrical properties on various substrates. As defined above, the high porosity of loose agglomerate printed lines is an obstacle to the fabrication of highly conductive microstructures. In turn, highly porous microstructures from agglomerates can be promising for gas sensor applications, where a high specific surface area of a gas sensitive material is important. Thus, in this work, using the method of \"dry\" AJP, a gas-sensitive layer was formed on a measuring substrate made of alumina. The gas-sensitive layer based on copper nanoparticles was a low (less than 5 \(\upmu\)m) printed line with a width of about 150 \(\upmu\)m, deposited on the measuring electrodes, heated from \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Alumina (Al\({}_{2}\)O\({}_{3}\))** & **Borosilicate Glass (BG)** & **Silicon (Si)** \\ \hline Electrical resistivity \(\rho\), & & & \\ \(\upmu\Omega\cdot\)cm & 5.1 \(\pm\) 0.6 & 5.3 \(\pm\) 0.7 & 5.1 \(\pm\) 0.8 \\ \hline Shrinkage \(S\), \% & 44 \(\pm\) 10 & 38 \(\pm\) 9 & 32 \(\pm\) 8 \\ \hline Elemental composition of & & Cu—98.6 & \\ sintered lines, wt.\% & & Ag—1.0 & \\  & & O—0.4 & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Electrical resistivity, shrinkage, and elemental composition of sintered lines made of spherical copper-based nanoparticles. below using a platinum heater in the form of a meander, see Figure 8a. Further, the formed structure was tested using the \"Mikrogaz-F\" gas-mixing system for the detection of gaseous impurities of ammonia and carbon monoxide, see Table 1. Based on the test results, the dependences of the resistance of the gas-sensitive layer on the concentration of NH\({}_{3}\) and CO were determined; see Figure 8b-d. The resistance of the sensors is decreased with increase of the working temperature, suggesting that the gas-sensitive layer possess characteristics of semiconductors. Figure 8c,d show the sensor response to NH\({}_{3}\) and CO, correspondingly, at the working temperature of 270 \({}^{\circ}\)C. The increase of resistance under influence of the target gases is related to the sensing mechanism of _p_-type metal oxide semiconductors [47]. The chemical reaction between the target gases and surface oxygen removes the chemisorbed oxygen molecules releasing free electrons. In case of _p_-type copper oxide, release of electrons leads to recombination between holes and electrons, decrease of number of free holes, increase of potential barriers between nanoparticles and the corresponding increase in the resistance. The dependences of the relative change of resistance on the concentration of NH\({}_{3}\) and CO gases are shown in Figure 8b. These dependencies can be fitted by a power law as usual for metal oxide sensors [48]. Figure 8c,d presents the resistance transients upon exposure to NH\({}_{3}\) and carbon oxide. The observed response and recovery times (250-300 s) agree well with those reported in [37]. Response of the sensor fabricated based on non-sintered oxidized copper nanoparticles to ammonia and carbon monoxide concentration of 40 ppm is about 20% and 80%, respectively.