## 3 Results
### Microstructure of Laser Sintered NP Arrays  
Due to the fact that the laser radiation energy enters the NP array through the surface, the sintered sample microstructure is expected to be inhomogeneous in depth. Therefore, to clarify the mechanisms of the laser sintering process, we studied the internal microstructure of sintered silver NP arrays in the form of lines for their transverse fractures by SEM. Figure 5a-i shows typical SEM images for transverse line fractures sintered by laser radiation in optimal regimes for each type of laser corresponding to the highest average line conductivities. Figure 5: _Cont._  
The SEM image in Figure 5a shows a general view of a sintered line characteristic fracture over the substrate for an optimal sintering regime corresponding to 980 nm wavelength. Figure 5b-i shows SEM images for the external fracture area, including the border of the sample surface, and the internal fracture area, obtained with a larger magnification in the areas marked with the red frames in the characteristic image of the line fracture general view (Figure 5a). The image analysis of sintered line fractures allows to see a common mechanism for the four types of laser radiation. The external surface of the sample fractures contains almost a non-porous zone of densely sintered material. In the images this zone is highlighted between two yellow dotted lines (Figure 5b,d,f,h). Deeper in the array after the pore-free zone there follows a transition zone with separate submicron pores. Further to the fracture middle part there is a microstructure with an approximate uniform submicron porosity (Figure 5c,e,g,i). The thickness of the non-porous layer, achieved from SEM images of sample fractures for the four types of laser radiation used, is presented in Table 2.  
It should be noted that the sintered pore-free layer thickness range for all types of lasers is 0.5-1 \(\upmu\)m. This layer differs from the rest of the fracture microstructure by the absence of submicron pores and by the large crystallite average sizes compared to the porous zone in the depth of the samples. The measurement results are given in Table 2. Crystallite sizes measurements in the zones discussed were done using fractures SEM images with crystallites approximated by an equivalent circle with an equal area. The average crystallite size was attained by averaging the sizes of many individual crystallites in amount of over 400. Comparing the parameters of microstructures sintered by different types of laser radiation (Table 2), we note that the thickness of the sintered pore-free layer as well as the average crystallite sizes increase  
Figure 5: SEM images of fractures of silver NP lines sintered in optimal modes: (**a**) The general view of the fracture of the line on a substrate sintered by radiation with a wavelength 980 nm; (**b,d,f,h**) External fracture areas including the surface borders of the sample; (**c,e,g,i**) Internal fracture areas. Sintering was done by pulsed laser sources with wavelengths (**b,c**) 527 nm, (**d,e**) 1054 nm and CW laser sources with wavelengths (**f,g**) 527 nm, (**h,i**) 980 nm. during the transition from pulsed laser sintering to CW sintering. The thickness also grows with an increase in the wavelength of pulsed laser radiation. In this case, the average crystallite sizes grow with an increase in the wavelength of both pulsed and CW laser radiation. Within each sample, the average crystallite sizes in the non-porous layer are much larger than for crystallites in the porous zone. It should be pointed out that the surface of the samples under the study is characterized by a low RMS roughness in the range of 40-70 nm, same for sintered and untreated samples, measured by AFM in the area \(10\times 10\)\(\upmu\)m\({}^{2}\). The use of polished silicon substrates in the experiments made it possible to study the fractures microstructure of sintered lines by SEM.