# A Review on the Deformation Behavior of Silver Nanowire Networks under Many Bending Cycles
## 2 Concept of a Cyclic Bending Tester
This refers to how grain boundaries tend to exist at thermally fused junctions, and individual silver nanowires and cracks were formed as a result of the focused strain on those grain boundaries. Figure 4: Numerical simulation results on deformation behavior of nanowire meshes with different densities under tensile strain. Reproduced with permission from ref. [38]. Copyright 2016, Elsevier. Figure 3: **(a)** Ion beam images of silver thin film with a thickness of 1.5 \(\upmu\)m taken by a focused ion beam (FIB), **(b)** an enlarged image at a black dot square in (c), and cross-sectional SEM image of a silver thin film damaged by repeated deformation. Reproduced with permission from ref. [32]. Copyright 1969, Elsevier. Based on previous research, it can be presumed that a silver nanowire network without thermal junctions has better mechanical stability under repeated deformation because the strain is focused in a smaller area. Figure 6 shows the result of the cyclic bending test of the silver nanowire network without a thermal annealing process. In the un-annealed silver nanowire network, the resistance exponentially reduced as the strain increased. In addition, even after the declination of the resistance was saturated, the resistance did not increase but maintained its decreased level. Such resistance reduction was also observed for the annealed silver nanowire network at the initial bending cycles as shown in Figure 2c. The reduction in the resistance resulted from the formation of mechanical welding between silver nanowires. As Figure 7 shows, the silver nanowires were stacked to form  
Figure 5: SEM images of annealed silver nanowire networks (**a**) before bending, (**b**) after imposing 10,000 bending cycles, and (**c,d**) after 500,000 bending cycles, high magnification and low magnification, respectively. Reproduced with permission from ref. [28]. Copyright 2014, Wiley. Figure 6: Cyclic bending test results of un-annealed silver nanowire networks. Reproduced with permission from ref. [28]. Copyright 2014, Wiley. a sort of mechanical weld between silver nanowires. This kind of mechanical weld was formed due to the plastic deformation of the silver nanowires caused by the strain [28,41]. Similar to thermal junctions, the mechanical weld formed strain-induced adhesion between silver nanowires that provided sufficient electrical conductivity. However, unlike thermal junctions, the strain was less focused at the junctions because no grain boundaries were formed, and fewer failures occurred as a result. Therefore, it is expected that the fabrication of a silver nanowire network with higher mechanical stability is enabled by the creation of mechanical welds between silver nanowires. [PERSON] et al. studied the resistance increase in the silver nanowire electrode during bending fatigue testing as a function of the density of the nanowire network and compared this against the behavior commonly observed in a Cu thin film electrode [42]. The silver nanowire network showed a smaller increase in fractional resistance (\(\Delta\)R/R) with an increase in the density of the network during cyclic bending; hence, the dense silver nanowire network is more reliable as shown in Figure 8b. Cu thin films showed the opposite trend of enhanced reliability for thinner films (Figure 8a). The opposite trend in the effect of thickness or density variation in the silver nanowire network can be understood by considering the electrical percolation behavior of a network. First, it was noted that the silver nanowire network results in the random distribution of failures at the junctions (point-type failure), where the Cu film results in fatigue-induced line crack propagation perpendicular to the bending direction (Figure 8c). The failure in the silver nanowire network was modeled as a grid to numerically calculate the expected resistance for high- and low-density networks, which indicates smaller \(\Delta\)R/R for a higher network density because more electrical pathways are present, as shown in Figure 8d.