Conductance-strain behavior in silver-nanowire composites: network properties of a tunable strain sensor.

Highly flexible and conductive nano-composite materials are promising candidates for stretchable and flexible electronics. We report on the strain-resistance relation of a silver-nanowire photopolymer composite during repetitive stretching. Resistance measurements reveal a gradual change of the hysteretic resistance curves towards a linear and non-hysteretic behavior. Furthermore, a decrease in resistance and an increase in electrical sensitivity to strain over the first five stretching cycles can be observed. Sensitivity gauge factors between 10 and 500 at 23% strain were found depending on the nanowire concentration and stretching cycle. We model the electrical behavior of the investigated silver nanowire composites upon repetitive stretching considering the strain induced changes in the local force distribution within the polymer matrix and the tunnel resistance between the nanowires by using a Monte Carlo method.

Electrical and network properties of flexible silver-nanowire composite electrodes under mechanical strain.

Flexible and conductive silver-nanowire photopolymer composites are fabricated and studied under mechanical strain. The initial resistances of the unstretched flexible composites are between 0.27 Ω mm-1 and 1.2 Ω mm-1 for silver-nanowire concentrations between 120 µg cm-2 and 40 µg cm-2. Stretching of the samples leads to an increased resistance by a factor of between 72 for 120 µg cm-2 and 343 for 40 µg cm-2 at elongations of 23%. In order to correlate network morphology and electrical properties, micrographs are recorded during stretching. The Fiber Image Network Evaluation (FINE) algorithm determines morphological silver-nanowire network properties under stretching. For unstretched and stretched samples, an isotropic nanowire network is found with only small changes in fiber orientation. Monte-Carlo simulations on 2D percolation networks of 1D conductive wires and the corresponding network resistance due to tunneling of electrons at nanowire junctions confirm that the elastic polymer matrix under strain exhibits forces in agreement with Hooke's law. By variation of a critical force distribution the resistance curves are accurately reproduced. This results in a model that is dominated by quantum-mechanical tunneling at nanowire junctions explaining the electrical behavior and the sensitivity of nanowire-composites with different filler concentrations under mechanical strain.