Interfaces play an important role in crystalline plasticity as they affect strength and often serve as obstacles to dislocation motion. Here we investigate effects of grain and nanotwin boundaries on uniaxial strength of 500 nm diameter Cu nanopillars fabricated by e-beam lithography and electroplating. Uniaxial compression experiments reveal that strength is lowered by introducing grain boundaries and significantly rises when twin boundaries are present. Weakening is likely due to the activation of grain-boundary-mediated processes, while impeding dislocation glide can be responsible for strengthening by twin boundaries.
Concentrator photovoltaic (CPV) modules operate in extreme conditions, including enhanced solar flux, elevated operating temperature, and frequent thermal cycling. Coupled with active environmental species such as oxygen and moisture, the operating conditions pose a unique materials challenge for guaranteeing operational lifetimes of greater than 25 years. Specifically, the coatings and encapsulants used in the optical elements are susceptible to environmental degradation during operation. For example, the interfaces must remain in contact to prevent optical attenuation and thermal runaway. We developed fracture mechanics based metrologies to characterize the adhesion of the silicone encapsulant and its adjacent surfaces, as well as the cohesion of the encapsulant. Further, we studied the effects of weathering on adhesion using an outdoor concentrator operating in excess of 1100 times the AM1.5 direct irradiance and in indoor environmental chambers with broadband ultraviolet (UV) irradiation combined with controlled temperature and humidity. We observed a sharp initial increase in adhesion energy followed by a gradual decrease in adhesion as a result of both outdoor concentrator exposure and indoor UV weathering. We characterized changes in mechanical properties and chemical structures using XPS, FTIR, and DMA to understand the fundamental connection between mechanical strength and the degradation of the silicone encapsulant. We developed physics based models to explain the change in adhesion and to predict operational lifetimes of the materials and their interfaces.
Alternating layers of organic and oxide thin films used as diffusion barriers in emerging flexible device technologies are vulnerable to degradation under the influence of mechanical stresses, temperature cycling, photodegradation, and chemically active environmental species. Delamination of the internal organic to oxide interfaces often limits the operational lifetime of the barrier system. We demonstrate a method for increasing the adhesion of organic and oxide thin films by generating nanostructures at the interface. We show that the adhesion of an acrylate to silicon oxide model system can be increased by up to an order of magnitude (from ∼2 J/m(2) to 24 J/m(2)). By altering the diameter and depth of the patterns in the model systems, the adhesion energy can be changed, and the delamination pathway can be controlled. In addition, we show that a patterned interface maintains a higher adhesion than its planar counterpart for all durations of UV-A and UV-B exposure.
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