2 research outputs found

    Strain-Driven Thermal and Optical Instability in Silver/Amorphous-Silicon Hyperbolic Metamaterials

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    Hyperbolic metamaterials show exceptional optical properties, such as near-perfect broadband absorption, due to their geometrically-engineered optical anisotropy. Many of their proposed applications, such as thermophotovoltaics or radiative cooling, require high-temperature stability. In this work, Ag/a-Si multilayers are examined as a model system for the thermal stability of hyperbolic metamaterials. Using a combination of nanotomography, finite element simulations, and optical spectroscopy, the thermal and optical instability of the metamaterials is mapped. Although the thermal instability initiates at 300 degrees C, the hyperbolic dispersion persists up to 500 degrees C. Direct finite element simulations on tomographical data provide a route to decouple and evaluate interfacial and elastic strain energy contributions to the instability. Depending on stacking order the instability's driving force is either dominated by changes in anisotropic elastic strain energy due thermal expansion mismatch or by minimization of interfacial energy. These findings open new avenues to understand multilayer instability and pave the way to design hyperbolic metamaterials able to withstand high temperatures.ISSN:2195-107

    Filament extrusion-based additive manufacturing of NiTi shape memory alloys

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    Integrated additive manufacturing of actuators based on active materials could potentially replace conventional motors in numerous applications across disciplines like biomedical engineering, robotics, or aerospace. In this work, extrusion-based additive manufacturing of functional NiTi shape memory alloys is demonstrated via 3D printing of filaments consisting of thermoplastic binder and metal powder. Two alloys are fabricated, one showing superelastic, the other showing shape memory properties at room temperature. The microstructures of both alloys are characterized and set into perspective with the measured thermo-mechanical properties. The 3D-printed NiTi showed a shape memory strain of 1.9 %, respectively a superelastic strain of 1.3 % for an applied strain of 4 %. To enlarge the shape memory strain actuator geometries are designed, fabricated, and tested. The results of this study may find application in the field of additive manufacturing of active structures, also referred to as 4D printing. Commonly, polymeric materials are used in such structures, which often suffer from poor mechanical properties and durability. The use of metallic materials as it is investigated in this work could help to overcome these limitations.ISSN:0264-1275ISSN:1873-419