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Novel three-dimensional (3D) hollow aero-silicon nano- and microstructures, namely, Si-tetrapods (Si-T) and Si-spheres (Si-S) were synthesized by a sacrificial template approach for the first time. The new Si-T and Si-S architectures were found as most temperature-stable hollow nanomaterials, up to 1000 °C, ever reported. The synthesized aero-silicon or aerogel was integrated into sensor structures based on 3D networks. A single microstructure Si-T was employed to investigate electrical and gas sensing properties. The elaborated hollow microstructures open new possibilities and a wide area of perspectives in the field of nano- and microstructure synthesis by sacrificial template approaches. The enormous flexibility and variety of the hollow Si structures are provided by the special geometry of the sacrificial template material, ZnO-tetrapods (ZnO-T). A Si layer was deposited onto the surface of ZnO-T networks by plasma-enhanced chemical vapor deposition. All samples demonstrated p-type conductivity; hence, the resistance of the sensor structure increased after introducing the reducing gases in the test chamber. These hollow structures and their unique and superior properties can be advantageous in different fields, such as NEMS/MEMS, batteries, dye-sensitized solar cells, gas sensing in harsh environment, and biomedical applications. This method can be extended for synthesis of other types of hollow nanostructures.

This Review covers the fundamentals of operation and scaling of elastocaloric cooling devices as well as current developments of elastocaloric shape‐memory alloy (SMA) films and the engineering of SMA film‐based cooling devices. Sputter‐deposited TiNiCuCo alloys showing ultra‐low fatigue enable unique functional properties such as tailored transformation temperature gradients. Two substantially different concepts for the development of elastocaloric cooling demonstrators are discussed. One concept relies on heat transfer by mechanical contact between the elastocaloric SMA film and solid heat sink and source elements. The second concept makes use of the heat transfer between the elastocaloric SMA film and a heat transfer fluid, including the advanced technology of active regeneration. Demonstrators based on a single SMA film reach device temperature spans of 14 K and a high specific cooling power of up to 18 W g−1. The performance characteristics are compared with other solid‐state caloric cooling technologies.

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Elastocaloric cooling demands for ultra-low functional and structural fatigue in combination with a high effect size and low energy input. Recent advances in fine-grained sputtered Ti-rich Ti54Ni34Cu12 and Ti54.7Ni30.7Cu12.3Co2.3 alloys show that a high fatigue resistance can be achieved. Ti54Ni34Cu12 shows a good compatibility (λ2 ∼ 0.9905) with coherent Ti2Cu precipitates, whereas Ti54.7Ni30.7Cu12.3Co2.3 shows a near perfect compatibility (λ2 ∼ 1.00083) but no Ti2Cu and lower transition temperatures. To differentiate whether the crystallographic compatibility or Ti2Cu precipitates influence the functional properties more, a TiNiCuCo alloy with a large expected fraction of Ti2Cu precipitates was chosen. In this work, freestanding Ti52.8Ni22.2Cu22.5Co2.5 films are fabricated by a multilayer sputter deposition approach. They show stable superelasticity for more than 2 × 107 cycles with almost no degradation. Temperature-dependent x-ray diffraction and scanning transmission electron microscopy-high-angle annular dark-field imaging investigations identify that a perfect crystallographic compatibility (λ2 ∼ 0.994 instead of 1) is not needed for high cyclic stability when combined with a small grain size (∼300 nm) and Ti2Cu precipitates. In situ x-ray diffraction studies of the stress-induced transformation reveal the presence of non-transformed austenite well above the superelastic plateau and an eased transformation perpendicular to the loading direction. In agreement with XRD studies, the adiabatic temperature change shows an increase with increasing strain up to −12.2 K for the reverse transformation. The material shows a stable isothermal entropy change of −21.8 J kg−1 K−1 over a wide range of 40 K. The average COPmat reaches a value of 11.2, which makes Ti52.8Ni22.2Cu22.5Co2.5 highly suitable for elastocaloric cooling applications.

Elastocaloric cooling demands for temperature changes larger than 30 K to become an alternative to classical vapour compression cooling systems. The principle of active regeneration allows to exceed the materials intrinsic adiabatic temperature change. The resulting temperature gradient of the cooling demonstrator leads to a change in the thermomechanical response along the regenerator bed, according to the Clausius-Clapeyron equation. A lowered efficiency as well as an increased probability of early breakdown of the device due to functional and especially structural fatigue are the result. These changes in the thermomechanical response are especially present in NiTi and NiTiCu-based systems with a coefficient @σ @T of about 7 and 10 MPa K À1 , respectively. To address these issues for future applications a change in the transformation temperature along the shape memory alloy film, adapted to the temperature gradient of the regenerator, is required. Cobalt is well known to reduce the transformation temperatures, while maintaining the functional stability of the TiNiCu-based films. In this study multilayer dcmagnetron sputtering is used to fabricate TiNiCuCo films with a cobalt concentration gradient along the samples, which can be precisely tuned by changing the sputter conditions. Transformation gradients of 0.3 K mm À1 are obtained, showing the same functional stability and adiabatic temperature changes of about À11 K as their counterparts without transformation gradient. Under isothermal conditions, a sloped transformation plateau is observed corresponding to a highly directed martensitic transformation.

Conductive serpentine interconnects comprise fundamental building blocks (e.g., electrodes, antennas, wires) of many stretchable electronic systems. Here we present the first numerical and experimental studies of freestanding thin-film TiNiCuCo superelastic alloys for stretchable interconnects. The electrical resistivity of the austenite phase of a Ti53.3Ni30.9Cu12.9Co2.9 thin-film at room temperature was measured to be 5.43×10-7 Ω m, which is larger than reported measurements for copper thin-films (1.87×10-8 Ω m). Structuring the superelastic conductor to limit localized strain using a serpentine geometry led to freestanding interconnects that could reach maximum serpentine elongations of up to 153%. Finite element analysis (FEA) simulations predicted that superelastic serpentine interconnects can achieve significantly larger (~5X–7X) elastic elongations than copper for the same serpentine geometry. FEA predictions for stress distribution along the TiNiCuCo serpentine interconnect were experimentally verified by infrared imaging and tensile testing experiments. The superior mechanical advantages of TiNiCuCo were paired with the high electronic conductivity of copper, to create Cu/TiNiCuCo/Cu serpentine composites that were demonstrated to serve as freestanding electrical interconnects between two LEDs. The results presented in this manuscript demonstrate that thin-film superelastic alloys are a promising material class to improve the performance of conductors in stretchable and flexible electronics.

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