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In situ measurements are a pivotal extension of conventional transmission electron microscopy (TEM). By means of the shape memory alloy NiTi thin film Functional Grids were produced for in situ straining as alternative or at least complement of expensive commercial holders. Due to the martensite-austenite transition temperature straining effects can be observed by use of customary heating holders in the range of 50 to 100   °C. The grids can be produced in diversified designs to fit for different strain situations. Micro tensile tests were performed and compared with finite element simulations to estimate the applied forces on the sample and to predict the functionality of different grid designs. As a first example of this Functional Grid technology, we demonstrate the impact of applying a strain to a network of ZnO tetrapods. [ABSTRACT FROM AUTHOR]

The positive influence of crystallographic compatibility on the thermal transformation stability has been already investigated extensively in the literature. However, its influence on the stability of the shape memory effect or superelasticity used in actual applications is still unresolved. In this investigation sputtered films of a highly compatible TiNiCuCo composition with a transformation matrix middle eigenvalue of 1 ± 0.01 are exposed to thermal as well as to superelastic cycling. In agreement with previous results the thermal transformation of this alloy is with a temperature shift of less than 0.1K for 40 cycles very stable; on the other hand, superelastic degradation behaviour was found to depend strongly on heat treatment parameters. To reveal the transformation dissimilarities between the differently heat-treated samples, the microstructure has been analysed by transmission electron microscopy, in situ stress polarization microscopy and synchrotron analysis. It is found that good crystallographic stability is not a sufficient criterion to avoid defect generation which guarantees high superelastic stability. For the investigated alloy, a small grain size was identified as the determining factor which increases the yield strength of the composition and decreases the functional degradation during superelastic cycling. [ABSTRACT FROM AUTHOR]

The elastocaloric effect that occurs during the stress-induced martensitic transformation in shape memory alloys is a promising mechanism in view of solid state cooling applications. It also allows for downscaling to feature sizes in the μm range, thus, being attractive for micro-cooling applications using thin film materials. In this study, elastocaloric properties of TiNi and TiNiCu films and their relation to functional fatigue were investigated. Both materials show similar effect sizes, their fatigue behavior is however different. While the temperature change in TiNi degrades by a factor of two within 150 cycles, no significant elastocaloric fatigue was found in TiNiCu. [ABSTRACT FROM AUTHOR]

State of the art minimally invasive treatments and diagnostics of neurological and cardiovascular diseases demand for flexible instruments and implants that enable sensing and stimulation of bioelectric signals. Besides medical applications, implantable bioelectronic brain-computer interfaces are envisioned as the next step in communication and data transfer. Conventional microelectrode arrays used for these types of applications are based on polymer substrates that are not suitable for biostable, rigid and self-expanding devices. Here, we present fully integrated bioelectrodes on superelastic NiTi carriers fabricated by microsystem technology processes. The insulation between the metallic NiTi structure and the Pt electrode layer is realized by different oxide layers (SiO x , TaO x and Yttrium stabilized Zirconia YSZ). Key properties of bioelectronic implants such as dissolution in body fluids, biocompatibility, mechanical properties and bioelectrical sensing/stimulation capabilities have been investigated by in vitro methods. Particular devices with YSZ are biostable and biocompatible, enabling sensing and stimulation. The major advantage of this system is the combination of medically approved materials and novel fabrication technology that enables miniaturization and integration beyond the state-of-the-art processes. The results demonstrate that this functionalization of superelastic NiTi is an enabling technology for the development of new kinds of bioelectronic devices. • NiTi-based microdevices with integrated bioelectrodes prepared by a novel MEMS fabrication process. • Assessment of biostability, biocompatibility and mechanical stability of SiO x and TaO x , YSZ as electrical isolation layer. • Sensing and stimulation properties were investigated by electrochemical in vitro experiments. • Micro-electrodes on the flexible NiTi-based device show similar properties compared to conventional Pt electrodes. [ABSTRACT FROM AUTHOR]

The exploitation of the elastocaloric effect in superelastic shape memory alloys (SMA) for cooling applications shows a promising energy efficiency potential but requires a better understanding of the non-homogeneous martensitic phase transformation. Temperature profiles on sputter-deposited superelastic $${\mathrm {Ti_{55.2}Ni_{29.3}Cu_{12.7}Co_{2.8}}}$$ shape memory alloy thin films show localized release and absorption of heat during phase transformation induced by tensile deformation with a strong rate dependence. In this paper, a model for the simulation of the thermo-mechanically coupled transformation behavior of superelastic SMA is proposed and its capability to reproduce the mechanical and thermal responses observed during experiments is shown. The procedure for experiment and simulation is designed such that a significant temperature change from the initial temperature is obtained to allow potential cooling applications. The simulation of non-local effects is enabled by the use of a model based on the one-dimensional Müller-Achenbach-Seelecke model, extended by 3D mechanisms such as lateral contraction and by non-local interaction, leading to localization effects. It is implemented into the finite element software COMSOL Multiphysics, and comparisons of numerical and experimental results show that the model is capable of reproducing the localized transformation behavior with the same strain rate dependency. Additionally to the thermal and the mechanical behavior, the quantitative prediction of cooling performance with the presented model is shown. [ABSTRACT FROM AUTHOR]

The time-resolved strain and temperature profiles of magnetron-sputtered pseudoelastic TiNi films of 20 μm thickness are investigated during tensile stress cycling for different strain rates. Based on simultaneous infrared thermography and digital image correlation, a good correlation between the evolution of temperature profiles and the local formation and propagation of Lüders-like strain bands is observed. The evolution of temperature profiles can be quantitatively described by a phenomenological Tanaka-type model of the martensitic transformation combined with a heat transfer model taking into account the local interaction between transformed and untransformed regions in the transformation kinetics. The investigated film samples exhibit an undercooling of −16 K upon mechanical unloading under adiabatic conditions at a strain rate of 0.2 s −1 . A coefficient of performance of 7.7 is determined. Due to the high surface-to-volume ratio, the films show fast heat exchange in air of the order of 1.5 s. Based on these results, the prospects of SMA film-based elastocaloric cooling are discussed. [ABSTRACT FROM AUTHOR]