Gunther Eggeler

Thermal Stabilization of NiTiCuV Shape Memory Alloys: Observations During Elastocaloric Training

The paper presents novel findings observed during the training process of superelastic, elastocalorically optimized Ni–Ti-based shape memory alloys (SMA). NiTiCuV alloys exhibit large latent heats and a small mechanical hysteresis, which may potentially lead to the development of efficient solid-state-based cooling processes. The paper starts with a brief introduction to the underlying principles of elastocaloric cooling, illustrating the effect by means of a typical thermodynamic cycle. It proceeds with the description of a custom-built testing platform that allows observation of temperature profiles and heat transfer between SMA and heat source/sink during high-loading-rate tensile tests. Similar to other SMA applications, a training process is necessary in order to guarantee stable performance. This well-known mechanical stabilization affects the stress–strain hysteresis and the cycle-dependent evolution of differential scanning calorimetry results. In addition, it can be shown here that the training is also accompanied by a cycle-dependent evolution of temperature profiles on the surface of an SMA ribbon. The applied training leads to local temperature peaks with intensity, number, and distribution of the temperature fronts showing a cycle dependency. The homogeneity of the elastocaloric effect has a significant influence on the efficiency of elastocaloric cooling process and is a key aspect of the specific material characterization.

[001] preferentially-oriented 2D tungsten disulfide nanosheets as anode materials for superior lithium storage

Rechargeable lithium ion batteries (LIBs) have transformed portable electronics and will play a crucial role in transportation, such as electric vehicles. For higher energy storage in LIBs, two issues should be addressed, that is, the fundamental understanding of the chemistry taking place in LIBs and the discovery of new materials. Here we design and fabricate two-dimensional (2D) WS2 nanosheets with preferential [001] orientation and perfect single crystalline structures. Being used as an anode for LIBs, the WS2-nanosheet electrode exhibits a high specific capacity, good cycling performance and excellent rate capability. Considering the controversy in the lithium storage mechanism of WS2, ex-situ X-ray diffraction (XRD), Raman and X-ray photoelectron spectroscopy (XPS) analyses clearly verify that the recharge product (3.0 V vs. Li+/Li) of the WS2 electrode after fully discharging to 0.01 V (vs. Li+/Li) tends to reverse to WS2. More remarkably, the [001] preferentially-oriented 2D WS2 nanosheets are also promising candidates for applications in photocatalysis, water splitting, and so forth.

Atomic layer-by-layer construction of Pd on nanoporous gold via underpotential deposition and displacement reaction

Atomic layer-by-layer construction of Pd on nanoporous gold (NPG) has been investigated through the combination of underpotential deposition (UPD) with displacement reaction. It has been found that the UPD of Cu on NPG is sensitive to the applied potential and the deposition time. The optimum deposition potential and time were determined through potential- and time-sensitive stripping experiments. The NPG-Pd electrode shows a different voltammetric behavior in comparison to the bare NPG electrode, and the deposition potential was determined through the integrated charge control for the monolayer UPD of Cu on the NPG-Pd electrode. Five layers of Pd were constructed on NPG through the layer-by-layer deposition. In addition, the microstructure of the NPG-Pdx (x = 1, 2, 3, 4 and 5) films was probed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). The microstructural observation demonstrates that the atomic layers of Pd form on the ligament surface of NPG through epitaxial growth, and have no effect on the nanoporous structure of NPG. In addition, the hydrogen storage properties of the NPG-Pdx electrodes have also been addressed.

On the widths of the hysteresis of mechanically and thermally induced martensitic transformations in Ni–Ti-based shape memory alloys

Abstract It is well known that a good crystallographic compatibility between austenite and martensite in Ni–Ti-based shape memory alloys results in narrow thermal hystereses (e.g. Ball and James, Arch. Ration. Mech. Anal., 1987). The present work suggests that a good crystallographic fit is moreover associated with a small mechanical hysteresis width, observed during a forward and reverse stress-induced transformation. Furthermore, shape memory alloys with a good crystallographic fit show smaller transformation strains. The results obtained in the present study suggest that these correlations are generic and apply to binary Ni–Ti (with varying Ni contents) and quaternary Ni–Ti–Cu–X (X = Cr, Fe, V) alloys. For binary Ni–Ti, it was observed that Ni-rich compositions (good lattice fit) show a lower accummulation of irreversible strains during pseudoelastic cycling.

Functional Fatigue and Tension–Compression Asymmetry in [001]-Oriented Co49Ni21Ga30 High-Temperature Shape Memory Alloy Single Crystals

Conventional shape memory alloys cannot be employed for applications in the elevated temperature regime due to rapid functional degradation. Co–Ni–Ga has shown the potential to be used up to temperatures of about 400xa0°C due to a fully reversible superelastic stress–strain response. However, available results only highlight the superelastic response for single cycle tests. So far, no data addressing cyclic loading and functional fatigue are available. In order to close this gap, the current study reports on the cyclic degradation behavior and tension–compression asymmetry in [001]-oriented Co49Ni21Ga30 single crystals at elevated temperatures. The cyclic stress-strain response of the material under displacement controlled superelastic loading conditions was found to be dictated by the number of active martensite variants and different resulting stabilization effects. Co–Ni–Ga shows a large superelastic temperature window of about 400xa0°C under tension and compression, but a linear Clausius–Clapeyron relationship could only be observed up to a temperature of 200xa0°C. In the present experiments, the samples were subjected to 1000 cycles at different temperatures. Degradation mechanisms were characterized by neutron diffraction and transmission electron microscopy. The results in this study confirm the potential of these alloys for damping applications at elevated temperatures.

Microstructural evolution in a Ti – Ta high-temperature shape memory alloy during creep

Abstract Alloys based on the titanium–tantalum system are considered for application as high-temperature shape memory alloys due to their martensite start temperatures, which can surpass 200 °C. In the present work we study the evolution of microstructure and the influence of creep on the phase transformation behavior of a Ti70Ta30 (at.%) high-temperature shape memory alloy. Creep tests were performed in a temperature range from 470 to 530 °C at stresses between 90 and 150 MPa. The activation energy for creep was found to be 307 kJ mol−1 and the stress exponent n was determined as 3.7. Scanning and transmission electron microscopy investigations were carried out to characterize the microstructure before and after creep. It was found that the microstructural evolution during creep suppresses subsequent martensitic phase transformations.

Cyclic degradation of titanium–tantalum high-temperature shape memory alloys — the role of dislocation activity and chemical decomposition

Titanium–tantalum shape memory alloys (SMAs) are promising candidates for actuator applications at elevated temperatures. They may even succeed in substituting ternary nickel–titanium high temperature SMAs, which are either extremely expensive or difficult to form. However, titanium–tantalum alloys show rapid functional and structural degradation under cyclic thermo-mechanical loading. The current work reveals that degradation is not only governed by the evolution of the ω-phase. Dislocation processes and chemical decomposition of the matrix at grain boundaries also play a major role.