3D Printed Microstructures Erasable by Darkness

To advance the applications of direct laser writing (DLW), adaptability of the printed structure is critical, prompting a shift toward printing structures that are comprised of different materials, and/or can be partially or fully erased on demand. However, most structures that contain these features are often printed by complex processes or require harsh developing techniques. Herein, a unique photoresist for DLW is introduced that is capable of printing 3D microstructures that can be erased by exposure to darkness. Specifically, microstructures based on light-stabilized dynamic materials are fabricated that remain stable when continously irradiated with green light, but degrade once the light source is switched off. The degradation and light stabilization properties of the printed materials are analyzed in-depth by time-lapse scanning electron microscopy. It is demonstrated that these resists can be used to impart responsive behavior onto the printed structure, and –critically– as a temporary locking mechanism to control the release of moving structural features.     

Thermoelectric properties of hot-pressed Ruddlesden-Popper phases CaO(CaMnO3)m

We investigated the Ruddlesden-Popper series CaO(CaMnO₃)_m with m = 1, 2, 3, ∞, to study the impact of the varying amounts of CaO layers on their thermoelectric properties. Previous studies showed that highly dense samples are difficult to obtain due to the refractory nature of these materials. In this study, we managed to obtain dense pellets during a classical hot-pressing step, if and only if the samples were subjected to extended ball-milling prior to pressing, resulting in crystallite sizes of 30–35 nm after hot-pressing. The sample with the largest amount of CaO layers (m = 1) had the lowest electrical and thermal conductivity, and the highest Seebeck coefficient, as predicted. Ultimately the perovskite CaMnO₃ (m = ∞, no CaO layers) exhibited the best thermoelectric properties.     

Carrier Localization on the Nanometer-Scale limits Transport in Metal Oxide Photoabsorbers

Metal oxides are considered as stable and low-cost photoelectrode candidates for hydrogen production by photoelectrochemical solar water splitting. However, their power conversion efficiencies usually suffer from poor transport of photogenerated charge carriers, which has been attributed previously to a variety of effects occurring on different time and length scales. In search for common understanding and for a better photo-conducting metal oxide photoabsorber, CuFeO₂, α-SnWO₄, BaSnO₃, FeVO₄, CuBi₂O₄, α-Fe₂O₃, and BiVO₄ are compared. Their kinetics of thermalization, trapping, localization, and recombination are monitored continuously 100 fs–100 µs and mobilities are determined for different probing lengths by combined time-resolved terahertz and microwave spectroscopy. As common issue, we find small mobilities < 3 cm²V^-1s^-1. Partial carrier localization further slows carrier diffusion beyond localization lengths of 1–6 nm and explains the extraordinarily long conductivity tails, which should not be taken as a sign of long diffusion lengths. For CuFeO₂, the localization is attributed to electrostatic barriers that enclose the crystallographic domains. The most promising novel material is BaSnO₃, which exhibits the highest mobility after reducing carrier localization by annealing in H₂. Such overcoming of carrier localization should be an objective of future efforts to enhance charge transport in metal oxides.     

Low-Cycle-Fatigue Performance of Stress-Aged EN AW-7075 Alloy

The effect of a novel heat treatment, that is, aging under superimposed external stress, on the fatigue performance and microstructural evolution of a high-strength aluminum alloy (EN AW-7075) is presented. Stress aging, a combination of heat treatment and superimposed external stress, can enhance the mechanical properties of EN AW-7075 under monotonic loading due to the acceleration of precipitation kinetics. Scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) reveal that a longer aging time and the presence of superimposed stress both promote the formation and growth of precipitates, that is, the precipitation of strengthening η´ precipitates. This is confirmed by differential scanning calorimetry (DSC) heating experiments of stressless and stress-aged states. Furthermore, stress aging leads to a reduction of dimensions of precipitate-free zones near grain boundaries. Cyclic deformation responses (CDRs) and half-life hysteresis loops are evaluated focusing on the low-cycle fatigue (LCF) performance of different conditions. A noticeable cyclic hardening seen in case of the specimens aged for a short time indicates the occurrence of dynamic strain aging (DSA). Eventually, stress aging allows for an enhancement of the monotonic mechanical properties of EN AW-7075 without degrading the cyclic performance in the LCF regime.     

Enhancing the Predictive Capabilities of Microstructure Simulations of PBF-LB/M by an Evaluation of Nucleation Theories

Metallurgical and Materials Transactions A - Powder Bed Fusion of Metals using a Laser Beam (PBF-LB/M) has proven to be a competitive manufacturing technology to produce customized parts with a...     

VDES R-Mode: Vulnerability analysis and mitigation concepts

VDES R-Mode aims at providing a contingency maritime positioning and navigation system when the operation of Global Navigation Satellite Systems (GNSS) is disrupted. However, VDES R-Mode, similarly to GNSS, can itself also be subject to different types of attacks, such as jamming or spoofing. In this paper, we evaluate the vulnerabilities of VDES R-Mode and discuss the effectiveness and cost of different types of countermeasures. The outcome of this cost-benefit analysis is a recommendation to introduce authentication for the navigation messages of R-Mode using the Timed Efficient Stream Loss-Tolerant Authentication (TESLA) protocol.