Inherent features of wavelets and pulse coupled networks

Biologically inspired image/signal processing like the pulse coupled neural network (PCNN) and the wavelet (packet) transforms are described. The two methods are applied to two-dimensional data in order to demonstrate the features of each method, pinpoint differences as well as similarities. The inherent properties (with respect to filtering, segmentation, etc.) of the two approaches with respect to detectors for physics experiments as well as remote sensing are discussed.     

Ramping down a clinical 3 T scanner: a journey into MRI and MRS at 0.75 T

Object

Lower-field MR is reemerging as a viable, potentially cost-effective alternative to high-field MR, thanks to advances in hardware, sequence design, and reconstruction over the past decades. Evaluation of lower field strengths, however, is limited by the availability of lower-field systems on the market and their considerable procurement costs. In this work, we demonstrate a low-cost, temporary alternative to purchasing a dedicated lower-field MR system.

Materials and Methods

By ramping down an existing clinical 3 T MRI system to 0.75 T, proton signals can be acquired using repurposed ¹³C transmit/receive hardware and the multi-nuclei spectrometer interface. We describe the ramp-down procedure and necessary software and hardware changes to the system.

Results

Apart from presenting system characterization results, we show in vivo examples of cardiac cine imaging, abdominal two- and three-point Dixon-type water/fat separation, water/fat-separated MR Fingerprinting, and point-resolved spectroscopy. In addition, the ramp-down approach allows unique comparisons of, e.g., gradient fidelity of the same MR system operated at different field strengths using the same receive chain, gradient coils, and amplifiers.

Discussion

Ramping down an existing MR system may be seen as a viable alternative for lower-field MR research in groups that already own multi-nuclei hardware and can also serve as a testing platform for custom-made multi-nuclei transmit/receive coils.

     

Investigations on the polymorphism of K4CaSi6O15 at elevated temperatures

In the present study, single crystals and polycrystalline material of K₄CaSi₆O₁₅ were prepared from solid-state reactions between stoichiometric mixtures of the corresponding oxides/carbonates. Heat capacity (C_p) measurements above room temperature using a differential scanning calorimeter indicated that two thermal effects occurred at approximately T₁ = 462 K and T₂ = 667 K, indicating the presence of structural phase transitions. The standard third-law entropy of K₄CaSi₆O₁₅ was determined from low-temperature C_p’s measured by relaxation calorimetry using a Physical Properties Measurement System and amounts to S°(298K) = 524.3 ± 3.7 J·mol⁻¹·K⁻¹. For the 1^st transition, the enthalpy change ΔH^tr1 = 1.48 kJ·mol and the entropy change ΔS^tr1 = 3.25 J·mol⁻¹·K⁻¹, whereas ΔH^tr2 = 3.33 kJ·mol⁻¹ and ΔS^tr2 = 5.23 J·mol⁻¹·K⁻¹ were determined for the 2^nd transition. The compound was further characterized by in-situ single-crystal X-ray diffraction between ambient temperature and 1063 K. At 773 K, the high-temperature phase stable above T₂ has the following basic crystallographic data: monoclinic symmetry, space group P2₁/c, a = 6.9469(4) Å, b = 9.2340(5) Å, c = 12.2954(6) Å, β = 93.639(3)°, V = 787.13(7) ų, Z = 2. It belongs to the group of interrupted framework silicates and is based on tertiary (Q³-type) [SiO₄]-tetrahedra. Together with the octahedrally coordinated Ca-cations, a three-dimensional mixed polyhedral network structure is formed, in which the remaining K-ions provide charge balance by occupying voids within the net. The intermediate temperature modification stable between T₁ and T₂ shows a (3+2)-dimensional incommensurately modulated structure that is characterized by the following q-vectors: q₁ = (0.057, 0.172, 0.379), q₂ = (-0.057, 0.172, -0.379). The crystal structures of the high- and the previously studied ambient temperature polymorph (space group Pc) are topologically equivalent and show a group-subgroup relationship. The index of the low- in the high-symmetry group is six and involves both, losses in translation as well as point group symmetry. The distortion is based on shifts of the different atom species and tilts of the 4- and 6-fold coordination polyhedra. Actually, for some of the oxygen atoms, the displacements exceed 0.5 Å. A more detailed analysis of the distortions relating to both structures has been performed using mode analysis, which revealed that the primary distortion mode transforms according to the Λ₁ irreducible representation of P2₁/c. However, other modes with smaller distortion amplitudes are also involved.     

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.     

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.