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Influence of a 265 °C heat treatment on the residual stress state of a PBF-LB/M AlSi10Mg alloy
(2022)
Laser Powder Bed Fusion (PBF-LB/M) additive manufacturing (AM) induces high magnitude residual stress (RS) in structures due to the extremely heterogeneous cooling and heating rates. As the RS can be deleterious to the fatigue resistance of engineering components, great efforts are focused on understanding their generation and evolution after post-process heat treatments.
In this study, one of the few of its kind, the RS relaxation induced in an as-built PBF-LB/M AlSi10Mg material by a low-temperature heat treatment (265 & DEG;C for 1 h) is studied by means of X-ray and neutron diffraction. Since the specimens are manufactured using a baseplate heated up to 200 & DEG;C, low RS are found in the as-built condition.
After heat treatment a redistribution of the RS is observed, while their magnitude remains constant. It is proposed that the redistribution is induced by a repartition of stresses between the alpha-aluminium matrix and the silicon phase, as the morphology of the silicon phase is affected by the heat treatment.
A considerable scatter is observed in the neutron diffraction RS profiles, which is principally correlated to the presence (or absence) of pockets of porosity developed at the borders of the chessboard pattern.
Despite the impressive performance and incredible promise for a variety of applications, the wide-scale commercialization of graphene is still behind its full potential. One of the main challenges is related to preserving graphene's unique properties upon transfer onto practically desirable substrates. In this work, few-layer graphene sheets deposited via liquid-phase transfer from copper onto a quartz substrate have been studied using a suite of experimental techniques, including scanning electron microscopy (SEM), Raman spectroscopy, admittance spectroscopy, and four-point probe electrical measurements. SEM measurements suggest that the transfer of graphene from copper foil to quartz using the aqueous solution of ammonium persulfate was accompanied by unintentional etching of the entire surface of the quartz substrate and, as a result, the formation of microscopic facet structures covering the etched surface of the substrate. As revealed by Raman spectroscopy and the electrical measurements, the transfer process involving the etching of the copper foil in a 0.1 M solution of (NH4)(2)S2O8 resulted in its p-type doping. This was accompanied by the appearance of an electronic gap of 0.022 eV, as evidenced by the Arrhenius analysis. The observed increase in the conductance of the samples with temperature can be explained by thermally activated carrier transport, dominating the scattering processes.
New continuum and polarization observations of the Cygnus Loop with FAST. II. Images and analyses
(2022)
We present total-intensity and polarized-intensity images of the Cygnus Loop supernova remnant (SNR) observed by the Five-hundred-meter Aperture Spherical radio Telescope. The high angular-resolution and high-sensitivity images enable us to thoroughly compare the properties of the northern part with the southern part of the SNR. The central filament in the northern part and the southern part have a similar foreground rotation measure, meaning their distances are likely similar. The polarization analysis indicates that the random magnetic field is larger than the regular field in the northern part, but negligible in the southern part. The total-intensity image is decomposed into components of various angular scales, and the brightness-temperature spectral index of the shell structures in the northern part is similar to that in the southern part in the component images. All the evidence suggests that the northern and southern parts of the Cygnus Loop are situated and thus evolved in different environments of interstellar medium, while belonging to the same SNR.
We analyze diffusion processes with finite propagation speed in a non-homogeneous medium in terms of the heterogeneous telegrapher's equation. In the diffusion limit of infinite-velocity propagation we recover the results for the heterogeneous diffusion process. The heterogeneous telegrapher's process exhibits a rich variety of diffusion regimes including hyperdiffusion, ballistic motion, superdiffusion, normal diffusion and subdiffusion, and different crossover dynamics characteristic for complex systems in which anomalous diffusion is observed. The anomalous diffusion exponent in the short time limit is twice the exponent in the long time limit, in accordance to the crossover dynamics from ballistic diffusion to normal diffusion in the standard telegrapher's process. We also analyze the finite-velocity heterogeneous diffusion process in presence of stochastic Poissonian resetting. We show that the system reaches a non-equilibrium stationary state. The transition to this non-equilibrium steady state is analyzed in terms of the large deviation function.
Context. Galactic cosmic rays (CRs) are widely assumed to arise from diffusive shock acceleration, specifically at shocks in supernova remnants (SNRs). These shocks expand in a complex environment, particularly in the core-collapse scenario as these SNRs evolve inside the wind-blown bubbles created by their progenitor stars. The CRs at core-collapse SNRs may carry spectral signatures of that complexity. Aims. We study particle acceleration in the core-collapse SNR of a progenitor with an initial mass of 60 M-circle dot and realistic stellar evolution. The SNR shock interacts with discontinuities inside the wind-blown bubble and generates several transmitted and reflected shocks. We analyse their impact on particle spectra and the resulting emission from the remnant. Methods. To model the particle acceleration at the forward shock of a SNR expanding inside a wind bubble, we initially simulated the evolution of the pre-supernova circumstellar medium (CSM) by solving the hydrodynamic equations for the entire lifetime of the progenitor star. As the large-scale magnetic field, we considered parameterised circumstellar magnetic field with passive field transport. We then solved the hydrodynamic equations for the evolution of a SNR inside the pre-supernova CSM simultaneously with the transport equation for CRs in test-particle approximation and with the induction equation for the magnetohydrodynamics in 1D spherical symmetry. Results. The evolution of a core-collapse SNR inside a complex wind-blown bubble modifies the spectra of both the particles and their emission on account of several factors including density fluctuations, temperature variations, and the magnetic field configuration. We find softer particle spectra with spectral indices close to 2.5 during shock propagation inside the shocked wind, and this softness persists at later evolutionary stages. Further, our calculated total production spectrum released into the interstellar medium demonstrates spectral consistency at high energy (HE) with the injection spectrum of Galactic CRs, which is required in propagation models. The magnetic field structure effectively influences the emission morphology of SNRs as it governs the transportation of particles and the synchrotron emissivity. There is rarely a full correspondence of the intensity morphology in the radio, X-ray, and gamma-ray bands.
Wrapped up
(2022)
A huge number of bacterial species are motile by flagella, which allow them to actively move toward favorable environments and away from hazardous areas and to conquer new habitats. The general perception of flagellum-mediated movement and chemotaxis is dominated by the Escherichia coli paradigm, with its peritrichous flagellation and its famous run-and-tumble navigation pattern, which has shaped the view on how bacteria swim and navigate in chemical gradients. However, a significant amount-more likely the majority-of bacterial species exhibit a (bi)polar flagellar localization pattern instead of lateral flagella. Accordingly, these species have evolved very different mechanisms for navigation and chemotaxis. Here, we review the earlier and recent findings on the various modes of motility mediated by polar flagella.
A recent advancement in the field of neuromodulation is to adapt stimulation parameters according to prespecified biomarkers tracked in real-time. These markers comprise short and transient signal features, such as bursts of elevated band power. To capture these features, instantaneous measures of phase and/or amplitude are employed, which inform stimulation adjustment with high temporal specificity. For adaptive neuromodulation it is therefore necessary to precisely estimate a signal's phase and amplitude with minimum delay and in a causal way, i.e. without depending on future parts of the signal. Here we demonstrate a method that utilizes oscillation theory to estimate phase and amplitude in real-time and compare it to a recently proposed causal modification of the Hilbert transform. By simulating real-time processing of human LFP data, we show that our approach almost perfectly tracks offline phase and amplitude with minimum delay and is computationally highly efficient.
In contrast to molecular-dipole polymers, such as PVDF, ferroelectrets are a new class of flexible spatially heterogeneous piezoelectric polymers with dosed or open voids that act as deformable macro-dipoles after charging.
With a spectrum of manufacturing processes being developed to engineer the heterogeneous structures, ferroelectrets are made with attractive piezoelectric properties well-suited for applications, such as pressure sensors, acoustic transducers, etc.
However, the sources of the macro-dipole charges have usually been the same, microscopic dielectric barrier discharges within the voids, induced when the ferroelectrets are poled under a large electric field typically via a so-called corona poling, resulting in the separation and trapping of opposite charges into the interior walls of the voids.
Such a process is inherently self-limiting, as the reverse internal field from the macro-dipoles eventually extinguishes the microdischarges, resulting in limited density of ions and not too high overall piezoelectric performance. Here, a new method to form ferroelectrets with gigantic electroactivity is proposed and demonstrated with the aid of an external ion booster.
A laminate consisting of expanded polytetrafluoroethylene (ePTFE) and fluorinated-ethylene-propylene (FEP) was prefilled with bipolar ions produced externally by an ionizer and sequentially poled to force the separation of positive and negative ions into the open fibrous structure, rendering an impressive piezoelectric d(33)( )coefficient of 1600 pC/N-an improvement by a factor of 4 in comparison with the d(33) of a similar sandwich poled with nonenhanced corona poling.
The (pre)filling dearly increases the ion density in the open voids significantly. The charges stored in the open-cell structure stays at a high level for at least 4 months. In addition, an all-organic nanogenerator was made from an ePTFE-based ferroelectret, with conducting poly(3,4-ethylene dioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) coated fabric electrodes.
When poled with this ion-boosting process, it yielded an output power twice that of a similar sample poled in a conventional corona-only process. The doubling in output power is mainly brought about by the significantly higher charge density achieved with the aid of external booster.
Furthermore, aside from the bipolar ions, extra monopolar ions can during the corona poling be blown into the open pores by using for instance a negative ionic hair dryer to produce a unipolar ePTFE-based ferroelectret with its d(33) coefficient enhanced by a factor of 3. Ion-boosting poling thus unleashes a new route to produce bipolar or unipolar open-cell ferroelectrets with highly enhanced piezoelectric response.
For an effectively one-dimensional, semi-infinite disordered system connected to a reservoir of tracer particles kept at constant concentration, we provide the dynamics of the concentration profile.
Technically, we start with the Montroll-Weiss equation of a continuous time random walk with a scale-free waiting time density.
From this we pass to a formulation in terms of the fractional diffusion equation for the concentration profile C(x, t) in a semi-infinite space for the boundary condition C(0, t) = C-0, using a subordination approach.
From this we deduce the tracer flux and the so-called breakthrough curve (BTC) at a given distance from the tracer source.
In particular, BTCs are routinely measured in geophysical contexts but are also of interest in single-particle tracking experiments.
For the "residual' BTCs, given by 1- P(x, t), we demonstrate a long-time power-law behaviour that can be compared conveniently to experimental measurements.
For completeness we also derive expressions for the moments in this constant-concentration boundary condition.
Solar filaments often erupt partially. Although how they split remains elusive, the splitting process has the potential of revealing the filament structure and eruption mechanism. Here we investigate the pre-eruption splitting of an apparently single filament and its subsequent partial eruption on 2012 September 27. The evolution is characterized by three stages with distinct dynamics. During the quasi-static stage, the splitting proceeds gradually for about 1.5 hr, with the upper branch rising at a few kilometers per second and displaying swirling motions about its axis. During the precursor stage that lasts for about 10 minutes, the upper branch rises at tens of kilometers per second, with a pair of conjugated dimming regions starting to develop at its footpoints; with the swirling motions turning chaotic, the axis of the upper branch whips southward, which drives an arc-shaped extreme-ultraviolet front propagating in a similar direction. During the eruption stage, the upper branch erupts with the onset of a C3.7-class two-ribbon flare, while the lower branch remains stable. Judging from the well-separated footpoints of the upper branch from those of the lower one, we suggest that the pre-eruption filament processes a double-decker structure composed of two distinct flux bundles, whose formation is associated with gradual magnetic flux cancellations and converging photospheric flows around the polarity inversion line.