Many different polymers and polymer composites are used for engineering applications in which friction and wear are critical issues. This article briefs (a) the importance of polymer tribology in ...general, (b) the special design principles of polymer composites for low friction and wear under sliding against smooth metallic counterparts, and (c) synergistic effects of nano-particles and traditional fillers and fibers for an optimal tribological performance. Based on these fundamental aspects, the article reviews traditional applications of polymeric tribo-components in mechanical and automotive engineering, including slide elements in textile machines, filament wound bushings for harsh environments, cages of high-precision ball bearings in dental turbines, and hybrid bushings in Diesel fuel injection pumps. A following chapter on special developments of tribo-components outlines (a) ways to achieve electrical conductivity of polymer bearings, (b) the enhancement of self-lubrication and self-healing potential by the incorporation of micro-capsules into the polymer matrix, (c) modern additive manufacturing methods for friction and wear loaded polymer parts, (d) the application and properties of high temperature polymer coatings, and (e) the composition and use of polymer composites under friction at cryogenic temperature conditions. Keywords: Polymer, Composites, Tribology, Friction, Wear, Electrical conductivity, Self-healing, Additive manufacturing, Cryogenic temperature, Coatings, Bearings, Multifunctionality
Common Explosion Mechanism for Type Ia Supernovae Mazzali, Paolo A; Röpke, Friedrich K; Benetti, Stefano ...
Science (American Association for the Advancement of Science),
02/2007, Volume:
315, Issue:
5813
Journal Article
Peer reviewed
Open access
Type Ia supernovae, the thermonuclear explosions of white dwarf stars composed of carbon and oxygen, were instrumental as distance indicators in establishing the acceleration of the universe's ...expansion. However, the physics of the explosion are debated. Here we report a systematic spectral analysis of a large sample of well-observed type Ia supernovae. Mapping the velocity distribution of the main products of nuclear burning, we constrain theoretical scenarios. We find that all supernovae have low-velocity cores of stable iron-group elements. Outside this core, nickel-56 dominates the supernova ejecta. The outer extent of the iron-group material depends on the amount of nickel-56 and coincides with the inner extent of silicon, the principal product of incomplete burning. The outer extent of the bulk of silicon is similar in all supernovae, having an expansion velocity of ~11,000 kilometers per second and corresponding to a mass of slightly over one solar mass. This indicates that all the supernovae considered here burned similar masses and suggests that their progenitors had the same mass. Synthetic light-curve parameters and three-dimensional explosion simulations support this interpretation. A single explosion scenario, possibly a delayed detonation, may thus explain most type Ia supernovae.
We present results for a suite of 14 three-dimensional, high-resolution hydrodynamical simulations of delayed-detonation models of Type Ia supernova (SN Ia) explosions. This model suite comprises the ...first set of three-dimensional SN Ia simulations with detailed isotopic yield information. As such, it may serve as a data base for Chandrasekhar-mass delayed-detonation model nucleosynthetic yields and for deriving synthetic observables such as spectra and light curves. We employ a physically motivated, stochastic model based on turbulent velocity fluctuations and fuel density to calculate in situ the deflagration-to-detonation transition probabilities. To obtain different strengths of the deflagration phase and thereby different degrees of pre-expansion, we have chosen a sequence of initial models with 1, 3, 5, 10, 20, 40, 100, 150, 200, 300 and 1600 (two different realizations) ignition kernels in a hydrostatic white dwarf with a central density of 2.9 × 109 g cm−3, as well as one high central density (5.5 × 109 g cm−3) and one low central density (1.0 × 109 g cm−3) rendition of the 100 ignition kernel configuration. For each simulation, we determined detailed nucleosynthetic yields by post-processing 106 tracer particles with a 384 nuclide reaction network. All delayed-detonation models result in explosions unbinding the white dwarf, producing a range of 56Ni masses from 0.32 to 1.11 M. As a general trend, the models predict that the stable neutron-rich iron-group isotopes are not found at the lowest velocities, but rather at intermediate velocities (∼3000-10 000 km s−1) in a shell surrounding a 56Ni-rich core. The models further predict relatively low-velocity oxygen and carbon, with typical minimum velocities around 4000 and 10 000 km s−1, respectively.
Sub-Chandrasekhar mass white dwarfs accreting a helium shell on a carbon-oxygen core are potential progenitors of normal Type Ia supernovae. This work focuses on the details of the onset of the ...carbon detonation in the double detonation sub-Chandrasekhar model. In order to simulate the influence of core-shell mixing on the carbon ignition mechanism, the helium shell and its detonation are followed with an increased resolution compared to the rest of the star treating the propagation of the detonation wave more accurately. This significantly improves the predictions of the nucleosynthetic yields from the helium burning. The simulations were carried out with the A
REPO
code. A carbon-oxygen core with a helium shell was set up in one dimension and mapped to three dimensions. We ensured the stability of the white dwarf with a relaxation step before the hydrodynamic detonation simulation started. Synthetic observables were calculated with the radiative transfer code A
RTIS
. An ignition mechanism of the carbon detonation was observed, which received little attention before. In this “scissors mechanism”, the impact the helium detonation wave has on unburnt material when converging opposite to its ignition spot is strong enough to ignite a carbon detonation. This is possible in a carbon enriched transition region between the core and shell. The detonation mechanism is found to be sensitive to details of the core-shell transition and our models illustrate the need to consider core-shell mixing taking place during the accretion process. Even though the detonation ignition mechanism differs form the converging shock mechanism, the differences in the synthetic observables are not significant. Though they do not fit observations better than previous simulations, they illustrate the need for multi-dimensional simulations.
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► Li–S battery was investigated during cycling by means of EIS. ► An equivalent circuit is proposed for interpretation of impedance contributions. ► Diminution of charge transfer ...resistance at middle frequency due to cell degradation. ► High charge transfer resistance at high frequency due high polysulfide concentration. ► Formation of isolating layer after cycling (AFM).
The electrochemical behavior of lithium–sulfur (Li–S) batteries was investigated by means of electrochemical impedance spectroscopy (EIS). Measurements were performed in equidistant charge intervals at different depths of discharge and charge during the first cycle. Additionally, the degradation of the cells was analyzed for up to 50 cycles. An equivalent electrical circuit is proposed to simulate the electrochemical processes and to quantify the impedance contributions of Li–S batteries. EIS as a function of the cycle number shows an increased capacity for fading correlated with a decrease in the charge transfer resistance of the cathode. Atomic force microscopy (AFM) was also used to provide information about changes in the electrical conductivity of the cathode surface as they are related to the building of isolating films.
During the common envelope (CE) phase, a giant star in a binary system overflows its Roche lobe and unstable mass transfer leads to a spiral-in of the companion, resulting in a close binary system or ...in a merger of the stellar cores. Dynamo processes during the CE phase have been proposed as a mechanism to generate magnetic fields that are important for forming magnetic white dwarfs (MWDs) and for shaping planetary nebulae. Here, we present the first magnetohydrodynamics simulations of the dynamical spiral-in during a CE phase. We find that magnetic fields are strongly amplified in the accretion stream around the 1 M⊙ companion as it spirals into the envelope of a 2 M⊙ RG. This leads to field strengths of 10–100 kG throughout the envelope after 120 d. The magnetic field amplification is consistent with being driven by the magnetorotational instability. The field strengths reached in our simulation make the magnetic field interesting for diagnostic purposes, but they are dynamically irrelevant. They are also too small to explain the formation of the highest fields found in MWDs, but may be relevant for luminous red novae, and detecting magnetic fields in these events would support the scenario as proposed here.
Hydrodynamical simulations of stellar interactions require stable models of stars as initial conditions. Such initial models, however, are difficult to construct for giant stars because of the wide ...range in spatial scales of the hydrostatic equilibrium and in dynamical timescales between the core and the envelope of the giant. They are needed for, e.g., modeling the common envelope phase where a giant envelope encompasses both the giant core and a companion star. Here, we present a new method of approximating and reconstructing giant profiles from a stellar evolution code to produce stable models for multi-dimensional hydrodynamical simulations. We determine typical stellar stratification profiles with the one-dimensional stellar evolution code mesa. After an appropriate mapping, hydrodynamical simulations are conducted using the moving-mesh code arepo. The giant profiles are approximated by replacing the core of the giant with a point mass and by constructing a suitable continuation of the profile to the center. Different reconstruction methods are tested that can specifically control the convective behaviour of the model. After mapping to a grid, a relaxation procedure that includes damping of spurious velocities yields stable models in three-dimensional hydrodynamical simulations. Initially convectively stable configurations lead to stable hydrodynamical models while for stratifications that are convectively unstable in the stellar evolution code, simulations recover the convective behaviour of the initial model and show large convective plumes with Mach numbers up to 0.8. Examples are shown for a 2 M⊙ red giant and a 0.67 M⊙ asymptotic giant branch star. A detailed analysis shows that the improved method reliably provides stable models of giant envelopes that can be used as initial conditions for subsequent hydrodynamical simulations of stellar interactions involving giant stars.
•Electrochemical impedance spectra measured during the first and second lithiation.•SEI formation was studied at elevated and lower temperatures.•Arrhenius behavior calculated from the measured ...impedance spectra
To optimize lithium-ion batteries it is important to understand the formation of the solid electrolyte interphase (SEI) occurring at the graphitic anode during the first cycles. In this study we measured electrochemical impedance spectra at equidistant voltage intervals during first and second lithiation (charging process). The distribution of relaxation times (DRT) was calculated from the measured impedance spectra. Based on the DRT calculations an equivalent circuit was set up to evaluate the different impedance contributions. In addition to measurements at room temperature we also studied SEI behavior at elevated and lowered temperatures. Above 55°C we monitored an unwanted thermally induced electrolyte decomposition. Below −10°C an incomplete SEI formation was observed resulting in an increased impedance during the second lithiation. The activation energies of the different processes were calculated assuming Arrhenius behavior.
ABSTRACT The common envelope (CE) phase is an important stage in binary stellar evolution. It is needed to explain many close binary stellar systems, such as cataclysmic variables, SN Ia progenitors, ...or X-ray binaries. To form the resulting close binary, the initial orbit has to shrink, thereby transferring energy to the primary giant's envelope that is hence ejected. The details of this interaction, however, are still not understood. Here, we present new hydrodynamic simulations of the dynamical spiral-in forming a CE system. We apply the moving-mesh code arepo to follow the interaction of a compact star with a red giant possessing a core. The nearly Lagrangian scheme combines advantages of smoothed particle hydrodynamics and traditional grid-based hydrodynamic codes and allows us to capture also small flow features at high spatial resolution. Our simulations reproduce the initial transfer of energy and angular momentum from the binary core to the envelope by spiral shocks seen in previous studies, but after about 20 orbits a new phenomenon is observed. Large-scale flow instabilities are triggered by shear flows between adjacent shock layers. These indicate the onset of turbulent convection in the CE, thus altering the transport of energy on longer timescales. At the end of our simulation, only 8% of the envelope mass is ejected. The failure to unbind the envelope completely may be caused by processes on thermal timescales or unresolved microphysics.
Sex and gender differences are of fundamental importance in most diseases, including chronic kidney disease (CKD). Men and women with CKD differ with regard to the underlying pathophysiology of the ...disease and its complications, present different symptoms and signs, respond differently to therapy and tolerate/cope with the disease differently. Yet an approach using gender in the prevention and treatment of CKD, implementation of clinical practice guidelines and in research has been largely neglected. The present review highlights some sex- and gender-specific evidence in the field of CKD, starting with a critical appraisal of the lack of inclusion of women in randomized clinical trials in nephrology, and thereafter revisits sex/gender differences in kidney pathophysiology, kidney disease progression, outcomes and management of haemodialysis care. In each case we critically consider whether apparent discrepancies are likely to be explained by biological or psycho-socioeconomic factors. In some cases (a few), these findings have resulted in the discovery of disease pathways and/or therapeutic opportunities for improvement. In most cases, they have been reported as merely anecdotal findings. The aim of the present review is to expose some of the stimulating hypotheses arising from these observations as a preamble for stricter approaches using gender for the prevention and treatment of CKD and its complications.
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