820 research outputs found
Horizon Mass Theorem
A new theorem for black holes is found. It is called the horizon mass
theorem. The horizon mass is the mass which cannot escape from the horizon of a
black hole. For all black holes: neutral, charged or rotating, the horizon mass
is always twice the irreducible mass observed at infinity. Previous theorems on
black holes are: 1. the singularity theorem, 2. the area theorem, 3. the
uniqueness theorem, 4. the positive energy theorem. The horizon mass theorem is
possibly the last general theorem for classical black holes. It is crucial for
understanding Hawking radiation and for investigating processes occurring near
the horizon.Comment: A new theorem for black holes is establishe
Warped product approach to universe with non-smooth scale factor
In the framework of Lorentzian warped products, we study the
Friedmann-Robertson-Walker cosmological model to investigate non-smooth
curvatures associated with multiple discontinuities involved in the evolution
of the universe. In particular we analyze non-smooth features of the spatially
flat Friedmann-Robertson-Walker universe by introducing double discontinuities
occurred at the radiation-matter and matter-lambda phase transitions in
astrophysical phenomenology.Comment: 10 page
Quantum Corrections to the Reissner-Nordstrom and Kerr-Newman Metrics: Spin 1
A previous evaluation of one-photon loop corrections to the energy-momentum
tensor has been extended to particles with unit spin and speculations are
presented concerning general properties of such forms.Comment: 21 pages, 1 Figur
Orbital Selective Magnetism in the Spin-Ladder Iron Selenides BaKFeSe
Here we show that the 2.80(8) {\mu}B/Fe block antiferromagnetic order of
BaFe2Se3 transforms into stripe antiferromagnetic order in KFe2Se3 with a
decrease in moment to 2.1(1) {\mu}B/Fe. This reduction is larger than expected
from the change in electron count from Ba to K, and occurs with
the loss of the displacements of Fe atoms from ideal positions in the ladders,
as found by neutron pair distribution function analysis. Intermediate
compositions remain insulating, and magnetic susceptibility measurements show a
suppression of magnetic order and probable formation of a spin-glass. Together,
these results imply an orbital-dependent selection of magnetic versus bonded
behavior, driven by relative bandwidths and fillings.Comment: Final versio
INSIDE – In-situ Diagnostics in Water Electrolysers
In this joint R&D project supported by the EU Fuel Cell and Hydrogen Joint Undertaking, an electrochemical in-situ diagnostics tool for the monitoring of locally resolved current densities in polymer electrolyte membrane fuel cells, is adapted to three different water electrolysis technologies. The developed tools allow correlating performance issues and ageing processes with local anomalies. The corresponding mechanisms are investigated with ex-situ analytics.
The patented segmented printed circuit board (PCB) for the monitoring of current density distributions in PEM based fuel cells is used and steadily improved at DLR. Applications are specific degradation mechanisms and optimisation of operation parameters. The real time technology allows, e. g., to observe and mitigate local deactivation of the fuel cell due to condensing water or irreversible local ageing. It has already been adapted for the use in Redox-Flow Battery systems and is ready for the next development step.
In water electrolysis, the technological boundaries are different to that of fuel cells, but similarly, there is need for systematic optimisation by locally resolved in-situ analytics and, in particular for an on-line diagnostics tool. The challenges for the adaptation of the segmented board technology to chemical and physical environment are different for each of the three involved technologies:
- Alkaline water electrolysis
- Proton exchange membrane based water electrolysis
- Anion exchange membrane based water electrolysis
For each technology, pH and chemical ambience, pressure temperature, bubble formation, and typical range of current densities hold different requirements to layout and corrosion stability. The proof of concept has already been shown in PEM based electrolysis
Formation of desert rose structures in vacuum plasma sprayed electrodes for alkaline electrolysis
The EU FCH-JU RESelyser project is concerned with the development of high pressure, high efficiency and low cost alkaline water electrolysers that can be operated variably and intermittently to meet the demands for integration into energy networks relying on fluctuating renewable energy. The project utilizes NiAlMo alloy electrodes produced at the German Aerospace Center (DLR) by vacuum plasma spraying (VPS). VPS results in a heterogeneous microstructure consisting of a multitude of intermetallic phase sub domains and pores. Prior to electrolysis operation the electrodes are activated by leaching of Al and some Al containing intermetallic phases leaving micrometer pores and nanometer dendritic pores increasing the surface area available for the electrolysis reactions.
The vacuum plasma sprayed electrodes were analyzed by high resolution SEM and TEM before and after electrolysis operation and after storage in water. Analyses of cross sections and electrode surfaces revealed desert rose like nano flake structures on the surface and in the pores on several electrodes. The formation of the desert rose structure appeared to be related to the electrolysis operation as well as the duration of storage in distilled water. The size of the faceted flakes varied from tens of nm to a couple of µm where the thickness varied from a few nm to ~50 nm. The desert rose structure was confirmed by TEM to consist primarily of NiO and Al2NiO4 like phases (similar lattice parameters). The possible implications for the application and performance of the electrodes are discussed
Elucidating the Performance Limitations of Alkaline Electrolyte Membrane Electrolysis: Dominance of Anion Concentration in Membrane Electrode Assembly
Anion exchange membrane water electrolyzers (AEMWEs) offer a cost-effective technology for producing green hydrogen. Here, an AEMWE with atmospheric plasma spray non-precious metal electrodes was tested in 0.1 to 1.0 M KOH solution, correlating performance with KOH concentration systematically. The highest cell performance was achieved at 1.0 M KOH (ca. 0.4 A cm−2 at 1.80 V), which was close to a traditional alkaline electrolysis cell with ≈6.0 M KOH. The cell exhibited 0.13 V improvement in the performance in 0.30 M KOH compared with 0.10 M KOH at 0.5 A cm−2. However, this improvement becomes more limited when further increasing the KOH concentration. Electrochemical impedance and numerical simulation results show that the ohmic resistance from the membrane was the most notable limiting factor to operate in low KOH concentration and the most sensitive to the changes in KOH concentration at 0.5 A cm−2. It is suggested that the effect of activation loss is more dominant at lower current densities; however, the ohmic loss is the most limiting factor at higher current densities, which is a current range of interest for industrial applications
Cosmological Constant, Conical Defect and Classical Tests of General Relativity
We investigate the perihelion shift of the planetary motion and the bending
of starlight in the Schwarzschild field modified by the presence of a
-term plus a conical defect. This analysis generalizes an earlier
result obtained by Islam (Phys. Lett. A 97, 239, 1983) to the case of a pure
cosmological constant. By using the experimental data we obtain that the
parameter characterizing the conical defect is less than
and , respectively, on the length scales associated with such
phenomena. In particular, if the defect is generated by a cosmic string, these
values correspond to limits on the linear mass densities of and
, respectively.Comment: 9 pages, no figures, revte
Active gravitational mass and the invariant characterization of Reissner-Nordstrom spacetime
We analyse the concept of active gravitational mass for Reissner-Nordstrom
spacetime in terms of scalar polynomial invariants and the Karlhede
classification. We show that while the Kretschmann scalar does not produce the
expected expression for the active gravitational mass, both scalar polynomial
invariants formed from the Weyl tensor, and the Cartan scalars, do.Comment: 6 pages Latex, to appear in General Relativity and Gravitatio
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