60 research outputs found
Degradation of metallic surfaces under space conditions, with particular emphasis on hydrogen recombination processes
The widespread use of metallic structures in space technology brings risk of
degradation which occurs under space conditions. New types of materials
dedicated for space applications, that have been developed in the last decade,
are in majority not well tested for different space mission scenarios. Very
little is known how material degradation may affect the stability and
functionality of space vehicles and devices during long term space missions.
Our aim is to predict how the solar wind and electromagnetic radiation degrade
metallic structures. Therefore both experimental and theoretical studies of
material degradation under space conditions have been performed. The studies
are accomplished at German Aerospace Center (DLR) in Bremen (Germany) and
University of Zielona G\'{o}ra (Poland). The paper presents the results of the
theoretical part of those studies. It is proposed that metal bubbles filled
with Hydrogen molecular gas, resulting from recombination of the metal free
electrons and the solar protons, are formed on the irradiated surfaces. A
thermodynamic model of bubble formation has been developed. We study the
creation process of -bubbles as function of, inter alia, the metal
temperature, proton dose and energy. Our model has been verified by irradiation
experiments completed at the DLR facility in Bremen. Consequences of the bubble
formation are changes of the physical and thermo-optical properties of such
degraded metals. We show that a high surface density of bubbles (up to
) with a typical bubble diameter of m will
cause a significant increase of the metallic surface roughness. This may have
serious consequences to any space mission. Changes in the thermo-optical
properties of metallic foils are especially important for the solar sail
propulsion technology, ..
The DLR Complex Irradiation Facility (CIF)
The DLR Institute of Space Systems in Bremen has built a new facility to study the behavior of materials under complex irradiation and to estimate their degradation in a space environment. It is named Complex Irradiation Facility (CIF). CIF allows simultaneously irradiating samples with three light sources for the simulation of the spectrum of solar electromagnetic radiation. The light sources are a solar simulator with a Xe-lamp (wavelength range 300-1200nm), a deuterium-UV-source (112-200nm), and an Argon-gas-jet-VUV-simulator. The latter allows irradiating samples with shorter wavelengths below the limitation of any window material. The VUV-simulator has been validated at the PTB (Physikalisch Technische Bundesanstalt) in Berlin by calibration that uses synchrotron radiation in the wavelength range between 40 and 400nm. Beside the different light sources CIF provides also electron and proton sources. Electrons and protons are generated in a low energy range from 1 to 10 keV with currents from 1 to 100 nA and in a higher range from 10 to 100 keV with 0.1 to 100 µA. Both particle sources can be operated simultaneously. In order to model temperature variations as appear in free space, the sample can be cooled down to liquid Nitrogen and heated up to about 450 K during irradiation.
The complete facility has been manufactured in UHV-technology with metal sealing. It is free of organic compounds to avoid self-contamination. The different pumping systems achieve a final pressure of 1*1010 mbar (empty sample chamber)
Besides the installed radiation sensors that control the stability of the various radiation sources and an attached mass spectrometer for analyzing the outgassing processes in the chamber, the construction of CIF allows adding other in-situ measurement systems to measure parameters that are of the user’s interest. We are currently planning to develop an in-situ measurement system in order to determine changes in the optical properties of the samples caused by irradiation. Within this paper we will show the design of CIF in more detail and discuss the performance of the various radiation sources
Neutron star cooling in transiently accreting low mass binaries: a new tool for probing nuclear matter
We explore, using an exact cooling code, the thermal evolution of a neutron
star undergoing episodes of intense accretion, alternated by long periods of
quiescence (e.g. Soft X-Ray Transients). We find that the soft component of the
quiescent luminosity of Aql X-1, 4U 1608-522 and of SAX J1808.4-3658 can be
understood as thermal emission from a cooling neutron star with negligible
neutrino emission. In the case of Cen X-4 strong neutrino emission from the
inner core is necessary to explain the observation: this may indicate that the
neutron star of Cen X-4 is heavier than 1.4 Msun. This study opens the
possibility of using the quiescent emission of Soft X-Ray Transients as a tool
for probing the core superfluidity in relation to the mass of the neutron star.Comment: 5 pages, 3 embedded figures, latex style included. To appear in
"Evolution of Binary and Multiple Stars", 2001, eds. Ph. Podsiadlowski, S.
Rappaport, A. R. King, F. D'Antona & L. Burderi (San Francisco: ASP
H2 Blister formation on metallic surfaces - a candidate for fegradation processes in space
H2-blisters are metal bubbles filled with hydrogen molecular gas resulting from recombination processes of protons in metal lattice. Bubble formation depends on many physical parameters, for instance: proton energy, proton flux, or the temperature of an exposed sample. Up to now no metallic sample that has been exposed to conditions prevalent in the interplanetary medium has been returned to Earth. Therefore, a direct evidence that blistering appears in space is missing. However, blistering is certainly a candidate of degradation processes which may occur in space. It could play an important role in the solar sail technology, where the performance of the sail is significantly affected by both the sail geometry but especially by optical properties of sail materials. Thus, both theoretical and laboratory studies of the blistering process have to be performed.
The here presented model simulates the growth of molecular hydrogen bubbles on metallic surfaces. Additionally, it calculates the decrease of reflectivity of the by blistering degraded foils. First theoretical results show that the reflectivity of an Aluminum foil decreases by about 27% for a bubble surface density of 1500 cm-2 and an average bubble radius of 100 μm. Therefore, if blistering occurs, the propulsion performance of any sail-craft will be decreased by a significant factor
Heating of the Real Polar Cap of Radio Pulsars
The heating of the real polar cap surface of radio pulsars by the bombardment
of ultra-relativistic charges is studied. The real polar cap is a significantly
smaller area within or close by the conventional polar cap which is encircled
by the last open field lines of the dipolar field . It is surrounded
by those field lines of the small scale local surface field that
join the last open field lines of in a height of cm
above the cap. As the ratio of radii of the conventional and real polar cap
, flux conservation requires . For
rotational periods s, G creates a strong electric
potential gap that forms the inner accelerating region (IAR) in which charges
gain kinetic energies eV. This sets an upper limit for
the energy that back flowing charges can release as heat in the surface layers
of the real polar cap. Within the IAR, which is flown through with a dense
stream of extremely energetic charges, no stable atmosphere of hydrogen can
survive. Therefore, we consider the polar cap as a solidified "naked" surface
consisting of fully ionized iron ions. We discuss the physical situation at the
real polar cap, calculate its surface temperatures as functions of
and , and compare the results with X-ray observations of radio pulsars.Comment: Published in MNRA
Molecular Hydrogen Bubbles Formation on Thin Vacuum Deposited Aluminum Layers after Proton Irradiation
Metals are the most common materials used in space
technology. Metal structures, while used in space, are
subjected to the full spectrum of the electromagnetic Radiation together with particle irradiation. Hence, they undergo degradation. Future space missions are planned to
proceed in the interplanetary space, where the protons of
the solar wind play a very destructive role on metallic surfaces.Unfortunately, their real degradation behavior is to
a great extent unknown.
Our aim is to predict materials’ behavior in such a
destructive environment. Therefore both, theoretical
and experimental studies are performed at the German
Aerospace Center (DLR) in Bremen, Germany.
Here, we report the theoretical results of those studies.
We examine the process of H2-bubble formation
on metallic surfaces. H2-bubbles are metal caps filled
with Hydrogen molecular gas resulting from recombination
processes of the metal free electrons and the solar
protons. A thermodynamic model of the bubble growth
is presented. Our model predicts e.g. the velocity of that
growth and the reflectivity of foils populated by bubbles.
Formation of bubbles irreversibly changes the surface
quality of irradiated metals. Thin metallic films are especially sensitive for such degradation processes. They are used e.g. in the solar sail propulsion technology. The Efficiency of that technology depends on the thermo-optical
properties of the sail materials. Therefore, bubble formation
processes have to be taken into account for the
planning of long-term solar sail missions
Design and performance of a vacuum-UV simulator for material testing under space conditions
This paper describes the construction and performance of a VUV-simulator that
has been designed to study degradation of materials under space conditions. It
is part of the Complex Irradiation Facility at DLR in Bremen, Germany, that has
been built for testing of material under irradiation in the complete UV-range
as well as under proton and electron irradiation. Presently available
UV-sources used for material tests do not allow the irradiation with
wavelengths smaller than about nm where common Deuterium lamps show an
intensity cut-off. The VUV-simulator generates radiation by excitation of a
gas-flow with an electron beam. The intensity of the radiation can be varied by
manipulating the gas-flow and/or the electron beam.
The VUV simulator has been calibrated at three different gas-flow settings in
the range from nm to nm. The calibration has been made by the
Physikalisch-Technische Bundesanstalt (PTB) in Berlin. The measured spectra
show total irradiance intensities from to mW (see Table
4.2) in the VUV-range, i.e. for wavelengths smaller than nm. They exhibit
a large number of spectral lines generated either by the gas-flow constituents
or by metal atoms in the residual gas which come from metals used in the source
construction. In the range from nm to nm where Deuterium lamps are
not usable, acceleration factors of to Solar Constants are reached
depending on the gas-flow setting. The VUV-simulator allows studies of general
degradation effects caused by photoionization and photodissociation as well as
accelerated degradation tests by use of intensities that are significantly
higher compared to that of the Sun at AU
Recycling neutron stars to ultra short periods: a statistical analysis of their evolution in the mu-P plane
We investigate the statistical evolution of magnetic neutron stars, recycled
in binary systems, simulating synthetic populations. To bracket uncertainties,
we consider a soft (FP) and a stiff (PS) equation of state (EoS) for nuclear
matter and explore the hypothesis that the magnetic field is confined in the
stellar crust. We follow the magneto-rotational evolution within a simple
recycling scenario, including the possibility of magnetospheric propeller.
We find the presence of a tail in the period distribution of the synthetic
populations at periods shorter than 1.558 ms, the minimum detected so far. For
the soft EoS the recycling gives rise to a spin distribution which is
increasing monotonically toward short periods and a clear ``barrier'' forms at
the minimum period for the onset of mass shedding. For the stiff EoS the
distribution is flatter displaying a broad maximum about 2-4 ms.
The estimated fraction of neutron stars spinning close to their shedding
limit over the millisecond pulsar population is found to be significant.
Crustal magnetic field decay models predict the existence of massive (M>1.4
M_sun) rapidly spinning neutron stars with very low magnetic moment.Comment: 34 pages, 2 tables, 9 figures, Latex. Accepted (5 Jul 99) for
publication in the Astrophysical Journal Supplement
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