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 $\rm{H_2}$-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 $10^8$ $\rm{cm^{-2}}$) with a typical bubble diameter of $\sim 0.4$$\rm{\mu}$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

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

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

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 $\vec{B}_d$. It is surrounded by those field lines of the small scale local surface field $\vec{B}_s$ that join the last open field lines of $\vec{B}_d$ in a height of $\sim 10^5$ cm above the cap. As the ratio of radii of the conventional and real polar cap $R_{dip}/R_{pc}\sim 10$, flux conservation requires $B_s/B_d\sim 100$. For rotational periods $P\sim 0.5$ s, $B_s\sim 10^{14}$ G creates a strong electric potential gap that forms the inner accelerating region (IAR) in which charges gain kinetic energies $\sim 3\times 10^{14}$ 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 $T_s$ as functions of $B_s$ and $P$, and compare the results with X-ray observations of radio pulsars.Comment: Published in MNRA

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 $115$ 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 $40$ nm to $410$ nm. The calibration has been made by the Physikalisch-Technische Bundesanstalt (PTB) in Berlin. The measured spectra show total irradiance intensities from $24$ to $58$ mW$\rm{m^{-2}}$ (see Table 4.2) in the VUV-range, i.e. for wavelengths smaller than $200$ 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 $40$ nm to $120$ nm where Deuterium lamps are not usable, acceleration factors of $3$ to $26.3$ 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 $1$ AU

The Complex Irradiation Facility at DLR-Bremen

All material exposed to interplanetary space conditions are subject to degradation processes. For obvious reasons there is a great interest to study these processes for materials that are used in satellite construction. However, also the influence of particle and electromagnetic radiation on the weathering of extraterrestrial rocks and on organic and biological tissues is the research topic of various scientific disciplines. To strengthen the comprehensive and systematic investigation of degradation processes a new laboratory, the complex irradiation facility (CIF), has been designed, set up, tested, and put into operation at the DLR-Institute of Space Systems in Bremen (Germany). The CIF allows the simultaneous irradiation with three light sources and with a dual beam irradiation system for the bombardment of materials with electrons and protons having energies up to 100 keV. It is eminently suitable to perform a large variety of irradiation procedures that are similar to those which appear at different distances to the Sun. This paper is devoted to potential users in order to inform them about the capabilities of the CIF

Time Evolution of the 3-D Accretion Flows: Effects of the Adiabatic Index and Outer Boundary Condition

We study a slightly rotating accretion flow onto a black hole, using the fully three dimensional (3-D)numerical simulations. We consider hydrodynamics of an inviscid flow, assuming a spherically symmetric density distribution at the outer boundary and a small, latitude-dependent angular momentum. We investigate the role of the adiabatic index and gas temperature, and the flow behaviour due to non-axisymmetric effects. Our 3-D simulations confirm axisymmetric results: the material that has too much angular momentum to be accreted forms a thick torus near the equator and the mass accretion rate is lower than the Bondi rate. In our previous study of the 3-D accretion flows, for gamma=5/3, we found that the inner torus precessed, even for axisymmetric conditions at large radii. The present study shows that the inner torus precesses also for other values of the adiabatic index: gamma=4/3, 1.2 and 1.01. However, the time for the precession to set increases with decreasing gamma. In particular, for gamma=1.01 we find that depending on the outer boundary conditions, the torus may shrink substantially due to the strong inflow of the non-rotating matter and the precession will have insufficient time to develop. On the other hand, if the torus is supplied by the continuous inflow of the rotating material from the outer radii, its inner parts will eventually tilt and precess, as it was for the larger gamma's.Comment: 19 pages, 19 figures; accepted to ApJ; version with full resolution figures may be downloaded from http://users.camk.edu.pl/agnes/publ_en.htm

Thermo-Optical Property Degradation of ITO-Coated Aluminized Polyimide Thin Films Under VUV and Low-Energy Proton Radiation

We studied thermo-optical property degradation of indium tin oxide (ITO)-coated aluminized polyimide thin films under exposure to vacuum ultraviolet radiation and low-energy (3 and 5 keV) protons during ground tests using the Complex Irradiation Facility at the DLR site in Bremen. Changes in solar absorption and thermal emission coefficients caused by the irradiation were analyzed. We report a significant increase in solar absorptance of the samples irradiated by protons. We also attempted to identify any defects on the surface of the samples. The study was motivated by a unique opportunity that is provided by the Complex Irradiation Facility to study the degradation effects induced by exposure to protons with an energy below 10 keV and short-wavelength light below 115 nm

Proton Spectra for the Interplanetary Space Derived From Different Environmental Models

Knowledge about the space radiation environment is crucial for the design and selection of materials and components used for space applications. This environment is characterized not only by the Sun’s electromagnetic radiation but also by charged particles categorized into solar wind, solar energetic particles (SEP) and galactic cosmic rays (GCR). Especially for material engineering and qualification testing, differential and integral spectra for particle energies ranging from keVs to GeVs are required. Up to now, a wide variety of models is available, whereas it is difficult to keep the overview. Although, e.g., the European Cooperation for Space Standardization (ECSS) standard includes instructions on how to investigate particle radiation, it does not provide an overall view. This paper shall support those in need of a comprehensive overview and provide comprehensive information about proton radiation spectra that can potentially be of use for space engineering tasks ranging from mission analysis to material and component design as well as qualification testing. The publicly accessible platforms OLTARIS, SPENVIS, and OMERE were examined for available proton spectra to be used. Exemplary, the particle radiation of solar cycle 23 is considered, which comprehends the years 1996–2008. A common drawback of the available models is their restriction to the MeV-range. Particularly when materials are directly exposed to the space environment, low energetic particles, specifically, the keV-range, are of high interest, since these particle transfer all their energy to the material. Therefore, additional data sources were used in order to include the usually neglected low energy protons into the derived spectrum. The data was transferred to common set of units and eventually could be compared and merged together. This includes a comparison of the most common models, incorporating data foundation, applicability, and accessibility. As a result, extensive and continues spectra are fitted that take all different models with its different energies and fluxes into account. Each covered year is represented by a fitted spectrum including confidence level as applicable. For solar active and quite times spectra are provided