De-Ling Liu

Synergistic effects of contamination and low energy space protons on solar cell current output

It is well known that solar cell coverglass materials are subject to darkening, or transmission degradation, due to interaction with protons. Our recent laboratory test results have shown that the transmission of coverglasses, once contaminated with organic molecular films, can be further degraded upon space proton irradiation (20–400 keV). The coverglass transmission loss occurs in the short wavelength region, thus multi-junction solar cells are expected to be particularly susceptible to such synergistic effects of contamination and proton irradiation when the top junction is the current limiting junction. In the previous work, AR/ITO coverglass materials, commonly used for space solar arrays, were photo-deposited with the model contaminant DC704. The contaminated coverglass samples were subsequently irradiated with a simulated 15-year geosynchronous orbit low energy proton radiation environment at 5-year increments. The progression of coverglass transmission change was characterized before and after each process. The measured coverglass transmission data were then convolved with the solar cell quantum efficiency and solar spectrum to determine the coverglass darkening effects on solar cell performance. Taking into account space proton radiation effects and the time dependent contaminant film accumulation process, our preliminary analysis indicates that, over a 15-year mission life, approximately 3.7 % solar cell current loss could be attributed to a beginning-of-life (BOL) contaminant film of 100 Å, with no additional on-orbit film growth. For a BOL film of 100 Å and additional film growth while on orbit, the end-of-life (EOL) solar cell current loss due to contamination is approximated at 5.5% for EOL 200 Å, and 7.3% for EOL 300 Å.

Effect of concurrent UV irradiation and contamination on silver-coated teflon radiator surface (Conference Presentation)

Silver coated Teflon (SCT) has been used as a radiator material for spacecraft thermal control. In order to reduce the specular reflection, an attempt was made to roughen the heritage smooth SCT surface via sanding, leading to abraded surfaces. The objective of this study is to gain insight into the relative thermal performance degradation of smooth and abraded SCT radiator materials under identical exposure of concurrent UV irradiation and contaminant deposition. Contaminant molecules outgassed from representative spacecraft materials were deposited onto the smooth and abraded SCT samples with quartz crystal microbalances (QCMs) in close proximity to monitor real-time contaminant deposition. Thermal performance degradation is characterized by measuring solar absorptance () change on the SCT samples before and after contaminant film accumulation. Atomic force microscope (AFM) was used to examine the extent of surface roughness before and after contaminant deposition on smooth SCT samples. The preliminary findings indicate that less contamination accumulation was observed on SCT surfaces in comparison to the gold coated crystal surface of QCMs. In addition, the roughness of SCT surface appears to play a role in contributing a more pronounced  change, suggesting the possibility of faster performance degradation of the abraded SCT materials in comparison to that of smooth SCT surfaces.

Percent area coverage through image analysis

The notion of percent area coverage (PAC) has been used to characterize surface cleanliness levels in the spacecraft contamination control community. Due to the lack of detailed particle data, PAC has been conventionally calculated by multiplying the particle surface density in predetermined particle size bins by a set of coefficients per MIL-STD-1246C. In deriving the set of coefficients, the surface particle size distribution is assumed to follow a log-normal relation between particle density and particle size, while the cross-sectional area function is given as a combination of regular geometric shapes. For particles with irregular shapes, the cross-sectional area function cannot describe the true particle area and, therefore, may introduce error in the PAC calculation. Other errors may also be introduced by using the lognormal surface particle size distribution function that highly depends on the environmental cleanliness and cleaning process. In this paper, we present PAC measurements from silicon witness wafers that collected fallouts from a fabric material after vibration testing. PAC calculations were performed through analysis of microscope images and compare them to values derived through the MIL-STD-1246C method. Our results showed that the MIL-STD-1246C method does provide a reasonable upper bound to the PAC values determined through image analysis, in particular for PAC values below 0.1.

Outgassing study of spacecraft materials and contaminant transport simulations

Contamination control plays an important role in sustaining spacecraft performance. One spacecraft degradation mechanism involves long-term on-orbit molecular outgassing from spacecraft materials. The outgassed molecules may accumulate on thermal control surfaces and/or optics, causing degradation. In this study, we performed outgassing measurements of multiple spacecraft materials, including adhesives, Nylon Velcro, and other assembly materials through a modified ASTM E595 test method. The modified ASTM E595 test had the source and receiver temperature remained at 125°C and 25°C, respectively, but with prolonged outgassing periods of two weeks. The condensable contaminants were analyzed by Fourier Transform Infrared Spectroscopy (FTIR) and Gas Chromatography/Mass Spectrometry (GC/MS) to determine their spectral transmission and chemical composition. The FTIR spectra showed several spacecraft materials, primarily adhesives and potting materials, exhibiting slight absorption from contaminants consisting of hydroxyl groups and carboxylic acids. To gain insight into molecular contaminant transport, simulations were conducted to characterize contaminant accumulation inside a hypothetical space system cavity. The simulation indicated that contaminant molecules bouncing inside the hypothetical payload cavity can lead to deposition on colder surfaces, even though large openings are available to provide venting pathways for escaping to space. The newly established molecular contaminant transport simulation capability holds the promise of providing quantitative guidance for future spacecraft and its venting design.

Molecular transport modeling for spaceborne instrument contamination prediction

We present a finite element model for the prediction of molecular contamination through narrow pathways in a hypothetical spaceborne instrument using the commercially available COMSOL Multiphysics software. The free molecular flow module of COMSOL uses the angular coefficient method as an alternative to particle based methods. In the angular coefficient method, the microscopic dynamical aspect of the material transport problem is reduced to a macroscopic problem by calculating emission and incident fluxes at each surface rather than the trajectories of individual molecules. The model was validated by comparing the simulated and experimentally measured pressure differential between two chambers separated by a mechanical test structure. The mechanical test structure was designed to exhibit narrow pathways with characteristic size that can be found on spaceborne optomechanical structures. It is shown that materials can slowly migrate through these pathways in a spaceborne instrument to cause noticeable performance degradation within a time scale of a few months. The model for material transport through the test structure was also verified using a stochastic method. To simulate water infiltration through narrow pathways of a hypothetical spaceborne instrument, nominal payload temperature profile was used in addition to setting empirical input parameters such as the desorption energy of water and the outgassing rate of water from multilayer insulator thermal blankets to the appropriate surfaces in the modeling domain. The rate of growth of ice films on low temperature optical components and how optical performance can be degraded over time are discussed in this paper.

Spacecraft materials HCl susceptibility assessments

The susceptibility of spacecraft materials to HCl exposure was investigated in light of concerns to potential contamination during evolved expendable launch vehicle (EELV) overflight scenarios. Overflight refers to the circumstance where one spacecraft, resident on a launch pad, may be exposed to HCl generated from an earlier solid rocket launch at an adjacent pad. One aspect of the overflight risk assessments involves spacecraft materials susceptibility to HCl exposure. This study examined a wide range of spacecraft materials after being exposed to HCl vapor in a well-characterized facility. Sample thermal/optical and electrostatic dissipation properties, as well as surface chemical and morphological features, were characterized before and after the HCl exposure. All materials tested, except for indium tin oxide (ITO) coated Kapton film, showed no significant degradation after HCl exposure of up to 4800 ppb-hr. The ITO coated Kapton sample showed slight signs of degradation after being exposed to 500 ppb-hr HCl, as the surface resistance was increased by a factor of 5. However, the potential HCl dose inside the payload fairing (PLF) was estimated to be far below 500 ppb-hr in an EELV overflight event. These results, along with other relevant laboratory test data on the HCl removal efficiency of the filtration media used on the launch sites, provide the technical rationale that properly filtered air as the PLF purge should pose little risk in terms of HCl contamination under EELV overflight scenarios.

Identification of collected volatile condensable material (CVCM) from ASTM E595 of silicone damper fluid

Polydimethylsiloxane damping fluids used for structural deployment mechanisms are not required to be low outgassing. During normal use, these damping fluids are typically encapsulated; however, an unintentional leak may occur which would cause an undesirable contamination at the leak point and form volatile condensable that could reach contamination-sensitive surfaces, degrading the performance of satellites. The collected volatile condensable material (CVCM) at 25 °C from ASTM E595 of a damping fluid, MeSi-300K, was < 0.10%, when the damping fluid was maintained at 125 °C for 24 hours under 10-6 Torr vacuum. MeSi-300K viscosity is 300,000 cSt, which indicates an average molecular weight (MW) of 204,000. This large MW polymer would contain about 2,756 dimethyl siloxane (DMS) units in the chain. These long chains are not expected to be volatile; however, during manufacture, linear chains and cyclic compounds of a smaller number of DMS units produced are volatile. Gas chromatography mass spectrometry (GC-MS) was used to identify the CVCM. Characterization of these materials revealed that the CVCM contained higher MW siloxanes, straight chain and cyclic, in the range of 682 to 1196 (9 to 16 DMS units), whereas CVCM from spacequalified, silicone-based materials have lower MW, 222 to 542 (3 to 7 DMS units). Consequently, contamination from MeSi-300K material would produce greater amounts of higher-MW siloxanes than space-qualified silicones. These higher-MW species would be harder to remove by evaporation and could remain on sensitive surfaces.

Analysis of particulates on tape lift samples

Particle counts on tape lift samples taken from a hardware surface exceeded threshold requirements in six successive tests despite repeated cleaning of the surface. Subsequent analysis of the particle size distributions of the failed tests revealed that the handling and processing of the tape lift samples may have played a role in the test failures. In order to explore plausible causes for the observed size distribution anomalies, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and time-of-flight secondary ion mass spectrometry (ToF-SIMS) were employed to perform chemical analysis on collected particulates. SEM/EDX identified Na and S containing particles on the hardware samples in a size range identified as being responsible for the test failures. ToF-SIMS was employed to further examine the Na and S containing particulates and identified the molecular signature of sodium alkylbenzene sulfonates, a common surfactant used in industrial detergent. The root cause investigation suggests that the tape lift test failures originated from detergent residue left behind on the glass slides used to mount and transport the tape following sampling and not from the hardware surface.

Assessment of Particle Deposition inside Payload Fairing from Launch Vehicle Plume Contribution

Concerns were raised for potential payload contamination inside payload faring (PLF) contributed from the soot particles in the launch vehicle ignition plume. Soot particles, once ingested into PLF through vents, can pose potential payload contamination risks due to their light absorbing characteristics. To gain insights into the extent of soot particle contamination inside the PLF, analytical calculations and laboratory experiments were performed using a PLF simulator to determine the rate of soot particle deposition onto surfaces. The analysis assumed a non-venting setting as the worst case scenario, in which particles were trapped inside the PLF simulator and allowed to deposit onto available surfaces. Soot particles were briefly introduced inside a PLF mockup and after the soot generation source ceased, particle deposition rates were examined by measuring the particle concentration decay as a function of time. Based on the experimentally determined particle deposition rates and other parameters including the venting scenarios, the impact of soot particle deposition for the full scale PLF and payload was evaluated. The effects of soot particles contamination were also studied, and pronounced transmission degradation toward the UV region on a fused silica substrate was observed.

Infiltration of supermicron aerosols into a simulated space telescope

Purging is a common scheme to protect sensitive surfaces of payloads and spacecraft from airborne contaminant intrusion during ground assembly, integration, and launch vehicle encapsulation. However, the purge for space volumes must be occasionally interrupted. Thus it is important to gain insights into the transport of ambient particles penetrating through vent holes and entering the interior of a confined space system, such as a space telescope, during a purge outage. This study presents experimental work performed to measure time-dependent aerosol concentration changes during a purge outage. The laboratory results from the aerosol experiments were compared with a mass balance based mechanistic model which had been experimentally validated for aerosols ranging from 0.5 to 2 μm. The experimental data show that the steady-state aerosol concentration inside a simulated space telescope (SST) is governed by the surrounding particle concentration, SST air exchange rate, and the particle deposition rate.