Hyperthermal Atomic Oxygen Research Articles

Nano-roughness effect on the interfacial thermal oxidation and surface catalytical recombination characteristics for silicon carbide based materials under non-equilibrium flow

With the rapid development of high-speed aircraft, the demanding for light, strong, reliable and high temperature resistive thermal protection material (TPM) is becoming urgent. High temperature material interface, such as silicon carbide (SiC) ceramic matrix composite, has received strong interest recently due to its high mechanical and thermochemical stability. For reactive interface development with nano-roughness under extremely hyperthermal non-equilibrium flow conditions for TPM applications, however, has not been reported yet. This work performs a systematic reactive molecular dynamics (RMD) study of the SiC surface morphology effect withstanding hyperthermal atomic oxygen (AO) impact via a gas–solid interface reaction model. The results show that a self-healing trend of the defect-rich silicon carbide surface with original nano-roughness is revealed towards a flatter interface accompanying with the oxidation process. The surface catalytic recombination coefficient is more sensitivity to the surface defects when wall temperature Tw > 500 K. Nano-roughness structure can also vary the main thermochemical reaction path at the thermochemical reactive interface: For the nano-groove surface model, the thermal oxidation reaction becomes dominant, while more pronounced surface catalytic recombination characteristics are revealed for the flat surface model. The mechanisms of roughness effect, surface temperature and gas impacting angle on the surface catalytic recombination, surface oxidation, and interface evolution are revealed, which advance our microscopic understanding of SiC interfacial performance with nano-roughness under non-equilibrium flow conditions.

Modeling Equilibration Dynamics of Hyperthermal Products of Surface Reactions Using Scalable Neural Network Potential with First-Principles Accuracy.

Equilibration dynamics of hot oxygen atoms following the dissociation of O2 on Pd(100) and Pd(111) surfaces are investigated by molecular dynamics simulations based on a scalable neural network potential enabling first-principles description of the interaction of O2 and O interacting with variable Pd supercells. By analyzing hundreds of trajectories with appropriate initial sampling, the measured distance distribution of equilibrated atom pairs on Pd(111) is well reproduced. However, our results on Pd(100) suggest that the ballistic motion of hot atoms predicted previously is a rare event under ideal conditions, while initial molecular orientation and surface thermal fluctuation could significantly affect the overall postdissociation dynamics. On both surfaces, dissociated hyperthermal oxygen atoms primarily locate their nascent positions and experience a similar random walk motion nearby.

Interaction mechanisms between Atomic Oxygen and materials: investigation on reflected beam

Atomic Oxygen (AO) is the main component of the residual atmosphere present at low earth orbit. Multiple reflections can locally enhance or lower erosion rate in case of complex geometry. The need for precise estimate of hyperthermal AO flux is thus crucial at critical target embedded into realistic assembly. Industrial modeling tools (example Atomox module in ESABASE2 and SYSTEMA toolboxes) may account for multiple reflections with simplistic approach (ray-tracing) and a set of semi-empirical parameters. In this context, an original geometrical setup was designed in order to expose target surfaces with normal and tilted incidences and after one reflection on a set of materials of interest (black conductive thermal coatings, coverglass, surface treatment…). Candidate materials have been selected in view to compare numerical results and experimental ones. Results of two test campaigns highlight the effect of incidence angle on erosion rate (higher erosion with tilted beam) and provide an estimate of residual fluence after one reflection (in the 4-10 % range depending on material type). First computations with engineering codes have been successfully conducted, the erosion patterns being rather well described with some basic hypotheses on material parameters (absorption, specular/diffuse reflection) and taken into account the reflection on adjacent metallic surfaces.

Polyimide degradation under atomic oxygen attack

Procedures are developed for accelerated laboratory tests of spacecraft polymers for resistance to long-term exposure to hyperthermal atomic oxygen (AO) with the use of high-energy AO ions of a rarefied plasma and Kapton-H polyimide as the reference material. The parameters that characterize physico-chemical, thermo-optical, and aerodynamic AO – Kapton-H interaction (surface roughness, solar absorptance, and normal infrared emittance) are determined at different values of the AO fluence. To do this, use was made of the data of tests onboard spacecraft in the Earth's atmosphere and the authors and others' laboratory tests. The normal and the tangential momentum transfer coefficient and the energy transfer coefficient are reported as a function of the AO bombardment angle. The obtained data on the AO-induced degradation of the parameters that characterize AO–polyimide dynamic interaction allow one to predict the behavior of the polyimide thermo-optical and aerodynamic properties during a long-term service in the Earth's upper atmosphere and ionosphere.

Hyperthermal erosion of thermal protection nanocomposites under atomic oxygen and N2 bombardment

Against the hyperthermal atomic oxygen (AO) bombardment, mitigation of the surface recession has been the main issue in designing surface shielding materials in low earth orbital (LEO) environment. Meanwhile, the role of nitrogen molecules (N2) in LEO and sub-LEO on disintegration of the shielding materials has rarely been studied despite their abundance and non-negligible colliding energy. This study for the first time compared the surface chemistry and disintegration of Kapton polyimide-based nanocomposites under AO and N2 bombardment conditions using a reactive molecular dynamics (MD) simulation. The heat transfer from the surface to the behind layer and the associated boundary condition for the bombardment simulation were primarily discussed to provide the optimum bombardment simulation setup. Polyhedral oligomeric silsesquioxane (POSS), carbon nanotube (CNT) and graphene were respectively blended with Kapton to evaluate the damage mitigation efficacy. The addition of all types of additives to Kapton induced a notable thermo-protection effect. AO bombardment resulted in a sequential disintegration of oxidation and desorption, whereas the surface recession of nanocomposites by exposure to N2 collision was mainly caused by physical desorption. It was also noteworthy that the surface recession caused by N2 bombardment was comparable to that by AO attack at the same fluence.

Sensitivity Analysis of the Catalysis Recombination Mechanism on Nanoscale Silica Surfaces.

A deep understanding of surface catalysis recombination characteristics is significant for accurately predicting the aeroheating between hypersonic non-equilibrium flow and thermal protection materials, while a de-coupling sensitivity analysis of various influential factors is still lacking. A gas–solid interface (GSI) model with a hyperthermal flux boundary was established to investigate the surface catalysis recombination mechanisms on nanoscale silica surfaces. Using the reactive molecular dynamics (RMD) simulation method, the effects of solid surface temperature, gas incident angle, and translational energy on the silica surface catalysis recombination were qualified under hyperthermal atomic oxygen (AO), atomic nitrogen (AN), and various AN/AO gas mixtures’ influence. It can be found that, though the Eley–Rideal (E–R) recombination mechanism plays a dominant role over the Langmuir–Hinshelwood (L–H) mechanism for all the sensitivity analyses, a non-linear increasing pattern of AO recombination coefficient γO2 with the increase in incident angle θin and translational energy Ek is observed. Compared with the surface catalysis under hyperthermal AO impact, the AN surface adsorption fraction shows an inverse trend with the increase in surface temperature, which suggests the potential inadequacy of the traditional proportional relationship assumptions between the surface adsorption concentration and the surface catalysis recombination coefficient for other species’ impact instead of AOs. For the incoming bi-component AO/AN gas mixtures, the corresponding surface catalysis coefficient is not the simple superposition of the effects of individual gases but is affected by both the intramolecular bond energies (e.g., O2, N2) and intermolecular energies (e.g., Si/N, Si/O).

Molecular insight of the interface evolution of silicon carbide under hyperthermal atomic oxygen impact

One of the key challenges faced by hypersonic flying is the complex thermal–mechanical–chemical coupling effect between thermal protection materials and non-equilibrium flow environment. Silicon carbide (SiC) has drawn much attention due to its superior physical and chemical characteristics, and its performance under hyperthermal atomic oxygen (AO) impact, however, is still little known. This work investigates the effects of various SiC crystalline polytypes, surface temperature, and crystal orientations on the SiC interface evolution by hyperthermal AO collisions via the reactive molecular dynamics method. The results showed that SiC surface erosion is highly dependent on the temperature and the presence of different interfacial structures. In the range of 500–2000 K, the proceeding of the passive oxidation advances the amorphous SiO2/SiC interface and the formation of SixOy phase weakens the surface catalytic characteristics and mechanical properties. The presence of defects, such as dangling bonds at the gas–solid interface, caused by different surface orientations affects the anti-erosion capabilities of SiC significantly, which may limit its further wide applications.

Atomistic-scale investigations of hyperthermal oxygen–graphene interactions via reactive molecular dynamics simulation: The gas effect

Hyperthermal atomic oxygen (AO) bombardment to thermal protection system surface has been identified to impact the aerodynamic heating significantly, due to complex chemical reactions at the gas–solid interface, e.g., surface catalysis recombination, oxidation, and ablation. Previous investigations have focused on the surface effects of the AO collision process, while the influence of impacting gas characteristics remains unclear under various non-equilibrium aerodynamic conditions. This work conducts a reactive molecular dynamics (RMD) study of AO collisions over graphene surface, by considering the incoming gas at different translational energies (0.1 ≤ Ek ≤ 10 eV), incident angles (θ = 15°, 30°, 45°, 60°, 75°, and 90°), and O/O2 ratios (χO2 = 0.00, 0.25, 0.50, 0.75, and 1.00). The RMD results indicate that for AO normal incidence, the predominant reactive products of O2, CO, and CO2 molecules are produced due to the synergistic catalytic recombination and surface ablation reaction effects. A maximum recombination performance is identified around 5-eV AO incidence. For off-normal AO incidence, the recombination coefficient increases with the increase in incidence angle from 15° to 60° due to the larger perpendicular components of translational energy and then decreases smoothly. With the increase in O2 mole fraction, the surface reflection probabilities increase, which result in the decrease in both catalytic recombination and ablation activities. Via revealing the atomistic-scale mechanism of gas effects on the surface under hypersonic non-equilibrium conditions, this work sheds light for the future design and optimization of thermal protection materials.

Direct Dynamics Simulations of Hyperthermal O(3P) Collisions with Pristine, Defected, Oxygenated, and Nitridated Graphene Surfaces

We report here an extensive direct dynamics study on the collisions of hyperthermal (14.9 kcal mol–1) atomic oxygen with a variety of graphene surfaces to explore possible reaction channels. Severa...

POSS-enhanced colorless organic/inorganic nanocomposite (CORIN®) for atomic oxygen resistance in low earth orbit

Copolymers and blends of polyhedral oligomeric silsesquioxane (POSS) and polyimides have been shown to resist atomic-oxygen (AO) attack, making them promising candidates as space-durable materials in the harsh oxidizing environment of low Earth orbit (LEO). CORIN® is a commercially available and colorless POSS–polyimide which has the desirable but uncommon property among polyimides that it may be dissolved in a solvent and applied at room temperature, thus making it possible to have a widely applicable clear coating for protection from AO in LEO. The physical properties of CORIN® have been largely characterized, but its AO resistance has received limited study. We have thus used a hyperthermal AO beam to expose CORIN® and other POSS-containing polyimides, 7.3 wt% and 9.1 wt% Si7O9 trisilanolphenyl POSS blended with poly(pyromellitic dianhydride-co-4,4′-oxydianiline), to characterize their resistance to AO attack. CORIN® was found to have a particularly low erosion yield that is ≲1% that of the ubiquitous satellite material, Kapton, which agrees favorably with a result from an experiment on the International Space Station. The other POSS-containing polymers exhibited somewhat higher erosion yields of 1.4 and 2.2% that of Kapton, respectively. Surface characterization of CORIN® showed that it remained relatively smooth during AO exposure and implied that a passivating silicon oxide was formed. The comparison of the AO effects on CORIN® and the other POSS–polyimides suggests that the net effect of AO on such POSS–polyimides depends on the density of Si atoms as well as the reactivity of the organic component of the polymer. This work adds to our confidence in the use of POSS-containing polymers in LEO and, especially, in very LEO, where the necessity of protecting satellite materials and components from AO attack is critical.

Polyhedral oligomeric silsesquioxane polyimide nanocomposites for color filters and flexible conductive films

AbstractColor filters and conductive films are widely used in spacecraft, while the lack of lightweight, flexibility, and atomic oxygen (AO) durability confine their potential applications in low earth orbit. In this study, a clear poly(amic acid) with an empirical 20 wt% polyhedral oligomeric silsesquioxane (POSS) solid content is designed for transparency and AO durability. Red, green, blue, and yellow dyes are reinforced with small amounts of 1–5 wt% in POSS polyimides for color filters. A silver nanowire network film is infiltrated onto the POSS polyimide for conductive film. Erosion depth upon hyperthermal AO exposure, surface morphology, surface chemistry, optical transparency, and conductivity have been systematically investigated. The erosion yields of all 20 wt% POSS polyimides decrease by an order of magnitude when subjected to 2.32 ± 0.05 and 2.39 ± 0.06 × 1020 atoms cm−2 AO fluences, as passivating SiOx networks are formed on film surface. The small‐amount dye additives into polyimides do not introduce obvious changes in AO durability and surface chemistry. The silver nanowire infiltrated POSS polyimide film shows a 65.7% transmittance at 550 nm and a sheet resistance of 8.50 ± 0.36 Ohm square−1. This study reveals promising attempts of POSS‐polyimide‐based color filters and flexible conductive films for potential space applications.

A reactive molecular dynamics study of hyperthermal atomic oxygen erosion mechanisms for graphene sheets

Carbon-based composite materials are widely used in the aerospace field due to their light weight and excellent physical/chemical properties. The mechanisms of the erosion process, e.g., surface catalysis and ablation, during the impact of oxygen atoms, however, remain unclear. In this study, the surface catalysis and ablation behavior during the erosion process of hyperthermal atomic oxygens were achieved through the molecular dynamics method with the reactive force field potential. The concomitant impacts of energy flux density of energetic oxygen atoms, the presence of multiple layers beneath the graphene sheet, and the morphology of graphite surfaces, i.e., graphite basal plane, armchair (AC) edge surface, and zigzag edge surface, respectively, were discussed. The results show that the adsorption of oxygen atoms dominates at the beginning by generating O2 molecules, suggesting the importance of surface catalytic for any ablation study. A unique “layer-by-layer” ablation phenomenon by hyperthermal atomic oxygen is observed for multi-layered graphite slab, and the ablation rate reduces as the number of graphene layers increases. The morphology/structure of the surface shows significant effects on the ablation rate, with AC surfaces showing the largest etching rate and the basal one showing the lowest. The low binding energies of the AC edge are responsible for the difficulty in the formation of stable functional group structures to resist the etching of high-enthalpy oxygen atoms. Such revelation of the detailed surface catalysis and ablation mechanism at the atomistic scale provides insight into design of future materials for the augmentation of the thermal protection effect.

Atomic oxygen durable ultra-black polyimide nanocomposite films in solar spectrum

Black thermal control blankets have been widely used in spacecraft for eliminating stray light around high-resolution optical instruments. However, the limited absorption in visible light range, the fast degradation upon hyperthermal atomic oxygen (AO) in low Earth orbit (LEO), and the poor adhesion of black coatings with substrate confined their applications into board solar spectrum and multi-layer interface engineering. In this work, a nanocomposite polyimide film containing a trisilanolphenyl POSS, a dye of solvent black 34, and a carbon black of CABOT M800 has been designed. With an ingredient of 20 wt% POSS, 5 wt% dye, and 10 wt% carbon black, the nanocomposite polyimide film PPIBC10 exhibited an ultra-low transmittance near zero in the range 200–2000 nm, a low reflectance below 5% in UV range and below 2% in VIS-IR range, and a low erosion yield of 0.49 ± 0.11 × 10−24 cm3 atom−1 upon a 2.27 × 1020 atoms cm−2 hyperthermal AO exposure, with an absorptance of 0.983, an emissivity of 0.86, and a coefficient of thermal expansion of 20 ppm oC−1. SEM and XPS analysis indicated that a uniform SiOx passivating network was formed on surface upon the hyperthermal AO exposure. The tensile strength decreased with increasing additives, and kept stable upon the AO exposure, for there was only sub-micrometer in erosion depth. The thermal expansion coefficients of the nanocomposite films are comparable to that of Kapton, which benefits interface engineering. The small granularity of tens of micrometers and good solubility in poly(amic acid) of additives POSS, dye, and carbon black benefit potential manufacture. This study suggests an ultra-black AO durable polyimide film for potential applications in LEO.

Theoretical Studies of the Reactions between Hyperthermal O(3P) and Graphite: Holes and the Second Layer

Theoretical direct dynamic calculations on the basis of density functional theory are used to investigate hyperthermal collisions between O(3P) and highly oriented pyrolytic graphite (HOPG). The simulations suggest the HOPG surface becomes functionalized with epoxide groups as in previous works. Also, incoming O atoms can react at the surface to form O2 by way of an Eley–Rideal mechanism when the surface exposed to colliding O atoms is already functionalized with epoxides. A second layer of pristine graphene, included in the model, absorbs collision energy and significantly reduces the rates of reactions that liberate carbon from the surface, compared to single-layer models. Semiquinone functional groups are thought to be common at graphitic sheet edges and holes and to be a major source of CO and CO2. We deploy a model to explore the behavior of semiquinones when exposed to hyperthermal oxygen atoms. These groups are found to lead to CO and to play a role in the formation of epoxide groups on the underla...

Quantifying the effects of hyperthermal atomic oxygen and thermal fatigue environments on carbon nanotube sheets for space-based applications

The effects of atomic oxygen and thermal fatigue on two different types of carbon nanotube sheets were studied. One set was treated with nitric acid, while the other set was left untreated. Monotonic tensile tests were performed before and after exposure to determine the effects of either exposure type on the sheets’ mechanical properties. Electrical conductivity and electromagnetic interference measurements were recorded to determine the effects of AO-exposure and thermal cycling on the sheets’ electrical properties. Neither exposure type affected the sheets’ specific strengths. Both exposure types increased the sheets’ specific stiffnesses and decreased the sheets’ strains at failure. The electrical conductivity of both sheets decreased due to the different exposure types, while the EMI shielding effectiveness was unaffected. Scanning electron microscopy was used to observe any changes in the sheets’ surface morphologies, while energy-dispersive X-ray spectroscopy was used to determine the effects of AO on the sheets’ chemical makeup.

Resistance of nanoclay reinforced epoxy composites to hyperthermal atomic oxygen attack

Due to outstanding mechanical properties, heat resistance, and relatively facile production, nanoclay reinforced epoxy composites (NCRE composites) have been suggested as candidate materials for use on external surfaces of spacecraft residing in the low Earth orbit (LEO) environment. The resistance of the NCRE composites to bombardment by atomic oxygen (AO), a dominant component of the LEO environment, has been investigated. Four types of samples were used in this study. They were pure epoxy (0 wt% nanoclay content), and NCRE composites with different loadings of nanoclay—1 wt%, 2 wt%, and 4 wt%. Etch depths decreased with increasing nanoclay content, and for the 4 wt% samples it ranged from 28% to 37% compared to that of pure epoxy. X-ray photoelectron spectroscopy (XPS) indicates that after AO bombardment, relative area of C–C/C–H peak decreased, while the area of the C–O, ketones peaks increased, and the oxidation degree of surfaces increased. New carbon-related component carbonates were detected on nanoclay containing composite surfaces. Scanning electron microscopy indicates that aggregates formed on nanoclay-containing surfaces after AO bombardment. The sizes and densities of aggregates increased with nanoclay content. The combined erosion depths, XPS and SEM results indicate that although all the studied surfaces got eroded and oxidized after AO bombardment, the nanoclay containing composites showed better AO resistance compared to pure epoxy, because the produced aggregates on surface potentially act as a physical “shield”, effectively retarding parts of the surface from further AO etching.

Hyperthermal atomic oxygen durable transparent silicon-reinforced polyimide

A clear poly(amic acid) was reinforced by a trisilanolphenyl polyhedral oligomeric silsesquioxane (POSS) by direct dissolution, and transparent silicon-reinforced polyimide (Si-RPI) films with different POSS loadings were obtained after curing, showing high transmittance of >90% within 380–800 nm. The Si-RPI films were exposed to a ground hyperthermal atomic oxygen (AO) beam. The erosion depths and derived erosion yields of the materials decreased with POSS loadings. At a 20 wt% POSS loading, the Si-RPI showed an erosion yield of 0.13 × 10−24 cm3 atom−1 at a fluence of 2.79 × 1020 O atoms cm−2. Surface morphology and element composition characterization on Si-RPI indicated that SiOx-based passivating layers were formed on surfaces upon the hyperthermal AO attack. This study suggests a facile way of reinforcing Si into transparent polyimide for a promising candidate of spacecraft coating material operating in low Earth orbit.

Effects of hyperthermal atomic oxygen on a cyanate ester and its carbon fiber-reinforced composite

The durability of cyanate ester (CE) to hyperthermal atomic oxygen (AO) attack in low Earth orbit may be enhanced by the addition of carbon fiber to form a carbon fiber-reinforced cyanate ester composite (CFCE). To investigate the durability of CFCE relative to CE, samples were exposed to a pulsed hyperthermal AO beam in two distinct types of experiments. In one type of experiment, samples were exposed to the beam, with pre- and post-characterization of mass (microbalance), surface topography (scanning electron microscopy (SEM)), and surface chemistry (X-ray photoelectron spectroscopy (XPS)). In the second type of experiment, the beam was directed at a sample surface, and volatile products that scattered from the surface were detected in situ with the use of a rotatable mass spectrometer detector. CFCE exhibited less mass loss than pure CE with a given AO fluence, confirming that the incorporation of carbon fiber adds AO resistance to CE. Erosion yields of CE and CFCE were 2.63 ± 0.16 × 10−24 and 1.46 ± 0.08 × 10−24 cm3 O-atom−1, respectively. The reduced reactivity of CFCE in comparison to CE was manifested in less oxidation of the CFCE surface in XPS measurements and reduced CO, CO2, and OH reaction products in beam-surface scattering experiments. The surface topographical images collected by SEM implied different surface deterioration processes for CE and CFCE. A change of surface topography with increasing AO fluence for CE indicated a threshold AO fluence, above which the erosion mechanism changed qualitatively. CFCE showed almost intact carbon fibers after relatively low AO fluences, and while the fibers eventually eroded, they did not erode as rapidly as the CE component of the composite.

Development and validation of a finite-rate model for carbon oxidation by atomic oxygen

Molecular beam experiments of hyperthermal oxygen atoms scattering from a vitreous carbon surface were simulated and analyzed using direct simulation Monte Carlo (DSMC). The purpose of this study was to construct and validate a surface chemistry model in DSMC by matching observed features in experimental time-of-flight (TOF) and angular distribution data. A variety of reaction mechanisms including adsorption, desorption, and several types of Langmuir-Hinshelwood (LH) reactions are used to characterize the surface processes in the DSMC surface chemistry framework. This framework is used to numerically simulate the beam-surface scattering experiments of oxygen on vitreous carbon with surface temperatures ranging from 600 to 2000 K. Detailed analysis of the experimental data including TOF and angular distributions, and product fluxes derived from these data, is used to propose modifications to the carbon surface oxidation model developed by Poovathingal et al. This revised model successfully captures the important features in the experimental TOF distributions, including: (i) the impulsively scattered and thermally desorbed components in the oxygen (O) TOF distributions, and (ii) the relatively slow long-tail components in the carbon monoxide (CO) TOF distributions. Comparisons between the simulated (DSMC) and experimental TOF and angular distributions, as well as product fluxes, show excellent agreement.

Resistance of POSS Polyimide Blends to Hyperthermal Atomic Oxygen Attack.

Copolymers of polyhedral oligomeric silsesquioxane (POSS) and polyimide (PI) have shown remarkable resistance to atomic oxygen (AO) attack and have been proposed as replacements for Kapton on the external surfaces of spacecraft in the harsh oxidizing environment of low Earth orbit (LEO). POSS PI blends would be an economical alternative to the copolymers if they also resisted AO attack. Thus, blends of trisilanolphenyl (TSP) POSS and PI with different weight percentages of the Si7O9 POSS cage were cast into films and exposed to a hyperthermal AO beam, and they were characterized in terms of their recession, mass loss, surface morphology, and surface chemistry. In order to compare the AO resistance of the blends with POSS PI copolymers, samples of previously studied copolymers were also investigated in parallel with the blends. For all POSS PI materials, the AO resistance increased with increasing AO fluence and with increasing POSS cage loading. At similar POSS cage loadings and exposure conditions, the TSP POSS PI blends showed comparable erosion yields to the POSS PI copolymers, with specific samples of blends and copolymers achieving erosion yields as low as 0.066 × 10-24 cm3 atom-1 with an AO fluence of 5.93 × 1020 O atoms cm-2. SEM and XPS analyses indicated that passivating SiOx layers were formed on the surfaces of all POSS-containing polymers during AO exposure. Thus, a TSP POSS PI blend is proposed as a low-cost variant of a POSS polyimide for use in extreme oxidizing environments, such as LEO.