Vanessa J. Murray

Atomic-Oxygen-Durable and Electrically-Conductive CNT-POSS-Polyimide Flexible Films for Space Applications

In low Earth orbit (LEO), hazards such as atomic oxygen (AO) or electrostatic discharge (ESD) degrade polymeric materials, specifically, the extensively used polyimide (PI) Kapton. We prepared PI-based nanocomposite films that show both AO durability and ESD protection by incorporating polyhedral oligomeric silsesquioxane (POSS) and carbon nanotube (CNT) additives. The unique methods that are reported prevent CNT agglomeration and degradation of the CNT properties that are common in dispersion-based processes. The influence of the POSS content on the electrical, mechanical, and thermo-optical properties of the CNT-POSS-PI films was investigated and compared to those of control PI and CNT-PI films. CNT-POSS-PI films with 5 and 15 wt % POSS content exhibited sheet resistivities as low as 200 Ω/□, and these resistivities remained essentially unchanged after exposure to AO with a fluence of ∼2.3 × 10(20) O atoms cm(-2). CNT-POSS-PI films with 15 wt % POSS content exhibited an erosion yield of 4.8 × 10(-25) cm(3) O atom(-1) under 2.3 × 10(20) O atoms cm(-2) AO fluence, roughly one order of magnitude lower than that of pure PI films. The durability of the conductivity of the composite films was demonstrated by rolling film samples with a tight radius up to 300 times. The stability of the films to thermal cycling and ionizing radiation was also demonstrated. These properties make the prepared CNT-POSS-PI films with 15 wt % POSS content excellent candidates for applications where AO durability and electrical conductivity are required for flexible and thermally stable materials. Hence, they are suggested here for LEO applications such as the outer layers of spacecraft thermal blankets.

Kinematics and dynamics of atomic-beam scattering on liquid and self-assembled monolayer surfaces

We have conducted investigations of the energy transfer dynamics of atomic oxygen and argon scattering from hydrocarbon and fluorocarbon surfaces. In light of these results, we appraise the applicability and value of a kinematic scattering model, which views a gas-surface interaction as a gas-phase-like collision between an incident atom or molecule and a localized region of the surface with an effective mass. We have applied this model to interpret the effective surface mass and energy transfer when atoms strike two different surfaces under identical bombardment conditions. To this end, we have collected new data, and we have re-examined existing data sets from both molecular-beam experiments and molecular dynamics simulations. We seek to identify trends that could lead to a robust general understanding of energy transfer processes induced by collisions of gas-phase species with liquid and semi-solid surfaces.

Molecular simulations of surface ablation using reaction probabilities from molecular beam experiments and realistic microstructure

Molecular simulations are performed of high temperature dissociated oxygen reacting with an idealized carbon-carbon composite material, where the microstructure is resolved. The Direct Simulation Monte Carlo (DSMC) method is used to simulate the convection and diffusion of reactants towards the microstructure and the transport of surface reaction products away from the microstructure. Simulations are performed with and without gas-phase chemical reactions in order to determine the relative importance of gas-surface reactions compared to gas-phase reactions next to the material surface. The simulations incorporate reaction probabilities for individual gas-surface collisions based on new reactive scattering data obtained in a molecular beam facility. The molecular beam experiments clearly indicate that a majority of surface reaction products were produced through thermal mechanisms. The experiments provide detailed data on the relative magnitude of O, O2, CO, and CO2 scattering from a representative material sample, made of vitreous carbon. For a gas-surface temperature of 800K, it is found from the simulations that despite CO being the dominant surface reaction product, a gas-phase exchange reaction forms significant CO2 within the microstructure region. The amount of CO2 production within the microstructure region is shown to be dependent on the local Knudsen number, based on the exposed microstructure height. Finally, preliminary simulations are performed for a real CarbonCarbon (C-C) surface. The surface topology is obtained through X-ray microtomography of an ablated C-C sample, which is triangulated and used directly within a DSMC simulation of the gas-surface interaction.

Molecular Simulation of Carbon Ablation Using Beam Experiments and Resolved Microstructure

Molecular simulations with a resolved surface microstructure were performed for high-temperature dissociated oxygen reacting with a carbon–carbon composite material. The direct simulation Monte Carlo method was used to simulate the convection and diffusion of reactants toward the microstructure and the transport of surface reaction products away from the microstructure. Simulations were performed with and without gas-phase chemical reactions to determine the relative importance of gas–surface reactions compared to gas-phase reactions next to the material surface. The simulations incorporated reaction probabilities for individual gas–surface collisions based on new reactive scattering data obtained from molecular beam experiments. The experiments provide detailed dynamical data on the scattering of O, O2, CO, and CO2 from a representative material sample, made of vitreous carbon. These experiments indicate that a majority of surface reaction products were produced through thermal mechanisms. For a gas–surf...

Finite-Rate Oxidation Model for Carbon Surfaces from Molecular Beam Experiments

An oxidation model for carbon surfaces has been developed where the gas–surface reaction mechanisms and corresponding rate parameters are based solely on observations from recent molecular beam exp...

Study of non-reactive scattering from graphene using molecular beam experiments and molecular dynamics

Conventional gas surface interaction (GSI) models and molecular dynamics (MD) are used to generate the gas scattering angle and post-collision kinetic energy distribution, which is compared with the experimental values obtained from molecular beam scattering experiments. While conventional GSI models were unable to capture the experimental scattering distributions, MD simulations were able to generate reasonable agreement. MD simulations also allowed the trajectories to be classified into single, multiple collisions with escape, and multiple collisions without escape events.

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.