Kumiko Yokota

Volume Diffusion of Atomic Oxygen in α-SiO2 Protective Coating

The minimum thickness for an amorphous silicon dioxide (α-SiO2) protective coating required to prevent volume diffusion of atomic oxygen in a low Earth orbit (LEO) was evaluated by measuring the oxide thickness formed on Si(001) wafers in a hyperthermal atomic oxygen beam. The thickness of oxide film was measured by x-ray photoelectron spectroscopy. The diffusion length of atomic oxygen in α-SiO2 at temperatures between 297 K and 493 K, where exterior surfaces of a spacecraft may be heated in LEO, shows temperature and flux dependences, i.e. the diffusion length of atomic oxygen increases with increasing temperature and beam flux. It was also demonstrated that the atomic oxygen fluence is not a primary factor of the diffusion length since the oxide growth obeys a parabolic law. The ground-based testing condition to evaluate performances of protective coatings are also discussed, based on the experimental data obtained in the experiments.

Oxidation Properties of Hydrogen-Terminated Si(001) Surfaces Following Use of a Hyperthermal Broad Atomic Oxygen Beam at Low Temperatures

The oxidation properties of an H-terminated Si(001) surface following the use of a hyperthermal broad atomic oxygen beam have been studied at low temperatures. Effects of sample temperature, flux and energy of the impinging atomic oxygen on the oxidation of silicon were investigated by X-ray photoelectron spectroscopy. It was confirmed that oxide growth at the surface was achieved even at room temperature and the oxide thickness reached a terminal thickness of a few nanometers depending upon the oxidation conditions. The oxidation process was divided into two stages: the fast oxidation stage and the subsequent slow oxidation stage. It was also confirmed that the reaction yield of atomic oxygen with Si increased with increasing translational energy. The characteristic feature of beam oxidation was explained by the effect of energy accommodation from the impinging oxygen atom to the surface silicon atom.

Nonlinear phenomena in the mass loss of polyimide films under hyperthermal atomic oxygen beam exposure

Erosion phenomenon of polyimide film under the hyperthermal atomic oxygen beam exposure, which is a simulated low Earth orbit space environment, has been investigated. The polyimide film was spin-coated on a sensor crystal of a quartz crystal microbalance, and the mass of the film was measured under the atomic oxygen beam exposure. The spin-coated polyimide film which was exposed to a 4.7 eV atomic oxygen beam showed a mass gain at the beginning of the reaction and then steady-state mass loss followed. The experimental results of the mass change was analysed by the computational model, and the results showed that the carbon abstraction rate at the oxygen-adsorbed sites was two orders higher than that at the unoxidized polyimide surface. The computational results suggested that a large fraction of the carbon abstraction reaction occurred in the oxygen-adsorbed site through a Langmuir–Hinshelwood reaction mechanism.

Degradation Property of Commercially Available Si-containing Polyimide in Simulated Atomic Oxygen Environments for Low Earth Orbit

Real-time measurement of the erosion rate of a commercially available Si-containing polyimide (BSF30) under hyperthermal atomic oxygen (AO) beam exposure condition, which simulates the AO environment in low Earth orbit (LEO), was performed. It was found that the erosion rate of BSF30 decreased with increasing AO fluence and it reached as low as 4% of the standard PMDA-ODA polyimide. X-ray photoelectron spectroscopy confirmed that the surface of AO-exposed BSF30 was covered by a SiO2 layer which functioned as a protective coating. In contrast, an SiO2 surface layer thick enough to protect bulk BSF30 was not formed by thermal AO, which was generated by vacuum ultraviolet exposure in an O2 atmosphere. Exposure to hyperthermal AO collision in LEO can also form an SiO2 layer which enables the surface to be self-healing and is desirable for a polyimide that would be used in LEO.

Comparison of Polyethylene and Polyimide as a Fluence Monitor of Atomic Oxygen

Synergistic effect on atomic oxygen-induced erosion of polyethylene and polyimide with 172 nm monochromatic ultraviolet exposure was compared. The erosion of these materials was measured by a quartz crystal microbalance under 5 eV simultaneous atomic oxygen and ultraviolet exposures. The effect of simultaneous ultraviolet exposure on the atomic oxygen-induced erosion was observed at the ultraviolet intensity of 0:5–2:6 10 15 mJ=atom for polyethylene. In contrast, the similar effect was observed in polyimide at the relative ultraviolet intensities one order greater than that of polyethylene. It was found that the effect of 172 nm ultraviolet was different in polyimide and polyethylene, i.e., photoinduced erosion was observed only for polyethylene. It was concluded that polyimide is less sensitive with simultaneous ultraviolet exposure during atomic oxygen bombardment, and thus evaluated as a better material for measuring atomic oxygen fluence.

Synergistic Study on Atomic Oxygen-Induced Erosion of Polyethylene with Vacuum Ultraviolet

Ground-based experimental results on atomic oxygen-induced erosion of polyethylene with simultaneous vacuum ultraviolet (VUV) are reported. In situ mass-loss measurement was made using a quartz crystal microbalance with amass resolution of 2 ng. A laser detonation atomic oxygen beam source was used to simulate collision energy of atomic oxygen with materials in low Earth orbit. To change relative intensity of atomic oxygen and VUV, the sample was rotated with an axis perpendicular both to the axes of atomic oxygen and VUV. It was observed that the erosion rate of polyethylene was increased 30-100% by a simultaneous VUV exposure depending on the incident angle of atomic oxygen. No contribution of temperature increase was confirmed in the polyethylene erosion. The experimental results suggested that two reaction pathways exist in the simultaneous atomic oxygen and VUV exposure conditions. It is suggested that the synergistic effect on polyethylene erosion is emphasized in the exposure condition where relative atomic oxygen flux is low against VUV flux.

Oxidation of Si(001) with a hyperthermal O-atom beam at room temperature: Suboxide distribution and residual order structure

Synchrotron radiation photoelectron spectroscopy (SR-PES) and crystal truncation rod (CTR) scattering profiles were used to investigate an ultrathin SiO2 overlayer on a Si(001) surface formed by a 5eV O-atom beam at room temperature. The SR-PES spectra indicated that the suboxides in the O-atom-beam oxidized film were concentrated on the SiO2 surface rather than at the Si∕SiO2 interface. The CTR scattering data of the O-atom-beam oxidation film had a lower intensity near (11L) (0.3<L<0.8), suggesting a lower content of the SiO2 ordered structure in the oxide film. An inverse diffusion of the interstitial Si atoms in the oxidation kinetics can explain the data.

Comparison of Macro and Microtribological Property of Molybdenum Disulfide Film Exposed to LEO Space Environment

Macro- and microtribological properties of the MoS2 film exposed to atomic oxygen, ultraviolet rays and radiation both in low earth orbit (LEO) and in ground-based facility were evaluated relevance to micro/nano satellites. MoS2 samples are exposed to LEO space environment by the space environment exposure device experiment on international space station. Laser-detonation atomic oxygen beam source was used for atomic oxygen simulation on the ground. X-ray photoelectron spectroscopy and energy dispersive spectroscopy measurements suggested that electron beam and ultraviolet exposure did not affect chemical structure of MoS2 surfaces. However, atomic oxygen-exposed and flight samples showed surface oxidation. It was found that the macroscopic friction coefficient of the flight sample was similar to that of the control sample. In contrast, remarkable increase in friction coefficient in microscopic properties was observed.

Spatially resolved wettability control of polymer surfaces using laser-detonated hyperthermal atomic beams

The fundamental properties of hyperthermal atomic oxygen or atomic fluorine-exposed polyimide and polyethylene surfaces, based on the analytical results by X-ray photoelectron spectroscopy and contact angle measurements, are reported. It was observed that CF, CF2 or CF3 moieties were formed at the atomic-fluorine-exposed polymer surfaces depending on the fluence, whereas carbonyl and carboxyl groups were formed at the atomic oxygen-exposed polymer surfaces. Advancing contact angles of water can be controlled from 60 to 150 degrees by varying the atomic oxygen or atomic fluorine fluence. It was also demonstrated that a selected area on the target surface could be processed with this method using a fine metal mask. The behavior of a water droplet on polymer surfaces could be controlled by the spatially resolved surface modification using hyperthermal atomic beams.

Synchrotron Radiation Photoelectron Emission Study of SiO2 Film Formed by Hyperthermal O-Atom Beam at Room Temperature

An ultrathin SiO2 overlayer on a Si(001) surface formed by a 5 eV O-atom beam at room temperature was analyzed by synchrotron radiation photoelectron spectroscopy (SR-PES). SR-PES spectra clearly indicated that the SiO2 layer formed by a hyperthermal O-atom beam at room temperature contains a small amount of suboxides compared with that formed by high-temperature oxidation in O2 atmosphere. Quantitative analysis suggests that the thickness of the structural transformation layer was less than a monolayer and the amount of suboxides was independent of the film thickness. The translational energy dependence of SR-PES spectra suggests that the reaction probability with a Si-atom increases with the translational energy of the O atoms in the range between 1 to 5 eV. The role of inverse diffusion of interstitial Si atoms in the kinetics of hyperthermal O-atom-beam oxidation is suggested.