Atomic Oxygen Plasma Research Articles

Monitoring the gradual change in oxidation state during surface oxidation or reduction of uranium oxides by photoemission spectroscopy of the 5f states

Photoelectron spectroscopy study of the U5f emission gives valuable insight into the surface oxidation mechanism of uranium oxides. Its intensity is directly related to the electron count nf, which decreases with increasing oxidation number (U(IV): nf=2; U(V): nf=1; U(VI): nf=0). n5f can be quantified by analysing the U5f/U4f intensity ratio and using a standard of known composition. In addition, the 5f emission has a characteristic multiplet shape, directly related to n5f, which can be used to distinguish the 5f2 and 5f1 configuration of U(IV) and U(V), respectively. Three independent methods are used to determine the surface oxidation state: the U5f/U4f intensity ratio, the relative intensities of the U4f oxide shifted peaks, and the O1s/U4f intensity ratio. The first two reveal the concentration of the U in each oxide, the third indicates the total concentration of oxygen. These methods are applied to follow the surface modification of UO2 films when exposed to various oxidative conditions: molecular and atomic oxygen and water plasma at 400°C. In addition, the reduction of UO3 by atomic H is studied. Molecular oxygen oxidizes UO2 to UO2+x(x = 0.22), containing both U(IV) and U(V). Atomic oxygen also oxidizes U(IV) to U(V) at low dosages, but then continues oxidizing U(V) to U(VI) (UO3) at high dosages. Conversely, atomic hydrogen reduces UO3. In the early phase of reduction U(V) forms exclusively – no U(IV) is observed. Water plasma first transforms almost all UO2 (surface and subsurface) into U(V). With further plasma exposure the surface is oxidized to about 80% U(VI) and 20% U(V). Up to this point, a small fraction of U(IV) remains at the surface. Once it disappears, the surface oxidation stops and further water plasma exposure now leads to surface reduction into U(V) (the 5f1 peak increases again). Despite the reduction at high dosage, the O1s/U4f intensity ratio keeps increasing.

A Magnetically Filtered Atomic Oxygen Plasma Source for Low-Earth-Orbit Simulation

A magnetically filtered oxygen plasma source has been developed and characterized for the purpose of simulating the low-Earth-orbit environment. The source uses a magnetic ring cusp geometry coupled with a transverse magnetic filter to produce a plasma stream consisting of streaming atomic oxygen ions with energy, density, and ion species composition corresponding to orbital conditions at altitudes ranging from 275 to 550 km. Plasma properties downstream of the source are dependent on the operational settings for source flow rate and discharge current. The properties are examined as a function of these operating parameters to optimize the production of ions with energy and density relevant for low-Earth-orbit satellite drag applications. At each source operational setting presented in this work, the ion energy distributions are measured 0.55 m downstream of the source using a retarding potential analyzer, and the streaming ions were shown to have most probable energies ranging from 2.8 to 7 eV or 5800 to . In addition, a Wein filter () and a residual gas analyzer are used to measure the ion species percent composition as a function of the operational conditions. Source flow rate and discharge current settings are presented that allow for the composition of atomic oxygen ions to be adjusted from 63 to 90% of the total plasma ion population.

Material erosion measurements and expected operational lifetime of a deployable photon sieve payload

Spacecraft operating in low-Earth orbit are subjected to a number of hazardous environmental constituents that can lead to decreased system performance and reduced operational lifetimes. Due to their thermal, optical, and mechanical properties, polymers are used extensively in space systems; however they are particularly susceptible to material erosion and degradation as a result of exposure to the LEO environment. The focus of this research is to examine the material erosion and mass loss experienced by a custom Kapton-like polyimide due to exposure in a simulated low-earth orbit environment. The deployable membrane telescope design discussed in this research is named Peregrine and is the scientific payload for FalconSat-7, a 3U cubesat designed and developed at the United States Air Force Academy (USAFA) for the purpose of demonstrating the capability of deployable membrane telescope technology on a nano-satellite platform. In addition to the polymer samples, chrome, silver and gold specimens will be examined to measure the oxidation rate and act as a control specimen, respectively. A magnetically filtered atomic oxygen plasma source has previously been developed and characterized for the purpose of simulating the low-Earth orbit environment. The plasma source can be operated at a variety of discharge currents and gas flow rates, of which the plasma parameters downstream of the source are dependent. The characteristics of the generated plasma were examined as a function of these operating parameters to optimize the production of O+ ions with energy relevant to LEO applications. The erosion yield of the Kapton-like polyimide was experimentally determined to be 2.84 × 10−24 cm3 per atom which is in close agreement when compared to the on-orbit measurement for reaction efficiency of Kapton HN. The experimentally determined reaction rate for the Kapton-like polyimide was used to estimate the operational lifetime of the photon sieve during the solar conditions expected beginning in May 2019. The effective lifetime is estimated between 126 and 153 days.

Oxygen Vacancies Engineering of Iron Oxides Films for Solar Water Splitting

Hematite (α-Fe2O3) can be considered as one of the top candidates to act as photoanode in the framework of clean hydrogen production through solar water splitting. The O:Fe ratio, that in this material plays a crucial role in the definition of its photoelectrochemical properties, has been investigated in detail. For this purpose, we examined thermal magnetite oxidation and hematite reduction as two possible routes to produce semiconducting iron oxides layers with controlled stoichiometry. We report on properties of single crystalline nanometric films elaborated by atomic oxygen plasma assisted molecular beam epitaxy as model systems to disentangle structural phase transition effects from pure stoichiometry ones. We provide new insights into the mechanisms related to hematite properties modifications and their correlation with photocurrent changes upon the presence of oxygen vacancies and phase mixing with magnetite, with respect to the vacancies concentration regimes. We show on one hand that crystallographic structure mixing appears as strongly detrimental for photoanodes synthesis whatever the oxygen vacancies concentration. On the other hand, oxygen vacancies in the optimal concentration range, while preserving the α-Fe2O3 corundum phase, is highly favorable for solar water splitting, inducing a substantial reduction of 0.2 V for the onset potential and an overall photocurrent increase of 50% with respect to stoichiometric hematite. The present study demonstrates more generally the possibility of using oxygen vacancies as a degree of freedom for the optimization of hematite photoanodes.

Antiferromagnetic long-range spin ordering in Fe- andNiFe2-dopedBaTiO3multiferroic layers

We report on the Fe doping and on the comparative Ni-Fe codoping with composition close to $\mathrm{NiF}{\mathrm{e}}_{2}$ of fully oxidized $\mathrm{BaTi}{\mathrm{O}}_{3}$ layers (\ensuremath{\sim}20 nm) elaborated by atomic oxygen plasma assisted molecular beam epitaxy; specifically any role of oxygen vacancies can be excluded in our films. Additionally to the classical in situ laboratory tools, the films were thoroughly characterized by synchrotron radiation x-ray diffraction and x-ray absorption spectroscopy. For purely Fe-doped layers, the native tetragonal perovskite structure evolves rapidly toward cubiclike up to 5% doping level above which the crystalline order disappears. On the contrary, low codoping levels $(\ensuremath{\sim}5%\mathrm{NiF}{\mathrm{e}}_{2})$ fairly improve the thin film crystalline structure and surface smoothness; high levels (\ensuremath{\sim}27%) lead to more crystallographically disordered films, although the tetragonal structure is preserved. Synchrotron radiation magnetic dichroic measurements reveal that metal clustering does not occur, that the Fe valence evolves from ${\mathrm{Fe}}^{2+}$ for low Fe doping levels to ${\mathrm{Fe}}^{3+}$ for high doping levels, and that the introduction of Ni favors the occurrence of the ${\mathrm{Fe}}^{2+}$ valence in the films. For the lower codoping levels it seems that ${\mathrm{Fe}}^{2+}$ substitutes ${\mathrm{Ba}}^{2+}$, whereas ${\mathrm{Ni}}^{2+}$ always substitutes ${\mathrm{Ti}}^{4+}$. Ferromagnetic long-range ordering can be excluded with great sensitivity in all samples as deduced from our x-ray magnetic absorption circular dichroic measurements. On the contrary, our linear dichroic x-ray absorption results support antiferromagnetic long-range ordering while piezoforce microscopy gives evidence of a robust ferroelectric long-range ordering showing that our films are excellent candidates for magnetic exchange coupled multiferroic applications.

High-pressure cold gas dynamic (CGD)-sprayed alumina-reinforced aluminum coatings for potential application as space construction material

Alumina–aluminum MMC coatings were deposited by cold gas dynamic spraying onto aluminum alloy (Al 7570) substrates. The microstructure, phase composition, thermal conductivity, microhardness, and thermal optical properties (solar absorptance αs and thermal emittance ε) of four types of coating were determined on as-sprayed samples and samples subjected to atomic oxygen (AO) plasma, intended to simulate conditions in outer space environment. In addition, time-of-flight (ToF) secondary ion mass spectrometry (SIMS) was applied to determine the responses of the coatings to oxidation by AO plasma. The results indicate that the very low thermal conductivity (λ), low solar absorptance (αs), comparatively high infrared emittance (ε) and oxidation stability of the investigated coatings may provide adequate protection as environmental control coatings of space-bound materials against deleterious effects of space environment within the Low Earth Orbit (LEO) and beyond.

Chemistry and structure of BaTiO3 ultra-thin films grown by different O2 plasma power

We present a study of the chemical and atomic properties of 5nm TiO2-terminated BaTiO3 (001) epitaxial films on Nb-doped SrTiO3, as a function of the atomic oxygen plasma power for film growth. Lower plasma power produces non-stoichiometric films with oxygen vacancies and Ti3+ ions. The larger Ti3+ ion radius and the in-plane clamping gives rise to an increase in the out-of-plane lattice parameter. XPS measures the Ti3+ concentration and the concomitant increase in dissociative water uptake in the film, giving rise to on-top OH− adsorption on surface Ti, proton adsorption on surface oxygen, and a near surface Ba-OH environment.

Surface composition of BaTiO3/SrTiO3(001) films grown by atomic oxygen plasma assisted molecular beam epitaxy

We have investigated the growth of BaTiO3 thin films deposited on pure and 1% Nb-doped SrTiO3(001) single crystals using atomic oxygen assisted molecular beam epitaxy and dedicated Ba and Ti Knudsen cells. Thicknesses up to 30 nm were investigated for various layer compositions. We demonstrate 2D growth and epitaxial single crystalline BaTiO3 layers up to 10 nm before additional 3D features appear; lattice parameter relaxation occurs during the first few nanometers and is completed at ∼10 nm. The presence of a Ba oxide rich top layer that probably favors 2D growth is evidenced for well crystallized layers. We show that the Ba oxide rich top layer can be removed by chemical etching. The present work stresses the importance of stoichiometry and surface composition of BaTiO3 layers, especially in view of their integration in devices.

Formation of ZnO Nanosheets by Oxidizing Zn Film

In this work, we report a simple method for fabricating nanostructured ZnO by directly exposing thin Zn film to an atomic oxygen plasma produced by plasma-enhanced chemical vapor deposition (PECVD). Thin Zn films (∼500-nm thickness) were exposed to an atomic oxygen plasma at 380 ◦C for 30 min, and well-defined ZnO nanosheets of about 5 nm thickness are shown to be produced, forming a nanoflower-like structure as analyzed by using field-emission electron microscopy (FESEM). The nanosheets were characterized by using transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction spectroscopy (XRD), and photoluminescence (PL) measurements. The resulting nanosheets exhibited wurtzite hexagonal structures grown along the [10-10] direction and emitted both ultraviolet and green light centered near 380 nm and 540 nm, respectively. A possible explanation is discussed.

Oxygen-passivated enhancement of photoluminescence from SiO 2 films containing Si nanocrystals

SiO 2 films containing Si nanocrystals were exposed to atomic oxygen (O) plasma at different temperatures. The photoluminescence intensity from the films increases with increasing exposure time, and then saturates at a level that depends on the exposure temperature. There are no significant changes in the PL spectrum shapes and the O-induced PL is fully recovered to its original state by thermal annealing. The results are likely to be due to a balance between defect passivation and depassivation, with the activation energy for the passivation reaction being larger than that for the depassivation reaction.

Permeation mechanisms of pulsed microwave plasma deposited silicon oxide films for food packaging applications

Silicon oxide barrier layers are deposited on polyethylene terephthalate as permeation barriers for food packaging applications by means of a low pressure microwave plasma. Hexamethyldisiloxane (HMDSO) and oxygen are used as process gases to deposit SiOx coatings via pulsed low pressure plasmas. The layer composition of the coating is investigated by Fourier transform infrared spectroscopy and energy dispersive x-ray spectroscopy to show correlations with barrier properties of the films. The oxygen permeation barrier is determined by the carrier gas method using an electrochemical detector. The transition from low to high barrier films is mapped by the transition from organic SiOxCyHz layers to quartz-like SiO1.7 films containing silanol bound hydrogen. A residual permeation as low as J = 1 ± 0.3 cm3 m−2 day−1 bar−1 is achieved, which is a good value for food packaging applications. Additionally, the activation energy Ep of oxygen permeation is analysed and a strong increase from Ep = 31.5 kJ mol−1 for SiOx CyHz-like coatings to Ep = 53.7 kJ mol−1 for SiO1.7 films is observed by increasing the oxygen dilution of HMDSO:O2 plasma. The reason for the residual permeation of high barrier films is discussed and coating defects are visualized by capacitively coupled atomic oxygen plasma etching of coated substrates. A defect density of 3000 mm−2 is revealed.

プラズマトーチ応用の超音速燃焼初期促進に関する気体分子運動論的解析 第２報：活性基のモル流束および熱流束の影讐

The 1st report clarified the superiority of the atomic oxygen injected from a plasma torch with the diffusive mode operation for initiating supersonic combustion. In this paper the features of combustion enhancement by various combinations of plasma mole flux and heat flux are investigated. The effectiveness of each combination is evaluated by H2O production efficiency. A molecular dynamical numerical simulation technique with the reactional molecular collision model is applied for the analysis of multi-species reaction system. The results of parametric study show that the effect of atomic oxygen plasma is attributed to the mole flux, whilst the strong dependence on the heat flux is observed for the atomic hydrogen plasma.

BSCCO thin films obtained by MBE coevaporation method

We obtained as-grown Bi2Sr2Cu1Ox (2201) superconducting thin films using a MBE system consisting of two electron guns and two effusion cells. The samples were obtained evaporating simultaneously the single elements on heated substrates and oxygenating them during the growth by using an atomic oxygen plasma source (OPS). Sticking coefficients of each element were determined as a function of the substrate temperature and oxygen partial pressure. In order to monitor the surface structure, Reflection High Energy Electron Diffraction (RHEED) analysis was employed during the deposition process. The chemical composition was obtained by Energy Dispersion Spectroscopy (EDS) while the final structure was studied by X-ray diffraction patterns.

Tantalum oxide films on silicon grown by tantalum evaporation in atomic oxygen

Tantalum pentoxide (Ta 20 5) thin layers were grown on Si(100) by evaporation of Ta in an atomic oxygen plasma. The physical and electrical properties of 1000 Å-thick as-deposited films were evaluated. Films grown at 650 °C without the presence of the plasma (molecular oxygen) were oxygen-deficient and poorly crystallized, whereas layers grown in the plasma (atomic oxygen) were stoichiometric (within 5%) and crystallized in the β-Ta 20 5 phase. A minimum flux of atomic oxygen during deposition, more than that required for growing films within 5% correct stoichiometry, was crucial for the suppression of the film leakage current. The electrical characterization of amorphous Ta 20 5 films grown at 400 °C resulted in I– V curves exhibiting an ohmic behaviour at low electric fields, <2 MV cm −1 (resistivity 10 14 Ωcm), and a leakage current density less than 10 −8 A cm −2 at 1 MV cm −1. The measured relative dielectric constant of amorphous Ta 2O 5 films was 26.

Growth and characterization of yttrium oxide thin layers on silicon deposited by yttrium evaporation in atomic oxygen

Polycrystalline yttrium oxide films have been grown under high vacuum conditions by evaporation of yttrium in an atomic oxygen plasma. Well-oxidized uniform layers in the thickness interval 30–170 nm were deposited on silicon over a large temperature range (80–800 °C), Film surfaces were very smooth with average surface roughness 1.6 nm for 70 nm films grown at 350 °C. No impurity phases were found by X-ray diffraction even for the highest growth temperatures. The layers exhibited a strong (111) texture for growth temperatures ⩾ 300 °C irrespective of the substrate orientation. High frequency capacitance-voltage measurements on aluminium/yttrium oxide/silicon capacitors showed metal-insulator-semiconductor operation with large hysteresis effects in the depletion region. The calculated values of the relative dielectric constant for the capacitor structures were 14 and 11 for 100 nm thick yttrium oxide films grown at 350 °C and 750 °C, respectively. The breakdown strength of the capacitors was found to be 3.8 MV cm −1.

Atomic Oxygen Treatment for Non-Contact Removal of Organic Protective Coatings from Painting Surfaces

Current techniques for removal of varnish (lacquer) and other organic protective coatingsfrom paintings involve contact with the surface. This contact can remove pigment, or alter the shape and location of paint on the canvas surface. A thermal energy atomic oxygen plasma, developed to simulate the space environment in low Earth orbit, easily removes these organic materials. Uniform removal of organic protective coatings from the surfaces ofpaintings is accomplished through chemical reaction. Atomic oxygen will not react with oxides so that most paint pigments will not be affected by the reaction. For paintings containing organic pigments, the exposure can be carefully timed so that the removal stops just short of the pigment. Color samples of Alizarin Crimson, Sap Green, and Zinc White coated with Damar lacquer were exposed to atomic oxygen. The lacquer was easily removed fromall of the samples. Additionally, no noticeable change in appearance was observed after the lacquer was reapplied. The same observations were made on a painted canvas test sampleobtained from the Cleveland Museum of Art. Scanning electron microscope photographs showed a slight microscopic texturing of the vehicle after exposure. However, there was no removal or disturbance of the paint pigment on the surface. It appears that noncontact cleaning using atomic oxygen may provide a viable alternative to other cleaning techniques. Itis especially attractive in cases where the organic protective surface cannot be acceptably or safely removed by conventional techniques.

Properties of Yttrium Oxide Thin Films on Silicon (100) Prepared by Evaporation of Yttrium in Atomic Oxygen

AbstractThin layers of Y2O3 have been prepared on silicon (100) by an activated reactive evaporation process involving evaporation of metal Y in an atomic oxygen plasma. The presence of the oxygen plasma was found to be crucial for the formation of homogeneous Y2O3 films on Si. The formation of Y2O3 films on Si (100) at different substrate temperatures was investigated. X-ray diffraction analysis showed that Y2O3 films formed between 300 °C and 650 °C were (111) textured while Y2O3 prepared at lower substrate temperatures (80 °C) exhibited mixed orientations. Rutherford backscattering spectrometry indicated that films were stoichiometric. No pronounced channeling was observed in films grown at 350 °C, suggesting polycrystalline film structures. Atomic force microscopy revealed very smooth surface morphologies with average surface roughness &lt; 20 Å for films 700 Å thick deposited at 350 °C. Secondary ion mass spectroscopy indicated the abundance of intermediate layers in the film-substrate interface.

Layer by layer growth of epitaxial films of Bi2Sr2CuO6: Structural and electrical properties related to the oxydation level

Bi 2 Sr 2 CuO 6 thin films were deposited on SrTiO 3 (100) single crystals by atomic layer epitaxy. The layer by layer deposition process is controled by RHEED oscillations. Bi and Sr are evaporated from Knudsen cells while Cu is evaporated using an electron gun. An atomic oxygen plasma source was used to oxidize the films. The substrate temperature is as low as 470°C. Atomic concentrations are obtained from RBS and EDX measurements. The obtained films are stoechiometric, oriented and crisallized. AFM measurements show a typical roughness of +/− 5Å. The modulation structure periodicity is studied from the RHEED pattern. The correlations between the oxidation level, the modulation reciprocal vector q ∗ , the c axis lattice parameter and the room temperature of the films is described. The films exhibit a metallic behaviour at low or high oxidation level, while they are insulating in the medium oxidation level. Possible origins of this behaviour are discussed.

ESCA study of the effect of hydrocarbon contamination on poly(tetrafluoroethylene) exposed to atomic oxygen plasma

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTESCA study of the effect of hydrocarbon contamination on poly(tetrafluoroethylene) exposed to atomic oxygen plasmaMorton A. Golub, Theodore. Wydeven, and Robert D. CormiaCite this: Langmuir 1991, 7, 5, 1026–1028Publication Date (Print):May 22, 1991Publication History Published online1 May 2002Published inissue 22 May 1991https://doi.org/10.1021/la00053a034RIGHTS & PERMISSIONSArticle Views103Altmetric-Citations23LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (332 KB) Get e-Alerts Get e-Alerts

BiSrCaCuO films made by coevaporation: Influence of the initial composition

BiSrCaCuO films were prepared under ultrahigh vacuum by coevaporation of the pure elements. The copper was deposited from an electron gun, and the other components were evaporated from Knudsen cells. In situ oxidation of the films was performed by an atomic oxygen plasma source. The films were annealed in different conditions, and we discuss the proportion of 2212 and 2223 phases obtained related to the resistive transition. This composition depends on the initial composition determined by Rutherford Backscattering, and on the deposition conditions. Films were also prepared by flux modulation at the atomic layer scale. The results obtained by this way are promising. The main difficulty in order to reach the right stoichiometry is related to the variations of the sticking coefficient of the bismuth. This problem will be discussed.