Electrode phenomena in high energy density discharges as applied to plasma acceleration problems
Electrode surface structure and morphology changes in high energy density discharges as applied to plasma acceleration problems
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Observations of actively forming lava tubes and associated structures, Hawaii, part 2
A ground examination is made of lave tubes and channels. The surface morphology and the changes noted through lava flow activity are cited, and compared to earlier aerial observations. The lava activity was believed to be caused by a small lava lake exposed by the collapse of a crust covering it. Drainage of the lake was caused by a fissure erruption. New tubes or extensions of existing ones were noted from the flow. Molten lava was not seen in any tubes examined on the ground, but some of the flows were not sufficiently cooled to allow subsurface examination and survey of the tubes.
Greeley, R.↗
The role of NaCl in flame chemistry, in the deposition process, and in its reactions with protective oxides as related to hot corrosion
Sodium chloride is believed to be the primary source of turbine engine contamination that contributes to hot corrosion. The behavior of NaCl-containing aerosols ingested with turbine intake air is very complex; some of the NaCl may vaporize during combustion while some may remain as particulates. The NaCl can lead to Na2SO4 formation by several possible routes or it can contribute to corrosion directly. Hydrogen or oxygen atom reaction with NaCl(c) was shown to result in the release of Na(g). Gaseous NaCl in flames can be partially converted to gaseous Na2SO4 by homogeneous reactions. The remaining gaseous NaCl and other Na-containing molecules can act as sodium carriers for condensate deposition of Na2SO4 on cool surfaces. A frozen boundary layer theory was developed to predict the rates of deposition. The condensed phase NaCl can be converted directly to condensed Na2SO4 by reaction with sulfur oxides and O2. Reaction of gaseous NaCl with Cr2O3 results in the vapor phase transport of chromium by the formation of complex Cr-containing gaseous molecules. Similar gaseous complexes are formed with molybdenum. The presence of gaseous NaCl was shown to affect the oxidation kinetics of Ni-Cr alloys. It also causes changes in the surface morphology of Al2O3 scales formed on Al-containing alloys.
Kohl, F. J.↗
The effects of atomic oxygen on the thermal emittance of high temperature radiator surfaces
Radiator surfaces on high temperature space power systems such as SP-100 space nuclear power system must maintain a high emittance level in order to reject waste heat effectively. One of the primary materials under consideration for the radiators is carbon-carbon composite. Since carbon is susceptible to attack by atomic oxygen in the low earth orbital environment, it is important to determine the durability of carbon composites in this environment as well as the effect atomic oxygen has on the thermal emittance of the surface if it is to be considered for use as a radiator. Results indicate that the thermal emittance of carbon-carbon composite (as low as 0.42) can be enhanced by exposure to a directed beam of atomic oxygen to levels above 0.85 at 800 K. This emittance enhancement is due to a change in the surface morphology as a result of oxidation. High aspect ratio cones are formed on the surface which allow more efficient trapping of incident radiation. Erosion of the surface due to oxidation is similar to that for carbon, so that at altitudes less than approximately 600 km, thickness loss of the radiator could be significant (as much as 0.1 cm/year). A protective coating or oxidation barrier forming additive may be needed to prevent atomic oxygen attack after the initial high emittance surface is formed. Textured surfaces can be formed in ground based facilities or possibly in space if emittance is not sensitive to the orientation of the atomic oxygen arrival that forms the texture.
Rutledge, Sharon K.↗
Low earth orbit durability evaluation of Haynes 188 solar receiver material
The effects of elevated-temperature vacuum and elevated-temperature atomic oxygen exposure on the mass, surface chemistry, surface morphology, and optical properties of Haynes 188, a possible heat receiver material for space-based solar dynamic power systems, have been studied. Pristine and surface modified Haynes 188 were exposed to vacuum less than or equal to 10 exp -6 torr at 820 C for 5215.5 h, and to atomic oxygen in an air plasma asher at 34 and 827 C for fluences up to 5.6 x 10 exp 21 atoms/sq cm. Results obtained indicate that vacuum heat treatment caused surface morphology and chemistry changes with corresponding optical property changes. Atomic oxygen exposure caused optical property changes which diminished with time. Mass changes are considered to be negligible for both exposures.
De Groh, Kim K.↗
Further investigations of experiment A0034 atomic oxygen stimulated outgassing
Thermal control coatings within the recessed compartments of LDEF Experiment A0034 experienced the maximum leading edge fluence of atomic oxygen with considerably less solar UV radiation exposure than top-surface mounted materials of other LDEF experiments on either the leading or the trailing edge. This combination of exposure within A0034 resulted in generally lower levels of darkening attributable to solar UV radiation than for similar materials on other LDEF experiments exposed to greater cumulative solar UV radiation levels. Changes in solar absorptance and infrared thermal emittance of the exposed coatings are thus unique to this exposure. Analytical results for other applications have been found for environmentally induced changes in fluorescence, surface morphology, light scattering, and the effects of coating outgassing products on adjacent mirrors and windows of the A0034 experiment. Some atmospheric bleaching of the thermal control coatings, in addition to that presumably experience during reentry and recovery operations, has been found since initial post-flight observations and measurements.
Linton, Roger C.↗
Preliminary Evaluation of Polyarylate Dielectric Films for Cryogenic Applications
Polymeric materials are used extensively on spacecraft and satellites in electrical power and distribution systems, as thermal blankets and optical surface coatings, as well as mechanical support structures. The reliability of these systems when exposed to the harsh environment of space is very critical to the success of the mission and the safety of the crew in manned-flight ventures. In this work, polyarylate films were evaluated for potential use as capacitor dielectrics and wiring insulation for cryogenic applications. Two grades of the film were characterized in terms of their electrical and mechanical properties before and after exposure to liquid nitrogen (-196 C). The electrical characterization consisted of capacitance and dielectric loss measure Cents in the frequency range of 50 Hz to 100 kHz, and volume and surface resistivities. The mechanical measurements performed included changes in tensile (Young's modulus, elongation-at-break, and tensile strength) and structural properties (dimensional change, weight, and surface morphology). The preliminary results, which indicate good stability of the polymer after exposure to liquid nitrogen, are presented and discussed.
Patterson, Richard L.↗
Fabrication of Monolithic Sapphire Membranes for High Tc Bolometer Array Development
This paper examines the effectiveness of Pt/Cr thin film masks for the architecture of monolithic membrane structures in r-plane sapphire. The development of a pinhole-free Pt/Cr composite mask that is resistant to hot H2SO4:H3PO4 etchant, will lead to the fabrication of smooth sapphire membranes whose surfaces are well-suited for the growth of low-noise high Tc films. In particular, the relationship of thermal annealing conditions on the Pt/Cr composite mask system to: (1) changes in the surface morphology and elemental concentration of the Pt/Cr thin film layers and (2) etch pit formation on the sapphire surface will be presented.
Pugel, D. E.↗
Fabrication of Monolithic Sapphire Membranes for High T(sub c) Bolometer Array Development
This paper examines the effectiveness of Pt/Cr thin film masks for the architecture of monolithic membrane structures in r-plane single crystal sapphire. The development of a pinhole-free Pt/Cr composite mask that is resistant to boiling H2SO4:H3PO4 etchant will lead to the fabrication of smooth sapphire membranes whose surfaces are well-suited for the growth of low-noise high Tc films. In particular, the relationship of thermal annealing conditions on the Pt/Cr composite mask system to: (1) changes in the surface morphology (2) elemental concentration of the Pt/Cr thin film layers and (3) etch pit formation on the sapphire surface will be presented.
Pugel, D. E.↗
Crack Growth Behavior in the Threshold Region for High Cycle Fatigue Loading
This paper describes the results of a research program conducted to improve the understanding of fatigue crack growth rate behavior in the threshold growth rate region and to answer a question on the validity of threshold region test data. The validity question relates to the view held by some experimentalists that using the ASTM load shedding test method does not produce valid threshold test results and material properties. The question involves the fanning behavior observed in threshold region of da/dN plots for some materials in which the low R-ratio data fans out from the high R-ratio data. This fanning behavior or elevation of threshold values in the low R-ratio tests is generally assumed to be caused by an increase in crack closure in the low R-ratio tests. Also, the increase in crack closure is assumed by some experimentalists to result from using the ASTM load shedding test procedure. The belief is that this procedure induces load history effects which cause remote closure from plasticity and/or roughness changes in the surface morphology. However, experimental studies performed by the authors have shown that the increase in crack closure is a result of extensive crack tip bifurcations that can occur in some materials, particularly in aluminum alloys, when the crack tip cyclic yield zone size becomes less than the grain size of the alloy. This behavior is related to the high stacking fault energy (SFE) property of aluminum alloys which results in easier slip characteristics. Therefore, the fanning behavior which occurs in aluminum alloys is a function of intrinsic dislocation property of the alloy, and therefore, the fanned data does represent the true threshold properties of the material. However, for the corrosion sensitive steel alloys tested in laboratory air, the occurrence of fanning results from fretting corrosion at the crack tips, and these results should not be considered to be representative of valid threshold properties because the fanning is eliminated when testing is performed in dry air.
Forman, R. G.↗
Investigation of Bipropellant Plume-Induced Contamination Effects on Coverglass Materials
Contamination and degradation of external spacecraft materials by unburned and partially combusted species from bipropellant thruster plumes has long been observed as a key component of the induced space environment. Space shuttle flight experiments and returned flight hardware from the International Space Station (ISS) have both experienced microscopic impact features induced by high-velocity thruster plume droplets. Analytical results have shown that droplet impingement angle relative to a receiving surface plays a key role in the surface damage. Although impacts with normal impingement angles contribute more severely to surface degradation than highly oblique angles, surface effects at higher impingement angles should not be dismissed. Thruster plume-induced materials degradation is a complex phenomenon that depends on a variety of parameters, including but not limited to material type, system temperature and pressure, plume composition, and thruster firing specifications such as number of pulses, pulse duration, and sample distance from the thruster. For space applications, attaining the vacuum pressure and temperature conditions necessary for flight-like plume expansion and exposure conditions is not a trivial task. The German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt, DLR) is a facility uniquely capable of simulating such conditions. Test coupons were exposed to bipropellant thruster firings under high vacuum at the DLR facility. Percent area coverage (PAC) and droplet size distributions were evaluated for the uncoated and coated solar array coverglass materials over a range of impingement angles (0̊ to 75̊). A post-test imaging workflow was developed that aimed to quantify changes in sample surface morphology obtained from scanning electron microscopy (SEM) images using the Image Processing and Analysis in Java (ImageJ) tool; an opensource image processing software. The goal was to create a framework through which to evaluate the effect of bipropellant-induced PAC and droplet size distribution on solar array coverglass optical transmission losses. Understanding this relationship is important because optical transmission losses are known to lead to current reduction in solar power generation systems. In addition to the development of surface characterization workflows, valuable lessons learned as they pertain to future investigations and experiments will be discussed. The authors hope that sharing these lessons will facilitate more utilization of DLR’s unique capabilities as well as open the conversation for how best to address experimental characterization of flight-like plume expansion and its impacts on materials surface degradation effects.
Gateway↗
Selected results for metals from LDEF experiment A0171
Metal specimens in disk type and ribbon configurations of interest to various programs at the Marshall Space Flight Center were exposed to the LEO environment for 5.8 years on Long Duration Exposure Facility (LDEF) Experiment A0171. Most of the metals flown were well heat sunk in the LDEF experiment tray which experienced benign temperatures, but a few metals were thermally isolated allowing them to experience greater thermal extremes. All metal specimens whose preflight weights were known showed a weight change as a result of exposure. Optical property and mass changes are attributed principally to atomic oxygen exposures. Silver and copper were grossly affected whereas tantalum, molybdenum, and several preoxidized alloys were the least affected. Metals contained in this experiment are shown. Results including mass, surface morphology, and optical property changes from selected evaluations of these metals are presented.
Whitaker, Ann F.↗
Ion-Implanted Diamond Films and Their Tribological Properties
This paper reports the physical characterization and tribological evaluation of ion-implanted diamond films. Diamond films were produced by microwave plasma, chemical vapor deposition technique. Diamond films with various grain sizes (0.3 and 3 microns) and roughness (9.1 and 92.1 nm r.m.s. respectively) were implanted with C(+) (m/e = 12) at an ion energy of 160 eV and a fluence of 6.72 x 10(exp 17) ions/sq cm. Unidirectional sliding friction experiments were conducted in ultrahigh vacuum (6.6 x 10(exp -7)Pa), dry nitrogen and humid air (40% RH) environments. The effects of C(+) ion bombardment on fine and coarse-grained diamond films are as follows: the surface morphology of the diamond films did not change; the surface roughness increased (16.3 and 135.3 nm r.m.s.); the diamond structures were damaged and formed a thin layer of amorphous non-diamond carbon; the friction coefficients dramatically decreased in the ultrahigh vacuum (0.1 and 0.4); the friction coefficients decreased slightly in the dry nitrogen and humid air environments.
Wu, Richard L. C.↗
Novel inverse oxide/metal catalysts for methanol synthesis: impact of oxide–metal interactions and reversible morphological changes
Inverse oxide/metal catalysts have proved to be excellent systems for the generation of methanol by CO 2 hydrogenation or the partial oxidation of methane. These systems can exhibit unique structural and chemical properties due to the nano size of the oxide component and strong oxide–metal interactions. Recent studies for ZnO/Cu, CeO 2 /Cu, TiO 2/ Cu, MgO/Cu, In 2 O 3 /Cu, and In 2 O 3 /Au catalysts have shown large variations in the composition and morphology of the oxide overlayer as a function of temperature and chemical environment. These oxide–metal interfaces are able to react with CO 2 and CH 4 at room temperature, and both reactants have a strong influence on the physical and chemical properties of the catalysts. Under reaction conditions, switches between oxide–metal and metal–metal interfaces can take in the catalyst surface. A dynamic behavior that can be linked to a high selectivity for methanol production over systems such as ZnO/Cu, CeO 2 /Cu and In 2 O 3 /Au. Finally, this type of correlation deserves additional systematic studies since it could be a powerful tool for designing highly efficient catalysts for methanol synthesis.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
A fundamental approach to adhesion: Synthesis, surface analysis, thermodynamics and mechanics
Various surface preparations for titanium 6-4 alloy were studied. An anodizing method was investigated, and compared with the results of other chemical treatments, namely, phosphate/fluoride, Pasa-Jell and Turco. The relative durability of the different surface treatments was assessed by monitoring changes in surface chemistry and morphology occasioned by aging at 505 K (450 F). Basic electron spectroscopic data were collected for polyimide and polyphenylquinoxaline adhesives and synthetic precursors. Fractographic studies were completed for several combinations of adherend, adhesive, and testing conditions.
Chen, W.↗
Analysis of the Surface Morphology and Chemical Composition of Zr-Nb3Sn Alloys with Zr Different Concentrations
The Zr inclusion in Nb3Sn can improve the high magnetic field performance of Nb3Sn radio frequency cavities. The focus of this project is to characterize the surface properties and chemical composition of Zr-Nb3Sn. Nb3Sn was doped with two different concentrations of Zr. ~0.5%, and ~24% Zr. Various spectroscopy techniques were used to examine how the surface morphology and chemical composition changes with increasing Zr fraction in Nb3Sn.
Sue, Micah↗
L-Band H Polarized Microwave Emission During the Corn Growth Cycle
Hourly L-band (1.4 GHz) horizontally (H) polarized brightness temperatures (T(sub B))'s measured during five episodes (more than two days of continuous measurements) of the 2002 corn growth cycle are analyzed. These T(sub B)'s measurements were acquired as a part of a combined active/passive microwave field campaign, and were obtained at five incidence and three azimuth angles relative to the row direction. In support of this microwave data collection, intensive ground sampling took place once a week. Moreover, the interpretation of the hourly T(sub B)'s could also rely on the data obtained using the various automated instruments installed in the same field. In this paper, the soil moisture and temperature measured at fixed time intervals have been employed as input for the tau-omega model to reproduce the hourly T(sub B). Through the calibration of the vegetation and surface roughness parameterizations, the impact of the vegetation morphological changes on the microwave emission and the dependence of the soil surface roughness parameter, h(sub r), on soil moisture are investigated. This analysis demonstrates that the b parameter, appearing in the representation of the canopy opacity, has an angular dependence that varies throughout the growing period and also that the parameter hr increases as the soil dries in a portion of the dry-down cycle. The angular dependence of the b parameter imposes the largest uncertainty on T(sub B) simulations near senescence as the response of b to the incidence is also affected by the crop row orientation. On the other hand, the incorporation of a soil moisture dependent h(sub r) parameterization was responsible for the largest error reduction of T(sub B) simulations in the early growth cycle.
Joseph, A. T.↗
Why ionizing radiation enhances surface wettability
Radiation-Induced Surface Activation is an inherent phenomenon where surfaces that are exposed to gamma irradiation are observed to undergo an increase in wettability. This increase in wettability as a result of the ionizing radiation exposure has so far been demonstrated to have a pronounced impact on Leidenfrost temperature and two-phase fluid dynamics. Test results from previous experiments have shown that incorporation of this effect on heat transfer equipment design may increase the thermal-hydraulic margin leading to higher thermal efficiency. However, the mechanism behind the increased wettability is not clearly understood. In the present work, three different materials (Zircaloy-4, 316 stainless steel, and copper) were exposed at two different dose rates with use of two different gamma irradiation facilities. A detailed surface characterization on the post-irradiated samples is carried out to understand the changes in surface chemistry, wettability and surface morphology. It is observed from the experiments that the increase in wettability upon irradiation depended on the total dose and not on the dose rate. Moreover, localized oxidation and porosity induced by radiolysis was seen to be the predominant mechanism behind increased wettability which leads to improved Leidenfrost temperature.
Seshadria, Arunkumar↗