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NASA NTRS2002

Laminar Soot Processes (LSP)

This is the final report of a research program considering the structure and the soot surface reaction properties of laminar nonpremixed (diffusion) flames. The study was limited to ground-based measurements of buoyant laminar jet diffusion flames at pressures of 0.1-1.0 atm. The motivation for the research is that soot formation in flames is a major unresolved problem of combustion science that influences the pollutant emissions, durability and performance of power and propulsion systems, as well as the potential for developing computational combustion. The investigation was divided into two phases considering the structure of laminar soot-containing diffusion flames and the soot surface reaction properties (soot surface growth and oxidation) of these flames, in turn. The first phase of the research addressed flame and soot structure properties of buoyant laminar jet diffusion flames at various pressures. The measurements showed that H, OH and O radical concentrations were generally in superequilibrium concentrations at atmospheric pressure but tended toward subequilibrium concentrations as pressures decreased. The measurements indicated that the original fuel decomposed into more robust compounds at elevated temperatures, such as acetylene (unless the original fuel was acetylene) and H, which are the major reactants for soot surface growth, and that the main effect of the parent fuel on soot surface growth involved its yield of acetylene and H for present test conditions. The second phase of the research addressed soot surface reaction properties, e.g., soot surface growth and surface oxidation. It was found that soot surface growth rates in both laminar premixed and diffusion flames were in good agreement, that these rates were relatively independent of fuel type, and that these rates could be correlated by the Hydrogen-Abstraction/Carbon-Addition (HACA) mechanisms of Colket and Hall (1994), Frenklach et al. (1990,1994), and Kazakov et al. (1995). It was also found that soot surface oxidation rates were relatively independent of fuel type, were not correlated with O2, CO2, H2O and O collision rates but were correlated with the collision rates of OH with a collision efficiency of 0.14, in agreement with the early measurements in premixed flames of Neoh et al. (1980), after allowing for oxidation by O2 via the classical rate expression of Nagle and Strickland-Constable (1962).

Dai, Z.↗

DOE OSTI2025

Enhancing low-temperature sintering in the MgO-LiF system: Mechanistic insights

In the present article, we provide compelling evidence that minor (1 wt%) additions of micron and nanometre-sized LiF particles in MgO decompose leading to free Li diffusing into MgO surfaces enhancing vacancy production during direct current sintering. The addition of nanometre-sized LiF particles leads to a > 500 °C reduction in the sintering temperature and over 99 % theoretical density of final consolidated compacts. Correlating differential scanning calorimetry with in-situ x-ray diffraction, together with Schottky calculations, post sintering electron microscopy and laser induced breakdown spectroscopy, we uncover critical insights into this impressive reduction in sintering temperature. Our quantitative analysis reveals that MgO and LiF interact at low temperatures with the diffusion of Li into the surface of MgO particles due to the intrinsic structural disorder of the LiF and MgO crystallites. Nanometre-sized LiF particles were found to react the most at low temperatures due to their increased structural disorder. Our multimodal characterization points to a Li-promoted densification and sintering mechanism. This impressive reduction in sintering temperature can be harnessed to promote low-temperature fabrication of MgO-based composites for technological applications.

36 MATERIALS SCIENCE↗

DOE OSTI2021

RF surface resistance tuning of superconducting niobium via thermal diffusion of native oxide

Recently, Nb superconducting radio frequency cavities vacuum heat treated between 300 and 400°C for a few hours have exhibited very high quality factors (~5 × 10 10 at 2.0 K). Secondary ion mass spectrometry measurements of O, N, and C show that this enhancement in RF surface conductivity is primarily associated with interstitial O alloying via dissolution and diffusion of the native oxide. We use a theory of oxide decomposition and O diffusion to quantify previously unknown parameters crucial in modeling this process. RF measurements of a vacuum heat-treated Nb superconducting radio frequency cavity confirm the minimized surface resistance (higher Q 0 ) previously expected only from 800 °C diffusive alloying with N.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

NASA NTRS1990

Uses of Auger and x ray photoelectron spectroscopy in the study of adhesion and friction

Three studies are described characterizing the possible contributions of surface science to tribology. These include surface contamination formed by the interaction of a surface with the environment, contaminants obtained with diffusion of compounds, and surface chemical changes resulting from selective thermal evaporation. Surface analytical tools such as Auger electron spectroscopy (AES) and x ray photoelectron spectroscopy (XPS) incorporated directly into adhesion and friction systems are primarily used to define the nature of tribological surfaces before and after tribological experimentation and to characterize the mechanism of solid-to-solid interaction. Emphasis is on fundamental studies involving the role of surfaces in controlling the adhesion and friction properties of materials emerging as a result of the surface analyses. The materials which were studied include metals and ceramics such as elemental metals, amorphous alloys (metallic glasses), and silicon-based ceramics.

Miyoshi, Kazuhisa↗

NASA NTRS1998

An Integrated Tool for the Coupled Thermal and Mechanical Analysis of Pyrolyzing Heatshield Materials

Materials that pyrolyze at elevated temperature have been commonly used as thermal protection materials in hypersonic flight, and advanced pyrolyzing materials for this purpose continue to be developed. Because of the large temperature gradients that can arise in thermal protection materials, significant thermal stresses can develop. Advanced applications of pyrolytic materials are calling for more complex heatshield configurations, making accurate thermal stress analysis more important, and more challenging. For non-pyrolyzing materials, many finite element codes are available and capable of performing coupled thermal-mechanical analyses. These codes do not, however, have a built-in capability to perform analyses that include pyrolysis effects. When a pyrolyzing material is heated, one or more components of the original virgin material pyrolyze and create a gas. This gas flows away from the pyrolysis zone to the surface, resulting in a reduction in surface heating. A porous residue, referred to as char, remains in place of the virgin material. While the processes involved can be complex, it has been found that a simple physical model in which virgin material reacts to form char and pyrolysis gas, will yield satisfactory analytical results. Specifically, the effects that must be modeled include: (1) Variation of thermal properties (density, specific heat, thermal conductivity) as the material composition changes; (2) Energy released or absorbed by the pyrolysis reactions; (3) Energy convected by the flow of pyrolysis gas from the interior to the surface; (4) The reduction in surface heating due to surface blowing; and (5) Chemical and mass diffusion effects at the surface between the pyrolysis gas and edge gas Computational tools for the one-dimensional thermal analysis these materials exist and have proven to be reliable design tools. The objective of the present work is to extend the analysis capabilities of pyrolyzing materials to axisymmetric configurations, and to couple thermal and mechanical analyses so that thermal stresses may be efficiently and accurately calculated.

Pronchick, Stephen W.↗

DOE OSTI2019

Ti interstitial flows giving rutile TiO 2 reoxidation process enhancement in (001) surface

In this work we revisited ab initio evaluations of the barrier energies along the possible diffusion paths of the defects in rutile TiO 2 by using a diffusion Monte Carlo method. We found that Ti interstitials hopping along the c axis are identified as the major diffusion directing to the (001) surface, contradicting any of the previous density functional theory studies. Our finding reasonably explains recent experiments reporting that the photocatalytic activity in the (001) surface is superior to that in the (110) surface: Here, the faster Ti diffusion directing to the (001) surface leads to the better self-compensation ability and maintains its photocatalytic activity.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Weak scaling of the contact distance between two fluctuating interfaces with system size

A pair of flat parallel surfaces, each freely diffusing along the direction of their separation, will eventually come into contact. If the shapes of these surfaces also fluctuate, then contact will occur when their centers-of-mass remain separated by a nonzero distance ℓ. An example of such a situation is the motion of interfaces between two phases at conditions of thermodynamic coexistence, and in particular the annihilation of domain wall pairs under periodic boundary conditions. Here we present a general approach to calculate the probability distribution of the contact distance ℓ and determine how its most likely value ℓ* depends on the surfaces' lateral size L. Using the Edward-Wilkinson equation as a model for interfaces, we demonstrate that ℓ* scales weakly with system size, i.e., the dependence of ℓ* on L for both (1+1)- and (2+1)-dimensional interfaces is such that lim L→∞ (ℓ*/L) = 0. In particular, for (2+1)-dimensional interfaces ℓ* is an algebraic function of logL, a result that is confirmed by computer simulations of slab-shaped domains formed under periodic boundary conditions. Overall, this weak scaling implies that such domains remain topologically intact until ℓ becomes very small compared to the lateral size of the interface, contradicting expectations from equilibrium thermodynamics.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2019

Advanced low-cost intermetallic coatings for molten salt pump impeller. Final report

1.0 INTRODUCTION The use of solar energy to produce steam for the generation of electricity is an established technology, with multiple designs for concentrating solar power (CSP) steam generators available commercially and a number of such units currently operating. As part of the effort to improve the overall efficiency of this technology, designers have developed thermal energy storage (TES) mechanisms that employ molten salt to capture a portion of the solar energy available during the day-time hours and make that energy available for steam generation during periods when the sun’s energy is not available. In current TES systems, nitrate salts are used extensively as the medium of energy exchange. However, due to the chemical instability of these salts at temperatures above 600ºC, they are not being considered for use in the next more efficient generation of CSP plants, in which the energy exchange fluids will be required to operate in the range 600ºC to 800ºC. At these higher temperatures, molten chloride salts, such as KCl-MgCl2, are viewed as promising alternatives to the nitrate salts, but, of course, these chloride-based salts are highly corrosive to the engineering alloys commonly used in the construction of pressure vessels, heat exchangers, pumps and piping at high temperatures. Hence, a solution to the problem of material degradation is essential if the next generation of CSP plants is to be commercially viable (Ref 1). In order to effectively address the problem of molten chloride salt corrosion at high temperatures (≥ 750ºC) and thereby to reduce the cost of molten salt tanks and related apparatus, various options have been considered, with one of the most promising being an innovative coatings system developed and applied onto low alloy ferritic or austenitic stainless material. Development of such a coating system would reduce the capital costs of thermal energy storage (TES) and increase the overall efficiency of the concentrating solar power (CSP) system. Although high strength nickel base alloys are well suited for many high temperature applications, they have significant limitations when exposed to molten chloride salt conditions and they are very expensive. Monolithic materials such as the austenitic stainless steels (e.g., SS310 and SS347) and the more common nickel base alloys (e.g., IN625 and IN800H) have exhibited high corrosion rates in molten chloride salt environments (Ref1). Molten salt corrosion of IN625 in salts containing chlorides showed a catastrophic corrosion rate of 1.111”/year at 800ºC (Ref1). Hence, an innovative coatings solution to mitigate molten chloride salt corrosion is needed. Of the many coating techniques available in the market, no one technique appears to be capable of producing effective protection from molten salt corrosion at a relatively low cost. There are thermal spray coating techniques, in particular the twin wire-arc technology, that are attractive when considering the coating of large molten salt tanks because the coatings application rate is high, the overall application cost is low and repairs to the coating can readily be made either in the shop or in the field. However, the twin wire arc coating materials currently available in the market have certain inherent limitations, including porosity and mechanical bonding to the substrate, which limit their effectiveness in providing protection against molten chloride salt corrosion at high temperatures. What is required is an approach that combines the economy and versatility of thermal spray wire-arc technology with the superior resistance of advanced alloys to effectively combat molten chloride salt corrosion at elevated temperatures. Applied Thermal Coatings, Inc. (ATC) has investigated an innovative coatings technology to produce dual layer cermet/ceramic coatings that will be dense and impervious to any corrosive molten salts/fluids, thereby resisting the effects of molten salt corrosion. Development of coatings using this technology will make it possible to fabricate the molten salt systems using a relatively low-cost base material, which will reduce the overall cost of the power generating system and enhance the commercial viability of solar power. 2.0 PROJECT OVERVIEW Of the coating technologies available for the production of a cost-effective method of protection from the effects of high-temperature chloride attack, there are two that offer specific advantages for the CSP/TES application, particularly when used in a way that effectively complements the strengths of each coating method. The first of these is electroless nickel plating, through which a nickel-rich layer is produced on the surface of a component through chemical reduction. The second coating technology is aluminizing, in which aluminum is diffused into the surface of a component and, depending on the composition of the substrate, aluminides are formed. The objective of this project has been to demonstrate the feasibility of a low- cost coating for the protection of pump impellers used for the inductive transport of molten salts. Initial interest focused on an inter-metallic nickel aluminide coating and subsequently, for economic reasons, consideration also was given to a coating composed of iron aluminides. Applying such coatings to low alloy or austenitic stainless-steel substrates will provide the superior resistance to molten chloride salt corrosion and blade tip erosion that normally can be achieved only through the use of more highly alloyed – and, therefore, more expensive - nickel-base materials and it will do so at a substantially lower cost. The coating systems evaluated were designed to take advantage either of the combined strengths of the electroless nickel plating process and the aluminizing process to produce a nickel-aluminide intermetallic coating or to rely solely on the aluminizing process to produce, in conjunction with iron from the substrate, a protective layer consisting of iron aluminides. With respect to the nickel aluminide protection, this has been achieved by first depositing a layer of nickel-rich alloy onto an austenitic stainless substrate using the electroless nickel plating process and then aluminizing the nickel- coated surface to produce the nickel aluminide coating. With respect to the iron aluminide protection, this has been achieved by aluminizing bare austenitic stainless steel. Testing has supported the conclusion that the developed coatings will resist attack by molten chloride salts at high temperatures (550°C to 750°C) and have sufficient hardness to resist blade tip erosion.The steps taken to develop either a low-cost intermetallic nickel aluminide coating using electroless nickel plating and aluminizing or a low-cost iron aluminide coating using aluminizing only, and to demonstrate the quality of the coatings and their effectiveness against molten chloride salt corrosion and erosion at high temperatures (≤ 750ºC) have been as follows: 1. Design of the optimum electroless nickel plating and aluminizing mixtures using advanced thermodynamic analysis; 2. Development of processing parameters for the electroless nickel plating and aluminizing processes, to be used either together or separately; 3. Production of specimens coated using the designated electroless nickel plating process; 4. Production of samples with intermetallic nickel aluminide coating or iron aluminide coating; 5. Investigation of the performance of the intermetallic nickel aluminide coating and the iron aluminide coating when subjected to molten chloride salts corrosion at high temperatures (≥ 750ºC); and, 6. Metallurgical characterization of the microstructure of the coatings, both prior to and after exposure to molten salt environments at elevated temperatures.

14 SOLAR ENERGY↗

NASA NTRS1993

Critical review of electrical conductivity measurements and charge distribution analysis of magnesium oxide

The electrical conductivity sigma of MgO single crystals shows a sharp increase at 500-800 C, in particular of sigma surface, generally attributed to surface contamination. Charge Distribution Analysis (CDA), a new technique providing information on fundamental properties that was previously unavailable, allows for the determination of surface charges, their sign and associated internal electric field. Data on 99.99% purity, arc-fusion grown MgO crystals show that mobile charge carriers start to appear in the bulk of the MgO crystals between 200 and 400 C when sigma (measured by conventional techniques) is in t he 10(exp -14) to 10(exp -16) /omega/cm range. Above 500 C, as sigma increases to 10(exp -6) to 10(exp -7)/omega/cm, more charges appear giving rise to a strong positive surface charge supported by a strong internal field. This indicates that charges are generated in the bulk and diffuse to the surface by an internally controlled process. On the basis of their positive sign they are identified as holes (defect electrons). Because of the low cation content of these very pure MgO crystals, theses holes cannnot be associated with transition metal impurties. Instead, they are associated with the O(2-) sublattice, e.g. consist of O(-) states or positive holes. This conclusion is supported by magnetic susceptibility data showing the appearance of 1000 +/- 500 ppm paramagnetic species between 200-500 C. The magnetic data are consistent with strongly coupled, diamagnetic O(-) pairs below 200-500 C, chemically equivalent to peroxy anions, O2(2-), and probably associated with cation vacancies in the MgO matrix. The formation of O2(2-) in arc-fusion grown MgO crystals is very unexpected because of the highly reducing growth conditions. Their presence implies an internal redox reaction involving dissolved 'water' by which OH(-) pairs convert to O2(2-) plus H2 molecules. This redox conversion is supported by mass spectroscopic measurements of the H2 release from highly OH(-)-doped, finely divided MgO and by wet-chemical analysis of its oxidant concentration.

Freund, Friedemann↗

NASA NTRS2014

Spring 2014 Internship Diffuser Data Analysis

J-2X engine testing on the A-2 test stand at the NASA John C. Stennis Space Center (SSC) has recently concluded. As part of that test campaign, the engine was operated at lower power levels in support of expanding the use of J-2X to other missions. However, the A-2 diffuser was not designed for engine testing at the proposed low power levels. To evaluate the risk of damage to the diffuser, computer simulations were created of the rocket engine exhaust plume inside the 50ft long, water-cooled, altitude-simulating diffuser. The simulations predicted that low power level testing would cause the plume to oscillate in the lower sections of the diffuser. This can possibly cause excessive vibrations, stress, and heat transfer from the plume to the diffuser walls. To understand and assess the performance of the diffuser during low power level engine testing, nine accelerometers and four strain gages were installed around the outer surface of the diffuser. The added instrumentation also allowed for the verification of the rocket exhaust plume computational model. Prior to engine hot-fire testing, a diffuser water-flow test was conducted to verify the proper operation of the newly installed instrumentation. Subsequently, two J-2X engine hot-fire tests were completed. Hot-Fire Test 1 was 11.5 seconds in duration, and accelerometer and strain data verified that the rocket engine plume oscillated in the lower sections of the diffuser. The accelerometers showed very different results dependent upon location. The diffuser consists of four sections, with Section 1 being closest to the engine nozzle and Section 4 being farthest from the engine nozzle. Section 1 accelerometers showed increased amplitudes at startup and shutdown, but low amplitudes while the diffuser was started. Section 3 accelerometers showed the opposite results with near zero G amplitudes prior to and after diffuser start and peak amplitudes to +/- 100G while the diffuser was started. Hot-Fire Test 1 strain gages showed different data dependent on section. Section 1 strains were small, and were in the range of 50 to 150 microstrain, which would result in stresses from 1.45 to 4.35 ksi. The yield stress of the material, A-285 Grade C Steel, is 29.7 ksi. Section 4 strain gages showed much higher values with strains peaking at 1600 microstrain. This strain corresponds to a stress of 46.41 ksi, which is in excess of the yield stress, but below the ultimate stress of 55 to 75 ksi. The decreased accelerations and strain in Section 1, and the increased accelerations and strain in Sections 3 and 4 verified the computer simulation prediction of increased plume oscillations in the lower sections of the diffuser. Hot-Fire Test 2 ran for a duration of 125 seconds. The engine operated at a slightly higher power level than Hot-Fire Test 1 for the initial 35 seconds of the test. After 35 seconds the power level was lowered to Hot-Fire Test 1 levels. The acceleration and strain data for Hot-Fire Test 2 was similar during the initial part of the test. However, just prior to the engine being lowered to the Hot-Fire Test 1 power level, the strain gage data in Section 4 showed a large decrease to strains near zero microstrain from their peak at 1500 microstrain. Future work includes further strain and acceleration data analysis and evaluation.

Laigaie, Robert T.↗

DOE OSTI2023

Quantifying the Impact of Electric Fields on the Local Structure and Migration of Potassium Ions at the Orthoclase (001) Surface

Developing an understanding of the response of mineral/water interfaces to applied electric fields is central to detecting and interpreting signatures of interfacial processes in the subsurface. Here we focus density functional theory calculations on understanding K + cation binding and diffusion across the (001) surface of orthoclase feldspar under various applied electric fields with and without surface hydration. The calculations reveal how water ligands labilize surface K + cations for diffusion while also increasing their sensitivity to electric field effects on the binding energy at different surface sites. The calculations also show how the direction and strength of the electric field systematically affect surface cation mobility, sorption, and hydration behavior. Specifically, electric fields directed toward the surface increase the potassium binding energies, facilitate K + diffusion across multiple binding sites, and increase the propensity of surface K + ions to lose their hydration shells. Furthermore, the findings help fill a major knowledge gap into the impact of electric fields on mineral/water interface structure and dynamics more generally, featuring in this case a commonly found type of feldspar involved in a multitude of atmospheric and geochemical processes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2019

W/TaC/SiC sandwich stack for high temperature applications

High-temperature stability of the tungsten/tantalum carbide/silicon carbide (W/TaC/SiC) system was examined and compared to the tungsten/silicon carbide (W/SiC) system. Interfacial reactions, surface morphology, and roughness evolutions on thermal annealing at 1000 °C were systematically investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). For the W/SiC system, WC, W 2 C and W 5 Si 3 phases were observed due to severe chemical reaction after 24 h annealing, which led to an inhomogeneous surface structure with several large craters and small pores, thereby increasing surface roughness greatly. On the other hand, there was no tungsten silicide and/or carbide formation upon annealing with a TaC diffusion barrier. The surface structure remained nearly unchanged, and small changes in surface roughness with annealing time, which could be due to the growth of W grains, were noted. Furthermore, both the as-deposited and the 24 h annealed W/TaC/SiC samples presented an excellent interfacial bonding with a dense uniform interface. The results indicate that TaC is a promising diffusion barrier in the W/SiC system for high temperature applications.

36 MATERIALS SCIENCE↗

DOE OSTI2019

Characterization of local morphology and availability of triple-phase boundaries in solid oxide cell electrodes

The performance of solid oxide cells is known to be dependent upon the density of three phase boundaries (TPB), but the potential for improving their effective electrocatalytic activity by morphological adjustments is imprecisely known. A spilling algorithm was developed to characterize the surfaces available for diffusion at TPBs. It scans each slice in a 3-D imaging dataset to measure the interfaces between the solid and the pore phases at each TPB. Because of the stereological approach, these surfaces are defined as “available lengths” (L A ). The measurement was tested on artificial packed spheres structures with controlled properties and a percolation theory-based model before application to a real Ni-YSZ. The L A distributions cover 2 orders of magnitude. The subset shorter than the extent of diffusion profiles reported in the literature is in the range of 3% and 20% for Ni and YSZ, respectively, suggesting possible limitations of their effective electrocatalytic properties. The average L A is larger on YSZ than on Ni, which is a trend opposite to the phase diameter. The available length analysis revealed microstructural characteristics that stem from the manufacturing route and cannot be identified by the inspection of standard metric and topological properties. A strong correlation between the available length and the extension of TPB lines is observed for Ni but not for YSZ, despite the predominance of convex shapes, which likely originates from the Ni reduction. This suggests possibilities for controlling the available length by the manufacturing route, depending specifically on the electrocatalytic properties of the phases in composite materials.

3-D electron microscopy↗

DOE OSTI2023

Diffusion-assisted growth of periodic patterns on metal surfaces

Periodic patterns formed on various solid surfaces could seriously affect the properties of materials. Understanding the underlying mechanism is a key step to apply and control pattern formation in many fields. In this work, periodic patterns induced by low energy plasma exposures on tungsten surfaces are observed and related new formation mechanisms are developed. Results from experiments and computer simulations demonstrate that atom diffusion accelerated by low energy and high flux exposures is the dominant mechanism for pattern formation. Additionally, a similar pattern formation has been confirmed by plasma exposure experiments on molybdenum. Based on this mechanism, patterns with controllable size and shape have been successfully prepared on tungsten surface by adjusting plasma exposure parameters, which may provide a new understanding of pattern formation and related surface damage in tungsten induced by plasma exposures.

36 MATERIALS SCIENCE↗

NASA NTRS1983

An EBIC equation for solar cells

When an electron beam of a scanning electron microscope (SEM) impinges on an N-P junction, the generation of electron-hole pairs by impact ionization causes a characteristic short circuit current I(sc) to flow. The I(sc), i.e., EBIC (electron beam induced current) depends strongly on the configuration used to investigate the cell's response. In this paper the case where the plane of the junction is perpendicular to the surface is considered. An EBIC equation amenable to numerical computations is derived as a function of cell thickness, source depth, surface recombination velocity, diffusion length, and distance of the junction to the beam-cell interaction point for a cell with an ohmic contact at its back surface. It is shown that the EBIC equation presented here is more general and easier to use than those previously reported. The effects of source depth, ohmic contact, and diffusion length on the normalized EBIC characteristic are discussed.

Luke, K. L.↗

NASA NTRS1982

Friction and surface chemistry of some ferrous-base metallic glasses

The friction properties of some ferrous-base metallic glasses were measured both in argon and in vacuum to a temperature of 350 C. The alloy surfaces were also analyzed with X-ray photoelectron spectroscopy to identify the compounds and elements present on the surface. The results of the investigation indicate that even when the surfaces of the amorphous alloys, or metallic glasses, are atomically clean, bulk contaminants such as boric oxide and silicon dioxide diffuse to the surfaces. Friction measurements in both argon and vacuum indicate that the alloys exhibit higher coefficients of friction in the crystalline state than they do in the amorphous state.

Miyoshi, K.↗

NASA NTRS1994

Lifetimes of Electrodes for AMTEC Cells

The lifetime of an AMTEC electrode depends on the rate of grain growth, which in turn depends on the surface self-diffusion coefficient of the electrode material under AMTEC operating conditions. Grain growth rates for molybdenum and platinum-tungsten alloy electrodes have been determined, and have been used to predict operating lifetimes of AMTEC electrodes. For lifetimes of 10 years of more, Mo may be used in AMTEC cells only at operating temperatures under 1100 K. Pt(sub 2.5)W electrodes may be used at much higher temperatures, up to 1300 K.

lifetime surface self-diffusion coefficient↗

NASA NTRS1978

Development of Surfaces Optically Suitable for Flat Solar Panels

Three areas of research in the development of flat solar panels are described. (1) A reflectometer which can separately evaluate the spectral and diffuse reflectivities of surfaces was developed. The reflectometer has a phase locked detection system. (2) A coating composed of strongly bound copper oxide that is formed by an etching process performed on an aluminum alloy with high copper content was also developed. Because of this one step fabrication process, fabrication costs are expected to be small. (3) A literature search was conducted and conclusions on the required optical properties of flat plate solar collectors are presented.

Desmet, D.↗