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Temperature rise of micron sized silver particles on carbon film due to electron beam heating, setting up heat balance equation
Particles from a variety of silver powders were investigated by scanning electron microscopy and particle size analyses. Particle size distribution curves and volume population graphs were prepared for these silver powders and for glass powders with optimum, extra fine and coarse particle sizes. The viscosity at a given shear rate and slope of viscosity over a range of shear rates were determined for thick film pastes made with these powders. Because of particle anomalies and variations, the need for flexibility to achieve the best printing qualities for silver pastes was evident. It was established that print quality, dried and fired film density and optimum contact of silver particles with silicon, important for cell electrical output, could be achieved by adjusting the slope of viscosity that fell outside of the range, -0.550 to -0.650. This was accomplished through organic vehicle technology that permitted a change in the slope of viscosity, up or down, while maintaining a constant silver and total solids content.
The high spectral and spatial resolution thermal infrared (TIR) data to be acquired from the upcoming Mars Observer-Thermal Emission Spectra (TES) mission will map the composition and texture of the Martian sediments. To prepare for these data, portions of two remote sensing experiments were conducted to test procedures for extracting surface property information from TIR data. Reported here is the continuing analysis of Thermal Infrared Multispectral Scanner (TIMS) data, field emission spectra, laboratory Fourier Transform Infrared (FTIR) reflectance spectra, and field observations with respect to the physical characteristics (composition, emissivity, etc.) of Silver Lake playa in southern California.
Silver biocide offers a potential advantage over iodine, the current state of the art in US spacecraft disinfection technology, in that silver can be safely consumed by the crew. Low concentrations of silver (<500 ppb) have been shown to kill bacteria in water systems and keep it safe for potability. Silver does not require hardware to remove it from a water system, and therefore can provide a simpler means for disinfecting water. The Russian segment of the International Space Station has utilized an electrochemically generated silver solution, which is colloidal in nature. To be able to reliably provide a silver biocide to drinking water by electrochemical means would reduce mass required for removing another biocide such as iodine from the water. This would also aid in crew time required to replace iodine removal cartridges. Future long term missions would benefit from electrochemically produced silver as the biocide could be produced on demand and requires only a small concentration to be effective. Since it can also be consumed safely, there is less mass in removal hardware and little consumables required for production. The goal of this project initially is to understand the nature of the electrochemically produced silver, the particle sizes produced by the electrochemical cell and the effect that voltage adjustment has on the particle size. In literature, it has been documented that dissolved oxygen and pH have an effect on the ionization of the electrochemical silver so those parameters would be measured and possibly adjusted to understand their effect on the silver.
An investment casting process has been developed to produce net-shape, superconducting ceramics. In this work, a factorial experiment was performed to determine the critical process parameters for producing cast YBa2Cu3O7 ceramics with optimum properties. An analysis of variance procedure indicated that the key variables in casting superconductive ceramics are the particle size distribution and sintering temperature. Additionally, the interactions between the sintering temperature and the other process parameters (e.g., particle size distribution and the use of silver dopants) were also found to influence the density, porosity, and critical current density of the fired ceramics.
Experiments have shown the stability enhancement of polymeric sensing films on mixing the polymer with colloidal filler particles (submicron-sized) of carbon black, silver, titanium dioxide, and fumed silicon dioxide. The polymer films are candidates for potential use as sensing media in micro/nano chemical sensor devices. The need for stability enhancement of polymer sensing films arises because such films have been found to exhibit unpredictable changes in sensing activity over time, which could result in a possible failure of the sensor device. The changes in the physical properties of a polymer sensing film caused by the sorption of a target molecule can be measured by any of several established transduction techniques: electrochemical, optical, calorimetric, or piezoelectric, for example. The transduction technique used in the current polymer stability experiments is based on piezoelectric principles using a quartz-crystal microbalance (QCM). The surface of the QCM is coated with the polymer, and the mass uptake by the polymer film causes a change in the oscillating frequency of the quartz crystal. The polymer used for the current study is ethyl cellulose. The polymer/ polymer composite solutions were prepared in 1,3 dioxolane solvent. The filler concentration was fixed at 10 weight percent for the composites. The polymer or polymer composite solutions were cast on the quartz crystal having a fundamental frequency of about 6 MHz. The coated crystal was subjected to a multistage drying process to remove all measurable traces of the solvent. In each experiment, the frequency of oscillation was measured while the QCM was exposed to clean, dry, flowing air for about 30 minutes, then to air containing a known concentration of isopropanol for about 30 minutes, then again to clean dry air for about 30 minutes, and so forth. This cycle of measurements for varying isopropanol concentrations was repeated at intervals for several months. The figure depicts some of the sensing film stability results for ethyl cellulose polymer, ethyl cellulose-carbon black, and ethyl cellulose-silicon dioxide composite systems. An ethyl cellulose film exhibited a marked decline in response in the first few months of study and settled to a steady average response after about four months. However, response varied widely around the average response for ethyl cellulose film. In contrast, ethyl cellulose- carbon black and ethyl cellulose-silicon dioxide composites also declined in the early months, but showed more repeatable sensing film activity after the initial decline. Similar trends were observed in experiments for ethyl cellulose-titanium dioxide and ethyl cellulose-silver composites.
Uniform and nonuniform size and energy glass bead and silver alloy and aluminum powder particles effects on sediment volume and viscosity in propylene oxide and mineral oil
Madidrop is a rectangular ceramic tablet originally designed to dose a 10-20-liter volume of water with silver to act as a biocide for remote communities with unreliable water sources. Silver as a biocide is a proposed method to control bacteria on the International Space Station or future Artemis missions. This would replace iodine currently used due to issues with a dual biocide system in place between Russian and US segments and the extra cost of iodine removal required before consumption. NASA has been reviewing this Madidrop technology and its potential application with a water reclamation system on orbit to dose silver after purifying the water. While there are a couple alternative methods to dosing silver on station, this method is like the existing passive dosing method used currently to dose iodine. Tests conducted characterize the manufacturer’s original intent to leave the tablet in a tank for a specified amount of time before removing and testing alternative passive flow-through methods. Passive flow testing involved two options: keeping the tablet intact or crushing the tablet into smaller particle sizes to increase the surface area while varying particle sizes. The results of these tests revealed a large amount of silver output at first that quickly reduced to a constant lower output within a reasonable dosing concentration range. The initial high concentration peaks after a couple days of quiescence and could be useful for shocking the initial tank water contents. Madidrop used as a passive particle doser could act as a stand-in replacement for iodine in future spaceflight water reclamation systems.
Madidrop is a rectangular ceramic tablet originally designed to dose a 10-20-liter volume of water with silver to act as a biocide for remote communities with unreliable water sources. Silver as a biocide is a proposed method to control bacteria on the International Space Station or future Artemis missions. This would replace iodine currently used due to issues with a dual biocide system in place between Russian and US segments and the extra cost of iodine removal required before consumption. NASA has been reviewing this Madidrop technology and its potential application with a water reclamation system on orbit to dose silver after purifying the water. While there are a couple alternative methods to dosing silver on station, this method is like the existing passive dosing method used currently to dose iodine. Tests conducted characterize the manufacturer’s original intent to leave the tablet in a tank for a specified amount of time before removing and testing alternative passive flow-through methods. Passive flow testing involved two options: keeping the tablet intact or crushing the tablet into smaller particle sizes to increase the surface area while varying particle sizes. The results of these tests revealed a large amount of silver output at first that quickly reduced to a constant lower output within a reasonable dosing concentration range. The initial high concentration peaks after a couple days of quiescence and could be useful for shocking the initial tank water contents. Madidrop used as a passive particle doser could act as a stand-in replacement for iodine in future spaceflight water reclamation systems.
The effects of BaF2-CaF2 particle size and size distribution on PS304 feedstock powder flowability have been investigated. Angular BaF2-CaF2 eutectic powders were produced by comminution and classified by screening to obtain 38 to 45 microns 45 to 106 microns, 63 to 106 microns, 45 to 53 microns, 63 to 75 microns, and 90 to 106 microns particle size distributions. The fluorides were added incrementally from 0 to 10 wt% to the other powder constituents of the PS304 feedstock: nichrome, chromia, and silver powders. The flow rate of the powder blends decreased linearly with increasing concentration of the fluorides. Flow was degraded with decreasing BaF2-CaF2 particle size and with increasing BaF2-CaF2 particle size distribution. A semiempirical relationship is offered to describe the PS304 powder blend flow behavior. The Hausner Ratio confirmed the funnel flow test results, but was slightly less sensitive to differences in BaF2-CaF2 particle size and size distribution. These findings may have applicability to other powders that do not flow easily, such as ceramic powders.
A superconductor, of composition Y(Ba,K,Na)2Cu3O(x)/F(y) and a composite of composition Y(Ba,K,Na)2Cu3O(x)/F(y) + Ag, with changing K, Na and F content but a constant silver content (Ag = 10 mass%) was prepared using a single heat treatment. the resulting material was ground in a corundum lined mill, separated to particle size fractions of 0-40 micron, 0-63 micron and 63-900 micron and explosively compacted, using an explosive pressure of 10(exp 4) MPa and a subsequent heat treatment. Best results were obtained with the 63-900 micron fraction of composition Y(Ba(1.95) K(0.01)Cu3O(x)F(0),(05)/Ag: porosity less than 0.01 cu cm/g and current density 2800 A/sq cm at 77K.
Nitric and hydrochloric acids have been postulated to condense in the winter polar stratosphere to become an important component of polar stratospheric clouds. One implication is that the removal of NO(y) from the gas phase by this mechanism allows high Cl(x) concentrations to react with O3, because the formation of ClNO3 is inhibited. Contributions of NO3 and Cl to the stratospheric aerosol were determined during the 1987 Airborne Antarctic Ozone Experiment by testing for the presence of nitrates and chlorides in the condensed phase. Aerosol particles were collected on four 500 micron diameter gold wires, each pretreated differently to give results that were specific to certain physical and chemical aerosol properties. One wire was carbon-coated for concentration and size analyses by scanning electron microscopy; X-ray energy dispersive analyses permitted the detection of S and Cl in individual particles. Three more wires were coated with Nitron, barium chloride and silver nitrate, respectively, to detect nitrate, sulfate and chloride in aerosol particles. All three ions, viz., sulfates, nitrates and chlorides were detected in the Antarctic stratospheric aerosol. In terms of number concentrations, the aerosol was dominated by sulfates, followed by chlorides and nitrates. An inverse linear regression can be established between nitrate concentrations and ozone mixing ratio, and between temperature and nitrates.
The effects of environmental humidity on the flow characteristics of PS304 feedstock have been investigated. Angular and spherical BaF2-CaF2 powder was fabricated by comminution and by atomization, respectively. The fluorides were added incrementally to the nichrome, chromia, and silver powders to produce PS304 feedstock. The powders were dried in a vacuum oven and cooled to a Tom temperature under dry nitrogen. The flow of the powder was studied from 2 to 100 percent relative humidity (RH) The results suggest that the feedstock flow is slightly degraded with increasing humidity below 66 percent RH and is more affected above 66 percent RH. There was no flow above 88 percent RH. Narrower particle size distributions of the angular fluorides allowed flow up to 95 percent RH. These results offer guidance that enhances the commercial potential for this material system.
Locating low frequency radio observatories on the lunar surface has a number of advantages, including positional stability and a very low ionospheric radio cutoff. Here, we describe the Radio Observatory on the lunar Surface for Solar studies (ROLSS), a concept for a low frequency, radio imaging interferometric array designed to study particle acceleration in the corona and inner heliosphere. ROLSS would be deployed during an early lunar sortie or by a robotic rover as part of an unmanned landing. The preferred site is on the lunar near side to simplify the data downlink to Earth. The prime science mission is to image type II and type III solar radio bursts with the aim of determining the sites at and mechanisms by which the radiating particles are accelerated. Secondary science goals include constraining the density of the lunar ionosphere by measuring the low radio frequency cutoff of the solar radio emissions or background galactic radio emission, measuring the flux, particle mass, and arrival direction of interplanetary and interstellar dust, and constraining the low energy electron population in astrophysical sources. Furthermore, ROLSS serves a pathfinder function for larger lunar radio arrays. Key design requirements on ROLSS include the operational frequency and angular resolution. The electron densities in the solar corona and inner heliosphere are such that the relevant emission occurs below 10 M Hz, essentially unobservable from Earth's surface due to the terrestrial ionospheric cutoff. Resolving the potential sites of particle acceleration requires an instrument with an angular resolution of at least 2 deg at 10 MHz, equivalent to a linear array size of approximately one kilometer. The major components of the ROLSS array are 3 antenna arms, each of 500 m length, arranged in a Y formation, with a central electronics package (CEP) at their intersection. Each antenna arm is a linear strip of polyimide film (e.g., Kapton(TradeMark)) on which 16 single polarization dipole antennas are located by depositing a conductor (e.g., silver). The arms also contain transmission lines for carrying the radio signals from the science antennas to the CEP. Operations would consist of data acquisition during the lunar day, with data downlinks to Earth one or more times every 24 hours.
Locating low frequency radio observatories on the lunar surface has a number of advantages. Here, we describe the Radio Observatory for Lunar Sortie Science (ROLSS), a concept for a low frequency, radio imaging interferometric array designed to study particle acceleration in the corona and inner heliosphere. ROLSS would be deployed during an early lunar sortie or by a robotic rover as part of an unmanned landing. The prime science mission is to image type II and type III solar radio bursts with the aim of determining the sites at and mechanisms by which the radiating particles are accelerated. Secondary science goals include constraining the density of the lunar ionosphere by searching for a low radio frequency cutoff of the solar radio emissions and constraining the low energy electron population in astrophysical sources. Furthermore, ROLSS serves a pathfinder function for larger lunar radio arrays. Key design requirements on ROLES include the operational frequency and angular resolution. The electron densities in the solar corona and inner heliosphere are such that the relevant emission occurs below 10 MHz, essentially unobservable from Earth's surface due to the terrestrial ionospheric cutoff. Resolving the potential sites of particle acceleration requires an instrument with an angular resolution of at least 2 deg, equivalent to a linear array size of approximately 500 meters. Operations would consist of data acquisition during the lunar day, with regular data downlinks. The major components of the ROLSS array are 3 antenna arms arranged in a Y shape, with a central electronics package (CEP). Each antenna arm is a linear strip of polyimide film (e.g., Kapton (TM)) on which 16 single polarization dipole antennas are located by depositing a conductor (e.g., silver). The arms also contain transmission lines for carrying the radio signals from the science antennas to the CEP.
Direct evidence is reported for the simultaneous occurrence of Ostwald ripening and short-distance cluster mobility during annealing of discontinuous metal films on clean amorphous substrates. The annealing characteristics of very thin particulate deposits of silver on amorphized clean surfaces of single-crystalline thin graphite substrates have been studied by in situ transmission electron microscopy (TEM) under controlled environmental conditions in the temperature range from 25 to 450 C. It was possible to monitor all stages of the experiments by TEM observation of the same specimen area. Slow Ostwald ripening was found to occur over the entire temperature range, but the overriding surface transport mechanism was short-distance cluster mobility. This was concluded from in situ observations of individual particles during annealing and from measurements of cluster size distributions, cluster number densities, area coverages, and mean cluster diameters.
A chemical method of synthesis and application of coating materials that are especially suitable for surface-enhanced Raman spectroscopy (SERS) has been developed. The purpose of this development is to facilitate the utilization of the inherently high sensitivity of SERS to detect chemicals of interest (analytes) in trace amounts, without need for lengthy sample preparation. Up to now, the use of SERS has not become routine because the methods available have not been able to reproduce sampling conditions and provide quantitative measurements. In contrast, the coating materials of the present method enable analysis with minimum preparation of samples, and SERS measurements made using these materials are reproducible and reversible. Moreover, unlike in methods investigated in prior efforts to implement SERS, sampling is not restricted to such specific environments as electrolytes or specific solvents. The coating materials of this method are porous glasses, formed in sol-gel processes, that contain small particles of gold or silver metal. Materials of this type can be applied to the sample-contact surfaces of a variety of sampling and sensing devices, including glass slides, glass vials, fiber-optic probes, and glass tubes. Glass vials with their insides coated according to this method are particularly convenient for SERS to detect trace chemicals in solutions: One simply puts a sample solution containing the analyte(s) into a vial, then puts the vial into a Raman spectrometer for analysis. The chemical ingredients and the physical conditions of the sol-gel process have been selected so that the porous glass formed incorporates particles of the desired metal with size(s) to match the wavelength(s) of the SERS excitation laser in order to optimize the generation of surface plasmons. The ingredients and processing conditions have further been chosen to tailor the porosity and polarity of the glass to optimize the sample flow and the interaction between the analyte(s) and the plasmon field that generates Raman photons. The porous silica network of a sol-gel glass creates a unique environment for stabilizing SERS-active metal particles. Relative to other material structures that could be considered for SERS, the porous silica network offers higher specific surface area and thus greater interaction between analyte molecules and metal particles. Efforts to perform SERS measurements with the help of sampling devices coated by this method have been successful. In tests, numerous organic and inorganic chemicals were analyzed in several solvents, including water. The results of the tests indicate that the SERS measurements were reproducible within 10 percent and linear over five orders of magnitude. One measure of the limits of detectability of chemicals in these tests was found to be a concentration of 300 parts per billion. Further development may eventually make it possible to realize the full potential sensitivity of SERS for detecting some analytes in quantities as small as a single molecule.
There are many challenges involved in deep-space exploration, but several of these can be mitigated, or even solved, by the development of a coating that reflects most of the Sun’s energy, yet still provides far-infrared heat emission. Such a coating would allow non-heat-generating objects in space to reach cryogenic temperatures without using an active cooling system. This would benefit deep-space sensors that require low temperatures, such as the James Webb Telescope focal plane array. It would also allow the use of superconductors in deep space, which could lead to magnetic energy storage rings, lossless power delivery, or perhaps a large-volume magnetic shield against galactic cosmic radiation. However, perhaps the most significant enablement achieved from such a coating would be the long-term, deep space storage of cryogenic liquids, such as liquid oxygen (LOX). In our Phase I NIAC study, we realized that a combination of scattering particles and a silver backing could yield a highly effective, very broadband, reflector that could potentially reflect more than 99.9% of the Sun’s irradiant power. We developed a sophisticated model of this reflector and theoretically showed that cryogenic temperatures could be achieved in deep space at one astronomical unit (1 AU) from the Sun. We showed how this new reflector could minimize heat conduction into the cryogenic tanks by coating the tank support struts. We then modelled a strawman architecture for a mission to Mars, using a coated LOX tank, coated struts, and infrared shields, to show that with our new coating it would be possible to maintain liquid oxygen passively. As a result of this work a patent application was generated and a paper published in Optics Letters. Our Phase II NIAC study had two primary goals, to develop a rigid version of the cryogenic selective surface proposed in Phase I and to test its performance in a simulated deep space environment. During the first year of the project the work concentrated on developing rigid tiles of BaF2, leading to tiles as large as 4 inches in diameter that transmitted very little visible light. In addition, during the first year a simulated deep space environment was created using a vacuum chamber and cryocooler. Using this facility, we showed that our BaF2 tiles absorbed less than ¼% of 375 nm radiation, a significant milestone for the work. During the second year of the project, we continued to develop the BaF2 tiles and we put significant effort into the construction of a deep space environment where we could project simulated solar radiation onto a sample. In the spring of 2018, we conducted our first solar simulator test with BaF2 and saw about 3.6% absorption. This is better than the state-of-the-art, but disappointing since predictions were for much lower absorption. We, erroneously, attributed this absorption to water retention by the BaF2, and decided to change materials. We considered several oxides and settled on yttrium oxide (Y2O3) for further development, because it is broadband, lightweight, has high index, and is hydrophobic. In July 2018 we conducted our first test of a rigid tile of Y2O3 in the simulated deep space environment and saw significant absorption again. We then realized that the issue was not water, but mid-wave radiation passing through the tile and being absorbed by the temperature sensor and the varnish used to hold it in place. We wrapped the sensor in silver foil, re-ran the test, and saw much lower absorption; only 1.1%. We then re-ran the BaF2 tile and saw 1.4% absorption. These values are almost adequate to maintain LOX in deep space, but we suspect that there are still issues in our test apparatus; we suspect thermocouple wires may be absorbing radiation. Further, post-NIAC, testing will better determine the performance of our new solar reflector. In order to restrict the size of this report, we will only briefly describe topics that we have previously published, allowing us to devote more time to new material. So minimal material will be devoted to modeling the material and deep space cryogenic storage, while longer sections will cover our material development, simulated deep space testing, and new applications. The Launch Service Program (LSP) requested that we explore ways to use this new coating to maintain LOX in low Earth Orbit and that work is described. In addition, the Nuclear Thermal Propulsion (NTP) Program asked us to explore ways to reduce the heat load for liquid hydrogen, resulting in the development of a spray-on version of the coating that should significantly improve in-space multi-layer insulation performance.