Preparation and Hydrogen Absorption/Desorption of Nanoporous Palladium Thin Films

Nanoporous Pd (np-Pd) was prepared by co-sputtering Pd-Ni alloy films onto Si substrates, followed by chemical dealloying with sulfuric acid. X-ray diffractometry and chemical analysis were used to track the extent of dealloying. The np-Pd structure was changed from particle-like to sponge-like by diluting the sulfuric acid etchant. Using suitable precursor alloy composition and dealloying conditions, np-Pd films were prepared with uniform and open sponge-like structures, with interconnected ligaments and no cracks, yielding a large amount of surface area for reactions with hydrogen. Np-Pd films exhibited shorter response time for hydrogen absorption/desorption than dense Pd films, showing promise for hydrogen sensing

Composite membrane fabrication with nanoporous metallic films

Magnetron sputtering is a physical vapor deposition method widely used for deposition of thin films of different materials on a variety of substrate materials. Sputtering allows fine control of the film thickness and composition through co-sputtering from multiple target materials. As part of this study thin films have been sputtered on top of membrane substrates. Microfiltration, ultrafiltration, and nanofiltration membranes have been investigated as substrates for thin film deposition. The resulting composite membranes have remained permeable under testing with deionized water. The base nanofiltration membrane showed permeability of 9.75 LMH/bar, while the membrane-film composite had a permeability of 2.76 LMH/bar. Thin films of metallic alloys deposited in this way can be made nanoporous through a process called dealloying. The process involves the removal of the less noble component of an alloy by an etchant creating an open nanoporous structure. The pores created by this method commonly vary from a few nanometers to a few hundred nanometers. This research focuses on using magnetron sputtering to deposit precursor metallic alloy films from 100 to 250nm thick on top of porous membrane substrates. These dense precursor films are then dealloyed to produce pore/ligament structures of approximately 10nm characteristic size. In these studies iron and palladium were chosen as a precursor alloy. A portion of the iron is etched away with sulfuric acid to generate an open nanoporous structure. Fe/Pd nanoparticles have been used with success to dechlorinate various chlorinated organic compounds (COCs) for wastewater treatment purposes. Nanoporous Fe/Pd films have shown similar activity in batch testing towards PCB degradation as nanoparticles. Taken together this means the composite membrane produced by fabricating a high surface area, porous Fe/Pd film on top of a membrane substrate shows promise both as a catalyst and as a platform for separations. This project is funded by NIH-NIEHS-SRC and by NSF KY EPSCOR at the University of Kentucky

Interplay of Composition, Structure, and Electron Density of States in W-Os Cathode Materials and Relationship with Thermionic Emission

The presence and composition of W-Os alloys have been found to significantly affect the thermionic emission properties of Os-coated tungsten dispenser cathodes. However, the comprehensive understanding of structure–property relationships needed to design improved tungsten cathodes with larger thermionic emission is still lacking. In this study, composition–structure–property relationships governing thermionic emission from W-Os alloys were investigated using quantum mechanical calculations. Low-energy W-Os atomic configurations at various compositions were determined from first-principles calculations based on density functional theory in combination with cluster expansion calculations. Electronic properties were investigated in terms of the electron density of states. The relative position of the Fermi level with respect to peaks and pseudogaps in the density of states for different W-Os structures can be used to explain, at least in part, observed variations in thermionic emission from Os-coated tungsten dispenser cathodes

Desorption from Hot Scandate Cathodes: Effects on Vacuum Device Interior Surfaces after Long-Term Operation

Scandate cathodes have exhibited superior emission properties compared to current state-of-the-art “M-type” thermionic cathodes. However, their integration into vacuum devices is limited in part by a lack of knowledge regarding their functional lifespan and behavior during operation. Here, we consider thermal desorption from scandate cathodes by examining the distribution of material deposited on interior surfaces of a sealed vacuum device after ~26,000 h of cathode operation. XPS, EDS, and TEM analyses indicate that on the order of 1 wt.% of the initial impregnate is desorbed during a cathode’s lifetime, Ca does not desorb uniformly with time, and little to no Sc desorbs from the cathode surfaces (or does so at an undetectable rate). Findings from this first-ever study of a scandate cathode after extremely long-time operation yield insight into the utility of scandate cathodes as components in vacuum devices and suggest possible effects on device performance due to deposition of desorption products on interior device surfaces

Correlation Between Microstructure and Thermionic Electron Emission from Os-Ru Thin Films on Dispenser Cathodes

Osmium-ruthenium films with different microstructures were deposited onto dispenser cathodes and subjected to 1000 h of close-spaced diode testing. Tailored microstructures were achieved by applying substrate biasing during deposition, and these were evaluated with scanning electron microscopy, x-ray diffraction, and energy dispersive x-ray spectroscopy before and after close-spaced diode testing. Knee temperatures determined from the close-spaced diode test data were used to evaluate cathode performance. Cathodes with a large {10-11} Os-Ru film texture possessed comparatively low knee temperatures. Furthermore, a low knee temperature correlated with a low effective work function as calculated from the close-spaced diode data. It is proposed that the formation of strong {10-11} texture is responsible for the superior performance of the cathode with a multilayered Os-Ru coating

Composition and Work Function Relationship in Os–Ru–W Ternary Alloys

Os–Ru thin films with varying concentrations of W were sputter deposited in order to investigate their structure–property relationships. The films were analyzed with x-ray diffraction to investigate their crystal structures, and a Kelvin probe to investigate their work functions. An Os–Ru–W film with ∼30 at. % W yielded a work function maximum of approximately 5.38 eV. These results align well with other studies that found work function minima from thermionic emission data on M-type cathodes with varying amounts of W in the coatings. Furthermore, the results are consistent with other work explaining energy-level alignment and charge transfer of molecules on metal oxides. This may shed light on the mechanism behind the “anomalous effect” first reported by Zalm et al., whereby a high work function coating results in a low work function for emitting cathode surfaces. An important implication of this work is the potential for the Kelvin probe to evaluate the effectiveness of dispenser cathode coatings

Composite metallic nanofoam structures

Metallic nanofoams made of metals such as nickel (Ni) or gold (Au) with ligament sizes on the order of 10’s to 100’s of nm’s exhibit several remarkable properties as a consequence of their low relative density and high specific surface area, such as outstanding strength to weight ratios, enhanced plasmonic behavior and size-effect-enhanced catalytic behavior. However, these metallic nanofoams suffer from macroscopically brittle behavior due to plastic deformation in individual ligaments. With little or no barriers for slip, work-hardening is not possible within ligaments and extremely localized plasticity, once initiated, leads to a few ligaments necking and what appears to be macroscopically brittle failure of the structure under load. Many of the nanofoams produced from metals were originally formed via dealloying. Recently both simulations and experiments have identified that layered ligaments of metallic foams can exhibit significantly improved strength and hardening in Ni-Au core-shell foams[1]. Simulations of Cu-Ni predict that this material combination will exhibit pseudo-elastic behavior and eliminate the macroscopic brittle failure [2]. However, using a metallic foam as a substrate for subsequent layered metallic films limits the amount of metallic layers that can be deposited because the initial foam must have a minimum amount of material (often a solid fraction of approximately 25%). Using a significantly less dense foam as a template should allow for subsequent multilayer growth that would enable larger numbers of layers, and therefore a possible increase in overall strength to total ligament diameter. Single layers have been demonstrated in a prior study [3]. Pulse electroplating from a nickel sulfamate electrolyte bath was used to deposit alternating layers of Ni and Cu. The bath consisted of 90 g l-1 Ni, 0.9 g l-1 Cu and 30 g l-1 boric acid (pH 3-3.5). This solution allows for alternating Ni and Cu layers to be alternately plated by varying the applied voltage, the end layer is actually an alloy of mostly Cu with Ni, and then mostly Ni. Low density foams were selected as a template for subsequent deposition. These included 2% volume carbon nanotubes as well as electrospun carbon fibers. Typical structures are shown in Figure 1. The resulting foams were then indented using flat punch nanoindentation and the effective modulus increased by a factor of three and the elastic recovery after indentation increased substantially (to about one half the original impression depth). The presentation will describe the processing method, the structural changes that occur when the films transition from layer by layer to island growth, and the resulting properties of the foam

Near-Surface Material Phases and Microstructure of Scandate Cathodes

Scandate cathodes that were fabricated using the liquid-solid process and that exhibited excellent emission performance were characterized using complementary state-of-the-art electron microscopy techniques. Sub-micron BaAl2O4 particles were observed on the surfaces and edges of tungsten particles, as seen in cross-section samples extracted from the scandate cathode surface regions. Although several BaAl2O4 particles were observed to surround smaller Sc2O3 nanoparticles, no chemical mixing of the two oxides was detected, and in fact the distinct oxide phases were separately verified by chemical analysis and also by 3D elemental tomography. Nanobeam electron diffraction confirmed that the crystal structure throughout W grains is body-centered cubic, indicating that they are metallic W and did not experience noticeable changes, even near the grain surfaces, as a result of the numerous complex chemical reactions that occur during cathode impregnation and activation. 3D reconstruction further revealed that internal Sc/Sc2O3 particles tend to exhibit a degree of correlated arrangement within a given W particle, rather than being distributed uniformly throughout. Moreover, the formation of Sc/Sc2O3 particles within W grains may arise from W surface roughening that occurs during the liquid-solid synthesis process

Supporting variables for biological effects measurements in fish and blue mussels

Biological effects measurements in fish and blue mussel are fundamental in marine environmental monitoring. Nevertheless, currently used biomarkers may be confounded by basic physiological phenomena, such as growth, reproduction, and feeding, as well as thereby associated physiological variation. Here, we present a number of supporting variables, which are essential to measure in order to obtain reliable biological effects data, facilitate their interpretation, and make valid comparisons. For fish, these variables include: body weight, body length, condition, gonad maturation status, various somatic indices, age, and growth. For blue mussels, these variables include: volume, flesh weight, shell weight, and condition. Also, grossly visible anomalies, lesions, and parasites should be recorded for both fish and blue mussels. General confounding factors and their effects are described, as well as recommendations for how to handle themPostprint