The alloy-design strategy of combining multiple elements in near-equimolar ratios has shown great potential for producing exceptional engineering materials, often known as 'high-entropy alloys'. Understanding the elemental distribution, and, thus, the evolution of the configurational entropy during solidification, is undertaken in the present study using the Al 1.3 CoCrCuFeNi model alloy. Here we show that, even when the material undergoes elemental segregation, precipitation, chemical ordering and spinodal decomposition, a significant amount of disorder remains, due to the distributions of multiple elements in the major phases. The results suggest that the high-entropy alloy-design strategy may be applied to a wide range of complex materials, and should not be limited to the goal of creating single-phase solid solutions.
In this paper we discuss the effect of background pressure and synthesis temperature on the graphene crystal sizes in chemical vapor deposition (CVD) on copper catalyst. For the first time, we quantitatively demonstrate a fundamental role of the background pressure and provide the activation energy for graphene nucleation in atmospheric pressure CVD (9 eV), which is substantially higher than for low pressure CVD (4 eV). We attribute the difference to a greater importance of copper sublimation in low pressure CVD, where severe copper evaporation likely dictates the desorption rate of active carbon from the surface. At atmospheric pressure, where copper evaporation is suppressed, the activation energy is assigned to the desorption energy of carbon clusters instead. The highest possible temperature, close to the melting point of copper, should be used for large single crystal graphene synthesis. Using these conditions, we have synthesized graphene single crystals approaching 1 mm in size. Single crystal nature of synthesized graphene was confirmed by low energy electron diffraction. We also demonstrate that CVD of graphene at temperatures below 1000 oC shows higher nucleation density on (111) than on (100) and (101) copper surfaces but there is no identifiable preference at higher temperatures.
Growth and microstructure of a protective or nonprotective SiO2 scale and the subsequent volatilization of scale formed on high‐purity chemical vapor deposited (CVD) SiC and nuclear‐grade SiC/SiC composites have been studied during high‐temperature 100% steam exposure. The environmental parameters of interest were temperature from 1200°C to 1700°C, pressure of 0.1 to 2 MPa and flow velocities of 0.23 to 145 cm/s. Scale microstructure was characterized via electron microscopy and X‐ray diffractometry. The Arrhenius dependence of the parabolic oxidation and linear volatilization rate constants were determined. The linear volatilization rate exhibited a strong dependence on steam partial pressure with a weaker dependence on flow velocity. At high steam pressures, the oxide scale developed substantial porosity, which significantly accelerated material recession. The dominant oxide phase for the conditions studied was cristobalite. The oxidation behavior of SiC/SiC composite was strongly dependent on the state of the surface, specifically whether steam could find easy entry into the material via surface‐exposed interface layers. For the case where these as‐machined interfaces were surface coated with matrix CVD SiC, composite recession was found to be essentially that of high‐purity CVD SiC.
Plasmonics is a rapidly growing field, yet imaging of the plasmonic modes in complex nanoscale architectures is extremely challenging. Here we obtain spatial maps of the localized surface plasmon modes of high-aspect-ratio silver nanorods using electron energy loss spectroscopy (EELS) and correlate to optical data and classical electrodynamics calculations from the exact same particles. EELS mapping is thus demonstrated to be an invaluable technique for elucidating complex and overlapping plasmon modes.
Equiatomic FeNiCoCrMn alloy has been reported to exhibit promising strength and ductility at cryogenic temperature and deformation mediated by nano-twining appeared to be one of the main reasons. We use the FeNiCoCrMn alloy as a base alloy to seek further improvement of its mechanical properties by alloying additional elements, i.e., interstitial carbon. The effects of carbon on microstructures, mechanical properties and twinning activities were investigated in two different temperatures (77 and 293 K). With addition of 0.5 at.% C, the high entropy alloy still remains entirely single phase face-centered cubic (FCC) crystal structure. The materials can be cold rolled and recrystallized to produce a microstructure with equiaxed grains. Both strain hardening rate and strength are enhanced while high uniform elongations to fracture (~ 70% at 77 K and ~ 40% at 293 K) are still maintained. The increased strain hardening and strength could be caused by the promptness of deformation twinning in C-containing high entropy alloys.
Optimized conditions for imaging and spectroscopic/elemental mapping of thin perfluorosulfonic acid (PFSA) ionomer layers in fuel cell electrodes by scanning transmission electron microscopy (STEM) have been investigated. The proper conditions were first identified using model systems of either Nafion ionomer-coated nanostructured thin film catalysts or thin films on nanoporous Si. These analysis conditions were then applied in a quantitative study of the ionomer through-layer loading for two differently-prepared electrode catalyst layers using electron energy loss (EELS) and energy dispersive X-ray spectroscopy (EDS) in the STEM. The electron-beam induced damage to the PFSA ionomer was quantified by following the fluorine mass loss with electron dose/exposure and was mitigated by several orders of magnitude using cryogenic specimen cooling and a higher incident electron voltage. Multivariate statistical analysis was applied to the analysis of both EELS and EDS spectrum images for data de-noising and unbiased separation of the independent components related to the catalyst, ionomer, and support distributions within the catalyst layers.Perfluorosulfonic acid (PFSA) ionomer is a key component within the electrode layers of polymer electrolyte fuel cells (PEFCs). The PEFC electrode layer is typically constructed at a ∼10 μm thickness and is comprised of a dispersed Pt nanoparticle catalyst supported on a highly structured carbon black support with a distributed PFSA ionomer film. This percolating solid polyelectrolyte in the electrode provides an efficient proton transport path to the active catalyst sites. The carbon and polymer occupy ∼20% volume fraction each in the electrode, which leaves ∼50-60% pore volume for transport of the reactant hydrogen/air and product water to/from the active Pt sites.Both the uniformity of the PFSA ionomer loading on a 100-nm length scale and the uniformity of the actual film thickness distribution surrounding the carbon support and catalyst nanoparticles on a 1-nm length scale are critical to electrode performance, and quantitative measurements of both these properties are highly desired. Scanning transmission electron microscopy (STEM) is an attractive tool for characterizing the distribution of ionomer within PEFC electrodes, especially when coupled with spectroscopic techniques such as electron energy loss spectroscopy (EELS) or energy dispersive X-ray spectroscopy (EDS). 1 While electron microscopy is more than capable of fulfilling the spatial resolution and chemical sensitivity requirements necessary for analysis of the PFSA ionomer, further method optimization of the STEM imaging and analysis parameters is required due to the beam-sensitive nature of the ionomer films.Fluorinated compounds, such as the PFSA ionomer, can be highly sensitive to electron beam radiation damage. 2 The high electron doses needed to acquire spectroscopic maps by either EDS or EELS can induce severe structural and chemical changes to the ionomer within the electrode, as previously demonstrated on PEF...
The family of layered carbides and nitrides known as MAX phase ceramics combine many attractive properties of both ceramics and metals due to their nanolaminate crystal structure and are promising potential candidates for application in future nuclear reactors. This investigation examines the effects of energetic heavy ion (5.8 MeV Ni) irradiations on polycrystalline samples of Ti3SiC2, Ti3AlC2, and Ti2AlC. The irradiation conditions consisted of midrange ion doses between 10 and 30 displacements per atom at temperatures of 400 and 700 C, conditions relevant to application in future nuclear reactors and a relatively un-explored regime for this new class of materials. Following irradiation, a comprehensive analysis of radiation response properties was compiled using grazing incidence X-ray diffraction (XRD), nanoindentation, scanning electron microcopy (SEM), and transmission electron microscopy (TEM). In all cases, XRD and TEM analyses confirm the materials remain fully crystalline although the intense atomic collisions induce significant damage and disorder into the layered crystalline lattice. Xray diffraction and nanoindentation show this damage is manifest in anisotropic swelling and hardening at all conditions and in all materials, with the aluminum based MAX phase exhibiting significantly more damage than their silicon counterpart. In all three materials there is little damage dependence on dose, suggesting saturation of radiation damage at levels below 10 displacements per atom, and significantly less retained damage at higher temperatures, suggesting radiation defect annealing. SEM surface analysis showed significant grain boundary cracking and loss of damage tolerance properties in the aluminum-based MAX phase irradiated at 400 C, but not in the silicon counterpart. TEM analysis of select samples suggest that interstitials are highly mobile while vacancies are immobile and that all three materials are in the so-called point defect swelling regime between 400 and 700 C. All results are consistent with previous work involving traditional and MAX phase ceramics. Results show that the aluminum MAX phases are not fit for application near 400 C and that the silicon MAX phase is overall more damage tolerant.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.