Design and Development of Stable Nanocrystalline High-Entropy Alloy: Coupling Self-Stabilization and Solute Grain Boundary Segregation Effects.

Grain growth is prevalent in nanocrystalline (NC) materials at low homologous temperatures. Solute element addition is used to offset excess energy that drives coarsening at grain boundaries (GBs), albeit mostly for simple binary alloys. This thermodynamic approach is considered complicated in multi-component alloy systems due to complex pairwise interactions among alloying elements. Guided by empirical and GB-segregation enthalpy considerations for binary-alloy systems, a novel alloy design strategy, the "pseudo-binary thermodynamic" approach, for stabilizing NC-high entropy alloys (HEAs) and other multi-component-alloy variants is proposed. Using Al25Co25Cr25Fe25 as a model-HEA to validate this approach, Zr, Sc, and Hf, are identified as the preferred solutes that would segregate to HEA-GBs to stabilize it against growth. Using Zr, NC-Al25Co25Cr25Fe25 HEAs with minor additions of Zr are synthesized, followed by annealing up to 1123 K. Using advanced characterization techniques- in situ X-ray diffraction (XRD), scanning/transmission electron microscopy (S/TEM), and atom probe tomography, nanograin stability due to coupling self-stabilization and solute-GB segregation effects is reported in HEAs up to substantially high temperatures. The self-stabilization effect originates from the preferential GB-segregation of constituent HEA-elements that stabilizes NC-Al25Co25Cr25Fe25 up to 0.5Tm (Tm-melting temperature). Meanwhile, solute-GB segregation originates from Zr segregation to NC-Al25Co25Cr25Fe25 GBs; this results in further stabilization of the phase and grain-size (≈14 nm) up to ≈0.58 and ≈0.64Tm, respectively.

A Holistic Approach for Elucidating Local Structure, Dynamics, and Speciation in Molten Salts with High Structural Disorder.

To examine ion solvation, exchange, and speciation for minority components in molten salts (MS) typically found as corrosion products, we propose a multimodal approach combining extended X-ray absorption fine structure (EXAFS) spectroscopy, optical spectroscopy, ab initio molecular dynamics (AIMD) simulations, and rate theory of ion exchange. Going beyond conventional EXAFS analysis, our method can accurately quantify populations of different coordination states of ions with highly disordered coordination environments via linear combination fitting of the EXAFS spectra of these coordination states computed from AIMD to the experimental EXAFS spectrum. In a case study of dilute Ni(II) dissolved in the ZnCl2+KCl melts, our method reveals heterogeneous distributions of coordination states of Ni(II) that are sensitive to variations in temperature and melt composition. These results are fully explained by the difference in the chloride exchange dynamics at varied temperatures and melt compositions. This insight will enable a better understanding and control of ion solubility and transport in MS.

X-ray scattering reveals ion clustering of dilute chromium species in molten chloride medium.

Enhancing the solar energy storage and power delivery afforded by emerging molten salt-based technologies requires a fundamental understanding of the complex interplay between structure and dynamics of the ions in the high-temperature media. Here we report results from a comprehensive study integrating synchrotron X-ray scattering experiments, ab initio molecular dynamics simulations and rate theory concepts to investigate the behavior of dilute Cr3+ metal ions in a molten KCl-MgCl2 salt. Our analysis of experimental results assisted by a hybrid transition state-Marcus theory model reveals unexpected clustering of chromium species leading to the formation of persistent octahedral Cr-Cr dimers in the high-temperature low Cr3+ concentration melt. Furthermore, our integrated approach shows that dynamical processes in the molten salt system are primarily governed by the charge density of the constituent ions, with Cr3+ exhibiting the slowest short-time dynamics. These findings challenge several assumptions regarding specific ionic interactions and transport in molten salts, where aggregation of dilute species is not statistically expected, particularly at high temperature.

Radiation-Assisted Formation of Metal Nanoparticles in Molten Salts.

Knowledge of structural and thermal properties of molten salts is crucial for understanding and predicting their stability in many applications such as thermal energy storage and nuclear energy systems. Probing the behavior of metal contaminants in molten salts is presently limited to either foreign ionic species or metal nanocrystals added to the melt. To bridge the gap between these two end states and follow the nucleation and growth of metal species in molten salt environment in situ, we use synchrotron X-rays as both a source of solvated electrons for reducing Ni2+ ions added to ZnCl2 melt and as an atomic-level probe for detecting formation of zerovalent Ni nanoparticles. By combining extended X-ray absorption fine structure analysis with X-ray absorption near edge structure modeling, we obtained the average size and structure of the nanoparticles and proposed a radiation-induced reduction mechanism of metal ions in molten salts.

Structure and dynamics of the molten alkali-chloride salts from an X-ray, simulation, and rate theory perspective.

Molten salts are of great interest as alternative solvents, electrolytes, and heat transfer fluids in many emerging technologies. The macroscopic properties of molten salts are ultimately controlled by their structure and ion dynamics at the microscopic level and it is therefore vital to develop an understanding of these at the atomistic scale. Herein, we present high-energy X-ray scattering experiments combined with classical and ab initio molecular dynamics simulations to elucidate structural and dynamical correlations across the family of alkali-chlorides. Computed structure functions and transport properties are in reasonably good agreement with experiments providing confidence in our analysis of microscopic properties based on simulations. For these systems, we also survey different rate theory models of anion exchange dynamics in order to gain a more sophisticated understanding of the short-time correlations that are likely to influence transport properties such as conductivity. The anion exchange process occurs on the picoseconds time scale at 1100 K and the rate increases in the order KCl < NaCl < LiCl, which is in stark contrast to the ion pair dissociation trend in aqueous solutions. Consistent with the trend we observe for conductivity, the cationic size/mass, as well as other factors specific to each type of rate theory, appear to play important roles in the anion exchange rate trend.

Connections between the Speciation and Solubility of Ni(II) and Co(II) in Molten ZnCl2.

Understanding the factors that control solubility and speciation of metal ions in molten salts is key for their successful use in molten salt reactors and electrorefining. Here, we employ X-ray and optical absorption spectroscopies and molecular dynamics simulations to investigate the coordination environment of Ni(II) in molten ZnCl2, where it is poorly soluble, and contrast it with highly soluble Co(II) over a wide temperature range. In solid NiCl2, the Ni ion is octahedrally coordinated, whereas the ZnCl2 host matrix favors tetrahedral coordination. Our experimental and computational results show that the coordination environment of Ni(II) in ZnCl2 is disordered among tetra- and pentacoordinate states. In contrast, the local structure of dissolved Co(II) is tetrahedral and commensurate with the ZnCl2 host's structure. The heterogeneity and concomitant large bond length disorder in the Ni case constitute a plausible explanation for its lower solubility in molten ZnCl2.

Elucidating Ionic Correlations Beyond Simple Charge Alternation in Molten MgCl2-KCl Mixtures.

The development of technologies for nuclear reactors based on molten salts has seen a big resurgence. The success of thermodynamic models for these hinges in part on our ability to predict at the atomistic level the behavior of pure salts and their mixtures under a range of conditions. In this letter, we present high-energy X-ray scattering experiments and molecular dynamics simulations that describe the molten structure of mixtures of MgCl2 and KCl. As one would expect, KCl is a prototypical salt in which structure is governed by simple charge alternation. In contrast, MgCl2 and its mixtures with KCl display more complex correlations including intermediate-range order and the formation of Cl--decorated Mg2+ chains. A thorough computational analysis suggests that intermediate-range order beyond charge alternation may be traced to correlations between these chains. An analysis of the coordination structure for Mg2+ ions paints a more complex picture than previously understood, with multiple accessible states of distinct geometries.

Sample environment for in situ synchrotron corrosion studies of materials in extreme environments.

A new in situ sample environment has been designed and developed to study the interfacial interactions of nuclear cladding alloys with high temperature steam. The sample environment is particularly optimized for synchrotron X-ray diffraction studies for in situ structural analysis. The sample environment is highly corrosion resistant and can be readily adapted for steam environments. The in situ sample environment design complies with G2 ASTM standards for studying corrosion in zirconium and its alloys and offers remote temperature and pressure monitoring during the in situ data collection. The use of the in situ sample environment is exemplified by monitoring the oxidation of metallic zirconium during exposure to steam at 350 °C. The in situ sample environment provides a powerful tool for fundamental understanding of corrosion mechanisms by elucidating the substoichiometric oxide phases formed during the early stages of corrosion, which can provide a better understanding of the oxidation process.

Monolithic aerogels of silver modified cadmium sulfide colloids.

Monolithic mesoporous aerogels comprising cadmium sulfide nanoparticles partially coated with metallic silver (CdS-Ag) are synthesized, and it is determined that the concentration of silver has a significant impact on the resultant CdS-Ag aerogel morphology and porosity.

Synthesis of cobalt oxide aerogels and nanocomposite systems containing single-walled carbon nanotubes.

The synthesis and characterization of porous nanostructured cobalt oxide (Co3O4) aerogels using epoxide addition method is described. Cobalt-containing monoliths were obtained by sol-gel processing of an alcoholic cobalt chloride solution with propylene oxide as the gelation agent. The alcogels were dried by supercritical CO2 fluid extraction to obtain the highly porous amorphous cobalt (II) aerogels. To enhance and control the structural properties of the aerogels, single-walled carbon nanotubes (SWCNT) were incorporated into the cobalt (II) aerogel network to form a homogenous composite material. The resulting materials were characterized using powder X-ray diffraction and nitrogen adsorption/desorption analysis. The detailed microstructure of aerogel networks was investigated using scanning and transmission electron microscopy, which showed significant structural changes induced by the incorporation of SWCNT to the cobalt aerogel. Annealing of the aerogel materials at 600 degrees C yields a highly crystalline well-faceted Co3O4 network.

Exploration of the use of novel SiO2 nanocomposites doped with fluorescent Eu3+/sensitizer complex for latent fingerprint detection.

Silicon dioxide-based nanocomposites have shown great potential in novel chemical and biological sensor development due to their large loading capacity and high surface area to volume ratio for trapping molecular complexes of various sizes. However, their potential applications in forensic science, latent fingerprint detection in particular, were still unclear. In this study, we have succeeded in trapping the highly fluorescent and photo-stable Eu3+ metal ions/sensitizer complex in silicon dioxide-based nanocomposites, doped xerogels, using the sol-gel method. We have tested the spectroscopic properties of Eu3+ in several combinations of rare earth sensitizers and different derivatives of silicon dioxide nanoporous templates. Our results indicated that the use of 1,10-phenanthroline (OP) sensitizer in tetraethoxysilane (TEOS) template provides the best fluorescently doped xerogels applicable for latent fingerprint detections on various forensic relevant materials, including metal foil, glass, plastic, colored paper, and a green tree leaf. The fabrication procedure, UV/vis and fluorescence characterizations, and fingerprint labeling results of these new Eu3+/OP/TEOS nanocomposites are presented in this exploratory study.