Kink bands promote exceptional fracture resistance in a NbTaTiHf refractory medium-entropy alloy.

Single-phase body-centered cubic (bcc) refractory medium- or high-entropy alloys can retain compressive strength at elevated temperatures but suffer from extremely low tensile ductility and fracture toughness. We examined the strength and fracture toughness of a bcc refractory alloy, NbTaTiHf, from 77 to 1473 kelvin. This alloy's behavior differed from that of comparable systems by having fracture toughness over 253 MPa·m1/2, which we attribute to a dynamic competition between screw and edge dislocations in controlling the plasticity at a crack tip. Whereas the glide and intersection of screw and mixed dislocations promotes strain hardening controlling uniform deformation, the coordinated slip of <111> edge dislocations with {110} and {112} glide planes prolongs nonuniform strain through formation of kink bands. These bands suppress strain hardening by reorienting microscale bands of the crystal along directions of higher resolved shear stress and continually nucleate to accommodate localized strain and distribute damage away from a crack tip.

Resolving protein-mineral interfacial interactions during in vitro mineralization by atom probe tomography.

Organic macromolecules exert remarkable control over the nucleation and growth of inorganic crystallites during (bio)mineralization, as exemplified during enamel formation where the protein amelogenin regulates the formation of hydroxyapatite (HAP). However, it is poorly understood how fundamental processes at the organic-inorganic interface, such as protein adsorption and/or incorporation into minerals, regulates nucleation and crystal growth due to technical challenges in observing and characterizing mineral-bound organics at high-resolution. Here, atom probe tomography techniques were developed and applied to characterize amelogenin-mineralized HAP particles in vitro, revealing distinct organic-inorganic interfacial structures and processes at the nanoscale. Specifically, visualization of amelogenin across the mineralized particulate demonstrates protein can become entrapped during HAP crystal aggregation and fusion. Identification of protein signatures and structural interpretations were further supported by standards analyses, i.e., defined HAP surfaces with and without amelogenin adsorbed. These findings represent a significant advance in the characterization of interfacial structures and, more so, interpretation of fundamental organic-inorganic processes and mechanisms influencing crystal growth. Ultimately, this approach can be broadly applied to inform how potentially unique and diverse organic-inorganic interactions at different stages regulates the growth and evolution of various biominerals.

Extended Shear Deformation of the Immiscible Cu-Nb Alloy Resulting in Nanostructuring and Oxygen Ingress with Enhancement in Mechanical Properties.

Deformation processing of immiscible systems is observed to disrupt thermodynamic equilibrium, often resulting in nonequilibrium microstructures. The microstructural changes including nanostructuring, hierarchical distribution of phases, localized solute supersaturation, and oxygen ingress result from high-strain extended deformation, causing a significant change in mechanical properties. Because of the dynamic evolution of the material under large strain shear load, a detailed understanding of the transformation pathway has not been established. Additionally, the influence of these microstructural changes on mechanical properties is also not well characterized. Here, an immiscible Cu-4 at. % Nb alloy is subjected to a high-strain shear deformation (∼200); the deformation-induced changes in the morphology, crystal structure, and composition of Cu and Nb phases as a function of total strain are characterized using transmission electron microscopy and atom probe tomography. Furthermore, a multimodal experiment-guided computational approach is used to depict the initiation of deformation by an increase in misorientation boundaries by crystal plasticity-based grain misorientation modeling (strain ∼0.6). Then, co-deformation and nanolamination of Cu and Nb are envisaged by a finite element method-based computational fluid dynamic model with strain ranging from 10 to 200. Finally, the experimentally observed amorphization of the severely sheared supersaturated Cu-Nb-O phase was validated using the first principle-based simulation using density functional theory while highlighting the influence of oxygen ingress during deformation. Furthermore, the nanocrystalline microstructure shows a >2-fold increase in hardness and compressive yield strength of the alloy, elucidating the potential of deformation processing to obtain high-strength low-alloyed metals. Our approach presents a step-by-step evolution of a microstructure in an immiscible alloy undergoing severe shear deformation, which is broadly applicable to materials processing based on friction stir, extrusion, rolling, and surface shear deformation under wear and can be directly applied to understanding material behavior during these processes.

Onset of High Methane Combustion Rates over Supported Palladium Catalysts: From Isolated Pd Cations to PdO Nanoparticles.

Industrial low-temperature methane combustion catalyst Pd/Al2O3 suffers from H2O-induced deactivation. It is imperative to design Pd catalysts free from this deactivation and with high atomic efficiency. Using a small-pore zeolite SSZ-13 as support, herein we report well-defined Pd catalysts with dominant active species as finely dispersed Pd cations, uniform PdO particles embedded inside the zeolite framework, or PdO particles decorating the zeolite external surface. Through detailed reaction kinetics and spectroscopic and microscopic studies, we show that finely dispersed sites are much less active than PdO nanoparticles. We further demonstrate that H2O-induced deactivation can be readily circumvented by using zeolite supports with high Si/Al ratios. Finally, we provide a few rational catalyst design suggestions for methane oxidation based on the new knowledge learned in this study.

Correlating nanoscale secondary ion mass spectrometry and atom probe tomography analysis of uranium enrichment in metallic nuclear fuel.

Accurate measurements of 235U enrichment within metallic nuclear fuels are essential for understanding material performance in a neutron irradiation environment, and the origin of secondary phases (e.g. uranium carbides). In this work, we analyse 235U enrichment in matrix and carbide phases in low enriched uranium alloyed with 10 wt% Mo via two chemical imaging modalities-nanoscale secondary ion mass spectrometry (NanoSIMS) and atom probe tomography (APT). Results from NanoSIMS and APT are compared to understand accuracy and utility of both approaches across length scales. NanoSIMS and APT provide consistent results, with no statistically significant difference between nominal enrichment (19.95 ± 0.14 at% 235U) and that measured for metal matrix and carbide inclusions.

Chemical short-range order in derivative Cr-Ta-Ti-V-W high entropy alloys from the first-principles thermodynamic study.

The development of high-entropy alloys (HEAs) focuses on exploring compositional regions in multi-component systems with all alloy elements in equal or near-equal atomic concentrations. Initially it was based on the main idea that high mixing configurational entropy contributions to the alloy free energy could promote the formation of a single solid solution phase. By using the ab-initio based Cluster Expansion (CE) Hamiltonian model constructed for the quinary bcc Cr-Ta-Ti-V-W system in combination with Monte Carlo (MC) simulations, we show that the phase stability and chemical short-range order (SRO) of the equiatomic quinary and five sub-quaternary systems, as well as their derivative alloys, can dramatically change the order-disorder transition temperatures (ODTT) as a function of alloy compositions. In particular, it has been found, that the equiatomic quaternary Ta-Ti-V-W and Cr-Ta-Ti-W alloys had the lowest order-disorder transition temperature (500 K) among all the analysed equiatomic compositions. In all investigated alloy systems, the strongest chemical ordering has been observed between Cr and V, which led to the conclusion that decreasing the concentration of either Cr or V might be beneficial in terms of decreasing the ODTT. It also predicts that increasing concentration of Ti significantly decreases the ODTT. Our analysis of chemical SRO as a function of alloy composition allows to understand the microstructure evolution of HEAs as a function of temperature in excellent agreement with available experimental observations. Importantly, our free energy of mixing and SRO calculations predict that the origin of precipitates formed by Cr- and V-rich in the sub-quaternary Cr-Ta-V-W system is driven by the thermodynamics. The modelling results are in an excellent agreement with experimental observation of Cr and V segregation in the W0.38Ta0.36Cr0.15V0.11 alloy which in turns shows an exceptional radiation resistance.

Rapid Response High Temperature Oxygen Sensor Based on Titanium Doped Gallium Oxide.

Real-time monitoring of combustion products and composition is critical to emission reduction and efficient energy production. The fuel efficiency in power plants and automobile engines can be dramatically improved by monitoring and controlling the combustion environment. However, the development of novel materials for survivability of oxygen sensors at extreme environments and demonstrated rapid response in chemical sensing is a major hindrance for further development in the field. Gallium oxide (Ga2O3), one among the wide band gap oxides, exhibit promising oxygen sensing properties in terms of reproducibility and long term stability. However, the oxygen sensors based on ß-Ga2O3 and other existing materials lack in response time and stability at elevated temperatures. In this context, we demonstrate an approach to design materials based on Ti-doped Ga2O3, which exhibits a rapid response and excellent stability for oxygen sensing at elevated temperatures. We demonstrate that the nanocrystalline ß-Ga2O3 films with 5% Ti significantly improves the response time (~20 times) while retaining the stability and repeatability in addition to enhancement in the sensitivity to oxygen. These extreme environment oxygen sensors with a rapid response time and sensitivity represent key advancement for integration into combustion systems for efficient energy conversion and emission reduction.

Nanoscale Spatially Resolved Mapping of Uranium Enrichment.

Spatially resolved analysis of uranium (U) isotopes in small volumes of actinide-bearing materials is critical for a variety of technical disciplines, including earth and planetary sciences, environmental monitoring, bioremediation, and the nuclear fuel cycle. However, achieving subnanometer-scale spatial resolution for such isotopic analysis is currently a challenge. By using atom probe tomography-a three-dimensional nanoscale characterisation technique-we demonstrate unprecedented nanoscale mapping of U isotopic enrichment with high sensitivity across various microstructural interfaces within small volumes (~100 nm3) of depleted and low-enriched U alloyed with 10 wt% molybdenum that has different nominal enrichments of 0.20 and 19.75% 235U, respectively. We map enrichment in various morphologies of a U carbide phase, the adjacent γ-UMo matrix, and across interfaces (e.g., carbide/matrix, grain boundary). Results indicate the U carbides were formed during casting, rather than retained from either highly enriched or depleted U feedstock materials. The approach presented here can be applied to study nanoscale variations of isotopic abundances in the broad class of actinide-bearing materials, providing unique insights into their origins and thermomechanical processing routes.

Injection of oxygen vacancies in the bulk lattice of layered cathodes.

Surfaces, interfaces and grain boundaries are classically known to be sinks of defects generated within the bulk lattice. Here, we report an inverse case by which the defects generated at the particle surface are continuously pumped into the bulk lattice. We show that, during operation of a rechargeable battery, oxygen vacancies produced at the surfaces of lithium-rich layered cathode particles migrate towards the inside lattice. This process is associated with a high cutoff voltage at which an anionic redox process is activated. First-principle calculations reveal that triggering of this redox process leads to a sharp decrease of both the formation energy of oxygen vacancies and the migration barrier of oxidized oxide ions, therefore enabling the migration of oxygen vacancies into the bulk lattice of the cathode. This work unveils a coupled redox dynamic that needs to be taken into account when designing high-capacity layered cathode materials for high-voltage lithium-ion batteries.

The limiting layer of fish scales: Structure and properties.

Fish scales serve as a flexible natural armor that have received increasing attention across the materials community. Most efforts in this area have focused on the composite structure of the predominately organic elasmodine, and limited work addresses the highly mineralized external portion known as the Limiting Layer (LL). This coating serves as the first barrier to external threats and plays an important role in resisting puncture. In this investigation the structure, composition and mechanical behavior of the LL were explored for three different fish, including the arapaima (Arapaima gigas), the tarpon (Megalops atlanticus) and the carp (Cyprinus carpio). The scales of these three fish have received the most attention within the materials community. Features of the LL were evaluated with respect to anatomical position to distinguish site-specific functional differences. Results show that there are significant differences in the surface morphology of the LL from posterior and anterior regions in the scales, and between the three fish species. The calcium to phosphorus ratio and the mineral to collagen ratios of the LL are not equivalent among the three fish. Results from nanoindentation showed that the LL of tarpon scales is the hardest, followed by the carp and the arapaima and the differences in hardness are related to the apatite structure, possibly induced by the growth rate and environment of each fish. STATEMENT OF SIGNIFICANCE: The natural armor of fish, turtles and other animals, has become a topic of substantial scientific interest. The majority of investigations have focused on the more highly organic layer known as the elasmodine. The present study addresses the highly mineralized external portion known as the Limiting Layer (LL). Specifically, the structure, composition and mechanical behavior of the LL were explored for three different fish, including the arapaima (Arapaima gigas), the tarpon (Megalops atlanticus) and the carp (Cyprinus carpio). Results show that there are significant differences in the surface morphology of the LL from posterior and anterior regions in the scales, and between the three species. In addition, the composition of the LL is also unique among the three fish. Results from nanoindentation showed that the LL of tarpon scales is the hardest, followed by the carp and the arapaima and the differences in hardness are related to the apatite structure, possibly induced by the growth rate and environment of each fish. In addition, a new feature was indentified in the LL, which has not been discussed before. As such, we feel this work is unique and makes a significant contribution to the field.

Reduced Magnetism in Core-Shell Magnetite@MOF Composites.

The magnetic susceptibility of synthesized magnetite (Fe3O4) microspheres was found to decline after the growth of a metal-organic framework (MOF) shell on the magnetite core. Detailed structural analysis of the core-shell particles using scanning electron microscopy, transmission electron microscopy, atom probe tomography, and57Fe-Mössbauer spectroscopy suggests that the distribution of MOF precursors inside the magnetic core resulted in the oxidation of the iron oxide core.

Three-Dimensional Nanoscale Mapping of State-of-the-Art Field-Effect Transistors (FinFETs).

The semiconductor industry has seen tremendous progress over the last few decades with continuous reduction in transistor size to improve device performance. Miniaturization of devices has led to changes in the dopants and dielectric layers incorporated. As the gradual shift from two-dimensional metal-oxide semiconductor field-effect transistor to three-dimensional (3D) field-effect transistors (finFETs) occurred, it has become imperative to understand compositional variability with nanoscale spatial resolution. Compositional changes can affect device performance primarily through fluctuations in threshold voltage and channel current density. Traditional techniques such as scanning electron microscope and focused ion beam no longer provide the required resolution to probe the physical structure and chemical composition of individual fins. Hence advanced multimodal characterization approaches are required to better understand electronic devices. Herein, we report the study of 14 nm commercial finFETs using atom probe tomography (APT) and scanning transmission electron microscopy-energy-dispersive X-ray spectroscopy (STEM-EDS). Complimentary compositional maps were obtained using both techniques with analysis of the gate dielectrics and silicon fin. APT additionally provided 3D information and allowed analysis of the distribution of low atomic number dopant elements (e.g., boron), which are elusive when using STEM-EDS.

The natural armors of fish: A comparison of the lamination pattern and structure of scales.

Fish scales exhibit a unique balance of flexibility, strength and toughness, which is essential to provide protection without encumbering locomotion. Although the mechanical behavior and structure of this natural armor are of recent interest, a comparison of these qualities from scales of different fish species has not been reported. In this investigation the armor of fish with different locomotion, size and protection needs were analyzed. Scales from the Arapaima gigas, the tarpon (Megalops atlanticus) and the carp (Cyprinus carpio) were compared in terms of the stacking sequence of individual plies and their microstructure. The scales were also compared with respect to anatomical position to distinguish site-specific functional differences. Results show that the lamination sequence of plies for the carp and tarpon exhibit a Bouligand structure with relative rotation of 75° between consecutive plies. The arapaima scales exhibit a cross-ply structure, with 90° rotation between adjacent plies. In addition, results indicate that the volume fraction of reinforcement, the number of plies and the variations in thickness with anatomical position are unique amongst the three fish. These characteristics should be considered in evaluations focused on the mechanical behavior.

Discerning the Location and Nature of Coke Deposition from Surface to Bulk of Spent Zeolite Catalysts.

The formation of carbonaceous deposits (coke) in zeolite pores during catalysis leads to temporary deactivation of catalyst, necessitating regeneration steps, affecting throughput, and resulting in partial permanent loss of catalytic efficiency. Yet, even to date, the coke molecule distribution is quite challenging to study with high spatial resolution from surface to bulk of the catalyst particles at a single particle level. To address this challenge we investigated the coke molecules in HZSM-5 catalyst after ethanol conversion treatment by a combination of C K-edge X-ray absorption spectroscopy (XAS), 13C Cross polarization-magic angle spinning nuclear magnetic resonance (CP-MAS NMR) spectroscopy, and atom probe tomography (APT). XAS and NMR highlighted the aromatic character of coke molecules. APT permitted the imaging of the spatial distribution of hydrocarbon molecules located within the pores of spent HZSM-5 catalyst from surface to bulk at a single particle level. 27Al NMR results and APT results indicated association of coke molecules with Al enriched regions within the spent HZSM-5 catalyst particles. The experimental results were additionally validated by a level-set-based APT field evaporation model. These results provide a new approach to investigate catalytic deactivation due to hydrocarbon coking or poisoning of zeolites at an unprecedented spatial resolution.

Role of 4-tert-Butylpyridine as a Hole Transport Layer Morphological Controller in Perovskite Solar Cells.

Hybrid organic-inorganic materials for high-efficiency, low-cost photovoltaic devices have seen rapid progress since the introduction of lead based perovskites and solid-state hole transport layers. Although majority of the materials used for perovskite solar cells (PSC) are introduced from dye-sensitized solar cells (DSSCs), the presence of a perovskite capping layer as opposed to a single dye molecule (in DSSCs) changes the interactions between the various layers in perovskite solar cells. 4-tert-Butylpyridine (tBP), commonly used in PSCs, is assumed to function as a charge recombination inhibitor, similar to DSSCs. However, the presence of a perovskite capping layer calls for a re-evaluation of its function in PSCs. Using TEM (transmission electron microscopy), we first confirm the role of tBP as a HTL morphology controller in PSCs. Our observations suggest that tBP significantly improves the uniformity of the HTL and avoids accumulation of Li salt. We also study degradation pathways by using FTIR (Fourier transform infrared spectroscopy) and APT (atom probe tomography) to investigate and visualize in 3-dimensions the moisture content associated with the Li salt. Long-term effects, over 1000 h, due to evaporation of tBP have also been studied. Based on our findings, a PSC failure mechanism associated with the morphological change of the HTL is proposed. tBP, the morphology controller in HTL, plays a key role in this process, and thus this study highlights the need for additive materials with higher boiling points for consistent long-term performance of PSCs.

A low-cost hierarchical nanostructured beta-titanium alloy with high strength.

Lightweighting of automobiles by use of novel low-cost, high strength-to-weight ratio structural materials can reduce the consumption of fossil fuels and in turn CO2 emission. Working towards this goal we achieved high strength in a low cost ß-titanium alloy, Ti-1Al-8V-5Fe (Ti185), by hierarchical nanostructure consisting of homogenous distribution of micron-scale and nanoscale α-phase precipitates within the ß-phase matrix. The sequence of phase transformation leading to this hierarchical nanostructure is explored using electron microscopy and atom probe tomography. Our results suggest that the high number density of nanoscale α-phase precipitates in the ß-phase matrix is due to ω assisted nucleation of α resulting in high tensile strength, greater than any current commercial titanium alloy. Thus hierarchical nanostructured Ti185 serves as an excellent candidate for replacing costlier titanium alloys and other structural alloys for cost-effective lightweighting applications.

Molecular structure and stability of dissolved lithium polysulfide species.

The ability to predict the solubility and stability of lithium polysulfide is vital in realizing longer lasting lithium-sulfur batteries. Herein we report combined experimental and computational analyses to understand the dissolution mechanism of lithium polysulfide species in an aprotic solvent medium. Multinuclear NMR, variable temperature ESR and sulfur K-edge XAS analyses reveal that the lithium exchange between polysulfide species and solvent molecules constitutes the first step in the dissolution process. Lithium exchange leads to de-lithiated polysulfide ions (Sn(2-)) which subsequently form highly reactive free radicals through dissociation reaction (Sn(2-) → 2Sn/2˙(-)). The energy required for the dissociation and possible dimer formation reactions of the polysulfide species is analyzed using density functional theory (DFT) based calculations. Based on these findings, we discuss approaches to optimize the electrolyte in order to control the polysulfide solubility.

Understanding Atom Probe Tomography of Oxide-Supported Metal Nanoparticles by Correlation with Atomic-Resolution Electron Microscopy and Field Evaporation Simulation.

Oxide-supported metal nanoparticles are widely used in heterogeneous catalysis. The increasingly detailed design of such catalysts necessitates three-dimensional characterization with high spatial resolution and elemental selectivity. Laser-assisted atom probe tomography (APT) is uniquely suited to the task but faces challenges with the evaporation of metal/insulator systems. Correlation of APT with aberration-corrected scanning transmission electron microscopy (STEM), for Au nanoparticles embedded in MgO, reveals preferential evaporation of the MgO and an inaccurate assessment of nanoparticle composition. Finite element field evaporation modeling is used to illustrate the evolution of the evaporation front. Nanoparticle composition is most accurately predicted when the MgO is treated as having a locally variable evaporation field, indicating the importance of considering laser-oxide interactions and the evaporation of various molecular oxide ions. These results demonstrate the viability of APT for analysis of oxide-supported metal nanoparticles, highlighting the need for developing a theoretical framework for the evaporation of heterogeneous materials.