Direct Observation of Trace Elements in Barium Titanate of Multilayer Ceramic Capacitors Using Atom Probe Tomography.

Accurately controlling trace additives in dielectric barium titanate (BaTiO3) layers is important for optimizing the performance of multilayer ceramic capacitors (MLCCs). However, characterizing the spatial distribution and local concentration of the additives, which strongly influence the MLCC performance, poses a significant challenge. Atom probe tomography (APT) is an ideal technique for obtaining this information, but the extremely low electrical conductivity and piezoelectricity of BaTiO3 render its analysis with existing sample preparation approaches difficult. In this study, we developed a new APT sample preparation method involving W coating and heat treatment to investigate the trace additives in the BaTiO3 layer of MLCCs. This method enables determination of the local concentration and distribution of all trace elements in the BaTiO3 layer, including additives and undesired impurities. The developed method is expected to pave the way for the further optimization and advancement of MLCC technology.

Tracking the Mn Diffusion in the Carbon-Supported Nanoparticles Through the Collaborative Analysis of Atom Probe and Evaporation Simulation.

Carbon-supported nanoparticles have been used widely as efficient catalysts due to their enhanced surface-to-volume ratio. To investigate their structure­property relationships, acquiring 3D elemental distribution is required. Here, carbon-supported Pt, PtMn alloy, and ordered Pt3Mn nanoparticles are synthesized and analyzed with atom probe tomography as model systems. A significant difference of Mn distribution after the heat-treatment was found. Finally, the field evaporation behavior of the carbon support was discussed and each acquired reconstruction was compared with computational results from an evaporation simulation. This paper provides a guideline for studies using atom probe tomography on the heterogeneous carbon-supported nanoparticle system that leads to insights toward a wide variety of applications.

Atom Probe Tomography Investigations of Ag Nanoparticles Embedded in Pulse-Electrodeposited Ni Films.

Atomic mapping of nanomaterials, in particular nanoparticles, using atom probe tomography (APT) is of great interest, as their properties strongly depend on shape, size, and composition. However, APT analyses of nanoparticles are extremely challenging, and there is an urgent need for developing robust and universally applicable sample preparation methods. Herein, we explored a method based on pulse electrodeposition to embed Ag nanoparticles in a Ni matrix and prepare APT specimens from the resulting composite film. By systematically varying the duty cycle during pulse electrodeposition, the dispersion and number density of the nanoparticles within the matrix was significantly enhanced as compared to DC electrodeposition. Several Ag nanoparticles were analyzed with APT from such samples. Shape distortions and biased compositions were observed for the Ag nanoparticles after applying a standard data reconstruction protocol. Numerical simulations of the field evaporation process showed that such artifacts were caused by a difference in the evaporation field of Ni and Ag and a local magnification effect. We expect such detrimental effects to be mitigated by a careful selection of the matrix material, matching the evaporation field of the nanoparticles. Furthermore, we anticipate that the method presented herein can be extended to a wider range of nanomaterials.

Elemental Sub-Lattice Occupation and Microstructural Evolution in γ/γ' Co-12Ti-4Mo-Cr Alloys.

We report on comparative atom probe tomography investigations of γ/γ'-forming Co­12Ti­4Mo­Cr alloys. Moderate additions of Cr (2 and 4 at%) reduced the γ/γ' lattice misfit and increased the γ' volume fraction of a Co­12Ti­4Mo alloy significantly. These microstructural changes were accompanied by changes in the elemental partitioning between γ and γ' and site-occupancy in γ'. Spatial distribution maps revealed that Mo occupied both Co and Ti sub-lattice sites in γ'. In agreement with the experimental data, thermodynamic calculations predicted a stronger tendency for Mo to occupy the Co-sites than for Cr and an increase in Cr fraction on the Ti-sites with increasing Cr content.

Atomic-Scale Mapping of Impurities in Partially Reduced Hollow TiO2 Nanowires.

The incorporation of impurities during the chemical synthesis of nanomaterials is usually uncontrolled and rarely reported because of the formidable challenge in measuring trace amounts of often light elements with sub-nanometer spatial resolution. And yet, these foreign elements (introduced by doping, for example) influence functional properties. We demonstrate how the hydrothermal growth and a partial reduction reaction on hollow TiO2 nanowires leads to the introduction of parts per millions of boron, sodium, and nitrogen. This doping explains the presence of oxygen vacancies and reduced Ti states at the surface, which enhance the functional properties of TiO2 . Our results were obtained on model metal oxide nanomaterials and they shed light on a general process that leads to the uncontrolled incorporation of trace impurities in TiO2 , thereby, having a strong effect on applications in energy-harvesting.

Fabrication of Atom Probe Tomography Specimens from Nanoparticles Using a Fusible Bi-In-Sn Alloy as an Embedding Medium.

We propose a new method for preparing atom probe tomography specimens from nanoparticles using a fusible bismuth-indium-tin alloy as an embedding medium. Iron nanoparticles synthesized by the sodium borohydride reduction method were chosen as a model system. The as-synthesized iron nanoparticles were embedded within the fusible alloy using focused ion beam milling and ion-milled to needle-shaped atom probe specimens under cryogenic conditions. An atom probe analysis revealed boron atoms in a detected iron nanoparticle, indicating that boron from the sodium borohydride reductant was incorporated into the nanoparticle during its synthesis.

Evaluation of Analysis Conditions for Laser-Pulsed Atom Probe Tomography: Example of Cemented Tungsten Carbide.

Cemented tungsten carbide has been analyzed using laser-pulsed atom probe tomography (APT). The influence of experimental parameters, including laser pulse energy, pulse repetition rate, and specimen base temperature, on the acquired data were evaluated from different aspects, such as mass spectrum, chemical composition, noise-to-signal ratio, and multiple events. Within all the applied analysis conditions, only 1 MHz pulse repetition rate led to a strong detector saturation effect, resulting in a largely biased chemical composition. A comparative study of the laser energy settings showed that an ~12 times higher energy was required for the less focused green laser of the LEAPTM 3000X HR system to achieve a similar evaporation field as the finer spot ultraviolet laser of the LEAPTM 5000 XS system.

(Nbx, Zr1-x)4AlC3 MAX Phase Solid Solutions: Processing, Mechanical Properties, and Density Functional Theory Calculations.

The solubility of zirconium (Zr) in the Nb4AlC3 host lattice was investigated by combining the experimental synthesis of (Nbx, Zr1-x)4AlC3 solid solutions with density functional theory calculations. High-purity solid solutions were prepared by reactive hot pressing of NbH0.89, ZrH2, Al, and C starting powder mixtures. The crystal structure of the produced solid solutions was determined using X-ray and neutron diffraction. The limited Zr solubility (maximum of 18.5% of the Nb content in the host lattice) in Nb4AlC3 observed experimentally is consistent with the calculated minimum in the energy of mixing. The lattice parameters and microstructure were evaluated over the entire solubility range, while the chemical composition of (Nb0.85, Zr0.15)4AlC3 was mapped using atom probe tomography. The hardness, Young's modulus, and fracture toughness at room temperature as well as the high-temperature flexural strength and E-modulus of (Nb0.85, Zr0.15)4AlC3 were investigated and compared to those of pure Nb4AlC3. Quite remarkably, an appreciable increase in fracture toughness was observed from 6.6 ± 0.1 MPa/m(1/2) for pure Nb4AlC3 to 10.1 ± 0.3 MPa/m(1/2) for the (Nb0.85, Zr0.15)4AlC3 solid solution.

Shear-induced mixing governs codeformation of crystalline-amorphous nanolaminates.

Deformation of ductile crystalline-amorphous nanolaminates is not well understood due to the complex interplay of interface mechanics, shear banding, and deformation-driven chemical mixing. Here we present indentation experiments on 10 nm nanocrystalline Cu-100 nm amorphous CuZr model multilayers to study these mechanisms down to the atomic scale. By using correlative atom probe tomography and transmission electron microscopy we find that crystallographic slip bands in the Cu layers coincide with noncrystallographic shear bands in the amorphous CuZr layers. Dislocations from the crystalline layers drag Cu atoms across the interface into the CuZr layers. Also, crystalline Cu blocks are sheared into the CuZr layers. In these sheared and thus Cu enriched zones the initially amorphous CuZr layer is rendered into an amorphous plus crystalline nanocomposite.

Segregation stabilizes nanocrystalline bulk steel with near theoretical strength.

Grain refinement through severe plastic deformation enables synthesis of ultrahigh-strength nanostructured materials. Two challenges exist in that context: First, deformation-driven grain refinement is limited by dynamic dislocation recovery and crystal coarsening due to capillary driving forces; second, grain boundary sliding and hence softening occur when the grain size approaches several nanometers. Here, both challenges have been overcome by severe drawing of a pearlitic steel wire (pearlite: lamellar structure of alternating iron and iron carbide layers). First, at large strains the carbide phase dissolves via mechanical alloying, rendering the initially two-phase pearlite structure into a carbon-supersaturated iron phase. This carbon-rich iron phase evolves into a columnar nanoscaled subgrain structure which topologically prevents grain boundary sliding. Second, Gibbs segregation of the supersaturated carbon to the iron subgrain boundaries reduces their interface energy, hence reducing the driving force for dynamic recovery and crystal coarsening. Thus, a stable cross-sectional subgrain size <10 nm is achieved. These two effects lead to a stable columnar nanosized grain structure that impedes dislocation motion and enables an extreme tensile strength of 7 GPa, making this alloy the strongest ductile bulk material known.

The maximum separation cluster analysis algorithm for atom-probe tomography: parameter determination and accuracy.

Atom-probe tomography is a materials characterization method ideally suited for the investigation of clustering and precipitation phenomena. To distinguish the clusters from the surrounding matrix, the maximum separation algorithm is widely employed. However, the results of the cluster analysis strongly depend on the parameters used in the algorithm and hence, a wrong choice of parameters leads to erroneous results, e.g., for the cluster number density, concentration, and size. Here, a new method to determine the optimum value of the parameter dmax is proposed, which relies only on information contained in the measured atom-probe data set. Atom-probe simulations are employed to verify the method and to determine the sensitivity of the maximum separation algorithm to other input parameters. In addition, simulations are used to assess the accuracy of cluster analysis in the presence of trajectory aberrations caused by the local magnification effect. In the case of Cu-rich precipitates (Cu concentration 40-60 at% and radius 0.25-1.0 nm) in a bcc Fe-Si-Cu matrix, it is shown that the error in concentration is below 10 at% and the error in radius is <0.15 nm for all simulated conditions, provided that the correct value for dmax, as determined with the newly proposed method, is employed.

Effect of Heat Treatment Temperature on the Crystallization Behavior and Microstructural Evolution of Amorphous NbCo1.1Sn.

Heat treatment-induced nanocrystallization of amorphous precursors is a promising method for nanostructuring half-Heusler compounds as it holds significant potential in the fabrication of intricate and customizable nanostructured materials. To fully exploit these advantages, a comprehensive understanding of the crystallization behavior of amorphous precursors under different crystallization conditions is crucial. In this study, we investigated the crystallization behavior of the amorphous NbCo1.1Sn alloy at elevated temperatures (783 and 893 K) using transmission electron microscopy and atom probe tomography. As a result, heat treatment at 893 K resulted in a significantly finer grain structure than heat treatment at 783 K owing to the higher nucleation rate at 893 K. At both temperatures, the predominant phase was a half-Heusler phase, whereas the Heusler phase, associated with Co diffusion, was exclusively observed at the specimen annealed at 893 K. The Debye-Callaway model supports that the lower lattice thermal conductivity of NbCo1.1Sn annealed at 893 K is primarily attributed to the formation of Heusler nanoprecipitates rather than a finer grain size. The experimental findings of this study provide valuable insights into the nanocrystallization of amorphous alloys for enhancing thermoelectric properties.

Doubled strength and ductility via maraging effect and dynamic precipitate transformation in ultrastrong medium-entropy alloy.

Demands for ultrahigh strength in structural materials have been steadily increasing in response to environmental issues. Maraging alloys offer a high tensile strength and fracture toughness through a reduction of lattice defects and formation of intermetallic precipitates. The semi-coherent precipitates are crucial for exhibiting ultrahigh strength; however, they still result in limited work hardening and uniform ductility. Here, we demonstrate a strategy involving deformable semi-coherent precipitates and their dynamic phase transformation based on a narrow stability gap between two kinds of ordered phases. In a model medium-entropy alloy, the matrix precipitate acts as a dislocation barrier and also dislocation glide media; the grain-boundary precipitate further contributes to a significant work-hardening via dynamic precipitate transformation into the type of matrix precipitate. This combination results in a twofold enhancement of strength and uniform ductility, thus suggesting a promising alloy design concept for enhanced mechanical properties in developing various ultrastrong metallic materials.

Tuning the C1 /C2 Selectivity of Electrochemical CO2 Reduction on Cu-CeO2 Nanorods by Oxidation State Control.

Ceria (CeO2 ) is one of the most extensively used rare earth oxides. Recently, it has been used as a support material for metal catalysts for electrochemical energy conversion. However, to date, the nature of metal/CeO2 interfaces and their impact on electrochemical processes remains unclear. Here, a Cu-CeO2 nanorod electrochemical CO2 reduction catalyst is presented. Using operando analysis and computational techniques, it is found that, on the application of a reductive electrochemical potential, Cu undergoes an abrupt change in solubility in the ceria matrix converting from less stable randomly dissolved single atomic Cu2+ ions to (Cu0 ,Cu1+ ) nanoclusters. Unlike single atomic Cu, which produces C1 products as the main product during electrochemical CO2 reduction, the coexistence of (Cu0 ,Cu1+ ) clusters lowers the energy barrier for C-C coupling and enables the selective production of C2+ hydrocarbons. As a result, the coexistence of (Cu0 ,Cu1+ ) in the clusters at the Cu-ceria interface results in a C2+ partial current density/unit Cu weight 27 times that of a corresponding Cu-carbon catalyst under the same conditions.

Pulsed laser-assisted additive manufacturing of Ti-6Al-4V for in-situ grain refinement.

Metal additive manufacturing (AM) enables rapid customization of complex parts. However, it leads to forming of columnar grain structures which give the AM parts anisotropic properties. In this study, we propose a pulsed laser-assisted AM (PLAAM) technique for in-situ grain refinement of Ti-6Al-4V parts. A nanosecond pulsed laser was focused onto a melt pool to generate a favorable environment for the promotion of fine equiaxed grains. The PLAAM technique provided an average prior-ß grain size of 549.6 µm, compared to that of 1297 µm provided by the conventional AM technique. Moreover, the maximum value of multiples of uniform distribution of the ß phase decreased from 16 to 7.7 when using the PLAAM technique, which indicates a weakened crystallographic texture. These changes confirm that the proposed PLAAM technique promotes finer and more equiaxed prior-ß grains. Furthermore, because the proposed technique is a non-contact technique, it can be applied to existing processes without adjusting tool paths.

Atom-Scale Chemistry in Chalcopyrite-Based Photovoltaic Materials Visualized by Atom Probe Tomography.

Chalcopyrite-based materials for photovoltaic devices tend to exhibit complex structural imperfections originating from their polycrystalline nature; nevertheless, properly controlled devices are surprisingly irrelevant to them in terms of resulting device performances. The present work uses atom probe tomography to characterize co-evaporated high-quality Cu(In,Ga)Se2 (CIGS) films on flexible polyimide substrates either with or without doping with Na or doping with Na followed by K via a post-deposition treatment. The intent is to elucidate the unique characteristics of the grain boundaries (GBs) in CIGS, in particular the correlations/anti-correlations between matrix elements and the alkali dopants. Various compositional fluctuations are identified at GBs irrespective of the presence of alkali elements. However, [Cu-poor and Se/In,Ga-rich] GBs are significantly more common than [Cu-rich and Se/In,Ga-poor] ones. In addition, the anti-correlations between Cu and the other matrix elements are found to show not only regular trends among themselves but also the association with the degree of alkali segregation at GBs. The Na and K concentrations exhibited a correlation at the GBs but not in the intragrain regions. Density functional theory calculations are used to explain the compositional fluctuations and alkali segregation at the GBs. Our experimental and theoretical findings not only reveal the benign or beneficial characteristics of the GBs of CIGS but also provide a fundamental understanding of the GB chemistry in CIGS-based materials.

Three-dimensional atomic mapping of ligands on palladium nanoparticles by atom probe tomography.

Capping ligands are crucial to synthesizing colloidal nanoparticles with functional properties. However, the synergistic effect between different ligands and their distribution on crystallographic surfaces of nanoparticles during colloidal synthesis is still unclear despite powerful spectroscopic techniques, due to a lack of direct imaging techniques. In this study, atom probe tomography is adopted to investigate the three-dimensional atomic-scale distribution of two of the most common types of these ligands, cetrimonium (C19H42N) and halide (Br and Cl) ions, on Pd nanoparticles. The results, validated using density functional theory, demonstrate that the Br anions adsorbed on the nanoparticle surfaces promote the adsorption of the cetrimonium cations through electrostatic interactions, stabilizing the Pd {111} facets. In contrast, the Cl anions are not strongly adsorbed onto the Pd surfaces. The high density of adsorbed cetrimonium cations for Br anion additions results in the formation of multiple-twinned nanoparticles with superior oxidation resistance.

Shear band-driven precipitate dispersion for ultrastrong ductile medium-entropy alloys.

Precipitation strengthening has been the basis of physical metallurgy since more than 100 years owing to its excellent strengthening effects. This approach generally employs coherent and nano-sized precipitates, as incoherent precipitates energetically become coarse due to their incompatibility with matrix and provide a negligible strengthening effect or even cause brittleness. Here we propose a shear band-driven dispersion of nano-sized and semicoherent precipitates, which show significant strengthening effects. We add aluminum to a model CoNiV medium-entropy alloy with a face-centered cubic structure to form the L21 Heusler phase with an ordered body-centered cubic structure, as predicted by ab initio calculations. Micro-shear bands act as heterogeneous nucleation sites and generate finely dispersed intragranular precipitates with a semicoherent interface, which leads to a remarkable strength-ductility balance. This work suggests that the structurally dissimilar precipitates, which are generally avoided in conventional alloys, can be a useful design concept in developing high-strength ductile structural materials.

Reducing Time to Discovery: Materials and Molecular Modeling, Imaging, Informatics, and Integration.

Multiscale and multimodal imaging of material structures and properties provides solid ground on which materials theory and design can flourish. Recently, KAIST announced 10 flagship research fields, which include KAIST Materials Revolution: Materials and Molecular Modeling, Imaging, Informatics and Integration (M3I3). The M3I3 initiative aims to reduce the time for the discovery, design and development of materials based on elucidating multiscale processing-structure-property relationship and materials hierarchy, which are to be quantified and understood through a combination of machine learning and scientific insights. In this review, we begin by introducing recent progress on related initiatives around the globe, such as the Materials Genome Initiative (U.S.), Materials Informatics (U.S.), the Materials Project (U.S.), the Open Quantum Materials Database (U.S.), Materials Research by Information Integration Initiative (Japan), Novel Materials Discovery (E.U.), the NOMAD repository (E.U.), Materials Scientific Data Sharing Network (China), Vom Materials Zur Innovation (Germany), and Creative Materials Discovery (Korea), and discuss the role of multiscale materials and molecular imaging combined with machine learning in realizing the vision of M3I3. Specifically, microscopies using photons, electrons, and physical probes will be revisited with a focus on the multiscale structural hierarchy, as well as structure-property relationships. Additionally, data mining from the literature combined with machine learning will be shown to be more efficient in finding the future direction of materials structures with improved properties than the classical approach. Examples of materials for applications in energy and information will be reviewed and discussed. A case study on the development of a Ni-Co-Mn cathode materials illustrates M3I3's approach to creating libraries of multiscale structure-property-processing relationships. We end with a future outlook toward recent developments in the field of M3I3.

An assessment of the homogeneity of nano-crystalline Fe-Cu powders as studied by means of APT.

In this contribution the homogeneity of mechanically alloyed Fe-Cu powders for two different compositions (Fe-10 and Fe-2.5at%Cu) has been systematically characterised by atom probe tomography. Since Fe-Cu exhibits the Invar effect, it is among the most attractive systems for technical application. Furthermore, this system is immiscible and characterised by a large positive heat of mixing. In combination with the widespread application and accessibility, this predestines Fe-Cu as a binary model alloy to elaborate the enforced nonequilibrium enhanced solubility for immiscible systems. Depending on the parameters composition and milling time, results on the extension of the solubility limit and on the homogeneity of the alloy are presented, discussed and compared to earlier works. Only for the alloy with lower Cu content and for the prolonged milling time of 50h, chemical homogeneity of the sample as measured by the atom probe was fully reached on the nano-scale. For all other parameter combinations homogeneity could not be achieved, even for long milling times and for those samples that appear to be homogeneous via X-ray analysis. Moreover, impurities were determined, mostly stemming from the fabrication procedure. The arrangement and homogeneity of the most common impurity, oxygen, was evaluated from atom probe data for different samples. Thus, the local concentration, segregation effects and the distribution of impurities could be quantified on the nano-scale, depending on the different nominal compositions and processing parameters. Additionally, structural information could be gained employing transmission electron microscopy and diffraction measurements.