Scientific Events

High-strength steels with a body-centered cubic (bcc) crystal structure are generally expected to exhibit limited ductility at low temperatures due to the ductile-to-brittle transition. In this talk, we show that some high-strength bcc steels can nevertheless display unexpectedly large macroscopic plasticity during tensile deformation at cryogenic temperatures, even below their transition regime. A systematic tensile testing campaign across temperatures and stress states reveals strongly coupled effects on damage and fracture, which are captured using a mechanism-informed continuum damage model with implications for structural materials in extreme environments. [more]

Sintered copper (Cu) nanoparticles have emerged as a promising substitute for sintered silver (Ag) nanoparticles in power electronics packaging, offering comparable electrical and thermal conductivities, superior mechanical strength, and lower cost. However, the complex interactions between microstructure evolution, interfacial bonding, and mechanical performance during sintering remain insufficiently understood. This research investigates the mechanical behavior, fracture mechanisms, and reliability of sintered Cu nanoparticles through a combination of microscale experiments and multiscale modeling. The studies revealed the anisotropic fracture toughness of sintered Cu nanoparticles, developed an Anand viscoplastic model to describe high-temperature deformation, and quantified interfacial strength while elucidating the effects of oxidation on bonding quality. Furthermore, the influence of particle morphology on mechanical properties was examined using micro-cantilever bending tests and phase-field fracture simulations. Overall, this work advances the understanding of sintered Cu nanoparticles and supports the development of reliable and cost-effective interconnect materials for next-generation power electronics. [more]

Metallic nanoparticles are utilized in a growing number of applications due to their unique and tunable properties. However, one of the primary tools used to tune bulk metals properties, recrystallization, is yet to be used in the case of nanoparticles. We studied pristine, single crystal platinum nanoparticles during a recrystallization annealing after deformation. We found that deformation causes a dramatic change in particles orientation, while annealing induced a plethora of different particle behaviors. Microstructurally, nucleation of new grains was observed, but in the smallest particles these new grains were quickly absorbed back into the deformed matrix. We describe a phenomenological kinetic model to explain the strong correlation between the particle properties and their annealing behavior. [more]

Comprehensive investigations of material properties in crystalline systems require information spanning atomistic to continuum scales. Mesoscale models play a crucial role in this context. They enable the study of large systems and long timescales while retaining microscopic details relevant to the targeted applications. This presentation illustrates recent results on key aspects of microstructure evolution in crystalline systems obtained via newly developed mesoscale frameworks for defects and interfaces. Using a bicrystallography-respecting continuum model that incorporates parameters obtained from atomistic methods, we first discuss phenomena associated with disconnection-mediated grain boundary (GB) motion, such as GB faceting and grain rotation. Then, through a phase-field formulation of this model, we demonstrate that internal stresses generated by disconnection flow (shear coupling) induce significant deviations from classical curvature-driven grain growth in microstructures. This provides a compelling explanation for the lack of correlation between GB velocity and curvature observed in recent experiments, identifying its primary cause. In the final section, an overview of other mesoscale frameworks that build upon the phase-field crystal model and its coarse-graining is provided. Representative results are presented, including the effects of temperature cycling on GB motion, as well as an emerging general framework for analyzing defect dynamics in systems with microscopic order, which extends beyond conventional crystals. [more]

Topological defects—dislocations, grain boundaries, and related features—play an essential role in determining the properties of crystalline materials. When crystallite or functional domain sizes shrink to the nanometer scale, these defects become dominant. To date, however, neither bottom‑up nor top‑down synthesis has provided a reliable means of controlling them. Here, we demonstrate delicate control over shell epitaxy on nanocrystals within thin films, producing three‑dimensionally organized nanocrystallites with uniform grain boundaries and associated defects. In these structures, the resulting 3D‑patterned strain field can be mapped with atomic precision and tuned to introduce targeted dislocations or disclinations. Using multiscale crystallography and spectroscopy, we show that the uniformity and discreteness of these defects provide a clear correlation between local structure and collective electrochemical performance—specifically, catalytic activity in oxygen evolution and reduction reactions. Finally, we outline how this nanocrystallite‑engineering approach is guiding the design of next‑generation functional materials for energy nanotechnology [more]

Understanding the geometry of nanomaterials at the atomic scale provides critical insights into local structural heterogeneities and their impact on functional properties. Since shapes vary from particle to particle, detailed analysis at the single-particle level is essential. In this talk, I will present a high-throughput pipeline that integrates deep learning-based segmentation with quantitative shape analysis of individual nanoparticles from high-resolution transmission electron microscopy (HRTEM) images. First, I will describe the application of convolutional neural networks (CNNs) to segment 727 HRTEM micrographs of cubic Co₃O₄ nanoparticles, enabling the extraction of shape descriptors from 441,067 particles. This automated workflow allows for population-wide statistical characterization, bridging local structural detail with large-scale analysis. Second, I will present a size-resolved shape analysis at subnanometer precision, highlighting a critical threshold, “onset radius”, that marks transitions in particle shape, such as surface faceting and a shift from thermodynamic to kinetic growth regimes. This bottom-up approach illustrates how machine learning and data-driven analysis can reveal previously unquantified trends, offering a generalizable framework for high-throughput materials characterization. [more]

Recent groundbreaking developments in aberration-corrected scanning transmission electron microscopy (STEM) combined with advanced vibrational electron energy-loss spectroscopy (EELS) techniques have fundamentally transformed the way atomic-scale lattice dynamics and phonon behaviors are studied. In this seminar, I will highlight our seminal work in developing and applying state-of-the-art, spatially and momentum-resolved vibrational EELS methodologies to directly visualize phonon modes at atomic resolution. Our approach enables the unprecedented observation of localized phonon phenomena at individual defects, interfaces, and nanostructures, profoundly advancing our understanding of phonon-defect interactions, thermal boundary conductance, and electron–phonon coupling in materials. I will present key examples from our recent studies, including the direct imaging of defect-localized vibrational modes, nanoscale mapping of interfacial phonons, and quantification of phonon momentum distributions in quantum dots and phonon-electron coupling at superconducting interfaces. These insights provide critical foundations for addressing fundamental challenges in thermal management, quantum materials engineering, and solid-state ionic devices. Ultimately, our innovations offer powerful tools to elucidate and engineer the atomic-scale behaviors that dictate the performance of next-generation functional materials and systems. [more]