Scientific Events

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]

Microstructures in many engineering alloys are predominantly influenced by solid-state phase transformations that occur during industrial processing; these transformations almost always proceed by nucleation and growth. Quantitative modelling of the process often requires detailed knowledge of the interfaces, notably the interfacial energies that determine nucleation barriers and the interfacial mobilities that control growth kinetics—both of which depend sensitively on the the interfacial structures. Beyond their role in transformation kinetics, interfacial structures and the accompanying orientation relationships (ORs) and interface orientations (IOs) are microstructural features in their own right and directly influence bulk properties. Based on extensive studies of diverse alloy systems, we have formulated a unified framework that rationalises the preferred interfaces and their reproducible ORs produced by phase transformations, by employing preferred interfacial structures of two hierarchical levels. At the fine (atomic) level, the interface adopts a low-energy, periodically matched configuration that minimises the nucleation barrier. Such matching is possible only for specific intrinsic ORs and IOs, thereby imposing the geometric constraints. The structures of the coarse level are characterized by singular interfacial defects. Their development, preferred under given phase transformation conditions, allows the OR and IO to deviate within certain limits from the intrinsic values. This talk will present general methods for correlating ORs and IOs with interfacial structures at both levels and will illustrate the approach with examples from several material systems [more]

The importance of different length scales in materials science is well-recognized and subject of intense interdisciplinary research efforts. In these developments, multiscale modelling approaches take a key role as these enable the prediction of the effective material response based on detailed microstructure representations. Against this background, we focus on a scale-bridging understanding of macroscale material interfaces and on the influence of microscale interfaces on the effective properties of continua. We make use of classic energy-based homogenisation approaches, extend these to material interfaces, and demonstrate the usefulness of the proposed generalised multiscale formulations by comparison with experimental data. [more]

The defactant concept allows to predict why at higher temperature the formation energy of vacancies, dislocations and surfaces is no longer decreased by hydrogen, because it is not trapped to these defects any more. Thus failure due to hydrogen embrittlement is not present at high temperatures. At low temperatures the diffusion of hydrogen to defect generated by deformation will be reduced and, therefore, the decrease of defect formation energy by segregated hydrogen will not occur. Based on these scenarios equations for crack growth or strain to failure are derived and compared with experimental result for power law creep, stress-strain tests and fatigue. [more]

Solid-state batteries (SSBs) promise to meet the increasing demand for safe, high-power, and high-capacity energy storage. SSBs with solid electrolytes (SEs) offer potential advantages over conventional lithium-ion batteries with liquid electrolytes. Their performance, however, strongly depends on the structure and composition of the various interfaces contained in the different materials, which also change upon electrochemical cycling. We use Scanning transmission electron microscopy (STEM), to quantify properties of interfaces in battery materials. When compared to image simulations, the information on the sample structure and composition derived from STEM data can be quantitative. Combining STEM with a fast, pixelated detector allows for the acquisition of a full diffraction pattern at each scan point. From this, four-dimensional STEM (4D-STEM) datasets are available, which can be used to generate different data, e.g. annular dark field (ADF) as well as (annular) bright field ((A)BF) images, angular resolved STEM (ARSTEM) or differential phase contrast (DPC) data. With the example of cathode, anode and different SE materials for battery applications (e.g., NCM, Si, LLZO, LATP), we track the formation of different phases of and defects within the materials in dependence on synthesis as well as cycling conditions of the material and derive ABF as well as BF images from 4D datasets. These are used to also obtain difference images (ABF-BF). It will be shown that the composition of the materials and especially the Lithium content can be derived from the contrast of the different atomic columns in the structure. This is possible by comparing the experimental data sets to state of the art multi-slice simulations. This contribution will summarize the material science aspects of the energy materials investigated but also elucidate the potential of quantitative 4D-STEM to investigate materials. [more]

The increasing demand on lightweight structures requires high-strength materials. However, with increasing strength many materials show an increasing susceptibility to hydrogen embrittlement. Hence, it is of vital interest to understand the mechanisms of hydrogen embrittlement. [more]

Due to its high diffusivity hydrogen atoms alloy with metals even at room temperature. At this temperature, the materials microstructure remains rather stable. When the system size is reduced to the nano-scale, microstructural defects as well as mechanical stress significantly affect the thermodynamics and kinetics properties of the system.[1-6] Effects will be demonstrated on Niobium-H and Palladium-H thin films.Hydrogen absorption in metal systems commonly leads to lattice expansion. The lateral expansion is hindered when the metal adheres to a rigid substrate, as for thin films. Consequently, high mechanical stresses arise upon hydrogen uptake. In theory, these stresses can reach about -10 GPa for 1 H/M. Usually, metals cannot yield such high stresses and deform plastically. Thereby, maximum compressive mechanical stress of -2 to -3 GPa is commonly measured for 100 nm Nb thin films adhered to Sapphire substrates. It will be shown that phase transformations change in the coherency state upon film thickness reduction. The coherency state affects the nucleation and growth behaviour of the hydride phase as well as the kinetics of the phase transformation.[1] It will be further demonstrated that plastic deformation can be hindered and even suppressed upon film thickness reduction. In this case the system behaves purely elastic and ultra-high stress of about -10 GPa can be experimentally reached.[2] These high mechanical stresses result in changes of the materials thermodynamics. In the case of Nb-H thin films of less than 8 nm thickness, the common phase transformation from the α-phase solid solution to the hydride phase is completely suppressed, at 300 K.[3,4,5] The experimental results go in line with the σDOS model that includes microstructural and mechanical stress effects on the chemical potential [6]. [1] V. Burlaka, K. Nörthemann, A. Pundt, „Nb-H Thin Films: On Phase Transformation Kinetics“, Def. Diff. Forum 371 (2017) 160. [2] M. Hamm, V. Burlaka, S. Wagner, A. Pundt, “Achieving reversibility of ultra-high mechanical stress by hydrogen loading of thin films”, Appl. Phys. Letters 106 (2015) 243108. [3] S. Wagner, A. Pundt, “Quasi-thermodynamic model on hydride formation in palladium-hydrogen thin films: Impact of elastic and microstructural constraints “, Int. J. Hydrog. Energy 41 (2016) 2727. [4] V. Burlaka, S. Wagner, M. Hamm, A. Pundt, “Suppression of phase transformation in Nb-H thin films below switchover-thickness”, Nano Letters 16 (2016) 6207. [5] S. Wagner, P. Klose, V. Burlaka, K: Nörthemann, M. Hamm, A. Pundt, Structural Phase Transitions in Niobium Hydrogen Thin Films: Mechanical Stress, Phase Equilibria and Critical Temperatures, Chem. Phys. Chem. 20 (2019) 1890–1904. [6] S. Wagner, A. Pundt, Hydrogen as a probe for defects in materials: Isotherms and related microstructures of palladium-hydrogen thin films, AIMS Materials Science 7 (2020), 399–419. [more]