Scientific Events

Two-dimensional (2D) materials, such as Graphene, hBN, and MoS₂, are promising candidates in a number of advanced functional and structural applications owing to their exceptional electrical, thermal, and mechanical properties. Understanding the mechanical properties of 2D materials is critically important for their reliable integration into future electronic, composite, and energy storage applications. In this talk, we will report our efforts to study the fracture behaviors of 2D materials. Our combined experiment and modelling efforts verify the applicability of the classic Griffith theory of brittle fracture to graphene [1]. Strategies on how to improve the fracture resistance in graphene, including a nanocomposite approach, and the implications of the effects of defects on mechanical properties of other 2D atomic layers will be discussed [2, 3]. More interestingly, stable crack propagation in monolayer 2D h-BN is observed and the corresponding crack resistance curve is obtained for the first time in 2D crystals [4]. Inspired by the asymmetric lattice structure of h-BN, an intrinsic toughening mechanism without loss of high strength is validated based on theoretical efforts, enabling stable crack propagation not seen in graphene. Finally, we will also discuss some of our recent efforts in evaluating the mechanical properties of 2D covalent organic frameworks (COFs) [5, 6] and the fracture behaviors of ultrathin van der Waals solids [7] [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]

Abstract: Some of the most pressing challenges in engineering science arise when materials are adjacent to one another - from the bottom of an impacting droplet to the separating faces of a crack. Here I will discuss two vignettes on these important topics: first, a calibrated, nano-scale, direct measurement of the intervening air film at the critical juncture of contact formation during droplet impact, and second, the toughening that can result from geometric complexity at the tip of a propagating crack. These seemingly disparate systems are deeply connected on a variety of levels, from their sensitivity to defects to the propagating singularity that defines the mathematical problem of contact line and crack propagation. The talk will conclude with a discussion of perspectives and some puzzles that remain open despite recent progress. [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]

Half-Heusler compounds exhibit significant potential in thermoelectric applications for power generation up to 1000 K, notwithstanding the substantial challenges posed by the cost of constituent elements and the imperative to augment the average thermoelectric figure-of-merit (zT_ave) for more practical applications. Overcoming these obstacles demands the advancement of high-performance p-type TaFeSb thermoelectric materials with diminished Ta content. This investigation systematically explores the quaternary-phase space encompassing Ta, Nb, V, and Ti to ascertain an optimal composition, namely Ta_0.42Nb_0.3V_0.15Ti_0.13FeSb. This composition is characterized by a remarkable reduction in Ta concentration coupled with an enhancement in zT, peaking at 1.23 at 973 K. Moreover, the integration of a high-mobility secondary phase, InSb, fosters enhancements in both the Seebeck coefficient and electrical conductivity, resulting in a 23% augmentation in the average power factor, while concurrently suppressing lattice thermal conductivity. The optimized composite, Ta_0.42Nb_0.3V_0.15Ti_0.13FeSb-(InSb)_0.015, achieves a peak zT value of 1.43 at 973 K and a zT_ave of 1 from 300 K to 973 K, thereby setting a precedent among p-type half-Heusler materials. Additionally, a single-leg device demonstrates a peak efficiency of approximately 8% under a temperature difference of 823 K vs. 303 K. These findings underscore the substantial potential of the proposed material design and fabrication methodologies in fostering efficient and sustainable thermoelectric applications. [more]

High entropy materials are materials with 5 or more principle components within crystalline lattices. High entropy metal chalcogenides are a recent (since 2016) development in this area that have shown exceptional promise in both thermoelectric energy conversion and electrocatalysis. However, these materials remain limited in both compositional range and characterisation. In this talk the compositional boundaries of high entropy metal sulfides will be explored, along with advanced and comprehensive characterisation techniques and our initial explorations into acidic hydrogen evolution electrocatalysis. [more]

Thermoelectric materials can realize waste heat recovery and solid-state refrigeration, providing sustainable solutions to the energy crisis and environmental pollution. The performance of thermoelectric materials is gauged by the transport of electrons and phonons. Materials with fast electron movement but slow phonon propagation would be ideal thermoelectrics. These transport behaviors of carriers are influenced by the intrinsic chemical bonding mechanism and structural defects of materials. Understanding the bonding and microstructures of materials is of paramount importance to improve their thermoelectric properties. Atom probe tomography (APT) provides a unique combination of characterizing chemical bonding and lattice defects, being a very powerful tool in the study of thermoelectric materials. It has been revealed that many of the high-performance thermoelectric chalcogenides utilize metavalent bonding (MVB), which can be distinguished from other bonding mechanisms by the unconventionally high value (>60%) of “probability of multiple events (PME)” measured by APT. Thus, many new compounds with high thermoelectric performance can be designed by tailoring their chemical bonds, and APT is an indispensable technique to corroborate the bonding transition. Owing to the high spatial and chemical resolution of APT, the local change of chemical bonding at defects such as grain boundaries and precipitates can also be detected by APT. This enables us to better understand the role of chemical bonding in regulating the electron and phonon transport across individual defects. The results in turn provide insights into the tailoring of thermoelectric properties by manipulating the local chemical bonds. For example, the thermoelectric properties of polycrystalline SnSe have been significantly improved by removing the stiff Sn-O bonds at the grain boundary as directly observed by APT. In contrast, the strong bonding connection between thermoelectric and interfacial materials enables a high-efficiency and durable thermoelectric device. Based on APT, in conjunction with other characterization techniques, we can explore some uncharted territories in the design of thermoelectric materials. [more]