Mario Scheel

Investigating lithium-ion battery materials during overcharge-induced thermal runaway: an operando and multi-scale X-ray CT study

Catastrophic failure of lithium-ion batteries occurs across multiple length scales and over very short time periods. A combination of high-speed operando tomography, thermal imaging and electrochemical measurements is used to probe the degradation mechanisms leading up to overcharge-induced thermal runaway of a LiCoO2 pouch cell, through its interrelated dynamic structural, thermal and electrical responses. Failure mechanisms across multiple length scales are explored using a post-mortem multi-scale tomography approach, revealing significant morphological and phase changes in the LiCoO2 electrode microstructure and location dependent degradation. This combined operando and multi-scale X-ray computed tomography (CT) technique is demonstrated as a comprehensive approach to understanding battery degradation and failure.

The role of local instabilities in fluid invasion into permeable media

Wettability is an important factor which controls the displacement of immiscible fluids in permeable media, with far reaching implications for storage of CO2 in deep saline aquifers, fuel cells, oil recovery, and for the remediation of oil contaminated soils. Considering the paradigmatic case of random piles of spherical beads, fluid front morphologies emerging during slow immiscible displacement are investigated in real time by X-ray micro–tomography and quantitatively compared with model predictions. Controlled by the wettability of the bead matrix two distinct displacement patterns are found. A compact front morphology emerges if the invading fluid wets the beads while a fingered morphology is found for non–wetting invading fluids, causing the residual amount of defending fluid to differ by one order of magnitude. The corresponding crossover between these two regimes in terms of the advancing contact angle is governed by an interplay of wettability and pore geometry and can be predicted on the basis of a purely quasi–static consideration of local instabilities that control the progression of the invading interface.

Ultra Fast Tomography: New Developments for 4D Studies in Material Science

X-ray tomography has become a widely used 3D characterization technique in materials science using either laboratory tomographs or large X-ray facilities. The two main improvements in the last decade are the decrease of the spatial resolution down to tens of nanometers and also the decrease in acquisition time of a complete scan down to 1 second with 2 µm spatial resolution. The aim of this presentation is to focus on the second point. We will present the technical problems arising due to ultra fast acquisition (development of specific sample environment) and its application in material science. We will show, that thanks to ultra fast tomography, it is now possible to investigate material science problems in 4D (crack propagation in metals and high temperature deformation).

Electromigration Mechanism of Failure in Flip-Chip Solder Joints Based on Discrete Void Formation

In this investigation, SnAgCu and SN100C solders were electromigration (EM) tested, and the 3D laminography imaging technique was employed for in-situ observation of the microstructure evolution during testing. We found that discrete voids nucleate, grow and coalesce along the intermetallic compound/solder interface during EM testing. A systematic analysis yields quantitative information on the number, volume, and growth rate of voids, and the EM parameter of DZ*. We observe that fast intrinsic diffusion in SnAgCu solder causes void growth and coalescence, while in the SN100C solder this coalescence was not significant. To deduce the current density distribution, finite-element models were constructed on the basis of the laminography images. The discrete voids do not change the global current density distribution, but they induce the local current crowding around the voids: this local current crowding enhances the lateral void growth and coalescence. The correlation between the current density and the probability of void formation indicates that a threshold current density exists for the activation of void formation. There is a significant increase in the probability of void formation when the current density exceeds half of the maximum value.

Real-Time Hard X-ray Imaging

Using hard X-rays for high-speed and ultra high-speed imaging has enormous potential to visualize the interior of opaque systems as they change with time. Exposure times below one nanosecond for ultra high-speed imaging are accessible when synchrotron light sources are employed and this provides a non-destructive method of in-motion radiography. The polychromatic radiation of insertion devices in combination with X-ray phase contrast has proven to be suited for acquisition rates up to the MHz range. This chapter outlines the basic principles of indirect hard X-ray imaging detectors for real-time imaging, and other detection schemes and sources of radiation are briefly discussed. The potential of using hard X-rays for high-speed imaging is demonstrated with application examples from soft matter physics and materials processing.