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Atomic Scale Defects: Structure and Function

Periodic Reporting for period 3 - AtoFun (Atomic Scale Defects: Structure and Function)

Reporting period: 2020-03-01 to 2021-08-31

Atomic scale defects play a key role in determining the behaviour of all crystalline materials, profoundly modifying mechanical, thermal and electrical properties. Many current technological applications make do with phenomenological descriptions of these effects; yet myriad intriguing questions about the fundamental link between defect structure and material function remain.

Transmission electron microscopy revolutionised the study of atomic scale defects by enabling their direct imaging. The novel coherent X-ray diffraction techniques developed in this project promise a similar advancement, making it possible to probe the strain fields that govern defect interactions in 3D with high spatial resolution (<10 nm). They will allow us to clarify the effect of impurities and retained gas on dislocation strain fields, shedding light on opportunities to engineer dislocation properties. The exceptional strain sensitivity of coherent diffraction will enable us to explore the fundamental mechanisms governing the behaviour of ion-implantation-induced point defects that are invisible to TEM. While we concentrate on dislocations and point defects, the new techniques will apply to all crystalline materials where defects are important. Our characterisation of defect structure will be combined with laser transient grating measurements of thermal transport changes due to specific defect populations. This unique multifaceted perspective of defect behaviour will transform our ability to devise modelling approaches linking defect structure to material function.

A deep, fundamental understanding of atomic scale defects and their effect on material function is an essential prerequisite for exploiting and engineering defects to enhance material properties for next generation power generation, energy storage and transport applications.
We have developed new coherent diffraction approaches that make it possible to probe with 20 nm 3D spatial resolution the full displacement field within micro-crystals. Using these techniques, we have studied the damage caused by focussed ion beam milling – a technique used extensively to prepare samples at the micro-and nano-scale. Importantly this has allowed us to identify approaches for effectively “cleaning up” FIB milling damage.

In parallel we have developed a new laser transient grating setup that allows high accuracy measurement of elastic properties and thermal transport at the micro- and nano-scale. Initial validation experiments have shown excellent performance of the setup and we are now looking forward to using it to examine the changes caused by crystal defects.
Our coherent X-ray diffraction work has shed unprecedented light on the complicated changes brought about by focussed ion beam milling. Importantly this technique is used extensively to prepare myriad samples, with many researchers not realising the profound changes it causes in the material. Our new research has put a spotlight on these changes and given the research community a tool to study and understand FIB damage in 3D. Looking ahead we will extend these coherent diffraction approaches to be able to study more complex objects and defect structure is “real”, rather than prototypical, materials.
Coherent X-ray diffraction patterns from a tungsten micro-crystal containing dislocations