Energy dependent structure of Xe ion tracks in YBCO and the effect on the superconductive properties in magnetic fields

The morphology and structure of damaged regions (tracks) produced by swift heavy 167, 77, and 46 MeV 132Xe23+ ions in YBCO-based second generation industrial high temperature superconductors wires (2G HTS) were studied using transmission electron microscopy. It was shown that ions produce tracks aligned along the ion trajectory but of morphology depending on their energy: continuous, nearly continuous, or discontinuous tracks like prolate ellipsoids of 10–15 nm in length and spheroids of 5 nm in diameter. The damaged regions of about 5 nm in diameter contain an amorphous material with a lower density compared to the pristine YBCO. The material density drops from 6300 kgm−3 for YBCO matrix to 3600 kgm−3 inside the tracks. Barium enrichment was found in the vicinity of the track at a distance of about 10 nm from the center of the defect. Ion damage tracks with different morphologies showed different effectivenesses as pinning centers. Tracks composed of discontinuous pinning centers—spheroids of about 5 nm in diameter—bring the best enhancement of the critical current density. It occurs for the lowest ion energy (46 MeV) in the range of energy loss of 8.9 keV/nm–4.7 keV/nm for Xe ions. The samples showed highest critical current densities of 56 MA/cm2 (4.2 K) and 3 MA/cm2 (77 K) in self-field, while in magnetic fields of 8 T, the values of 17 MA/cm2 (4.2 K) and 1.6 MA/cm2 (77 K) were achieved. The reduction of the superconducting volume corresponding to the amorphous radiation defects did not exceed 4% from the total sample volume.

Characterization at the Atomistic Level of Defective Structures in Complex Materials

Piyush Agrawal

Defects are key to enhance or deploy particular materials properties. In this thesis I present analyses of the impact of defects on the electronic structure of materials using combined experimental and theoretical Electron energy loss spectroscopy (EELS) in (Scanning) Transmission Electron Microscopy. The energy loss near-edge structure (ELNES) in EELS reflects the element-specific electronic structure providing insights into bonding characteristics of individual atomic species. New electron optical devices have boosted the analytical capabilities by which materials can be investigated with atomic resolution and single atom sensitivity using (scanning) transmission electron spectroscopy (STEM/TEM).With the help of aberration correctors for forming small electron probes, high intensity electron beams can nowadays be focused to clearly less than 100 pm which has enhanced the resolution and sensitivity in analytical scanning transmission electron microscopy (STEM). Various kinds of defects in different complex oxides were studied: point defects like oxygen vacancies in BiVO4 and SrMnO3, edge-dislocations in BiFeO3, and planar defects in GaAs. By comparison with experimental data, structures for these systems were proposed based on all-electron density functional theory (DFT) code Wien2k. By comparing theoretical calculations and experimental data, a pronounced surface reduction in the oxidation state of vanadium in BiVO4 from +5 to +4 was unveiled, which is due to a high density of oxygen vacancies, and its importance in potential application of BiVO4 in photoelectrochemical energy conversion. A similar study was performed on a series of SrMnO3 thin films with different epitaxial strain where theoretical investigations revealed the impact of oxygen non-stoichiometry and strain on the O-K ELNES. In the next study, molecular dynamics simulations were combined with FEFF-based EELS calculations and its comparison with experiments was helpful for the correct prediction of the edge dislocation core structure in BiFeO3. This study also confirmed the presence of Fe atoms in the core of the edge dislocation which possibly makes these defects ferromagnetic whereas the bulk structure is known to be antiferromagnetic. This thesis has established methodologies for utilizing different codes, illustrating how links between experimental and theoretical ELNES can be used in revealing structural information around defects and how defects affect materials properties. This tandem methodology of theory and experiments is applicable to various future materials where the reliable interpretation of EELS data is pivotal in unfolding mysteries of such technologically important materials.

EPFL2017

Transmission electron microscopy study of the β′ phase (Al-Mg-Si alloys) and QC phase (Al-Cu-Mg-Si alloys): ordering mechanism and crystallographic structure

Philippe Buffat, Cyril Cayron

The β′ and QC rod-shaped precipitates, observed in the Al-Mg-Si and Al-Cu-Mg-Si alloys, respectively, are studied by transmission electron microscopy (TEM). Superstructure images and microdiffraction patterns confirm the likeness between the β′ and QC phases. The microdiffraction patterns are compared with computed ones in the aim to refine a previous structural model based on the existence of a latent disordered phase QP constituted by sub-unit triangular clusters. The space group of the β′ and QC phase is found to be hexagonal P6̄2m. Besides its smaller hexagonal parameter (a = 6.7 nm for QC and a = 0.71 nm for β′), the QC structure differs from the β′ one due to the Cu atoms contained in addition to the Si atoms in the triangular sites of the clusters. The phase transition of QC (or β′) from the latent phase QP (or βP) is concluded to be displacive. The complete phase transition (QP → QC → Q) or (βP → β′ → B′) during the elaboration of the materials is discussed.

2000

Electron probing of oxygen-evolving oxide catalysts

Tzu-Hsien Shen

Water splitting is one of the cleanest ways to store energy. The production of hydrogen and oxygen gases can be utilized in fuel cells to generate electricity, power, and heat. In the water splitting process, the oxygen evolution reaction (OER), taking place at the anode, is the sluggish reaction limiting the efficiency. Electrocatalysts are used to reduce the kinetic barrier of OER. Development of high-performance and stable OER catalysts requires fundamental understanding of their electrocatalytic properties and, hence, their holistic characterization is indispensable. The objective of this thesis is to probe Co-based oxide catalysts for OER in an alkaline medium using advanced (scanning) transmission electron microscopy ((S)TEM) techniques. Starting with the surface characterization of a highly active OER oxide catalyst, Ba0.5Sr0.5Co0.8Fe0.2O3-d (BSCF), it is shown that the surfaces of the as-synthesized BSCF particles are Co/Fe rich and adopt a spinel-like structure with a reduced valence of Co ions. The Co/Fe spinel structure is further linked to the formation of the highly active Co(Fe)OOH at the BSCF surface. Further real-time probing of BSCF oxide catalysts using electrochemical liquid-cell TEM shows a switchable wetting behavior of Co-based oxide catalysts by analyzing the liquid movement around oxide particles. The interfacial interactions at the surface-electrolyte under potential cycling are correlated to electrowetting and hydrophilic surface transformation. Molecular oxygen evolution is detected qualitatively under OER conditions by identifying the O2 feature in electron energy-loss spectra. Quantification of the O2 using electron energy-loss spectroscopy is also demonstrated, pending further improvements.The work herein offers new insights into the characterization framework of oxide catalysts for OER using (S)TEM. The bulk to surface structure of the catalysts can be analyzed, and the interfacial interactions of single-particle catalysts can be probed in real-time. The findings aid in understanding the performance and stability of oxygen-evolving oxide catalysts with further prospects envisioned towards a complete quantitative probing of electrocatalytic reactions in real-time. Ultimately, quantitative characterization in electrochemical liquid-phase (S)TEM can provide understanding of surface-active sites and reaction kinetics of oxygen evolution catalysts.