1,721,384 research outputs found
Crystal structure of submicron-sized sulfur particles using 3D ED obtained in atmospheric conditions
No full textLithium-sulfur batteries are a promising candidate for the next generation of rechargeable batteries. Despite extensive research on this system over the last decade, a complete understanding of the phase transformations has remained elusive. Conventional in-situ powder X-ray diffraction has struggled to determine the unit cell and space group of the polysulfides formed during charge and discharge cycles due to the high solubility of these solid products in the liquid electrolyte. With the improvement in in-situ electrochemical set-ups dedicated to transmission electron microscopes, three-dimensional electron diffraction (3D ED) has the potential to capture the crystal structures of the polysulfides during cycling. In this work, the structure solution and refinement from 3D ED data of elemental sulfur, known to sublimate in the vacuum of transmission electron microscopes, is enabled through the use of an environmental cell with a micro-electromechanical system. This work represents the first step in characterizing sulfur's transformation into lithium polysulfides using in-situ 3D ED.Funding for this research was provided by: AgentschapInnoveren en Ondernemen (grant No. HBC.2021.0016); FondsWetenschappelijk Onderzoek (grant No. BOF TOP 38689);H2020 Marie Skłodowska-Curie Actions (grant No. 956099);Hercules fund (grant No. FWO I003218N).
Acknowledgements
The authors acknowledge the support by SIM (StrategicInitiative Materials in Flanders) and VLAIO (Flemishgovernment agency Flanders Innovation and Entrepreneur-ship) within the SBO project FuGels in the SIM researchprogram SIMBA (Sustainable and Innovative Materials forBatteries). Further financial support is acknowledged from theHercules fund and the University of Antwerp. This project hasalso received funding from the European Union’s Horizon2020 research and innovation program under a Marie-Sklo-dowska-Curie grant agreement. The authors are grateful toSaleh Gholam for the in-house scripting of the microscope forcontinuous rotation data acquisition and pre-process datahandling
Study of the effect of cation substitution on the local structure and the properties of perovskites and Li-ion battery cathode materials
No full textAbstract: The type and arrangement of the atoms in a material, called the crystal structure, determine its physical properties. Thus to gain control over materials properties, the first step is to determine these crystal structures. During my PhD, I have investigated the crystal structures of two main groups of materials, perovskites and Li-ion battery cathode materials, to which cation substitution was applied in an attempt to induce and enhance specific properties. For the perovskites, which can have a wide variety of properties depending on small structural changes, we focussed on the magnetic properties, of great importance in nano-electronics. With the Li-ion batteries, we tried to find new cathode materials with enhanced electrochemical performance. The crystal structures of these new materials were investigated using advanced transmission electron microscopy (TEM) in both reciprocal space (electron diffraction) and direct space (imaging techniques), combined with energy-dispersive X-ray spectroscopy (EDX). Based on these studies, we proposed and refined models for the new structures. With these models, we were able to explain the variations in the properties of these materials. The study of the relation between structure and properties has yielded fundamental knowledge, applicable for the optimization of the properties of the investigated materials as well as of related materials
In situ 3D electron diffraction to investigate redox reactions of perovskite-based energy materials
No full textAbstract: Climate change is one of the biggest challenges for science in the twenty-first century, and the development of new technologies that lower the carbon footprint has never been more pressing. Many of these alternative energy applications involve inorganic crystalline solids exposed to redox conditions. In this research area, the development of in situ 3D electron diffraction at elevated temperatures in gas and vacuum unlocks great new potential, as it can unravel atomic structure transformations in operational conditions. That way, we can investigate the origins of e.g. high performance and degradation properties. A particularly interesting group of energy materials is based on the perovskite structure, with ABX3 as chemical formula. This project studied the influence of high temperature reducing atmospheres on the atomic structure of two perovskite-based energy materials : LaxSr2 12xMnO4 12\u1d6ff and (Ca,Sr)(Fe,Mn)O3 12\u1d6ff. The first group of materials \u2013 i.e. Ruddlesden-Popper LaxSr2 12xMnO4 12\u1d6ff \u2013 have good performance properties as symmetric solid oxide fuel cell electrodes. With 3D electron diffraction, we found new incommensurately modulated structures for La0.25Sr1.75MnO4 12\u1d6ff and La0.5Sr1.5MnO4 12\u1d6ff upon annealing in hydrogen gas, which have never been reported before. Further, we discovered differences in defect structure and ordering that can be linked to previously unexplained differences in electrical conductivity with lanthanum concentration. Next, (Ca,Sr)(Fe,Mn)O3 12\u1d6ff was studied, which is an oxygen carrier in the CLOU process. This is a carbon capturing combustion technique that inherently separates expelled CO2 from air. The performance properties of CaMnO3 12\u1d6ff can be improved by A or B site doping with e.g. strontium or iron. Using in situ 3D electron diffraction, we discovered that this is connected to differences in ordering of oxygen vacancies for the undoped versus doped materials. Further, we could successfully solve and refine the structure of CaMnO2.75 for the first time ever. However, at the highest temperatures during in situ experiments in gas atmospheres, a reaction of the samples with the Si3N4 heating chips gave rise to a SiO2 shell around the crystals. This caused experimental limitations and discrepancies between in situ and ex situ annealing results. Therefore, a systematic investigation was performed into the potential of a protective graphene coating. Still, this research illustrated how a combination of various in situ and ex situ electron microscopy techniques led to new insights in the structure-property relations of redox-based energy materials
Topotactic oxidative and reductive control of the structures and properties of layered manganese oxychalcogenides
No full textTopotactic modification, by both oxidation and reduction, of the composition, structures, and magnetic properties of the layered oxychalcogenides Sr(4)Mn(3)O(7.5)CU(2)Ch(2) (Ch = S, Se) is described. These Mn3+ compounds are composed of alternating perovskite-type strontium manganese oxide slabs separated by anti-fluorite-type copper chalcogenide layers and are intrinsically oxide deficient in the central layer of the perovskite slabs. The systems are unusual examples of perovskite-related compounds that may topotactically be both oxidized by fluorination and reduced by deintercalation of oxygen from the oxide-deficient part of the structure. The compounds exhibit antiferromagnetic ordering of the manganese magnetic moments in the outer layers of the perovskite slabs, while the other moments, in the central layers, exhibit spin-glass-like behavior. Fluorination has the effect of increasing the antiferromagnetic ordering temperature and the size of the ordered moment, whereas reduction destroys magnetic long-range order by introducing chemical disorder which leads to both further disorder and frustration of the magnetic interactions in the manganese oxide slab
Refining short-range order parameters from diffuse electron scattering
No full textAbstract: Electrons, X-rays and neutrons that pass through a thin crystalline sample will be diffracted. Diffraction patterns of crystalline materials contain Bragg reflections (sharp discrete intensity maxima) and diffuse scattering (a weak continuous background). The Bragg reflections contain information about the average crystal structure (the type of atoms and the average atomic positions), whereas the diffuse scattering contains information about the short-range order (deviations from the average crystal structure that are ordered on a local scale). Because the properties of many materials depend on the short-range order, refining short-range order parameters is essential for understanding and optimizing material properties. The refinement of short-range order parameters has previously been applied to the diffuse scattering in single-crystal X-ray and single-crystal neutron diffraction data but not yet to the diffuse scattering in single-crystal electron diffraction data. In this work, we will verify the possibility to refine short-range order parameters from the diffuse scattering in single-crystal electron diffraction data. Electron diffraction allows to acquire data on submicron-sized crystals, which are too small to be investigated with single-crystal X-ray and single-crystal neutron diffraction. In the first part of this work, we will refine short-range order parameters from the one-dimensional diffuse scattering in electron diffraction data acquired on the lithium-ion battery cathode material Li1.2Ni0.13Mn0.54Co0.13O2. The number of stacking faults and the twin percentages will be refined from the diffuse scattering using a Monte Carlo refinement. We will also describe a method to determine the spinel/layered phase ratio from the intensities of the Bragg reflections in electron diffraction data. In the second part of this work, we will refine short-range order parameters from the three-dimensional diffuse scattering in both single-crystal electron and single-crystal X-ray diffraction data acquired on Nb0.84CoSb. The correlations between neighbouring vacancies and the displacements of Sb and Co atoms will be refined from the diffuse scattering using a Monte Carlo refinement and a three-dimensional difference pair distribution function refinement. The effect of different experimental parameters on the spatial resolution of the observed diffuse scattering will also be investigated. Finally, the model of the short-range Nb-vacancy order in Nb0.84CoSb will also be applied to LiNi0.5Sn0.3Co0.2O2
Characterization of defects, modulations and surface layers in topological insulators and structurally related compounds
No full textAbstract: Topological insulators, a new class of fascinating materials, are ideally bulk insulating and surface conducting. They are intensely studied due to their special physics and possible future applications, such as in spintronics. Adding defects (point-, line-, planar- and 3D defects), modulating the structure or interfacing the material with another material is a common practice to obtain the desired properties for applications. In this thesis, different kinds of adaptations of topological insulators and structurally related materials were studied. The atomic structure of the materials was determined, because this will affect the properties of the material. Different transmission electron microscopy techniques, complemented with first-principles calculations using the VASP code, were used to characterize the structures. The first part of the thesis concerns the characterization of the bulk structure of topological insulators. Bi2Se3 and (Bi1-xInx)2Se3 showed atomic mobility around and across the van der Waals gap between the quintuple layers. The topological insulator-normal insulator Sb2(Te1-xSex)3 showed a different substitution order than reported in literature. Some members of the GemBi2nTe(m+3n) series were trigonal layered structures with l-layered (l=7,9,11,5-7) building blocks instead of the five-layered building blocks for Bi2Se3 and others were rock salt structures with planar defects. The second part of the thesis reveals the structure and chemical composition of the oxidized layers and sublayers of Bi2Te3, Sb2Te3, (Sb0.55Bi0.45)2Te3 and GemBi2nTe(m+3n) and oxidation mechanisms were proposed. Also the structure and chemical composition of the interface between the (approximately) 20 nm thick Fe layer and Bi2Te3 is shown, unfolding the intermediate 3.5 nm amorphous FeTe interface layer, where excessive Bi migrates to the shallow bulk forming septuple layers of Bi3Te4. In the last part of the thesis, the bulk structures of two structurally related materials, \u3b1-GeTe and Fe2Ge3, were solved: \u3b1-GeTe, showed a planar defect structure, while Fe2Ge3 was incommensurately modulated
Quantitative electron diffraction tomography for structure characterization of cathode materials for Li-ion batteries
No full textSingle phase and composite multiferroic thin films and heterostructures from aqueous chemical solution deposition
No full textMagnetoelectric multiferroics as materials with coexistence of two or more ferroic properties and coupling between the ferroic orders present not only the vast playground for different areas of fundamental research but they also have a great potential for memory and logic device applications. Offering possibilities towards miniaturization of electronic devices and at the same time increasing their performances, these multifunctional materials, especially in thin film form, have received enormous interest from the scientific community in the past decade. However, multiferroic materials with room temperature functionalities still remain a challenge. Besides the weak magnetoelectric coupling, bismuth ferrite (BiFeO3), currently the only known intrinsic multiferroic at room temperature is also facing issues with phase formation and thermal stability. Contradictory reports on phase purity and thermal stability of this compound have resulted in many open questions whereby some of them are related to the processing conditions of BiFeO3 thin films at elevated temperature. On the other hand, the composite multiferroics combining ferroelectric and ferri/ferromagnetic phases are considered as a promising alternative to single-phase multiferroic, although the new compositions and structures that would result in a material with enhanced magnetoelectric properties are still appealing. The question that we address throughout this work is how we can correlate synthesis, processing, structure and final properties of multiferroic materials. The main aim of the work presented in this thesis was to deposit and investigate different single phase as well as composite multiferroic thin films and heterostructures using the aqueous chemical solution deposition method as costeffective and easily accessible way. Furthermore, the additional aim was to contribute to understanding the phase formation and stability of BiFeO3 film depending on different processing parameters where particular emphasis was placed on the appropriate choice of the underlying substrate. The second part of the conducted work aims for creating self-organized composite heterostructures with various constituent phases using the flexibility of solution chemistry and further to investigate the potential of such solution-derived nanostructures in the field of multiferroics.Magnetoelektrische multiferroïcs zijn materialen waarbij gelijktijdig een ferro elektrisch en (anti)ferro magnetisch moment optreedt en waarbij er een koppeling is tussen beide momenten zodat de magnetisatiegraad met een extern elektrisch veld kan gecontroleerd worden en de polarisatiegraad met een extern magnetisch veld. Daarom zijn deze materialen erg interessant voor toepassingen in multifunctionele onderdelen waar één enkel element verschillende taken moet kan vervullen, hetgeen dan leidt tot miniaturisatie van elektronica in combinatie met verhoogde prestaties. In de uitdaging om multiferroïsche materialen te ontwikkelen op kamertemperatuur heeft het onderzoek twee richtingen genomen. De eerste richting spits zich toe op de intrinsieke enkelfasige multiferroïca waar dunne films bestaande uit bismut ferraat (BiFeO3) zeer veelbelovend zijn. De tweede richting ontwikkelt composiet multiferroïca met aparte ferro elektrische en magnetische fasen. Bismut ferraat kent echter problemen naar fasevorming, fasezuiverheid en temperatuursstabiliteit die een negatieve invloed hebben naar de uiteindelijke materiaaleigenschappen en de toepassingsgeschiktheid van het gesynthetiseerde materiaal. Bovendien is het aantal alternatieven voor enkelfasige intrinsieke multiferroïcs die actief zijn bij kamertemperatuur zeer schaars zodat steeds meer nadruk komt te liggen op het ontwikkelen van nieuwe composieten en nieuwe mengstructuren met verbeterde magnetoelektrische eigenschappen. In deze thesis wordt uitgegaan van waterige oplossingen als goedkoop en milieuvriendelijk alternatief voor de afzetting van dunne films van bismut ferraat alsook van composieten. In het eerste deel van het project ligt de nadruk op het begrijpen van de invloed van de verschillende procesparameters en het onderliggende substraat op de fasenvorming en temperatuursstabiliteit van de BiFeO3 dunne films. In het tweede deel worden composietstructuren gecreëerd met behulp van zelfassemblage en de flexibiliteit van natchemische depositie en wordt het potentieel van natchemisch gedeponeerde nanostructuren in de multiferroïca onderzocht
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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