Thickening of T1 Precipitates during Aging of a High Purity Al−4Cu−1Li−0.25Mn Alloy

The age hardening response of a high-purity Al–4Cu–1Li–0.25Mn alloy (wt. %) during isothermal aging without and with an applied external load was investigated. Plate shaped nanometer size T1 (Al2CuLi) and θ′ (Al2Cu) hardening phases were formed. The precipitates were analyzed with respect to the development of their structure, size, number density, volume fraction and associated transformation strains by conducting transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) studies in combination with geometrical phase analysis (GPA). Special attention was paid to the thickening of T1 phase. Two elementary types of single-layer T1 precipitate, one with a Li-rich (Type 1) and another with an Al-rich (Defect Type 1) central layer, were identified. The results show that the Defect Type 1 structure can act as a precursor for the Type 1 structure. The thickening of T1 precipitates occurs by alternative stacking of these two elementary structures. The thickening mechanism was analyzed based on the magnitude of strain associated with the precipitation transformation normal to its habit plane. Long-term aging and aging under load resulted in thicker and structurally defected T1 precipitates. Several types of defected precipitates were characterized and discussed. For θ′ precipitates, a ledge mechanism of thickening was observed. Compared to the normal aging, an external load applied to the peak aged state leads to small variations in the average sizes and volume fractions of the precipitates.DFG, 237105621, SPP 1713: Stark gekoppelte thermo-chemische und thermo-mechanische Zustände in Angewandten Materialie

Electron microscopy investigations of metal-support interaction effects in M/Y2O3 and M/ZrO2 thin films (M=Cu, Ni)

Model systems of the clean and pure oxides Y2O3 and ZrO2, as well as Cu/Cu2O and Ni/NiO particles embedded in the respective oxides have been used to study the reduction behavior of the oxides and the eventually associated metal-support interaction effects in oxide-supported systems. Particular emphasis has also been given to the influence of the phase transformation in ZrO2-containing systems on metal-support interaction. Whereas Y2O3 has been found to be an outstandingly structurally and thermally stable oxide even upon reduction in hydrogen up to 1073 K, ZrO2 was found to undergo a series of phase transformations from amorphous ZrO2 to polycrystalline tetragonal ZrO2 (673 K) and subsequently to monoclinic ZrO2 (above 873 K). Both phase transformations were found to be basically dependent on gas partial pressure and annealing rate. However, substantial reduction of the oxides did not take place during the phase transformations. In turn, both Cu- and Ni-containing systems were not observed to be substantially affected by any (strong) metal-support interaction effects such as encapsulation by sub-stoichiometric oxides or reductive formation of intermetallic phases, at least up to temperatures of 1073 K. Equally, for the ZrO2-containing systems, also the phase transformations occurring at elevated temperatures did not cause structural or thermo-chemical alterations of the Cu or Ni-particles. Differences in the metal-support interaction between Cu- and Ni-particles have only been obtained in the structural “reference” systems, that is, if supported on SiO2. Whereas Cu/Cu2O particles on SiO2 are basically unaffected by the reductive treatment at elevated temperatures, a Ni3Si2 intermetallic phase is formed if SiO2-supported Ni/NiO particles are treated in hydrogen at 673 K and above.(VLID)2852394Accepted versio

Oriented attachment explains cobalt ferrite nanoparticle growth in bioinspired syntheses

Oriented attachment has created a great debate about the description of crystal growth throughout the last decade. This aggregationbased model has successfully described biomineralization processes as well as forms of inorganic crystal growth, which could not be explained by classical crystal growth theory. Understanding the nanoparticle growth is essential since physical properties, such as the magnetic behavior, are highly dependent on the microstructure, morphology and composition of the inorganic crystals. In this work, the underlying nanoparticle growth of cobalt ferrite nanoparticles in a bioinspired synthesis was studied. Bioinspired syntheses have sparked great interest in recent years due to their ability to influence and alter inorganic crystal growth and therefore tailor properties of nanoparticles. In this synthesis, a short synthetic version of the protein MMS6, involved in nanoparticle formation within magnetotactic bacteria, was used to alter the growth of cobalt ferrite. We demonstrate that the bioinspired nanoparticle growth can be described by the oriented attachment model. The intermediate stages proposed in the theoretical model, including primary-building-block-like substructures as well as mesocrystal-like structures, were observed in HRTEM measurements. These structures display regions of substantial orientation and possess the same shape and size as the resulting discs. An increase in orientation with time was observed in electron diffraction measurements. The change of particle diameter with time agrees with the recently proposed kinetic model for oriented attachmen

The theory and application of inelastic coherence in the electron microscope

Zsfassung in dt. SpracheIn der vorliegenden Arbeit werden zwei auf den ersten Blick unabhängig erscheinende Techniken, "Energieverlust magnetisch chiraler Dichroismus" (EMCD) und "Energieverluste durch kanalisierte Elektronen" (ELCE), unter dem gemeinsamen Prinzip der interferometrischen Elektronenenergieverlustspektrometrie (EELS) beschrieben. Dazu werden die theoretischen Formulierungen der interferometrischen EELS, ELCE und EMCD dargestellt. Sowohl für ELCE als auch für EMCD werden Simulationen durchgeführt, die elastische und inelastische Streueffekte verbinden, um den Einfluss der dynamischen Beugung und Strahlkonvergenz auf die experimentellen Ergebnisse zu untersuchen. Weiters wird EMCD im Rahmen von ausführlichen TEM-Untersuchungen von unterschiedlichen Heusler-Verbindungen und Magnetit verwendet. Dabei wird die Zuverlässigkeit in Hinblick auf den täglichen Gebrauch der Technik diskutiert. Die ausgewählten Materialien zeigen einen magnetostrukturellen Phasenübergang, der unter Verwendung von EMCD in-situ untersucht werden kann, um mehr Informationen über die Änderung der magnetischen Eigenschaften zu erhalten. Die Verbindung von Simulationen und Experimenten ermöglicht es, den magnetischen Phasenübergang für Materialien in der Spintronic maßzuschneidern. Die ELCE Technik wird angewendet um die Änderung der Feinstruktur in Rutil bei Anregung von Endzuständen unterschiedlichen Charakters ortsspezifisch zu untersuchen. Kombinierte Blochwellen- und DFT-Simulationen zeigen exzellente Übereinstimmung mit den experimentellen Spektren. Die vorliegende Arbeit zeigt, dass die Kombination von Simulationen dynamischer Beugungseffekte und Berechnungen der elektronischen Struktur notwendig ist um die Ergebnisse von ELCE und EMCD Messungen zu interpretieren. Diese Techniken können in zukünftigen Experimenten angewendet werden um z.B. die magnetischen Eigenschaften von Oberflächen und Grenzflächen zu untersuchen und um ortsabhängige Informationen über die Bindungen in Kristallen zu erhalten. Sie stellen hochentwickelte Mittel für elektronenmikroskopische Analysen in Gebieten wie funktionelle Materialien, Spintronics und Katalyse bereit.In this work, two seemingly unrelated techniques, "energy-loss magnetic chiral dichroism" (EMCD) and "energy losses by channelled electrons"(ELCE), are described under the unifying principle of interferometric electron energy loss spectrometry (EELS). To this end, the theoretical formulations of interferometric EELS, ELCE and EMCD are presented. For both, ELCE and EMCD, simulations combining elastic and inelastic scattering effects are performed to discuss the influence of dynamical diffraction and beam convergence on the experimental results. Furthermore, EMCD is applied in course of a thorough TEM investigation of different Heusler alloys as well as magnetite, discussing its reliability concerning a daily use. The chosen materials exhibit a magnetostructural phase transition which can be investigated in-situ using EMCD gaining knowledge about the changes of the magnetic properties. Combining simulations and experiments paves the way for tailoring of the magnetic phase transition of materials for use in spintronics. The ELCE technique is applied to site-specifically investigate the change of the fine-structure as final states of different character are probed in rutile. It is shown that a combined Bloch wave and DFT simulation exhibits excellent agreement with the experimental spectra. The presented work shows that the combination of simulations of dynamical diffraction effects and electronic structure calculations is necessary to interpret results of ELCE and EMCD measurements. These techniques can be applied in future experiments to investigate for example the magnetic properties at surfaces and interfaces and to gain site-specific information about the bonding situation in crystals. Thus, they provide sophisticated means for electron microscopical analyses in fields like functional materials, spintronics and catalysis.14

Fine structure and site specific energy loss spectra of NiO

Zsfassung in dt. SpracheMittels des Simulationsprogramms WIEN2k wurden Berechnungen der elektronischen Struktur von NiO durchgeführt. Die mit verschiedenen Austausch-Korrelationsfunktionalen erhaltenen Ergebnisse wurden mit experimentellen Daten verglichen. Man sieht, dass das modifizierte Becke-Johnson Austauschpotential die beste Wahl für das Übergangsmetalloxid NiO ist.Weiters wurde die Wechselwirkung zwischen dem Elektronenstrahl und der Probe mittels Energieverlust von kanalisierten Elektronen untersucht.Die Berechnungen, durchgeführt mit einem Simulationsprogramm basierend auf dem Blochwellenformalismus und dem gemischten dynamischen Formfaktor, wurden mit Experimenten verglichen und zeigen sehr gute Übereinstimmung.Zusammenfassend kann gesagt werden, dass es die jüngsten Fortschritte bei Simulationsprogrammen erlauben, die mit komplizierten Methoden in einer neuen Generation von korrigierten Elektronenmikroskopen erhaltenen experimentellen Daten besser zu erklären.Electronic structure calculations for NiO were performed using the ab-initio simulation package WIEN2k. The results obtained using different exchange-correlation functionals were compared to experimental data. It is shown that the modified Becke-Johnson exchange potential is the best choice for the transition-metal oxide NiO.Furthermore, the probe-target interaction was investigated by means of energy loss by channelled electrons. The calculations conducted using a simulation software based on the Bloch-wave formalism and the mixed dynamic form factor were compared to experiments. It is shown that the measurements are in very good agreement with the simulations.Thus, it is shown that recent advances in simulation software allow for explaining high precision experimental data obtained by sophisticated techniques in a new generation of corrected electron microscopes.8

Investigation of Electrocatalysts Produced by a Novel Thermal Spray Deposition Method.

Common methods to produce supported catalysts include impregnation, precipitation, and thermal spray techniques. Supported electrocatalysts produced by a novel method for thermal spray deposition were investigated with respect to their structural properties, elemental composition, and electrochemical performance. This was done using electron microscopy, X-ray photoelectron spectroscopy, and cyclic voltammetry. Various shapes and sizes of catalyst particles were found. The materials exhibit different activity towards oxidation and reduction of Fe. The results show that this preparation method enables the selection of particle coverage as well as size and shape of the catalyst material. Due to the great variability of support and catalyst materials accessible with this technique, this approach is a useful extension to other preparation methods for electrocatalysts

Experimental realization of a semiconducting full-Heusler compound: Fe2TiSi

Meinert M, Geisler M, Schmalhorst J-M, et al. Experimental realization of a semiconducting full-Heusler compound: Fe2TiSi. Physical Review B. 2014;90(8): 85127