Kinetics of Solid-State Reactions in Heterogeneous Catalysis from Time-Resolved X-Ray Absorption Spectroscopy

The potentials of X-ray absorption spectroscopy (XAS) (quantitative phase composition and average valence together with a short-range order structure analysis) combined with a time-resolution in the second range make time-resolved (TR-) XAS a powerful tool for investigating the reactivity of solids in catalysis and solid-state chemistry. General aspects of TR-XAS investigations are discussed (i.e., instrumentation, data analysis). In addition, some experiments illustrate how the kinetics of solid-state reactions in heterogeneous catalysis can be elucidated from TR-XAS studies.     

Fiber‐draw‐induced elongation and break‐up of particles inside the core of a silica‐based optical fiber

Particles in the core of optical fibers are widely studied to tailor or to improve optical properties. The analysis of nanoparticles embedded in silica‐based optical fiber allowed new observations of the evolution of amorphous particles during fiber drawing. Even at the nanoscale, competition between viscous stresses and surface tension on the particles induces elongation and even break‐up of particles during the process. Indeed, particles between 140 and 200 nm diameter inside the preform can break up in fragments with diameters down to 60 nm inside the drawn fiber. Break‐up of particles appears as a new “top‐down” strategy to produce small particles. These observations are promising for micro/nanostructured and multiphasic optical fibers.     

On morphology and strain field of Ge/Si(001) Islands according to TEM phase imaging method

The mechanism driving Germanium islands nucleation and self-assembly is an important effect for opto-electronic applications, still not fully understood. We demonstrate that the new transmission electron microscopy phase imaging method provides insights on the distribution of strain and composition fields in and around the islands on rather large areas. The method consists of retrieving the phase from a focus series of plane view images. The phase image is representative of morphology, composition and strain. The results show that whatever the islands size and shape is, a maximum compressive strain is obtained at the apex of the islands compensated by a maximum tensile strain in the substrate close to the islands perimeter. The maximum compressive strain is associated to a larger Ge concentration. The distribution of tensile strain varies with the shape of the islands: for square base pyramidal "hut" islands, a maximum tensile strain is obtained at the four corners of the pyramid base and for "dome" islands, the tensile strain is less pronounced and affects almost the whole island perimeter. These results are consistent with the higher strain relaxation level of "dome" islands in comparison to those of "hut" islands.     

Interfacial magnetic coupling between Fe nanoparticles in Fe–Ag granular alloys

The role of the interface in mediating interparticle magnetic interactions has been analysed in Fe50Ag50 and Fe55Ag45 granular thin films deposited by the pulsed laser deposition technique (PLD). These samples are composed of crystalline bcc Fe (2–4 nm) nanoparticles and fcc Ag (10–12 nm) nanoparticles, separated by an amorphous Fe50Ag50 interface, occupying around 20% of the sample volume, as determined by x-ray diffraction (XRD), x-ray absorption spectroscopy (XAS), and high resolution transmission electron microscopy (HRTEM). Interfacial magnetic coupling between Fe nanoparticles is studied by dc magnetization and x-ray magnetic circular dichroism (XMCD) measurements at the Fe K and Ag L2,3 edges. This paper reveals that these thin films present two magnetic transitions, at low and high temperatures, which are strongly related to the magnetic state of the amorphous interface, which acts as a barrier for interparticle magnetic coupling.     

TEM investigations of the oxide layers formed on a 316L alloy in simulated PWR environment

Atomic scale observations of the oxide formed on stainless steels, under simulated nuclear reactor conditions, are performed to estimate the oxide layer contribution on stress corrosion cracking (SCC) mechanisms. A duplex oxide composed of a chromium enriched inner layer (Fe_1.5Cr_1.5O₄) and an outer layer composed of magnetite crystallites (Fe₃O₄) is found. The oxide layer structure evolves from amorphous, for oxidation times of 1 min, to nano-crystalline at 2 min and mono-crystalline after 5 h. IFFT images, calculated from Cs-corrected HRTEM images recorded on grains oriented in the 〈111〉 direction, highlight a double network of dislocations with ½ 〈10-1〉 and ½ 〈?110〉 Burgers vectors. This network leads to the decrease in non-relaxed deformation and favors an epitaxial growth between steel and oxide. Both crystal structure transformations and epitaxial relations between metal and oxide have provided relevant information which contributed to progress on SCC modeling.     

Properties of a high resolution negative photoresist

The recent advantages in designing large and very large integrated circuits have imposed stringent requirements on the processes used in the production of semiconductor devices. The requirements on photoresists have been increased not only with regard to improved exposure tools but also in view of other process steps such as plasma etching, ion implantation, etc. A new developed negative photoresist, Selectilux N, combines the well-known advantages of a cyclized polyisoprene-based resist with the capability to resolve features in the 1 to 2 micron range and a high thermal stability. The main properties are: high photospeed to reduce the influence of oxygen and to prevent thickness loss, high contrast to achieve good resolution in projection exposure, low flow distortion up to 240 C to ensure good image stability, low swelling during the development to prevent reduction of the resolution. Photospeed and contrast can be derived from the characteristic curve of Selectilux N which is obtained by the determination of the remaining thickness of the resist vs exposure energy.     

Zustandsestimation in Netzen mit Stations- und Netztopologiefehlern

Übersicht Die Zustandsestimation basiert so, wie sie heute durchgeführt wird, auf der Hypothese, daß die Topologie des Netzes richtig modelliert ist. Entspricht sie jedoch nicht mehr der im Prozeßrechner abgespeicherten Topologie, können die Netzzustandsgrößen nicht mehr mit ausreichender Genauigkeit estimiert werden. Alle Widersprüche werden dann zunächst auf grobe Meßfehler zurückgeführt. Diese vermeindlichen Meßfehler werden deshalb mit Hilfe einer Prozedur zur Elimination grober Meßfehler aus dem Datensatz entfernt, bis schließlich die lokale Redundanz an der Stelle des Topologiefehlers verschwindet. Nach einer Zustandsestimation mit dem reduzierten Meßwertesatz können deshalb keine Widersprüche mehr wahrgenommen werden. Trotzdem ist das Estimationsergebnis für die Führung eines Netzes nicht brauchbar, denn die Ursache der Widersprüche, nämlich der Topologiefehler, ist nach wie vor im Datensatz enthalten. In diesem Aufsatz wird eine Methode vorgestellt, die die Wahrnehmung und Lokalisation beliebiger Stationstopologiefehler und die Identifikation des korrekten Schaltungszustands ermöglicht.

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State estimation with a network containing substation and network topology errors

Summary State estimation as it is carried out nowadays is based on the hypothesis that the topology of the network has been correctly modelled. If however the topology of the network does not correspond to the topology stored in the process computer, it is then not possible to estimate an accurate state of the network. All inconsistencies caused by such a topological error are traced back to the gross measurement errors. Thus procedures for measurement error treatment eliminate all suspicious measurements up to the local redundancy of measurements disappears near the substation where the topological error has occured. When estimating the state of the network using the reduced measurement set, inconsistencies are no more detectable. Nevertheless the estimated state is not a usable data base for the control of the network. This is because the original inconsistencies are caused by a topology which has been erroneously modelled and this error is still in the data set. In this paper a method based on WLS-estimators is proposed which is able to detect and locate and topology error in substations and to identify the correct topology.

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Liste der verwendeten Symbole J Zielfunktion des Estimationsproblems - J _U Zielfunktion des umgebenden Netzes einer topologiefehlerverdächtigen Station - J _U,i Zielfunktion des umgebenden Netzes der Stationi - J _M Zielfunktion des Mininetzes - J _M,i Zielfunktion des Mininetzes der Stationi bei Verwendung der Schaltungsvariante - J _ges Summe der Zielfunktion des umgebenden Netzes und der Zielfunktion des Mininetzes - J _ges,i Summe der Zielfunktion des umgebenden Netzes der Stationi und der Zielfunktion des Mininetzes bei Verwendung der Variante - h Vektor der Meßgleichungen - R Matrix der Kovarianzen der Messungen - v Vektor der Meßfehler - x Vektor der unbekannten Zustandsgrößen - z Vektor der Meßwerte - _e maximale Einspeiseleistung bei Simulationsuntersuchungen - _Li relative Lokalisationshäufigkeit eines topologischen Fehlers in Stationi - _Ii relative Identifikationshäufigkeit des korrekten Schaltungszustandes in der topologiefehlerbehafteten Stationi - Gesamtanzahl an Simulationen - v _Li Anzahl an Simulationen mit positiver Lokalisation des topologischen Fehlers in Stationi - v _Ii Anzahl an Simulationen mit positiver Identifikation des topologischen Fehlers in Stationi - Schaltungsvariante