57 research outputs found
Magnetic Properties of Uranium Based Ferromagnetic Superconductors
No full textFerromagnetism and superconductivity have long been thought to be mutually exclusive. Recently however it was found that the compounds UGe2, URhGe and UIr belong to a class of materials in which ferromagnetisme and superconductivity appear simultaneously. One characteristic property of these compounds is the existence of strong correlations between the magnetic moments of the uranium ions and the conduction electrons. These correlations lead to unusual magnetic properties at low temperatures. By applying external pressure the magnetic correlations can be varied. The fact that superconductivity in these materials is found only for those pressures, at which the magnetic correlations are strongest, indicates that the effective attracting force between the conduction electrons responsible for superconductivity has a magnetic origin. In this research the magnetic correlations of the ferromagnetic superconductors are investigated in order to better understand the unusual coexistence of ferromagnetism and superconductivity. Besides the dilatometry, specific heat, magnetization, and three-dimensional neutron depolarization techniques, the muon spin relaxation (muSR) technique is frequently used in the study of de magnetic properties of the ferromagnetic superconductors. The muon experiments indicate that unusual excitations exist in these materials which are possibly responsible for the superconductivity.Applied Science
Microstructure in powders: Spin-echo small-angle neutron scattering measurements
No full textApplied Science
Direct Oxidation of Benzene to Phenol: investigation of the active iron species in [Fe,Al]MFI catalysts by 57Fe Mössbauer spectroscopy
No full textSteam-treated isomorphously substituted [Fe,Al]MFI zeolite is known to exhibit superior catalytic performance in the direct oxidation of benzene to phenol, using N2O as oxidant (BTOP). However, despite extensive efforts, the nature of the active sites in the [Fe,Al]MFI catalyst for the BTOP reaction is still largely unknown. However, recent investigations have shown strong indication that extra-framework iron species in the [Fe,Al]MFI zeolite are the catalytically active constituents in the BTOP reaction. To elucidate the active sites, this study investigates the relationship between the structure and activity of [Fe,Al]MFI catalysts in the BTOP reaction. Several isomorphously substituted [Fe,Al]MFI zeolites, varying in iron and aluminum concentrations, were successfully prepared via hydrothermal synthesis. All as-synthesized [Fe,Al]MFI zeolites were enriched with 57Fe isotope to enhance the Mauer effect, and thus providing an excellent signal-to-noise ratio. The evolution of iron from framework to extra-framework position during sequential post-activation treatments is investigated with 57Fe Mauer spectroscopy. In addition, the redox property of the extra-framework iron species formed after steam-treatment is studied by in situ Fe K-edge XANES. The catalytic performance of steam-treated [Fe,Al]MFI zeolites, with varying iron and aluminum concentrations, is evaluated in the direct oxidation of benzene to phenol, using N2O as oxidant, as well as in the N2O decomposition. From characterization and activity data, it can be inferred that (i) not all extra-framework iron species formed after steam-treatment of the [Fe,Al]MFI zeolites are active in the BTOP reaction; (ii) active iron species in BTOP reaction are preferably formed in samples with low iron concentration, while higher N2O conversions were achieved for [Fe,Al]MFI catalysts with increasing iron loading; (iii) the occurrence of catalyst deactivation, most likely due to coke formation, is more favorable in the presence of aluminum; while in the N2O decomposition, catalysts with higher aluminum concentration exhibit superior performance; and, (iv) aluminum does not play a role in the BTOP catalysis. Finally, in situ 57Fe Mauer studies show that there are several active sites for the direct oxidation of benzene to phenol. These active sites are most likely small clusters of iron species with low nuclearity (e.g. enzyme-like systems). Thus, this heterogeneous catalyst is simply heterogeneous in terms of the extra-framework iron species formed after steam-treatment, which makes it difficult to establish a direct relationship between structure and activity in the BTOP reaction.Applied Science
Instrumentation to handle thermal polarized neutron beams
No full textIn this thesis we investigate devices needed to handle the polarization of thermal neutron beams: Ï/2-flippers (to start/stop Larmor precession) and Ï-flippers (to reverse polarization/precession direction) and illustrate how these devices are used to investigate the properties of matter and of the neutron. The central theme is: demonstration - for the full thermal spectrum - of a special mode of Larmor precession (called "zero-field"-precession) over the neutron beam path length between two "adiabatic Radio-Frequency flippers". This technique enables one to build neutron spin-echo spectrometers with exceedingly large precession angles, without applying magnetic fields over long distances (meters). As an extension of this demonstration, a prototype Spin-Echo instrument for Small Angle Neutron Scattering (SESANS) was built, based on similar flippers with skew magnet poles. The viability of this instrument was demonstrated by measurements in quartz and limestone. As a spin-off of this work, interference was produced between "ordinary" precession and "zero-field"-precession. From this interference the 4Ï periodicity of the spinor for the neutron could be explicitly demonstrated.Applied Science
Spin-echo development for a time-of-flight neutron reflectometer
No full textNeutron scattering techniques are a useful tool to determine structures in samples with sizes ranging from 1 x 10-10 up to 1 x 10-5 meter. Unfortunately powerful neutron sources are scares and expensive, which demands an efficient use of the available neutrons. This thesis focuses mainly on the practical realisation of a combination of two neutron scattering techniques; neutron reflectometry and neutron spin-echo angle labelling. This special combination will not only result in more efficient neutron scattering measurements but will also open up possibilities for new types of measurements. OffSpec is a new dedicated spin-echo reflectometer which is designed to accommodate the fusion of the two techniques and is located at the Second Target Station at ISIS in the United Kingdom. It will be ready for user operation mid 2009. Delft University of Technology has been responsible for the development and production of the spin-echo components for OffSpec. A flexible mechanical design results in five different applications for this spectrometer: ultra small-angle scattering by spin-echo, high intensity measurements on in-plane - nanometre to micrometre - correlations, high resolution measurements for bent or wavy samples, possibility to separate specular and off-specular scattering within a detector pixel, and inelastic spin-echo in transmission or in reflection. A large part of this thesis work consists of the design and build of these spin-echo components. A dedicated beam at the Reactor Institute Delft was used to test the components and built a number of prototypes spectrometers. These prototypes were used to perform proof of principle measurements for several different instrumental modes which are described in this thesis.Radiation, radionuclides & reactorsApplied Science
Observation and modeling of biological colloids with neutron scattering techniques and Monte Carlo simulations
No full textIn this study non-invasive neutron scattering techniques are used on soft condensed matter, probing colloidal length scales. Neutrons penetrate deeply into matter and have a different interaction with hydrogen and deuterium, allowing for tunable contrast using light and heavy water as solvents. The mesoscopic structure of materials is determined by measuring the elastic scattering of neutrons over small angles. Spin-echo small angle neutron scattering (SESANS) and the reciprocal-space equivalent ultra small angle neutron scattering (USANS) have been used to investigate the structure of colloidal suspensions and gels by measuring the projections of the density-density correlation function and of the scattering function. A hollow sphere model is developed and used to investigate liposomes and E. coli bacteria. The sizes of liposomes and E. coli and their hollow sphere nature were confirmed. Particle size and size distribution have been measured for milk, and the change in typical length scale during gelation into yoghurt was measured kinetically. 3D Monte Carlo simulations of colloidal aggregation have been performed using a varying reactivity, ranging from reaction limited (RLCA) to diffusion limited cluster-cluster aggregation (DLCA), to study the effect on structure and formation. A comparison is made between simulated structures and SESANS measurements by calculating the density correlation function. A relaxation time is introduced into the simulated reactivity to control aggregation speed. The increase of typical sizes during gelation at low reactivity was consistently observed in measurements and simulations, but longest length scales could not be accurately simulated. A transition from RLCA to DLCA occurs if the typical reaction time is below typical monomer diffusion time. The increase in coordination number becomes larger when aggregation remains reaction limited. The optimal sample thickness for a SESANS measurement is derived as a function of scattering cross-section and linear attenuation coefficient. The optimal sample thickness is at about 0.8 scattering events for neutron transparent samples, whereas it is the 1/e-length when samples are neutron opaque.Radiation, Radionuclides & ReactorsApplied Science
Organometallics in confined geometries. Ferrocene in zeolite Y
No full textZeolites are crystalline aluminosilicates with a three-dimensional open framework structure of channels and cavities of molecular dimensions. Zeolites can thus act as molecular sieves, making catalytic centres in the interior of the zeolite only accessible to molecules of the correct size. Such catalytic centres, especially clusters of transition metals, can be incorporated in zeolites by vapour phase insertion of volatile compounds, such as organometallics. An often used zeolite is zeolite Y, which contains especially large cavities, called supercages. We study the inclusion of the organometallic ferrocene, Fe(C5H5)2, molecules in the supercages of Y-type zeolites. Ferrocene consists of an iron atom "sandwiched" by two identical parallel cyclopentadienyl, C5H5, rings. The scope of this thesis is to determine the structure and dynamics of ferrocene in zeolite Y, and to illuminate the interaction between the ferrocene molecules and the zeolite supercages. We look into differences between a single ferrocene molecule, solid (crystalline) ferrocene and ferrocene in a zeolite supercage, thus illustrating the influence of a surrounding geometry on an encaged molecule. We focus on two types of zeolites, Na55Y and K55Y.Interfaculty Reactor Institut
High-pressure sulfidation of hydrotreating catalysts: Genesis and properties of the active phase
No full textThe global petroleum demand is projected to increase in the next decennia as oil is expected to remain the primary source of energy around the globe contributing to approximately 40% of the total consumption. The production of high-quality fuels from crude oil involves physical separation steps but also catalytic processes to remove contaminants (hydrotreating), shift the boiling point (alkylation and hydrocracking) and increase fuel quality (isomerisation). In hydrotreating sulfur, nitrogen, oxygen and metal atoms are removed from the different petroleum streams and unsaturated hydrocarbons are hydrogenated. The main reasons for refineries to perform hydrotreating are of environmental and economic nature. Besides protection of downstream catalysts from poisoning by sulfur, stringent environmental legislation has been aimed at the reduction of sulfur oxide emissions from fuel combustion. The environmental regulations on transportation fuel quality and the diminishing supplies of lighter types of crude oil necessitate further improvement of hydrotreating processes. An important approach is then to further optimize the catalytic activity of hydrotreating catalysts. In spite of the substantial progress made in the fundamental understanding of the active phase morphology, the metal-support interaction and reaction mechanisms, many details about the nature and stability of the active sites have not yet been elucidated. One specific issue not widely addressed is the influence of the sulfidation pressure on the active phase in hydrotreating catalysts. In industrial practice, catalysts are brought in their active, sulfided form at elevated pressure. The main objective of the present study was to understand the effect of the sulfidation pressure on the active phase structure in CoMo and NiW catalystsApplied Science
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