Diffusion des rayons X aux petits angles

La diffusion des rayons X aux petits angles (SAXS de l’anglais : Small Angle X-rays Scattering) est une technique expérimentale qui permet d’étudier les propriétés structurelles des matériaux à une échelle allant de 1 à . Cette technique se base sur l’interaction élastique des photons avec les nuages électroniques. Les photons sont diffusés en traversant l’échantillon et fournissent des informations sur la fluctuation des densités électroniques dans la matière hétérogène. Les longueurs d’onde des rayons X se situent entre 10-8 à 10-12 mètres, ce qui est du même ordre de grandeur que les distances interatomiques, permettant un patron de diffraction. Celui-ci correspond à la structure atomique et réside dans la région des petits angles, d’où la technique tient son nom. Principe Le rayonnement monochromatique incident de vecteur d’onde k_i traverse l’échantillon. L’intensité diffusée est collectée en fonction de l’angle de diffusion θ. Les interactions élastiques

Visualisation of magnetic domain structures and magnetisation processes in Goss-oriented, high permeability steels using neutron grating interferometry

Benedikt Karl-Josef Gerhard Betz

Industrial transformers cores are built from stacked sheets of an iron silicon alloy, called laminations. The magnetic domain structures of these highly anisotropic electrical steels with a sharp (110)[001]-texture, the so-called Gosstexture, determines the magnetic properties of transformers. Commonly used investigation techniques for these laminations are inductive B-H-measurements. This technique reveals global magnetic properties such as hysteresis, remanence, saturation and losses. Locally resolved information about the underlying domain structure cannot be obtained. In this thesis, the neutron grating interferometer (nGI) is used to investigate the domain structures in Goss-oriented (GO) electrical steels at the Swiss Spallation Neutron Source using the cold neutron imaging facility ICON. In contrast to the attenuation based transmission image, the dark-field image (DFI) is related to multiple refraction of unpolarised neutrons at magnetic domain walls. Thereby the use of the DFI allows for the visualisation of bulk magnetic domain structures in two and three dimensions with a spatial resolution of down to 70 um in a field of view of 64mm x 64mm. The DFI is used for the visualisation of the locally resolved response of the bulk magnetic domain structures under the influence of externally applied magnetic fields. In the first part, the domain formation and growth along the initial magnetisation curve (0-6000 A/m) up to saturation in static DC magnetic fields was studied. For decreasing field values, the visualisation of the recurrence of the hysteretic domain structure down to the remanent (0 A/m) state is given. A correlation of the grain orientation and the corresponding basic and supplementary domain structure is given. In the second part, the DFI is used to investigate the response of magnetic domain walls to dynamic AC magnetic excitations. The visualisation of the domain wall motion under influence of alternating magnetic fields is performed. In detail, scans combining varying levels of an offset DC (0-30 A/m), oscillation amplitude A (0-1500 A/m), and oscillation frequency f (0-200 Hz) are conducted. By increasing the amplitude while maintaining constant values of DC and f, the transition from a frozen domain wall structure to a mobile one is recorded. Vice versa, increasing f while keeping A and DC constant led to the reverse transition from a mobile domain wall structure into a frozen one. It is shown that varying both, A and f shifts the position of transition region. Higher frequencies require higher oscillation amplitudes to overcome the freezing. The DFI allows for the analysis of the laminations coatings impact to the magnetic structures. To visualise the stress effect of the coating to the underlying domain formation an uncoated lamination is investigated with stepwise increasing applied external tensile stresses up to 20MPa. The domain configurations of the intermediate stress states are imaged and the original domain structure of the coated state is reproduced. To verify the results, complementary experiments using Kerr microscopy, Faraday imaging, small-angle neutron scattering and Laue X-ray diffraction were performed. The here presented findings allow for new insights into macromagnetism and enable new approaches in the field of descriptive models for bulk macromagnetism. Furthermore the results have the potential to further improve the properties of GO-steels used in industrial transformer applications.

EPFL2016

Polyphenols as Morphogenetic Agents for the Controlled Synthesis of Mesoporous Silica Nanoparticles

Jialuo Luo, Alina Osypova, René M. Rossi, Fabien Sorin

Non-surfactant-induced synthesis of mesoporous silica nanoparticles (MSNPs) is gaining increasing interest because of their low toxicity and simple purification compared to conventional surfactant-based methods. Tannic acid (TA), considered as a glucose-derived polyphenol, was first employed a few years ago and has attracted great research interest. Despite recent progress, the mechanisms resulting in the porous structure remain to be elucidated. In this work, we have employed TA and four structurally related polyphenols (gallic acid, ethyl gallate, eudesmic acid, and quercetin) to elucidate the effect of the chemical structure and properties of polyphenols on their templating ability. Our results unravel the mechanism of MSNP formation templated by TA, which form a supramolecular framework as the skeleton for the silica species to attach. The structure of the supramolecular network results in irregular pores. Additionally, the pK(a) value of the templates may be accounted for the particle size. Small-angle X-ray scattering was used to provide precise information on the morphology, especially the porosity of the resulting MSNPs in addition to electron microscopy, dynamic light scattering, nitrogen adsorption and desorption Brunauer-Emmett-Teller method, and thermogravimetric analysis.

AMER CHEMICAL SOC2019

Unravelling the precipitation pathway of biorelevant minerals: from fundamental studies to biomedical applications

Agnese Carino

The formation pathway of biominerals archetypes, namely calcium carbonate (CaC) and calcium phosphates (CaPs), was investigated using a multidisciplinary approach. The time-resolved analytical data were collected using a controlled-composition method and interpreted using a computational tool. Some basic-science questions were identified arising from long-lasting debates on the details about the solid formation mechanism. Some of these questions are here addressed by the accurate experimental data collection, the application of new cutting-edge techniques, and the development of a rigorous thermodynamic-kinetic model for the entire solid formation process from solution. The time-resolved data were collected analysing the aqueous species in-situ. In parallel, the amount of chemicals added into the reactor, to maintain iso-pH conditions, was mathematically correlated to the bonded carbonate or phosphate ions, allowing the collection of two independent series of data and the assessment of both the cation and the anion during the solid formation process. This analytical approach went beyond the state-of-the-art. The complex mechanism was partitioned into its sub-processes, and a new strategy was identified to indirectly disclose the nature of the building units involved in the solid growth. Some assumptions embedded in the derivation of the nucleation theory did not compromise the theoretical validity of the framework, and the thermodynamic quantities expressed in the formulation have to be properly related to the nucleating phase and not extended as intensive properties of the solid. As main scientific results, for both CaC and CaPs systems, the experimental data are consistent with a classical nucleation and crystallisation mechanism, where primary, secondary, and diffusion limited growth are taken into account. The associated thermodynamic and kinetic quantities, such as the surface energy for primary and secondary nucleation as well as the nucleation and growth rate are reported and critically discussed in this dissertation. The models allowed detailed investigations in the pre-nucleation zone with the aim of evaluating the presence of clusters at controlled T, pH, ionic strength, and cation/anion ratio against saturation level. An experimental setup was developed to study clusters in-situ: a reactive horizontal micrometric pulsation-free liquid jet was investigated with a focused synchrotron X-ray beam, and the small angle scattering signals were collected for both CaC and CaPs systems. Preliminary results confirm the presence of clusters and attempts to identify their size distribution are ongoing. The agreement between experimental data, standard and cutting-edge analytical investigations, and modelling results allowed the definition of a plausible solid formation pathway, for both the investigated systems, based on the nucleation theory framework, in which the plausible influence of clusters was assessed. Beside the basic-science activities, some applications in the biomedical field were also evaluated. Among them, the controlled growth of CaPs phase on pre-treated Ti implants, e.g., dental implants, was achieved. This application opened exciting perspectives toward the production and the commercialisation of implants characterised by a faster healing time and improved stability. The scientific results were published in peer-reviewed papers and an international patent application; two additional papers are under finalisation.