Diffusion des rayons X | EPFL Graph Search

La diffusion des rayons X (X-ray scattering en anglais) est une technique d'analyse basée sur la diffusion des ondes de rayons X par une substance. Alors que la diffraction des rayons X ne peut être utilisée qu'avec des substances cristallines, la diffusion des rayons X peut être utilisée pour des substances cristallines ou amorphes. La diffusion des rayons X est basée sur l'interaction des rayons X avec les électrons des atomes. Techniques La diffusion des rayons X peut être élastique ou inélastique. Trois techniques principales de diffusion élastique des rayons X sont connues : Parmi les techniques de diffusion inélastique des rayons X on peut citer la diffusion inélastique résonante de rayons X. Utilisation La diffusion des rayons X donne des informations sur la forme, la taille et l’orientation de matières ayant des dimensions ne dépassant pas l'échelle du micromètre comme :

les grandes molécules, exemples : les protéines et les polymères ;
les nanopa

les grandes molécules, exemples : les protéines et les polymères ;
les nanopa

In Search of Ferroelectricity in Antiferroelectric Lead Zirconate

Kaushik Vaideeswaran

Since the prediction of antiferroelectricity and the subsequent discovery of PbZrO3, the description of antiferroelectric behaviour in materials has been constantly modified to account for the latest properties identified in antiferroelectric materials. Adding to this refinement, the current study considers Lead Zirconate (PbZrO3) as a prototypical antiferroelectric and through it, aims to understand the origin of antiferroelectricity, and the possibility of localised ferroelectric behaviour inside the material. The observation and control of the occurence of such localised ferroelectricity is attempted and the mechanical behaviour of such ferroelectric structures is simulated. Antiferroelectrics are defined as materials which undergo a phase transition from one centrosymmetric phase to another while being accompanied by a dielectric anomaly at the transition. The origin of antiferroelectricity as observed in the prototypical antiferroelectric PbZrO3, involves multiple lattice instabilities with interactions that remained only partially deciphered. The current study provides for an explanation of the lattice dynamics occuring in PbZrO3 through results obtained from a combination of scattering techniques such as Inelastic XRay, Thermal Diffused and Brillouin scattering. The primary structural instability associated with the antiparallel lead displacements is seen to be coupled with the ferroelectric order parameter, and to the order parameter related to the oxygen octahedral rotations. On the appearance of the structural order parameter, these interactions result in the formation of the antiferroelectric phase of PbZrO3 as known previously. The premises for such interactions and their validations are provided, while explaining all aspects associated with the antiferroelectric phase transition occuring in PbZrO3 including the dielectric anomaly. One significance of the scenario with competing order parameters lies in the possible appearance of the subdued order parameters in regions where the primary order parameter is absent. The conditions for the disappearance of this order parameter in the case of PbZrO3 is discussed, and structural domain boundaries are shown to be potential regions for the local observation of otherwise subdued ferroelectricity. Aimed at the observation of localised ferroelectric structures in an otherwise antiferroelectric material, growth of epitaxial thin films of PbZrO3 is undertaken using Pulsed Laser Deposition. The growth parameters are varied to control the crystalline orientation and the defect concentration in the films through the control of the interfacial strain. In this process, a technique for the tunable variation of the epitaxial strain using a single composition buffer layer has been documented. The antiferroelectric films are then utilised for the observation of regions of disrupted symmetry, for the observation of localised ferroelectricity. The distribution of such regions in the thin films, as well as the anomalous interaction of different types of domain walls with crystalline defects are observed and analysed. Alongside, the response of such localised ferroelectric structures to external electric fields is simulated. The conditions necessary for sufficiently large displacements of such structures, large enough for potential observation using Scanning Probe Microscopy techniques, have been listed.

EPFL2015

Simulations d'une structure cristalline incommensurable à caractère désordonné

Eric Germaneau

Incommensurate crystal structures do not display three-dimensional periodicity. Discovered in the 1960's, a mathematical model has been established in 1974 only. Incommensurate structures belong to the big family of aperiodic structures. The modulation of incommensurate structures manifests itself as satellite reflections around the main peaks on X-rays diffractograms. Short range order correlations in crystals lead to diffuse scattering between Bragg peaks in the diffraction pattern. The computing power available today allows for very time consuming simulations of the incommensurate or disordered crystal structures. Simulations based on Monte-Carlo and the molecular dynamic techniques have been performed. The former is based on stochastic algorithms, whereas the latter is deterministic. This PhD thesis strove to combine these two methods, using a four-isomermodel, in order to study the incommensurate, disordered polymorph of p-azoxydiphenetol (PAP), which crystallises below 356 K. One of the main features of the PAP molecule is its dative bond. A Monte-Carlo program, using true empirical potentials, has been developed in order to explain the behaviour of PAP. This code (6000 lines), written in C, uses the GSL and OpenMP libraries. Using numerical and experimental tools, this thesis provides explanations for the origin, from an atomic point of view, of the incommensurate and disordered behaviour of PAP. Calculations have been performed on the clusters of École Polytechnique Fédérale de Lausanne. Simulated diffraction patterns have been compared to experimental data, collected before and during this study. Moreover, a set of differential scanning calorimetry (DSC) measurements has been carried out during this work, the results of which have been analysed based on Arrhenius's law. For a better understanding of the role played by the dative bond, and the relation between the complexity of the molecule and the structure, DFT and ab initio calculations have been performed. This work has successfully woven the results from different experimental and numerical techniques into a comprehensive model for this very complex system indeed.

EPFL2008

Deformation behavior of nanoporous polycrystalline silver layers sintered by low temperature bonding: experiment and simulation

Saba Zabihzadeh

Integrated circuit packaging technology has become a prime design consideration for the develop-ment of electronic system concepts. One key issue is the bonding layer between chip and substrate. Currently, high-lead solder materials are being used, which apart from their environmental problem, limit the reliable operation temperature of devices to temperatures lower than 175ºC. Low-temperature bonding by sintering nanograin sized silver is emerging as a promising replacement in power electronics modules working at high temperatures. Although silver has been studied as bonding material since three decades, it has not been widely used in the packaging industry which might be ascribed to the limited knowledge on its mechanical behavior. In this work, the microstructure of thin layers of nanoporous silver sintered under different conditions are studied and linked to the mechanical behavior (at room temperature) using X-ray diffraction in-situ deformation tests and finite element simulations. First the microstructure of several samples such as grain size distribution, defect structures, porosity, pore morphology and ligament size distribution are investigated and are related to the corresponding sintering conditions. Electron microscopy, high resolution X-ray imaging, and X-ray diffraction techniques are the employed characterization methods. We found a strong dependency of the microstructure on the sintering conditions. Afterward, series of X-ray diffraction in-situ continuous, load-unload and stress relaxation tensile tests are performed to study the deformation mechanisms at room temperature. All samples exhibit small elongation and a large hysteresis in the stress-strain curve. From the change in the peak position upon unloading the residual elastic strain inside the sample is calculated while the evolution of the peak broadening provides information on the recovery mechanisms and induced dislocations. A significant recovery in the peak position and peak broadening after unloading and during relaxation is observed for all specimens. This is attributed to the presence of the pores. We found that the density of generated dislocations during tensile deformation depends on the porous structure. The results suggest that there exist two contributions to deformation: one related to the pores and other to intra-granular dislocations. The competition between both mechanisms is discussed in terms of porosity and grain size. Finally, 3D FE microstructure-based simulations are performed to allow a better correlation between the porous morphology and mechanical behavior. The 3D reconstructed images of nanoporous silver obtained from X-ray ptychography are used as an input to create finite element meshes of the actual porous morphology. For homogenization purpose, a representative volume element approach is chosen. The apparent elastoplastic response of the bulk silver is extracted from the porous samples using inverse homogenization method. The simulations allow distinguishing between the interplay and role of the ligament size, pore morphology and porosity, and provide a better comprehension on the experimental observations. We show that the proposed model has a predictive character for general nanoporous silver microstructures.

EPFL2016