Quantifying the Solution Structure of Metal Nanoclusters Using Small-Angle Neutron Scattering

Metal nanoclusters are a unique class of synthetic material, as their crystal structures can be resolved using X-ray diffraction, and their chemical formula can be precisely determinated from mass spectroscopy. However, a complete structure characterization by these two techniques is often a challenging task. Here, we utilize small-angle neutron scattering (SANS) to directly quantify the key structure parameters of a series of silver and gold nanoclusters in solution. The results not only correlate well to their crystallographic structures, but also allow the quantification of the counterions layer surrounding charged nanoclusters in solution. Furthermore, when combining with X-ray scattering, it is possible to estimate the molecular weight of both the metal core and the ligand shell of nanoclusters. This work offers an alternative characterization tool for nanoclusters without the requirement of crystallization or gas phase ionization.

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

Influence of Residual Stresses on Fatigue Response of Welded Tubular K-Joints

Claire Acevedo

Seeking light and transparent bridge designs, engineers and architects have found an efficient and artistic way to fulfill their requirements: steel tubular bridges. In these modern tubular truss bridges, welded K-joints have been shown to be critically susceptible to fatigue failure induced by repeated traffic loads. Despite the research devoted to the study of fatigue during the last 35 years, especially in the offshore oil industry, estimating their fatigue resistance of tubular truss bridges is difficult and solutions are approximate. This is due to the complexity of the parameters influencing bridge fatigue strength, including: 1) complex loadings, 2) applied stress concentrations in joints, 3) welding imperfections, 4) welding residual stresses, and 5) size effects. Some design rules and studies are available to estimate these effects; however, residual stresses in tubular K-joints remain unknown as well as their influence on fatigue crack propagation. Residual stresses are well known to have detrimental influence on fatigue crack propagation in other metallic structures, and further study is a necessary for improving the understanding of K-joint fatigue behaviour. To address this problem, the main objective of this research is to assess, experimentally and numerically, welding residual stresses and their influence on the fatigue response of K-joints. In this thesis, experimental measurements of the residual stress fields were investigated through neutron-diffraction, hole-drilling and X-ray methods. Results have shown that, principal residual stresses are oriented transversely, i.e. perpendicularly to the weld direction, both at and near the surface. Residual stresses can reach the S355 steel yield strength at the weld toe. This orientation is particularly detrimental for fatigue because it is also the orientation of the principal applied stresses, and as a consequence high residual and applied stresses superimpose. Moreover, it is proved that a restraining effect occurs in the gap region in-between the braces remaining the critical residual stresses high in the gap region. Two large scale tubular truss beams were subjected to high-cycle fatigue in order to assess the effect of residual stresses on fatigue response of welded tubular K-joints. These tests revealed that tensile residual stresses do not affect the fatigue crack growth in details loaded in tension (hot-spot 1), whereas they play a significant role in details loaded in compression (hot-spot 1c). Tensile residual stresses enable crack opening, making the fatigue cycle partly or entirely effective to crack propagation. Based on these tests and previous tests from the ICOM test database, it is recommended to design all (tension or compression) K-joints using a strength curve Srhs Srhs – Nf category 100 (for T = 16 mm). A thermo-mechanical finite element model was also developed to numerically reproduce the residual stress creation (using ABAQUS and MORFEO codes). Once the numerical results were validated by comparison with experimental results, the finite element model was used to model other K-joint geometries. A geometrical parametric study was performed to identify the most influential parameters affecting the residual stress distribution (technological size effect). It is found that a raise of the wall chord thickness T (proportional scaling) or of the wall thickness τ = t/T (non-proportional scaling) strongly increases the magnitude of tensile transverse residual stresses. Based on this parametric study, residual stress distributions with depth for transverse, longitudinal and radial directions are proposed. Finally, an extended finite element method (X-FEM) model was used to propagate a fatigue crack under residual and applied stresses. An analytical model based on the effective stress intensity factor range ΔKeff (fracture mechanics) and combined with the proposed distribution for residual stresses was also established to predict fatigue crack propagation in joints loaded in tension and in compression.

EPFL2011

The success story of crystallography

Dieter Schwarzenbach

Diffractionists usually place the birth of crystallography in 1912 with the first X-ray diffraction experiment of Friedrich, Knipping and Laue. This discovery propelled the mathematical branch of mineralogy to global importance and enabled crystal structure determination. Knowledge of the geometrical structure of matter at atomic resolution had revolutionary consequences for all branches of the natural sciences: physics, chemistry, biology, earth sciences and material science. It is scarcely possible for a single person in a single article to trace and appropriately value all of these developments. This article presents the limited, subjective view of its author and a limited selection of references. The bulk of the article covers the history of X-ray structure determination from the NaCl structure to aperiodic structures and macromolecular structures. The theoretical foundations were available by 1920. The subsequent success of crystallography was then due to the development of diffraction equipment, the theory of the solution of the phase problem, symmetry theory and computers. The many structures becoming known called for the development of crystal chemistry and of data banks. Diffuse scattering from disordered structures without and with partial long-range order allows determination of short-range order. Neutron and electron scattering and diffraction are also mentioned.