Application of Synchrotron Radiation Techniques for Model Validation of Advanced Structural Materials

Synchrotron radiation techniques represent powerful tools to characterize materials down to the nanometer level. This paper presents a survey of the state-of-the-art synchrotron-based techniques which are particularly well-suited for investigating materials properties. Complementary X-ray absorption techniques such as extended X-ray absorption fine structure (EXAFS), X-ray magnetic circular dichroism (XMCD), photoemission electron microscopy (PEEM) are used to address the individual local atomic structure and magnetic moments in Fe–Cr model systems. The formation of atomic clusters/precipitates in such systems is also investigated by means of scanning transmission X-ray microscopy (STXM). Such advanced analytical techniques can not only offer valuable structural and magnetic information on such systems, they can also serve for validating computational calculations performed at different time and length scales which can help improve materials lifetime predictions.

Synchrotron Light Techniques for the Investigation of Advanced Nuclear Reactor Structural Materials

Didier Gavillet, Wolfgang Hoffelner, Manuel Alexandre Pouchon

In the frame of the Generation IV initiative, different structural material candidates are investigated at the Paul Scherrer Institute. These are oxide dispersion strengthened (ODS) steels, intermetallic materials and ceramic composite materials. The response of the material to different potential loads (irradiation, temperature …) is addressed in a multi-scale approach, both, modelling wise and also experimentally. The investigation of each scale delivers at least a qualitative understanding of possibly evolving damage in the material and also delivers a validation of the corresponding scale on the modelling side. From the experimental side, the lower end of the scale, the atomistic and structural level, can be investigated by conventional techniques, as for example transmission electron microscopy (TEM) and X-ray diffraction (XRD). However, the use of synchrotron radiation techniques offers an ideal, complementary way to investigate the material structure and other properties. This paper presents applications in the field of the ODS research, where the structural behaviour of the nanoscopic dispersoids can selectively be investigated, although only being present with roughly 5 wt % in the matrix. A study showing the structural behaviour of these oxide particles as a function of irradiation illustrates the potential of the extended X-ray absorption fine structure (EXAFS) technique. Using X-ray magnetic circular dichroism (XMCD), which is a difference-signal of two X-ray absorption spectra recorded for positive and negative helicities of the beam, the magnetic structure and some magnetic parameters, can be resolved. An example shows, how this can be applied to understand (Fe,Cr) systems, which is the base alloy of the investigated ODS steel. The results deliver an important crosscheck for modelling. Beside the presentation of these techniques, this paper shows how beamline techniques can serve nuclear research, with possibly activated materials. At the Paul Scherrer Institute, a sample holder for highly active materials has been developed, and has already served for EXAFS measurements at the Swiss Light Source (SLS). The set-up of this sample holder is briefly presented here.

Nuclear Energy Agency, Organisation for Economic Co-operation and Development2007

Investigation of low-dimensional supramolecular architectures by low-temperature scanning tunneling microscopy

Sylvain Clair

In this thesis we report investigations on supramolecular architectures assembled at well-defined metal surfaces. Supramolecular chemistry is dedicated to the study and use of non-covalent interactions to build highly organized molecular arrangements, aiming ultimately at creating systems with tailored properties and useful functions. The adaptation of its powerful principles to fabricate molecular systems at surfaces has already been proven to be very promising; the extended possibilities for the choice of on the one hand molecules with defined size, shape, symmetry and function, and on the other hand substrates with controlled composition, symmetry and patterning allow for a quasi-infinite tuning of the structure and properties of the respective assemblies. The Scanning Tunneling Microscope (STM) is a tool particularly efficient for the nanoscale elucidation of supramolecular systems at surfaces, simultaneously providing topographic and spectroscopic information at the single-atom level. In this regard, a new Low-Temperature STM (LT-STM) operational at temperatures in the range 5 to 400 K has been constructed. A detailed description of the instrument is provided and the demonstration of its exquisite performance represents a major achievement. In addition to local STM studies, complementary experiments were performed with a synchrotron radiation source using integral techniques, namely X-ray Absorption Spectroscopy (XAS) and X-ray Magnetic Circular Dichroism (XMCD). We present results obtained with the linear organic linker 1,4-benzenedicarboxylic acid (terephthalic acid - TPA) on the Au(111), Cu(111) and Cu(100) surfaces. The three-dimensional design principles of supramolecular chemistry could be successfully adapted to these low-dimensional systems. TPA molecules adsorbed on Au(111) organize in two-dimensional molecular sheets, wherein the typical one-dimensional hydrogen-bonded chain motif is found. We show that the inhomogeneities induced by the surface reconstruction can be used to examine the response of the supramolecular self-assembly. In particular, variations in the hydrogen bond length of up to 20% are encountered. On Cu(111), a strict commensurability of the supramolecular sheet to the substrate is not possible because of the reduced lattice spacing of the latter, and the creation of defects is analyzed. Due to their peculiar reactivity, the elbow sites of the reconstructed Au(111) surface act as nucleation centers in the epitaxial growth of transition metals (Fe, Co, Ni), leading to the formation of regular arrays of nanoscale metallic islands. We take advantage of this patterning to steer the formation of metallosupramolecular nano-architectures. Co-deposited TPA molecules attack both Fe and Co clusters and metal-carboxylate linkages evolve. Depending on the deposition parameters, nanoporous grids, mesoscopically organized stripes or truly one-dimensional linkages are obtained. Furthermore, the dynamics of the formation of the nanogrids are monitored in situ. Finally, we report investigations of the magnetic and electronic properties of surface-supported coordination architectures by means of XAS and XMCD. We studied Fe-terephthalate systems engineered on a Cu(100) substrate. Both mononuclear Fe(TPA)4 compounds and diiron centers structures in 2D Fe-TPA lattices exhibit a paramagnetic behavior. Moreover, we demonstrate the decisive influence of the ligand rather than the substrate on the electronic ground state of the metal centers, thus illustrating new vistas to effectively tailor the valence state and magnetic moment of transition metal atoms at surfaces.

EPFL2004

Investigation of low-dimensional supramolecular architectures by low-temperature scanning tunneling microscopy

Sylvain Clair

EPFL2004

Synchrotron Light Techniques for the Investigation of Advanced Nuclear Reactor Structural Materials

Didier Gavillet, Wolfgang Hoffelner, Manuel Alexandre Pouchon

Nuclear Energy Agency, Organisation for Economic Co-operation and Development2007