In-situ ASAXS and NMR Contrast Variation for Composite Materials

Small-angle x-ray scattering (SAXS) probes the difference between the electron density of different parts of a composite material. The technique characterisesthe structure of inhomogeneous materials on a nanometre to micrometrelength scale. A variant of the technique, anomalous SAXS or ASAXS, usesx-rays of a particular energy which coincides with the absorption edge ofan element present in the sample. This results in an emphasis on thoseparts of the sample which contain the edge element and therefore enablesstructural investigations with chemical selectivity. Recent advances insynchrotron technology, such as fast tunable monochromators and novelRapid detectors, to be implemented at the new Diamond source, will allowus to use ASAXS with complex materials and with good time resolution, sothat in-situ experiments while varying a process parameter become possiblefor the first time. This includes sintering experiments at high temperatureas well as studies of reactivity of solid materials such as refractoriesand catalysts with liquid or gaseous reactants.We adopt a similar strategy to enhance the chemical selectivity ofNuclear Magnetic Resonance (NMR) spectroscopy. While NMR is inherentlyelement specific, Cross Polarisation (CP) techniques, which transfermagnetisation between atoms of different elements, can be used to selectpairs of two different atoms. As a consequence, structural investigationsof particular areas in a complex material become possible. Examples aregrain surfaces in nano-crystalline ceramics, which have a high proportionof aluminium-hydrogen pairs due to adsorption of water. CP-NMR extendsthe length range of our structural studies down to the atomic scale.Both ASAXS and CP-NMR require some technique development to enable theirapplication to complex, real-world materials. We have chosen a numberof systems which are complex enough to demonstrate this point whilecontaining both an x-ray absorption edge within the energy range accessibleand two NMR-active nuclei for cross polarisation. In particular, wewill study:-a) the corrosion of refractories (Zr K edge, 1H-27Al NMR)b) ion exchange in LiNbO3 ferroelectric ceramic (Nb K edge, 23Na/7Li-93Nb/29Si NMR)c) co-precipitation of SnO2/SiO2 sol-gel nanocomposite (Sn L edge, 1H-29Si/119Sn)