Ralph Gilles

Scientific Review: The Structure Powder Diffractometer SPODI

The structure powder diffractometer SPODI at FRM II is a common project of Darmstadt University of Technology, Ludwig-Maximilians University Munich, and Technische Universität München. The main objectives of the instrument are to achieve a high Q-resolution and to provide a versatile and powerful sample environment. In addition to common applications of high-resolution powder diffraction to determine crystallographic and magnetic structures, the diffractometer SPODI is developed particularly for materials science applications. Prior to the construction, individual components as well as the whole instrument have been simulated by Monte Carlo technique applying the McStas software [1]. Results of these studies have been reported in [2,3].

Application of silver behenate powder for wavelength calibration of a SANS instrument – a comprehensive study of experimental setup variations and data processing techniques

Silver behenate powder [CH3(CH2)20COOAg] is one of the very few materials featuring cold neutron Bragg reflections in the angular range of 4–26° (2θ) accessible to small-angle neutron scattering (SANS) instruments. These reflections can be analysed with Braggs law to determine the wavelength of the primary beam produced by the monochromator of the SANS instrument. Compared with the usual time-of-flight (TOF) method for wavelength calibration, this method does not require any additional time-resolving equipment for the instrument. An extensive study has been performed to determine the optimum parameters for the experimental setup and for the data processing procedure. The results of wavelength calibrations using silver behenate and TOF measurements are compared.

Evaluation of anisotropic small-angle neutron scattering data; a faster approach

An improvement of the anisotropic small-angle neutron scattering (SANS) data evaluation program is presented. The program is particularly suited for treatment of data from dense precipitate systems like those appearing in single crystal Ni-base superalloys. A new model of the precipitate microstructure was implemented allowing for a significant shortening of the evaluation time which is necessary mainly for in-situ studies. The change concerns the mode in which the size distribution is calculated. The local random smearing of the size as well as the distance of particles was introduced, too, which leads to a more realistic look of the microstructure model. The form of individual cuboidal particle was changed as well. The characteristics of the new microstructure model with respect to the SANS data modeling are tested on several measured as well as simulated data.

A Combined SAXS/SANS Study for the in Situ Characterization of Ligand Shells on Small Nanoparticles: The Case of ZnO.

ZnO nanoparticles (NPs) have great potential for their use in, e.g., thin film solar cells due to their electro-optical properties adjustable on the nanoscale. Therefore, the production of well-defined NPs is of major interest. For a targeted production process, the knowledge of the stabilization layer of the NPs during and after their formation is of particular importance. For the study of the stabilizer layer of ZnO NPs prepared in a wet chemical synthesis from zinc acetate, only ex situ studies have been performed so far. An acetate layer bound to the surface of the dried NPs was found; however, an in situ study which addresses the stabilizing layer surrounding the NPs in a native dispersion was missing. By the combination of small angle scattering with neutrons and X-rays (SANS and SAXS) for the same sample, we are now able to observe the acetate shell in situ for the first time. In addition, the changes of this shell could be followed during the ripening process for different temperatures. With increasing size of the ZnO core (d(core)) the surrounding shell (d(shell)) becomes larger, and the acetate concentration within the shell is reduced. For all samples, the shell thickness was found to be larger than the maximum extension of an acetate molecule with acetate concentrations within the shell below 50 vol %. Thus, there is not a monolayer of acetate molecules that covers the NPs but rather a swollen shell of acetate ions. This shell is assumed to hinder the growth of the NPs to larger macrostructures. In addition, we found that the partition coefficient μ between acetate in the shell surrounding the NPs and the total amount of acetate in the solution is about 10% which is in good agreement with ex situ data determined by thermogravimetric analysis.

Monte Carlo simulations of the new small-angle neutron scattering instrument SANS-1 at the Heinz Maier-Leibnitz Forschungsneutronenquelle

A new small-angle scattering instrument SANS-1 will be installed on beamline NL 4a at the Heinz Maier-Leibnitz Forschungsneutronenquelle (FRM II). It is a joint venture between the Technische Universitat Munchen and the Geesthacht Neutron Facility (GENF). SANS-1 has been optimized to be one of the most intense and versatile small-angle scattering instruments within the boundaries of available space and interaction with neighbouring instruments. Using the program McStas, the dimensions and the features of the different optical components were investigated and compared for the final selection. A vertical S-shaped neutron guide, a tower with two possible selectors, one for medium resolution at high intensity and one for high resolution, and two optimized transmission polarizers are the main advantages of SANS-1 compared with traditional instruments at other facilities.

Counterion complexation of calixarene ligands in monolayers and micellar solutions

Abstract Calix[n]arenes and their derivatives are currently the object of several studies, due to their peculiar cavity, that is suitable for very specific and efficient complexation of ions and small organic molecules, with a high degree of efficiency and selectivity, and form host–guest systems in the solid and in the liquid state. Besides some very important applications as ion carriers and cages, they can be used for studying the counterion distribution in micellar aggregates formed by anionic amphiphiles such as alkyl-sulfates. In this paper we report the studies on the complexing properties of a new calixarene derivative, namely the 1,3-dioctyloxy-calix[4]arene-crown-6-ether (CAL), that shows a high selectivity for cesium ions, at the air/water interface and in aqueous micellar solutions of cesium dodecyl sulfate. Langmuir surface pressure (π/A) isotherms were performed in order to study the stability of CAL films and the effect of cesium and potassium ions on the monolayer properties. In addition, small-angle neutron-scattering (SANS) experiments were carried out in order to determine the structure of the micellar system, and the interactions between the ligand and the micellar interface. Our study shows that the charge screening at the micellar interface is the predominant phenomenon that rules over the ion transport across liquid membranes, that is usually performed by macrocyclic ligands.

Mesoscopic structures of triglyceride nanosuspensions studied by small-angle X-ray and neutron scattering and computer simulations.

Aqueous suspensions of platelet-like shaped tripalmitin nanocrystals are studied here at high tripalmitin concentrations (10 wt % tripalmitin) for the first time by a combination of small-angle X-ray and neutron scattering (SAXS and SANS). The suspensions are stabilized by different lecithins, namely, DLPC, DOPC, and the lecithin blend S100. At such high concentrations the platelets start to self-assemble in stacks, which causes interference maxima at low Q-values in the SAXS and SANS patterns, respectively. It is found that the stack-related interference maxima are more pronounced for the suspension stabilized with DOPC and in particular DLPC, compared to suspensions stabilized by S100. By use of the X-ray and neutron powder pattern simulation analysis (XNPPSA), the SAXS and SANS patterns of the native tripalmitin suspensions could only be reproduced simultaneously when assuming the presence of both isolated nanocrystals and stacks of nanocrystals of different size in the simulation model of the dispersions. By a fit of the simulated SAXS and SANS patterns to the experimental data, a distribution of the stack sizes and their volume fractions is determined. The volume fraction of stacklike platelet assemblies is found to rise from 70% for S100-stabilized suspensions to almost 100% for the DLPC-stabilized suspensions. The distribution of the platelet thicknesses could be determined with molecular resolution from a combined analysis of the SAXS and SANS patterns of the corresponding diluted tripalmitin (3 wt %) suspensions. In accordance with microcalorimetric data, it could be concluded that the platelets in the suspensions stabilized with DOPC, and in particular DLPC, are significantly thinner than those stabilized with S100. The DLPC-stabilized suspensions exhibit a significantly narrower platelet thickness distribution compared to DOPC- and S100-stabilized suspensions. The smaller thicknesses for the DLPC- and DOPC-stabilized platelets explain their higher tendency to self-assemble in stacks. The finding that the nanoparticles of the suspension stabilized by the saturated lecithin DLPC crystallize in the stable β-tripalmitin modification with its characteristic platelet-like shape is surprising and can be explained by the fact that the main phase transformation temperature for DLPC is, as for unsaturated lecithins like DOPC and S100, well below the crystallization temperature of the supercooled tripalmitin emulsion droplets.

Neutron and synchrotron probes in the development of Co-Re-based alloys for next generation gas turbines with an emphasis on the influence of boron additives

Nickel-based superalloys are the materials of choice in the hot section of current gas turbines, but they are reaching temperature limits constrained by their melting temperature range. Co–Re alloy development was prompted by a search for new materials for future gas turbines, where the temperature of application will be considerably higher. Addition of the very high melting point refractory metal Re to Co can increase the melting range of Co alloys to much higher temperatures than the commercial Co alloys in use today. The alloy development strategy is first discussed very briefly. In this program, model ternary and quaternary compositions were studied in order to develop a basic understanding of the alloy system. In situ neutron and synchrotron measurements (small and wide angle) at high temperatures were extensively used for this purpose and some selected results from the in situ measurements are presented. In particular, the effect of boron doping in Co–Re–Cr alloys and the stability of the TaC precipitates at high temperatures were investigated. A fine dispersion of TaC precipitates strengthens some Co–Re alloys, and their stability at the application temperature is critical for the long-term creep properties.

The measurement of internal strain in core–shell Ni3Si(Al)–SiOx nanoparticles

Internal defects and strain in nanoparticles can influence their properties and therefore measuring these values is relevant. Powder diffraction techniques (neutron and synchrotron) are successfully used to characterize internal strain in the core-shell Ni(3)Si(Al)-SiO(x) nanoparticles having mean diameters of approximately 80 nm. The nanoparticles, which are strain-free after extraction from the bulk alloys, develop internal strain on heating. Both micro- and macro-strains can be measured from the analysis of Bragg peak shift and broadening. It is identified that differences in thermal expansion coefficient of the metallic core and the amorphous shell of the nanoparticles, as well as partial disordering of the L1(2) ordered core phase, are the main causes of strain evolution. Synchrotron measurements also detected partial crystallization of the amorphous silica shell.

Ni3Si(Al)/a-SiOx core?shell nanoparticles: characterization, shell formation, and stability

We have used an electrochemical selective phase dissolution method to extract nanoprecipitates of the Ni(3)Si-type intermetallic phase from two-phase Ni-Si and Ni-Si-Al alloys by dissolving the matrix phase. The extracted nanoparticles are characterized by transmission electron microscopy, energy-dispersive x-ray spectrometry, x-ray powder diffraction, and electron powder diffraction. It is found that the Ni(3)Si-type nanoparticles have a core-shell structure. The core maintains the size, the shape, and the crystal structure of the precipitates that existed in the bulk alloys, while the shell is an amorphous phase, containing only Si and O (SiO(x)). The shell forms around the precipitates during the extraction process. After annealing the nanoparticles in nitrogen at 700 °C, the tridymite phase recrystallizes within the shell, which remains partially amorphous. In contrast, on annealing in air at 1000 °C, no changes in the composition or the structure of the nanoparticles occur. It is suggested that the shell forms after dealloying of the matrix phase, where Si atoms, the main constituents of the shell, migrate to the surface of the precipitates.