Hartmut Fuess

The synchrotron powder diffractometer at beamline B2 at HASYLAB/DESY: status and capabilities

The synchrotron powder diffraction beamline B2 at HASYLAB/DESY is described. The beamline is capable of high-resolution powder diffraction as well as time-resolved studies and offers several sophisticated ancillary equipments for special applications. A newly developed image-plate system allows for kinetic studies with good resolution in the minutes range. Numerous sample environments allow for various standard applications including structure solution, kinetic studies and in situ observations under flexible and well defined conditions. Representative examples are shown for these setups, which are also supported for experiments of external users.

Synthesis and characterization of carbon-supported Pt–Ru–WOx catalysts by spectroscopic and diffraction methods

Carbon-supported Pt–Ru–WOx/C catalysts for application in PEMFC anodes were synthesized by a modified Bönnemann method. Their electrocatalytic activity for the oxidation of H2/CO mixtures and CH3OH was measured by E/i-curves in PEM single cell arrangements under working conditions. Information about composition, microstructure and nanomorphology was obtained by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray fluorescence analysis (XFA) and transmission electron microscopy (TEM). X-ray diffraction data at room temperature show only one single Pt f.c.c. phase; no evidence of Ru, W and their oxides, respectively, is found. Hence, the presence of W and Ru as amorphous oxide species seems likely. Surface-sensitive XPS measurements detect Pt0, platinum oxide and hydroxide species, metallic Ru, ruthenium oxide, hydrous ruthenium oxide and WO3. For the crystalline platinum phase particle sizes of less than 2 nm were determined by TEM images and XRD patterns via solving the Scherrer equation. Temperature-dependent XRD measurements were performed to show the influence of ageing on the catalyst structure.

Field-induced phase transition in Bi1/2Na1/2TiO3- based lead-free piezoelectric ceramics

The origin of the electric field-induced strain in the polycrystalline ceramic 0.92Bi1/2Na1/2TiO3–0.06BaTiO3–0.02K1/2Na1/2NbO3 was investigated using in situ high-resolution X-ray and neutron diffraction techniques. The initially existing tetragonal phase with pseudocubic lattice undergoes a reversible phase transition to a significantly distorted rhombohedral phase under electric field, accompanied by a change in the oxygen octahedral tilting from a0a0c+ to a−a−a− and in the tilting angle. The polarization values for the tetragonal and rhombohedral phases were calculated based on the structural information from Rietveld refinements. The large recoverable electric field-induced strain is a consequence of a reversible electric field-induced phase transition from an almost nonpolar tetragonal phase to a ferroelectrically active rhombohedral phase.

Nanodomains in morphotropic lead zirconate titanate ceramics : On the origin of the strong piezoelectric effect

The outstanding piezoelectric properties of lead zirconate titanate (PZT) ceramics with compositions close to the morphotropic phase boundary of the quasibinary phase diagram of lead zirconate and lead titanate are still under debate. A combination of ex situ and in situ transmission electron microscopy and high resolution x-ray diffraction revealed that the extrinsic piezoelectric effect in morphotropic PZT is closely connected to the existence of nanodomains. The in situ transmission electron microscopy investigations with applied electric field show that mainly the nanodomains respond to the electric field while the microdomain structure does not change noticeably in our experiments.

Spinel‐Si3N4: Multi‐Anvil Press Synthesis and Structural Refinement

The third known polymorph of silicon nitride, which is cubic and was only recently discovered, has been prepared from two further, different precursors—Si2N2(NH) and a-Si3N4—in a high-pressure, high-temperature synthesis using multi-anvil presses. The synthesis and characterization of the products is described, which included a structural determination by Rietveld refinement of powder X-ray diffraction data. Spinel-type c-Si3N4 is significantly harder than the α and β phases and may possibly find applications as an ultrahard material.

Composition dependence of the domain configuration and size in Pb(Zr1−xTix)O3 ceramics

The composition dependent variation of domain configuration and size in Pb(Zr1−xTix)O3 (PZT) has been investigated in a detailed transmission electron microscopy study in the range of 0.40⩽x⩽0.55. Single phase composition, Pb(Zr0.45Ti0.55)O3 and Pb(Zr0.60Ti0.40)O3, the former belonging to the tetragonal, the latter to the rhombohedral phase, feature small microdomain widths coupled with a pronounced bimodal domain distribution. Samples with compositions around the morphotropic phase boundary exhibit a decrease of bimodal distribution and an increase in microdomain width associated with nanodomain formation. The investigation of micro- and nanodomains, as well as the bimodal distribution of microdomains in undoped PZT ceramics, with respect to composition, is reported. We define nanodomains as “domains arranged within microdomains possessing a width of a few nanometers.” The strict alternation of the two orientation variants of microdomains is denoted as “bimodal domain distribution,” and is characterized ...

Microstructural studies of nanocrystalline CeO2 produced by gas condensation

Abstract Nanocrystalline ceria powders prepared by inert gas condensation using thermal evaporation were characterized by high resolution transmission electron microscopy. No significant differences were observed between the powders collected in the different parts of the UHV chamber. The crystallite size distributions are narrow with maxima between 3 and 3.5 nm diameter. The crystallite growth was studied by X-ray diffraction line profile analysis. Transmission electron microscopy shows that particles develop cubeoctahedra shapes during annealing in the temperature range of 400 ° to 800 °C. The crystallites grow individually by a binary coalescence process and only very few grain boundaries were observed. The size of about 25–30% of all crystallites is not affected by sintering at 600 °C. Significant changes occur in the sample annealed at 800 °C when two populations of crystallites develop. The TEM and X-ray diffraction results agree very well. These studies were completed by BET measurements.

Characterization of differently synthesized Pt-Ru fuel cell catalysts by cyclic voltammetry, FTIR spectroscopy, and in single cells

Carbon-supported Pt-Ru (1:1) catalysts were synthesized from aqueous solutions of Pt(IV) and Ru(IV) salts by two different reductive methods and characterized in comparison to a commercial Pt-Ru/C catalyst purchased from E-TEK. Inc. The three catalysts differ in particle size. dispersion, and degree of alloying, as determined by X-ray diffraction and transmission electron microscopy. Cyclic voltammetry in different methanol concentrations and CO-stripping experiments were conducted to check their electrocatalytic activity. The results obtained are in good agreement with single-cell measurements using H 2 /CO mixtures with concentrations of 75 and 150 ppm CO. The synthesized catalysts show improved activities for low CO concentrations at 75°C cell temperature. In addition, for the synthesized catalysts only low CO coverages were found at the electrode surface by special in situ infrared reflectance techniques in contrast to the commercial one.

Characterization of different Pt–Ru catalysts by X-ray diffraction and transmission electron microscopy

Carbon-supported Pt–Ru (1:1)-catalysts have been synthesized by two reduction methods in an aqueous phase and in a third way in an organic solution and characterized in comparison to a commercial Pt–Ru/C catalyst purchased from E-TEK. X-ray diffraction and transmission electron microscopy were carried out on the different samples in the as-synthesized state and after heat-treatment at 500°C in nitrogen and air atmospheres respectively. Powder patterns of the different catalysts in the as-synthesized state reveal a fcc pattern with d values matching or close to platinum. No evidence of metallic Ru or any oxide phases was found. After heat-treatment in nitrogen Ru reflections occur in the synthesized catalyst samples, backing the supposition of separate platinum and ruthenium particles instead of a Pt–Ru alloy.

Hrtem study of nanocrystalline zirconia powders

Abstract Zirconia powders with grain sizes between 4 and 35 nm were synthesized by the gas condensation technique. Different qualities (grain size, grain size distribution) of powders have been observed and can be attributed to different synthesis conditions (mainly temperature gradients) within the UHV chamber. The existence of necks between single particles has been observed. These necks are due to a first sintering process during the oxidation of the metal/ suboxide clusters condensed on the walls in the synthesis chamber. Several stages of this sintering process are documented by high-resolution micrographs. Furthermore, some particles exhibit a shell-like structure with monoclinic ZrO2 in the outer region and tetragonal ZrO2 in the inner region. This is an indication of (i) the martensitic phase transition character of zirconia in general, and (ii) the existence of a critical particle size in n-ZrO2 responsible for the metastability of a tetragonal polymorph in nanosized zirconia powder. Lattice distortions in some directions can be concluded from image reconstruction of the digitized micrograph of a single nanosized ZrO2 particle.