Atomic level structure of Ge-Sb-S glasses: chemical short range order and long Sb-S bonds

The structure of Ge$_{20}$Sb$_{10}$S$_{70}$, Ge$_{23}$Sb$_{12}$S$_{65}$ and Ge$_{26}$Sb$_{13}$S$_{61}$ glasses was investigated by neutron diffraction (ND), X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS) measurements at the Ge and Sb K-edges as well as Raman scattering. For each composition, large scale structural models were obtained by fitting simultaneously diffraction and EXAFS data sets in the framework of the reverse Monte Carlo (RMC) simulation technique. Ge and S atoms have 4 and 2 nearest neighbors, respectively. The structure of these glasses can be described by the chemically ordered network model: Ge-S and Sb-S bonds are always preferred. These two bond types adequately describe the structure of the stoichiometric glass while S-S bonds can also be found in the S-rich composition. Raman scattering data show the presence of Ge-Ge, Ge-Sb and Sb-Sb bonds in the S-deficient glass but only Ge-Sb bonds are needed to fit diffraction and EXAFS datasets. A significant part of the Sb-S pairs has 0.3-0.4 {\AA} longer bond distance than the usually accepted covalent bond length (~2.45 {\AA}). From this observation it was inferred that a part of Sb atoms have more than 3 S neighbors.Comment: 23 pages, 6 figures, submitted to Journal of Alloys and Compound

Catalytic reactor for operando spatially resolved structure–activity profiling using high-energy X-ray diffraction

In heterogeneous catalysis, operando measurements probe catalysts in their active state and are essential for revealing complex catalyst structure–activity relationships. The development of appropriate operando sample environments for spatially resolved studies has come strongly into focus in recent years, particularly when coupled to the powerful and multimodal characterization tools available at synchrotron light sources. However, most catalysis studies at synchrotron facilities only measure structural information about the catalyst in a spatially resolved manner, whereas gas analysis is restricted to the reactor outlet. Here, a fully automated and integrated catalytic profile reactor setup is shown for the combined measurement of temperature, gas composition and high-energy X-ray diffraction (XRD) profiles, using the oxidative de­hydrogenation of C$_{2}$H$_{6}$ to C$_{2}$H$_{4}$ over MoO$_{3}$/γ-Al$_{2}$O$_{3}$ as a test system. The profile reactor methodology was previously developed for X-ray absorption spectroscopy and is here extended for operando XRD. The profile reactor is a versatile and accessible research tool for combined spatially resolved structure–activity profiling, enabling the use of multiple synchrotron-based characterization methods to promote a knowledge-based optimization of a wide range of catalytic systems in a time- and resource-efficient wa

Projetos de Aprendizagem Mediados por Ambientes Virtuais no Ensino de EngenhariaEl trica

As experiências relacionadas à metodologia de projetos de aprendizagem têm sua origem nas formulações de John Dewey no início do século XX. A concepção de projeto de aprendizagem defendida nesta pesquisa alia às formulações de Dewey as propostas da professora Lea Fagundes. Esta metodologia tem o aluno como responsável pela sua aprendizagem e reflete os conceitos construtivistas de Piaget. A integração e a construção de conhecimento via projetos é facilitada com o uso dos ambientes virtuais para apoio à aprendizagem que são softwares que permitem interações síncronas ou assíncronas além de possibilitar o registro de todo o caminho percorrido pelo estudante e de todas as atividades de uma disciplina. A partir de dados coletados, no período de 2003 a 2007, em atividades escolares de disciplinas do terceiro período letivo do curso de Engenharia Elétrica da Universidade Federal do Espírito Santo pretende-se analisar (a) em que medida os ambientes virtuais de aprendizagem e a apropriação das tecnologias de informação e comunicação acrescentam na formação do estudante de engenharia, em relação às práticas pedagógicas presenciais e (b) em que medida os projetos de aprendizagem contribuem para o enfrentamento dos problemas que temos, hoje, no ensino de engenharia elétric

Microscopic structure and dynamics of glass forming Zr2Co melts and the impact of different late transition metals on the melt properties

We studied the short-range order and the atomic dynamics of stable and undercooled binary Zr2Co alloy melts as well as their density and viscosity. The containerless processing technique of electrostatic levitation was used to achieve deep undercooling and to avoid contaminations. Static structure factors are determined by combining this technique with neutron and high energy X-ray diffraction. Co self-diffusion coefficients are measured by quasielastic neutron scattering. Our results reveal that the short-range order of the Zr2Co melts closely resembles that previously observed for Zr64Ni36. We consider this as the origin of the very similar melt dynamics of these two alloys at same temperatures. On the other hand, the difference in the structure and dynamics when compared with those of Zr2Cu and Zr2Pd shows clearly that not only the atomic sizes, but also electronic properties or chemical bonding have an important influence on the melt properties of Zr-based glass forming melts

In situ uniaxial pressure cell for x-ray and neutron scattering experiments

We present an in situ uniaxial pressure device optimized for small angle x-ray and neutron scattering experiments at low-temperatures and high magnetic fields. A stepper motor generates force, which is transmitted to the sample via a rod with an integrated transducer that continuously monitors the force. The device has been designed to generate forces up to 200 N in both compressive and tensile configurations, and a feedback control allows operating the system in a continuous-pressure mode as the temperature is changed. The uniaxial pressure device can be used for various instruments and multiple cryostats through simple and exchangeable adapters. It is compatible with multiple sample holders, which can be easily changed depending on the sample properties and the desired experiment and allow rapid sample changes

The Effect of pH on the Structure and Lateral Organization of Cardiolipin in Langmuir Monolayers

Cardiolipin (CL) is a unique anionic phospholipid with a dimeric structure. Two phosphatidyl moieties are connected by a central glycerol group. With its four alkyl chains, it has a large hydrophobic region and the charged hydrophilic head group is relatively small. Biological membranes exhibit CL exclusively in the inner bacterial and mitochondrial membranes. Alteration of the CL packing can lead to structural changes and membrane instabilities. One environmental influence is the change in pH. Since the acidic properties of the phosphate head groups remain still controversial in literature, this work is engaging the influence of pH on the ionization degree of CL. For comprehensive analyses, the well-known surface pressure (π) – molecular area (A) isotherm experiments were combined with total reflection X-ray fluorescence (TRXF) and grazing incidence X-ray diffraction (GIXD). Continuous ionization with a high CL packing density was observed in the monolayer over a wide pH range (3 to 8). At physiological pH, the head groups carry a charge of 1.5. No individual pKa values can be assigned to the two phosphate groups. Instead, a mutual influence is observed

Effect of non-mesogenic chiral terphenylate on the formulation of room temperature ferroelectric liquid crystal mixtures suitable for display applications

By varying the concentration of a chiral terphenylate dopant in a four-component achiral phenyl pyrimidine-based host mixture we have formulated six ferroelectric liquid crystal mixtures. Though the dopant is non-mesogenic it is observed that only 2 wt% of it induces polarity in the host mixture at 32 °C. With increasing concentration, the mixtures show ferroelectric behavior even below ambient temperature down to at least 12 °C. Optical polarizing microscopy, differential scanning calorimetry, dielectric and electro-optic studies reveal that with respect to temperature range, phase sequence, spontaneous polarization, optical tilt angle and switching time the formulated mixtures are suitable for ferroelectric based liquid crystal displays

Phase behaviour and structural properties of two members of biphenylyl benzoate chiral mesogenic series

Phase behaviour and structural properties of two members of biphenylyl benzoate chiral meso-genic series (2F3R and 3F3R) have been investigated. While both the compounds exhibit SmC*phase over a wide temperature range, 2F3R forms orthogonal SmA* from tilted SmC* on heatingbut 3F3R melts directly to the isotropic phase. The SmA* phase of 2F3R is found to have de vriescharacteristics with small effective layer contraction. Both the samples on cooling form hexagonalSmF* phase below SmC* phase. On further cooling soft crystal like hexagonal SmJ* phase isformed in 2F3R, undergoing a change in the tilt direction, but in 3F3R, SmG* phase is formedwithout any change in the tilt direction. A coexistence phase of (SmC*+SmF*) is also observed ina certain temperature range. Slight differences in the dipole moment and molecular conforma-tion of the two molecules give rise to a subtle change in the intermolecular interaction and playan important role in the appearance of different phases in the two compounds. Cell parametersof SmF*, SmG*, SmJ* phases have also been determined. Layer spacings, tilt angles, averageintermolecular spacings and correlation lengths have been measured. How some of theseproperties compare with other members of the series has been discussed