1,720,988 research outputs found
Disorder at the nanoscale in perovskite titanates and doped ceria: multiple approaches to modelling total scattering data
Get PDFCrystal structures can take a number of forms and their relationship with the physics of a solid lends itself to an even greater number of interpretations. Ferroelectricity, for example, can be predicted to occur in certain symmetries, but sometimes it can be hidden in the response to an electric field, in a peculiar electron- or nuclear-spin resonance, in the unfolding of a phase transition, or else. In any of these cases, the emergence of ferroelectricity is intimately related to subtle changes in the atomic positions with respect to a high-symmetry parent structure. As is the case with many other physical properties, this often implies that the crystal structure is disordered locally (as opposed to on average or long-range), but the magnitude and the time- and length-scales of disorder can vary dramatically between different materials.
This work combines multiple X-ray and neutron powder diffraction techniques to try to capture the most of the structural complexity hidden in the bulk of materials. First, reciprocal-space analysis by the Rietveld method allowed drawing the crystallographic phase diagram of a novel perovskite solid solution, praseodymium-doped strontium titanate (SPTO). SPTO is a perovskite that combines a centrosymmetric antiferrodistortive (AFD) structure and a large, diffuse dielectric response at room temperature, prompting the question of whether AFD and ferroelectric (FE) distortions coexist in the material. Subsequently, the Pair Distribution Function (PDF) of SPTO, representing the instantaneous spatial arrangement of the atoms in the material, was analysed using multiple approaches. X-ray PDF was fitted with structural models obtained by evaluating possible polar and rotational distortions of the cubic perovskite phase. The most accurate, a tetragonal structure derived from the long-range phase, showed a huge AFD distortion that could also be consistent with a local FE mode. To get a better insight into the local structure, neutron PDF data were collected and modelled by i) fitting the distorted tetragonal model over successive ranges in real space (`box-car� refinements) and ii) atomistic simulations using the Reverse Monte Carlo (RMC) method. This led to a model in which FE distortion caused by Pr-doping at the perovskite A-site spreads to the AFD framework of octahedra, resulting in distorted coordination of both A and B cations.
Neutron PDF data of undoped strontium titanate was also used in the development of another approach to PDF modelling, in which structural models representing a unique soft-mode distortion are evaluated statistically against the experimental neutron PDF. This symmetry-adapted approach evidenced that four soft modes among those suggested by theoretical phonon dispersion are active distortions in SrTiO3, and have signatures in the local structure determined experimentally. This approach is aimed at obtaining an unbiased local structural model of strontium titanate, in that symmetry lowerings are inferred directly from the amplitudes of the individual soft modes refined against the PDF instead of assessing the goodness of fit of an arbitrarily chosen model.
Whereas all these methods focused on a length scale of 10 �(1 nm), the last approach to PDF modelling presented in this work studies the coherence length of lower-symmetry nanometric domains in a solid solution by evaluating structural parameters up to 600 �(60 nm). Instead of titanates, though, this study involved yttrium-doped ceria: thanks to the extremely high structural coherence of crystalline ceria and owing to the high angular resolution of the instrument used, the data collected lend themselves very well to analysing the PDF well beyond the short range. The aim of this approach is to obtain a structural model for every slice of real space analysed, in order to describe in terms of order parameters how the local structure coalesces into the average crystallographic phase
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Sugar-Based Catalysts For Oxygen Reduction Reaction
No full textNon-noble metal catalysts for the electrochemical reduction of oxygen (ORR) in acidic and alkaline solutions have been produced by pyrolysis of a silica gel containing fructose, as a carbon source, and derivatives of guanidine, as nitrogen compounds. The importance of the formation of iron complexes with nitrogen precursors to obtain good oxygen reduction reaction (ORR) catalysts has been investigated by varying the functionalization of guanidine derivatives. The presence of secondary or tertiary amines on guanidine limits or hinders the formation of iron complexes and, as a result, the produced materials are not very active. Conversely, the presence of primary amines promotes the formation of iron complexes, and, during the pyrolysis, of iron nanoparticles and ordered graphitic planes, making the final material very active for ORR. An explanation of this behavior in terms of surface and bulk composition has been proposed. An attempt to find the ORR rate determining step for the samples in both alkaline and acidic solutions has been done
Combining control of branching and sulfonation in one-pot synthesis of random sulfonated polyarylethersulfones: effect on thermal stability and water retention
No full textIn functional polymers with tunable hydrophilic behaviour like sulfonated polyarylethersulfone (SPES) high ionic conductance can be obtained directly by increasing the concentration of sulfonic moieties along the macromolecular chain. This, however, often comes to the cost of excessive water sorption, which can lead to membrane rupture and consequent device failure. To overcome this drawback, and to reconcile high hydrophilicity with the high mechanical properties of aromatic polymers, we propose the use of SPES copolymers with a low degree of branching. A series of branched SPES was synthesized using homogeneous (one-pot) copolymerization by replacing an amount up to 0.9% mol of the difunctional monomer 4,4'-dihydroxydiphenyl with the trifunctional 1,3,5-trihydroxybenzene (THB). The polymers were characterized by H-1-NMR spectroscopy, intrinsic viscosity, water sorption measurements, thermogravimetric analysis and differential scanning calorimetry. Like linear SPES, branched SPES is totally amorphous and soluble in polar aprotic solvents, lending itself to easy fabrication of membranes and coatings. Besides reducing the water sorption and in-plane swelling of SPES membranes upon equilibration in liquid, branching also improves the retention of water in swollen membranes
Sulphonated Poly (Aryl Ether Sulphone) as electrode modifier in electroanalytical applications
No full textA general problem encountered during the use of solid electrodes is the lowering of the electroanalytical response, caused by partial blocking of the surface by adsorbable reaction products. The use of polymeric materials to modify and protect the electrodes can minimize such alteration, allowing the detection of different species also in complex matrixes. Among different polymers, Nafion® is one of the most used [1-3], although scarcely studied and not appropriately designed for electrode modification.
Sulphonated Poly (Aryl Ether Sulphones), commonly called S-PES, are engineered thermoplastic materials [4] that, due to their aromatic skeleton and charged sulphonic groups, present interesting properties: thermal and mechanical strength, resistance to oxidation and acid catalyzed hydrolysis, high glass transition temperature, good solubility in polar aprotic and halogenated solvents, radiation stability, low flammability and toughness, together with low costs.
In this work, sulphonated Poly (Aryl Ether Sulphones) (S-PES), prepared by copolymerization with sulphonated monomers, have been studied as new materials for the production of modified electrodes for electroanalytical applications. A comparison with the use of Nafion® has been also made.
After a complete characterization by Cyclic Voltammetry and Electrochemical Impedance Spectroscopy (EIS), the electroanalytical performances of the modified electrodes (sensitivity, LOD, LOQ, stability in air and solvent, ...) have been evaluated in function of different morphological and preparative parameters: the quantity and the form of the polymer (acidic, salt, with different macromolecular architecture, ...), different IECs, the method of drying, the casting solvent.
The results demonstrate that 1 % linear PES in the acidic form, dried at 25 °C in a oven, after deposition from a N-Methylpyrrolidone solution presents the best performances in terms of higher voltammetric peak currents, more stability and less resistive behaviour, superior to Nafion®, maintaining the partial electrochemical and chemical reversibility and the diffusive control.
[1] V. Pifferi, V. Marona, M. Longhi, L. Falciola, Electrochimica Acta, 109, (2013), 447-453.
[2] L.M. Moretto, F. Montagner, R. Ganzerla, P. Ugo, Anal Bioanal Chem, 405, (2013), 3603-3610.
[3] C.M.A. Brett, V.A. Alves, D.A. Fungaro, Electroanalysis, 13(3), (2001), 212-218.
[4] R.T.S. Muthu Lakshmi, J. Meier-Haack, K. Schlenstedt, H. Komber, V. Choudhary and I.K. Varma, Reactive and Functional Polymers, 66 (6), (2006), 634–644
Homogeneous synthesis and characterization of sulfonated polyarylethersulfones having low degree of sulfonation and highly hydrophilic behavior
No full textIn the present paper, the wettability of polyarylethersulfones (PESs) was promoted through the controlled introduction of sulfonic groups in PES polymeric chain. Homogeneous synthesis using a sulfonated comonomer introduced sulfonic groups while exerting a tight control over the degree of sulfonation (DS) and avoiding the undesired side reactions brought about by heterogeneous sulfonation reactions. A series of sulfonated polyarylethersulfones (SPESs) with very low amounts of sulfonic groups - 0.5, 0.75, and 1 meq SO3−*g−1 of polymer - was synthesized via polycondensation using 4,4’-difluorodiphenylsulfone, 4,4’-dihydroxydiphenyl and a sulfonated comonomer, 2,5-dihydroxybenzene-1-sulfonate potassium salt. The presence of sulfonic groups was confirmed by Fourier transform infrared (FTIR) spectra; the macromolecular structure and the real DS of SPESs were determined by 1H NMR; the molecular weights were investigated by size exclusion chromatography (SEC); ion exchange capacity (IEC) values, measured by potentiometric titration, are in good agreement with the experimental DS. The polyelectrolyte effect was studied on intrinsic viscosity (IV) measurements. The effect of DS on the thermal properties of SPES membranes was studied by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC); wetting properties were characterized by static water contact angle (SWCA) measurements on membranes obtained via solution casting. The results showed that an increased DS results in higher glass transition temperatures (Tg) and lower water contact angles, the latter dropping from (Formula presented.) to (Formula presented.) as DS was raised from 0 to 1.0 meq SO3−*g−1. This work demonstrates that homogeneous synthesis of SPES is an efficient way to prepare SPES membranes with tightly controlled DS and enhanced hydrophilic properties, in particular, an excellent hydrophilic SWCA even at very low amounts of sulfonic moieties
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
- …
