1,752 research outputs found
Energetics and crystal chemistry of Ruddlesden-Popper type structures in high T(sub c) ceramic superconductors
The formation of Ruddlesden-Popper type layers (alternating slabs of rocksalt and perovskite structures) is seen in these oxides which is similar in many respects to what is seen in the system Sr-Ti-O. However, it was observed that there are some significant differences, for example the rocksalt and perovskite blocks in new superconducting compounds are not necessarily electrically, unlike in Sr-Ti-O systems. This will certainly render an additional coulombic bonding energy between two different types of blocks and may well lead to significant differences in their structural chemistry. In the higher order members of the various homologous series, additional Cu-O planes are inserted in the perovskite blocks. In order for the unit cell to electrically neutral the net positive charge on rocksalt block (which remains constant throughout the homologous series) should be balanced by an equal negative charge on perovskite block. It, thus becomes necessary to create oxygen vacancies in the basic perovskite structure, when width of the perovskite slab changes on addition of extra Cu-O planes. Results of atomistic simulations suggest that these missing oxygen ions allow the Cu-O planes to buckle in these compounds. This is also supported by the absence of buckling in the first member of Bi-containing compounds in which there are no missing oxygen ions and the Sr-Ti-O series of compounds. Additional results are presented on the phase stability of polytypoid structures in these crystal chemically complex systems. The studies will focus on the determination of the location of Cu(3+) in the structures of higher order members of the La-Cu-O system and whether Cu(3+) ions or oxygen vacancies are energetically more favorable charge compensating mechanism
Hypercrosslinked materials
This chapter describes the chemistry of hypercrosslinked materials, and presents a description of their synthesis, defining physico-chemical features and their most important applications. The synthesis section will examine the different monomers, precursor polymers, reagents and synthetic strategies used to prepare hypercrosslinked materials. Each synthesis section also details the chemical and morphological properties of the hypercrosslinked materials and the main field of application
Hypercrosslinked materials : preparation, characterisation and applications
This review article provides an overview of hypercrosslinking technology. In particular, it covers the preparation and characterisation of hypercrosslinked materials and their applications. The synthesis section examines the different monomers, precursor polymers and reagents used to prepare hypercrosslinked materials, but also the different synthetic approaches disclosed in the literature. The various chemical modification reactions relevant to this area are also reviewed. Several examples of applications for hypercrosslinked materials are described; these applications are grouped into thematic areas such as chromatography, gas storage and the trapping of organic contaminants
Fermi Surface as the Driving Mechanism for Helical Antiferromagnetic Ordering in Gd-Y Alloys
The first direct experimental evidence for the Fermi surface (FS) driving the
helical antiferromagnetic ordering in a gadolinium-yttrium alloy is reported.
The presence of a FS sheet capable of nesting is revealed, and the nesting
vector associated with the sheet is found to be in excellent agreement with the
periodicity of the helical ordering.Comment: 4 pages, 4 figure
Thermodynamic Structure of the Solar Corona: Tomographic Reconstructions and MHD Modeling
We carry out a study of the global three-dimensional (3D) structure of the
electron density and temperature of the quiescent inner solar corona () by means of tomographic reconstructions and magnetohydrodynamic
simulations. We use differential emission measure tomography (DEMT) and the
Alfv\'en Wave Solar Model (AWSoM), in their latest versions. Two target
rotations were selected from the solar minimum between solar cycles (SCs) 23
and 24 and the declining phase of SC 24. We report in quantitative detail on
the 3D thermodynamic structure of the core and outer layers of the streamer
belt, and of the high latitude coronal holes (CH), as revealed by the DEMT
analysis. We report on the presence of two types of structures within the
streamer belt, loops with temperature decreasing/increasing with height (dubbed
down/up loops), as reported first in previous DEMT studies. We also estimate
the heating energy flux required at the coronal base to keep these structures
stable, found to be or order , consistently with
previous DEMT and spectroscopic studies. We discuss how these findings are
consistent with coronal dissipation of Alfv\'en waves. We compare the 3D
results of DEMT and AWSoM in distinct magnetic structures. We show that the
agreement between the products of both techniques is the best so far, with an
overall agreement , depending on the target rotation and the
specific coronal region. In its current implementation the ASWsoM model can not
reproduce down loops though. Also, in the source region of the fast and slow
components of the solar wind, the electron density of the AWSoM model increases
with latitude, opposite to the trend observed in DEMT reconstructions
Segre Types of Symmetric Two-tensors in n-Dimensional Spacetimes
Three propositions about Jordan matrices are proved and applied to
algebraically classify the Ricci tensor in n-dimensional Kaluza-Klein-type
spacetimes. We show that the possible Segre types are [1,1...1], [21...1],
[31\ldots 1], [z\bar{z}1...1] and degeneracies thereof. A set of canonical
forms for the Segre types is obtained in terms of semi-null bases of vectors.Comment: 14 pages, LaTeX, replaced due to a LaTex erro
Bioactive sol-gel glasses at the atomic scale: the complementary use of advanced probe and computer modelling methods
Sol-gel synthesised bioactive glasses may be formed via a hydrolysis condensation reaction, silica being introduced in the form of tetraethyl orthosilicate (TEOS) and calcium is typically added in the form of calcium nitrate. The synthesis reaction proceeds in an aqueous environment; the resultant gel is dried, before stabilisation by heat treatment. These
materials, being amorphous, are complex at the level of their atomic-scale structure, but their bulk properties may only be properly understood on the basis of that structural insight. Thus, a full understanding of their structure : property relationship may only be achieved
through the application of a coherent suite of leading-edge experimental probes, coupled with the cogent use of advanced computer simulation methods. Using as an exemplar a calcia-silica sol-gel glass of the kind developed by Larry Hench, to whose memory this paper is dedicated, we illustrate the successful use of high-energy x-ray and neutron scattering (diffraction) methods, magic-angle spinning solid state NMR, and molecular dynamics simulation as components to a powerful methodology for the study of amorphous materials
Does ohmic heating influence the flow field in thin-layer electrodeposition?
In thin-layer electrodeposition the dissipated electrical energy leads to a
substantial heating of the ion solution. We measured the resulting temperature
field by means of an infrared camera. The properties of the temperature field
correspond closely with the development of the concentration field. In
particular we find, that the thermal gradients at the electrodes act like a
weak additional driving force to the convection rolls driven by concentration
gradients.Comment: minor changes: correct estimation of concentration at the anode,
added Journal-re
British HIV Association/British Association for Sexual Health and HIV/British Infection Association adult HIV testing guidelines 2020
Experimental investigation of the initial regime in fingering electrodeposition: dispersion relation and velocity measurements
Recently a fingering morphology, resembling the hydrodynamic Saffman-Taylor
instability, was identified in the quasi-two-dimensional electrodeposition of
copper. We present here measurements of the dispersion relation of the growing
front. The instability is accompanied by gravity-driven convection rolls at the
electrodes, which are examined using particle image velocimetry. While at the
anode the theory presented by Chazalviel et al. describes the convection roll,
the flow field at the cathode is more complicated because of the growing
deposit. In particular, the analysis of the orientation of the velocity vectors
reveals some lag of the development of the convection roll compared to the
finger envelope.Comment: 11 pages, 15 figures, REVTEX 4; reference adde
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