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71.
Freita JC Emmerich FG Cernicchiaro GR Sampaio LC Bonagamba TJ 《Solid state nuclear magnetic resonance》2001,20(1-2):61-73
13C high-resolution solid-state nuclear magnetic resonance (NMR) was employed to study carbon materials prepared through the thermal decomposition of four different organic precursors (rice hulls, endocarp of babassu coconut, peat, and PVC). For heat treatment temperatures (HTTs) above about 600 C, all materials presented 13C NMR spectra composed of a unique resonance line associated with carbon atoms in aromatic planes. With increasing HTT a continuous broadening of this resonance and a diamagnetic shift in its central frequency were verified for all samples. The evolution of the magnitude and anisotropy of the magnetic susceptibility of the heat-treated carbon samples with HTT explains well these findings. It is shown that these results are better understood when a comparison is made with the features of the 13C NMR spectrum of polycrystalline graphite, for which the magnetic susceptibility effect is also present and is much more pronounced. 相似文献
72.
M.H. Radke de Cuba H. Emmerich S. Gemming 《The European physical journal. Special topics》2007,149(1):43-56
A hybrid scheme is developed to describe vicinal surface growth during epitaxy on two different time and
length scales. For this purpose this algorithm combines two modules based on a continuum and an atomistic approach.
The continuum module is realized by a phase-field-model which traces back to the Burton–Cabrera–Frank theory, the
atomistic module is based on the anisotropic Ising model which is mapped onto a lattice-gas model. The latter provides thermal
density fluctuations resulting in adatom clustering. With increasing temperature the probability for island nucleation on
the
terraces decreases according to 1-p where p is an Arrhenius-type activation probability which prevents clusters from becoming
islands. Within this framework it is possible to find the transition from a rough surface at low temperatures to an evenly
stepped
surface at high temperatures where slight step meandering is observed. Furthermore two competing mechanisms of step bunching
are
investigated within this scale bridging algorithm: alternating anisotropic diffusion and different Ehrlich–Schwoebel barriers
at
the step edges. It is shown that a simulation of step bunching displaying the full variety of phenomena observed in experiments
can only be achieved by the consideration of different time and length scales. 相似文献
73.
A new two-scale model for liquid phase epitaxy is presented
which
enables the numerical simulation of processes with microstructures having an
arbitrarily small scale.
It is based on a BCF-model for epitaxial growth, a Navier–Stokes system and
a convection-diffusion equation. The application of a homo- genization approach
leads to a separation of scales; the resulting two-scale model
consists of macroscopic partial differential equations for fluid flow and
solute diffusion in the fluid volume, coupled to microscopic BCF-models.
The two-scale model can be discretized using grids that are
independent of the scale of the microstructure.
Numerical experiments based on a phase field version of the
BCF model are presented; the results illustrate the
physical relevance of the model. 相似文献
74.
G. Krishnamoorthy H. Emmerich V. Chalupecký 《The European physical journal. Special topics》2007,148(1):19-26
In this article, graphene is investigated with respect to its electronic properties when introduced into field effect devices
(FED). With the exception of manual graphene deposition, conventional top-down CMOS-compatible
processes are applied. Few and monolayer graphene sheets are
characterized by scanning electron microscopy, atomic force
microscopy and Raman spectroscopy. The electrical properties of
monolayer graphene sandwiched between two silicon dioxide films are studied. Carrier mobilities in graphene
pseudo-MOS structures are compared to those obtained from
double-gated Graphene-FEDs and silicon metal-oxide-semiconductor field-effect-transistors (MOSFETs). 相似文献
75.
76.
Freitas JC Schettino MA Emmerich FG Wong A Smith ME 《Solid state nuclear magnetic resonance》2007,32(4):109-117
The sodium environments in porous carbon materials prepared from NaOH activation of a char were investigated by means of multiple-field solid-state 23Na NMR measurements, carried out at magnetic fields of 4.7, 8.45 and 14.1 T, with single-pulse excitation and magic angle spinning (MAS). The recorded spectra showed a relatively featureless resonance with linewidth and peak shift strongly dependent on the magnetic field strength and on the hydration level of the samples. The existence of second-order quadrupolar effects was inferred, although the structural disorder and the mobile character associated with the Na environment precluded the direct observation of typical quadrupolar features in the MAS NMR spectra. The analysis of the spectra collected at multiple magnetic fields yielded the values of −2.8 ppm for the isotropic chemical shift and 1.8 MHz for the quadrupole coupling constant, which were interpreted as due to Na+ ions bonded to oxygenated groups at the edges of the graphene planes within the carbon pore network. 相似文献
77.
Emmerich Haimer Martin Wendland Kerstin Schlufter Katrin Frankenfeld Peter Miethe Antje Potthast Thomas Rosenau Falk Liebner 《Macromolecular Symposia》2010,294(2):64-74
Bacterial cellulose aerogels overcome the drawback of shrinking during preparation by drying with supercritical CO2. Thus, the pore network of these gels is fully accessible. These materials can be fully rewetted to 100% of its initial water content, without collapsing of the structure due to surface tension of the rewetting solvent. This rehydration property and the high pore volume of these material rendered bacterial cellulose aerogels very interesting as controlled release matrices. Supercritical CO2 drying, the method of choice for aerogel preparation, can simultaneously be used to precipitate solutes within the cellulose matrix and thus to load bacterial cellulose aerogels with active substances. This process, frequently termed supercritical antisolvent precipitation, is able to perform production of the actual aerogel and its loading in one single preparation step. In this work, the loading of a bacterial cellulose aerogel matrix with two model substances, namely dexpanthenol and L-ascorbic acid, and the release behavior from the matrix were studied. A mathematical release model was applied to model the interactions between the solutes and the cellulose matrix. The bacterial cellulose aerogels were easily equipped with the reagents by supercritical antisolvent precipitation. Loading isotherms as well as release kinetics indicated no specific interaction between matrix and loaded substances. Hence, loading and release can be controlled and predicted just by varying the thickness of the gel and the solute concentration in the loading bath. 相似文献
78.
79.
The crystal structure of the new polar compound caesium hydrogen (L)‐aspartate, Cs(C4H6NO4), (abbreviated: Cs(L ‐AspH)) was determined from single crystal X‐ray diffraction data; it comprises two crystallographically different L ‐AspH anions that are connected via caesium cations to form a three dimensional framework. The Cs ions are irregularly sevenfold [Cs1O7] respectively eightfold [Cs2O8] coordinated to all α– and β– carboxylate oxygen atoms. Cs(L ‐AspH) represents a novel structure type of its own, as do most compounds of (L)‐aspartic acid with inorganic constituents. A brief summary of such structurally known aspartates is given. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
80.
Sackers E Osswald T Weber K Keller M Hunkler D Wörth J Knothe L Prinzbach H 《Chemistry (Weinheim an der Bergstrasse, Germany)》2006,12(24):6242-6254
As part of a study to achieve selective oligo(poly)bromination-ultimately perbromination-of the dodecahedral C(20) skeleton, the extent and direction of the ionic bromination of dodecahedrene and 1,16-dodecahedradiene were explored. Along sequences of Br(+) additions/deprotonations and allylic rearrangements, up to ten hydrogen atoms were substituted (traces of C(20)H(x)Br(10)). Tetrabromododecahedrenes obtained under defined conditions in up to 50 % total yield with three and four allylic bromine substituents protecting the extremely bent C==C bonds, proved highly unreactive even towards oxygen but reacted rapidly with CH(2)N(2). Upon electron impact ionization (MS) of the newly secured oligo(poly)bromododecahedra(e)nes, sequential loss of the substituents ended generally in polyunsaturated dodecahedranes (in the extreme C(20)H(4), "tetrahydro-C(20) fullerenes"). Only subsequently did skeletal fragmentations occur. From X-ray crystal-structure analyses, more information was obtained on the structural response of the dodecahedral skeleton to the strain induced by the voluminous substituents. As Appendix, the forcing radical bromination of 1,6-dibromododecahedrane and exploratory cis-beta-HBr/cis-beta-Br(2) eliminations in bromododecahedranes with [Fe(2)(CO)(9)], P(2)F/[FeCp(2)] and [Fe(tmeda)Cp*Cl] (in situ protection) are presented. 相似文献