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951.
Motivated by recent attenuation experiments on finely grained samples, we reanalyse the Raj-Ashby model of grain-boundary sliding. Two linearly elastic layers having finite thickness and identical elastic constants are separated by an interface (grain boundary) whose location is a given periodic function of position. Dissipation is confined to that interfacial region. It is caused by two mechanisms: a slip (boundary sliding) viscosity, and grain-boundary diffusion, with corresponding Maxwell relaxation times tv and td. Owing to the assumption of a given, time-independent interface, the resulting boundary-value problem (b.v.p.) is linear and time-separable. The response to time-periodic forcing depends on angular frequency ω, on the ratio M=tv/td of Maxwell times, and on the characteristic interface slope. The b.v.p. is solved using a perturbation method valid for small slopes. To relate features of the mechanical loss spectrum previously studied in isolation, we first discuss the solution as a function of M. Motivated by experiments, we then emphasize the case M?1 in which the relaxation times are widely separated. The loss spectrum then always has two major features: a frequency band 1?ωtd?M-1 within which the loss varies relatively weakly with ω; and a loss maximum at ωtd∼M-1 due to the slip viscosity. If corners on the interface are sufficiently rounded, those two universal features are separated by a third feature: between them, there is a strong minimum whose location is (entirely) independent of slip viscosity. The existence of that minimum has not previously been reported. These features are likely to occur even in solutions for finite interface slopes, because they are a consequence of the separation of timescales. The precise form of the spectrum in the weakly varying band must, however, be slope-dependent because it is controlled by stress singularities occurring at corners, and the strength of those singularities depends on the angle subtended by the corner. 相似文献
952.
Juliet A. Gerrard Susie J. Meade Jonathan C. Morris Peter J. Steel 《Acta Crystallographica. Section C, Structural Chemistry》2000,56(10):1251-1253
The orthorhombic form of 2‐hydroxycyclopent‐2‐enone, C5H6O2, consists of chains of hydrogen‐bonded molecules aligned along a twofold screw axis. The monoclinic form contains two independent molecules, which have different orientations of the hydroxyl proton, and which assemble into ribbons along a twofold screw axis. 相似文献
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Papri Bhattacharya Zachariah M. Heiden Geoffrey M. Chambers Samantha I. Johnson R. Morris Bullock Michael T. Mock 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(34):11744-11750
Catalysts for the oxidation of NH3 are critical for the utilization of NH3 as a large‐scale energy carrier. Molecular catalysts capable of oxidizing NH3 to N2 are rare. This report describes the use of [Cp*Ru(PtBu2NPh2)(15NH3)][BArF4], (PtBu2NPh2=1,5‐di(phenylaza)‐3,7‐di(tert‐butylphospha)cyclooctane; ArF=3,5‐(CF3)2C6H3), to catalytically oxidize NH3 to dinitrogen under ambient conditions. The cleavage of six N?H bonds and the formation of an N≡N bond was achieved by coupling H+ and e? transfers as net hydrogen atom abstraction (HAA) steps using the 2,4,6‐tri‐tert‐butylphenoxyl radical (tBu3ArO.) as the H atom acceptor. Employing an excess of tBu3ArO. under 1 atm of NH3 gas at 23 °C resulted in up to ten turnovers. Nitrogen isotopic (15N) labeling studies provide initial mechanistic information suggesting a monometallic pathway during the N???N bond‐forming step in the catalytic cycle. 相似文献
954.
Tommy Whewell Valerie R. Seymour Kieran Griffiths Nathan R. Halcovitch Aamod V. Desai Russell E. Morris A. Robert Armstrong John M. Griffin 《Magnetic resonance in chemistry : MRC》2022,60(5):489-503
Conjugated alkali metal dicarboxylates have recently received attention for applications as organic anode materials in lithium- and sodium-ion batteries. In order to understand and optimise these materials, it is important to be able to characterise both the long-range and local aspects of the crystal structure, which may change during battery cycling. Furthermore, some materials can display polymorphism or hydration behaviour. NMR crystallography, which combines long-range crystallographic information from diffraction with local information from solid-state NMR via interpretation aided by DFT calculations, is one such approach, but this has not yet been widely applied to conjugated dicarboxylates. In this work, we evaluate the application of NMR crystallography for a set of model lithium and sodium dicarboxylate salts. We investigate the effect of different DFT geometry optimisation strategies and find that the calculated NMR parameters are not systematically affected by the choice of optimisation method, although the inclusion of dispersion correction schemes is important to accurately reproduce the experimental unit cell parameters. We also observe hydration behaviour for two of the sodium salts and provide insight into the structure of an as-yet uncharacterised structure of sodium naphthalenedicarboxylate. This highlights the importance of sample preparation and characterisation for organic sodium-ion battery anode materials in particular. 相似文献
955.
Steven T. Parslow Najlaa K. Almazrouei Michael I. Newton Elizabeth R. Dye Robert H. Morris 《Magnetic resonance in chemistry : MRC》2019,57(9):695-699
Moisture content of foodstuffs are typically assessed by Titration or Near Infrared Spectroscopy, which are labour-intensive as a manual measurement or costly when automated. Magnetic resonance offers a method for moisture evaluation but is also normally costly. In this work, we revisit Look and Locker's “Tone Burst” experiment with a marginal oscillator to evaluate moisture content of powdered–skimmed milk subjected to increased humidity. We refer to this technique as the Transient Effect Determination of Spin–Lattice (TEDSpiL) relaxation times. Moisture content in the samples ranged from 0–12% as determined from the weight gained by the dry powder when re-suspended in water to reach a concentration of 40% w/v. The relaxation properties of re-hydrated samples were measured with a CW NMR sensor. Solutions made up from powders with a higher retained moisture content provided lower measured relaxation values providing a method of measuring the moisture content of the powder. This technique provides a moisture measurement in under 5∼s compared with several minutes for the equivalent pulsed method using low-field hardware. 相似文献
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Balister Paul Bollobás Béla Lee Jonathan D. Morris Robert Riordan Oliver 《Archiv der Mathematik》2019,112(4):371-375
Archiv der Mathematik - We present a short and purely combinatorial proof of Linnik’s theorem: for any $$\varepsilon >0$$ there exists a constant $$C_\varepsilon $$ such that for any... 相似文献
960.
Duarte A Slutsky M Hanrahan G Mello CM Bazan GC 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(3):756-759
Electrostatic nanoassemblies were employed to identify bacterial growth conditions. They comprise a cationic conjugated oligoelectrolyte and fluorescein-tagged ssDNA and were optimized with a hybrid, computational neural network model. The photoluminescence spectra contained the oligomer and sensitized fluorescein emission. The spectra changed depending on the growth history of the bacteria introduced (see figure). 相似文献