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61.
Free volume quantities proposed earlier by Boyer and Simha in connection with the glass transition are reformulated by taking into account the temperature dependence of the thermal expansivities α l and αg for the liquid and the glass, respectively. This necessitates an extrapolation of the liquid to temperatures below Tg which is performed by means of the reduced volume-temperature function established and given a theoretical foundation previously. For the glass, low temperature experimental data, encompassing all relaxations occuring below Tg, are required. Two polymer series are examined in detail, namely, poly(methacrylates) and poly(vinyl) alkyl ethers, where αg has been measured between at least 30°K and Tg. Results for poly(methylacrylate) and poly(styrene) are also given. The systematic decrease in the product (αl - αg) · T|T=Tg with increasing length of the side chain noted previously is considerably reduced but not eliminated when the appropriately corrected expression is substituted instead. However, the free volume fraction related to the quantity αlT|T=Tg remains more nearly invariant in the polymers analyzed. An alternative treatment is discussed which considers an occupied volume expanding below Tg by a mechanism of thermal vibrations solely. Experimental and theoretical means of obtaining this quantity arc suggested. 相似文献
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Vera M Leirer Christian Wienbruch Isabella Paul-Jordanov Stephan Kolassa Thomas Elbert Iris T Kolassa 《BMC neuroscience》2010,11(1):113
Background
The hippocampus is a brain region that is particularly affected by age-related morphological changes. It is generally assumed that a loss in hippocampal volume results in functional deficits that contribute to age-related cognitive decline. In a combined cross-sectional behavioural and magnetoencephalography (MEG) study we investigated whether hippocampal-associated neural current flow during a transverse patterning task - which requires learning relational associations between stimuli - correlates with age and whether it is modulated by cognitive competence. 相似文献65.
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We study regularity properties related to Cohen, random, Laver, Miller and Sacks forcing, for sets of real numbers on the \({\varvec{\Delta}^1_3}\) level of the projective hieararchy. For \({\varvec{\Delta}^1_2}\) and \({\varvec{\Sigma}^1_2}\) sets, the relationships between these properties follows the pattern of the well-known Cichoń diagram for cardinal characteristics of the continuum. It is known that assuming suitable large cardinals, the same relationships lift to higher projective levels, but the questions become more challenging without such assumptions. Consequently, all our results are proved on the basis of ZFC alone or ZFC with an inaccessible cardinal. We also prove partial results concerning \({\varvec{\Sigma}^1_3}\) and \({\varvec{\Delta}^1_4}\) sets. 相似文献
68.
Dr. Yuzhou Wu Prof. Dr. Fedor Jelezko Prof. Dr. Martin B Plenio Prof. Dr. Tanja Weil 《Angewandte Chemie (International ed. in English)》2016,55(23):6586-6598
The currently available techniques for molecular imaging capable of reaching atomic resolution are limited to low temperatures, vacuum conditions, or large amounts of sample. Quantum sensors based on the spin‐dependent photoluminescence of nitrogen‐vacancy (NV) centers in diamond offer great potential to achieve single‐molecule detection with atomic resolution under ambient conditions. Diamond nanoparticles could also be prepared with implanted NV centers, thereby generating unique nanosensors that are able to traffic into living biological systems. Therefore, this technique might provide unprecedented access and insight into the structure and function of individual biomolecules under physiological conditions as well as observation of biological processes down to the quantum level with atomic resolution. The theory of diamond quantum sensors and the current developments from their preparation to sensing techniques have been critically discussed in this Minireview. 相似文献
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Johannes Oschwald Dr. David Reger Dr. Stefan Frühwald Vera Warmbrunn Prof. Dr. Andreas Görling Prof. Dr. Norbert Jux Prof. Dr. Thomas Drewello 《Chemphyschem》2023,24(21):e202300496
Gas-phase complexes of [n]helicenes with n=6, 7 and 8 and the silver(I) cation are generated utilizing electrospray ionization mass spectrometry (ESI-MS). Besides the well-established [1 : 1] helicene/Ag+-complex in which the helicene provides a tweezer-like surrounding for the Ag+, there is also a [2 : 1] complex formed. Density functional theory (DFT) calculations in conjunction with energy-resolved collision-induced dissociation (ER-CID) experiments reveal that the second helicene attaches via π-π stacking to the first helicene, which is part of the pre-formed [1 : 1] tweezer complex with Ag+. For polycyclic aromatic hydrocarbons (PAHs) of planar structure, the [2 : 1] complex with silver(I) is typically structured as an Ag+-bound dimer in which the Ag+ would bind to both PAHs as the central metal ion (PAH–Ag+–PAH). For helicenes, the Ag+-bound dimer is of similar thermochemical stability as the π-π stacked dimer, however, it is kinetically inaccessible. Coronene (Cor) is investigated in comparison to the helicenes as an essentially planar PAH. In analogy to the π-π stacked dimer of the helicenes, the Cor−Ag+−Cor−Cor complex is also observed. Competition experiments using [n]helicene mixtures reveal that the tweezer complexes of Ag+ are preferably formed with the larger helicenes, with n=6 being entirely ignored as the host for Ag+ in the presence of n=7 or 8. 相似文献
70.
R. M. Parr A. Fajgelj R. Dekner H. Vera Ruiz F. P. Carvalho P. P. Povinec 《Analytical and bioanalytical chemistry》1998,360(3-4):287-290
For many years the International Atomic Energy Agency (IAEA) has been promoting analytical quality assurance and quality control in its Member States with emphasis on measurands that are amenable to analysis by nuclear and related techniques, i.e. radionuclides, trace elements, and stable isotopes. This paper reviews briefly the rationale for some of these activities, particularly in relation to the needs of participants in developing countries arising out of co-ordinated research programmes, technical co-operation projects and global and regional networks. Emphasis is given to biological and environmental reference materials with a matrix of natural origin. Also described are some activities arising out of the requirements of ISO-25 and other relevant international quality standards. 相似文献