The formation mechanisms for the structural hierarchy in geological media are discussed. It is shown that the formation of such a hierarchy is caused by certain external and internal circumstances. The first ones consist in the fact that, because of external actions, the Earth’s poles continuously execute the translational and rotational motions responsible for a regular structure of rock fracture and for the appearance of the scale factor √2. The second ones consist in the fact that, under the action of many random factors and because of external actions, during the formation of geological media there appear dissipative structures and, hence, some self-similar fractal structures are formed. 相似文献
The hexa-4-dodecyloxybenzoyl derivative 1 of azacrown [18]-N6 was originally reported to have a 'tubular' mesophase on the basis of its large central ring and 6-fold symmetry. Starting in the mesophase, annealing of 1 under a cover slip results in formation of a new crystalline phase that melts directly to an isotropic liquid at the temperature previously observed for the mesophase to isotropic transition. Thus the phase behaviour of 1 is kinetically controlled. The analogous hexa-3,4-bisdodecyloxybenzoyl derivative 2 of azacrown [18]-N6 has no kinetic limitations to its phase changes and has an enantiotropic columnar liquid crystalline phase. We have synthesized side chain copolysiloxanes with a (CH2)11 spacer and 75-84 per cent by weight of the same 4-dodecyloxybenzoyl-[18]-N6 mesogen. The polysiloxanes also display a liquid crystalline phase. 相似文献
There are growing research interests in flax fibers due to their renewable ‘green’ origin and high strength. However, these natural fibers easily absorb moisture and have poor adhesion with polymer matrix leading to low interfacial strength for the composites. A hybrid chemical treatment technique combining alkali (sodium hydroxide) and silane treatments is adopted in the current study to modify flax fibers for improved performances of flax/polypropylene composites. Changes in chemical composition, microstructure, wettability, surface morphology, crystallinity and tensile properties of single flax fiber before and after chemical treatments were comprehensively characterized using techniques including SEM, FTIR, AFM, XRD, micro-fiber tester, etc. It was found that hemicellulose and lignin at the fiber surface were removed due to alkali treatment, which helped to reduce moisture absorption of the composites. Alkali-treated flax fibers were later subjected to silane treatment, which helped to improve the compatibility between flax fiber and polypropylene matrix. After alkali-silane hybrid chemical treatment, moisture absorption of the composites was further decreased. At the same time, the interfacial bonding strength between flax and polypropylene is significantly enhanced. All these results validate the great advantage of the hybrid chemical treatment approach for flax/polypropylene composites, which has the potential to promote the application of chemical treatment techniques in the plant fiber composite industry.
In this paper, we report a new method for coarse-grained elastic normal-mode analysis. The purpose is to overcome a long-standing problem in the conventional analysis called the tip effect that makes the motional patterns (eigenvectors) of some low-frequency modes irrational. The new method retains the merits of a conventional method such as not requiring lengthy initial energy minimization, which always distorts structures, and also delivers substantially more accurate low-frequency modes with no tip effect for proteins of any size. This improvement of modes is crucial for certain types of applications such as structural refinement or normal-mode-based sampling. 相似文献
The copper-based catalysts have been generally regarded as high-performance catalysts for CO_2 hydrogenation toward methanol,while the production of ethanol via C–C coupling on the copper-based catalysts is still challenging. Herein, we report a new catalyst where Cu nanoparticles are embedded in the carbon support with abundant defect sites, achieving a high selectivity for ethanol in the CO_2 hydrogenation. The experiments coupled with the theoretical studies show a clear map where carbon defects serve as anchor sites that can stabilize interfacial copper species, and interfacial Cu sites with low coordination numbers can adsorb two C_1 species and later convert them to a C_2 species via a hydrogenation-induced coupling reaction. Further adjacent Cu atoms of interfacial Cu sites can facilitate OH reduction reactions via the Cu–Cu bridge adsorption to assist the formation of ethanol. Especially, those specific active sites easily disappear in the reducing conditions and during the reaction, the major product can transform from ethanol to methanol. 相似文献
The emergence of atomically precise metal nanoclusters with unique electronic structures provides access to currently inaccessible catalytic challenges at the single-electron level. We investigate the catalytic behavior of gold Au25(SR)18 nanoclusters by monitoring an incoming and outgoing free valence electron of Au 6s1. Distinct performances are revealed: Au25(SR)18− is generated upon donation of an electron to neutral Au25(SR)180 and this is associated with a loss in reactivity, whereas Au25(SR)18+ is generated from dislodgment of an electron from neutral Au25(SR)180 with a loss in stability. The reactivity diversity of the three Au25(SR)18 clusters stems from different affinities with reactants and the extent of intramolecular charge migration during the reactions, which are closely associated with the valence occupancies of the clusters varied by one electron. The stability difference in the three clusters is attributed to their different equilibria, which are established between the AuSR dissociation and polymerization influenced by one electron. 相似文献