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81.
From a microscopic point of view, various natural and engineering materials consist of individual grains, whose motion strongly influence the macroscopic material behaviour. Exemplarily, one may look at the development of shear zones in natural granular materials, such as sand, occurring as a result of local grain dislocations and the transition of the granulate from a denser to a looser packing. The intuitive modelling approach for granular assemblies is consequently the consideration of each grain as a rigid particle. In a numerical framework, this leads to the Discrete Element Method (DEM), wherein the motion of each particle can be obtained solving Newton's equations for each particle. The present contribution discusses the basic fundaments of modelling granular material on the microscopic scale by use of the DEM. Special interest is taken to the constitutive choice of the governing particle-to-particle contact forces, as they have to account for plastic material behaviour as well as for assumptions concerning particle shape, size and distribution. As engineering problems are regularly described on the macroscale by means of continuum mechanics, a homogenisation strategy transfers the information from the microscale towards continuum quantities via volume averaging. Therefore, characteristic Representative Elementary Volumes (REV) are constructed by an ensemble of particles, where each particle can be chosen as the centre of a REV. (© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
82.
Sami Mustapha 《偏微分方程通讯》2013,38(1-2):245-275
Abstract We study the regularity of the free boundary in the two membranes problem. We prove that around any point the free boundary is either a C 1, α surface or a cusp, as in the obstacle problem. We also prove C 1, 1 regularity for the pair of functions solving the problem. 相似文献
83.
84.
Background
Neurogenesis continues to occur throughout life but dramatically decreases with increasing age. This decrease is mostly related to a decline in proliferative activity as a result of an impoverishment of the microenvironment of the aged brain, including a reduction in trophic factors and increased inflammation.Results
We determined that human umbilical cord blood mononuclear cells (UCBMC) given peripherally, by an intravenous injection, could rejuvenate the proliferative activity of the aged neural stem/progenitor cells. This increase in proliferation lasted for at least 15 days after the delivery of the UCBMC. Along with the increase in proliferation following UCBMC treatment, an increase in neurogenesis was also found in the aged animals. The increase in neurogenesis as a result of UCBMC treatment seemed to be due to a decrease in inflammation, as a decrease in the number of activated microglia was found and this decrease correlated with the increase in neurogenesis.Conclusion
The results demonstrate that a single intravenous injection of UCBMC in aged rats can significantly improve the microenvironment of the aged hippocampus and rejuvenate the aged neural stem/progenitor cells. Our results raise the possibility of a peripherally administered cell therapy as an effective approach to improve the microenvironment of the aged brain. 相似文献85.
Hui Shen Guocheng Deng Sami Kaappa Tongde Tan Ying‐Zi Han Sami Malola Shui‐Chao Lin Boon K. Teo Hannu Hkkinen Nanfeng Zheng 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(49):17895-17899
Surface organic ligands play a critical role in stabilizing atomically precise metal nanoclusters in solutions. However, it is still challenging to prepare highly robust ligated metal nanoclusters that are surface‐active for liquid‐phase catalysis without any pre‐treatment. Now, an N‐heterocyclic carbene‐stabilized Au25 nanocluster with high thermal and air stabilities is presented as a homogenous catalyst for cycloisomerization of alkynyl amines to indoles. The nanocluster, characterized as [Au25(iPr2‐bimy)10Br7]2+ (iPr2‐bimy=1,3‐diisopropylbenzimidazolin‐2‐ylidene) ( 1 ), was synthesized by direct reduction of AuSMe2Cl and iPr2‐bimyAuBr with NaBH4 in one pot. X‐ray crystallization analysis revealed that the cluster comprises two centered Au13 icosahedra sharing a vertex. Cluster 1 is highly stable and can survive in solution at 80 °C for 12 h, which is superior to Au25 nanoclusters passivated with phosphines or thiols. DFT computations reveal the origins of both electronic and thermal stability of 1 and point to the probable catalytic sites. This work provides new insights into the bonding capability of N‐heterocyclic carbene to Au in a cluster, and offers an opportunity to probe the catalytic mechanism at the atomic level. 相似文献
86.
Planar reconstruction patterns at the zigzag and armchair edges of graphene were investigated with density-functional theory. It was unexpectedly found that the zigzag edge is metastable and a planar reconstruction spontaneously takes place at room temperature. The reconstruction changes electronic structure and self-passivates the edge with respect to adsorption of atomic hydrogen from a molecular atmosphere. 相似文献
87.
Sami M. Al-Jaber 《International Journal of Theoretical Physics》2008,47(7):1853-1864
We compute the energy eigenvalues for the N-dimensional harmonic oscillator confined in an impenetrable spherical cavity. The results show their dependence on the size
of the cavity and the space dimension N. The obtained results are compared with those for the free N-dimensional harmonic oscillator, and as a result, the notion of fractional dimensions is pointed out. Finally, we examine
the correlation between eigenenergies for confined oscillators in different dimensions. 相似文献
88.
α–Fe2O3/TiO2 Composite powders have been prepared by high energy ball-milling for different times. The composites were studied using Mössbauer Spectroscopy (MS) and X-ray diffraction (XRD). The patterns of XRD show broadening in the diffraction peaks, indicating a decrease in the particle size of the composites with milling time. Also, the XRD patterns show an evolving new structural phase correlated with an evolving Titanium ferrite species with milling time. Mössbauer Spectroscopy shows the evolving titanium ferrite species characterized by a quadrupole doublet at the expense of the α–Fe2O3 represented by the magnetic sextet. The doublet corresponding to the Ti-ferrite phase dominates the Mössbauer spectra at long milling time (greater than 100 h of milling). 相似文献
89.
90.