Let (M, g) be a 4-dimensional Einstein Riemannian manifold. At each point p of M, the tangent space admits a so-called Singer-Thorpe basis (ST basis) with respect to the curvature tensor R at p. In this basis, up to standard symmetries and antisymmetries, just 5 components of the curvature tensor R are nonzero. For the space of constant curvature, the group O(4) acts as a transformation group between ST bases at TpM and for the so-called 2-stein curvature tensors, the group Sp(1) ? SO(4) acts as a transformation group between ST bases. In the present work, the complete list of Lie subgroups of SO(4) which act as transformation groups between ST bases for certain classes of Einstein curvature tensors is presented. Special representations of groups SO(2), T2, Sp(1) or U(2) are obtained and the classes of curvature tensors whose transformation group into new ST bases is one of the mentioned groups are determined. 相似文献
The complexes [PdX2Py]2(di-NHC) (X = Br or Cl) in which di-NHC represents a di-N-heterocyclic carbene, featuring a rigid phenylene spacer between the carbene units, have been prepared from reactions of the corresponding diimidazolium halide salts with PdCl2 in pyridine. The molecular structures of three of the complexes were determined by X-ray diffraction studies. The influences of different substitutions and of the halide ligand (Br or Cl) on the structure and reactivity of the complexes have been studied. The catalytic activity of the binuclear palladium complexes was tested in the Mizoroki–Heck reaction of styrene with bromobenzene. 相似文献
A novel 5,15-di-[4-carboxylatomethoxy]phenyl-10,20-diphenylporphyrin, its copper complex and the corresponding metalloporphyrin-TiO2 photocatalyst were synthesized and characterized by DRS, SEM, XRD, and FT–IR. The photocatalytic effects of anataseTiO2 impregnated with this copper(II) porphyrin was investigated by photodegradation of 4-nitrophenol(4-NP) in aqueous solution under Xenon lamp irradiation. The results indicated that the photoactivity of copper(II) porphyrin-TiO2 composite was evidently enhanced by the interaction between carboxyl of the porphyrin molecule and hydroxyls anchored on the TiO2. Futhermore, the copper(II) carboxylic porphyrin displayed good adsorption behavior and activity of the dye-sensized TiO2 system. 相似文献
The high-dimensional (that is, three-dimensional (3D)) assembly of nanomaterials is an effective means of improving their properties; however, achieving this assembly at the atomic level remains challenging. Herein, we obtained a novel nanocluster, [Au8Ag57(Dppp)4(C6H11S)32Cl2]Cl (Dppp=1,3-bis(diphenylphosphino)propane) showing a 3D octameric assembly mode involving the kernel penetration of eight complete icosahedral Au@Ag10Au2 units for the first time. The atomically precise structure was determined by single-crystal X-ray diffraction, and further confirmed by thermogravimetric analysis, X-ray photoelectron spectroscopy, and electrospray ionization mass spectrometry measurements. Furthermore, ligand-induced transformation prompted the conversion of [Au8Ag57(Dppp)4(C6H11S)32Cl2]Cl, with complete octameric fusion into [Au8Ag55(Dppp)4(C6H11S)34][BPh4]2, with incomplete octameric fusion. These observations will hopefully facilitate further research on the assembly of M13 nanobuilding blocks. 相似文献
Crystallography Reports - For structures with a C2PO2 skeleton, a comparison of “single-and-half” phosphorus–oxygen bonds with single and double phosphorus–oxygen bonds is... 相似文献
Nonlinear Dynamics - A new nonlinear adaptive control strategy for electrohydraulic active suspensions with the hysteretic leaf spring is proposed to improve suspension performances of heavy... 相似文献
The paper presents a new concept of absorbing car body vibrations, which consists in a modification of the construction of the classical mono-tube hydraulic shock absorber by the introduction of an additional inner cylinder with an auxiliary piston. By making an appropriate selection of the system parameters, one may obtain the damping force characteristics dependent on the excitation amplitude and frequency. In the case of driving on a good-quality road surface, the shock absorber displays the soft characteristics which are desired as far as the driving comfort is concerned. In the case of worse-quality roads or while overcoming large obstacles, the hard characteristics ensure a higher level of safety and protect the shock absorber from getting damaged. The developed nonlinear model makes it possible to effectively analyse the system responses to harmonic, impulse and random excitations. On the basis of the analysis of the impact of harmonic excitations on the driving comfort and safety indexes, one may estimate the optimal values of the shock absorber construction parameters. Impulse and random excitations are applied in order to finally verify the effectiveness of the operation of the proposed shock absorber.
A design of ultrathin crystalline silicon solar cells patterned with α-NaEr_(0.2)Y_(0.8)F_4 upconversion nanosphere(NSs) arrays on the surface was proposed. The light trapping performance ofα-NaEr_(0.2)Y_(0.8)F_4 NSs with different ratios of sphere diameter to sphere pitch was systematically studied by COMSOL Multiphysics. The influence of different NS diameters and ratio to the average optical absorption of ultrathin crystalline silicon solar cell was calculated, as well as the short circuit current densities. The results show that the average optical absorption of solar cells with 2.33 μm silicon covered by α-NaEr_(0.2)Y_(0.8)F_4 NSs of 100 nm in diameter and 5.2 in ratio has improved by 8.5% compared to planar silicon solar cells with the same thickness of silicon. The light trapping performance of different thicknesses of silicon solar cells with the optimized configuration of NSs was also discussed. The results indicate that our structure enhances the light absorption. The presented model will be the basis for further simulations concerning frequency upconversion of α-NaEr_(0.2)Y_(0.8)F_4 materials. 相似文献
Experimental Mechanics - Kinetics of cell sickling and morphological change have been recognized as important parameters that are correlated closely with altered blood rheology and vasoocclusion in... 相似文献