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31.
Pr(BO2)3 and PrCl(BO2)2: Two Praseodymium meta‐Borates in Comparison Single‐crystalline PrCl(BO2)2 can be obtained by the reaction of praseodymium, Pr6O11 and PrCl3 with a small excess of B2O3 in evacuated silica tubes after seven days at 850 °C. If NaCl is additionally used as flux, single crystals of Pr(BO2)3 dominate the main product. Both praseodymium(III) meta‐borates are air and water stable. The crystals of PrCl(BO2)2 emerge as long, thin, pale green needles which tend to severe twinning due to their fibrous habit. The crystal structure (triclinic, P1¯; a = 420.56(4), b = 655.42(7), c = 808.34(8) pm, α = 82.361(8), β = 89.173(9), γ = 71.980(7)°, Z = 2) exhibits zigzag chains {[(B1)ot1/1Oe2/2(B2)Ot1/1Oe2/2]2−} (≡ {[BO2]−}) of corner‐linked [BO3]3− triangles with syndiotactic orientation of the terminal oxygen atoms which are running parallel to the [100] direction. The Pr3+ cations are surrounded by three Cl− and seven O2− anions with the shape of a tetracapped trigonal prism. The green, transparent crystals of Pr(BO2)3 (monoclinic, C2/c; a= 984.98(9), b = 809.57(8), c = 641.02(6) pm, β = 126.783(9)°, Z = 4) appear either lath‐shaped or rather spherical. In the crystal structure the B3+ cations reside both in trigonal planar as well as in tetrahedral coordination of oxygen atoms. Both types of borate polyhedra ([BO3]3− and [BO4]5−) are linked via corners to form chains of the composition {[(B2)‐Ot1/1Oe2/2(B1)Oe4/2(B2)Ot1/1Oe2/2]3−} (≡ {[BO2]−}) which run parallel [101]. The coordination sphere of the Pr3+ cations consists of ten oxide anions which build up a bicapped square antiprism. 相似文献
32.
Sm4S3[Si2O7] and NaSm9S2[SiO4]6: Two Sulfide Silicates with Trivalent Samarium The sulfide silicates Sm4S3[Si2O7] and NaSm9S2[SiO4]6 are obtained as light yellow transparent crystals by the reaction of Sm, Sm2O3, S, and SiO2 with fluxing SmCl3 or NaCl, respectively, in suitable molar ratios in fused evacuated silica tubes (850 °C, 7 d). Tetragonal crystals of Sm4S3[Si2O7] (I41/amd; Z = 8; a = 1186.4(1); c = 1387.0(2) pm) with ecliptically conformed [Si2O7]6–‐groups of corner sharing [SiO4]‐tetrahedra are formed. These double tetrahedra as well the sulfide anions (S2–) coordinate two crystallographically independent metal cations. They provide coordination numbers of 8 + 1 (5 S2– and 3 + 1 O2–) for Sm1 and 9 (3 S2– and 6 O2–) for Sm2. NaSm9S2[SiO4]6 crystallizes hexagonally (P63/m; Z = 1; a = 975.32(9); c = 676.46(7) pm) in a modified bromapatite‐type structure. The coordination spheres about the two crystallographically different Sm3+ cations are built up by oxygen atoms of the orthosilicate units ([SiO4]4–) and sulfide anions (S2–). As a result, Sm1 and Sm2 have coordination numbers of 9 and 8, respectively. Na+ and (Sm1)3+ occupy the position 4 f in a molar ratio of 1 : 3 whereas the lower coordinated (Sm2)3+ occupies the 6 h position. 相似文献
33.
Sm3Cl[SiO4]2: A Chlorine‐poor Chloride Orthosilicate of Samarium Pale yellow, plate‐like single crystals of Sm3Cl[SiO4]2 (orthorhombic, Pnma; a = 701.74(8), b = 1800.8(2), c = 626.63(7) pm; Z = 4) are obtained upon the reaction of SmCl3, Sm2O3 and SiO2 (”︁Kieselgel”︁”︁) in 1 : 4 : 6 molar ratios, most advantageously in the presence of substantial amounts of NaCl as fluxing agent, after seven days at 850 °C in evacuated silica ampoules. The B‐type crystal structure (isotypic with e. g. Yb3Cl[SiO4]2) contains discrete orthosilicate tetrahedra [SiO4]4– which form anionic double layers ({(Sm1)2[SiO4]2}2–) with (Sm1)3+. These are alternatingly sheethed along [010] with cationic monolayers ({(Sm2)Cl}2+) consisting of (Sm2)3+ and Cl–. Both crystallographically independent Sm3+ cations exhibit coordination numbers of eight (Sm1: 1 Cl + 7 O; Sm2: 2 Cl + 6 O) with respect to the involved electronegative particles. 相似文献
34.
On the pollution effect of quasi‐compressibility methods in magneto‐hydrodynamics and reactive flows
Andreas Prohl 《Mathematical Methods in the Applied Sciences》1999,22(17):1555-1584
The study of the asymptotics of quasi‐compressibility methods is an essential tool to construct and improve numerical schemes in hydrodynamics, like e.g., stabilized finite element methods or time‐splitting projection methods. The goal of this work is to illustrate different asymptotical solution behaviour for equations in magneto‐hydrodynamics and those that describe reactive flows for standard quasi‐compressibility methods that influences the design of numerical algorithms. Copyright © 1999 John Wiley & Sons, Ltd. 相似文献
35.
M. Wanke M. Franz M. Vetterlein G. Pruskil B. Hpfner C. Prohl I. Engelhardt P. Stojanov E. Huwald J.D. Riley M. Dhne 《Surface science》2009,603(17):2808-2814
We report on angle-resolved photoelectron spectroscopy results of thin dysprosium-silicide layers formed on Si(1 1 1), taken with a toroidal analyzer allowing to image the energy surfaces in k||-space. At monolayer dysprosium coverages, where hexagonal DySi2 grows with a 1×1 superstructure, electron pockets are observed at the points with highly anisotropic effective masses, and around a hole pocket at the point also an anisotropic dispersion is found. The band filling of these two bands amounts to one, indicating an even number of electrons assigned to the surface unit cell. Similar features are found for multilayer coverages, where hexagonal Dy3Si5 layers are formed with a superstructure. Here, the influence of zone folding effects due to the reconstructed layers in the bulk silicide is only weak because of the high surface sensitivity of the experiments. 相似文献
36.
In the paper methods for the coupling of different meshed local domains of a product model, which occur in an adaptivity process are compared. The properties of the applied coupling methods are demonstrated on finite element models from structural mechanics to point out the advantages and disadvantages of the methods in a model adaptivity process in the field of automotive engineering. (© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
37.
We propose and compare two classes of convergent finite element based approximations of the nonstationary Nernst–Planck–Poisson
equations, whose constructions are motivated from energy versus entropy decay properties for the limiting system. Solutions
of both schemes converge to weak solutions of the limiting problem for discretization parameters tending to zero. Our main
focus is to study qualitative properties for the different approaches at finite discretization scales, like conservation of
mass, non-negativity, discrete maximum principle, decay of discrete energies, and entropies to study long-time asymptotics. 相似文献
38.
Ghattas W Hess CR Iacazio G Hardré R Klinman JP Réglier M 《The Journal of organic chemistry》2006,71(22):8618-8621
An efficient preparation of pure ethyl Z- and E-alpha,alpha-difluoro-4-phenyl-3-butenoate 1a and 1b together with the corresponding acids 2a and 2b is described. The procedures involve stereocontrolled additions of *CF2CO2Et to phenylacetylene or beta-bromostyrene. Compound 1a is easily obtained by addition of *CF2CO2Et to phenylacetylene via a mechanism where the stereochemistry is controlled by an electron-transfer process to produce predominantly the Z vinyl anion. The product 1b is obtained by *CF2CO2Et addition-elimination to Z- or E-beta-bromostyrenes via a mechanism where the stereochemistry is controlled by steric factors in the conformational equilibration of the intermediates. 相似文献
39.
Alfio Grillo Raphael Prohl Gabriel Wittum 《Continuum Mechanics and Thermodynamics》2016,28(1-2):579-601
We present a poroplastic model of structural reorganisation in a binary mixture comprising a solid and a fluid phase. The solid phase is the macroscopic representation of a deformable porous medium, which exemplifies the matrix of a biological system (consisting e.g. of cells, extracellular matrix, collagen fibres). The fluid occupies the interstices of the porous medium and is allowed to move throughout it. The system reorganises its internal structure in response to mechanical stimuli. Such structural reorganisation, referred to as remodelling, is described in terms of “plastic” distortions, whose evolution is assumed to obey a phenomenological flow rule driven by stress. We study the influence of remodelling on the mechanical and hydraulic behaviour of the system, showing how the plastic distortions modulate the flow pattern of the fluid, and the distributions of pressure and stress inside it. To accomplish this task, we solve a highly nonlinear set of model equations by elaborating a previously developed numerical procedure, which is implemented in a non-commercial finite element solver. 相似文献
40.
We study the size of the set of ergodic directions for the directional billiard flows on the infinite band ${\mathbb{R}\times [0,h]}$ with periodically placed linear barriers of length 0 < λ < h. We prove that the set of ergodic directions is always uncountable. Moreover, if λ/h ∈ (0, 1) is rational, the Hausdorff dimension of the set of ergodic directions is greater than 1/2. In both cases (rational and irrational) we construct explicitly some sets of ergodic directions. 相似文献