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991.
Hau-Riege SP Chapman HN Krzywinski J Sobierajski R Bajt S London RA Bergh M Caleman C Nietubyc R Juha L Kuba J Spiller E Baker S Bionta R Sokolowski Tinten K Stojanovic N Kjornrattanawanich B Gullikson E Plönjes E Toleikis S Tschentscher T 《Physical review letters》2007,98(14):145502
At the recently built FLASH x-ray free-electron laser, we studied the reflectivity of Si/C multilayers with fluxes up to 3 x 10(14) W/cm2. Even though the nanostructures were ultimately completely destroyed, we found that they maintained their integrity and reflectance characteristics during the 25-fs-long pulse, with no evidence for any structural changes over lengths greater than 3 A. This experiment demonstrates that with intense ultrafast pulses, structural damage does not occur during the pulse, giving credence to the concept of diffraction imaging of single macromolecules. 相似文献
992.
Burda JV Toro-Labbé A Gutiérrez-Oliva S Murray JS Politzer P 《The journal of physical chemistry. A》2007,111(13):2455-2457
The reaction force F(Rc) of a chemical or physical process is given by the negative derivative of the potential energy V(Rc) along the intrinsic reaction coordinate Rc. F(Rc) unambiguously and naturally divides the activation barrier in each direction into two contributions, one of which has been found to reflect preparative structural factors, Eact,prep, and the other corresponds to the first part of the transition to products, Eact,trans. We have analyzed F(Rc) for an SN2 substitution reaction in both the gas and aqueous phases. Although the overall forward and reverse activation barriers are significantly lowered by the solvent, the Eact,trans are very little affected. Thus the increased rates that are predicted for this process in aqueous solution can be attributed to the solvent facilitating the structural effects in the preparative stages, decreasing the Eact,prep. This example shows how the reaction force decomposition of activation barriers can help to elucidate the roles played by external factors, e.g., solvents. 相似文献
993.
The paper presents the theory of the discontinuous Galerkin finite element method for the space-time discretization of a linear
nonstationary convection-diffusion-reaction initial-boundary value problem. The discontinuous Galerkin method is applied separately
in space and time using, in general, different nonconforming space grids on different time levels and different polynomial
degrees p and q in space and time discretization, respectively. In the space discretization the nonsymmetric interior and boundary penalty
approximation of diffusion terms is used. The paper is concerned with the proof of error estimates in “L
2(L
2)”-and “
”-norms, where ɛ ⩾ 0 is the diffusion coefficient. Using special interpolation theorems for the space as well as time discretization,
we find that under some assumptions on the shape regularity of the meshes and a certain regularity of the exact solution,
the errors are of order O(h
p
+ τ
q
). The estimates hold true even in the hyperbolic case when ɛ = 0. 相似文献
994.
Cirić-Marjanović G Blinova NV Trchova M Stejskal J 《The journal of physical chemistry. B》2007,111(9):2188-2199
Phenosafranine and safranine have been oxidized with ammonium peroxydisulfate in acidic aqueous solution. The oxidative coupling of both safranines was proved by gel permeation chromatography demonstrating the presence of oligomeric chains of mass-average molar masses 6500 and 4500 g mol-1 for polyphenosafranine and polysafranine, respectively. A theoretical study of the mechanism of safranine and phenosafranine polymerization was based on the MNDO-PM3 semiempirical quantum chemical computations of the heat of formation of dimeric reaction intermediates, taking into account solvation effects. The study of the redox properties of the hydrated safranines and their reactive species shows that nitrenium cations are the main reactive species generated by the oxidation of the parent safranines with a two-electron oxidant, peroxydisulfate, in the initiation phase. The dominant dimers of safranines are formed by N-C coupling reactions between nitrenium cations and the parent safranines. The main coupling reactions of phenosafranine are N-C2 (C8) and N-C4 (C6); N-C4 (C6) is the dominant coupling mode for safranine. The molecular structure of oligosafranines has been studied by FTIR, Raman, and UV-vis spectroscopies. Besides prevalent unoxidized monomeric units, polymerization products of safranines contain also the iminoquinonoid and newly formed fused phenazine units. 相似文献
995.
Experimental and theoretical evidence for the proximity effect as a basic mechanism of the hydrogen bond cooperativity was obtained in a model system. Hydrogen bond (HB) interaction between poly(4-vinylpyridine) (PVP) and selected acids as HB donors was studied using PFG NMR self-diffusion measurements, 1H NMR longitudinal relaxation and quantum-mechanical DFT calculations. Bivalent HB donors, such as glutaric (GA) and adipic acid (AA), were compared to univalent donors 4-chlorobutyric acid, 4-acetylbutyric acid and 5-chlorovaleric acid. PFG NMR established substantially larger HB equilibrium constants for AA and, in particular, GA than for univalent donors, thus indicating cooperativity of COOH groups in bivalent donors. According to the values of these constants, the fraction of the transiently bound GA and AA molecules, which are bound by two hydrogen bonds, is 0.70 and 0.63, respectively. This result, which means substantial cooperativity in particular in GA, was then independently verified by a relaxation study comparing longitudinal relaxation rates of univalent and bivalent donors. Analysis of relaxation led to the same probabilities that HB of one COOH group of a bivalent donor will be accompanied by HB of another COOH group of the same molecule, namely 0.70 for GA and 0.63 for AA. Such cooperativity must be due to the proximity effect, i.e., the lowering of the entropy demand of the next binding by the motional restriction imposed by the already existing bonds. This conclusion is in excellent agreement with DFT calculations on the interaction of GA with a model vinylpyridine dimer, which predict preference of double binding of the same GA molecule over that of two GA molecules and show that this preference is due to a substantially lower entropy demand. 相似文献
996.
Using dissipative particle dynamics, we investigate the behavior of a binary mixture, exhibiting demixing in a bulk phase, confined in slit-like pores with walls modified by the stripes of tethered brush of chains. Our main interest is to determine possible morphologies that can be formed inside the pore, depending on the geometrical parameters characterizing the system (the size of the pore and the width of the stripes). In order to describe the observed morphologies we calculate several characteristics, as the density and local temperature profiles, the radii of gyration for the attached polymers, and the minimum polymer-polymer distances in the direction parallel and perpendicular to the pore walls. The summary of our findings is presented as a sketch of the diagram of morphologies. 相似文献
997.
Kubinec R Blaško J Górová R Addová G Ostrovský I Amann A Soják L 《Journal of chromatography. A》2011,1218(13):1767-1774
Isomer mixtures of monomethyl branched saturated C7-C23 fatty acid methyl esters (FAME) were prepared by performing a methylene insertion reaction to the straight chain FAME and this study model was completed by using commercially available standards of C4-C7 FAME. The equivalent chain lengths (ECL) of all 220 C4-C23 monomethyl branched FAME on OV-1 stationary phase were measured, achieving an average repeatability of ±0.0004 ECL units. The monomethyl branched FAME was identified by GC on the basis of regularity of the fractional chain lengths (FCL) dependence on the number of carbon atoms (C(z)) of individual homologous series of methyl 2-, 3-, …, 21-FAME. The prediction of retention of the first homologues, having the new position of methyl group beginning at higher carbon atoms number, and analogously for the second, third, fourth, and other members of the homologous series, allowed the dependence FCL=f(C(z)) for the first and subsequent members of beginning homologous of monomethyl derivatives of FAME. The identification was confirmed by mass spectrometry. All of the methyl isomers of FAME, which could not be completely separated by gas chromatography due to having a methyl group in surroundings of the middle of the carbon chain, were resolved by mass spectrometry using deconvolution in a SIM-mode. Measured gas chromatographic and mass spectrometric data were applied for identification of the monomethyl branched saturated FAME in tongue coating. 相似文献
998.
Futera Z Koval T Leszczynski J Gu J Mitoraj M Srebro M Burda JV 《The journal of physical chemistry. A》2011,115(5):784-794
This study focuses on the first step of interaction between DNA and the paddle-wheel dirhodium complex. The ammonia molecule was used to model the oligonucleotide sequence. The reaction was considered in neutral and acidic conditions, in gas phase, and in solvent, using the COSMO model. Molecular structures of the complexes were optimized in both models at the B3PW91/6-31G(d) level. The B3LYP functional and aug-cc-pvdz basis set were employed for single-point energy determination and electron distribution analyses. It was shown that in neutral solution the replacement of axial aqua ligand is mildly exoergic. The reaction is characterized by a relatively low activation barrier (10-12 kcal/mol), and, according to Eyring transition state theory, it proceeds very quickly. The breaking of the Rh-O(ac) bond in neutral solution is mildly endoergic (less than 1 kcal/mol) with an activation barrier of about 21 kcal/mol. However, this process can occur much more spontaneously (ΔG of -14 kcal/mol) when the dirhodium complex is protonated at the acetyl oxygen in remote position. 相似文献
999.
Martin Stěnička Vladimír Pavlínek Petr Sáha Natalia V. Blinova Jaroslav Stejskal Otakar Quadrat 《Colloid and polymer science》2011,289(4):409-414
Particles of polyaniline protonated with perfluorooctanesulfonic acid provided a material with hydrophobic surface. This property
enabled its perfect dispersion in silicone oil due to its good compatibility with the hydrophobic medium. In contrast, in
a suspension of hydrophilic polyaniline particles doped with sulfamic acid, strong interactions of particles prevailed, which
led to the formation of entangled chains of aggregated particles in suspension. The difference in structural properties of
suspensions exists already in the absence of electric field and significantly influences their electrorheological behavior
after application of electric field. The formation of electrorheological structure has been monitored by recording time dependences
of the shear stress and the electric current passing through the flowing suspensions. 相似文献
1000.
Experimental verification of our previous numerical simulation of wall effects on the terminal falling velocity of spherical
particles moving slowly along the axis of a cylindrical vessel filled with a Carreau model fluid is presented. Dependences
of the wall correction factor F
W on the sphere to tube ratio d/D and on the dimensionless Carreau model parameters m, Λ, and η
r were obtained using a finite element method. Calculated data of the wall correction factor were compared with the results
of our new falling sphere experiments. The experiments were carried out in six types of cylindrical Perspex columns (16 mm,
21 mm, 26 mm, 34 mm, 40 mm, and 90 mm in diameter) filled with aqueous solutions of polymers exhibiting different degrees
of shear thinning and elasticity. Seventeen types of spherical particles (1–8 mm in diameter) made of glass, ceramics, steel,
lead, and tungsten carbide were used for the drop tests. Measurements of the liquid flow curves, primary normal stress differences,
oscillatory, creep and recovery, stress relaxation, and stress growth tests were carried out on the rheometer Haake MARS (Thermo
Scientific). A good agreement between numerically and experimentally obtained F
W data was found. 相似文献