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51.
Ulrich Dierkes 《manuscripta mathematica》1986,56(3):313-331
Plateau's problem for surfaces of prescribed mean curvature will be considered in regions which are not necessarily H-convex. 相似文献
52.
Ulrich Schafft 《manuscripta mathematica》1986,57(1):33-48
It is shown that the third Chern-number of a semistable Rk-3-vector bundle on a smooth hypersurface of degree 3 in 4 can be bounded by the first and the second Chern-number of the bundle. 相似文献
53.
Summary Coordination compounds of the S2N2 molecule including methods for their preparation, reactivities, i.r. data, structures, and aspects of chemical bonding are reviewed. Methods of synthesis include reactions of S2N2, S4N4 or (NSCl)3 with metal halides, metal complexes such as carbonyls, or even metals themselves. In all cases, the planar S2N2 ring is coordinated, usuallyvia both, of its nitrogen atoms so that S2N2 acts as a bridging ligand between two metal centres; short contact distances imply that halogen atoms linked to the metal atoms show some interaction with the sulphur atoms. The stability of S2N2 is greatly enhanced by coordination. In the i.r. spectra, two characteristic S2N2 vibrations assist identification of the S2N2 species, a ring stretching mode being observable atca. 850 cm–1 and the out-of-plane deformation at 450–490 cm–1. 相似文献
54.
Dr. Ulrich Mayer 《Monatshefte für Chemie / Chemical Monthly》1978,109(2):421-433
A model has been developed for calculating the enthalpy, entropy and free energy change associated with the creation of cavities in a liquid the size of which corresponds to the volume occupied by a solvent molecule. The molar enthalpy change H
cav equals the molar enthalpy of vaporization of the liquid, the free energy change G
cav is given by G
cav=–RT ln (V
m
·p
eq
/RT) (V
m
=molar volume,p
eq
=equilibrium vapor pressure) and is related to the standard free energy of vaporization. This relationship provides an estimate of the free energy of cavity formation required to accomodate a substrate in the liquid. It has been shown, that the free energy of solvation of a substrate can be dissected into different contributions accounting for (1) the concentration dependence of partial molar free energy quantities, (2) the formation of holes in the solvent, (3) the existence of specific, short range solute-solvent interactions and (4) the dielectric polarization of the medium. Application of this concept leads to an equation of the general form G
S
–G
R
=a(DN
S
–DN
R
)+b(AN
S
–AN
R
)+c(G
vp
oS
–G
vp
oR
), where G represents the free energy of reaction or activation,DN the donor number,AN the acceptor number and G
vp
o
the standard free energy of vaporization of a solventS and a reference solventR, resp.
Mit 3 Abbildungen 相似文献
Mit 3 Abbildungen 相似文献
55.
Different transformations of the functions fixed on a heterocyclic molecule were made. It is thus possible to quickly determine the number of functions of each type present in the molecule: e.g. (i) all the functions are silylated and (ii) the alcohols are acetylated whilst the sulfates and acids are silylated. Mass spectra of atractylate, apoatractylate and gummiferine have been studied and the substituents are counted and enumerated. 相似文献
56.
Ulrich Häussermann Magnus BoströmPer Viklund Östen RappTherese Björnängen 《Journal of solid state chemistry》2002,165(1):94-99
The intermetallic compounds FeGa3 and RuGa3 were prepared from the elements using a Ga flux and their structures were refined from single-crystal X-ray data. Both compounds crystallize with the FeGa3 structure type (tetragonal, space group P42/mnm, Z=4). Electrical resistivity measurements revealed a semiconducting behavior for FeGa3 and RuGa3, which is in contrast to the good metallic conductivity observed for the isotypic compound CoGa3. The origin of the different electronic properties of these materials was investigated by first-principle calculations. It was found that in compounds adopting the FeGa3 structure type the transition metal atoms and Ga atoms interact strongly. This opens a d-p hybridization bandgap with a size of about 0.31 eV in the density of states at the Fermi level for 17-electron compounds (i.e., FeGa3 and RuGa3). The electronic structure of CoGa3 (an 18-electron compound) displays rigid band behavior with respect to FeGa3. As a consequence, the Fermi level in CoGa3 becomes located above the d-p hybridization gap which explains its metallic conductivity. 相似文献
57.
Two protocols for functionalization of glass supports with hexaethylene glycol (HEG)-linked oligonucleotides were developed.
The first method (standard amidite protocol) made use of the 2-cyanoethyl-phosphoramidite derivative of 4,4′-dimethoxytrityl-protected
HEG. This was first coupled to the support by standard solid-phase phosphoramidite chemistry followed by extension with a
thymidylic acid icosanucleotide. Stepwise addition of the linker phosphoramidite graduated at 1% (relative to the total sites
available) perstep at 50°C resulted in an optimal yield of immobilized oligonucleotides at a density of 2.24 × 1010 strands/mm2. This observed loading maximum lies well below the theoretical maximum loading owing to nonspecific adsorption of HEG on
the glass and subsequent blocking of reactive sites. Surface loadings as high as 3.73 × 1010/mm2 and of excellent sequence quality were achieved with a reverse amidite protocol. The support was first modified into a 2-cyanoethyl-N,N-diisopropylphosphoramidite analog followed by coupling with 4,4′-dimethoxytrityl-protected HEG. This protocol is conveniently
available when using a conventional DNA synthesizer. The reverse amidite protocol allowed for control of the surface loading
at values suitable for subsequent analytical applications that make use of immobilized oligonucleotides as probes for selective
hybridization of sample nucleic acids of unknown sequence and concentration. 相似文献
58.
Meier MA de Gans BJ van den Berg AM Schubert US 《Rapid communications in mass spectrometry : RCM》2003,17(20):2349-2353
Recently, a new multiple-layer matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) sample spotting technique for poly(ethylene glycol), offering improved analysis possibilities, was described. In this contribution the application of ink-jet printing to automated, multiple-layer MALDI-TOFMS sample preparation of synthetic polymers is presented, allowing accurate deposition of matrix, additive and analyte solutions. The new sample preparation technique was evaluated for poly(ethylene glycol) as well as poly(methyl methacrylate) standards, and optimized settings for both synthetic polymers have been obtained. 相似文献
59.
Ulrich Burkert 《Tetrahedron》1979,35(5):691-695
The geometries and energies of polymethyl-1,3-dioxanes were studied by molecular mechanics calculations. Buttressing effects of the Me groups are discussed. The chair/twist conformational equilibrium of 1,3-dioxanes having two syn-axial Me groups in the chair were calculated, and a twist form (the 1,4-twist) is found to be more stable than the chair only for 9 and 14, chair and 2,5-twist form are of comparable energies for 10 and 13, and the chair is considerably favored in 11 and 12. The chair/1,4-twist energy difference of 1 was calculated to be only 16.4 kJ mol?1. Ring inversion of 1 goes through a transition state with C-C-C-O coplanar with a calculated activation enthalpy of 28.3 kJ mol?1. 相似文献
60.
Martin Gilges Heribert Husmann Maria-Helen Kleemiß Stephan R. Motsch Gerhard Schomburg 《Journal of separation science》1992,15(7):452-457
Mixtures of several basic proteins have been used to test CZE capillaries with surfaces modified by new pretreatment procedures; the performance obtained has been compared with that achieved using capillaries treated by procedures described in the literature. It has been shown that addition of non-ionic polyvinylalcohols (PVA) to CZE buffer solutions deactivates even bare, i.e. untreated, fused silica surfaces and renders them suitable for separations of basic proteins. The performance obtained from such surfaces was comparable with that of capillaries modified by the more elaborate procedures of etching, silanol derivatization, and/or adsorptive coating (again with polymers). A home-made device is described which enables derivatization and coating reactions to be performed on fused silica capillaries under an inert atmosphere, i.e. one free from oxygen and water. 相似文献