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871.
C. Ascheron R. Flagmeyer G. Otto J. Vogt H. U. Frey 《Crystal Research and Technology》1987,22(12):1497-1503
On GaP single crystals bombarded with 0.3 MeV and 1.0 MeV protons the fluence and depth dependences of radiation damage are studied by means of particle induced X-ray emission spectrometry under chaneling conditions (PIXE-C) and the Rutherford backscattering/channeling technique (RBS-C). It is demonstrated that PIXE-C is suitable to study depth profiles of damage density by the combination with the bevelling technique. 相似文献
872.
Christian Lach Philipp Müller Holger Frey Rolf Mülhaupt 《Macromolecular rapid communications》1997,18(3):253-260
Synthesis of hyperbranched polycarbosilanes by hydrosilylation of triallylsilane is reported. Soluble high molecular weight polymer was obtained in an uncontrolled bulk reaction. Addition of 2-(10-decen-1-yl)-1,3-oxazoline (i) afforded control over degree of polymerization (DP) and (ii) resulted in hyperbranched macromonomers. 17000 ≥ M w ≥ 800 (GPC (PS)) was obtained. 29Si NMR spectroscopy allowed identification of dendritic, branched, linear, and terminal silicon branch-points. A modified definition for the degree of branching yielded a value of 0.48 ± 0.05 (theory: 0.44). 相似文献
873.
874.
Hyperbranched aliphatic copolyesters have been prepared by copolymerization of ε‐caprolactone with 2,2‐bis(hydroxymethyl)butyric acid, catalyzed by immobilized Lipase B from Candida antarctica (Novozyme 435) under mild conditions. Via this novel combination of ring‐opening AB polymerization and AB2 polycondensation, the degree of branching (DB) and, consequently, the density of functional end groups can be controlled by the comonomer ratio in the feed (0 < DB < 0.33). 相似文献
875.
Garrelt P. van Der Laan Matthijs P. De Haas Holger Frey Martin Mller 《Macromolecular Symposia》1995,96(1):219-228
The charge transport properties of a series of symmetrically substituted mesomorphic poly(di-n-alkylsilylene)s are studied using the pulse-radiolysis time resolved microwave conductivity (PR-TRMC) technique. The observed conductivities for these polymers could be correlated with different backbone conformations present both in the crystalline solid phase and in the liquid crystalline mesophase. The transition from the solid phase to the mesophase is accompanied by a disordering of the silicon backbone that results in a decrease of the conductivity of up to two orders of magnitude. The charge carrier mobilities found varied from 5×10−5 m2 /Vs for the all-trans conformation in the solid phase to 6×10−7 m2 /Vs for the disordered backbone conformation in the mesophase. The anisotropic radiation-induced conductivity observed for aligned poly(di-n-hexylsilylene) samples demonstrate that charge carrier migration takes place preferentially in the direction of the polymer backbone. 相似文献
876.
Dr. Daniel M. Wanner Patrick M. Becker Simon Suhr Dr. Nick Wannenmacher Dr. Slava Ziegler Justin Herrmann Dr. Felix Willig Julia Gabler Khushbu Jangid Dr. Juliane Schmid Andreas C. Hans Dr. Wolfgang Frey Prof. Dr. Biprajit Sarkar Prof. Dr. Johannes Kästner Prof. Dr. René Peters 《Angewandte Chemie (International ed. in English)》2023,62(36):e202307317
Pyrazolones represent an important structural motif in active pharmaceutical ingredients. Their asymmetric synthesis is thus widely studied. Still, a generally highly enantio- and diastereoselective 1,4-addition to nitroolefins providing products with adjacent stereocenters is elusive. In this article, a new polyfunctional CuII-1,2,3-triazolium-aryloxide catalyst is presented which enables this reaction type with high stereocontrol. DFT studies revealed that the triazolium stabilizes the transition state by hydrogen bonding between C(5)−H and the nitroolefin and verify a cooperative mode of activation. Moreover, they show that the catalyst adopts a rigid chiral cage/pore structure by intramolecular hydrogen bonding, by which stereocontrol is achieved. Control catalyst systems confirm the crucial role of the triazolium, aryloxide and CuII, requiring a sophisticated structural orchestration for high efficiency. The addition products were used to form pyrazolidinones by chemoselective C=N reduction. These heterocycles are shown to be valuable precursors toward β,γ’-diaminoamides by chemoselective nitro and N−N bond reductions. Morphological profiling using the Cell painting assay identified biological activities for the pyrazolidinones and suggest modulation of DNA synthesis as a potential mode of action. One product showed biological similarity to Camptothecin, a lead structure for cancer therapy. 相似文献
877.
Christoph Hahn Moritz Rauschenbach Holger Frey 《Angewandte Chemie (International ed. in English)》2023,62(28):e202302907
We report the first anionic polymerization of 1-vinylcyclohexene (VCH). This structure may be considered as an intermediate between dienes and styrene. The polymerization of this cyclic 1,2-disubstituted 1,3-diene proceeded quantitatively in cyclohexane at 25 °C with sec-butyllithium as an initiator. The obtained polymers have well-controlled molecular weights in the range of 5 to 142 kg mol−1, controlled by the molar ratio of monomer and initiator, with narrow molecular weight distributions (Đ<1.07–1.20). In situ 1H NMR kinetic characterization revealed a weak gradient structure for the copolymers of styrene and VCH, (rSty=2.55, rVCH=0.39). P(VCH) obtained in cyclohexane with sec-BuLi as an initiator showed both 1,4- and 3,4-incorporation mode (ratio: 64 : 36). It was demonstrated that the microstructure of the resulting P(VCH) can be altered by the addition of a modifier (THF), resulting in increasing 3,4-microstructure (up to 78 %) and elevated glass-transition temperature up to 89 °C. Thus, the monomer VCH polymerizes carbanionically like a diene, however leading to rigid polymers with high glass transition temperature, which provides interesting options for combination with other dienes to well-defined polymer architectures and materials. 相似文献
878.
879.
Hyperbranched polyglycerols exceeding Mn > 15,000 g/mol (DPn > 270) have been prepared, using a modified version of the synthetic protocol reported earlier. In the optimized process polydispersities recorded by SEC remained narrow, with Mw/Mn < 1.7 over a large molecular weight range. The drastic increase of viscositity in the course of the reaction, was found to be responsible for the increased fraction of cycles (MALDI‐TOF MS) at high molecular weights. 相似文献
880.