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141.
Tinnemans SJ Kox MH Sletering MW Nijhuis TA Visser T Weckhuysen BM 《Physical chemistry chemical physics : PCCP》2006,8(20):2413-2420
In continuation of our previous work on the applicability of the G(R(infinity)) correction factor for the quantification of Raman spectra of coke during propane dehydrogenation experiments (Phys. Chem. Chem. Phys., 2005, 7, 211), research has been carried out on the potential of this correction factor for the quantification of supported metal oxides during reduction experiments. For this purpose, supported chromium oxide catalysts have been studied by combined in situ Raman and UV-Vis spectroscopy during temperature programmed reduction experiments with hydrogen as reducing agent. The goal was to quantify on-line the amount of Cr(6+) in a reactor based on the measured in situ Raman spectra. During these experiments, a significant temperature effect was observed, which has been investigated in more detail with a thermal imaging technique. The results revealed a temperature 'on the spot' that can exceed 100 degrees C. It implies that Raman spectroscopy can have a considerable effect on the local reaction conditions and explains observed inconsistencies between the in situ UV-Vis and Raman data. In order to minimize this heating effect, reduction of the laser power, mathematical matching of the spectroscopic data, a different cell design and a change in reaction conditions has been evaluated. It is demonstrated that increasing the reactor temperature is the most feasible method to solve the heating problem. Next, it allows the application of in situ Raman spectroscopy in a reliable quantitative way without the need of an internal standard. 相似文献
142.
Beale AM Grandjean D Kornatowski J Glatzel P de Groot FM Weckhuysen BM 《The journal of physical chemistry. B》2006,110(2):716-722
A CrAPO-5 molecular sieve has been investigated with X-ray absorption spectroscopy (EXAFS-XANES) as dehydrated material and after loading with water and ammonia to unravel the coordination geometries of Cr3+ in the framework of a microporous crystalline aluminophosphate, more particularly of the AFI-type. A comparison of the XANES data, a preedge analysis with crystal field multiplet calculations and EXAFS data, pointed toward the presence of framework Cr3+ which, on dehydration, takes on a distorted tetrahedral coordination state. Due to the 3d3 configuration of Cr3+, this unusual tetrahedral coordination environment strongly tends to transform into the more stable 6-fold coordination geometry by binding two extraframework water molecules during hydration. In the presence of ammonia, tetrahedral Cr3+ readily transforms into a 5-fold coordination geometry by binding one ammonia molecule. Therefore, depending on the environmental conditions, the Cr3+ ions can occur in a 4-, 5-, or 6-fold coordination. This observation underlines the flexibility of transition metal ions, such as Cr3+, to cope with unusual coordination geometries in inorganic hosts, making them interesting as potential active sites in heterogeneous catalysis. 相似文献
143.
Bert Schroer 《Foundations of Physics》2010,40(12):1800-1857
The crossing property is perhaps the most subtle aspect of the particle-field relation. Although it is not difficult to state
its content in terms of certain analytic properties relating different matrixelements of the S-matrix or formfactors, its
relation to the localization- and positive energy spectral principles requires a level of insight into the inner workings
of QFT which goes beyond anything which can be found in typical textbooks on QFT. This paper presents a recent account based
on new ideas derived from “modular localization” including a mathematic appendix on this subject. Its main achievement is
the proof of the crossing property from a two-algebra generalization of the KMS condition. 相似文献
144.
Jacek Brona Vasily Cherepanov Konstantin Romanyuk Bert Voigtl?nder 《Surface science》2010,604(3-4):424-427
Alternating deposition of Ge and Si in the step-flow growth regime using Bi acting as a surfactant can lead to a spontaneous formation of one atomic layer deep pits in the area of surface covered by Ge. During Si growth Ge atoms of the epitaxial 2D Ge layer move to Si step edges where stronger bonds with Si atoms are formed. Appropriate growth conditions can suppress or enhance the pit formation effect and consequently a new type of self-organized nanostructures can be formed. 相似文献
145.
146.
Bert J. Debusschere Youssef M. Marzouk Habib N. Najm Blane Rhoads Dimitris A. Goussis Mauro Valorani 《Combustion Theory and Modelling》2013,17(1):173-198
This paper presents a novel tabulation strategy for the adaptive numerical integration of chemical kinetics using the computational singular perturbation (CSP) method. The strategy stores and reuses CSP quantities required to filter out fast dissipative processes, resulting in a non-stiff chemical source term. In particular, non-parametric regression on low-dimensional slow invariant manifolds (SIMs) in the chemical state space is used to approximate the CSP vectors spanning the fast chemical subspace and the associated fast chemical time-scales. The relevant manifold and its dimension varies depending on the local number of exhausted modes at every location in the chemical state space. Multiple manifolds are therefore tabulated, corresponding to different numbers of exhausted modes (dimensions) and associated radical species. Non-parametric representations are inherently adaptive, and rely on efficient approximate-nearest-neighbor queries. As the CSP information is only a function of the non-radical species in the system and has relatively small gradients in the chemical state space, tabulation occurs in a lower-dimensional state space and at a relatively coarse level, thereby improving scalability to larger chemical mechanisms. The approach is demonstrated on the simulation of homogeneous constant pressure H2–air and CH4–air ignition, over a range of initial conditions. For CH4–air, results are shown that outperform direct implicit integration of the stiff chemical kinetics while maintaining good accuracy. 相似文献
147.
W. Kimpe P. Janssens K. Bert S. Wackens J.L. Goeman J. Van der Eycken 《Tetrahedron》2019,75(45):130416
A small library of novel chiral monodentate phosphoramidite ligands, characterized by a dihydroazepine-biferrocene backbone was prepared. In order to obtain this biferrocene substructure, a mild and efficient homocoupling was developed. This allowed to synthesize the dihydroazepine ligand precursor and the phosphoramidite ligands with good overall yields and high enantiomeric excess. These ligands were successfully tested in a rhodium(I)-catalyzed hydrogenation of activated olefins. 相似文献
148.
Zhongrui Chen Maxime Bert Simon Pascal Gabriel Canard Olivier Siri 《Tetrahedron letters》2019,60(38):151024
The transamination reaction of 2,5-diaminobenzoquinonediimine (QDI) with ethylenediamine gave fluorescent 1,2,3,4-tetrahydropyrazino[2,3-g]quinoxaline (1). When the same reaction was carried out with N,N’-bis(aminoethyl)-1,3-propanediamine, a novel cationic quinoxalinium species (2) was isolated, which can be further condensed with p-cyanobenzaldehyde to afford a benzimidazolo-fused quinoxaline dye (3) that is a water-soluble fluorophore in the UV–visible range. 相似文献
149.
Prof. Dr. Bert M. Weckhuysen Zafer Öztürk Rogier P. Brand Jelle M. Boereboom Dr. Florian Meirer 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(34):8070-8084
Surface-mounted metal–organic frameworks (SURMOFs) are crystalline films of MOFs and have garnered a great deal of attention in the past years. So far, thin-film MOF research has been mainly focused on the synthesis and the exploration of potential applications of these materials, while a detailed understanding of their growth is still lacking. In this report evidence is provided for the inter-grown nature of surface-mounted thin films of Zn-ZIF-8 (SURZIF-8; ZIF=zeolitic imidazolate framework). Two distinct SURZIF-8 thin films have been made through layer-by-layer (LBL) growth after applying 20 and 50 LBL cycles. They have been characterized with atomic force microscopy (AFM) and Raman micro-spectroscopy. A detailed analysis of the Raman mapping data, inter alia using principal component analysis (PCA), revealed the existence of phase boundaries within the 20-cycle thin film, while the 50-cycle thin film is chemically more homogeneous. To further analyze these chemical heterogeneities, density functional theory (DFT) calculations were performed of three theoretical models providing spectroscopic fingerprints of the molecular vibrations associated with the Zn-ZIF-8 thin films. Based on these calculations and the experimental data distinct vibrational markers indicative for the presence of defects sites were identified. 相似文献
150.
Modular forms and three-loop superstring amplitudes 总被引:1,自引:0,他引:1
We study a proposal of D'Hoker and Phong for the chiral superstring measure for genus three. A minor modification of the constraints they impose on certain Siegel modular forms leads to a unique solution. We reduce the problem of finding these modular forms, which depend on an even spin structure, to finding a modular form of weight 8 on a certain subgroup of the modular group. An explicit formula for this form, as a polynomial in the even theta constants, is given. We checked that our result is consistent with the vanishing of the cosmological constant. We also verified a conjecture of D'Hoker and Phong on modular forms in genus 3 and 4 using results of Igusa. 相似文献