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991.
Pablo Ríos Javier Borge Francisco Fernndez de Crdova Giuseppe Sciortino Agustí Lleds Amor Rodríguez 《Chemical science》2021,12(7):2540
The concept of metal–ligand cooperation opens new avenues for the design of catalytic systems that may offer alternative reactivity patterns to the existing ones. Investigations of this concept with ligands bearing a boron center in their skeleton established mechanistic pathways for the activation of small molecules in which the boron atom usually performs as an electrophile. Here, we show how this electrophilic behavior can be modified by the ligand trans to the boron center, evincing its ambiphilic nature. Treatment of diphosphinoboryl (PBP) nickel–methyl complex 1 with bis(catecholato)diboron (B2Cat2) allows for the synthesis of nickel(ii) bis-boryl complex 3 that promotes the clean and reversible heterolytic cleavage of dihydrogen leading to the formation of dihydroborate nickel complex 4. Density functional theory analysis of this reaction revealed that the heterolytic activation of H2 is facilitated by the cooperation of both boryl moieties and the metal atom in a concerted mechanism that involves a Ni(ii)/Ni(0)/Ni(ii) process. Contrary to 1, the boron atom from the PBP ligand in 3 behaves as a nucleophile, accepting a formally protic hydrogen, whereas the catecholboryl moiety acts as an electrophile, receiving the attack from the hydride-like fragment. This manifests the dramatic change in the electronic properties of a ligand by tuning the substituent trans to it and constitutes an unprecedented cooperative mechanism that involves two boryl ligands in the same molecule operating differently, one as a Lewis acid and the other one as a Lewis base, in cooperation with the metal. In addition, reactivity towards different nucleophiles such as amines or ammonia confirmed the electrophilic nature of the Bcat moiety, allowing the formation of aminoboranes.A bis(boryl)nickel complex promotes the facile and reversible activation of H2 through a cooperative mechanism that involves the metal and both boryl moieties in a concerted five-center process. 相似文献
992.
Mulder FM Stride J Picken SJ Kouwer PH de Haas MP Siebbeles LD Kearley GJ 《Journal of the American Chemical Society》2003,125(13):3860-3866
Discotic molecules have planar, disklike polyaromatic cores that can self-assemble into "molecular wires". Highly anisotropic charge transfer along the wires arises when there is sufficient intermolecular overlap of the pi-orbitals of the molecular cores. Discotic materials can be applied in molecular electronics, field-effect transistors, and-recently with record quantum efficiencies-photovoltaics (Schmidt-Mende, L.; Fechtenk?tter, A.; Müllen, K.; Moons, E.; Frien, R. H.; MacKenzie, J. D. Science 2001, 293, 1119). A combination of quasielastic neutron scattering (QENS) measurements with molecular dynamics simulations on the discotic molecule hexakis(n-hexyloxy)triphenylene (HAT6) shows that the dynamics of the cores and tails of discotic molecules are strongly correlated. Core and tail dynamics are not separated, the system being characterized by overall in-plane motion, on a time scale of 0.2 ps, and softer out-of-plane motions at 7 ps. Because charge transfer between the molecules is on similar time scales, these motions are relevant for the conducting properties of the materials. Both types of motion are dominated by van der Waals interactions. Small-amplitude in-plane motions in which the disks move over each other are almost entirely determined by tail/tail interactions, these also playing an important role in the out-of-plane motion. The QENS measurements reveal that these motions are little changed by passing from the columnar phase to the isotropic liquid phase, just above the clearing temperature. The model of four HAT6 molecules in a column reproduces the measured QENS spectrum of the liquid phase, suggesting that correlations persist within the liquid phase over about this number of disks. 相似文献
993.
Grażyna Janowska Teresa Mikołajczyk Dorota Wołowska-Czapnik M. Boguń 《Journal of Thermal Analysis and Calorimetry》2005,82(1):129-135
The paper discusses the thermal properties of alginate fibres made from alginic acid or sodium alginate and from alginates
substituted with divalent metal ions during the fibre-forming stage. Alginate fibres with an addition of silica nanoparticles
have also been examined. The selection of fibre-forming parameters was intended to obtain the best either sorption or strength
properties depending on the specific fibre application. Thermal curves of the fibres under investigations obtained by under
air atmosphere and differential scanning calorimetry (DSC) under neutral gas atmosphere have been interpreted from the view
of physical and chemical changes in the fibre-forming material. Based on thermogravimetric curves, the fibre thermal stability
indices have been determined. It has been found that the addition of silica nanoparticles exerts a positive influence on the
thermal properties of the examined fibres. 相似文献
994.
Pyrazoles as molecular probes to study the properties of co-crystals by solid state NMR spectroscopy
Concepción López Rosa M. Claramunt M. Ángeles García José Elguero 《Central European Journal of Chemistry》2004,2(4):660-671
Equimolar mixtures of 3,5-dimethylpyrazole (1) with four NH-imidazoles (2–5) have been studied by13C and 15N CPMAS NMR and by DSC. In three cases, the solid mixture behaves as the sum of the individual components [imidazole (2), 2-methylimidazole (3) and 2,4(5)-dimethylimidazole (5)]. In one case [4,5-dimethylimidazole (4)], the mixture corresponds to a new species in which the dynamic behavior of1 no longer exists. 相似文献
995.
Summary. Phenoxyacetic acid distribution in two-phase systems n-aliphatic hydrocarbon (C5–C8) – water and its dimerization in organic phase were investigated. The values of distribution coefficient (D
HR), distribution constant (K
D), and dimerization constant (K
dim) of acid were obtained. The empirical correlations of these quantities with Hildebrand solubility parameter of organic solvents were established. The influence of pH of the aqueous phase as well as the polarity of the applied organic solvents on phenoxyacetic acid physical chemistry in
the two-phase systems was described. 相似文献
996.
Ronaldo Santos da Silva Maria Inês Basso Bernardi Antonio Carlos Hernandes 《Journal of Sol-Gel Science and Technology》2007,42(2):173-179
In this work, we have studied the influence of the pH on the synthesis and structural properties of the Ba0.77Ca0.23TiO3 nanopowders synthesized by a modified polymeric precursor method, in order to achieve non-agglomerated powders. Synthesis,
morphology, thermal reactions, crystallite and average particle size of the synthesized powders were investigated through
thermal analysis (DTA/TG), X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM) and Infrared spectroscopy.
In summary, Ba0.77Ca0.23TiO3 nanopowders were synthesized for the first time at a relative low temperature (500 °C). It was also found that the alkalinity
and acidity of the solution presented a great influence on the powder properties. The best results were obtained from solutions
with pH = 8.5 and 11 whose nanopowders presented weakly agglomerate, with homogeneous particle size and a narrow size distribution
(30–40 nm). This behavior could be explained based on the FT-IR results in which it was possible to see the increased of the
chelation in higher pHs. 相似文献
997.
Raquel Peña-Alonso Gian Domenico Sorarù 《Journal of Sol-Gel Science and Technology》2007,43(3):313-319
The sol–gel method has been used for the synthesis of borosilicate gels from mixtures of methyltriethoxysilane (MTES) and
dimethyldiethoxysilane (DMDES) and boric acid. The use of boric acid, B(OH)3 allows the hydrolysis and condensation of hybrid silicon alkoxides without further addition of water or catalyst. The use
of difunctional silicon units, –(CH3)2SiO– promote the formation, during the sol–gel process, of linear oligomers which facilitate fiber drawing before gelation.
Gel characterization performed by FT-IR, XRD, TG-DTA and DCS analysis indicates the formation of a mixed network with incorporation
of the boron units via =B-O-Si≡ bridges. The formation of borosiloxane bonds seems favored by the presence of DMDES. SiBOC
glasses were obtained after pyrolysis of the borosilicate gels in argon atmosphere at 1000 °C. TG-DTA study indicates that
the ceramic yield decreases by increasing the amount of DMDES. Gel fibers were successfully prepared from convenient partially-aged
solutions by hand drawing. Pyrolysis of the obtained gel fibers under argon atmosphere at 1000 °C open the possibility to
produce SiBOC homogeneous glass fibers with diameter as low as 10 μm. 相似文献
998.
Controlled drug delivery remains a research focus for public health to enhance patient compliance, drug efficiency and reduce
the side effects of drugs. Pectin, an edible plant polysaccharide, has been shown to be useful for the construction of drug
delivery systems for specific drug delivery. Several pectin derived formulations have been developed in our laboratory and
tested in vitro, ex vivo, and in vivo for the ability to deliver bioactive substances for therapeutic purposes in the context of interactions with living tissues.
Pectin derivatives carrying primary amine groups were more mucoadhesive and have shown potential in nasal drug delivery and
other mucosal drug delivery. Pectin derivatives with highly esterified galacturonic acid residues are more hydrophobic and
able to sustain the release of incorporated fragrances for a prolonged duration. Less esterified pectin derivatives are able
to penetrate deeper into the skin and may be useful in aromatherapy formulations. Pectin, in combination with zein, a corn
protein, forms hydrogel beads. The bound zein restricts bead swelling and retains the porosity of the beads; the pectin networks
shield the zein from protease attack. The complex beads are ideal vehicles for colon-specific drug delivery. Studies presented
in this paper indicate the flexibility and possibility to tailor pectin macromolecules into a variety of drug delivery systems
to meet different clinical requirements.
Mention of trade names or commercial products in this article is solely for the purpose of providing specific information
and does not imply recommendation or endorsement by the US Department of Agriculture. 相似文献
999.
Alan Rigter Jan PM Langeveld Drophatie Timmers-Parohi Jorg G Jacobs Peter LJM Moonen Alex Bossers 《BMC biochemistry》2007,8(1):6
Background
The common event in transmissible spongiform encephalopathies (TSEs) or prion diseases is the conversion of host-encoded protease sensitive cellular prion protein (PrPC) into strain dependent isoforms of scrapie associated protease resistant isoform (PrPSc) of prion protein (PrP). These processes are determined by similarities as well as strain dependent variations in the PrP structure. Selective self-interaction between PrP molecules is the most probable basis for initiation of these processes, potentially influenced by chaperone molecules, however the mechanisms behind these processes are far from understood. We previously determined that polymorphisms do not affect initial PrPC to PrPSc binding but rather modulate a subsequent step in the conversion process. Determining possible sites of self-interaction could elucidate which amino acid(s) or amino acid sequences contribute to binding and further conversion into other isoforms. To this end, ovine – and bovine PrP peptide-arrays consisting of 15-mer overlapping peptides were probed with recombinant sheep PrPC fused to maltose binding protein (MBP-PrP). 相似文献1000.
J. Seixas de Melo J. Pina H.D. Burrows R.E. Di Paolo A.L. Maanita 《Chemical physics》2006,330(3):449-456
A comprehensive photophysical and spectroscopic study of a new class of p-phenylenevinylene oligomers (PPV-trimers) possessing different alkyl and alkyloxy sidechain substituents and different end groups (aldehyde, CC, phenylene and anthracene units) was undertaken in solution at room temperature (293 K), low temperature (77 K) and in thin films. The study comprises absorption, emission and triplet–triplet absorption spectra, together with quantitative measurements of quantum yields (fluorescence, intersystem crossing, internal conversion and singlet oxygen formation) and lifetimes. The data allow the determination of rate constants for all decay processes. From these, several conclusions could be drawn. Changing from alkyl to alkyloxy substituents does not change fluorescence and internal conversion yields but decreases the (already small) intersystem crossing yield. The introduction of anthracene at the terminal ends of the PPV-trimers leads to the lowest fluorescence yield reported in this study. Of particular importance is the fact that the fluorescence quantum yields in films are of the same order of magnitude as those in solution, which suggests the potential for use of these oligomers for light-emitting device applications. With one of the alkyloxy derivatives, a more detailed study of the early part of the fluorescence decay was made, and it was found that upon excitation a fast conformational relaxation process of the initially excited oligomer occurs, leading to a more planar conjugation segment. 相似文献