全文获取类型
收费全文 | 9141篇 |
免费 | 185篇 |
国内免费 | 46篇 |
专业分类
化学 | 5901篇 |
晶体学 | 71篇 |
力学 | 292篇 |
数学 | 1676篇 |
物理学 | 1432篇 |
出版年
2021年 | 97篇 |
2020年 | 94篇 |
2019年 | 79篇 |
2018年 | 70篇 |
2017年 | 63篇 |
2016年 | 143篇 |
2015年 | 135篇 |
2014年 | 154篇 |
2013年 | 519篇 |
2012年 | 371篇 |
2011年 | 478篇 |
2010年 | 264篇 |
2009年 | 239篇 |
2008年 | 480篇 |
2007年 | 445篇 |
2006年 | 441篇 |
2005年 | 433篇 |
2004年 | 341篇 |
2003年 | 326篇 |
2002年 | 354篇 |
2001年 | 148篇 |
2000年 | 134篇 |
1999年 | 102篇 |
1998年 | 95篇 |
1997年 | 118篇 |
1996年 | 147篇 |
1995年 | 87篇 |
1994年 | 102篇 |
1993年 | 108篇 |
1992年 | 104篇 |
1991年 | 101篇 |
1990年 | 96篇 |
1989年 | 80篇 |
1988年 | 92篇 |
1987年 | 93篇 |
1986年 | 68篇 |
1985年 | 149篇 |
1984年 | 145篇 |
1983年 | 120篇 |
1982年 | 135篇 |
1981年 | 149篇 |
1980年 | 148篇 |
1979年 | 125篇 |
1978年 | 132篇 |
1977年 | 127篇 |
1976年 | 117篇 |
1975年 | 104篇 |
1974年 | 106篇 |
1973年 | 105篇 |
1972年 | 65篇 |
排序方式: 共有9372条查询结果,搜索用时 250 毫秒
991.
George A. Anastassiou 《Mathematical and Computer Modelling》2011,53(5-6):1111-1132
Here we study the univariate quantitative approximation of real and complex valued continuous functions on a compact interval or all the real line by quasi-interpolation hyperbolic tangent neural network operators. This approximation is derived by establishing Jackson type inequalities involving the modulus of continuity of the engaged function or its high order derivative. Our operators are defined by using a density function induced by the hyperbolic tangent function. The approximations are pointwise and with respect to the uniform norm. The related feed-forward neural network is with one hidden layer. 相似文献
992.
We study the forcing operators on MTL‐algebras, an algebraic notion inspired by the Kripke semantics of the monoidal t ‐norm based logic (MTL). At logical level, they provide the notion of the forcing value of an MTL‐formula. We characterize the forcing operators in terms of some MTL‐algebras morphisms. From this result we derive the equality of the forcing value and the truth value of an MTL‐formula (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
993.
Franco I Solomon GC Schatz GC Ratner MA 《Journal of the American Chemical Society》2011,133(39):15714-15720
We present a model molecular system with an unintuitive transport-extension behavior in which the tunneling current increases with forced molecular elongation. The molecule consists of two complementary aromatic units (1,4-anthracenedione and 1,4-anthracenediol) hinged via two ether chains and attached to gold electrodes through thiol-terminated alkenes. The transport properties of the molecule as it is mechanically elongated in a single-molecule pulling setting are computationally investigated using a combination of equilibrium molecular dynamics simulations of the pulling with gDFTB computations of the transport properties in the Landauer limit. Contrary to the usual exponential decay of tunneling currents with increasing molecular length, the simulations indicate that upon elongation electronic transport along the molecule increases 10-fold. The structural origin of this inverted trend in the transport is elucidated via a local current analysis that reveals the dual role played by H-bonds in both stabilizing π-stacking for selected extensions and introducing additional electronic couplings between the complementary aromatic rings that also enhance tunneling currents across the molecule. The simulations illustrate an inverted electromechanical single-molecule switch that is based on a novel class of transport-extension behavior that can be achieved via mechanical manipulation and highlight the remarkable sensitivity of conductance measurements to the molecular conformation. 相似文献
994.
Mecinović J Snyder PW Mirica KA Bai S Mack ET Kwant RL Moustakas DT Héroux A Whitesides GM 《Journal of the American Chemical Society》2011,133(35):14017-14026
The hydrophobic effect, the free-energetically favorable association of nonpolar solutes in water, makes a dominant contribution to binding of many systems of ligands and proteins. The objective of this study was to examine the hydrophobic effect in biomolecular recognition using two chemically different but structurally similar hydrophobic groups, aliphatic hydrocarbons and aliphatic fluorocarbons, and to determine whether the hydrophobicity of the two groups could be distinguished by thermodynamic and biostructural analysis. This paper uses isothermal titration calorimetry (ITC) to examine the thermodynamics of binding of benzenesulfonamides substituted in the para position with alkyl and fluoroalkyl chains (H(2)NSO(2)C(6)H(4)-CONHCH(2)(CX(2))(n)CX(3), n = 0-4, X = H, F) to human carbonic anhydrase II (HCA II). Both alkyl and fluoroalkyl substituents contribute favorably to the enthalpy and the entropy of binding; these contributions increase as the length of chain of the hydrophobic substituent increases. Crystallography of the protein-ligand complexes indicates that the benzenesulfonamide groups of all ligands examined bind with similar geometry, that the tail groups associate with the hydrophobic wall of HCA II (which is made up of the side chains of residues Phe131, Val135, Pro202, and Leu204), and that the structure of the protein is indistinguishable for all but one of the complexes (the longest member of the fluoroalkyl series). Analysis of the thermodynamics of binding as a function of structure is compatible with the hypothesis that hydrophobic binding of both alkyl and fluoroalkyl chains to hydrophobic surface of carbonic anhydrase is due primarily to the release of nonoptimally hydrogen-bonded water molecules that hydrate the binding cavity (including the hydrophobic wall) of HCA II and to the release of water molecules that surround the hydrophobic chain of the ligands. This study defines the balance of enthalpic and entropic contributions to the hydrophobic effect in this representative system of protein and ligand: hydrophobic interactions, here, seem to comprise approximately equal contributions from enthalpy (plausibly from strengthening networks of hydrogen bonds among molecules of water) and entropy (from release of water from configurationally restricted positions). 相似文献
995.
We demonstrate a simple method to tune and pattern the wettability of polydimethylsiloxane (PDMS) to generate microfluidic mimics of heterogeneous porous media. This technique allows one to tailor the capillary forces at different regions within the PDMS channel to mimic multi-phase flow in oil reservoirs. In this method, UV-ozone treatment is utilized to oxidize and hydrophilize the surface of PDMS. To maintain a stable surface wettability, the oxidized surfaces are immersed in water. Additionally, the use of a photomask makes it convenient to pattern the wettability in the porous media. A one-dimensional diffusive reaction model is established to understand the UV-ozone oxidation as well as hydrophobic recovery of oxidized PDMS surfaces. The modeling results show that during UV-ozone, surface oxidation dominates over diffusion of low-molecular-weight (LMW) species. However, the diffusivity of LMW species plays an important role in wettability control of PDMS surfaces. 相似文献
996.
Nethravathi C Rajamathi JT George P Rajamathi M 《Journal of colloid and interface science》2011,354(2):793-797
A new anionic clay, α-magnesium hydroxide, was synthesized by hydrolysis of magnesium acetate in propylene glycol. The structure of this α-hydroxide is similar to that of hydrotalcites. It consists of positively charged magnesium hydroxide layers arising out of partial protonation of the hydroxyl groups of the [Mg(OH)(2)] layers and loosely held anions in the interlayer region. As expected it ages readily in water to give β-magnesium hydroxide, brucite. While anion-exchange reactions of α-magnesium hydroxide could not be carried out in aqueous medium a number of anion-exchange reactions could be carried out successfully in ethanol medium. 相似文献
997.
A basic method for dissociation and fractionation of monoclonal IgG heavy and light chain is described. It employs less noxious and hazardous reagents than the classical mercaptoethanol/propionic acid process and replaces size exclusion chromatography with cation exchange on a monolith to improve productivity. Significant scope remains to refine the conditions. The method can be applied to other disulfide bonded proteins with significant affinity for cation exchangers. 相似文献
998.
Nunes AP Peres AE de Araujo AC Valadão GE 《Journal of colloid and interface science》2011,361(2):632-638
The reverse apatite flotation with fatty acids has been widely used for the reduction of phosphorus content of magmatic origin iron ores. However, the occurrence of phosphorus intensely disseminated as secondary minerals such as wavellite renders the anionic reverse flotation a challenge. Zeta potential measurements and microflotation tests of wavellite with the use of anionic and cationic collectors were carried out in this work. The wavellite's IEP value was achieved at pH 4.5. Below the IEP value, the surface positively charged sites are made up of aluminum ions. The species H(+), Al(OH)(2)(+), Al(OH)(2+), Al(3+), OH(-), H(2)PO(4)(-), HPO(4)(2-), and PO(4)(3-) play a role in the protonation and deprotonation reactions that will determine the wavellite-solution interface properties. The highest values of wavellite's floatability under basic pH conditions were achieved in the presence of cationic collectors (1 × 10(-4) mol L(-1)). The formation of surface complexes and the precipitation of insoluble salt of aluminum onto wavellite surface seems to be the most likely hypothesis for the chemical nature interactions between amines and wavellite. The surface formation of aluminum oleate on the wavellite's surface seems to be the most probable hypothesis for the adsorption mechanism and the resultant high floatability of wavellite between pH 7.5 and pH 10.0 in the presence of sodium oleate (1 × 10(-4) mol L(-1)). The results showed that the cationic reverse flotation of secondary phosphates is a promising route to reduce the phosphorus content of iron ores from deposits that underwent a supergene enrichment process, since wavellite floatability in the alkaline pH range, using amine as collector, was not significantly affected by the presence of corn starch. 相似文献
999.
We report a structural characterization using X-ray absorption spectroscopy of the molybdenum site of Escherichia coli YedY, a novel oxidoreductase related to be the sulfite oxidase family of molybdenum enzymes. We find that the enzyme can exist in Mo(V) and Mo(IV) oxidation states but cannot be readily oxidized to the Mo(VI) form. Mo(V) YedY has molybdenum coordination similar to that of sulfite oxidase, with one Mo═O at 1.71 ?, three Mo-S at 2.39 ?, and one Mo-OH at 2.09 ?, which elongates to 2.20 ? upon reduction to Mo(IV), indicating Mo-OH(2) coordination. The Mo(V) enzyme also possesses a long Mo-O coordination at 2.64 ?, which may be due to oxygen coordination by Asn-45 O(δ), with Mo-O(δ) approximately trans to the Mo═O group. A comparison with sulfite oxidase indicates that YedY possesses a much more uniform Mo-S coordination, with a maximum permitted deviation of less than 0.05 ?. Our results indicate that the YedY active site shows considerable similarity to but also important differences from that of reduced forms of sulfite oxidase. 相似文献
1000.
We report ab initio spectroscopic constants for the recently identified 1(5)Π(g) state of C(2) [P. Bornhauser, Y. Sych, G. Knopp, T. Gerber, and P. P. Radi, J. Chem. Phys. 134, 044302 (2011)]. The calculations are performed at the multi-reference configuration interaction level of theory with Davidson's correction using aug-cc-pV6Z basis sets and include core-valence correlation and relativistic corrections obtained with quadruple-zeta bases. Such treatment accurately reproduces the experimentally observed constants of the a(3)Π(u) and other states. Thus, we expect our calculated ω(e) value for the 1(5)Π(g) state to be within a few cm(-1), and rotational constants to be within 0.1% of experiment. Agreement with available spectroscopic data is excellent, with the calculations strongly suggesting that the 1(5)Π(g) vibrational level observed by Bornhauser et al. is v = 0. 相似文献