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991.
Labajos-Broncano L González-Martín ML Bruque JM González-García CM 《Journal of colloid and interface science》2001,233(2):356-360
Two experimental methods are usually employed to study liquid penetration in porous media. One of them is based on the measure of the height of the advance liquid front vs time, and the other one is based on the measure of the weight gained by the porous system due to the liquid penetration vs time. Generally, the experimental data obtained from these techniques are analyzed through Washburn's equation. However, depending on which of them is selected, different conditions, coming from the experimental method, are needed to be taken into account in order to get the correct application of Washburn's equation to the experimental data. Although these conditions are different for each method, we prove in this paper that only if these conditions are considered both techniques are equivalent to analyze imbibition experiments using Washburn's equation. Copyright 2001 Academic Press. 相似文献
992.
Rafael González Florentino Murrieta-Guevara Arturo Trejo 《Journal of solution chemistry》1986,15(10):791-801
We present in this work the results of measurements of excess volumes of mixing V
E
at 30°C over the whole composition range for sixteen mixtures of n-hexanenitrile, n-octanenitrile, n-decanenitrile, and n-dodecanenitrile mixed individually with n-heptane, n-decane, n-tridecane, and n-hexadecane. The experimental values of V
E
show a regular pattern of behavior for the four sets of binary systems. In each group of mixtures for a given n-alkanenitrile, the magnitude of V
E
decreases as the n-alkane chain-length decreases. For mixtures containing a common n-alkane, V
E
decreases with increasing n-alkanenitrile chain-length. In order to explain the observed behavior, we have used the Prigogine-Flory-Patterson theory, which divides V
E
into three different contributions. The agreement between the theoretical and experimental V
E
is reasonable for the systems studied. 相似文献
993.
Hilal M Parrado MF Rosa M Gallardo M Orce L Massa EM González JA Prado FE 《Photochemistry and photobiology》2004,79(2):205-210
UV-B radiation (280-320 nm) is harmful to living organisms and has detrimental effects on plant growth, development and physiology. In this work we examined some mechanisms involved in plant responses to UV-B radiation. Seedlings of quinoa (Chenopodium quinoa Willd.) were exposed to variable numbers of UV-B radiation doses, and the effect on cotyledons was studied. We analyzed (1) cotyledons anatomy and chloroplasts ultrastructure; (2) peroxidase activity involved in the lignification processes; and (3) content of photosynthetic pigments, phenolic compounds and carbohydrates. Exposure to two UV-B doses induced an increase in the wall thickness of epidermal cells, which was associated with lignin deposition and higher activity of the peroxidase. The chloroplast ultrastructure showed an appearance typical of plants under shade conditions, likely in response to reduced light penetration into the mesophyll cells due to the screening effect of epidermal lignin deposition. Exposure to UV-B radiation also led to (1) enhancement in the level of phenolics, which may serve a protective function; (2) strong increase in the fructose content, a fact that might be related to higher requirement of erythrose-4P as a substrate for the synthesis of lignin and phenolics; and (3) reduction in the chlorophyll concentration, evidencing alteration in the photosynthetic system. We propose that the observed lignin deposition in epidermal tissues of quinoa is a resistance mechanism against UV-B radiation, which allows growing of this species in Andean highlands. 相似文献
994.
Gómez-Lor B González-Cantalapiedra E Ruiz M de Frutos O Cárdenas DJ Santos A Echavarren AM 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(10):2601-2608
The C60 polyarenes 4, 5, 18 a, and 18 b have been synthesized from truxene by triple alkylation at C5, C10, and C15 followed by a palladium-catalyzed intramolecular arylation. The synthesis of "crushed fullerene" C60H30 (2) is the most efficient reported to date and proceeds in 33% overall yield. 相似文献
995.
Xavier Prat-Resina Mireia Garcia-Viloca Gerald Monard Angels González-Lafont José M. Lluch Josep Maria Bofill Josep Maria Anglada 《Theoretical chemistry accounts》2002,107(3):147-153
We propose a methodology to locate stationary points on a quantum mechanical/molecular mechanical potential-energy surface.
This algorithm is based on a suitable approximation of an initial full Hessian matrix, either a modified Broyden–Fletcher–Goldfarg–Shanno
or a Powell update formula for the location of, respectively, a minimum or a transition state, and the so-called rational
function optimization. The latter avoids the Hessian matrix inversion required by a quasi-Newton–Raphson method. Some examples
are presented and analyzed.
Received: 16 July 2001 / Accepted: 9 October 2001 / Published online: 9 January 2002 相似文献
996.
Silvina C. Pellegrinet María I. Colombo Sebastián A. Testero Manuel González Sierra Edmundo A. Rúveda 《The Chemical Educator》2002,7(3):155-158
In this paper we describe a new comprehensive laboratory experiment based on a chemical transformation of santonin. The experiment is designed as a puzzle-solving exercise for advanced undergraduate organic chemistry courses and consists of the structure elucidation of santonic acid, the product obtained by the action of strong alkalis on santonin. An enquiry-based approach helps the students achieve this goal by combining their knowledge of organic chemistry and spectroscopy together with mechanistic thinking. 相似文献
997.
Barone G Duca D Silvestri A Gomez-Paloma L Riccio R Bifulco G 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(14):3240-3245
Ab initio calculations at the Hartree-Fock level with full-geometry optimization using the 6-31G(d) basis set, and GIAO (gauge including atomic orbitals) (13)C NMR chemical shifts, are presented here as a support in the study of the stereochemistry of low-polar organic compounds having an open-chain structure. Four linear stereoisomers, fragments of a natural product previously characterized by experimental (13)C NMR spectra, which possesses three stereogenic centers, 11 carbon atoms, and 38 atoms in total, were considered. Conformational searches, by empirical force-field molecular dynamics, pointed out the existence of 8-13 relevant conformers per stereoisomer. Thermochemical calculations at the ab initio level in the harmonic approximation of the vibrational modes, allowed the evaluation, at 298.15 K, of the standard Gibbs free energy of the conformers. The (13)C NMR chemical shift of a given carbon atom in each stereoisomer was considered as the average chemical shift value of the same atom in the different conformers. The averages were obtained by the Boltzmann distribution, using the relative standard free energies as weighting factors. Computed parameters related to linear correlation plots of experimental (13)C chemical shifts versus the corresponding computed average data allowed us to distinguish among the four stereoisomers. 相似文献
998.
López de Alda Villaizán MJ García Falcón MS González Amigo S Simal Lozano J Lage Yusty MA 《Talanta》1996,43(9):1405-1412
The use of derivative constant-wavelength synchronous scan fluorimetry is reported for the determination of three polycyclic aromatic hydrocarbon pollutants in drinking water (linearity range 0.4-4 mug 1(-1)). The limits of detection (LD) and quantification (LQ) (mug 1(-1)) are 0.01 and 0.07 for benzo[b]fluoranthene, 0.03 and 0.12 for benzo[a]pyrene and 0.19 and 0.57 for indeno[1,2,3-cd]pyrene in the presence of three other pollutants, benzo[k]fluoranthene, benzo[ghi]perylene and fluoranthene. The precision (RSD = 10.6%) and recovery (>/= 85%) were satisfactory. 相似文献
999.
Paul Jara Nicolás Yutronic Guillermo González 《Journal of inclusion phenomena and macrocyclic chemistry》1995,22(3):203-210
The synthesis and structural aspects of urea host-guest inclusion compounds containing linear secondary alkylamines (dibutyl-,dipentyl-, dihexyl-, dioctyl-) at 25°C are reported. Elemental analysis,13C CP-MAS NMR and1H-NMR Spectroscopy, and Powder X-ray Diffraction Analysis confirm the inclusion process. The basic host structure of the products is similar to that of urea-hydrocarbon systems.13C MAS-NMR experiments show chemical shift differences for the confined guest molecule with respect to the liquid phase. Stoichiometry and |c
g| values for the inclusion compounds with dipentyl-and dihexylamine suggest a commensurate structure. 相似文献
1000.
González-Rodríguez J Pérez-Juan P de Castro MD 《Analytical and bioanalytical chemistry》2003,377(7-8):1190-1195
Solid residues from winemaking processes have been subjected to extraction with superheated water–ethanol mixtures. Identification and characterization of the extracted compounds were achieved by spectrophotometry, gas chromatography with either flame-ionization or mass detectors, and-high performance liquid chromatography with UV detection. Extraction was performed statically with single or repeated cycles. All variables affecting the extraction process have been studied and optimised. The extraction time and temperature were 65 min and 210 °C, respectively. Extracts comprised two phases—an aqueous phase, rich in phenolic compounds, and an oily phase, comprising mainly fatty acids. The method allows manipulation of extract composition by changing the applied pressure, temperature, water-to-ethanol ratio, and pH. The method is faster than traditional extraction procedures for obtaining valuable compounds from these residues. 相似文献