全文获取类型
收费全文 | 3294篇 |
免费 | 172篇 |
国内免费 | 14篇 |
专业分类
化学 | 2284篇 |
晶体学 | 5篇 |
力学 | 89篇 |
数学 | 613篇 |
物理学 | 489篇 |
出版年
2023年 | 31篇 |
2022年 | 77篇 |
2021年 | 100篇 |
2020年 | 77篇 |
2019年 | 77篇 |
2018年 | 68篇 |
2017年 | 55篇 |
2016年 | 158篇 |
2015年 | 116篇 |
2014年 | 134篇 |
2013年 | 194篇 |
2012年 | 268篇 |
2011年 | 308篇 |
2010年 | 145篇 |
2009年 | 126篇 |
2008年 | 217篇 |
2007年 | 196篇 |
2006年 | 187篇 |
2005年 | 173篇 |
2004年 | 141篇 |
2003年 | 115篇 |
2002年 | 109篇 |
2001年 | 36篇 |
2000年 | 30篇 |
1999年 | 25篇 |
1998年 | 24篇 |
1997年 | 22篇 |
1996年 | 24篇 |
1995年 | 8篇 |
1994年 | 18篇 |
1993年 | 22篇 |
1992年 | 18篇 |
1991年 | 13篇 |
1990年 | 13篇 |
1989年 | 14篇 |
1988年 | 11篇 |
1987年 | 9篇 |
1986年 | 13篇 |
1985年 | 9篇 |
1984年 | 9篇 |
1983年 | 5篇 |
1982年 | 6篇 |
1981年 | 8篇 |
1980年 | 10篇 |
1979年 | 7篇 |
1977年 | 6篇 |
1975年 | 3篇 |
1974年 | 3篇 |
1971年 | 3篇 |
1967年 | 6篇 |
排序方式: 共有3480条查询结果,搜索用时 15 毫秒
51.
An introduction to population approaches for optimization and hierarchical objective functions: A discussion on the role of tabu search 总被引:3,自引:0,他引:3
Pablo Moscato 《Annals of Operations Research》1993,41(2):85-121
Population approaches suitable for global combinatorial optimization are discussed in this paper. They are composed of a number of distinguishable individuals called "agents", each one using a particular optimization strategy. Periods of independent search follow phases on which the population is restarted from new configurations. Due to its intrinsic parallelism and the asynchronicity of the method, it is particularly suitable for parallel computers. Results on two test problems are presented in this paper. The individual search optimization strategies for each agent have been chosen having the basic characteristics of tabu search. This has been done in order to avoid mixing the hypothesized properties of these population approaches with those of more elaborate tabu search strategies, but remarking on its main characteristics. A set of four test problem "landscapes" is discussed and their use to improve and benchmark the results by using tabu search as the individual optimization strategy within a population heuristic is suggested and explored. The application of tabu search to new problem areas, like molecular biology, is also investigated. 相似文献
52.
den Brok MW Nuijen B Miranda E Floriano P Munt S Manzanares I Beijnen JH 《Journal of chromatography. A》2003,1020(2):251-258
ES-285 x HCl [(2S,3R)-2-amino-3-octadecanol hydrochloride] is a novel investigational anticancer agent, which has shown in vitro and in vivo cytotoxic activity against various tumor cell lines with selectivity for certain solid tumors. The pharmaceutical development of ES-285 x HCl warranted the availability of an assay for the quantification and purity determination of ES-285 x HCl active pharmaceutical ingredient (API) and its pharmaceutical dosage form. A liquid chromatographic method (LC) comprising of derivatisation of ES-285 x HCl with phenylisothiocyanate and UV-detection was developed. The method was found to be linear, precise and accurate. The assay also proved selectivity as determined by analysing ES-285 x HCl in combination with 15 analogues and in combination with hydroxypropyl-beta-cyclodextrin, the excipient used in the lyophilised pharmaceutical dosage form. Stress testing showed that the degradation products were separated from the parent compound, confirming its stability indicating capacity. The method was found robust as determined with design of experiments (DoE), which made it possible to predict system suitability responses in worst case experimental conditions and to define criteria for system suitability testing. 相似文献
53.
Different transformations of chiral epoxy esters 1 afford two different amino gamma-butyrolactones 2 and 6, and amino gamma-butenolides 8, by different nucleophilic opening-closing processes. [reaction: see text] 相似文献
54.
Tedesco E Della F Favaretto L Barbarella G Albesa-Jové D Pisignano D Gigli G Cingolani R Harris KD 《Journal of the American Chemical Society》2003,125(40):12277-12283
The "rigid-core" material 3,5-dimethyl-2,3'-bis(3-methylthiophene)-dithieno[3,2-b:',3'-d]thiophene-4,4-dioxide (DTTOMe4) has the highest photoluminescence ever reported for thiophene-based molecules in the solid state. We report the structure of this material, determined directly from powder X-ray diffraction data using the Genetic Algorithm method for structure solution, followed by Rietveld refinement, and the structural properties are discussed in relation to the structures of the corresponding subsystems DTTO and DTTOMe. While the crystal structures of the latter compounds contain cofacial dimers, the crystal structure of DTTOMe4 comprises layers of molecules aligned in an antiparallel fashion. Intermediate neglect of differential overlap with single configuration interaction (INDO/SCI) calculations on the intermolecular interactions in the three crystal structures show that the different solid-state photoluminescence efficiencies of DTTOMe4, DTTOMe, and DTTO cannot be correlated with the different types of dipole-dipole alignment in the solid state. Instead, photoluminescence efficiencies correlate well with the rate of formation of nonradiatively decaying charge-transfer pairs upon photoexcitation. Because of larger intermolecular distances in DTTOMe4, the photoluminescence is less effectively quenched by charge-transfer processes than in DTTOMe and DTTO. 相似文献
55.
Haque SA Palomares E Cho BM Green AN Hirata N Klug DR Durrant JR 《Journal of the American Chemical Society》2005,127(10):3456-3462
In this paper we focus upon the electron injection dynamics in complete dye-sensitized nanocrystalline metal oxide solar cells (DSSCs). Electron injection dynamics are studied by transient absorption and emission studies of DSSCs and correlated with device photovoltaic performance and charge recombination dynamics. We find that the electron injection dynamics are dependent upon the composition of the redox electrolyte employed in the device. In a device with an electrolyte composition yielding optimum photovoltaic device efficiency, electron injection kinetics exhibit a half time of 150 ps. This half time is 20 times slower than that for control dye-sensitized films covered in inert organic liquids. This retardation is shown to result from the influence of the electrolyte upon the conduction band energetics of the TiO2 electrode. We conclude that optimum DSSC device performance is obtained when the charge separation kinetics are just fast enough to compete successfully with the dye excited-state decay. These conditions allow a high injection yield while minimizing interfacial charge recombination losses, thereby minimizing "kinetic redundancy" in the device. We show furthermore that the nonexponential nature of the injection dynamics can be simulated by a simple inhomogeneous disorder model and discuss the relevance of our findings to the optimization of both dye-sensitized and polymer based photovoltaic devices. 相似文献
56.
Lainez A del Burgo P Junquera E Aicart E 《Langmuir : the ACS journal of surfaces and colloids》2004,20(14):5745-5752
Speed of sound, density, conductivity, and fluorescence spectroscopy experiments were run to analyze the mixed aggregation process of a nonionic-cationic surfactant system in aqueous media at 298.15 K. The mixed system comprises a nonionic surfactant, n-octyl-beta-D-glucopyranoside (OBG), and a cationic surfactant, tetradecyltrimethylammonium bromide (C14TAB), with 8 and 14 carbon atoms on the hydrophobic tails, respectively. From these data, the total and partial critical micellar concentrations, the total and partial aggregation numbers, apparent molar volumes and isentropic compressibilities, hydration numbers, and the corresponding changes in the latest properties due to the mixed aggregation process were determined. Pure and mixed micelles were analyzed from a geometrical point of view by determining the packing parameter of the aggregates. Furthermore, the experimental characterization of both the monomeric and micellar phases was completed with a theoretical study of the mixed micellization phenomena studied herein, by means of some of the most relevant theoretical models. 相似文献
57.
Giorgio Montaudo Emilio Scamporrino Daniele Vitalini Roberto Rapisardi 《Journal of polymer science. Part A, Polymer chemistry》1992,30(4):525-532
The low molecular weight compounds formed by partial ozonolysis of poly(isoprene) and poly(chloroprene) were analyzed by fast atom bombardment mass spectrometry (FABMS). In the poly(chloroprene) case, the ozonized mixtures were treated with piperidine before the MS analysis to transform in amide end groups the reactive acyl chlorides formed by the cleavage of double bonds along the main chain. Only one family of compounds having carboxyl and ketone or carboxyl and amide end groups were obtained from the ozonolysis of poly(isoprene) and poly(chloroprene), respectively. The assigned structures were confirmed by FAB-MS analysis of the GPC separation fractions [poly(chloroprene)] or by FAB-MS of the KOH-doped ozonolysis mixtures [poly(isoprene)]. It has been also ascertained, by GPC experiments, that poly(chloroprene) decomposes more rapidly than poly(isoprene) and poly(butadiene). 相似文献
58.
Amparo Fuertes Emilio Escrivá Carmen Munoz Jaime Alamo Aurelio Beltrán-Porter Daniel Beltrán-Porter 《Transition Metal Chemistry》1987,12(1):62-68
Summary Knowledge of the kinetic and thermodynamic behaviour of aqueous solutions containing two divalent cations andtrans-1,2-cyclohexanediaminetetraacetate has been used to design synthetic pathways to ordered bimetallic complexesPart 1: E. Escriva, A. Fuertes and D. Beltran,Transition Met. Chem.,9, 184 (1984). 相似文献
59.
Mary E. R. McHale Ann-Sofi M. Kauppila Joyce R. Powell Pablo Otero Jr. Melani Jayasekera William E. Acree Jr. 《Journal of solution chemistry》1996,25(3):295-302
Experimental solubilities are reported for benzil dissolved in six binary mixtures containing dibutyl ether with hexane, heptane, octane, cyclohexane, methylcyclohexane, and 2,2,4-trimethylpentane at 25°C. Results of these measurements are compared to the predictions of equations developed previously for solubility in systems of nonspecific interactions. The most successful equation in terms of goodness of fit involved a volume fraction average of the excess Gibbs energies relative to the Flory-Huggins model, and predicted the experimental solubilities in the six systems studied to within an overall average absolute deviation of 3.4% and with a maximum deviation of 6.0%. 相似文献
60.
A computational study of hydration, solution structure, and dynamics in dilute carbohydrate solutions 总被引:1,自引:0,他引:1
We report results from a molecular simulation study of the structure and dynamics of water near single carbohydrate molecules (glucose, trehalose, and sucrose) at 0 and 30 degrees C. The presence of a carbohydrate molecule has a number of significant effects on the microscopic water structure and dynamics. All three carbohydrates disrupt the tetrahedral arrangement of proximal water molecules and restrict their translational and rotational mobility. These destructuring effects and slow dynamics are the result of steric constraints imposed by the carbohydrate molecule and of the ability of a carbohydrate to form stable H bonds with water, respectively. The carbohydrates induce a pronounced decoupling between translational and rotational motions of proximal water molecules. 相似文献