全文获取类型
收费全文 | 6364篇 |
免费 | 293篇 |
国内免费 | 15篇 |
专业分类
化学 | 5100篇 |
晶体学 | 46篇 |
力学 | 94篇 |
数学 | 776篇 |
物理学 | 656篇 |
出版年
2023年 | 68篇 |
2022年 | 103篇 |
2021年 | 121篇 |
2020年 | 121篇 |
2019年 | 140篇 |
2018年 | 143篇 |
2017年 | 111篇 |
2016年 | 263篇 |
2015年 | 242篇 |
2014年 | 232篇 |
2013年 | 401篇 |
2012年 | 512篇 |
2011年 | 523篇 |
2010年 | 289篇 |
2009年 | 242篇 |
2008年 | 438篇 |
2007年 | 449篇 |
2006年 | 368篇 |
2005年 | 349篇 |
2004年 | 277篇 |
2003年 | 252篇 |
2002年 | 247篇 |
2001年 | 72篇 |
2000年 | 54篇 |
1999年 | 49篇 |
1998年 | 31篇 |
1997年 | 59篇 |
1996年 | 62篇 |
1995年 | 34篇 |
1994年 | 28篇 |
1993年 | 33篇 |
1992年 | 23篇 |
1991年 | 28篇 |
1990年 | 19篇 |
1989年 | 20篇 |
1988年 | 14篇 |
1987年 | 18篇 |
1986年 | 23篇 |
1985年 | 29篇 |
1984年 | 31篇 |
1983年 | 16篇 |
1982年 | 12篇 |
1981年 | 14篇 |
1980年 | 17篇 |
1979年 | 10篇 |
1978年 | 17篇 |
1977年 | 12篇 |
1976年 | 10篇 |
1974年 | 7篇 |
1973年 | 11篇 |
排序方式: 共有6672条查询结果,搜索用时 0 毫秒
41.
Dr. María Jose Capitán Dr. Jesús Álvarez Dr. Yang Wang Dr. Roberto Otero Dr. Manuel Alcamí Prof. Fernando Martín Prof. Rodolfo Miranda 《Chemphyschem》2013,14(14):3294-3302
The structure and stability of adenine crystals and thin layers has been studied by using scanning tunneling microscopy, X‐ray diffraction, and density functional theory calculations. We have found that adenine crystals can be grown in two phases that are energetically quasi‐degenerate, the structure of which can be described as a pile‐up of 2D adenine planes. In each plane, the structure can be described as an aggregation of adenine dimers. Under certain conditions, kinetic effects can favor the growth of the less stable phase. These results have been used to understand the growth of adenine thin films on gold under ultra‐high vacuum conditions. We have found that the grown phase corresponds to the α‐phase, which is composed of stacked prochiral planes. In this way, the adenine nanocrystals exhibit a surface that is enantiopure. These results could open new insight into the applications of adenine in biological, medical, and enantioselective or pharmaceutical fields. 相似文献
42.
Michael V. Turturro Daniela María Vélez Rendón Fouad Teymour Georgia Papavasiliou 《大分子反应工程》2013,7(2):107-115
Poly(ethylene glycol) diacrylate (PEGDA) hydrogels are extensively used as scaffolds in tissue engineering. The ability to spatially control hydrogel properties is critical for designing scaffolds that direct cell behavior and tissue regeneration. To this end, we have recently developed a polymerization technique, perfusion‐based frontal photopolymerization, to generate tunable gradients in PEG hydrogels. This study explores the effects of polymerization conditions on the velocity of the propagating front and its influence on gradients in hydrogel swelling. Alterations in photoinitiator perfusion rate result in the largest variations in frontal velocity and in the magnitude of the swelling gradient among all polymerization conditions investigated.
43.
A synthesis of β-stannyl esters from ester enolates and iodomethyl (tri-n-butyl) stannane (3) is described. 相似文献
44.
The synthesis of densely functionalized pyrrolo[2,1-a]isoquinolines was accomplished using the organocatalytic [3+2] cycloaddition between enals and azomethine ylides under iminium catalysis developed in our group some years ago as the key step. The cycloaddition proceeds smoothly yielding the corresponding pyrrolidine as a single endo diastereoisomer with enantiopurity in the range of 94–96% and further manipulations of the obtained cycloadducts using high yielding reaction protocols and a final mesylation/intramolecular N-alkylation sequence allowed the synthesis of target compounds in which the integrity of all stereocenters installed during the first step is maintained. 相似文献
45.
Clarimma Sessa Héctor Bagán Jose Francisco García 《Analytical and bioanalytical chemistry》2014,406(26):6735-6747
Mid-infrared fiberoptics reflectance spectroscopy (mid-IR FORS) is a very interesting technique for artwork characterization purposes. However, the fact that the spectra obtained are a mixture of surface (specular) and volume (diffuse) reflection is a significant drawback. The physical and chemical features of the artwork surface may produce distortions in the spectra that hinder comparison with reference databases acquired in transmission mode. Several studies attempted to understand the influence of the different variables and propose procedures to improve the interpretation of the spectra. This article is focused on the application of mid-IR FORS and multivariate calibration to the analysis of easel paintings. The objectives are the evaluation of the influence of the surface roughness on the spectra, the influence of the matrix composition for the classification of unknown spectra, and the capability of obtaining pigment composition mappings. A first evaluation of a fast procedure for spectra management and pigment discrimination is discussed. The results demonstrate the capability of multivariate methods, principal component analysis (PCA), and partial least squares discrimination analysis (PLS-DA), to model the distortions of the reflectance spectra and to delimitate and discriminate areas of uniform composition. The roughness of the painting surface is found to be an important factor affecting the shape and relative intensity of the spectra. A mapping of the major pigments of a painting is possible using mid-IR FORS and PLS-DA when the calibration set is a palette that includes the potential pigments present in the artwork mixed with the appropriate binder and that shows the different paint textures. Graphical Abstract
? 相似文献
46.
do Nascimento Ticiano Gomes Borges Arthur Luy Tavares Ferreira de Almeida Lara Mendes Ribeiro Êurica Adélia Nogueira Silva Fernanda Geny Calheiros da Costa Silva Valdemir do Nascimento Prata Ana Paula Basílio-Júnior Irinaldo Diniz Goulart Marília Oliveira Fonseca Morilla Demetrius Peireira de Freitas Johnnatan Duarte Aragão Cícero Flávio Soares 《Journal of Thermal Analysis and Calorimetry》2022,147(21):11949-11963
Journal of Thermal Analysis and Calorimetry - This work prepared and characterized microcapsule of Uncaria tomentosa (UT) in order to standardize a spray-dryer Uncaria tomentosa extract. The UT... 相似文献
47.
48.
Fabricio R. Bisogno Rosario Fernndez Jose María Lassaletta Gonzalo de Gonzalo 《Molecules (Basel, Switzerland)》2021,26(2)
Room temperature ionic liquids (RTILs) have been widely used as (co)solvents in several catalytic processes modifying, in most of the cases, the catalyst activity and/or the selectivity for the studied reactions. However, there are just a few examples of their use in hydrogen bonding organocatalysis. In this paper, we show the positive effect of a set of imidazole-based ionic liquids ([bmim]BF4 and [hmim]PF6) in the enantioselective addition of formaldehyde tert-butylhydrazone to prochiral α-keto esters catalyzed by a sugar-based chiral thiourea. Reactions performed in the presence of low percentages of RTILs led to an increase of the catalyst activity, thereby making possible to work at lower temperatures. Thus, the chiral tert-butyl azomethyl tertiary alcohols could be obtained with moderate to good conversions and higher enantioselectivities for most of the studied substrates when using up to 30 vol% of [hmim]PF6 as a cosolvent in processes performed in toluene. 相似文献
49.
We report on an optical arrangement capable of compensating angular dispersion of paraxial wave fields developed by diffractive optical elements (DOEs). Schematically, the system is a beam expander in which two phase-only zone plates have been inserted, remaining afocal the coupled system. The DOE, which induces a continuous set of dispersive tilted plane waves, is placed at a specific position within the proposed setup providing an output spectrum with achromatic angular deviation. A directional matching between phase fronts and pulse fronts of output wave packets is demonstrated. 相似文献
50.
Given an excess demand function of an economy, say Z(p), a stable price adjustment mechanism (SPAM) guarantees convergence of solution path p(t,p0) to an equilibrium peq solution of Z(p)=0. Besides, all equilibrium points of Z(p) are asymptotically stable. Some SPAMs have been proposed, including Newton and transpose Jacobian methods. Despite this powerful stability property of SPAMs, their acceptation in the economics community has been limited by a lack of interpretation. This paper focuses on this issue. Specifically, feedback control theory is used to link SPAMs and price dynamics models with control inputs, which match the economically intuitive Walrasian Hypothesis (i.e., prices change with excess demand sign). Under mild conditions, it is shown the existence of a feedback function that transforms the price dynamics into a desired SPAM. Hence, a SPAM is interpreted as a fundamental (e.g., Walrasian) price dynamics under the action of a feedback function aimed to stabilize the equilibrium set of the excess demand function. 相似文献