全文获取类型
收费全文 | 2744篇 |
免费 | 145篇 |
国内免费 | 10篇 |
专业分类
化学 | 2314篇 |
晶体学 | 12篇 |
力学 | 25篇 |
数学 | 285篇 |
物理学 | 263篇 |
出版年
2023年 | 21篇 |
2022年 | 36篇 |
2021年 | 34篇 |
2020年 | 51篇 |
2019年 | 39篇 |
2018年 | 28篇 |
2017年 | 22篇 |
2016年 | 101篇 |
2015年 | 103篇 |
2014年 | 88篇 |
2013年 | 140篇 |
2012年 | 206篇 |
2011年 | 234篇 |
2010年 | 140篇 |
2009年 | 85篇 |
2008年 | 184篇 |
2007年 | 169篇 |
2006年 | 157篇 |
2005年 | 165篇 |
2004年 | 162篇 |
2003年 | 127篇 |
2002年 | 114篇 |
2001年 | 68篇 |
2000年 | 37篇 |
1999年 | 34篇 |
1998年 | 27篇 |
1997年 | 32篇 |
1996年 | 29篇 |
1995年 | 27篇 |
1994年 | 17篇 |
1993年 | 18篇 |
1992年 | 21篇 |
1991年 | 12篇 |
1990年 | 14篇 |
1989年 | 9篇 |
1988年 | 23篇 |
1987年 | 15篇 |
1986年 | 7篇 |
1985年 | 13篇 |
1984年 | 18篇 |
1983年 | 5篇 |
1982年 | 6篇 |
1981年 | 13篇 |
1980年 | 4篇 |
1979年 | 4篇 |
1978年 | 13篇 |
1977年 | 4篇 |
1976年 | 4篇 |
1975年 | 3篇 |
1969年 | 5篇 |
排序方式: 共有2899条查询结果,搜索用时 0 毫秒
201.
Monika Schaub Barbara Seth Christine Alewell 《Rapid communications in mass spectrometry : RCM》2009,23(2):313-318
The stable oxygen isotope signature (δ18O) of soil is expected to be the result of a mixture of components within the soil with varying δ18O signatures. Thus, the δ18O of soils should provide information about the soil's substrate, especially about the relative contribution of organic matter versus minerals. As there is no standard method available for measuring soil δ18O, the method for the measurement of single components using a high‐temperature conversion elemental analyser (TC/EA) was adapted. We measured δ18O in standard materials (IAEA 601, IAEA 602, Merck cellulose) and soils (organic and mineral soils) in order to determine a suitable pyrolysis temperature for soil analysis. We consider a pyrolysis temperature suitable when the yield of signal intensity (intensity of mass 28 per 100 µg) is at a maximum and the acquired raw δ18O signature is constant for the standard materials used and when the quartz signal from the soil is still negligible. After testing several substances within the temperature range of 1075 to 1375°C we decided to use a pyrolysis temperature of 1325°C for further measurements. For the Urseren Valley we have found a sequence of increasing δ18O signatures from phyllosilicates to upland soils, wetland soils and vegetation. Our measurements show that the δ18O values of upland soil samples differ significantly from wetland soil samples. The latter can be related to the changing mixing ratio of the mineral and organic constituents of the soil. For wetlands affected by soil erosion, we have found intermediate δ18O signatures which lie between typical signatures for upland and wetland sites and give evidence for the input of upland soil material through erosion. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
202.
203.
A Novel Approach for Introducing Bio-Materials Into Cells 总被引:1,自引:0,他引:1
A novel approach was developed to introduce biological materials into cells for gene transfection and gene therapy applications. The method is based on the technique of electrospraying bio-materials into cells. A prototype apparatus was constructed for a feasibility study. The features of the gene transfector include: (1) A dual-capillary assembly to spray suspensions of biological materials. The outer capillary provided sheathing liquid that controlled the charge level on individual particles without altering the properties of suspensions. (2) An air–CO2 gas mixture was used for suppressing possible corona discharge and kept the same gas composition as those in incubators. (3) The designed chamber enabled the spray to operate at reduced pressure for increasing sprayed particle velocity. In the feasibility study, both suspensions of plasmid and plasmid-coated gold particles were used. The plasmid used was the commercially available Enhanced Green Fluorescent Protein gene. COS-1 cells were used as the target and the liquid media was evacuated immediately prior to the spraying process. Electrospraying was conducted at ambient pressure and the duration was no more than 2 min. After the spray transfection, the media was immediately replaced and the cell samples were returned to the incubator for 36 h. Transgene expression was detected by cellular fluorescence. This technology promises to have great potential for gene transfection and therapy studies. 相似文献
204.
Armando Talavera-Alemán Mario A. Gómez-Hurtado Rosa E. del Río Jérôme Marrot Christine Thomassigny Christine Greck 《Tetrahedron letters》2017,58(30):2901-2903
The first study of photooxidation reaction of 6β-acetoxyvouacapane isolated from Caesalpinia platyloba is reported. The reaction yielded four new epoxy lactones, 6β-acetoxy-15,16α-epoxy-13-spirocassa-12,16-olide, 6β-acetoxy-15,16β-epoxy-13-spirocassa-12,16-olide, 6β-acetoxy-12,13β-epoxycassa-16,12-olide and 6β-acetoxy-12,13α-epoxycassa-16,12-olide. All the structures were supported by 1D and 2D NMR spectroscopy as well as mass spectrometry. The stereochemistry was established on the base of single crystal X-ray diffraction. 相似文献
205.
Zakaria Belhachmi Christine Bernardi Andreas Karageorghis 《Journal of Mathematical Fluid Mechanics》2004,6(2):121-156
This paper deals with the spectral element discretization
of the Navier-Stokes equations in a disk with discontinuous
boundary data, which is known as the driven cavity problem.
The numerical treatment does not involve any
regularization of these data. Relying on a variational
formulation in the primitive variables of
velocity and pressure, we describe a discretization of these equations and
derive error estimates in appropriate weighted Sobolev spaces.
We propose an algorithm to solve the
nonlinear discrete system and present numerical experiments to verify its
efficiency. 相似文献
206.
Unraveling the crystallization mechanism of zeolites remains an increasingly important challenge in chemistry. During the last decade, in situ spectroscopic methods have provided an unprecedented level of detail of the underlying molecular mechanisms and their kinetics. Magnetic resonance, vibrational and X-ray absorption techniques have emerged as principal tools for the in situ observation of crystallization. In this tutorial review, we discuss how these in situ methods have contributed to our understanding of the complex and diverse molecular processes that govern zeolite crystallization. 相似文献
207.
Awada H Grignard B Jérôme C Vaillant A De Coninck J Nysten B Jonas AM 《Langmuir : the ACS journal of surfaces and colloids》2010,26(23):17798-17803
We show experimentally and analytically that for single-valued, isotropic, homogeneous, randomly rough surfaces consisting of bumps randomly protruding over a continuous background, superhydrophobicity is related to the power spectral density of the surface height, which can be derived from microscopy measurements. More precisely, superhydrophobicity correlates with the third moment of the power spectral density, which is directly related to the notion of Wenzel roughness (i.e., the ratio between the real area of the surface and its projected area). In addition, we explain why randomly rough surfaces with identical root-mean-square roughness values may behave differently with respect to water repellence and why roughness components with wavelength larger than 10 μm are not likely to be of importance or, stated otherwise, why superhydrophobicity often requires a contribution from submicrometer-scale components such as nanoparticles. The analysis developed here also shows that the simple thermodynamic arguments relating superhydrophobicity to an increase in the sample area are valid for this type of surface, and we hope that it will help researchers to fabricate efficient superhydrophobic surfaces based on the rational design of their power spectral density. 相似文献
208.
Zoia V. Voitenko Oleksandr I. Halaev Sergey V. Kolotilov Christine Lepetit Bruno Donnadieu Remi Chauvin 《Tetrahedron》2010,66(41):8214-8222
Base- and acid-catalyzed nucleophilic addition of 11H-isoindolo[2,1-a]quinazoline-5-one to aromatic aldehydes and maleimides was investigated. The aldol adducts and condensation products were obtained stereoselectively. Main diastereomers of the Michael adducts were isolated in 74-89% yield, and converted by N-methylation to new stable α-substituted isoindole derivatives, for which 6-methylisoindolo[2,1-a]]quinazoline-5-one stands as the unsubstituted reference. The stability of the latter was monitored in moist aerated CDCl3 solution, and one of the oxidative hydrolysis product was characterized by X-ray diffraction analysis as the corresponding N-arylphthalimide. The reactivity of the unsubstituted 6-methylisoindolo[2,1-a]]quinazoline-5-one was also investigated with acetylenic Michael acceptors. Fully conjugated isoindole derivatives possessing an original pull-push-pull structure were obtained. The conformations and molecular orbitals of the dibenzoylacetylene adduct were studied at the DFT level of theory. Its static quadratic hyperpolarizabilty β0 was also calculated at the ZINDO level. 相似文献
209.
Christine Walbaum Ingo Pantenburg Peter Junk Glen B. Deacon Gerd Meyer Prof. Dr. 《无机化学与普通化学杂志》2010,636(8):1444-1446
Black single crystals of [Lu(Db18c6)(H2O)3(thf)6]4(I3)2(I5)6(I8)(I12) were obtained from lutetium, I2 and Db18c6 (dibenzo‐18‐crown‐6) in THF solution. In the bulky cation, Lu3+ is surrounded by nine oxygen atoms, six of Db18c6 and three of water molecules to which two THF molecules are attached each. Meanwhile, four polyiodide anions, (I3)–, (I5)–, (I8)2– and (I12)2–, in a 2:6:1:1 ratio form a three‐dimensional network and leave space for the bulky cations. 相似文献
210.