全文获取类型
收费全文 | 38205篇 |
免费 | 1421篇 |
国内免费 | 1304篇 |
专业分类
化学 | 17452篇 |
晶体学 | 362篇 |
力学 | 1848篇 |
综合类 | 122篇 |
数学 | 10080篇 |
物理学 | 11066篇 |
出版年
2023年 | 179篇 |
2022年 | 300篇 |
2021年 | 334篇 |
2020年 | 368篇 |
2019年 | 356篇 |
2018年 | 1453篇 |
2017年 | 1716篇 |
2016年 | 1162篇 |
2015年 | 962篇 |
2014年 | 1028篇 |
2013年 | 1351篇 |
2012年 | 3953篇 |
2011年 | 3160篇 |
2010年 | 2373篇 |
2009年 | 2038篇 |
2008年 | 1177篇 |
2007年 | 1136篇 |
2006年 | 1047篇 |
2005年 | 4808篇 |
2004年 | 4258篇 |
2003年 | 2613篇 |
2002年 | 829篇 |
2001年 | 570篇 |
2000年 | 266篇 |
1999年 | 386篇 |
1998年 | 247篇 |
1997年 | 176篇 |
1996年 | 176篇 |
1995年 | 141篇 |
1994年 | 140篇 |
1993年 | 118篇 |
1992年 | 251篇 |
1991年 | 231篇 |
1990年 | 182篇 |
1989年 | 158篇 |
1988年 | 134篇 |
1987年 | 88篇 |
1986年 | 74篇 |
1985年 | 67篇 |
1984年 | 39篇 |
1979年 | 34篇 |
1976年 | 82篇 |
1975年 | 41篇 |
1974年 | 42篇 |
1973年 | 48篇 |
1972年 | 40篇 |
1970年 | 36篇 |
1969年 | 41篇 |
1968年 | 38篇 |
1966年 | 41篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
The ability of bottlebrush block copolymers (BBCPs) to self-assemble into ordered large periodic structures could greatly expand the scope of photonic and membrane technologies. In this paper, we describe a two-step synthesis of poly(l-lactide)-b-polystyrene (PLLA-b-PS) BBCPs and their rapid thin-film self-assembly. PLLA chains were grown from exo-5-norbornene-2-methanol via ring-opening polymerization (ROP) of l-lactide to produce norbornene-terminated PLLA. Norbonene-terminated PS was prepared using anionic polymerization followed by a termination reaction with exo-5-norbornene-2-carbonyl chloride. PLLA-b-PS BBCPs were prepared from these two norbornenyl macromonomers by a one-pot sequential ring opening metathesis polymerization (ROMP). PLLA-b-PS BBCPs thin-films exhibited cylindrical and lamellar morphologies depending on the relative block volume fractions, with domain sizes of 46–58 nm and periodicities of 70–102 nm. Additionally, nanoporous templates were produced by the selective etching of PLLA blocks from ordered structures. The findings described in this work provide further insight into the controlled synthesis of BBCPs leading to various possible morphologies for applications requiring large periodicities. Moreover, the rapid thin film patterning strategy demonstrated (>5 min) highlights the advantages of using PLLA-b-PS BBCP materials beyond their linear BCP analogues in terms of both dimensions achievable and reduced processing time. 相似文献
2.
Julia K Pruns Jens-Peter Vietzke Manfred Strassner Claudius Rapp Ulrich Hintze Wilfried A K?nig 《Rapid communications in mass spectrometry : RCM》2002,16(3):208-211
A new solvent-free sample preparation method using silver trifluoroacetate (AgTFA) was developed for the analysis of low molecular weight paraffins and microcrystalline waxes by laser desorption/ionization time-of-flight mass spectrometry (LDI-TOFMS). Experiments show that spectral quality can be enhanced by dispersing AgTFA directly in liquid paraffins without the use of additional solvents. This preparation mixture is applied directly to the MALDI probe. Solid waxes could be examined by melting prior to analysis. The method also provides sufficiently reproducible spectra that peak area ratios between mono- and bicyclic alkane peaks indicated variations in the cycloalkane content of paraffin samples. Dehydrogenation of hydrocarbons observed during the desorption/ionization process was studied by analysis of alkane standards. 相似文献
3.
The versatile behavior of a trihydrazidophosphoradamantane allowing the synthesis of a variety of neutral, mono- or di-cationic water-soluble molecules of potential interest for biphasic catalysis is reported. 相似文献
4.
We build a metric space which is homeomorphic to a Cantor set but cannot be realized as the attractor of an iterated function system. We give also an example of a Cantor set K in R3 such that every homeomorphism f of R3 which preserves K coincides with the identity on K. 相似文献
5.
浅析二阶齐次线性变系数微分方程的一个可积类型 总被引:1,自引:0,他引:1
姬志飞 《应用数学与计算数学学报》2006,20(1):125-128
本文讨论了二阶齐次线性变系数微分方程的特殊形式,给出了这种微分方程的一个可积类型. 相似文献
6.
Milan Medved? 《Journal of Mathematical Analysis and Applications》2002,267(2):643-650
A criterion for the nonexplosion of solutions to semilinear evolution equations on Banach spaces is proved. The result is obtained by applying a modification of the Bihari type inequality to the case of a weakly singular nonlinear integral inequality. 相似文献
7.
Based on the Collins formula and irradiance moments definition, the propagation of the kurtosis parameter of super-Gaussian beams through a spherically aberrated lens is studied. Detailed numerical results are given. It is shown that, as compared with aberration-free super-Gaussian beams, the profile of spherically aberrated super-Gaussian beams can be leptokurtic, mesokurtic or platykurtic, depending on the aberration and propagation distance. The results for the spherically aberrated Gaussian beams can be dealt with as a special case treated in this paper. Finally, the advantage of our method is pointed out. 相似文献
8.
We consider the one-dimensional steady-state semiconductor deviceequations modelling a pnpn device. There are two relevant scalingsof the equations corresponding to small and large applied voltages.In both scalings, the semiconductor equations can be consideredas singularly perturbed. It turns out that the small-voltagescaling breaks down for current values between two saturationcurrents. In that interval, the large-voltage scaling has tobe employed. For both scalings, we derive the first-order termsof an asymptotic expansion and show that the reduced problemhas a solution. An example verifies that the current-voltagecurves obtained have the expected qualitative structure. 相似文献
9.
Summary Some estimators of maximum likelihood type are constructed for estimating functionals of one-dimensional Gibbs states. We also show that those estimators are strongly consistent, asymptotically normal and asymptotically efficient. 相似文献
10.
Burn‐in is a widely used method to improve the quality of products or systems after they have been produced. In this paper, we consider the problem of determining the optimal burn‐in time and optimal work size maximizing the long‐run average amount of work saved per time unit in the computer applications. Assuming that the underlying lifetime distribution of the computer has an initially decreasing or/and eventually increasing failure rate function, an upper bound for the optimal burn‐in time is derived for each fixed work size and a uniform (with respect to the burn‐in time) upper bound for the optimal work size is also obtained. Furthermore, it is shown that a non‐trivial lower bound for the optimal burn‐in time can be derived if the underlying lifetime distribution has a large initial failure rate. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献