全文获取类型
收费全文 | 7786篇 |
免费 | 1382篇 |
国内免费 | 829篇 |
专业分类
化学 | 5814篇 |
晶体学 | 12篇 |
力学 | 902篇 |
综合类 | 52篇 |
数学 | 2144篇 |
物理学 | 1073篇 |
出版年
2024年 | 4篇 |
2023年 | 94篇 |
2022年 | 129篇 |
2021年 | 202篇 |
2020年 | 389篇 |
2019年 | 212篇 |
2018年 | 268篇 |
2017年 | 231篇 |
2016年 | 372篇 |
2015年 | 442篇 |
2014年 | 470篇 |
2013年 | 673篇 |
2012年 | 522篇 |
2011年 | 549篇 |
2010年 | 490篇 |
2009年 | 550篇 |
2008年 | 592篇 |
2007年 | 527篇 |
2006年 | 492篇 |
2005年 | 453篇 |
2004年 | 405篇 |
2003年 | 466篇 |
2002年 | 179篇 |
2001年 | 187篇 |
2000年 | 175篇 |
1999年 | 138篇 |
1998年 | 132篇 |
1997年 | 130篇 |
1996年 | 124篇 |
1995年 | 84篇 |
1994年 | 65篇 |
1993年 | 43篇 |
1992年 | 53篇 |
1991年 | 37篇 |
1990年 | 26篇 |
1989年 | 22篇 |
1988年 | 8篇 |
1987年 | 12篇 |
1986年 | 7篇 |
1985年 | 11篇 |
1984年 | 9篇 |
1983年 | 7篇 |
1982年 | 3篇 |
1981年 | 1篇 |
1980年 | 4篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1976年 | 2篇 |
1957年 | 1篇 |
1936年 | 1篇 |
排序方式: 共有9997条查询结果,搜索用时 531 毫秒
281.
282.
A finite volume scheme preserving extremum principle for convection–diffusion equations on polygonal meshes
下载免费PDF全文
![点击此处可从《国际流体数值方法杂志》网站下载免费的PDF全文](/ch/ext_images/free.gif)
We propose a nonlinear finite volume scheme for convection–diffusion equation on polygonal meshes and prove that the discrete solution of the scheme satisfies the discrete extremum principle. The approximation of diffusive flux is based on an adaptive approach of choosing stencil in the construction of discrete normal flux, and the approximation of convection flux is based on the second‐order upwind method with proper slope limiter. Our scheme is locally conservative and has only cell‐centered unknowns. Numerical results show that our scheme can preserve discrete extremum principle and has almost second‐order accuracy. Copyright © 2017 John Wiley & Sons, Ltd. 相似文献
283.
284.
《Angewandte Chemie (International ed. in English)》2017,56(18):4936-4940
Herein, we describe the first catalytic asymmetric intramolecular [4+2] cycloaddition of in situ generated ortho‐quinone methides. In the presence of a confined chiral imidodiphosphoric acid catalyst, various salicylaldehydes react with dienyl alcohols to give transient ortho ‐quinone methide intermediates, which undergo an intramolecular [4+2] cycloaddition to provide highly functionalized furanochromanes and pyranochromanes in excellent diastereoselectivity and enantioselectivity. 相似文献
285.
286.
287.
Liangru Yang Yunfei Li Pu Mao Jinwei Yuan Yongmei Xiao 《Phosphorus, sulfur, and silicon and the related elements》2013,188(8):780-788
AbstractThe catalytic activity of a series of chiral heteroaryl coordinated chelating N-heterocyclic carbene (NHC) palladium complexes towards asymmetric allylic alkylation (AAA) were presented here. The effects of different N-substituents, NHC backbones and chelate rings on the catalytic activity and the enantioselectivity of the alkylation of (E)-1,3-diarylallyl acetates with dialkyl malonate were investigated. The results showed that, under the optimized conditions, complexes 3a, 3b, and 3i carrying the pyridinyl-coordinated five-membered chelate ring showed high catalytic activity and chiral induction efficiency. The corresponding alkylated products were obtained in high yields with moderate ee. Furthermore, it was found that the substituents of (E)-1,3-diarylallyl acetates and the type of the nucleophile affect the results as well. 相似文献
288.
Alma Viso Roberto Fernández de la Pradilla Ana García Carlos Guerrero-Strachan Marta Alonso Mariola Tortosa 《Phosphorus, sulfur, and silicon and the related elements》2013,188(5-6):1229-1234
Abstract Readily available p-tolylsulfinimines undergo highly stereoselective [3 + 2] cycloadditions with azomethine ylides generated from α -iminoesters and LDA to produce N-sulfinylimidazolidines. In the presence of Lewis acids, p-tolylsulfinimines react with glycine iminoester enolates to produce N-sulfinylimidazolidines, after cyclization of open chain intermediates. These mechanistically diverse processes take place with excellent regio-, stereo-, and facial selectivities, and the latter is opposite to most known reactions involving sulfinimines. Some of the resulting imidazolidines have been transformed into examples of a novel class of nonsymmetrical vicinal diamines using reductive and/or hydrolytic protocols. 相似文献
289.