全文获取类型
收费全文 | 54003篇 |
免费 | 668篇 |
国内免费 | 426篇 |
专业分类
化学 | 27388篇 |
晶体学 | 450篇 |
力学 | 2006篇 |
综合类 | 13篇 |
数学 | 13234篇 |
物理学 | 12006篇 |
出版年
2021年 | 180篇 |
2020年 | 228篇 |
2019年 | 293篇 |
2018年 | 1409篇 |
2017年 | 1640篇 |
2016年 | 1128篇 |
2015年 | 880篇 |
2014年 | 903篇 |
2013年 | 1934篇 |
2012年 | 4251篇 |
2011年 | 3754篇 |
2010年 | 2544篇 |
2009年 | 2134篇 |
2008年 | 1882篇 |
2007年 | 1986篇 |
2006年 | 1972篇 |
2005年 | 5747篇 |
2004年 | 5067篇 |
2003年 | 3286篇 |
2002年 | 1477篇 |
2001年 | 646篇 |
2000年 | 380篇 |
1999年 | 411篇 |
1998年 | 390篇 |
1997年 | 377篇 |
1996年 | 435篇 |
1995年 | 337篇 |
1994年 | 323篇 |
1993年 | 307篇 |
1992年 | 433篇 |
1991年 | 384篇 |
1990年 | 357篇 |
1989年 | 341篇 |
1988年 | 357篇 |
1987年 | 302篇 |
1986年 | 281篇 |
1985年 | 419篇 |
1984年 | 464篇 |
1983年 | 347篇 |
1982年 | 444篇 |
1981年 | 420篇 |
1980年 | 349篇 |
1979年 | 353篇 |
1978年 | 370篇 |
1977年 | 348篇 |
1976年 | 353篇 |
1975年 | 307篇 |
1974年 | 300篇 |
1973年 | 287篇 |
1972年 | 182篇 |
排序方式: 共有10000条查询结果,搜索用时 328 毫秒
21.
In response to the bio-activity found in fluorine-containing 4-alkyl-3, 5-pyridinedicarboxylates, a series of novel 4-substituted derivatives, not directly available by Hantzsch sequences, were prepared. Starting 4-alkylpyridines, 1 , were converted via enamine 2 to materials 3–8 . Derivatives 9–16 in turn were derived from aldehyde 3 , while acid derivatives 28–36 were prepared from 14 . Addition of oxygen, sulfur, and carbenoids effected conversion of 4-allylpyridine 16 to epoxy and cyclopropyl derivative 16–22 . A number of neighboring group effects were noted, including those forming the fused-ring systems 23-27 . 相似文献
22.
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
23.
24.
For ϕ a δ-subharmonic function, sharp results are obtained that connectA(r, ϕ), B(r, ϕ) andT(r, ϕ), whereA(r, ϕ)=inf|z|=r
ϕ(z),B(r, ϕ)=sup|z|=r
ϕ(z), andT(r, ϕ) is the Nevanlinna characteristics. 相似文献
25.
Could simple intraannular-arm macrocyclic systems exist in enantiopure stable forms? The effective synthesis of two representative compounds of such a class, their resolution into enantiomers, and experiments justifying their stability toward racemization are presented. 相似文献
26.
Ricaurte Rodríguez Manuel Nogueras Justo Cobo John N. Low Christopher Glidewell 《Acta Crystallographica. Section C, Structural Chemistry》2008,64(7):o376-o378
Molecules of the title compound, C12H13ClN4, are linked by two independent N—H...N hydrogen bonds into a chain of edge‐fused R22(8) rings. The significance of this study lies in its attempt to rationalize the patterns of supramolecular aggregation in the title compound and in a range of analogous 4,6‐disubstituted 2‐aminopyrimidines. 相似文献
27.
Micha? Chmielewski 《Tetrahedron letters》2004,45(31):6007-6010
Comparison of the anion binding properties of a series of uncharged macrocyclic tetraamides reveal significant effects of the receptor's size on the strength of its anion complexes. This study allowed for estimation of the optimal size of a macroring for complexation of common anions. 相似文献
28.
Volker John 《国际流体数值方法杂志》2004,44(7):777-788
This paper presents a numerical study of a two‐dimensional time‐dependent flow around a cylinder. Its main objective is to provide accurate reference values for the maximal drag and lift coefficient at the cylinder and for the pressure difference between the front and the back of the cylinder at the final time. In addition, the accuracy of these values obtained with different time stepping schemes and different finite element methods is studied. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
29.
John A. Ekaterinaris 《国际流体数值方法杂志》2004,45(11):1187-1207
A high‐order accurate, finite‐difference method for the numerical solution of incompressible flows is presented. This method is based on the artificial compressibility formulation of the incompressible Navier–Stokes equations. Fourth‐ or sixth‐order accurate discretizations of the metric terms and the convective fluxes are obtained using compact, centred schemes. The viscous terms are also discretized using fourth‐order accurate, centred finite differences. Implicit time marching is performed for both steady‐state and time‐accurate numerical solutions. High‐order, spectral‐type, low‐pass, compact filters are used to regularize the numerical solution and remove spurious modes arising from unresolved scales, non‐linearities, and inaccuracies in the application of boundary conditions. The accuracy and efficiency of the proposed method is demonstrated for test problems. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
30.
Michael A. Brook Yang Chen Kui Guo Zheng Zhang Wen Jin Anil Deisingh Jorge Cruz-Aguado John D. Brennan 《Journal of Sol-Gel Science and Technology》2004,31(1-3):343-348
The preparation of discrete polyol based silane precursors derived from glycerol by a simple one-pot process is described. These polyol-based silanes could be hydrolyzed under mild pH conditions and upon gelation resulted in the formation of optically clear, monolithic, mesoporous silica. The hydrolysis and condensation reactions lead to cure rates that are very sensitive to ionic strength, but are almost unaffected by pH in contrast to those of alkoxysilanes derived from primary alcohols such as Si(OEt)4. Residual glycerol in the silica monolith could be removed by washing, or could be left in the silica to reduce the magnitude of shrinkage during long term storage. The biocompatible glyceroxysilane precursors lead to materials that were able to retain the activity of entrapped enzymes over repeated cycles of use for periods of up to several months. 相似文献