全文获取类型
收费全文 | 34096篇 |
免费 | 225篇 |
国内免费 | 292篇 |
专业分类
化学 | 14254篇 |
晶体学 | 278篇 |
力学 | 1441篇 |
综合类 | 12篇 |
数学 | 9964篇 |
物理学 | 8664篇 |
出版年
2020年 | 67篇 |
2018年 | 1223篇 |
2017年 | 1468篇 |
2016年 | 788篇 |
2015年 | 634篇 |
2014年 | 554篇 |
2013年 | 800篇 |
2012年 | 3257篇 |
2011年 | 2512篇 |
2010年 | 1896篇 |
2009年 | 1622篇 |
2008年 | 749篇 |
2007年 | 796篇 |
2006年 | 786篇 |
2005年 | 4647篇 |
2004年 | 4081篇 |
2003年 | 2452篇 |
2002年 | 645篇 |
2001年 | 362篇 |
2000年 | 175篇 |
1999年 | 236篇 |
1998年 | 162篇 |
1997年 | 144篇 |
1996年 | 116篇 |
1995年 | 119篇 |
1994年 | 94篇 |
1993年 | 75篇 |
1992年 | 238篇 |
1991年 | 196篇 |
1990年 | 202篇 |
1989年 | 145篇 |
1988年 | 162篇 |
1987年 | 125篇 |
1986年 | 88篇 |
1985年 | 137篇 |
1984年 | 92篇 |
1983年 | 88篇 |
1982年 | 93篇 |
1981年 | 84篇 |
1979年 | 103篇 |
1978年 | 95篇 |
1977年 | 73篇 |
1976年 | 158篇 |
1975年 | 115篇 |
1974年 | 97篇 |
1973年 | 99篇 |
1972年 | 82篇 |
1970年 | 84篇 |
1967年 | 69篇 |
1966年 | 78篇 |
排序方式: 共有10000条查询结果,搜索用时 876 毫秒
1.
2.
Vereshchagin Anatoly N. Karpenko Kirill A. Elinson Michail N. Minaeva Alexandra P. Goloveshkin Alexander S. Hansford Karl A. Egorov Mikhail P. 《Molecular diversity》2020,24(4):1327-1342
Molecular Diversity - A novel five-component diastereoselective synthesis of polysubstituted 2-piperidinones is reported. The Knoevenagel condensation–Michael addition–Mannich cascade... 相似文献
3.
Dr. Man-Bo Li Daniels Posevins Dr. Karl P. J. Gustafson Dr. Cheuk-Wai Tai Dr. Andrey Shchukarev Dr. Youai Qiu Dr. Jan-E. Bäckvall 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(1):210-215
A highly selective and efficient oxidative carbocyclization/borylation of enallenols catalyzed by palladium immobilized on amino-functionalized siliceous mesocellular foam (Pd-AmP-MCF) was developed for diastereoselective cyclobutenol synthesis. The heterogeneous palladium catalyst can be recovered and recycled without any observed loss of activity or selectivity. The high diastereoselectivity of the reaction is proposed to originate from a directing effect of the enallenol hydroxyl group. Optically pure cyclobutenol synthesis was achieved by the heterogeneous strategy by using chiral enallenol obtained from kinetic resolution. 相似文献
4.
We prove the existence of global smooth solutions near a given steady state of the hydrodynamic model of the semiconductors
in a bounded domain with physical boundary conditions. The steady state and the doping profile are permitted to be of large
variation but the initial velocity must be small. Two cases are considered. In the first one the problem is three-dimensional,
the boundary conditions are insulating and the steady state velocity vanishes. In the second one, the problem is one-dimensional,
the boundary is of contact type and the steady state velocity does not vanish. 相似文献
5.
Ralf Freund Christos Allagiannis Peter Schnholzer Karl Bernauer 《Helvetica chimica acta》1994,77(3):615-621
A crude hydrate 6 and a crystalline hemiacetal 7 of glyoxylamide 4 were prepared from crotonamide 5 (Scheme 2). Particularly hemiacetal 7 , but also 6 and the ‘dimer’ 8 (obtained from 7 ) may serve as homochiral auxiliaries. The structure of 8 was determined by X-ray analysis. By arenesulfonyl halides, tryptimines 12–14 of 4 were diastereoselectively transformed into spirotricycles 15–17 and 19 . 相似文献
6.
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
7.
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
8.
In this paper we construct an example of a word metric on an infinite cyclic subgroup. This example shows that subexponential
distortion does not obstruct non-trivial growth of connected radii. This answers a question of Gromov [6]. The constructed
metric has other pathological properies. Specifically, its asymptotic cone depends on the choice of ultrafilter and scaling
sequence.
The work has been partially supported by the Swiss National Science Foundation. 相似文献
9.
The Cauchy Problem for the relativistic Vlasov-Maxwell equations is studied in three space dimensions. It is assumed that the initial data satisfy the required constraints and have compact support. If in addition the data have sufficiently smallC
2 norm, then a uniqueC
1 solution to this system is shown to exist on all of spacetime.Research supported in part by NSF DMS 85-20662 and NSF DMS 84-20957 相似文献
10.
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