全文获取类型
收费全文 | 31020篇 |
免费 | 222篇 |
国内免费 | 280篇 |
专业分类
化学 | 12376篇 |
晶体学 | 271篇 |
力学 | 1397篇 |
综合类 | 12篇 |
数学 | 9185篇 |
物理学 | 8281篇 |
出版年
2022年 | 34篇 |
2021年 | 51篇 |
2020年 | 66篇 |
2019年 | 48篇 |
2018年 | 1206篇 |
2017年 | 1462篇 |
2016年 | 792篇 |
2015年 | 640篇 |
2014年 | 538篇 |
2013年 | 759篇 |
2012年 | 3287篇 |
2011年 | 2496篇 |
2010年 | 1911篇 |
2009年 | 1596篇 |
2008年 | 702篇 |
2007年 | 738篇 |
2006年 | 722篇 |
2005年 | 4558篇 |
2004年 | 4042篇 |
2003年 | 2402篇 |
2002年 | 578篇 |
2001年 | 346篇 |
2000年 | 122篇 |
1999年 | 201篇 |
1998年 | 133篇 |
1997年 | 97篇 |
1996年 | 70篇 |
1995年 | 66篇 |
1994年 | 58篇 |
1993年 | 38篇 |
1992年 | 189篇 |
1991年 | 164篇 |
1990年 | 148篇 |
1989年 | 115篇 |
1988年 | 121篇 |
1987年 | 71篇 |
1986年 | 48篇 |
1985年 | 49篇 |
1984年 | 46篇 |
1979年 | 33篇 |
1978年 | 37篇 |
1976年 | 83篇 |
1975年 | 40篇 |
1974年 | 40篇 |
1973年 | 50篇 |
1972年 | 40篇 |
1970年 | 34篇 |
1969年 | 42篇 |
1968年 | 37篇 |
1966年 | 38篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
1.
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
2.
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.
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
5.
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
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.
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
8.
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
9.
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
10.
P. Santhana Gopala Krishnan Chaobin He Christine Tay Shang Shang 《Journal of polymer science. Part A, Polymer chemistry》2004,42(16):4036-4046
Two ladder‐like polysilsesquioxanes (LPS) containing side‐chain maleimide groups have been synthesized successfully by reacting N‐(4‐hydroxyphenyl)maleimide (HPM) with LPS containing 100 mol % of chloropropyl groups (Ladder A ) and 50 mol % of each methyl and chloropropyl group (Ladder B ). HPM was synthesized by reacting maleic anhydride with 4‐aminophenol, and the resulting amic acid was imidized using p‐toluenesulfonic acid as a catalyst (Scheme 1 ). The LPSs were characterized by Fourier transform infrared (FTIR), 1H nuclear magnetic resonance (NMR), proton‐decoupled 13C NMR, 29Si NMR, wide‐angle X‐ray diffraction (WAXD), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). Characterization indicated that these polymers had ordered ladder‐like structures with possible defects. These polymers were soluble in common solvents at ambient temperature, which suggested that they were not crosslinked. Both the polymers and the HPM were cured, and their kinetics were followed by dynamic DSC. The Ozawa and Kissinger methods were used to calculate activation energies for curing. Curing increased the temperature at which both 5% weight loss and maximum rate of weight loss were observed. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4036–4046, 2004 相似文献