全文获取类型
收费全文 | 31472篇 |
免费 | 1158篇 |
国内免费 | 5172篇 |
专业分类
化学 | 34402篇 |
晶体学 | 354篇 |
力学 | 131篇 |
综合类 | 248篇 |
数学 | 583篇 |
物理学 | 2084篇 |
出版年
2024年 | 50篇 |
2023年 | 309篇 |
2022年 | 931篇 |
2021年 | 864篇 |
2020年 | 857篇 |
2019年 | 896篇 |
2018年 | 762篇 |
2017年 | 951篇 |
2016年 | 1076篇 |
2015年 | 988篇 |
2014年 | 1190篇 |
2013年 | 2766篇 |
2012年 | 1985篇 |
2011年 | 1584篇 |
2010年 | 1372篇 |
2009年 | 1664篇 |
2008年 | 1761篇 |
2007年 | 1929篇 |
2006年 | 1797篇 |
2005年 | 1699篇 |
2004年 | 1566篇 |
2003年 | 1299篇 |
2002年 | 2625篇 |
2001年 | 877篇 |
2000年 | 786篇 |
1999年 | 644篇 |
1998年 | 524篇 |
1997年 | 557篇 |
1996年 | 507篇 |
1995年 | 488篇 |
1994年 | 404篇 |
1993年 | 356篇 |
1992年 | 339篇 |
1991年 | 242篇 |
1990年 | 185篇 |
1989年 | 193篇 |
1988年 | 153篇 |
1987年 | 103篇 |
1986年 | 74篇 |
1985年 | 65篇 |
1984年 | 80篇 |
1983年 | 35篇 |
1982年 | 57篇 |
1981年 | 36篇 |
1980年 | 34篇 |
1979年 | 25篇 |
1978年 | 29篇 |
1977年 | 8篇 |
1971年 | 32篇 |
1970年 | 18篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
Gang Wang Xiulin Zhu Zhenping Cheng Jian Zhu 《Journal of polymer science. Part A, Polymer chemistry》2006,44(9):2912-2921
A series of (di)picolinic acids and their derivates are investigated as novel complexing tridentate or bidentate ligands in the iron‐mediated reverse atom transfer radical polymerization of methyl methacrylate in N,N‐dimethylformamide at 100 °C with 2,2′‐azobisisobutyrontrile as an initiator. The polymerization rates and polydispersity indices (1.32–1.8) of the resulting polymers are dependent on the structures of the ligands employed. Different iron complexes may be involved in iron‐mediated reverse atom transfer radical polymerization, depending on the type of acid used. 1H NMR spectroscopy has been used to study the structure of the resulting polymers. Chain‐extension reactions have been performed to further confirm the living nature of this catalytic system. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2912–2921, 2006 相似文献
2.
meso-Tetra(n-hexyloxycarbonyl)porphyrin was found to be converted into porphine, the mother compound of porphyrins, in a 77% yield when heated in aqueous sulfuric acid at 180 °C over 30 min under an inert atmosphere. The observation demonstrates that the substituted porphyrin serves as a novel and useful precursor for porphine. 相似文献
3.
Juzo Oyamada 《Tetrahedron》2006,62(29):6918-6925
Synthesis of coumarins from phenols and propiolic acids was examined by using a Pt catalyst such as PtCl2/AgOTf, K2PtCl4/AgOTf, and K2PtCl4/AgOAc. Propiolic acid reacted even with less reactive phenols in trifluoroacetic acid to give coumarins and dihydrocoumarins. In the case of substituted propiolic acids, phenylpropiolic acid and 2-octynoic acid, the reactions proceeded selectively to afford coumarins in good to high yields. 相似文献
4.
Intramolecular (2+2) photocycloaddition of β-stilbazoles tethered by silyl chains took place with high efficiency. Complexation with dicarboxylic acid or catechol further enhanced both the efficiency and stereoselectivity. 相似文献
5.
Mariusz Gadzinowski Stanislaw Sosnowski 《Journal of polymer science. Part A, Polymer chemistry》2003,41(23):3750-3760
New biodegradable/biocompatible ABC block copolymers, poly(ethylene oxide)‐b‐poly(glycidol)‐b‐poly(L ,L ‐lactide) (PEO‐PGly‐PLLA), were synthesized. First, PEO‐b‐poly(1‐ethoxyethylglycidol)‐b‐PLLA was synthesized by a successive anionic ring‐opening copolymerization of ethylene oxide, 1‐ethoxyethylglycidyl ether, and L ,L ‐lactide initiated with potassium 2‐methoxyethanolate. In the second step, the 1‐ethoxyethyl blocking groups of 1‐ethoxyethylglycidyl ether were removed at weakly acidic conditions leaving other blocks intact. The resulting copolymers were composed of hydrophilic and hydrophobic segments joined by short polyglycidol blocks with one hydroxyl group in each monomeric unit. These hydroxyl groups may be used for further copolymer transformations. The PEO‐PGly‐PLLA copolymers with a molecular weight of PLLA blocks below 5000 were water‐soluble. Above the critical micellar concentration (ranging from 0.05 to1.0 g/L, depending on the composition of copolymer), copolymers formed macromolecular micelles with a hydrophobic PLLA core and hydrophilic PEO shell. The diameters of the micelles were about 25 nm. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3750–3760, 2003 相似文献
6.
The composite comprised of zinc oxide quantum dots and poly(amic acid) (PAAc) was prepared and studied by X-rays diffraction, X-ray photoelectron spectroscopy, light scattering, UV absorbance and UV fluorescence. The UV absorbance of the ZnO/PAAc composite was found to be much larger than that of its components taken separately. The fluorescence of the ZnO/PAAc composite was found to be shifted to longer wavelengthes in comparison with pure ZnO. The presence of the dopant dodecylbenzenesulfonic acid was found to affect the observed fluorescence. 相似文献
7.
Reaction of acetylene dicarboxaldehyde monoacetal with substituted Meldrum’s acid leads good yields in 2-pyranones-4-carboxaldehydes substituted in position 3. 相似文献
8.
Emmanuel DardennesÁrpád Kovács-Kulyassa Andrea RenzettiJanos Sapi Jean-Yves Laronze 《Tetrahedron letters》2003,44(2):221-223
Chiral pyrano- and pyrrolidino-fused tryptamines were prepared by a diasteroselective trimolecular condensation between indole, Garner's aldehyde and Meldrum's acid, followed by selective functional group transformations. 相似文献
9.
M. V. Veselovskaya S. V. Shilin M. M. Garazd V. P. Khilya 《Chemistry of Natural Compounds》2003,39(2):177-181
Furocoumarins modified by amino acids were prepared by condensation of the N-hydroxysuccinimide ester of 3-(2,3,5-trimethyl-7-oxofuro[3,2-g]chromen-6-yl)propanoic acid with amino acids. 相似文献
10.
The concept and application of phase-space reconstructions are reviewed. Fractional derivatives are then proposed for the purpose of reconstructing dynamics from a single observed time history. A procedure is presented in which the fractional derivatives of time series data are obtained in the frequency domain. The method is applied to the Lorenz system. The ability of the method to unfold the data is assessed by the method of global false nearest neighbors. The reconstructed data is used to compute recurrences and correlation dimensions. The reconstruction is compared to the commonly used method of delays in order to assess the choice of reconstruction parameters, and also the quality of results. 相似文献