全文获取类型
收费全文 | 60306篇 |
免费 | 9982篇 |
国内免费 | 1435篇 |
专业分类
化学 | 56748篇 |
晶体学 | 497篇 |
力学 | 973篇 |
综合类 | 1篇 |
数学 | 6743篇 |
物理学 | 6761篇 |
出版年
2023年 | 165篇 |
2022年 | 167篇 |
2021年 | 406篇 |
2020年 | 1582篇 |
2019年 | 2944篇 |
2018年 | 1475篇 |
2017年 | 1076篇 |
2016年 | 4113篇 |
2015年 | 4066篇 |
2014年 | 4013篇 |
2013年 | 5523篇 |
2012年 | 4386篇 |
2011年 | 3685篇 |
2010年 | 3691篇 |
2009年 | 3556篇 |
2008年 | 3719篇 |
2007年 | 3126篇 |
2006年 | 2710篇 |
2005年 | 2793篇 |
2004年 | 2556篇 |
2003年 | 2188篇 |
2002年 | 2762篇 |
2001年 | 1877篇 |
2000年 | 1765篇 |
1999年 | 741篇 |
1998年 | 323篇 |
1997年 | 329篇 |
1996年 | 394篇 |
1995年 | 275篇 |
1994年 | 316篇 |
1993年 | 274篇 |
1992年 | 264篇 |
1991年 | 230篇 |
1990年 | 231篇 |
1989年 | 201篇 |
1988年 | 186篇 |
1987年 | 178篇 |
1986年 | 175篇 |
1985年 | 253篇 |
1984年 | 274篇 |
1983年 | 188篇 |
1982年 | 212篇 |
1981年 | 225篇 |
1980年 | 224篇 |
1979年 | 180篇 |
1978年 | 175篇 |
1977年 | 197篇 |
1976年 | 164篇 |
1975年 | 165篇 |
1974年 | 130篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
211.
212.
The structure of the title adduct comprises a phenanthroline derivative 2-phenyl-imidazo[4,5-f]1,10-phenanthroline and a methanol.The composition of the crystalline adduct was characterized as C19H12N4.CH3OH.It belongs to orthorhombic system,space group Pna21 with a=1.3693(4)nm,b=2.2988(7)nm,c=0.51338(15)nm,V=1.6160(8)nm^3.Z=4,and final R1=0.0423.wR2=0.1012 .Crystal structure shows that all the 19 carbon atoms and 4 nitrogen atoms are coplanar.The bond length data indicated that a very extensive conjugation system was formed.This conjugation makes the compound being a potentially excellent energy transformer used for luminescent materials. 相似文献
213.
Hong‐Qing Wang Zhao‐Jie Liu Li‐Min Yang Ming‐Wu Ding 《Journal of heterocyclic chemistry》2004,41(3):393-397
A series of new 1H‐pyrazolo[3,4‐d]pyrimidin‐4(5H)‐one Derivatives 5 has been designed and regio‐selectively synthesized via a tandem aza‐Wittig reaction. The structures of all compounds prepared have been confirmed by 1H NMR, IR, EI‐MS spectroscopy and elemental analyses. The results of preliminary bioassay indicate that most compounds 5 possess an inhibition effect against Botrytis cinereapers and Pyricularia oryzae at the concentration of 50 mg/L. 相似文献
214.
215.
216.
Jean-Pierre Changeux Alain Connes M. B. DeBevoise Jean Petitot Mark Anspach 《Mathematical Intelligencer》2005,27(4):48-56
Conclusion The debate between Jean-Pierre Changeux and Alain Connes is one of the most, interesting to take place in recent years. It
re-frames in a very up-to-date context a whole series of traditional and difficult questions from the standpoint of the knowledge
and experience of two of the leading protagonists of contemporary science. To the choice presented by the neurobiologist between
a Platonist ontology and a neurocognitive psychology of mathematical activity, the mathematician replies with a conception
that is objective (neither ontological nor psychological) of the thoroughly consistent universe of mathematical idealities.
It is indeed in this three-sided arena that the major difficulties play themselves out. One of the great virtues of the book
is to cast a spotlight on this confrontation. 相似文献
217.
218.
Yong‐Jin Kim Jin‐Woong Kim Jung‐Eun Lee Jee‐Hyun Ryu Junoh Kim Ih‐Seop Chang Kyung‐Do Suh 《Journal of polymer science. Part A, Polymer chemistry》2004,42(22):5627-5635
Mesoporous polymer microspheres with gold (Au) nanoparticles inside their pores were prepared considering their surface functionality and porosity. The Au/polymer composite microspheres prepared were characterized by transmission electron microscope (TEM), X‐ray diffraction (XRD), and Brunauer–Emmett–Teller (BET) techniques. The results showed that the adsorption of Au nanoparticles could be increased by imparting the pore structure and surface‐functional groups into the supporting polymer microspheres (in this study, poly (ethylene glycol dimethacrylate‐co‐acrylonitrile) and poly (EGDMA‐co‐AN) system). Above all, from this study, it was established that the porosity of the polymer microspheres is the most important factor that determines the distribution and adsorption amount of face‐centered cubic (fcc) Au nanoparticles in the final products. Our study showed that the continuous adsorption of Au nanoparticles with the aid of the large surface area and surface interaction sites formed more favorably the Au/polymer composite microspheres. The BET measurements of Au/poly(EGDMA‐co‐AN) composite microspheres reveals that the adsorption of Au nanoparticles into the pores kept the pore structure intact and made it more porous. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5627–5635, 2004 相似文献
219.
Pedro lvarez‐Boo Jos Sergio Casas Alfonso Castieiras María Delfina Couce Eduardo Freijanes Eva Novoa Jos Sordo 《应用有机金属化学》2003,17(9):725-729
Reaction of dichloro‐ and dibromodimethyltin(IV) with 2‐(pyrazol‐1‐ylmethyl)pyridine (PMP) afforded [SnMe2Cl2(PMP)] and [SnMe2Br2(PMP)] respectively. The new complexes were characterized by elemental analysis and mass spectrometry and by IR, Raman and NMR (1H, 13C) spectroscopies. Structural studies by X‐ray diffraction techniques show that the compounds consist of discrete units with the tin atom octahedrally coordinated to the carbon atoms of the two methyl groups in a trans disposition (Sn? C = 2.097(5), 2.120(5) Å and 2.110(6), 2.121(6) Å in the chloro and in the bromo compounds respectively), two cis halogen atoms (Sn? Cl = 2.4908(16), 2.5447(17) Å; Sn? Br = 2.6875(11), 2.7464(9) Å) and the two donor atoms of the ligand (Sn? N = 2.407(4), 2.471(4) Å and 2.360(5), 2.455(5) Å). In both cases, the Sn? N(pyridine) bond length is markedly longer than the Sn? N(pyrazole) distance. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
220.
We give a number of characterizations of bodies of constant width in arbitrary dimension. As an application, we describe a way to construct a body of constant width in dimension n, one of its (n – 1)‐dimensional projection being given. We give a number of examples, like a four‐dimensional body of constant width whose 3D‐projection is the classical Meissner's body. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献