全文获取类型
收费全文 | 44708篇 |
免费 | 5418篇 |
国内免费 | 10090篇 |
专业分类
化学 | 46221篇 |
晶体学 | 2973篇 |
力学 | 591篇 |
综合类 | 300篇 |
数学 | 554篇 |
物理学 | 9577篇 |
出版年
2024年 | 50篇 |
2023年 | 355篇 |
2022年 | 750篇 |
2021年 | 1161篇 |
2020年 | 1484篇 |
2019年 | 1353篇 |
2018年 | 1346篇 |
2017年 | 1581篇 |
2016年 | 2123篇 |
2015年 | 1939篇 |
2014年 | 2645篇 |
2013年 | 4605篇 |
2012年 | 2928篇 |
2011年 | 2936篇 |
2010年 | 2585篇 |
2009年 | 2887篇 |
2008年 | 3005篇 |
2007年 | 3165篇 |
2006年 | 3205篇 |
2005年 | 2987篇 |
2004年 | 2912篇 |
2003年 | 2430篇 |
2002年 | 1754篇 |
2001年 | 1235篇 |
2000年 | 1164篇 |
1999年 | 1146篇 |
1998年 | 982篇 |
1997年 | 873篇 |
1996年 | 832篇 |
1995年 | 740篇 |
1994年 | 658篇 |
1993年 | 577篇 |
1992年 | 545篇 |
1991年 | 331篇 |
1990年 | 183篇 |
1989年 | 164篇 |
1988年 | 146篇 |
1987年 | 102篇 |
1986年 | 59篇 |
1985年 | 82篇 |
1984年 | 57篇 |
1983年 | 28篇 |
1982年 | 23篇 |
1981年 | 28篇 |
1980年 | 18篇 |
1979年 | 9篇 |
1978年 | 9篇 |
1977年 | 7篇 |
1976年 | 10篇 |
1975年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 365 毫秒
81.
研究了γ-辐照前后纯Y2SiO5和Eu3+掺杂的Y2SiO5晶体吸收光谱的变化,辐照后,未退火和氢气退火的纯Y2SiO5晶体在260~270 nm和320 nm波段产生了附加吸收峰,分别是由F心和O-心的吸收引起的;经过空气退火的纯YSO晶体中,由于消除了氧空位,因此辐照后没有出现色心吸收峰。在Eu3+∶Y2SiO5晶体中,不但有相同的F心和O-心吸收峰,而且还有Eu2+离子在300 nm和390 nm的吸收峰。随着辐照剂量的增加,色心附加吸收峰增强。空气退火能减少Eu3+∶Y2SiO5晶体中的色心,而氢气退火能增加色心。 相似文献
82.
Early stages of rare-earth metal (Yb and Eu) growth on a vicinal, single-domain Si(1 0 0)2 × 1 surface have been studied in the coverage range of 0.1-0.3 monolayer (ML) by low energy electron diffraction, scanning tunneling microscopy, and synchrotron radiation photoemission spectroscopy. We show that Yb induces the 2 × 3 periodicity in the whole range of coverage studied. The 2 × 3 reconstruction coexists with the local 3 × 2/4 × 2 structure at about 0.2 ML of Yb. In contrast, Eu forms the 3 × 2 periodicity at 0.1-0.2 ML, whereas this structure is converted into the 2 × 3 phase at about 0.3 ML. The atomic arrangement and electronic properties of these reconstructions and the adsorbate-mediated modification of surface morphology are investigated. 相似文献
83.
M. Faucher D. Garcia E. Antic-Fidancev M. Lemaitre-Blaise 《Journal of Physics and Chemistry of Solids》1989,50(12)
Some structural and spectroscopic features of rare earth orthoaluminates are examined.The trigonal→ orthorhombic transition is studied in a series of NdxSm1−xAlO3 compounds. The evolution of the crystal structure is followed by X-ray analysis and optical absorption. The free ion and crystal field parameters of Nd3+ (4f3 configuration) are determined in LuAlO3:Nd3+.The anomaly of the calculated splitting of the
levels is slight, but well characterized in NdAlO3. The spin correlated crystal field and orbitally correlated crystal field models are tested as well as an empirical correction which was proposed earlier. 相似文献
84.
YunXiaWANG LinJIA ZhenSHI 《中国化学快报》2003,14(6):561-564
The addition-hydrolysis reaction of benzimidazolium salt with some mono- and bifunctional amine nucleophiles is reported,and a novel method of biomimetic synthesis for formamides and heterocycle compounds is provided. 相似文献
85.
The title compound, [Cd(SeCN)2(pyCN)2]∞, adopts an extended one‐dimensional chain structure in which the neighboring cadmium atoms are bridged by two selenocyanate ions. The central cadmium atom has a distorted octahedral geometry defined by two isonicotinonitrile and four selenocyanate ions in a 4N2Se fashion. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
86.
In mononuclear HgI2[(C5H4N)3N], mercury is tetrahedrally coordinated by two nitrogen atoms of a tris(2‐pyridyl)amine ligand and two iodides. The coordination moieties are connected by weak intermolecular Hg(II)···I interactions to give a one‐dimensional structure. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
87.
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
88.
Gowravaram Sabitha G.S.Kiran Kumar ReddyK.Bhaskar Reddy J.S. Yadav 《Tetrahedron letters》2003,44(34):6497-6499
The three component condensation of an aldehyde, a β-keto ester and urea (thiourea) in the presence of a catalytic amount of VCl3 is disclosed for the solution phase synthesis of dihydropyrimidinones. The ease of synthesis and work-up allowed the parallel synthesis of a 48-membered library of dihydropyrimidinones quickly and efficiently in good yields. 相似文献
89.
Aiqing Zhang Xiangdan Li Changwoon Nah Keejun Hwang Myong‐Hoon Lee 《Journal of polymer science. Part A, Polymer chemistry》2003,41(1):22-29
A soluble aromatic polyimide was chloromethylated via a reaction with chloromethyl methyl ether in the presence of tin(IV) chloride to produce a new starting material for the modification of aromatic polyimides. The chemical structure of the resulting polymer was confirmed by 1H NMR and Fourier transform infrared spectroscopy. The maximum number of chloromethyl groups per repeat unit was 1.81. The chloromethylated polyimide was stable up to 250 °C and soluble in both chloroform and tetrahydrofuran. So that its utilization for further modification could be demonstrated, cinnamic acid was reacted with the formed polyimide, and it produced a new photosensitive polyimide with a cinnamoyl side chain. The photosensitivity of the resulting polyimide was investigated with ultraviolet spectroscopic methods. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 22–29, 2003 相似文献
90.
Lígia M. Rodrigues 《Tetrahedron》2004,60(40):8929-8936
Tetrapeptides containing one of a set of four different α,α-dialkyl glycines at the C-terminus were synthesized by conventional methods in solution and their conformational behavior investigated by 1H NMR spectroscopy in connection with molecular mechanics calculations. The results were consistent with conformations stabilized by a γ-turn in the case of compounds with alkyl groups larger than methyl, while the corresponding Aib derivative did not exhibit intramolecular hydrogen bonding. 相似文献