全文获取类型
收费全文 | 82010篇 |
免费 | 6211篇 |
国内免费 | 12170篇 |
专业分类
化学 | 49423篇 |
晶体学 | 2109篇 |
力学 | 1074篇 |
综合类 | 496篇 |
数学 | 13464篇 |
物理学 | 15251篇 |
综合类 | 18574篇 |
出版年
2024年 | 190篇 |
2023年 | 1091篇 |
2022年 | 2109篇 |
2021年 | 2088篇 |
2020年 | 2127篇 |
2019年 | 2200篇 |
2018年 | 1954篇 |
2017年 | 2276篇 |
2016年 | 2418篇 |
2015年 | 2291篇 |
2014年 | 3343篇 |
2013年 | 5937篇 |
2012年 | 4570篇 |
2011年 | 5452篇 |
2010年 | 4738篇 |
2009年 | 5746篇 |
2008年 | 5603篇 |
2007年 | 5993篇 |
2006年 | 5433篇 |
2005年 | 4629篇 |
2004年 | 4329篇 |
2003年 | 3660篇 |
2002年 | 3056篇 |
2001年 | 2436篇 |
2000年 | 2289篇 |
1999年 | 1992篇 |
1998年 | 1583篇 |
1997年 | 1446篇 |
1996年 | 1335篇 |
1995年 | 1262篇 |
1994年 | 1163篇 |
1993年 | 990篇 |
1992年 | 900篇 |
1991年 | 674篇 |
1990年 | 502篇 |
1989年 | 452篇 |
1988年 | 354篇 |
1987年 | 270篇 |
1986年 | 218篇 |
1985年 | 245篇 |
1984年 | 165篇 |
1983年 | 87篇 |
1982年 | 133篇 |
1981年 | 149篇 |
1980年 | 100篇 |
1979年 | 86篇 |
1978年 | 74篇 |
1977年 | 86篇 |
1976年 | 60篇 |
1973年 | 39篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
Gas-phase 21Ne nuclear magnetic resonance spectra were measured at the natural abundance of 21Ne isotope for samples consisting of pressurized neon up to 60 bar at room temperature and applying the magnetic field of the strength B0 = 11.7574 T. It showed that the nuclear magnetic resonance frequency is linearly dependent on the density of gaseous neon. The resonance frequency was extrapolated to the zero-density point, and it permitted the determination of the 21Ne nuclear magnetic moment, μ(21Ne) = 0.6617774(10) μN. The present value of μ(21Ne) is not influenced by the bulk magnetic susceptibility of neon and interactions between neon atoms; therefore, it is more precise and reliable than the previous result obtained for μ(21Ne). 相似文献
22.
《Journal of Saudi Chemical Society》2020,24(10):754-764
A kinetic study of the reactions of potentially bioactive 2-amino-4-arylthiazoles with highly reactive 4,6-dinitrobenzofuroxan (DNBF) is reported herein in acetonitrile solution. The complexation reaction was followed by recording the UV–vis spectra with time at λmax = 482 nm. Electronic effects of substituents influencing the rate of reaction have been studied using structure-reactivity relationships. It is shown that the Hammett plot relative to the reaction of DNBF with 2-amino-4-(4-chlorophenyl)thiazole exhibit positive deviation from the log k1 versus σ correlation, while it showed excellent linear correlation in terms of Yukawa–Tsuno equation. It has be noticed that the nonlinear Hammett plot observed for 2-amino-4-(4-chlorophenyl) thiazole is not attributed to a change in rate-determining step but is due to nature of electronic effect of substituent caused by the resonance of stabilization of substrates. The second-order rate constant (k1) relating to the bond C–C and C-N forming step of the complexation processes of DNBF with 4-substituted-aminothiazoles and 2-amino-5-methyl-4-phenylthiazole, respectively, is fit into the linear relationship log k = sN (N + E), thereby permitting the assessment of the nucleophilicity parameter (N) of the 2-amino-4-arylthiazoles of the range (4.90 < N < 6.85). 2-amino-4-arylthiazoles is subsequently ranked by positioning its reactivity on the general nucleophilicity scale developed recently by Mayr and coworkers (2003) leading an interesting and a direct comparison over a large domain of π-, σ -, and n-nucleophiles. The global electrophilicity/nucleophilicity reactivity indexes of the 2-amino-4-arylthiazoles have been investigated by means of a density functional theory (DFT) method. . 相似文献
23.
It is believed that there are more fundamental gauge symmetries beyond those described by the Standard Model of particle physics. The scales of these new gauge symmetries are usually too high to be reachable by particle colliders. Considering that the phase transition (PT) relating to the spontaneous breaking of new gauge symmetries to the electroweak symmetry might be strongly first order, we propose considering the stochastic gravitational waves (GW) arising from this phase transition as an indirect way of detecting these new fundamental gauge symmetries. As an illustration, we explore the possibility of detecting the stochastic GW generated from the PT of \begin{document}$ {\bf{B}}-{\bf{L}}$\end{document} ![]()
![]()
in the space-based interferometer detectors. Our study demonstrates that the GW energy spectrum is reachable by the LISA, Tianqin, Taiji, BBO, and DECIGO experiments only for the case where the spontaneous breaking of \begin{document}$ {\bf{B}}-{\bf{L}}$\end{document} ![]()
![]()
is triggered by at least two electroweak singlet scalars. 相似文献
24.
A string of four new hetero binuclear Ru(III) complexes of ferrocenecarboxaldehyde-4(N)-substituted thiosemicarbazones were synthesized and characterized by various spectral (infrared, ultraviolet–visible, Electron Paramagnetic Resonance (EPR) and High Resolution Mass Spectrometry (HR-MS) techniques. The binding abilities of the ligands/complexes with nucleic acid (calf thymus DNA, CT-DNA) and bovine serum albumin (BSA) were analyzed by absorption and emission titration methods. The complexes exhibited better DNA binding affinity than their parent ligands. The interaction with CT-DNA was found to be intercalative and with BSA static quenching mechanism was observed. All the synthesized Ru(III) complexes were subjected to study their in vitro cytotoxicity against MCF-7 (human breast cancer) and HT-29 (human colon cancer) cell lines. Among the four complexes, complex 3 [RuCp (FF-etsc)PPh3]Cl exhibited the highest cytotoxicity in MCF-7 cells and complex 4 [RuCp (FF-ptsc)PPh3]Cl was the most active on HT-29 cells. 相似文献
25.
一个基于Exchange的企业办公自动化系统的实现 总被引:1,自引:0,他引:1
介绍了一个基于Exchange的企业办公自动化系统的实现,其中包括系统目标、开发平台和工具的选择、系统的功能模块、关键技术以及系统的安全机制。 相似文献
26.
Summary Carbon deposits on the surface ofRu/Fe2O3 catalysts used in the water-gas shift reaction have been investigated by Auger Electron Spectrometry. A correlation has been
found between the thickness of the carbon deposit and the catalytic activity in WGSR. The carbon deposit covers the metallic
active centers and blocks their contact with reagents. The dotting of the iron oxide support with sodium has been found to
reduce the amount of carbon deposit. . 相似文献
27.
28.
The use of supercritical fluid carbon dioxide (SF-CO_2) in extraction of lanolin and its alcohol issuperior to the conventional solvent extractionmethod. Its distinctive advantages include highextractive ratio, nontoxic and nonflammablesolvents, and minimal by-product pollution. The 相似文献
29.
Abrupt climate change:Debate or action 总被引:4,自引:1,他引:3
CHENGHai 《科学通报(英文版)》2004,49(18):1997-2002
Global abrupt climate changes have been documented by various climate records, including ice cores, ocean sediment cores, lake sediment cores, cave deposits, loess deposits and pollen records. The climate system prefers to be in one of two stable states, i.e. interstadial or stadial conditions, but not in between. The transition between two states has an abrupt character. Abrupt climate changes are, in general, synchronous in the northern hemisphere and tropical regions. The timescale for abrupt climate changes can be as short as a decade. As the impacts may be potentially serious, we need to take actions such as reducing CO2 emissions to the atmosphere. 相似文献
30.
提出了基于B/S模式的水利水电工程造价管理系统,对B/S的体系结构、系统的总体结构设计、功能及功能设计进行了阐述,并说明了系统实现的关键技术. 相似文献