全文获取类型
收费全文 | 526篇 |
免费 | 2篇 |
国内免费 | 7篇 |
专业分类
化学 | 91篇 |
数学 | 28篇 |
物理学 | 29篇 |
综合类 | 387篇 |
出版年
2022年 | 6篇 |
2021年 | 3篇 |
2020年 | 3篇 |
2019年 | 62篇 |
2018年 | 8篇 |
2017年 | 8篇 |
2016年 | 5篇 |
2015年 | 6篇 |
2014年 | 22篇 |
2013年 | 19篇 |
2012年 | 35篇 |
2011年 | 36篇 |
2010年 | 28篇 |
2009年 | 31篇 |
2008年 | 40篇 |
2007年 | 23篇 |
2006年 | 47篇 |
2005年 | 15篇 |
2004年 | 28篇 |
2003年 | 13篇 |
2002年 | 11篇 |
2001年 | 9篇 |
2000年 | 14篇 |
1999年 | 4篇 |
1998年 | 1篇 |
1997年 | 1篇 |
1996年 | 1篇 |
1995年 | 3篇 |
1994年 | 4篇 |
1991年 | 1篇 |
1987年 | 1篇 |
1985年 | 8篇 |
1984年 | 5篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 5篇 |
1980年 | 5篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1977年 | 4篇 |
1976年 | 3篇 |
1975年 | 3篇 |
1974年 | 6篇 |
1973年 | 1篇 |
排序方式: 共有535条查询结果,搜索用时 46 毫秒
61.
Koos Duppen Laurens W. Molenkamp Jos B.W. Morsink Douwe A. Wiersma H.P. Trommsdorff 《Chemical physics letters》1981,84(3):421-424
Optical absorption and picosecond photon echo experiments are used to study the dephasing of pentacene in benzoic acid. It is shown that, while the absorption spectrum of pentacene is effected by proton transfer in the benzoic acid dimer, the dephasing is caused by elastic and inelastic phonon scattering processes. 相似文献
62.
Bruce J. West Ary L. Goldberger Galina Rovner Valmik Bhargava 《Physica D: Nonlinear Phenomena》1985,17(2):198-206
Under physiologic conditions, the AV junction is traditionally regarded as a passive conduit for the conduction of impulses from the atria to the ventricles. An alternative view, namely that subsidiary pacemakers play an active role in normal electrophysiologic dynamics during sinus rhythm, has been suggested based on nonlinear models of cardiac oscillators. A central problem has been the development of a simple but explicit mathematical model for coupled nonlinear oscillators relevant both to stable and perturbed cardiac dynamics. We use equations describing an analog electrical circuit with an external d.c. voltage source (V0) and two nonlinear oscillators with intrinsic frequencies in the ratio of 3:2, comparable to the SA node and AV junction rates. The oscillators are coupled by means of a resistor. 1:1 (SA:AV) phase-locking of the oscillators occurs over a critical range of V0. Externally driving the SA oscillator at increasing rates results in 3:2 AV Wenckebach periodicity and a 2:1 AV block. These findings appear with no assumptions about conduction time or refractoriness. This dynamical model is consistent with the new interpretation that normal sinus rhythm may represent 1:1 coupling of two or more active nonlinear oscillators and also accounts for the appearance of an AV block with critical changes in a single parameter such as the pacing rate. 相似文献
63.
徐宏图 《杭州师范学院学报(社会科学版)》2006,28(6):42-46
南宋时期的温州,同时产生了"温州杂剧"(南戏)与"永嘉学派"这两种先进文化,从而在中国的戏剧史、哲学史乃至整个文化史上写下了极其光辉的一页.它们主张"通商惠工",提倡"扶持商贾";批判腐败,提倡"事功";反对"理学"教条,提倡"民本"关怀及爱国主义,反对苛捐杂税;提倡勤劳致富、鼓励勤学成才等等,从而体现了相通之处.深入辨析其在精神深层的共同源头,有助于增进对浙江古代文化的整体认识. 相似文献
64.
近年来,全国各地广泛开展了消防“进社区、进学校、进企业、进农村”为主要内容的消防“四进”活动。本文针对开展此项工作中存在的问题提出了相关对策:积极创建工作模式,创新工作机制;领导重视,统一规划狠抓消防工作基础建设,提高学校自防自救能力;抓好学校消防宣传教育和培训,提高师生消防安全意识。 相似文献
65.
The school bus routing problem: A review 总被引:2,自引:0,他引:2
This paper aims to provide a comprehensive review of the school bus routing problem (SBRP). SBRP seeks to plan an efficient schedule for a fleet of school buses where each bus picks up students from various bus stops and delivers them to their designated schools while satisfying various constraints such as the maximum capacity of a bus, the maximum riding time of a student in a bus, and the time window of a school. This class of problem consists of different sub-problems involving data preparation, bus stop selection, bus route generation, school bell time adjustment, and bus scheduling. In this paper, the various assumptions, constraints, and solution methods used in the literature on SBRP are summarized. A list of issues requiring further research is also presented. 相似文献
66.
Tao Li Ping-Lin Li Xiang-Chao Luo Xu-Li Tang Guo-Qiang Li 《Tetrahedron letters》2019,60(30):1996-1998
Three new dibromopyrrole alkaloids, 9-N-methylcylindradine A (1), 1-N-methylugibohlin (2), nemoechine H (3), together with one known dibromopyrrole alkaloid N-methyldibromoisophakellin (4) were isolated from the South China Sea sponge Agelas nemoechinata. Their structures were elucidated by comprehensive spectroscopic methods including HRESIMS and NMR, and the absolute configuration of compound 1 was further confirmed by comparison of optical rotation. Compound 3 exhibited moderate cytotoxic activity against K562 and L-02 with IC50 values of 6.1 μM and 12.3 μM, respectively. 相似文献
67.
Thioacids and thioamino acids were synthesized in excellent yields from readily available acyl benzotriazoles and sodium hydrosulfide in water at room temperature. The new methodology features mild reaction conditions, high yields, short reaction times, and does not involve the use of organic solvents or bases. The reaction is eco-friendly, and the workup procedure is simple and does not require chromatographic separation. 相似文献
68.
Xinxin Yuan Lulu Wu Cuilian Xu Zhenliang Pan Lijun Shi Guoyu Yang Caixia Wang Sufang Fan 《Tetrahedron letters》2019,60(52):151329
A one-pot two-step reaction of 3-(trifluoroacetyl)coumarin and indole afforded trifluoromethyl-substituted bis(indolyl)methane compounds containing coumarin skeleton. The atomic economic and simply manipulative reaction involved premier treatment of reaction mixtures with Sc(OTf)3, followed by p-TSA in one-pot process. The reaction proceeded to give the title compounds in high yields (up to 95% yield). 相似文献
69.
The antibiotic pipeline has failed to keep pace with the rise of multidrug resistant tuberculosis and extensively drug-resistant tuberculosis pathogens. Naturally occurring peptides provide a rich source of lead compounds for developing novel pharmaceuticals with high selectivity and potency. Given the vast number of naturally-occurring bioactive cyclic peptides identified so far, the following digest highlights several cyclic peptides, discovered in the preceding decade, that exhibit promising activity against Mycobacterium tuberculosis. 相似文献
70.
Breanna M. Wong Greg Collinge Alyssa J.R. Hensley Yong Wang Jean-Sabin McEwen 《Progress in Surface Science》2019,94(2):100538
Bimetallic catalysts have demonstrated properties favorable for upgrading biofuel through catalytic hydrodeoxygenation. However, the design and optimization of such bimetallic catalysts requires the ability to construct accurate, predictive models of these systems. To generate a model that predicts the kinetic behavior of benzene adsorbed on Pt (1 1 1) and a Pt3Sn (1 1 1) surface alloy (Pt3Sn (1 1 1)), the adsorption of benzene was studied for a wide range of benzene coverages on both surfaces using density functional theory (DFT) calculations. The adsorption energy of benzene was found to correlate linearly with benzene coverage on Pt (1 1 1) and Pt3Sn (1 1 1); both surfaces exhibited net repulsive lateral interactions. Through an analysis of the d-band properties of the metal surface, it was determined that the coverage dependence is a consequence of the electronic interactions between benzene and the surface. The linear coverage dependence of the adsorption energy allowed us to quantify the influence of the lateral interactions on the heat of adsorption and temperature programmed desorption (TPD) spectra using a mean-field model. A comparison of our simulated TPD to experiment showed that this mean-field model adequately reproduces the desorption behavior of benzene on Pt (1 1 1) and Pt3Sn (1 1 1). In particular, the TPD correctly exhibits a broadening desorption peak as the initial coverage of benzene increases on Pt (1 1 1) and a low temperature desorption peak on Pt3Sn (1 1 1). However, due to the sensitivity of the TPD peak temperature to the desorption energy, precise alignment of experimental and theoretical TPD spectra demands an accurate calculation of the adsorption energy. Therefore, an analysis of the effect of the exchange-correlation functional on TPD modeling is presented. Through this work, we show the necessity of incorporating lateral interactions into theoretical models in order to correctly predict experimental behavior. 相似文献