全文获取类型
收费全文 | 5220篇 |
免费 | 930篇 |
国内免费 | 651篇 |
专业分类
化学 | 3760篇 |
晶体学 | 94篇 |
力学 | 254篇 |
综合类 | 32篇 |
数学 | 589篇 |
物理学 | 2072篇 |
出版年
2024年 | 5篇 |
2023年 | 112篇 |
2022年 | 185篇 |
2021年 | 222篇 |
2020年 | 258篇 |
2019年 | 211篇 |
2018年 | 208篇 |
2017年 | 195篇 |
2016年 | 280篇 |
2015年 | 291篇 |
2014年 | 333篇 |
2013年 | 440篇 |
2012年 | 533篇 |
2011年 | 507篇 |
2010年 | 341篇 |
2009年 | 348篇 |
2008年 | 331篇 |
2007年 | 344篇 |
2006年 | 275篇 |
2005年 | 197篇 |
2004年 | 169篇 |
2003年 | 121篇 |
2002年 | 123篇 |
2001年 | 90篇 |
2000年 | 99篇 |
1999年 | 100篇 |
1998年 | 67篇 |
1997年 | 45篇 |
1996年 | 66篇 |
1995年 | 28篇 |
1994年 | 33篇 |
1993年 | 37篇 |
1992年 | 35篇 |
1991年 | 23篇 |
1990年 | 27篇 |
1989年 | 26篇 |
1988年 | 17篇 |
1987年 | 20篇 |
1986年 | 9篇 |
1985年 | 9篇 |
1984年 | 14篇 |
1983年 | 8篇 |
1982年 | 7篇 |
1981年 | 2篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1957年 | 2篇 |
排序方式: 共有6801条查询结果,搜索用时 46 毫秒
991.
Bénard CP Geng Z Heuft MA VanCrey K Fallis AG 《The Journal of organic chemistry》2007,72(19):7229-7236
Four new classes of organic solvent soluble ethynylpentacene derivatives (2,9-, 2,10-, 2,6,9,13-, 2,6,10,13-) have been prepared by complementary, versatile, double Diels-Alder strategies. Functional groups on the A, C, and E rings can be manipulated to increase the solubility, modulate the electronics, and alter the solid-state packing. Cycloaddition reactions with diene 2 and 1,4,5,8-anthradiquinone (3) or ortho-quinodimethane 19 with 1-butyl-3-methylimidazolim iodide (18) as the iodide source (a significant improvement over NaI) and benzoquinone (20) followed by in situ aromatization afforded the quinones 4, 5, 21, and 22, respectively. For the 2,9- and 2,10- families, a one-pot desilylation/triflation was developed. Palladium(0) coupling and reductive aromatization afforded 2,9-di(triisopropylsilylethynyl)pentacene (10) and 2,10-di(triisopropylsilylethynyl)pentacene (11), respectively. Photodimerization of these pentacenes afforded the air-stable [4 + 4] cycloaddition pentacene precursors (tetrakisnaphthotricyclo[4.2.2.22,5]dodecanes, 12-15). Thermal cycloreversion of the dimers ( approximately 13 J/g, approximately 4 kcal/mol) produces the parent pentacenes (10 or 11). The tetrasubstituted family utilized a parallel route with extra versatility as the timing of the Grignard and palladium(0) coupling step may be varied depending upon the functional group combinations desired. The subsequent reactions provided the tetraethynylpentacenes 28-30, 33-35 (para-isomers), and 38 (meta-isomer). X-ray crystallography analysis of 28, 29, and 33 revealed various pi-pi stacked packing motifs that differ from the unfavorable herringbone pattern of pentacene. 相似文献
992.
Improving hydrophilicity and protein resistance of poly(vinylidene fluoride) membranes by blending with amphiphilic hyperbranched-star polymer 总被引:3,自引:0,他引:3
Zhao YH Zhu BK Kong L Xu YY 《Langmuir : the ACS journal of surfaces and colloids》2007,23(10):5779-5786
To endow hydrophobic poly(vinylidene fluoride) (PVDF) membranes with reliable hydrophilicity and protein resistance, an amphiphilic hyperbranched-star polymer (HPE-g-MPEG) with about 12 hydrophilic arms in each molecule was synthesized by grafting methoxy poly(ethylene glycol) (MPEG) to the hyperbranched polyester (HPE) molecule using terephthaloyl chloride (TPC) as the coupling agent and blended with PVDF to fabricate porous membranes via phase inversion process. The chemical composition changes of the membrane surface were confirmed by X-ray photoelectron spectroscopy (XPS), and the membrane morphologies were measured by scanning electron microscopy (SEM). Water contact angle, static protein adsorption, and filtration experiments were used to evaluate the hydrophilicity and anti-fouling properties of the membranes. It was found that MPEG segments of HPE-g-MPEG enriched at the membrane surface substantially, while the water contact angle decreased as low as 49 degrees for the membrane with a HPE-g-MPEG/PVDF ratio of 3/10. More importantly, the water contact angle of the blend membrane changed little after being leached continuously in water at 60 degrees C for 30 days, indicating a quite stable presence of HPE-g-MPEG in the blend membranes. Furthermore, the blend membranes showed lower static protein adsorption, higher water and protein solution fluxes, and better water flux recovery after cleaning than the pure PVDF membrane. 相似文献
993.
McLaughlin J Renzi D Parker K Wu Z 《The Journal of the Acoustical Society of America》2007,121(4):2438-2446
Two new experiments were created to characterize the elasticity of soft tissue using sonoelastography. In both experiments the spectral variance image displayed on a GE LOGIC 700 ultrasound machine shows a moving interference pattern that travels at a very small fraction of the shear wave speed. The goal of this paper is to devise and test algorithms to calculate the speed of the moving interference pattern using the arrival times of these same patterns. A geometric optics expansion is used to obtain Eikonal equations relating the moving interference pattern arrival times to the moving interference pattern speed and then to the shear wave speed. A cross-correlation procedure is employed to find the arrival times; and an inverse Eikonal solver called the level curve method computes the speed of the interference pattern. The algorithm is tested on data from a phantom experiment performed at the University of Rochester Center for Biomedical Ultrasound. 相似文献
994.
995.
996.
997.
998.
Geranylflavonoids are interesting natural products isolated in recent years from some traditional medicinal plants. Some of these flavanoids have been reported to exhibit hypotensive, antibacterial and antitumor effects1. With respect to the synthesis of geranylflavonoids, only the 8-geranylflavanone skeleton as found in compounds 12, 23 and 34 has not been yet prepared. 相似文献
999.
Dexing Kong 《偏微分方程(英文版)》1996,9(3):221-236
In this paper, we give a lower bound for the life-span of classical solutions to the Cauchy problem for first order nonlinear hyperbolic systems with small initial data, which is sharp, and give its application to the system of one-dimensional gas dynamics; for the Cauchy problem of the system of one-dimensional gas dynamics with a kind of small oscillatory initial data, we obtain a precise estimate for the life-span of classical solutions. 相似文献
1000.
测定了8个4-位取代的2,2,6,6-四甲基哌啶的自然丰度15N核磁共振谱及13C谱,发现4-位取代基对15N化学位移有一定的远程相互作用,13C化学位移与4-位卤素取代基的原子电负性有近似的线性关系。 相似文献