全文获取类型
收费全文 | 12546篇 |
免费 | 1117篇 |
国内免费 | 702篇 |
专业分类
化学 | 8439篇 |
晶体学 | 121篇 |
力学 | 499篇 |
综合类 | 51篇 |
数学 | 1484篇 |
物理学 | 3771篇 |
出版年
2023年 | 159篇 |
2022年 | 293篇 |
2021年 | 284篇 |
2020年 | 308篇 |
2019年 | 326篇 |
2018年 | 253篇 |
2017年 | 264篇 |
2016年 | 404篇 |
2015年 | 415篇 |
2014年 | 485篇 |
2013年 | 723篇 |
2012年 | 899篇 |
2011年 | 1025篇 |
2010年 | 651篇 |
2009年 | 677篇 |
2008年 | 769篇 |
2007年 | 734篇 |
2006年 | 648篇 |
2005年 | 561篇 |
2004年 | 533篇 |
2003年 | 455篇 |
2002年 | 405篇 |
2001年 | 334篇 |
2000年 | 273篇 |
1999年 | 208篇 |
1998年 | 162篇 |
1997年 | 139篇 |
1996年 | 175篇 |
1995年 | 132篇 |
1994年 | 151篇 |
1993年 | 124篇 |
1992年 | 118篇 |
1991年 | 98篇 |
1990年 | 107篇 |
1989年 | 85篇 |
1988年 | 95篇 |
1987年 | 65篇 |
1986年 | 73篇 |
1985年 | 95篇 |
1984年 | 69篇 |
1983年 | 68篇 |
1982年 | 49篇 |
1981年 | 46篇 |
1980年 | 47篇 |
1979年 | 34篇 |
1977年 | 37篇 |
1976年 | 37篇 |
1975年 | 34篇 |
1974年 | 48篇 |
1973年 | 44篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
Improved method for analyzing estrogens in water by liquid chromatography-electrospray mass spectrometry 总被引:1,自引:0,他引:1
An improved LC-electrospray ionization MS method was established for four estrogens (17beta-estradiol (E2), estriol (E3), estrone (E1), and ethynyl estradiol (EE)) in environmental water. Almost complete separation of all estrogens was achieved on a phenyl column with methanol/water as the mobile phase. Quantification was achieved in the negative ionization mode using selected ion monitoring. The instrumental detection limits were 20-30 ng/l for the four analytes. In Milli-Q spiked water, the recoveries of the four estrogens were 72-81%, which was similar to those found for river water spiked with the corresponding deuterated estrogens. The detection limits for the four estrogens in river water were 0.1-0.2 ng/l. The method was used to detect residual estrogens in the Tonghui River, which receives water from a municipal sewage treatment plant in Beijing; E1 (1.1 ng/l) and E2 (0.2 ng/l) were detected. 相似文献
72.
Six novel selenium-containing polyphenolic acid esters were synthesized and evaluated as antioxidants and 5-lipoxygenase inhibitors. Synthesis of the title compounds involved the Mitsunobu reaction of polyphenolic acids with 2-phenylselenoethanol. Compounds and were found to be very effective antioxidants and 5-lipoxygenase inhibitors with activity comparable to or better than caffeic acid (3,4-dihydroxycinnamic acid) phenethyl ester (CAPE). 相似文献
73.
74.
75.
In an attempt to elucidate the underlying mechanisms for iron toxicity in plants, the combined effects of iron overload and light intensities on the photosynthetic capacity of leaves were particularly focussed upon in this study, using mung bean seedlings grown under varied conditions regarding the supply of light and iron. The seedlings, when supplied with excess iron (up to 1.0 m M ) and low light (40 W/m2 ), did not suffer any loss of photosynthesis; further, the typical symptoms of iron toxicity, as shown in the leaves grown in sunlight at ca 450 W/m2 on an average, were not seen in those. Nonetheless, excess iron supply resulted in a marked increase in photosensitivity of the low light-adapted seedlings. A large portion of iron accumulated in chloroplasts by the supply of excess iron was found to be incorporated into thylakoids as nonheme iron (NHI), which acts as a potent sensitizer, photogenerating singlet oxygen (1 O2 ). The generation rate of 1 O2 from thylakoids linearly increased with increasing content of NHI; this was in parallel with the NHI content dependence of photoinactivation rates of photosynthetic electron transport and key enzymes of the Calvin cycle in chloroplasts. The results suggest that Fe-dependent photosensitization reactions, occurring via the 1 O2 mechanism, may be deeply involved in cellular processes leading to developing iron toxicity symptoms in plants. 相似文献
76.
光学活性化合物的工业合成 总被引:9,自引:0,他引:9
本文介绍了光学活性化合物研究现状及发展趋势。并从技术和经济角度论述了工业合成光学活性化合物的可能性 相似文献
77.
78.
Diastereoselective allylation of imine derived from N‐glyoxyloyl camphorpyrazolidinone and amines was treated with allyltributylstannane in the presence of triflic acid. The corresponding optically enriched homoallylic amines were obtained in reasonable to high material yields and with practical levels of stereoselectivity in 10 min at ambient temperature. 相似文献
79.
Li Jiang Fushen Lu Qing Chang Yang Liu Huibiao Liu Yuliang Li Wei Xu Guanglei Cui Junpeng Zhuang Xiaofang Li Shu Wang Yinglin Song Daoben Zhu 《Chemphyschem》2005,6(3):481-486
An ultrathin composite film containing anionic Ag-His complexes (His: L-Histidine) and oppositely charged BH-PPV was fabricated by means of the electrostatic layer-by-layer self-assembly technique. UV/Vis spectra showed a continuous deposition process of Ag-His complexes and BH-PPV. The film structure was characterized by using small-angle X-ray diffraction, AFM, and SEM. The nonlinear optical properties of the ultrathin film were studied by using the Z-scan technique with a laser duration of 8 ns at a wavelength of 532 nm. The film sample exhibited a strong nonlinear saturated absorption, with an alpha2 value of -3.9 x 10(-5) mW(-1) and a self-defocusing effect with an n2 value of -4.78 x 10 (-12) m2W(-1). 相似文献
80.
Shuang‐Jun Lin Chang‐Heng Tan Shan‐Hao Jiang Yi‐Ming Li Da‐Yuan Zhu 《Helvetica chimica acta》2006,89(11):2789-2793
Four new iridoids, buergerinins B–E ( 1 – 4 ), along with three known iridoids, were isolated from the roots of Scrophularia buergeriana. Their structures were identified on the basis of spectroscopic analysis. 相似文献