全文获取类型
收费全文 | 34071篇 |
免费 | 3664篇 |
国内免费 | 2519篇 |
专业分类
化学 | 24007篇 |
晶体学 | 337篇 |
力学 | 1790篇 |
综合类 | 159篇 |
数学 | 4329篇 |
物理学 | 9632篇 |
出版年
2023年 | 423篇 |
2022年 | 768篇 |
2021年 | 775篇 |
2020年 | 960篇 |
2019年 | 853篇 |
2018年 | 767篇 |
2017年 | 666篇 |
2016年 | 1128篇 |
2015年 | 1085篇 |
2014年 | 1314篇 |
2013年 | 1989篇 |
2012年 | 2367篇 |
2011年 | 2677篇 |
2010年 | 1579篇 |
2009年 | 1460篇 |
2008年 | 2025篇 |
2007年 | 1901篇 |
2006年 | 1975篇 |
2005年 | 1718篇 |
2004年 | 1458篇 |
2003年 | 1277篇 |
2002年 | 1246篇 |
2001年 | 695篇 |
2000年 | 589篇 |
1999年 | 610篇 |
1998年 | 506篇 |
1997年 | 493篇 |
1996年 | 569篇 |
1995年 | 465篇 |
1994年 | 455篇 |
1993年 | 348篇 |
1992年 | 340篇 |
1991年 | 312篇 |
1990年 | 264篇 |
1989年 | 225篇 |
1988年 | 254篇 |
1987年 | 208篇 |
1986年 | 167篇 |
1985年 | 254篇 |
1984年 | 265篇 |
1983年 | 195篇 |
1982年 | 254篇 |
1981年 | 238篇 |
1980年 | 231篇 |
1979年 | 208篇 |
1978年 | 198篇 |
1977年 | 181篇 |
1976年 | 182篇 |
1974年 | 162篇 |
1973年 | 155篇 |
排序方式: 共有10000条查询结果,搜索用时 515 毫秒
951.
Effects of ultraviolet B on epidermal morphology, shedding, lipid peroxide, and antioxidant enzymes in Cope's rat snake (Elaphe taeniura) 总被引:1,自引:0,他引:1
Cope's rat snakes (Elaphe taeniura) favor to expose under sunlight in order to increase their body temperature simultaneously increasing the risk of skin damage by ultraviolet B (UVB) irradiation. We have investigated the effects of UVB irradiation on their skin. Results show that the UVB transmission of the keratinous layer was only 5.1+/-0.36%. The peak of epidermal damage and malondialdehyde (MDA) content, a product of lipid peroxidation, simultaneously occurred 72-96, 48 or 24 h after exposure to 300, 500 and 800 mJ/cm2 of UVB radiation, respectively. Superoxide dismutase (SOD) activity was inhibited by UVB and the lowest activity occurred 24, 48, 12 and 12 h after exposure to 110, 300, 500 and 800 mJ/cm2 of UVB, respectively. SOD activity recovered later to some extent but mostly remained below control level. After exposure to different doses of UVB radiation, catalase (CAT) activity was inhibited immediately, and then gradually recovered and even increased to peak levels above control level. The highest CAT levels accompanied the most serious damage of skin morphology. Later on, CAT activity decreased and recovered again close to or below control level, which was accompanied by shedding off the damaged epidermal complex. This indicated that the epidermal damage induced by UVB is closely related to lipid peroxidation, where CAT acts as a primary antioxidant enzyme. Moreover, the keratinous layer protects the viable cell layer against UVB damage as well. 相似文献
952.
953.
954.
Quantum yield of the iodide-iodate chemical actinometer: dependence on wavelength and concentrations
Rahn RO Stefan MI Bolton JR Goren E Shaw PS Lykke KR 《Photochemistry and photobiology》2003,78(2):146-152
The quantum yield (QY) of the iodide-iodate chemical actinometer (0.6 M KI-0.1 M KIO3) was determined for irradiation between 214 and 330 nm. The photoproduct, triiodide, was determined from the increase in absorbance at 352 nm, which together with a concomitant measurement of the UV fluence enabled the QY to be calculated. The QY at 254 nm was determined to be 0.73 +/- 0.02 when calibration was carried out against a National Institute of Standards and Technology traceable radiometer or photometric device. At wavelengths below 254 nm the QY increased slightly, leveling off at -0.80 +/- 0.05, whereas above 254 nm the QY decreases linearly with wavelength, reaching a value of 0.30 at 284 nm. In addition, the QY was measured at different iodide concentrations. There is a slight decrease in QY going from 0.6 to 0.15 M KI, whereas below 0.15 M KI the QY drops off sharply, decreasing to 0.23 by 0.006 M KI. Calibration of the QY was also done using potassium ferrioxalate actinometry to measure the irradiance. These results showed a 20% reduction in QY between 240 and 280 nm as compared with radiometry. This discrepancy suggests that the QY of the ferrioxalate actinometer in this region of the spectrum needs reexamination. 相似文献
955.
The kinetic and thermodynamic parameters for the oxo and imido exchange reactions among MeReO(3), MeReO(2)(NR), MeReO(NR)(2), and MeRe(NR)(3) (R = 1-adamantyl, Ad; or 2,6-diisopropylphenyl, Ar) have been measured. The rate constant for the NAr series decreases from 0.27 to 0.0024 L mol(-1) s(-1) at 25 degrees C in benzene as the total number of participating imido groups increases from 2 to 4, indicating that steric effects play an important role in the kinetics of the ligand exchange reactions. But, with NAd, the values of k/L mol(-1) s(-1) are 0.2 (4 NAd), 100 (3 NAd), and 0.74 (2 NAd). The equilibrium constants, also subject to steric effects, are much larger than those predicted by ligand combination statistics and greatly favor the mixed oxo-imido compounds. The different steric demands by imido and oxo ligands are believed to be the main factor for the larger equilibrium constants because the equilibrium constant shows minimal dependence on temperature. The large negative activation entropies for the ligand exchange reactions are consistent with a metathesis mechanism featuring nearly concurrent interchange of oxo and imido groups. 相似文献
956.
Huaping Liu Guoan Cheng Ruiting Zheng Yong Zhao Changlin Liang 《Journal of molecular catalysis. A, Chemical》2005,230(1-2):17-22
The Ni/CNT catalyst was fabricated by directly dipping carbon nanotube precursors refluxed in 4 M of nitric acid into Ni electroless plating bath, and used to synthesize new carbon nanotubes. The experimental results indicate that the duration of acid-treatment of carbon nanotubes precursors exerts a great influence on the catalysis of Ni/CNT in the synthesis of carbon nanotubes and hence the structures of the new carbon nanotubes. When the carbon nanotubes precursors were refluxed for 0.5 h in 4 M of nitric acid, bamboo-shaped carbon nanotubes (BSCNT) or Y junction carbon nanotubes in the carbon products were obtained. As the duration of acid-treatment of carbon nanotubes precursors increased to 6 h, the as-prepared Ni/CNT displayed higher activity, and the carbon nanotube products were high pure without any Y junction structure or any separation layers in hollow. 相似文献
957.
Qing Z Wen-Chen Z Xiao-Xuan W Yang M 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2005,62(4-5):769-772
From the high-order perturbation formulas of EPR parameters (zero-field splitting D, g factors gparallel, gperpendicular and hyperfine structure constants Aparallel, Aperpendicular) based on the two spin-orbit coupling parameter model for 3d3 ions in trigonal symmetry, the EPR parameters of Cr3+ and V2+ ions in HfS2 crystals are calculated. From the calculations, it is found that the local trigonal distortion angle theta of impurity center in HfS2:Cr3+ is smaller than that in HfS2:V2+. The dominant cause of the small zero-field splitting |D| and g-anisotropy |Deltag|=|gparallel-gperpendicular| in HfS2:Cr3+ (compound to HfS2:V2+) is due to the small local trigonal distortion angle theta rather than to the small impurity-ligand distance R in HfS2:Cr3+. 相似文献
958.
James H. Looker Duane L. Shaneyfelt Wayne Halfar 《Journal of heterocyclic chemistry》1979,16(6):1281-1282
Condensation reactions of comenaldehyde methyl ether (I) with malonic acid, ethyl cyanoacetate, and cyanoacetamide to give β-(5-methoxy-4H-pyran-4-on-2-yl)acrylic acid (II), ethyl 2-cyano-3-(5-methoxy-4H-pyran-4-on-2-yl)propenoate (III), and 2-cyano-3-(5-methoxy-4H-pyran-4-on-2-yl)propenamide (IV), respectively, are described. Ultraviolet absorption spectra for 2-hydroxymethyl-5-methoxy-4H-pyran-4-one, I and II are presented. 相似文献
959.
Boyd AS Carroll JB Cooke G Garety JF Jordan BJ Mabruk S Rosair G Rotello VM 《Chemical communications (Cambridge, England)》2005,(19):2468-2470
We report the electrochemically tuneable intramolecular hydrogen bonding interactions between a covalently linked flavin-diamidopyridine unit. 相似文献
960.