全文获取类型
收费全文 | 21569篇 |
免费 | 616篇 |
国内免费 | 142篇 |
专业分类
化学 | 15332篇 |
晶体学 | 138篇 |
力学 | 428篇 |
数学 | 3282篇 |
物理学 | 3147篇 |
出版年
2022年 | 169篇 |
2021年 | 246篇 |
2020年 | 255篇 |
2019年 | 260篇 |
2018年 | 207篇 |
2017年 | 192篇 |
2016年 | 449篇 |
2015年 | 436篇 |
2014年 | 497篇 |
2013年 | 1004篇 |
2012年 | 1072篇 |
2011年 | 1345篇 |
2010年 | 703篇 |
2009年 | 624篇 |
2008年 | 1172篇 |
2007年 | 1166篇 |
2006年 | 1155篇 |
2005年 | 1188篇 |
2004年 | 1105篇 |
2003年 | 872篇 |
2002年 | 808篇 |
2001年 | 294篇 |
2000年 | 313篇 |
1999年 | 220篇 |
1998年 | 199篇 |
1997年 | 242篇 |
1996年 | 315篇 |
1995年 | 222篇 |
1994年 | 215篇 |
1993年 | 215篇 |
1992年 | 220篇 |
1991年 | 188篇 |
1990年 | 166篇 |
1989年 | 149篇 |
1988年 | 155篇 |
1987年 | 167篇 |
1986年 | 130篇 |
1985年 | 290篇 |
1984年 | 249篇 |
1983年 | 189篇 |
1982年 | 265篇 |
1981年 | 220篇 |
1980年 | 279篇 |
1979年 | 241篇 |
1978年 | 248篇 |
1977年 | 257篇 |
1976年 | 214篇 |
1975年 | 178篇 |
1974年 | 177篇 |
1973年 | 161篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Polyazido high-nitrogen compounds: hydrazo- and azo-1,3,5-triazine 总被引:13,自引:0,他引:13
Huynh MH Hiskey MA Hartline EL Montoya DP Gilardi R 《Angewandte Chemie (International ed. in English)》2004,43(37):4924-4928
102.
Enthalpies of solution have been measured from 5 to 85°C for aqueous tetraethyl- and tetrapropylammonium bromides, and the integral heat method is employed to evaluate
for these electrolytes over a wide temperature range. Data taken from the literature have been used to evaluate
for aqueous Bu4NBr over a similar temperature range. These data, along with similar data for Me4NBr, previously reported, have been used to evaluate absolute ionic heat capacities. While the absolute values agree only qualitatively with two other methods of division, the temperature dependences of the three methods essentially agree up to 65°C. Heat capacities due to structural effects on the solvent, obtained by subtracting the inherent heat capacities of the ions, are extraordinarily positive for all four tetraalkylammonium ions and have negative temperature coefficients, indicating that all four ions, including the tetramethylammonium ion, are structure-making ions. 相似文献
103.
BIOLOGICAL ACTIVITIES OF PHTHALOCYANINES-X. SYNTHESES AND ANALYSES OF SULFONATED PHTHALOCYANINES 总被引:5,自引:0,他引:5
Hasrat Ali Réjean Langlois J. Richard Wagner Nicole Brasseur Benoit Paquette Johan E. VAN Lier 《Photochemistry and photobiology》1988,47(5):713-717
Abstract— Synthetic methods to obtain selectively sulfonated metallo phthalocyanines are compared. Both condensation and direct sulfonation procedures lead to mixtures of mono- to tetrasulfonated products which are resolved by reverse phase liquid chromatography in buffered aqueous-methanol. The proportion of sulfonated derivatives is examined as a function of the starting reagents in the case of the condensation method, and as a function of the temperature and reaction time in the case of the direct sulfonation procedure. The number of sulfonate groups per phthalocyanine molecule is determined by oxidative degradation of the phthalocyanine ring followed by quantitative chromatographic analysis of the sulfophthalamide and phthalamide fragments. 相似文献
104.
William W. Paudler Richard A. Vandahm Young N. Park 《Journal of heterocyclic chemistry》1972,9(1):81-85
The diazaanalog of “cycl[3,2,2]azine”, “1,4-diazacycl[3,2,2]azine” (1,4,7b-triazacyclopent-[cd]indene) and its 2-methyl derivative were prepared. These compounds are subject to facile acid-catalyzed hydrolysis affording substituted imidazo[1,2-a]pyridines. 相似文献
105.
106.
Lichens have long been regarded as bioindicators of air pollution, and structural studies typically have indicated negative impacts in highly polluted areas. In this research, Parmelia sulcata and Platismatia glauca were collected from one clean and two polluted sites in the Pacific Northwest forests of the United States to investigate the anatomical and ultrastructural responses of relatively resistant lichens to moderate air pollution. Light microscopy of polluted materials revealed only slight increases in the algal cell proportions of the thallus, and a decrease in the fungal cells of the medulla. Using transmission electron microscopy, increased lipid droplets in the cytoplasm and an increase in the cell wall thickness of the photobionts were found in the polluted lichens. These results were compared with physiological data in which the net carbon uptake did not show any significant differences; however, the total chlorophyll content was heightened in the polluted samples. The increased total chlorophyll content and the absence of any changes in the algal cell proportions of the polluted samples suggest that the photobionts possessed a higher chlorophyll content per unit volume of the photobiont at polluted sites. The results also indicate that lichens have altered their storage allocation in different cellular compartments. This may be a result of symbiotic readjustment(s) between the photobiont and the mycobiont. In comparison with the physiological results from these two species, these changes do not represent damaging effects by low-level air pollution. 相似文献
107.
Ronald N. Warrener Richard P. Johnson Charles W. Jefford David A. Ralph 《Tetrahedron letters》1979,20(31):2939-2942
Structural assignments are made to the three adducts derived from the title reaction. 1H-n.m.r., 13C-n.m.r. and euroshift data are used to support the new structures. 相似文献
108.
The class
of orthomodular lattices which have only finitely many commutators is investigated. The following theorems are proved:
contains the block-finite orthomodular lattices. Every irreducible element of
is simple. Every element of
is a direct product of a Boolean algebra and finitely many simple orthomodular lattices. The irreducible elements of
which are modular, or are M-symmetric with at least one atom, have height two or less. 相似文献
109.
Nicole Fietkau Fran?ois G Chevallier Li Jiang Timothy G J Jones Richard G Compton 《Chemphyschem》2006,7(10):2162-2167
The size of a glass sphere positioned in the center of a microdisk electrode is determined by using a simple electrochemical procedure and is confirmed, additionally, by a microscopical measurement of the sphere at the time of the electrochemical measurement. The cyclic voltammetric response of the naked electrode and of the electrode with the sphere positioned in its center is recorded over a wide range of scan rates (0.002-1.5 V s(-1)). The size of the sphere is then determined by comparison of the experimental voltammogram with simulations for each individual scan rate. 相似文献
110.
Yu Gui Gu Xiaolin Zhang Richard F. Clark Stevan W. Djuric Zhenkun Ma 《Tetrahedron letters》2004,45(15):3051-3053
An intramolecular 1,3-dipolar cycloaddition of azomethine ylide, generated in situ via the reaction of C12-glycinate derivative of macrolide with formaldehyde, provided a novel tricyclic macrolide. The high stereoselectivity of this [2+3] reaction was achieved by introducing a suitable directing group at C-6 position of macrolide. 相似文献