全文获取类型
收费全文 | 23131篇 |
免费 | 2965篇 |
国内免费 | 1794篇 |
专业分类
化学 | 24210篇 |
晶体学 | 476篇 |
力学 | 57篇 |
综合类 | 50篇 |
数学 | 16篇 |
物理学 | 3081篇 |
出版年
2024年 | 59篇 |
2023年 | 280篇 |
2022年 | 1030篇 |
2021年 | 966篇 |
2020年 | 1304篇 |
2019年 | 930篇 |
2018年 | 714篇 |
2017年 | 633篇 |
2016年 | 1136篇 |
2015年 | 1100篇 |
2014年 | 1131篇 |
2013年 | 1769篇 |
2012年 | 1253篇 |
2011年 | 1119篇 |
2010年 | 1114篇 |
2009年 | 1079篇 |
2008年 | 1067篇 |
2007年 | 1065篇 |
2006年 | 1023篇 |
2005年 | 937篇 |
2004年 | 963篇 |
2003年 | 868篇 |
2002年 | 2068篇 |
2001年 | 525篇 |
2000年 | 432篇 |
1999年 | 383篇 |
1998年 | 388篇 |
1997年 | 285篇 |
1996年 | 477篇 |
1995年 | 452篇 |
1994年 | 205篇 |
1993年 | 137篇 |
1992年 | 167篇 |
1991年 | 109篇 |
1990年 | 92篇 |
1989年 | 93篇 |
1988年 | 77篇 |
1987年 | 44篇 |
1986年 | 55篇 |
1985年 | 62篇 |
1984年 | 51篇 |
1983年 | 30篇 |
1982年 | 29篇 |
1981年 | 26篇 |
1980年 | 15篇 |
1979年 | 16篇 |
1978年 | 13篇 |
1975年 | 10篇 |
1971年 | 48篇 |
1970年 | 25篇 |
排序方式: 共有10000条查询结果,搜索用时 156 毫秒
91.
The acute influences of arsenic compounds on the metabolism of porphyrins and heme in various organs of rats after oral or intratracheal administration of disodium arsenate (Na2HAsO4) and gallium arsenide (GaAs) were examined and compared. For the oral administration experiments, 21 or 84 mg of Na2HAsO4, or 2 or 4 g of GaAs, per cm3 saline per kg body weight of each animal was administered to Jcl: Wistar male rats and the organs were removed after exsanguination from the vein of the right axilla under anesthesia with ether, 16 h after administration. In the case of intratracheal administration, rats given 8.2 or 16.4 mg of Na2HAsO4, or 0.2 or 0.4 g GaAs per cm3 saline per kg body weight were examined under the same experimental conditions as for the administration route. Increase in the body weight of rats was suppressed after intratracheal administration of the two arsenic compounds. In these rats the hematocrit value increased significantly. These changes were not shown by the orally administered rats. Elevation in δ-aminolevulinate synthase (ALA-S, EC 2.3.1.37) activity in erythroblasts by Na2HAsO4 was much higher after intratracheal administration than after oral administration. Suppression in the activities of δ-aminolevulinate dehydratase (ALA-D, EC 4.2.1.24) and porphobilinogen deaminase (PBG-D, EC 4.3.1.8) in peripheral erythrocytes by Na2HAsO4 and GaAs were stronger by intratracheal administration than by the oral route. Influences of GaAs on the activity of PBG-D in rat liver were shown to be more effective by oral administration than by the intratracheal route. Oral administration of Na2HAsO4 and GaAs had a stronger suppression effect on the activities of ALA-D and PBG-D in rat kidney. It seems from these results that the different extents of the influence of arsenic compounds might depend on the routes of intake. 相似文献
92.
93.
94.
Koichi Itoh Takeji Takui 《Proceedings of the Japan Academy. Series B, Physical and biological sciences》2004,80(2):29
This review paper deals with an overview of molecule-based magnetism as a rapidly developing interdisciplinary field, topological symmetry rule as the first principle of spin alignment in organic open-shell systems in the ground state, the proposal of organic through-bond 1D and 2D ferro- and superparamagnets and the detection of the first organic high-spin molecule, m-phenylenebis(phenylmethylene) in the quintet ground state (S = 2), followed by extended organic high-spin systems with π-conjugation such as aromatic hydrocarbons having S = 3, 4, 5. The paper also describes a theoretical approach to the understanding of electronic spin structures of organic high-spin molecules by invoking both Heisenberg and Hubbard model Hamiltonians, weakly interacting intramolecular high-spin systems from both experimental and theoretical sides, the spin density distribution of the first organic high-spin molecule in terms of electron- nuclear multiple resonance spectroscopy and the detection and characterization of ionic high-spin hydrocarbons, emphasizing the establishment of high spin chemistry underlying organic molecular magnetism. 相似文献
95.
96.
《Journal of separation science》2003,26(5):402-408
When nitrated polynuclear aromatic hydrocarbons (NPAH) were recognised as long‐term toxic compounds and were also identified in several anthropogenic emissions and among the by‐products of photochemical smog they became objects of investigation in ambient air in both polluted and clean environments. In Italy the first investigations focussing on atmospheric NPAH were performed in the early eighties and encompassed both the canyon streets and dwelling areas of Rome. Because NPAH were indicated as markers of vehicle emission, new investigations were performed in other cities and also in rural and (nominally) unpolluted locations. In general, the first measurements of NPAH were episodic and could not be assumed to be strictly representative of the sites investigated. In contrast, nowadays consistent data bases on NPAH have been built up and are discussed in the technical literature, so that both neat concentrations and the yearly timed variation of the most important NPAH congeners have been measured in Italian air. Although concentrations of NPAH in air are usually much lower than those of PAH, owing to their very high potency in inducing mutations and tumours NPAH are regarded as having an important health impact on populations living in Italian urban areas. In this regard the genotoxicity of atmospheric aerosols is partly associated with nitrofluoranthenes and nitropyrenes whereas nitronaphthalenes and methylnitronaphthalenes seem to contribute to that of the gaseous organic fraction. Several civic and regional administrations are introducing control of atmospheric NPAH among ways of improving air quality in cities. 相似文献
97.
98.
99.
100.
A. O. Manyashin A. I. Fomenko V. N. Storozhenko N. T. Berberova 《Russian Journal of Electrochemistry》2003,39(11):1240-1244
Stoichiometry and kinetics of reactions of 2,6-diphenyl-4-chloropyrylium, 4-chloroflavylium, 4-bromoflavylium, and 4-iodoflavylium perchlorates with nucleophiles N,N-dimethylaniline and n-phenylenediamine are studied using cyclic voltammetry and spectroscopy. Nucleophilic substitution in these compounds proceeds via the formation of a charge-transfer complex, which converts into a radical ion pair as a result of the electron transfer. Heterolytic clevage of the C–Hal bond occurs at the stage of pyranyl (flavanyl) radical. 相似文献