全文获取类型
收费全文 | 23939篇 |
免费 | 1015篇 |
国内免费 | 1438篇 |
专业分类
化学 | 17016篇 |
晶体学 | 189篇 |
力学 | 438篇 |
综合类 | 10篇 |
数学 | 3714篇 |
物理学 | 5025篇 |
出版年
2023年 | 174篇 |
2022年 | 407篇 |
2021年 | 503篇 |
2020年 | 378篇 |
2019年 | 398篇 |
2018年 | 405篇 |
2017年 | 419篇 |
2016年 | 957篇 |
2015年 | 734篇 |
2014年 | 817篇 |
2013年 | 1851篇 |
2012年 | 1677篇 |
2011年 | 1860篇 |
2010年 | 1077篇 |
2009年 | 1257篇 |
2008年 | 1241篇 |
2007年 | 1154篇 |
2006年 | 974篇 |
2005年 | 973篇 |
2004年 | 1251篇 |
2003年 | 918篇 |
2002年 | 856篇 |
2001年 | 470篇 |
2000年 | 379篇 |
1999年 | 319篇 |
1998年 | 297篇 |
1997年 | 274篇 |
1996年 | 267篇 |
1995年 | 239篇 |
1994年 | 187篇 |
1993年 | 207篇 |
1992年 | 183篇 |
1991年 | 141篇 |
1990年 | 133篇 |
1989年 | 124篇 |
1988年 | 129篇 |
1987年 | 123篇 |
1986年 | 130篇 |
1985年 | 174篇 |
1984年 | 167篇 |
1983年 | 139篇 |
1982年 | 125篇 |
1981年 | 128篇 |
1980年 | 130篇 |
1979年 | 104篇 |
1978年 | 106篇 |
1977年 | 114篇 |
1976年 | 90篇 |
1974年 | 94篇 |
1973年 | 90篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
71.
N. Filipović-Vinceković D. Barišić N. Mašić S. Lulić 《Journal of Radioanalytical and Nuclear Chemistry》1991,148(1):53-62
The distribution of137Cs through soil layers has been investigated by field sampling. The investigation deals with137Cs concentration found after the Chernobyl accident. The relationship between important soil characteristics, total precipitation and cesium distribution are analyzed. Cesium is strongly sorbed in the top soil layer and its migration downward is limited. Measurements performed over a 3-year period revealed that approximately 20–38 per cent of total137Cs is located in the upper soil layer. 相似文献
72.
L Fišera U. A. R. Al-Timari P. Ertl N. Prónayová 《Monatshefte für Chemie / Chemical Monthly》1993,124(10):1019-1029
Summary Nitrones2 derived fromD-glucose oxime and benzaldehydes without employing any protection of hydroxyl group were isolated in pure state. The 1,3-dipolar cycloaddition of2 to N-arylmaleimides gave predominantly theanti isoxazolidines3 and was rationalized byZ/E isomerization of N-glycosylnitrones2. The structure and steric configuration of the products have been assigned on the basis of1H- and13C-NMR spectroscopy. AM1 calculations of the nitrones and MM2 calculations of the adducts were performed.
Darstellung und Stereoselektivität der 1,3-dipolaren Cycloaddition vonD-Glucose-abgeleiteten Nitronen an N-Arylmaleimiden
Zusammenfassung Die Nitrone2 wurden ausD-Glucoseoxim und Benzaldehyden ohne Schutz von Hydroxylgruppen in reinem Zustand erhalten. Die 1,3-dipolare Cycloaddition von2 an N-Arylmaleimiden ergab bevorzugt dieanti-Isoxazolidine3; dies wurde über eineZ/E-Isomerisierung der N-Glycosylnitrone2 rationalisiert. Struktur und Stereochemie wurden auf Basis von1H- und13C-NMR-Spektroskopie ermittelt. Außerdem wurden AM1-Berechnungen an den Nitronen und MM2-Rechnungen an den Addukten ausgeführt.相似文献
73.
The UV. irradiation of 17β-acetoxy-4α, 5α-epoxy-2-oxaandrostan-3-one ( 7 ) yields 17β-acetoxy-2-oxa-10(5 → 4)abeo-4ζ (H)-androsta-3,5-dione ( 11 ). A non-photochemical synthesis of 11 , proceeding in lower yield, is also described. 相似文献
74.
Eduardo Corts C. Eduardo Corts R. María P. A. Fuentes B. 《Journal of mass spectrometry : JMS》1992,27(8):903-904
The mass spectra of twelve 2-methyl-3-(o- and p-R-phenyl)-3,3a,4,5,6,7-hexahydro-2H-indazoles which have potentially useful pharmacological properties are presented. 相似文献
75.
The principle of tandem on-line continuous separation techniques as an alternative means of introducing samples into plasmas was applied to the development of a sensitive, selective and convenient method for the determination of arsenic by inductively coupled plasma atomic emission spectrometry (ICP-AES). Arsenic is continuously extracted as AsI3 into xylene from the sample dissolved in 0.1 M potassium iodide solution in 7.2 M hydrochloric acid. The xylene phase (containing the analyte) is continuously mixed on-line with NaBH4 in dimethylformamide and acetic acid solutions. Arsine is thus continuously generated directly from the organic phase and is separated in a gas—liquid separation device which prevents most of the xylene phase vapour from reaching the ICP. The system was optimized for the continuous extraction of AsI3, the direct generation of arsine from xylene and the final ICP determination of arsenic. Finally, the tandem on-line continuous separation ICP detection system was applied to the determination of arsenic in real samples (white metal, cast iron, cupro-nickel and orchard leaves standard materials). Very good agreement between the experimental results and the certified values was obtained. 相似文献
76.
Analysing experimentally determined P, ?, T data of N2, ethylene, propene, CO2 and Xe by a new mathematical technique, we present evidence that their isothermal pressure-density dependence exhibits behaviour characteristic of a Riemann-Hugoniot catastrophe, in analogy to a van der Waals gas for above and sub critical isotherms. 相似文献
77.
M. A. Aramendía V. Borau C. Jiménez J. M. Marinas A. Moreno F. J. Urbano 《Reaction Kinetics and Catalysis Letters》1995,56(1):87-96
We have studied the activity of Pd/sepiolite and Pd/SiO2–AlPO4 catalysts in cyclohexane aromatization. The catalysts were characterized by H2 chemisorption measurements. The influence of support and reduction temperature on the metal surface properties and activity of the catalysts is discussed. 相似文献
78.
C. Aslangul R. Constanciel R. Daudel L. Esnault E. V. Ludea 《International journal of quantum chemistry》1974,8(4):499-522
It is shown that any expectation value of any observable associated with a molecule is the sum of loge contributions and of loge pair contributions. This result provides a rigorous theoretical basis for the study of additive properties of molecules. It is demonstrated that molecular wave functions (exact or approximate) can be expressed as a sum of functions corresponding to the various electronic events. Furthermore any of these event functions can be expressed in terms of correlated loge functions. This expression suggests many kinds of variational procedures of calculating wave functions (known methods and new ones). The case in which noncorrelated completely localized loge functions are used is discussed. If continuous functions are used the variational equation reduces to a sum of independent variational equations, each one corresponding to a particular electronic event. This is not so when discontinuous functions are used or when a delocalized function is added to replace the correlation interloge function. The noncorrelated completely localized loge model is analyzed in more detail. It is seen that local spin operators can be introduced and that each event density operator is the product of the loge density operators. Therefore that model is an independent loge model. The corresponding generalized self-consistent field equations are derived. This treatment helps us to understand how a localized state of a molecule can produce an ion containing a delocalized region, a phenomenon which is sometimes at the origin of some misunderstanding in photoelectron spectroscopy. Finally it is seen how virtual loge functions can be introduced to describe excited states. 相似文献
79.
80.
Ângela F. S. S. Mendonça Dina T. R. Formigo Isabel M. S. Lampreia 《Journal of solution chemistry》2002,31(8):653-670
Solubilities of triethylamine in aqueous tetraethylammonium chloride solutions were measured at 20, 25, and 35°C. The molalities in Et4NCl of the aqueous solvents ranged from 0.03 to 1 mol-kg–1. The data were evaluated from density measurements using a vibrating-tube densimeter. At each temperature, least-squares method was used to fit experimental density data points to double polynomial equations of various degrees. Triethylamine molalities of the saturated aqueous phases were estimated by extrapolation from those equations. Experimental data were interpreted in terms of hydrophobic and electrostatic perturbed domains in the hydration shells of the noneleceory and of the cation of the salt, as a function of temperature and salt concentration. The conclusions obtained are consistent with previous volumetric studies. 相似文献