全文获取类型
收费全文 | 1469篇 |
免费 | 49篇 |
国内免费 | 5篇 |
专业分类
化学 | 955篇 |
晶体学 | 16篇 |
力学 | 36篇 |
数学 | 200篇 |
物理学 | 316篇 |
出版年
2023年 | 36篇 |
2022年 | 42篇 |
2021年 | 50篇 |
2020年 | 44篇 |
2019年 | 44篇 |
2018年 | 42篇 |
2017年 | 23篇 |
2016年 | 49篇 |
2015年 | 46篇 |
2014年 | 42篇 |
2013年 | 82篇 |
2012年 | 77篇 |
2011年 | 121篇 |
2010年 | 48篇 |
2009年 | 55篇 |
2008年 | 75篇 |
2007年 | 71篇 |
2006年 | 58篇 |
2005年 | 48篇 |
2004年 | 49篇 |
2003年 | 28篇 |
2002年 | 29篇 |
2001年 | 19篇 |
2000年 | 20篇 |
1999年 | 9篇 |
1998年 | 9篇 |
1997年 | 17篇 |
1996年 | 14篇 |
1995年 | 8篇 |
1994年 | 9篇 |
1993年 | 22篇 |
1992年 | 7篇 |
1991年 | 14篇 |
1990年 | 10篇 |
1988年 | 10篇 |
1987年 | 13篇 |
1986年 | 8篇 |
1985年 | 9篇 |
1984年 | 14篇 |
1983年 | 9篇 |
1982年 | 9篇 |
1981年 | 11篇 |
1980年 | 8篇 |
1979年 | 9篇 |
1978年 | 12篇 |
1977年 | 8篇 |
1976年 | 9篇 |
1975年 | 5篇 |
1974年 | 5篇 |
1973年 | 6篇 |
排序方式: 共有1523条查询结果,搜索用时 0 毫秒
1.
Audrey A S G Lonni Ieda S Scarminio Lucas M C Silva Dalva T Ferreira 《Analytical sciences》2003,19(7):1013-1017
Phytochemical investigation of the aerial parts of three Baccharis species (Asteraceae family) was performed using HPLC and chemometric methods, with the objective of distinguishing between three morphologically very similar species: Baccharis genistelloides Persoon var. trimera (Less.) DC, B. milleflora (Less.) DC and B. articulata (Lam.) Persoon. With the help of Principal Component Analysis (PCA) and variance weights, it was possible to characterize the chromatographic profiles of the alcoholic extracts of the three species. Application of Soft Independent Modeling of Class Analogy (SIMCA) and K-Nearest Neighbor (KNN) methods on a training set of 74 extracts resulted in models that correctly classified all eight samples in an independent test set. 相似文献
2.
Lucas Hernández Pedro Hernández José Vicente 《Fresenius' Journal of Analytical Chemistry》1993,345(11):712-715
Summary The determination of methyl-parathion (MPT), ortho (ONP), meta (MNP) and para nitrophenol (PNP) has been studied by differential pulse voltammetry with a carbon-paste electrode modified with 50% (w/w) of C18. A study of the influence of the pH in the preconcentration cell and the measurement cell was carried out for an electrode with 50% modifier and an accumulation time of 5 min. The voltammograms were recorded with a sweep rate of 40 mV s–1 and a pulse amplitude of 50 mV. With the optimum conditions of pH for both of the steps, various other variables were studied. The variables for each compound were optimized and the possibility of application to the determination of a mixture of the four compounds was investigated. The determination limits found for all the compounds are: 2 ng ml–1 for ONP, 5 ng ml–1 for MNP, 4.3 ng ml–1 for PNP and 7.9 ng ml–1 for MPT. The method was applied to samples of a small lake which gathers rain water and water filtered from land on which cereals are grown. 相似文献
3.
Palladium (0)-catalyzed allylic substitution reactions employing triorganoindium reagents have been investigated. In situ generated vinyl- and arylindiums react with substituted and unsubstituted cyclohex-2-enyl esters in the presence of 1-3 mol % Pd(2)(dba)(3) to produce vinyl- or arylcyclohexenes in moderate to excellent yields. The stereoselectivity of this process was also examined, and evidence is presented that the reaction proceeds with inversion of stereochemical configuration. 相似文献
4.
N-Aminophthalimide ( I ) reacted in refluxing isopropyl alcohol with a number of isothiocyanates to give the related 1:1 addition products, N-(3-substitutedthioureido)phthalimides III. On the other hand, heating I directly with an excess of neat arylisothiocyanates produced the N-arylphthalimides IV. As shown for IIIa, the 1:1 addition products are conveniently deblocked by the Ing-Manske procedure to yield the 4-substituted thiosemicarbazide. 相似文献
5.
6.
A structural study of a nonstoichiometric niobium-zirconium oxyfluoride with the ReO3 type structure
Within the ZrNbOF quaternary system a nonstoichiometric compound of the ReO3 structure is formed with composition limits given by MX2.9MX3.14. Crystals at the lower end of this range with a composition of Nb0.55Zr0.45O1.1F1.8 (MX2.9) have been investigated by optical and electron microscopy and by powder and single crystal X-ray diffraction in order to determine the mode by which the nonstoichiometry has been accommodated in the structure. It was found that the material does not contain crystallographic shear planes or have a perovskite bronze type of structure but contains vacancies in the anion lattice. It seems likely that these defects are associated into clusters with a definite structure rather than being isolated from each other. 相似文献
7.
Reaction of 1-oxo-2-formyl-3-dimethylamino-1H-naphtho[2,1-b]pyran with amidines, guanidine, O-methylisourea, S-methylisothiourea afforded 9-substituted 12-oxo-12H-naphtho[1′,2′:5,6]pyrano[2,3-d]pyrimidines. When the reaction with O-methylisourea was carried out in anhydrous pyridine, 10,20-dioxo-10H,20H-dinaphtho[1,2-e:1′,2′-e′][1,5]diazocino[2,3-b:6,7-b′]dipyran was formed. 相似文献
8.
9.
J. B. Natowitz S. Leray R. Lucas C. Ngô E. Tomasi C. Volant 《Zeitschrift für Physik A Hadrons and Nuclei》1986,325(4):467-474
A simple formula based on leading particle models of nucleon-nucleon collisions is used to evaluate the average momentum transfer in central heavy ion collisions at energies from 20 to 150 MeV/u. This formula, which reproduces a number of the features of the observed systematics, offers a straightforward explanation for the approximate universality of fractional linear momentum transfer as a function of velocity, the dependence of fractional momentum transfer on target mass and the greater fractional momentum transfer of the proton as compared to heavy ion projectiles. 相似文献
10.