全文获取类型
收费全文 | 13771篇 |
免费 | 333篇 |
国内免费 | 136篇 |
专业分类
化学 | 8080篇 |
晶体学 | 146篇 |
力学 | 529篇 |
综合类 | 9篇 |
数学 | 2345篇 |
物理学 | 3131篇 |
出版年
2022年 | 183篇 |
2021年 | 224篇 |
2020年 | 254篇 |
2019年 | 239篇 |
2018年 | 227篇 |
2017年 | 181篇 |
2016年 | 317篇 |
2015年 | 274篇 |
2014年 | 307篇 |
2013年 | 858篇 |
2012年 | 594篇 |
2011年 | 719篇 |
2010年 | 492篇 |
2009年 | 431篇 |
2008年 | 619篇 |
2007年 | 554篇 |
2006年 | 527篇 |
2005年 | 496篇 |
2004年 | 477篇 |
2003年 | 400篇 |
2002年 | 388篇 |
2001年 | 259篇 |
2000年 | 227篇 |
1999年 | 193篇 |
1998年 | 154篇 |
1997年 | 166篇 |
1996年 | 183篇 |
1995年 | 149篇 |
1994年 | 146篇 |
1993年 | 172篇 |
1992年 | 167篇 |
1991年 | 172篇 |
1990年 | 131篇 |
1989年 | 131篇 |
1988年 | 140篇 |
1987年 | 194篇 |
1986年 | 159篇 |
1985年 | 206篇 |
1984年 | 195篇 |
1983年 | 162篇 |
1982年 | 178篇 |
1981年 | 187篇 |
1980年 | 178篇 |
1979年 | 180篇 |
1978年 | 166篇 |
1977年 | 169篇 |
1976年 | 147篇 |
1975年 | 161篇 |
1974年 | 130篇 |
1973年 | 132篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
151.
[C3H3N2]+ ions have been generated under electron impact conditions from some monosubstituted pyrazoles C3H3N2R. Collision-induced dissociation (CID) mass spectra of deuterium-labelled precursors suggest that the majority of the [C3H3N2]+ ions formed from 1-nitro- and 4-bromo-pyrazole retain their cyclic structure, whereas the ions from 3(5)-bromopyrazole are mainly linear. This is confirmed by the relative values observed for the overall cross-sections for CID and for ion loss. An isotope effect of the order of 1.5–1.9 has been found for the collision-induced loss of H˙ from [C3H3N2]+, generated from 3(5)- and 4-bromopyrazole. 相似文献
152.
Two-photon induced fluorescence and resonance-enhanced photoionization have been observed in atomic sulfur originating from both the 3P2,1,0 and the 1D2 states. Sulfur atoms are generated by the sequential multiphoton dissociation of CS2 at probing wavelengths. The two-photon absorption process involves the 3 3P2,1,0 → 4 3P2,0,1 or the 3 1D2 → 4 1F3 transitions with resolution of the individual J″ → J′ transitions in most cases. Intensities of the 33PJ″ → 4 3PJ′ transitions have been compared with Hartree-Fock calculated transition probabilities from the analogous transitions in atomic oxygen. Photoionization is observed in a three-photon (two to resonance) ionization originating from the 3P2,1,0 and the 1D2 states. Induced fluorescence is observed at 167 and 180 nm which is dipole-allowed radiation from the intermediate 3S01 and 1D02 states, respectively. 相似文献
153.
B. M. Goldschmidt B. L. Van Duuren R. C. Goldstein 《Journal of heterocyclic chemistry》1976,13(3):517-519
The nucleophile 4-(p-nitrobenzyl)pyridine was allowed to react with four carcinogenic alkylating agents, chloromethyl methyl ether, bis(chloromethyl) ether, glycol sulfate and propane sultone, one carcinogenic acylating agent, N,N-dimethylcarbamyl chloride, and one noncarcinogenic electrophile, perchlorocyclobutenone. The structures of the major products formed, which are substituted pyridinium salts or 1,4-dihydropyridines, were determined. 相似文献
154.
Henk Van Der Poel Gerard Van Koten Kees Vrieze 《Journal of organometallic chemistry》1977,135(3):C63-C65
Complexes of di(t-butyl)diimine with PdCl2(PhCN)2 and with (CO)2RhCl dimer have been synthesized and characterized. The diimine ligand is monodentate bonded in PdCl2(t-butyldiimine)2, while in (t-butyldiimine)-[Rh(CO)2Cl]2 it bridges two Cl(CO)2Rh units. 相似文献
155.
Problems in the use of x-ray spectrometry for the fast multi-element analysis of environmental samples such as aerosols collected on filters arise from absorption of the fluorescence intensity in the filter and in the collected matter. A procedure is described for the determination of the elements sulphur to calcium in aerosols collected on Whatman 41 filters. Two methods are compared for deconvolution of the spectra. These are based on counting in fixed energy channels or on non-linear least-squares analysis. Corrections for absorption effects have been calculated. Calibration is done with thin film standard deposits. The accuracy of the method is better than 15%. Difficulties in ascertaining the chlorine content of the filter are discussed. The evaporation of the chlorine fraction from the filter was studied in detail. 相似文献
156.
R. Van der Linden F. De Corte J. Hoste 《Journal of Radioanalytical and Nuclear Chemistry》1974,20(2):729-743
The isotopes97Ru,103Ru and105Ru, produced by reactor irradiation of elemental ruthenium, were applied as triple comparators in the activation analysis
of rock FU-41, a basanitoid from Fuerteventura, Canary Island. The concentrations of the following elements were determined:
Sm, Sc, Fe, Co, Na, La, Hf, Eu, Th and Cr. The aim of this work was the experimental control of the error theory of the multiple
comparator method as well as the experimental check of the accuracy.
Presented at the 3rd Symposium on the Recent Developments in Neutron Activation Analysis, Cambridge (U.K.), 2–4 July 1973. 相似文献
157.
A fully automated sequential injection spectrophotometric method for the determination of phenylephrine hydrochloride in pharmaceutical preparations is reported. The method is based on the condensation reaction of the analyte with 4-aminoantipyrine in the presence of potassium ferricyanide. The absorbance of the condensation product was monitored at 503 nm. A linear relationship between the relative peak height and concentration was obtained in the range 0.5-17.5 mg l−1. The detection limit (as 3σ value) was 0.09 mg l−1 and repeatability was 0.8 and 0.6% at 2.5 and 5 mg l−1, respectively. Results obtained by this method agreed very well with those obtained by the AOAC official method. 相似文献
158.
Dale E. Van Sickle 《Journal of polymer science. Part A, Polymer chemistry》1972,10(2):355-368
The oxidation of both amorphous and crystalline polypropylene in benzene solution was studied at 100–130°C. tert-Butyl peroxide was used as an initiator. The kinetic behavior of the amorphous and crystalline forms differs slightly; the oxidation rate of the amorphous type is slower for a given polymer and initiator concentration. The oxidation rate of solutions of the crystalline form can be simply described by the expression: R0 = 1.87 × 1013 exp {?29,000/RT} [t-Bu2O2]0.58[polypropylene]0.73, mole/l.-min. Product analyses of the oxidized solutions are incomplete, but the results do show that only ~40% of the absorbed oxygen is present as hydroperoxide. Further, much of the hydroperoxide is present in low molecular weight polar fragments which are acetone-soluble. These results show that oxidized polypropylene cannot be regarded simply as “polypropylene hydroperoxide” with repeating hydroperoxide groups attached to the polymer chain in 1,3,5… (tertiary) positions. 相似文献
159.
Van den Bergh V Coeckelberghs H Vanhees I De Boer R Compernolle F Vinckier C 《Analytical and bioanalytical chemistry》2002,372(5-6):630-638
Biogenic non-methane hydrocarbons such as isoprene, alpha-pinene, and beta-pinene, are emitted by forests in very large quantities. To evaluate the role of alpha- and beta-pinene and their contribution to the global production of trace gases and especially aerosol precursors, a study of the oxidation mechanism of alpha- and beta-pinene with hydroxyl radicals must be conducted.The degradation products of both monoterpenes with hydroxyl radicals were identified and quantified in a fast-flow reactor. The products were collected on a liquid-nitrogen trap coated with a 2,4-DNPH solution to which two internal standards (benzaldehyde-2,4-DNPH and tolualdehyde-2,4-DNPH) had been added. The collection method was based on the in situ conversion of aldehyde and/or ketone compounds to their 2,4-dinitrophenylhydrazone derivatives. The derivatives were analyzed by HPLC-MS using APCI(-). TIC chromatograms and mass spectral data for the various oxidation products are presented.For alpha-pinene, pinonaldehyde is the most important degradation product, with smaller amounts of acetone, formaldehyde, campholenealdehyde, and acetaldehyde. For beta-pinene, nopinone and formaldehyde are the most abundant products, of almost equal importance, whereas acetone and acetaldehyde are minor compounds. 相似文献
160.
N. Van Dijk M. Noort S. Voelker G.W. Canters J.H. Van Derwalls 《Chemical physics letters》1980,71(3):415-423
We report absorption spectra from the ground state to the photoexcited triplet state of platinum porphin (PtP) in single crystals of n-octane (C8) and n-decane (C10) at 4.2 K, with and without a magnetic field. For PtP in C10 the same transition was studied in emission. From the experiments, values are derived of the spin-orbit coupling parameter Z, the crystal field splitting δ and the orbital angular momentum A for PtP in the two hosts: Z = 76 ± 2 cm?1 (C8, C10), δ = 71 ± 1 cm?1 (C8), 55 ± 1 cm?1 (C10) and A = 1.6 ± 0.1 (C8, C10). For the ratio of the in-plane and the z-polarized electric dipole transition moments we obtain ¦Mx,y¦/¦Mz¦=76± 0.3 (C8). 相似文献