全文获取类型
收费全文 | 6284篇 |
免费 | 322篇 |
国内免费 | 19篇 |
专业分类
化学 | 4691篇 |
晶体学 | 25篇 |
力学 | 133篇 |
数学 | 597篇 |
物理学 | 776篇 |
无线电 | 403篇 |
出版年
2023年 | 55篇 |
2022年 | 121篇 |
2021年 | 211篇 |
2020年 | 164篇 |
2019年 | 152篇 |
2018年 | 97篇 |
2017年 | 75篇 |
2016年 | 225篇 |
2015年 | 216篇 |
2014年 | 216篇 |
2013年 | 336篇 |
2012年 | 412篇 |
2011年 | 452篇 |
2010年 | 276篇 |
2009年 | 224篇 |
2008年 | 372篇 |
2007年 | 346篇 |
2006年 | 340篇 |
2005年 | 265篇 |
2004年 | 251篇 |
2003年 | 219篇 |
2002年 | 176篇 |
2001年 | 74篇 |
2000年 | 65篇 |
1999年 | 47篇 |
1998年 | 42篇 |
1997年 | 38篇 |
1996年 | 64篇 |
1995年 | 64篇 |
1994年 | 53篇 |
1993年 | 46篇 |
1992年 | 34篇 |
1991年 | 32篇 |
1990年 | 35篇 |
1989年 | 39篇 |
1988年 | 31篇 |
1987年 | 38篇 |
1986年 | 28篇 |
1985年 | 53篇 |
1984年 | 43篇 |
1983年 | 36篇 |
1982年 | 42篇 |
1981年 | 36篇 |
1980年 | 28篇 |
1978年 | 27篇 |
1977年 | 32篇 |
1975年 | 24篇 |
1966年 | 24篇 |
1965年 | 39篇 |
1964年 | 46篇 |
排序方式: 共有6625条查询结果,搜索用时 678 毫秒
201.
A series of isobaric and isomeric molecules related to 3,4methylenedioxymethamphetamine (3,4-MDMA) are prepared and evaluated as potential mass spectral equivalents to this controlled substance. These compounds have the potential to produce a mass spectrum equivalent to 3,4-MDMA, thus making mass spectrometry a nonconclusive method for confirming the identity of any one of the substances. The various isomeric forms of the methoxymethylphenethylamines and the methoxymethcathinones have mass spectra essentially equivalent to 3,4-MDMA, but the ethoxy substituted phenethylamines show a unique fragment at m/z 107. Gas chromatographic separation on nonpolar stationary phases successfully resolved these compounds from 3,4-MDMA, however only a limited set of side chain regioisomers and ring substitution patterns are evaluated in this initial study. 相似文献
202.
H. P. Peters D. Stauffer H. P. Hölters K. Loewenich 《Zeitschrift für Physik B Condensed Matter》1979,34(4):399-408
Monte Carlo simulation and series expansion shows the radius of gyration of large clusters withs sites each to vary ass
with0.56 in two and0.47 in three dimensions at the percolation threshold, and with(d=2)0.65 and(d=3)0.53 for random lattice animals (zero concentration). Clusters up tos=100 were used. The perimeter of random animals approaches 2.8s for larges on the simple cubic lattice. Monte Carlo simulation of the Eden process (growing animals) up tos=5,000 indicates a systematic variation of about ±0.05 for the effective exponent=(s) and thus suggests that the true asymptotic exponents may be compatible with the predictions of hyper-scaling. 相似文献
203.
Marcel Sickert Harald Weinstabl Brendan Peters Xiao Hou Mark Lautens 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2014,126(20):5247-5251
A palladium‐catalyzed intermolecular cross‐coupling of two aryl iodides is reported, giving polycyclic ring systems with a high level of convergence and efficiency. 相似文献
204.
Laura Pop Pierrik Lassalas László Csaba Bencze Monica Ioana Toşa Botond Nagy Florin Dan Irimie Christophe Hoarau 《Tetrahedron: Asymmetry》2012,23(6-7):474-481
Stereoselective preparative enzymatic acylation and hydrolysis/methanolysis of various C-substituted rac-thiazol-2-yl-methanols were achieved for the preparation of enantiopure or enantiomerically enriched, naturally occurring 2-hydroxymethylthiazoles. The absolute configurations of the resulting secondary alcohols were determined by a detailed 1H NMR study of Mosher’s derivatives. 相似文献
205.
206.
Maria G. Bettoli Lorenzo Cantelli Damiano Quarin Marzia Ravanelli Laura Tositti Ottavio Tubertini 《International journal of environmental analytical chemistry》2013,93(3-4):321-330
Abstract 210Pb is widely used to determine accumulation rates in order to obtain a time scale in environmental samples. The most accurate method uses the determination of 210Pb via its grand-daughter 210Po by alpha spectrometry. Unfortunately this method requires a complex wet-chemistry procedure to achieve the separation of 210Po from its matrix. In this work a simplified procedure for the chemical separation of 210Po is proposed and applied to three marine sediment cores and a 10 m snow core collected in Antarctica. The calculated sedimentation rates for marine sediments range from 0.053 to 0.071 cm y?1. The mean annual accumulation rate for the snow is 16.6 cm y?1 w.e. A comparison with literature data in the same region is given. 相似文献
207.
Nicolás D. Gómez Violeta D'Accurso V. Mariana Freytes Francisco A. Manzano Jorge Codnia M. Laura Azcárate 《国际化学动力学杂志》2013,45(5):306-313
An experimental setup that coupled IR multiple‐photon dissociation (IRMPD) and laser‐induced fluorescence (LIF) techniques was implemented to study the kinetics of the recombination reaction of dichlorocarbene radicals, CCl2, in an Ar bath. The CCl2 radicals were generated by IRMPD of CDCl3. The time dependence of the CCl2 radicals’ concentration in the presence of Ar was determined by LIF. The experimental conditions achieved allowed us to associate the decrease in the concentration of radicals to the self‐recombination reaction to form C2Cl4. The rate constant for this reaction was determined in both the falloff and the high‐pressure regimes at room temperature. The values obtained were k0 = (2.23 ± 0.89) × 10?29 cm6 molecules?2 s?1 and k∞ = (6.73 ± 0.23) × 10?13 cm3 molecules?1 s?1, respectively. 相似文献
208.
An expeditious synthetic method for the preparation of bis(indollyl)methane derivatives has been developed through a bismuth nitrate-catalyzed reaction of indole with various carbonyl compounds in mortar and pastle under solvent-free conditions. 相似文献
209.
Alternating copolymers of vinyl acetate (VAc) and maleic anhydride (MA) are produced preferentially when VAc and MA are copolymerized at moderate temperatures while random copolymers are produced at temperatures above 90°C. A charge-transfer complex (CTC) of VAc/MA exists at moderate temperatures but does not exist at temperatures above 90°C. 相似文献
210.
Dr. Maria Laura Tummino Dr. Roberto Nisticò Dr. Chiara Riedo Prof. Debora Fabbri Prof. Marta Cerruti Prof. Giuliana Magnacca 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(2):660-668
Biowaste-derived substances isolated from green compost (BBS-GC) are environmentally friendly reactants similar to humic substances, which contain multiple functionalities, that are suitable for adsorbing different kinds of pollutants in wastewater. Herein, sodium alginate (derived from brown algae) cross-linked with both Ca2+ ions and BBS-GC in the form of hydrogels and dried films are proposed as green, easy-to-form, and handleable materials for tertiary water treatments. The results show that both hydrogels and films are mechanically stable and can effectively remove differently charged dyes through an adsorption mechanism that can be described by the Freundlich model. BBS-GC-containing gels always performed better than samples prepared without BBS-GC, revealing that such unconventional materials can integrate waste valorization and water decontamination, potentially providing social and environmental benefits. 相似文献