全文获取类型
收费全文 | 7743篇 |
免费 | 1735篇 |
国内免费 | 833篇 |
专业分类
化学 | 2811篇 |
晶体学 | 83篇 |
力学 | 129篇 |
综合类 | 74篇 |
数学 | 1081篇 |
物理学 | 2721篇 |
无线电 | 3412篇 |
出版年
2024年 | 33篇 |
2023年 | 122篇 |
2022年 | 193篇 |
2021年 | 214篇 |
2020年 | 221篇 |
2019年 | 208篇 |
2018年 | 189篇 |
2017年 | 302篇 |
2016年 | 312篇 |
2015年 | 286篇 |
2014年 | 501篇 |
2013年 | 852篇 |
2012年 | 623篇 |
2011年 | 589篇 |
2010年 | 497篇 |
2009年 | 502篇 |
2008年 | 584篇 |
2007年 | 581篇 |
2006年 | 555篇 |
2005年 | 497篇 |
2004年 | 411篇 |
2003年 | 375篇 |
2002年 | 359篇 |
2001年 | 202篇 |
2000年 | 207篇 |
1999年 | 148篇 |
1998年 | 88篇 |
1997年 | 102篇 |
1996年 | 82篇 |
1995年 | 85篇 |
1994年 | 79篇 |
1993年 | 52篇 |
1992年 | 45篇 |
1991年 | 36篇 |
1990年 | 24篇 |
1989年 | 31篇 |
1988年 | 12篇 |
1987年 | 14篇 |
1986年 | 18篇 |
1985年 | 15篇 |
1984年 | 11篇 |
1983年 | 3篇 |
1982年 | 16篇 |
1981年 | 7篇 |
1980年 | 8篇 |
1979年 | 4篇 |
1977年 | 4篇 |
1976年 | 3篇 |
1974年 | 4篇 |
1973年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
221.
M. Katkevičs A. Kontijevskis I. Mutule E. Sūna 《Chemistry of Heterocyclic Compounds》2007,43(2):151-159
A 30-membered library of coumarins has been synthesized in a microwave-assisted Pechmann reaction using neat trifluoroacetic
acid both as an acidic reagent and a reaction medium. Alternatively, polymer-supported sulfonic acid Amberlyst-15 could also
be employed to facilitate the formation of coumarins. The use of a specially-built microwave synthesizer with liquid handling
tools rendered the automated synthesis of a coumarin library feasible.
__________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 200–208, February, 2007. 相似文献
222.
微波增压溶样-石墨炉原子吸收光谱法同时测定植物性样品中微量铅镉 总被引:7,自引:2,他引:7
使用微波增压溶样法,将样品用HNO3—HClO4溶解后,用石墨炉原子吸收光谱法同时测定植物性样品中铅和镉。以磷酸二氢铵—硝酸镁作为基体改进剂,可使灰化温度大大提高,克服了基体的干扰。铅和镉的回收率分别为95.3%—104.8%和96.2%—105.5%,RSD分别为4.8%-7.2%和4.3%-7.6%。 相似文献
223.
Todd B. Kreutzian Khalid S. A. Seraj Larry G. Anderson Donald C. Zapien 《Electroanalysis》2007,19(23):2479-2482
In this work, long optical path length thin‐layer electrochemical cell was constructed using indium‐tin oxide on glass as the electrode material. Iron release from ferritin adsorbed on the electrode was induced by applying a negative potential sweep in the presence of 1,10‐phenanthroline. The usefulness of spectroelectrochemistry as a means of determining the quantity of iron released from an adsorbed layer of ferritin is demonstrated. 相似文献
224.
HUA Guo-Pinga ZHU Xiao-Tongb ZHANG Jin-Pengb XU Jia-Ningb WANG Qianb JI Shun-Junc ZHANG Yongc TU Shu-Jiangb② a 《结构化学》2006,25(5):599-603
1 INTRODUCTION Various quinolone derivatives are known to dis- play interesting biological properties ranging from microbial activity to cytotoxicity[1]. They have been reported as antiviral (HIV-1)[2] and antitumor agents[3] as well as used as tubulin[4], topoisomerase[5] and thrombocyte inhibitors[6]. As a member of the quino- lone family, substituted N-phenyl-2-quinolones re-present the structural basis of many biologically active compounds, such as protein kinase inhibitors, immunodu… 相似文献
225.
改进的微波辅助无溶剂法提取薄荷和陈皮中的挥发油组分 总被引:1,自引:0,他引:1
An improved solvent free microwave extraction, in which a kind of microwave absorption medium (carbonyl iron powder) was used, was applied to the extraction of essential oil from dried menthol mint and orange peel without addition of any solvent and pretreatment. It took much less time of extraction (30 min) than microwave-assisted hydrodistillation (90 min) and conventional hydrodistillation (180 min). The kinds of chemical compositions in essential oil extracted by different methods were almost the same and such improved solvent free microwave extraction can be a feasible way in extraction of essential oil from dried plant materials. 相似文献
226.
227.
A. M. Wrobel B. Lamontagne M. R. Wertheimer 《Plasma Chemistry and Plasma Processing》1988,8(3):315-329
We have investigated O2/CF4 plasma etching of five commercial polymers: polyimide, polyamide, polyethylene terephthalate, polycarbonate and cured epoxy resin. A new large-area microwave plasma apparatus has been used in this work, but the same apparatus can also be used as a capacitively coupled radiofrequency (13.56 MHz) discharge reactor. The effect of operating parameters such as pressure, etchant gas composition, excitation frequency and sample temperature upon etch kinetics has been examined. We have observed distinct maxima in the etch rate as functions of pressure and CF4 concentration. Activation, energies evaluated from the Arrhenius plots fall in the range 0.04-0.2 eV, in agreement with data in the literature. Dry etch susceptibility of a given polymer correlates strongly with the degree of unsaturation in the polymer's structure 相似文献
228.
This paper investigates the performance of a quasioptical gyrotron, when the electron beam interacts with the radiation fields at harmonics of the gyrofrequency. The nonlinear equations of motion are obtained in the slow-timescale. The expression for the linear gain is derived and the conditions for excitation are given (frequency threshold, optimal operating point, bean current and resonator quality thresholds). In the nonlinear regime, it is shown that maximum efficiencies comparable to those at the fundemental (50%) are possible, albeit at a prohitively high radiation field amplitude, while realistically feasible field amplitudes can give somewhat smaller, but nevertheless still high efficiencies (15%). Finally, the results are suplemented by empirical scaling laws, useful for experimental designs. 相似文献
229.
Michael C. Bhm Joachim Schulte Rafael Ramírez 《International journal of quantum chemistry》2002,86(3):280-296
The absolute magnetic shieldings of benzene and ethylene have been theoretically studied under the conditions of thermal equilibrium, i.e., under explicit consideration of the nuclear degrees of freedom. For this purpose we have combined the Feynman path integral quantum Monte Carlo (PIMC) formalism with the gauge‐including atomic orbital (GIAO) approach in the Hartree–Fock (HF) approximation. The HF operator has been employed to derive the NMR parameters of the two hydrocarbons via an ensemble averaging over large sets of molecular configurations that are populated in thermal equilibrium. The nuclear fluctuations are responsible for a deshielding of the nuclei relative to the shieldings at the vibrationless minimum of the potential energy surface (PES). The influence of the nuclear degrees of freedom is largest for the isotropic part of the 13C shielding tensor. The theoretical results can be explained on the basis of simple geometrical considerations. The bond lengths in thermal equilibrium are larger than the bond lengths at the minimum of the PES. This length enhancement is the prerequisite for a deshielding of the nuclei in thermal equilibrium. The vibrational corrections of the nuclear magnetic resonance (NMR) parameters of benzene and ethylene are quantum driven; classical thermal degrees of freedom of the nuclei are of minor importance. Conceptual problems of theoretical studies of NMR parameters on the basis of a single molecular geometry are emphasized. The influence of the spatial uncertainty of the nuclei becomes decisive in molecules with light atoms. It is pointed out that the combination of the PIMC formalism with electronic Hamiltonians of state‐of‐the‐art quality renders possible accurate determinations of NMR parameters. © 2002 John Wiley & Sons, Inc. Int J Quantum Chem 86: 280–296, 2002 相似文献
230.
Baeva M. Luo X. Schäfer J. H. Uhlenbusch J. Zhang Z. 《Plasma Chemistry and Plasma Processing》1998,18(4):429-446
The present work deals with a pulsed microwave discharge in an Ar/CF
4
gas mixture under a low pressure (1–10 mbar). The discharge chamber developed has a cylindrical geometry with a coupling window alternatively made of quartz or alumina. The setup allows one to investigate the plasma–wall interactions (here etching of the quartz window) and the ignition process of the pulsed microwave plasma. Microwave pulses with a duration of 50–200 s and repetition rate between 1 and 10 kHz are typical for the experiments. The space-time behavior of the fluorine number density in the discharge has been investigated experimentally by optical actinometry. The discharge kinetics is modeled using electron-transport parameters and rate coefficients derived from solutions of the Boltzmann equation. Together with the solution of the continuity and electron balance equations and the rate equations describing the production of CF
x
(x=2, 3, 4) radicals and F atoms, a good agreement between experimental and theoretical data can be achieved. 相似文献