全文获取类型
收费全文 | 178429篇 |
免费 | 2888篇 |
国内免费 | 1197篇 |
专业分类
化学 | 93254篇 |
晶体学 | 3078篇 |
力学 | 7577篇 |
综合类 | 47篇 |
数学 | 20422篇 |
物理学 | 58136篇 |
出版年
2021年 | 1500篇 |
2020年 | 1742篇 |
2019年 | 1810篇 |
2018年 | 2224篇 |
2017年 | 2297篇 |
2016年 | 3531篇 |
2015年 | 2265篇 |
2014年 | 3523篇 |
2013年 | 8170篇 |
2012年 | 6398篇 |
2011年 | 7569篇 |
2010年 | 5465篇 |
2009年 | 5405篇 |
2008年 | 6971篇 |
2007年 | 6837篇 |
2006年 | 6461篇 |
2005年 | 5784篇 |
2004年 | 5342篇 |
2003年 | 4752篇 |
2002年 | 4633篇 |
2001年 | 5328篇 |
2000年 | 3991篇 |
1999年 | 3111篇 |
1998年 | 2606篇 |
1997年 | 2523篇 |
1996年 | 2360篇 |
1995年 | 2228篇 |
1994年 | 2190篇 |
1993年 | 2098篇 |
1992年 | 2369篇 |
1991年 | 2303篇 |
1990年 | 2250篇 |
1989年 | 2154篇 |
1988年 | 2151篇 |
1987年 | 2153篇 |
1986年 | 2030篇 |
1985年 | 2653篇 |
1984年 | 2692篇 |
1983年 | 2387篇 |
1982年 | 2496篇 |
1981年 | 2313篇 |
1980年 | 2285篇 |
1979年 | 2427篇 |
1978年 | 2462篇 |
1977年 | 2388篇 |
1976年 | 2436篇 |
1975年 | 2360篇 |
1974年 | 2327篇 |
1973年 | 2504篇 |
1972年 | 1623篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
831.
First experiences with the technique of acoustical levitation of droplets in the field of analytical and atmospheric chemistry
are reported. Acoustical levitation enables the contactless handling of solid and liquid microsamples. This avoids adsorption
of analyte at and desorption of contaminants from container walls especially for liquid samples. Common experiments of sample
preparation procedures were conducted in levitated drops like liquid/liquid extractions, solvent exchange, and analyte enrichment
by evaporation of the solvent. A first approach was made to use acoustical levitation for the simulation of atmospheric chemistry
situations.
Received: 10 October 1996 / Revised: 25 October 1996 / Accepted: 25 October 1996 相似文献
832.
Robert B. Hermann 《Journal of computational chemistry》1993,14(6):741-750
Using experimental solubilities and partial pressures for hydrocarbon solution in water and molecular dynamics calculations of hydrocarbon water interaction energies, hydrocarbon–water cavity potentials are obtained and then plotted vs. accessible surface area. The data used is mainly for aliphatic hydrocarbons, but benzene is included. Molecular dynamics calculations of pairs of hydrocarbon molecules together with the cavity potential curve are then used to obtain hydrophobic interaction free energies between the hydrocarbon pairs. While the cavity potential change is related to a change in surface area for hydrocarbon systems, the hydrocarbon–water interaction energy is not, so that the hydrophobic binding energy is not. The results are in agreement with previous results by a different method (R.B. Hermann, In Seventh Jerusalem Symposium on Quantum Pharmacology, E. Bergman and B. Pullman, Eds., D. Reidel, Dordrecht, 1974, p. 441) in that there is little or no solvent-induced binding free energy between small hydrocarbon molecules in a dilute aqueous solution. It is proposed that the cavity potential vs. accessible surface area curve obtained here can be used together with OPLS parameters to calculate both hydrocarbon–water solvation free energies and hydrophobic interactions. © 1993 John Wiley & Sons, Inc. 相似文献
833.
A quartz crystal microbalance has been proposed for studies on the temperature dependence of the linear rate of a reaction interface advance in topochemical reactions of the thermal decomposition of solids. A quartz crystal microbalance has been used in investigations of the CuSO4 · 5H2O dehydration. The data agree fairly well with those available in the literature. Advantages and disadvantages of the method proposed are discussed. 相似文献
834.
The oxidations of 1,2-polybutadiene by molecular oxygen, singlet oxygen, atomic oxygen and ozone have been studied using u.v. and i.r. spectroscopic methods. Some possible implications of the results of oxidation in the presence of singlet oxygen (parallel free radical oxidation) and atomic oxygen (formation of NO2 and its reaction with polymer) are discussed. Crosslinking was observed during all types of oxidation. A new mechanism involving formation of free radicals has been considered in detail. During ozonization of 1,2-polybutadiene, formation of formaldehyde and formic acid were detected. An ozonization mechanism has been proposed. 相似文献
835.
1:2-Benzanthracene and the 3′-methyl derivative have been prepared by Raney nickel reduction of the sulphuric esters of the leuco derivatives of 1:2-benzanthraquinone and 4′-chloro-3′-methyl-1:2-benzanthraquinone, followed by dehydrogenation. 3-Hydroxy-1:2-benzanthraquinone was methylated in the 4-position by formaldehyde, sodium hydrosulphite and sodium hydroxide solution (the Marschalk reaction). Simultaneous reduction of the nuclear hydroxyl and quinone groups was effected by Raney nickel reduction of the trisulphuric ester of 3:9:10-trihydroxy-4-methyl-1:2-benzanthracene, and the resultant hexahydro-4-methyl-1:2-benzanthracene was dehydrogenated to 4-methyl-1:2-benzanthracene. The preparation of 3:4:9:10-dibenzopyrene from Mayvat brilliant red AF by Raney nickel reduction of the sulphuric ester of the leuco derivative and subsequent dehydrogenation is described. 相似文献
836.
X. Y. Li G. J. Wang J. G. Sun Y. T. Zheng B. Yan H. T. Xie X. Wang 《Chromatographia》2007,65(1-2):13-18
To support preclinical pharmacokinetic investigation of 1-[4-[2-(4-bromobenzene-sulfonaminoethyl)phenylsufonyl]-3-(trans-4-methylcyclohexyl)urea
(G004), a rapid, sensitive and specific high-performance liquid chromatography–electrospray ionization mass spectrometry (LC–ESI-MS)
method was developed and validated. Glibenclamide was employed as internal standard. After liquid–liquid extraction the analyte
was analyzed on a Kromasil C18 column (150 × 2.0 mm i.d.) with a mobile phase consisted of acetonitrile–water (0.05% acetic acid), 30:70 (v/v). The flow
rate was 0.2 mL min−1. Detection was performed on a quadrupole mass spectrometer using an electrospray ionization interface and the selected-ion
monitoring (SIM) mode. The retention time was about 3.5 and 4.2 min for Glibenclamide and G004, respectively. The assay was
linear over the concentration range of 2.0–500.0 ng mL−1. Extraction Recovery of G004 in rat plasma was more than 87%. The intra- and inter-assay precision was lower than 11.5% (CV).
This validated method was successfully applied to the pharmacokinetics of G004 in rats. 相似文献
837.
M M Ayad G B El-Hefnawy G W Bahlol 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2002,58(1):161-166
The spectrophotometric and thermodynamic properties of different substituted methylnaphthalenes charge transfer (CT) complexes with tetracyanoethylene have been studied in carbon tetrachloride. The spectral characteristics of the CT bands have also been discussed in relation to the positions of methyl groups. The formation constants and the spectral properties of the complexes are markedly affected with the substitution position of the methyl groups. The ionization potentials of the donors are determined. 相似文献
838.
B. V. Potapkin M. A. Deminsky A. A. Fridman V. D. Rusanov 《Radiation Physics and Chemistry》1995,45(6):1081-1088
Theoretical investigation of the effect of molecular clusters and aerosol particles on non-equilibrium plasma flue gas cleaning was made in this paper. Two types of heterogeneous reactions in aerosol and clusters are considered. It was shown that in both cases these reactions are essential in the evaluation of chemical composition. As a result of theoretical approach and modelling, the optimum regime of plasma generation for essential decreasing of purification energy cost was established. 相似文献
839.
V. P. Perevalov L. I. Baryshnenkova M. A. Andreeva Yu. A. Manaev I. A. Denisova B. I. Stepanov V. I. Seraya 《Chemistry of Heterocyclic Compounds》1983,19(12):1322-1326
A method was developed for the synthesis of 5-bromo-1-methyl-4-nitropyrazole from 1-methylpyrazole. In the reaction with 25% aqueous ammonia at 180–190 °C, 5-bromo-1-methyl-4-nitropyrazole is readily converted to 5-amino-1-methyl-4-nitropyrazole; the production of 4-amino-1-methyl-5-nitropyrazole from 4-bromo-1-methyl-5-nitropyrazole requires the presence of a copper catalyst; under the same conditions in the amination of 4-bromo-1-methyl-3-nitropyrazole, 4-amino-1-methyl-3-nitro- and 1-methyl-3-nitropyrazoles are formed in a 23 ratio.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1672–1675, December, 1983. 相似文献
840.