全文获取类型
收费全文 | 1185篇 |
免费 | 62篇 |
国内免费 | 19篇 |
专业分类
化学 | 720篇 |
晶体学 | 3篇 |
力学 | 14篇 |
数学 | 220篇 |
物理学 | 309篇 |
出版年
2023年 | 6篇 |
2022年 | 20篇 |
2021年 | 31篇 |
2020年 | 47篇 |
2019年 | 35篇 |
2018年 | 24篇 |
2017年 | 22篇 |
2016年 | 57篇 |
2015年 | 47篇 |
2014年 | 43篇 |
2013年 | 77篇 |
2012年 | 82篇 |
2011年 | 78篇 |
2010年 | 64篇 |
2009年 | 40篇 |
2008年 | 52篇 |
2007年 | 56篇 |
2006年 | 54篇 |
2005年 | 67篇 |
2004年 | 40篇 |
2003年 | 31篇 |
2002年 | 35篇 |
2001年 | 35篇 |
2000年 | 21篇 |
1999年 | 14篇 |
1998年 | 13篇 |
1997年 | 10篇 |
1996年 | 9篇 |
1995年 | 7篇 |
1994年 | 5篇 |
1993年 | 10篇 |
1992年 | 5篇 |
1991年 | 4篇 |
1990年 | 7篇 |
1989年 | 8篇 |
1987年 | 6篇 |
1986年 | 3篇 |
1985年 | 7篇 |
1984年 | 6篇 |
1983年 | 11篇 |
1982年 | 10篇 |
1981年 | 4篇 |
1980年 | 8篇 |
1979年 | 8篇 |
1978年 | 8篇 |
1976年 | 5篇 |
1974年 | 6篇 |
1973年 | 4篇 |
1967年 | 2篇 |
1915年 | 2篇 |
排序方式: 共有1266条查询结果,搜索用时 171 毫秒
11.
Some features of the propagation of one-soliton successions in the third order dispersion region are presented for the case where the phase difference between adjacent pulses is rads. We show that this considerably improves the stability of the pulses in the zero-dispersion region. Walk-off can be diminished by increasing the number of pulses in the succession. These features may help optimize the bandwidth of non-linear fibre communication links. 相似文献
12.
F. Benedetti S. Bozzini M. Forchiassin G. Nardin G. Pitacco C. Russo E. Valentin 《Journal of heterocyclic chemistry》1989,26(2):301-305
The heterocyclization reaction of arylsulfonylhydrazones from alkyl substituted 1,2-cyclohexanediones is sensitive to the steric requirements of the alkyl groups. A mechanism for the cyclization in acidic medium is also reported. 相似文献
13.
Valentin Herbez 《Tetrahedron letters》2005,46(40):6797-6799
Polyethylene glycol tert-octylphenyl ether and polyoxyethylenesorbitan trioleate are polydisperse macromolecular detergent molecules, containing a single hydroxyl function, which was transformed by 1,1-carbonyldiimidazole into imidazole-detergent conjugates with a carbamate linkage. 相似文献
14.
Lepère M Valentin A Henry A Camy-Peyret C Blanquet G Populaire JC Mantz AW 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2002,58(11):2413-2419
We studied the R(0) line profile in the CH4 v4 band from room temperature to 188 K with N2 as a perturber, to 100 K with O2 as perturber, and from room temperature to 15 K using He as a perturber. The N2 and O2 measurements were performed over a total pressure range of 15-110 mbar, and for the He measurements the maximum sample pressure at 15 K was 1.10 mbar. Broadening parameters were determined, taking into account the confinement narrowing (Dicke effect), and narrowing parameters, deduced from the soft or hard collision model, are compared with the dynamic friction coefficient calculated values. We also obtained preliminary values for the temperature dependence of the N2, O2 and He line broadening parameters for this transition. 相似文献
15.
Vadim?B.?Fetisov Alexander?N.?Ermakov Galina?M.?Belysheva Andrey?V.?Fetisov Valentin?M.?Kamyshov Khiena?Z.?BraininaEmail author 《Journal of Solid State Electrochemistry》2004,8(8):565-571
The present paper deals with the electrochemical behavior of magnetite microcrystals in an acid medium. A voltammetric method employing a carbon-paste electroactive electrode (CPEE) with an organic binder was used. It was found that the cathodic voltammograms, which were recorded at different scan rates, formed a set bounded in the space of i–E parameters by a generalizing voltammetric curve corresponding to the effective potential scan rate eff. In other words, all curves are situated under one enveloping curve, just as the smaller dolls sit in the largest doll of a Russian doll. Reverse currents (a cathodic current in the anodic direction of the potential scan) were observed on the cyclic voltammogram. Forward and reverse currents obey the same laws and have one and the same generalizing curve, which could be taken as the magnetite characteristic. 相似文献
16.
Mladenov M Mirceski V Gjorgoski I Jordanoski B 《Bioelectrochemistry (Amsterdam, Netherlands)》2004,65(1):69-76
The electrode reaction of glutathione (GSH) at the hanging mercury drop electrode is studied by means of square-wave voltammetry (SWV). At potentials more positive than -0.350 V (vs. Ag/AgCl (3 mol/l KCl)) the oxidation of the mercury electrode in the presence of GSH leads to creation of a sparingly soluble mercury-GSH complex that deposits onto the electrode surface. Under cathodic potential scan, the deposited complex acts as a reducible reactant, giving raise to a well-defined cathodic stripping reversible SW voltammetric response. The electrode reaction can be described by the scheme: Hg(SG)(2(s))+e(-)+2H((aq))(+) = Hg((l))+2GSH((aq)). Thus, the electrode reaction provides information on both thermodynamics and kinetics of the chemical interactions of GSH with mercury. An experimental methodology for measuring the kinetics of the electrode reactions, based on the property known as "quasireversible maximum", is developed. The standard redox rate constant is 5.09, 5.75 and 5.22 cm s(-1) in a phosphate buffer at pH 5.6, 7.0 and 8.5, respectively, with a precision of +/-10%. The high rate of the electrode reaction reflects the strong affinity of GSH towards chemical interaction with mercury. The electrode reaction is particularly sensitive to the presence of heavy metal ions such as Cu(2+), Cd(2+), and Zn(2+.) The rate of the electrode reaction decreases significantly in the presence of these ions due to simultaneous interactions of GSH with the respective ion and mercury. 相似文献
17.
Recently several theoretical studies have examined oxygen adsorption on the clean, reduced TiO2(110) surface. However the photocatalytic behavior of TiO2 and the scavenging ability of oxygen are known to be influenced by the presence of surface hydroxyls. In this paper the chemistry of O2 on the hydroxylated TiO2 surface is investigated by means of first-principles total energy calculations and molecular dynamics (MD) simulations. The MD trajectories show a direct, spontaneous reaction between O2 and the surface hydroxyls, thus supporting the experimental hypothesis that the reaction does not necessarily pass through a chemisorbed O2 state. Following this reaction, the most stable chemisorbed intermediates are found to be peroxide species HO2 and H2O2. Although these intermediates are very stable on the short time scale of MD simulations, the energetics suggests that their further transformation is connected to a new 300 K feature observed in the experimental water temperature programmed desorption (TPD) spectrum. The participation of two less stable intermediate states, involving terminal hydroxyls and/or chemisorbed water plus oxygen adatoms, to the desorption process, is not supported by the total energy calculations. Analysis of the projected density of states, however, suggests the possibility that these intermediates have a role in completing the surface oxidation immediately before desorption. 相似文献
18.
Chiesa M Paganini MC Spoto G Giamello E Di Valentin C Del Vitto A Pacchioni G 《The journal of physical chemistry. B》2005,109(15):7314-7322
Paramagnetic centers at the surface of ionic oxides in the form of trapped electrons can be generated by exposure of the material to alkali metal or hydrogen atoms or of molecular hydrogen under UV irradiation. For many years, it has been assumed that the resulting paramagnetic centers consist of oxygen vacancies filled by one electron. High-resolution electron spin resonance spectra and ab initio quantum chemical calculations show that the paramagnetic centers consist of (H(+))(e(-)) electron pairs formed at morphological irregularities of the surface. At least three different kinds of (H(+))(e(-)) centers, [A], [B], and [C], have been identified with abundances of 80%, 10%, and 8%, respectively. In this work, we compare a wide set of measured and computed g-factors and hyperfine coupling constants of the unpaired electron with the surrounding (25)Mg, (17)O, and (1)H nuclei and we propose a general assignment of the centers. (H(+))(e(-)) pairs formed at Mg(4c) ions at steps and edges account for species [A], centers formed at Mg(4c) ions at reverse corners correspond to species [B], and species [C] originates from (H(+))(e(-)) pairs formed at Mg(3c) ions at corners and kinks. 相似文献
19.
Cimpeanu V Parvulescu VI Amorós P Beltrán D Thompson JM Hardacre C 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(18):4640-4646
Heterogeneous catalytic oxidation of a series of thioethers (2-thiomethylpyrimidine, 2-thiomethyl-4,6-dimethyl-pyrimidine, 2-thiobenzylpyrimidine, 2-thiobenzyl-4,6-dimethylpyrimidine, thioanisole, and n-heptyl methyl sulfide) was performed in ionic liquids by using MCM-41 and UVM-type mesoporous catalysts containing Ti, or Ti and Ge. A range of triflate, tetrafluoroborate, trifluoroacetate, lactate and bis(trifluoromethanesulfonyl)imide-based ionic liquids were used. The oxidations were carried out by using anhydrous hydrogen peroxide or the urea-hydrogen peroxide adduct and showed that ionic liquids are very effective solvents, achieving greater reactivity and selectivity than reactions performed in dioxane. The effects of halide and acid impurities on the reactions were also investigated. Recycling experiments on catalysts were carried out in order to evaluate Ti leaching and its effect on activity and selectivity. 相似文献
20.
1,2-Oxazine N-oxides derived from aminocyclohexenes open into the corresponding nitroalkylated trisubstituted enamines, whereas those derived from aminocyclopentenes give stable tetrasubstituted enamines. Both open-chain systems are easily hydrolyzed to the corresponding γ-nitrocycloalkanones. 相似文献