全文获取类型
收费全文 | 1019篇 |
免费 | 29篇 |
国内免费 | 4篇 |
专业分类
化学 | 643篇 |
力学 | 32篇 |
数学 | 205篇 |
物理学 | 172篇 |
出版年
2023年 | 4篇 |
2022年 | 14篇 |
2021年 | 20篇 |
2020年 | 21篇 |
2019年 | 20篇 |
2018年 | 13篇 |
2017年 | 13篇 |
2016年 | 39篇 |
2015年 | 32篇 |
2014年 | 31篇 |
2013年 | 56篇 |
2012年 | 55篇 |
2011年 | 55篇 |
2010年 | 49篇 |
2009年 | 38篇 |
2008年 | 57篇 |
2007年 | 55篇 |
2006年 | 42篇 |
2005年 | 40篇 |
2004年 | 31篇 |
2003年 | 37篇 |
2002年 | 38篇 |
2001年 | 17篇 |
2000年 | 18篇 |
1999年 | 19篇 |
1998年 | 15篇 |
1997年 | 16篇 |
1996年 | 14篇 |
1995年 | 10篇 |
1994年 | 12篇 |
1993年 | 11篇 |
1992年 | 12篇 |
1991年 | 9篇 |
1990年 | 7篇 |
1989年 | 4篇 |
1988年 | 10篇 |
1987年 | 10篇 |
1986年 | 4篇 |
1985年 | 7篇 |
1984年 | 10篇 |
1983年 | 5篇 |
1982年 | 7篇 |
1981年 | 5篇 |
1980年 | 8篇 |
1979年 | 5篇 |
1978年 | 6篇 |
1977年 | 10篇 |
1975年 | 5篇 |
1974年 | 5篇 |
1970年 | 4篇 |
排序方式: 共有1052条查询结果,搜索用时 31 毫秒
991.
992.
Bosisio Renato G. Nachman Manfred Spooner Jacques 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1974,2(4):273-276
Measurements on RF power absorption in microwave discharges at 2.45 GHz, at pressures from 1 to 30 Torr in N2 and from 1 to 500 Torr in Ar, are described. A linear slow-wave structure of the strapped-bar type was employed for coupling RF energy to the plasma. From measurements on the plasma volume and on the total power absorbed, the variation with gas pressure of the RF power density in the plasma was obtained. For an incident power of 1 kW, power densities as high as 2-3 W/cm3 over relatively large plasma volumes could be achieved. The experimental data were used to calculate the pressure dependence of the electron density in an argon plasma, for an incident power of 1 kW. 相似文献
993.
Renato Angelucci Rita Rizzoli Vincenzo Vinciguerra Maria Fortuna Bevilacqua Sergio Guerri Franco Corticelli Mara Passini 《Physica E: Low-dimensional Systems and Nanostructures》2007,37(1-2):11
In this paper, a site-selective catalytic chemical vapor deposition synthesis of carbon nanotubes on silicon-based substrates has been developed in order to get horizontally oriented nanotubes for field effect transistors and other electronic devices. Properly micro-fabricated silicon oxide and polysilicon structures have been used as substrates. Iron nanoparticles have been obtained both from a thin Fe film evaporated by e-gun and from iron nitrate solutions accurately dispersed on the substrates. Single-walled nanotubes with diameters as small as 1 nm, bridging polysilicon and silicon dioxide “pillars”, have been grown. The morphology and structure of CNTs have been characterized by SEM, AFM and Raman spectroscopy. 相似文献
994.
Renato G. Bettiol Paolo Piccione 《Calculus of Variations and Partial Differential Equations》2013,47(3-4):789-807
We study existence and non-existence of constant scalar curvature metrics conformal and arbitrarily close to homogeneous metrics on spheres, using variational techniques. This describes all critical points of the Hilbert–Einstein functional on such conformal classes, near homogeneous metrics. Both bifurcation and local rigidity type phenomena are obtained for 1-parameter families of U(n + 1), Sp(n + 1) and Spin(9)-homogeneous metrics. 相似文献
995.
996.
Renato Hisashi Okina Eliana Maíra Agostini Valle Fabio Ruiz Simões Lucia Codognoto 《Electroanalysis》2021,33(5):1264-1270
In this work, a glassy carbon electrode (GCE) modified with multiwalled carbon nanotubes functionalized with carboxylic groups (MWCNT−COOH) was used to determine the hormone estrone in seawater samples. Modification of the electrode was optimized using three successive 10-μL aliquots of the MWCNT−COOH dispersion in ethanol (1 : 5 mL). The cyclic voltammetry results showed an oxidation peak at 0.59 V with characteristics of an irreversible process, pH dependent and controlled by adsorption of species. The results of square-wave voltammetry showed that the intensities of peak currents for the MWCNT−COOH/GCE were about 2.5 times higher than for GCE. The calibration curve showed a linearity of 0.9981 and a sensitivity of 0.1521 μA/mol L−1. The limits of detection and quantification were 0.117 and 0.392 μmol L−1, respectively. The recovery obtained using seawater samples was 91%, indicating the applicability of the method in marine environments. 相似文献
997.
Spontaneous enzymatically mediated dynamic kinetic resolution of 8-amino-5,6,7,8-tetrahydroquinoline
A spontaneous dynamic kinetic resolution of 8-amino-5,6,7,8-tetrahydroquinoline 1 was observed in the presence of Candida antarctica Lipase B, in which a >60% yield of (R)-acetamide [(R)-2] was isolated from the racemic amine. The spontaneous formation of ketone 3, followed by a condensation/hydrolysis sequence with the remaining (S)-amine 1, via enamine 4, provides the necessary racemization pathway. 相似文献
998.
Cavallotti C Mancarella S Rota R Carrà S 《The journal of physical chemistry. A》2007,111(19):3959-3969
We have recently proposed that the addition of C2H2 to the cyclopentadienyl radical can lead to the rapid formation of the cycloheptatrienyl radical and, in succession, of the indenyl radical. These reactions represent an interesting and unexplored route for the enlargement of gas-phase cyclic species. In this work we report ab initio calculations we performed with the aim of investigating in detail the gas-phase reactivity of cycloheptatrienyl and indenyl radicals. We found that the reaction of the cycloheptatrienyl radical with atomic hydrogen can lead to its fast conversion into the more stable benzyl radical. This reaction pathway involves the intermediate formation of heptatriene, norcaradiene, and toluene. Successively we investigated whether this reaction mechanism can be extended to polycyclic aromatic hydrocarbons (PAHs). For this purpose we studied the reaction of C2H2 with the indenyl radical, which can be considered as a superior homologue of the cyclopentadienyl radical. This reaction proceeds through a pathway similar to that proposed for C5H5 but with a reaction rate about an order of magnitude smaller. The present calculations extend thus the previously proposed C5-C7-C9 mechanism to bicyclic PAH and suggest a fast route for the conversion of C5 into C6 cyclic radicals, mediated by the formation of C7 cyclic species. 相似文献
999.
A variable selection methodology based on genetic algorithm (GA) was applied in a bilinear least squares model (BLLS) with second-order advantage, in three distinct situations, for determination by HPLC-DAD of the pesticides carbaryl (CBL), methyl thiophanate (TIO), simazin (SIM) and dimethoate (DMT) and the metabolite phthalimide (PTA) in wine. The chromatographic separation was carried out using an isocratic elution with 50:50 (v/v) acetonitrile:water as mobile phase. Preprocessing methods were performed for correcting the chromatographic time shifts, baseline variation and background. The optimization by GA provided a significant reduction of the errors, where for SIM and PTA a decrease of three times the value obtained using all variables, and an improvement in the distribution of them, reducing the observed bias in the results were observed. Comparing the RMSEP of the optimized model with the uncertainty estimates of the reference values it is observed that GA can be a very useful tool in second-order models. 相似文献
1000.
Luigi Angiolini Tiziana Benelli Francesco Mauriello Renato Bozio Danilo Pedron 《European Polymer Journal》2007,43(8):3550-3561
Three series of optically active photochromic copolymers, deriving from methyl methacrylate (MMA) and the chiral bisazoaromatic monomers (S)-3-methacryloyloxy-1-[4′-phenylazo-(4-azobenzene)]-pyrrolidine [(S)-MPAAP], (S)-3-methacryloyloxy-1-[4′-cyanophenylazo-(4-azobenzene)]-pyrrolidine [(S)-MPAAP-C] and (S)-3-methacryloyloxy-1-[4′-nitrophenylazo-(4-azobenzene)]-pyrrolidine [(S)-MPAAP-N], have been prepared and characterized in solution and in the solid state with the aim to evaluate the effect on their chiroptical and thermal properties originated by the insertion of inactive MMA groups along the main chain. The optical activity displayed by the bisazo polymers is discussed in terms of extent of chiral conformation assumed by the macromolecules as a consequence of dipole-dipole interactions between the bisazoaromatic chromophores.The photoinduction of birefringence has been assessed on thin films of the investigated copolymers in order to evaluate their behaviour as materials for optical data storage. The results are interpreted in terms of copolymer composition and conformational stiffness of the bisazoaromatic chromophoric co-units, which are responsible for the optical response rates, and are compared to those of the similar derivatives containing only one azo bond. The observed enhanced thermal properties and the temporal stability appear of interest for a potential use of these materials in nano technologies for all-optical data manipulation and in optoelectronics. 相似文献