全文获取类型
收费全文 | 1844篇 |
免费 | 85篇 |
国内免费 | 19篇 |
专业分类
化学 | 1328篇 |
晶体学 | 19篇 |
力学 | 108篇 |
数学 | 205篇 |
物理学 | 288篇 |
出版年
2023年 | 21篇 |
2022年 | 63篇 |
2021年 | 88篇 |
2020年 | 61篇 |
2019年 | 93篇 |
2018年 | 85篇 |
2017年 | 63篇 |
2016年 | 108篇 |
2015年 | 71篇 |
2014年 | 104篇 |
2013年 | 182篇 |
2012年 | 129篇 |
2011年 | 121篇 |
2010年 | 96篇 |
2009年 | 77篇 |
2008年 | 91篇 |
2007年 | 67篇 |
2006年 | 55篇 |
2005年 | 36篇 |
2004年 | 30篇 |
2003年 | 29篇 |
2002年 | 31篇 |
2001年 | 13篇 |
2000年 | 15篇 |
1999年 | 7篇 |
1998年 | 17篇 |
1997年 | 12篇 |
1996年 | 8篇 |
1995年 | 6篇 |
1994年 | 13篇 |
1993年 | 8篇 |
1992年 | 6篇 |
1991年 | 7篇 |
1990年 | 11篇 |
1989年 | 13篇 |
1988年 | 12篇 |
1987年 | 10篇 |
1986年 | 5篇 |
1985年 | 10篇 |
1984年 | 12篇 |
1983年 | 8篇 |
1982年 | 11篇 |
1980年 | 7篇 |
1979年 | 6篇 |
1978年 | 5篇 |
1977年 | 3篇 |
1976年 | 5篇 |
1975年 | 4篇 |
1968年 | 2篇 |
1967年 | 3篇 |
排序方式: 共有1948条查询结果,搜索用时 15 毫秒
151.
152.
Cadmium sulfide nanoparticles with average size of ≈16 nm have been synthesized using chemical precipitation reaction of CdCl2 and Na2S in the presence of nonionic surfactant stabilized reverse emulsions. Sorbian monolaurate (Span 20) is used for the stabilization of emulsions. The influence of Span 20 on controlling the size and properties of CdS nanoparticles were studied. The obtained CdS nanoparticles were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Energy-dispersive x-ray (EDX), and X-ray diffraction (XRD). The optical properties of CdS nanoparticles are investigated systematically by UV-visible absorption spectroscopy. The blue shift in the absorption region and increase in the bandgap to the larger value (2.77 eV) are attributed to the nanosize of the obtained particles. 相似文献
153.
154.
Mahmoud Hachemane Mohamed Abdelwahab Benbitour Abdallah Smida 《Foundations of Physics》1997,27(4):579-594
Along the lines of a previous work, the geometrical structure of Hibert bundles describing extended quantum free particles
is repeated with Galilei external and internal independent symmetries. Then, in order to introduce the interaction, this structure
is extended by replacing configuration and momentum spaces by the socelled spaces of trajectories and extended velocity boosts,
respectively. These provide representations giving the probability amplitudes for the particle to follow certain trajectories.
The interaction can be introduced in the transformation law from functions on the space of trajectories (free dynamics) to
functions on spacetime (intracting dynamics). This transformation law, which makes use of a universal distribution, is seen
as a functional in our work according to a quantum functional theory which generalizes the ideas of de Broglie. Intertwining
of induced representations gives the free propagator in the space of trajectories and, henceforth, the propagator with interaction
in space-time for the extended particle. 相似文献
155.
Rare-earth doped fibers suffer from relatively high attenuation in comparison to conventional communication fibers. In order to improve the properties of such fibers, understanding of the mechanisms involved in scattering effects is of great importance. The effect of Nd-doping, Al-codoping and of the drawing temperature on the scattering effect was investigated for several different optical fibers. To this end a measuring setup was realized which allows absolute measurements of the scattering coefficient at different positions along a fiber. Also separation of total attenuation and scattering effects is possible. The presented results indicate scattering effects as primary source for increased attenuation compared with undoped fibers. 相似文献
156.
Mahmoud Abdel-Aty 《理论物理通讯》2002,37(6)
We investigate the entropy squeezing of the nonlinear k-quantum JC model. A definition of squeezing is presented for this system based on the quantum information theory. The utility of the definition is illustrated by examining squeezing in the information entropy of a nonlinear k-quantum two-level atom. The influence of the atomic coherence and the detuning parameter on the properties of the information entropy and squeezing of the atomic variables is examined. 相似文献
157.
Mahmoud Benkhalifa 《Journal of Pure and Applied Algebra》2007,210(2):343-362
The present paper is devoted to the classification problem of the quasi-isomorphism classes of free differential graded algebras (dgas) over a (P.I.D) R. We introduce the notion of coherent homomorphisms, perfect and quasi-perfect dgas (the Adams-Hilton model of simply connected CW-complex such that H∗(X,R) is free is a such a dga) and our first main result asserts that two perfect (quasi-perfect) dgas are quasi-isomorphic if and only if their Whitehead exact sequences are coherently isomorphic. Moreover we define the notion of a strong isomorphism between the Whitehead exact sequences and we show that two free R-dgas, of which their Whitehead exact sequences are strongly isomorphic, are quasi-isomorphic. 相似文献
158.
Semiconducting thin films of cadmium selenide have been grown by conventional thermal evaporation technique on glass substrate. Films evaporated at substrate temperature equal 523 K are stoichiometric and homogeneous. Effect of various growth parameters like rate of deposition and substrate temperature on the electrical properties has been studied in details. Also, the annealed at 673 K under vacuum for 1 h films have been analyzed for resistivity and Hall effect. 相似文献
159.
A series of experiments were carried out to study the degradation of methylene blue by the irradiation of ultrasound onto TiO(2) in aqueous solution. A statistically significant decrease in the concentration of methylene blue was observed after 60 min irradiation. While the reduction was 22% of the initial concentration without H(2)O(2), addition of H(2)O(2) significantly enhanced the degradation of methylene blue for the TiO(2) containing system (85% reduction of the initial concentration). The addition of H(2)O(2) had no effect on the methylene blue degradation when the system contained Al(2)O(3). The degradation ratio of methylene blue was dependent on the amount of TiO(2) and also the specific surface area of TiO(2) in the solution. The effects of radical scavenging agents on the degradation of methylene blue were also investigated for the system with TiO(2). It was found that the radical scavenging agents dimethyl sulfoxide (DMSO), methanol, and mannitol suppressed the degradation, with DMSO being the most effective. The effect of pH on the degradation of methylene blue was further investigated. An U-shaped change in the concentration of methylene blue in the presence of TiO(2) was observed along with the change in pH values (pH 3-12), and the highest degradation ratio was observed at around pH 7. In conclusion, ultrasound irradiation of TiO(2) in aqueous solution resulted in significant generation of hydroxyl radicals, and this process may have potential for the treatment of organic dyes in wastewater. 相似文献
160.
Maghsoodlou MT Habibi-Khorassani SM Heydari R Hassankhani A Marandi G Nassiri M Mosaddeg E 《Molecular diversity》2007,11(2):87-91
A new one-pot, simple and effective procedure is presented for the preparation of O-containing phosphorus ylides by the Michael
addition reaction of N-methylpyrrole-2-carbaldehydoxime, pyridin-2-carbaldehydoxime or acetophenonoxime with acetylenic esters. 相似文献