全文获取类型
收费全文 | 717篇 |
免费 | 23篇 |
国内免费 | 2篇 |
专业分类
化学 | 457篇 |
晶体学 | 2篇 |
力学 | 19篇 |
数学 | 98篇 |
物理学 | 166篇 |
出版年
2023年 | 5篇 |
2022年 | 8篇 |
2021年 | 20篇 |
2020年 | 10篇 |
2019年 | 8篇 |
2018年 | 8篇 |
2017年 | 12篇 |
2016年 | 23篇 |
2015年 | 19篇 |
2014年 | 23篇 |
2013年 | 40篇 |
2012年 | 39篇 |
2011年 | 50篇 |
2010年 | 30篇 |
2009年 | 20篇 |
2008年 | 48篇 |
2007年 | 50篇 |
2006年 | 44篇 |
2005年 | 52篇 |
2004年 | 30篇 |
2003年 | 20篇 |
2002年 | 25篇 |
2001年 | 9篇 |
2000年 | 8篇 |
1999年 | 6篇 |
1998年 | 5篇 |
1997年 | 8篇 |
1996年 | 8篇 |
1995年 | 4篇 |
1994年 | 8篇 |
1993年 | 7篇 |
1992年 | 9篇 |
1991年 | 6篇 |
1990年 | 5篇 |
1989年 | 6篇 |
1988年 | 7篇 |
1987年 | 6篇 |
1986年 | 2篇 |
1985年 | 8篇 |
1984年 | 6篇 |
1983年 | 3篇 |
1982年 | 6篇 |
1981年 | 5篇 |
1980年 | 6篇 |
1977年 | 3篇 |
1974年 | 2篇 |
1973年 | 3篇 |
1972年 | 2篇 |
1897年 | 1篇 |
1880年 | 1篇 |
排序方式: 共有742条查询结果,搜索用时 31 毫秒
731.
The relative energies of azaphosphiridine and its isomers, the ring stability towards valence isomerization, and the ring strain, as well as the kinetics and thermodynamics of possible ring‐opening reactions of PIII derivatives ( 1 – 5 ) and PV chalcogenides ( 6 – 9 ; O to Te), were studied at high levels of theory (up to CCSD(T)). The barrier to inversion at the nitrogen atom in the trimethyl‐substituted PIII derivative 5 increases from 12.11 to 15.25 kcal mol?1 in the P‐oxide derivative 6 (PV); the relatively high barrier to inversion at the phosphorus in 5 (75.38 kcal mol?1) points to a configurationally stable center (MP2/def2‐TZVPP//BP86/def2‐TZVP). The ring strain for azaphosphiridine 5 (av. 22.6 kcal mol?1) was found to increase upon P‐oxidation ( 6 ) (30.8 kcal mol?1; same level of theory). Various ring‐bond‐activation processes were studied: N‐protonation of PIII ( 5 ) and PV ( 6 , 7 ) derivatives leads to highly activated species that readily undergo P? N bond cleavage. In contrast, metal chlorides such as LiCl, CuCl, CuCl2, BeCl2, BCl3, AlCl3, TiCl3, and TiCl4 show little P? N bond activation in 5 and 7 . Remarkably, TiCl3 selectively activates the C? N bond, and induces stronger bond activation for PV ( 6, 7 ) than for PIII azaphosphiridines ( 5 ). The ring‐expanding rearrangement of PV azaphosphiridines 6 – 9 to yield PIII 1,3,2‐chalcogena‐azaphosphetidines 32 a – d is predicted to be preferred for the heavier chalcogenides 7 – 9 , but not for the P‐oxide 6 . The first comparative analysis of three bond strength parameters is presented: 1) the electron density at bond critical points, 2) Wiberg’s bond index, and 3) the relaxed force constant. This reveals the usefulness of these parameters in assessing the degree of ring bond activation. 相似文献
732.
The synthesis and electrochemical, optical, and cation-sensing properties of the ferrocene-triazole-pyridine triads 3 and 5 are presented. Azidoferrocene 1 and 1,1'-diazidoferrocene 4 underwent the "click" reaction with 2-ethynylpyridine to give the triads 3 and 5 in 81% and 68% yield, respectively. Electrochemical studies carried out in CH(3)CN in the presence of increasing amounts of Zn(2+), Ni(2+), Cd(2+), Hg(2+), and Pb(2+) metal cations, showed that the wave corresponding to the ferrocene/ferrocenium redox couple is anodically shifted by 70-130 mV for triad 3 and 167-214 mV for triad 5. The maximum shift of the ferrocene oxidation wave was found for 5 in the presence of Zn(2+). In addition, the low-energy band of the absorption spectra of 3 and 5 are red-shifted (Δλ = 5-10 nm) upon complexation with these metal cations. The crystal structures of compounds 3 and 5 and the complex [3(2)·Zn](2+) have been determined by single-crystal X-ray methods. (1)H NMR studies as well as density functional theory calculations have been carried out to get information about the binding sites that are involved in the complexation process. 相似文献
733.
N. Rubido C. Cabeza S. Kahan G.M. Ramírez Ávila Arturo C. Marti 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2011,62(1):51-56
Stable synchronous states of different order were analytically, numerically and
experimentally characterized in pulse-coupled light-controlled oscillators (LCOs). The
Master-Slave (MS) configuration was studied in conditions where different time-scale
parameters were tuned under varying coupling strength. Arnold tongues calculated
analytically – based on the piecewise two-time-scale model for LCOs – and obtained
numerically were consistent with experimental results. The analysis of the stability
pattern and tongue shape for (1 : n) synchronization was based on the
construction of return maps representing the Slave LCO evolution induced by the action of
the Master LCO. The analysis of these maps showed that both tongue shape and stability
pattern remained invariant. Considering the wide variation range of LCO parameters, the
obtained results could have further applications on ethological models. 相似文献
734.
Experimental measurements of the latency of transient evoked otoacoustic emission and auditory brainstem responses are compared, to discriminate between different cochlear models for the backward acoustic propagation of otoacoustic emissions. In most transmission-line cochlear models otoacoustic emissions propagate towards the base as a slow transverse traveling wave, whereas other models assume fast backward propagation via longitudinal compression waves in the fluid. Recently, sensitive measurements of the basilar membrane motion have cast serious doubts on the existence of slow backward traveling waves associated with distortion product otoacoustic emissions [He et al., Hear. Res. 228, 112-122 (2007)]. On the other hand, recent analyses of "Allen-Fahey" experiments suggest instead that the slow mechanism transports most of the otoacoustic energy [Shera et al., J. Acoust. Soc. Am. 122, 1564-1575 (2007)]. The two models can also be discriminated by comparing accurate estimates of the otoacoustic emission latency and of the auditory brainstem response latency. In this study, this comparison is done using human data, partly original, and partly from the literature. The results are inconsistent with fast otoacoustic propagation, and suggest that slow traveling waves on the basilar membrane are indeed the main mechanism for the backward propagation of the otoacoustic energy. 相似文献
735.
S. V. Gasilov A. Ya. Faenov T. A. Pikuz I. Yu. Skobelev F. Calegari C. Vozzi M. Nisoli G. Sansone G. Valentini S. De Silvestri S. Stagira 《JETP Letters》2008,87(5):238-242
The possibility of phase-contrast imaging of nanostructures has been analyzed with the use of a femtosecond-laser plasma as a spatially coherent soft x-ray source and a LiF crystal as an x-ray detector having both the submicron spatial resolution in a wide field of view and a high contrast. It is demonstrated that the spatial coherence length of radiation in the wavelength range 1–13 nm at a distance of 30 cm from the femtosecond-laser plasma source is ?1.5 μm. The achieved spatial coherence of the source is sufficient to obtain high-quality phase-contrast x-ray images of foils with various chemical compositions and a thickness of ?100 nm. 相似文献
736.
Calegari F Vozzi C Gasilov S Benedetti E Sansone G Nisoli M De Silvestri S Stagira S 《Physical review letters》2008,100(12):123006
The spatiotemporal effects generated in the wake of a laser filament propagating in nitrogen are investigated. At suitable time delays, a probe light pulse propagating along the wake experiences a strong spatial confinement and a noticeable spectral broadening at the same time. Numerical simulations, well reproducing the experimental findings, show the key role of the impulsive rotational Raman response in the observed phenomena. 相似文献
737.
An experimental demonstration of the mechanical transfer of orbital angular momentum to matter from acoustical vortices in free field is presented. Vortices with topological charges l=+/-1 and l=+/-2 were generated and a torsion pendulum was used to study the angular momentum transfer to hanging disks of several sizes. This allowed us to make a comparative study of the effective acoustical torque in terms of topological charge of the vortex, the disk radius, and its position along the main propagation axis. A theoretical discussion of the generated sound fields is also provided. 相似文献
738.
The electrochemical synthesis of poly(phenylmethylsilane) was carried out in a divided cell using a polymeric anion-exchange membrane. The solution contained phenylmethydichlorosilane (PhMeSiCl2), mixed solvent tetrahydrofuran + hexamethylphosphoramide, with tetrabutylammonium perchlorate as support electrolyte. The electrodes were stainless steel as the cathode, stainless steel as the sacrificial anode or platinum as the resistant anode. Poly(phenylmethylsilane) was obtained in yields from 1.5 to 4.5 % from phenylmethyldichlorosilane concentrations equal or higher than 1 M. The number-average molecular mass Mn of poly(phenylmethylsilane) was in the range from 5600 to 9500. A monomodal molecular weight distribution was obtained with a polydipersity Index of 2. The spectra of 1H, 13C, 29Si MMR and IR were determined. From the 29Si NMR analysis, the poly(phenylmethylsilane) showed a clear difference in the tacticity related to the chemical synthesis of the same polymer. The UV spectra showed a strong absorption in a 331 to 335 nm characteristic for the poly(phenylmethylsilane). 相似文献
739.
Senen Paz-Abuin Arturo Lopez-Quintela Mercedes Pazos Pellin Montserrat Varela Pilar Prendes 《Journal of polymer science. Part A, Polymer chemistry》1998,36(6):1001-1016
The epoxy-diamine cure process was studied. We found that the mechanism can be described in three steps: (i) initiation up to 20–25% of conversion. (ii) autoacceleration and (iii) inhibition. It has been observed that after the initiation there is a clear autoacceleration effect which has been explained in terms of free volume, considerations: the volume occupied by the products of reaction diminishes the available volume increasing the “local concentration” of the reactants and therefore the rate of reaction. The reaction was followed by FTIR (near infrared) as the main technique. We used a purified epoxy resin as well as phenyl glycidyl ether cured with m-xylenediamine. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1001–1016, 1998 相似文献
740.