全文获取类型
收费全文 | 2519篇 |
免费 | 25篇 |
国内免费 | 12篇 |
专业分类
化学 | 1958篇 |
晶体学 | 6篇 |
力学 | 30篇 |
数学 | 250篇 |
物理学 | 312篇 |
出版年
2016年 | 21篇 |
2015年 | 18篇 |
2014年 | 22篇 |
2013年 | 40篇 |
2012年 | 52篇 |
2011年 | 64篇 |
2010年 | 32篇 |
2009年 | 34篇 |
2008年 | 49篇 |
2007年 | 50篇 |
2006年 | 72篇 |
2005年 | 67篇 |
2004年 | 56篇 |
2003年 | 43篇 |
2002年 | 46篇 |
2001年 | 30篇 |
2000年 | 45篇 |
1999年 | 27篇 |
1998年 | 18篇 |
1997年 | 28篇 |
1996年 | 34篇 |
1994年 | 38篇 |
1993年 | 42篇 |
1992年 | 54篇 |
1991年 | 33篇 |
1990年 | 28篇 |
1989年 | 30篇 |
1988年 | 43篇 |
1987年 | 65篇 |
1986年 | 39篇 |
1985年 | 49篇 |
1984年 | 52篇 |
1983年 | 39篇 |
1982年 | 49篇 |
1981年 | 43篇 |
1980年 | 41篇 |
1979年 | 45篇 |
1978年 | 46篇 |
1977年 | 55篇 |
1976年 | 58篇 |
1975年 | 36篇 |
1974年 | 43篇 |
1973年 | 29篇 |
1972年 | 30篇 |
1971年 | 23篇 |
1970年 | 31篇 |
1967年 | 22篇 |
1966年 | 17篇 |
1932年 | 16篇 |
1909年 | 24篇 |
排序方式: 共有2556条查询结果,搜索用时 15 毫秒
991.
Baggerman J Jagesar DC Vallée RA Hofkens J De Schryver FC Schelhase F Vögtle F Brouwer AM 《Chemistry (Weinheim an der Bergstrasse, Germany)》2007,13(4):1291-1299
[2]- and [3]-rotaxanes with a tetraphenoxy perylene diimide core were synthesized. Hydrogen bonding between the wheel and the imide changes the optical properties of the perylene chromophore: the absorption and fluorescence spectra are red-shifted. The decay times of the rotaxanes are shorter in comparison with that of the axle. Single molecule fluorescence measurements reveal relatively narrow distributions of emission maxima and decay times. The averages are in agreement with ensemble measurements. The observed red shifts make the perylene diimide a suitable chromophore for sensing the position of the wheel on the axle. 相似文献
992.
Puntoriero F Ceroni P Balzani V Bergamini G Vögtle F 《Journal of the American Chemical Society》2007,129(35):10714-10719
We have studied the adducts formed by eosin (E) with a fourth generation dendrimer (D) that comprises 30 tertiary amine units in the interior and 32 naphthyl and 32 trans azobenzene units in the periphery. We have found that: (i) the all trans dendrimer D(32t) can be converted by irradiation with 365 nm light (Phi=0.12) into species containing, as an average, 4 trans and 28 cis azobenzene units, D(4t28c), that at 313 K undergoes a D(4t28c) --> D(32t) thermal back reaction (k = 7.0 x 10(-5) s(-1)); (ii) D(32t) and D(4t28c) extract 8 and, respectively, 6 eosin molecules from water at pH 7, yielding the species D(32t) subset 8E and D(4t28c) subset 6E; (iii) eosin uptake is significantly faster for D(32t) than for D(4t28c); (iv) irradiation at 365 nm of the D(32t) subset 8E species at 298 K leads to the release of two eosin molecules with formation of a photostable D(15t17c) subset 6E species (Phi = 0.15) that is also obtained from the back thermal reaction of D(4t28c) subset6E at 313 K (k = 2.7 x 10(-5) s(-1)); (v) thermal release of E from D(32t) subset 6E is much faster than from D(4t28c) subset 6E; and (vi) excitation of E in the adducts sensitizes the cis --> trans (but not the trans --> cis) isomerization. The results obtained show that the isomerization of the 32 peripheral azobenzene units controls to some extent the hosting capacity of the dendrimer and, viceversa, eosin molecules hosted in the dendrimer affect the isomerization process of its azobenzene units. 相似文献
993.
Vicinelli V Bergamini G Ceroni P Balzani V Vögtle F Lukin O 《The journal of physical chemistry. B》2007,111(24):6620-6627
We have investigated the fluorescence properties of dendrimers (Gn is the dendrimer generation number) containing four different luminophores, namely terphenyl (T), dansyl (D), stilbenyl (S), and eosin (E). In the case of T, the dendrimers contain a single p-terphenyl fluorescent unit as a core with appended sulfonimide branches of different size and n-octyl chains. In the cases of D and S, multiple fluorescent units are appended in the periphery of poly(propylene amine) dendritic structures. In the case of E, the investigated luminophore is noncovalently linked to the dendritic scaffold, but is encapsulated in cavities of a low luminescent dendrimer. Depending on the photophysical properties of the fluorescent units and the structures of the dendrimers, different mechanisms of fluorescence depolarization have been observed: (i) global rotation for GnT dendrimers; (ii) global rotation and local motions of the dansyl units at the periphery of GnD dendrimers; (iii) energy migration among stylbenyl units in G2S; and (iv) restricted motion when E is encapsulated inside a dendrimer, coupled to energy migration if the dendrimer hosts more than one eosin molecule. 相似文献
994.
995.
996.
Fritz C Cross I Moore BC Woodhouse J 《The Journal of the Acoustical Society of America》2007,122(6):3640-3650
This study is the first step in the psychoacoustic exploration of perceptual differences between the sounds of different violins. A method was used which enabled the same performance to be replayed on different "virtual violins," so that the relationships between acoustical characteristics of violins and perceived qualities could be explored. Recordings of real performances were made using a bridge-mounted force transducer, giving an accurate representation of the signal from the violin string. These were then played through filters corresponding to the admittance curves of different violins. Initially, limits of listener performance in detecting changes in acoustical characteristics were characterized. These consisted of shifts in frequency or increases in amplitude of single modes or frequency bands that have been proposed previously to be significant in the perception of violin sound quality. Thresholds were significantly lower for musically trained than for nontrained subjects but were not significantly affected by the violin used as a baseline. Thresholds for the musicians typically ranged from 3 to 6 dB for amplitude changes and 1.5%-20% for frequency changes. Interpretation of the results using excitation patterns showed that thresholds for the best subjects were quite well predicted by a multichannel model based on optimal processing. 相似文献
997.
Xiang-Zhao Kong Markus Holzner Fritz Stauffer Wolfgang Kinzelbach 《Experiments in fluids》2011,50(6):1659-1670
The main goal of this work is to implement and validate a visualization method with a given temporal/spatial resolution to
obtain the dynamic three-dimensional (3D) structure of an air plume injected into a deformable liquid-saturated porous medium.
The air plume develops via continuous air injection through an orifice at the bottom of a loose packing of crushed silica
grains. The packing is saturated by a glycerin-water solution having the same refractive index and placed in a rectangular
glass container. By using high-speed image acquisition through laser scanning, the dynamic air plume is recorded by sequential
tomographic imaging. Due to the overlap between adjacent laser sheets and the light reflection, air bubbles are multiply exposed
in the imaging along the scanning direction. Four image processing methods are presented for the removal of these redundant
pixels arising from multiple exposure. The respective results are discussed by comparing the reconstructed air plume volume
with the injected one and by evaluating the morphological consistency of the obtained air plume. After processing, a 3D dynamic
air flow pattern can be obtained, allowing a quantitative analysis of the air flow dynamics on pore-scale. In the present
experimental configuration, the temporal resolution is 0.1 s and the spatial resolution is 0.17 mm in plane and about 1 mm
out of plane of the laser sheet. 相似文献
998.
Riederer SK Bechlars B Herrmann WA Kühn FE 《Dalton transactions (Cambridge, England : 2003)》2011,40(1):41-43
A facile and straightforward synthetic procedure for the preparation of new chiral bis-1,2,4-triazolium salts and their corresponding rhodium(I) biscarbene complexes are reported. It is widely applicable for the synthesis of 1,2,4-triazolium salts. The new chiral biscarbenes represent promising ligands for transition metal-catalyzed asymmetric syntheses. Their first successful application is demonstrated by the rhodium-catalyzed hydrogenation of dimethylitaconate and methyl-2-acetamidoacrylate which yielded enantioselectivities of up to 61% ee. 相似文献
999.
1000.
Delay M Dolt T Woellhaf A Sembritzki R Frimmel FH 《Journal of chromatography. A》2011,1218(27):4206-4212
The rapid development of nanotechnology and the related production and application of nanosized materials such as engineered nanoparticles (ENP) inevitably lead to the emission of these products into environmental systems. So far, little is known about the occurrence and the behaviour of ENP in environmental aquatic systems. In this contribution, the influence of natural organic matter (NOM) and ionic strength on the stability and the interactions of silver nanoparticles (n-Ag) in aqueous suspensions was investigated using UV–vis spectroscopy and asymmetrical flow field-flow fractionation (AF4) coupled with UV–vis detection and mass spectrometry (ICP-MS). n-Ag particles were synthesized by chemical reduction of AgNO3 with NaBH4 in the liquid phase at different NOM concentrations. It could be observed that the destabilization effect of increasing ionic strength on n-Ag suspensions was significantly decreased in the presence of NOM, leading to a more stable n-Ag particle suspension. The results indicate that this behaviour is due to the adsorption of NOM molecules onto the surface of n-Ag particles (“coating”) and the resulting steric stabilization of the particle suspension. The application of AF4 coupled with highly sensitive detectors turned out to be a powerful method to follow the aggregation of n-Ag particle suspensions at different physical–chemical conditions and to get meaningful information on their chemical composition and particle size distributions. The method described will also open the door to obtain reliable data on the occurrence and the behaviour of other ENP in environmental aquatic systems. 相似文献