全文获取类型
收费全文 | 3961篇 |
免费 | 64篇 |
国内免费 | 56篇 |
专业分类
化学 | 2759篇 |
晶体学 | 4篇 |
力学 | 15篇 |
综合类 | 7篇 |
数学 | 10篇 |
物理学 | 1006篇 |
无线电 | 280篇 |
出版年
2023年 | 2篇 |
2022年 | 2篇 |
2021年 | 6篇 |
2020年 | 2篇 |
2019年 | 9篇 |
2018年 | 5篇 |
2017年 | 4篇 |
2016年 | 9篇 |
2014年 | 6篇 |
2013年 | 9篇 |
2012年 | 360篇 |
2011年 | 465篇 |
2010年 | 86篇 |
2009年 | 35篇 |
2008年 | 375篇 |
2007年 | 392篇 |
2006年 | 366篇 |
2005年 | 351篇 |
2004年 | 282篇 |
2003年 | 199篇 |
2002年 | 178篇 |
2001年 | 132篇 |
2000年 | 148篇 |
1999年 | 52篇 |
1998年 | 22篇 |
1997年 | 27篇 |
1996年 | 70篇 |
1995年 | 60篇 |
1994年 | 44篇 |
1993年 | 80篇 |
1992年 | 73篇 |
1991年 | 40篇 |
1990年 | 38篇 |
1989年 | 26篇 |
1988年 | 23篇 |
1987年 | 17篇 |
1986年 | 19篇 |
1985年 | 16篇 |
1984年 | 3篇 |
1983年 | 5篇 |
1982年 | 11篇 |
1981年 | 7篇 |
1980年 | 3篇 |
1979年 | 4篇 |
1976年 | 2篇 |
1968年 | 1篇 |
1966年 | 2篇 |
1958年 | 5篇 |
1957年 | 2篇 |
1948年 | 1篇 |
排序方式: 共有4081条查询结果,搜索用时 15 毫秒
991.
King JB Haneline MR Tsunoda M Gabbaï FP 《Journal of the American Chemical Society》2002,124(32):9350-9351
In the crystalline state, the trinuclear mercury(II) complex [(o-C(6)F(4)Hg)(3)*mu(3)-acetone] forms cofacial dimers which are held by two mercurophilic interactions of 3.51 A. In this form, the complex displays an intense photoluminescence with a sharp emission centered at 480 nm. 相似文献
992.
We report what we believe to be the first applications of numerical optimization algorithms to the design of diffractive elements that customize the fundamental mode profile of a laser system. Standard design techniques treat these elements as specific phase-conjugation devices, which leads to performance loss when they are quantized to permit fabrication. Numerical optimization can account for quantization of the element to increase the effective performance. Also, it is shown that allowing a slight increase in the intrinsic loss of the cavity can substantially increase the fidelity of the fundamental mode of the customized cavity. The good discrimination qualities of the mode-selection elements are shown to be unaffected by this process. 相似文献
993.
994.
995.
Vos MR Jardĺ GE Pallas AL Breurken M van Asselen OL Bomans PH Leclère PE Frederik PM Nolte RJ Sommerdijk NA 《Journal of the American Chemical Society》2005,127(48):16768-16769
Here we present a surfactant molecule (1) containing an ammonium headgroup, in which a bis-ureido group is incorporated in its hydrocarbon chain. Due to strong hydrogen bonding interactions, 1 forms well-defined highly ordered ribbon-like aggregates in water. Moreover, we demonstrate that these ribbons can be functionalized via a modular approach through molecular recognition of other bis-urea containing molecules. The dye disperse orange and biotin were coupled to matching bis-ureido groups and incorporated into the ribbon structure. The anchoring of different functionalities in a modular approach proved to be possible using the molecular recognition capabilities of the bis-ureido moiety, thereby opening possibilities to a wide range of applications. 相似文献
996.
The effect of substitution at the 4 and 3,5 positions in the pyridine ring of europium(III) pyridine-2,6-dicarboxylate complexes has been investigated with particular emphasis on sensitization of the Eu3+ ion. Sensitization of the Eu3+ 615-nm emission was achieved through excitation of the ligands in which the 4 substituent was -H, -OH, and -Cl and the 3,5 position was -H. In these cases, the ligand-to-Eu3+ ratio was confirmed as being 3:1. The sensitization was found to increase following substitution of the 4 position in the order Cl > H > OH. This is attributed to energy transfer occurring from the ligands into different Eu3+ intra-atomic energy levels, with spin selection rules governing the efficiency of this process. The Eu3+ luminescence lifetime was measured and found to vary from 1.16 to 2.90 ms depending on the excitation energy, ligand, and solvent. For the case of the 3,5-dibromo-4-hydroxy derivative, no sensitization was observed and a ligand-to-Eu3+ ratio of 1:1 was found. The solubility of these complexes in water and their long emission lifetime make them attractive for use as probes in biological systems. 相似文献
997.
998.
Electrochromism has been demonstrated by three novel electrochemically-reversible compounds and, which were synthesised by Pd catalysed cross-coupling of iodotrimethylTTF (6) and 2,6(7)-diiodoTTF (7) with the 2,5-diaryl-1,3,4-oxadiazole derivatives 1 and 2 containing terminal ethyne and butadiyne groups. 相似文献
999.
Ford SJ Delamore OJ Evans JS McIntyre GJ Johnson MR Radosavljević Evans I 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(52):14942-14951
Deuterated 3,5‐pyridinedicarboxylic acid exhibits reversible temperature‐induced deuteron migration of a magnitude unprecedented in this class of compounds. We used a combination of variable‐temperature powder and single‐crystal neutron diffraction and density functional theory (DFT)‐based computational methods to elucidate the origin of this remarkable behaviour. Single‐crystal neutron diffraction shows that between 15 and 300 K, the deuteron moves by 0.32(1) Å and the structure changes from a low‐temperature N? D???O form to a high‐temperature N???D? O form. Variable‐temperature powder neutron‐diffraction data, which was fitted by using parametric Rietveld refinement, show that this deuteron migration is due to an isosymmetric, first‐order phase transition that occurs by growth of the daughter phase in the parent‐phase matrix. Similar phase transitions are observed in two selectively deuterated forms of the material. DFT calculations demonstrate the role of phonons and show that vibrational free‐energy stabilisation, which plays a key role in the observed structural phase transitions, is more pronounced in the fully deuterated material and proportional to the mass of the molecule, that is, the level of deuteration. This is consistent with our experimental work, for which distinct crystallographic phase transitions were clearly observed for the three deuterated systems, but not for the fully protonated material. 相似文献
1000.
Batani D Baton SD Manclossi M Santos JJ Amiranoff F Koenig M Martinolli E Antonicci A Rousseaux C Le Gloahec MR Hall T Malka V Cowan TE King J Freeman RR Key M Stephens R 《Physical review letters》2005,94(5):055004
We study the propagation of fast electrons in a gas at different densities. A large relativistic electron current is produced by focusing a short-pulse ultrahigh-intensity laser on a metallic target. It then propagates in a gas jet placed behind the foil. Shadowgraphy in the gas shows an electron cloud moving at sub-relativistic average velocities. The experiment shows (i) the essential role of the density of background material for allowing propagation of fast electrons, (ii) the importance of the ionization phase which produces free electrons available for the return current, and (iii) the effect of electrostatic fields on fast-electron propagation. 相似文献