全文获取类型
收费全文 | 37954篇 |
免费 | 2024篇 |
国内免费 | 857篇 |
专业分类
化学 | 25345篇 |
晶体学 | 373篇 |
力学 | 1417篇 |
综合类 | 52篇 |
数学 | 3796篇 |
物理学 | 9852篇 |
出版年
2023年 | 283篇 |
2022年 | 305篇 |
2021年 | 727篇 |
2020年 | 715篇 |
2019年 | 747篇 |
2018年 | 566篇 |
2017年 | 564篇 |
2016年 | 1093篇 |
2015年 | 1011篇 |
2014年 | 1232篇 |
2013年 | 2138篇 |
2012年 | 2744篇 |
2011年 | 3047篇 |
2010年 | 1884篇 |
2009年 | 1747篇 |
2008年 | 2417篇 |
2007年 | 2294篇 |
2006年 | 2129篇 |
2005年 | 1943篇 |
2004年 | 1736篇 |
2003年 | 1395篇 |
2002年 | 1333篇 |
2001年 | 982篇 |
2000年 | 830篇 |
1999年 | 532篇 |
1998年 | 402篇 |
1997年 | 420篇 |
1996年 | 495篇 |
1995年 | 379篇 |
1994年 | 414篇 |
1993年 | 393篇 |
1992年 | 375篇 |
1991年 | 297篇 |
1990年 | 260篇 |
1989年 | 221篇 |
1988年 | 223篇 |
1987年 | 175篇 |
1986年 | 160篇 |
1985年 | 256篇 |
1984年 | 176篇 |
1983年 | 160篇 |
1982年 | 168篇 |
1981年 | 132篇 |
1980年 | 126篇 |
1978年 | 112篇 |
1977年 | 122篇 |
1976年 | 130篇 |
1975年 | 134篇 |
1974年 | 127篇 |
1973年 | 145篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Micropatterning of a semicrystalline poly(vinylidene fluoride) (PVDF) solution was performed by a temperature controlled capillary micromolding where the rate of solvent evaporation was controlled by substrate temperature. In order to choose proper solvents for micropatterning, we have investigated the solubility of PVDF in various organic solvents and crystal structures of the PVDF bulk films cast from the solvents. The films prepared from the polar solvents such as dimethylformamide (DMF), dimethyl sulfoxide (DMSO) dominantly showed γ type crystals regardless of preparation temperature, while the films from tetrahydrofuran (THF) exhibit α type crystals and the ones from acetone and methyl ethyl ketone (MEK) show the characteristics of both α- and γ-PVDF. The quality of micropatterns and shapes of the PVDF crystals in the patterns significantly depend on solvent evaporation rates. Micropatterns of PVDF formed in DMF at 120 °C showed the best uniformity in shape. Crystals of the PVDF nucleated at the center regions of microchannels tended to be elongated with the b-axis of γ-PVDF crystals along the channels as the concentration of the solution decreased. In contrast, crystals nucleated at the corner regions of the channels had their b-axis oriented perpendicular to the channels. In line patterns with the width of 2 μm, the corner nucleated crystals were dominant and a resulting bamboo-like crystalline microstructure was observed in which the b-axis of γ-PVDF crystals, fast growth direction, is oriented normal to the microchannels. The crystal structures of the bulk films and the micropatterns were characterized by X-ray diffractometer, Fourier transform infrared spectroscope in Attenuated Total Reflection mode, Polarized Optical and Scanning Electron Microscope. 相似文献
992.
Jung-Hoon Oh Mun-Chual Rho Hyeong Kyu Lee Jae Nyoung Kim Sang-Hun Jung 《Tetrahedron letters》2004,45(38):7077-7079
Asymmetric synthesis of four possible stereoisomers of (9,10)-epoxyheptadecan-4,6-diyn-3-one was accomplished, and the absolute configuration of the naturally occurring (9R,10S)-epoxyheptadecan-4,6-diyn-3-one (1) was elucidated. 相似文献
993.
Yu-Ping Zhang Kwang-Pill Lee Hyun-Joo Noh Ali Md Showkat Seong-Ho Choi 《Microchemical Journal》2004,78(2):99-106
An appropriate optimization strategy should be used to find a desired resolution or selectivity with a minimum number of experiments in a limited time, which could assure the baseline separation of all target compounds. It was usually realized by means of a specialized computer program. In this paper, mapping optimization method and overlapping resolution mapping were compared for the optimization of a reversed-phase high-performance liquid chromatography (HPLC) isocratic separation of neutral compounds. The calculated resolutions and separation time of 7 to 10 experiments are fitted by different equations, which were used to build a contour plot with a minimum effective resolution and maximum retention time as a function of a mobile phase composition. The balance between resolution and analysis time could be easily realized by the overlapping of the final overlapping resolution mapping and analysis time mapping. The validity of the two methods was confirmed by some typical experiments. The models are simple, visual, and common without theoretical arithmetic. 相似文献
994.
The self-assembly of metallosupramolecules from reactions of flexible 2-pyridyl ligands and silver salts is described. When 1,3-bis(2-pyridyl)propane (L1), tris[(2-pyridyl)methyl]methane (L2), and 1,3-bis(2-pyridyl)-2-tolylpropane (L3) are used in combination with silver ions, novel discrete metallocyclic complexes are formed in crystals. Moreover, the self-assembly of 1,3-bis(2-pyridyl)-2-phenylpropane (L4) with silver nitrate yields a coordination polymer. The examination of its solution shows that this coordination polymer is formed via the solution-based discrete metallocyclic species. 相似文献
995.
Much improved catalytic carbon-carbon bond-forming reactions between aryl chlorides and Grignard reagents has been achieved using nickel(II) ion on nanoporous carbon. 相似文献
996.
Keeyung Lee 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1997,40(1):164-166
The electronic structure and magnetism of 13 atom clusters of ruthenium, rhodium and palladium having face centered cubic(fcc) geometry has been studied using a Gaussian orbital basis and the local spin density approximation. Calculations were done for the lattice spacings relevant to the bulk crystal lattice. Using the fixed moment states as input potentials, as many as 3 self-consistent states were obtained for these clusters. The 3 converged states of Rh13 cluster is found to have magnetic moments of 0.69 μB , 1.00 μB and 1.46 μB . Out of these states, 0.69 μB moment state is found to be the ground state. But the total energy difference between the 0.69 μB and 1.00 μB state is very small. The 1.46 μB moment state coincides with the state reported previously by other authors which was obtained using the discrete variational method. The experimentally observed moment was around 0.47 μB . Our calculated moment is closer to the experimentally observed moment than the previously reported moment, but is still a bit larger. Ru13 cluster is also found to have large moments, and 3 self-consistent states are also obtained for this cluster. The 3 magnetic moments of the Ru13 cluster are 0.46 μB , 0.62 μB and 1.08 μB . Out of these states, 0.62 μB moment state is found to be the ground state. For the Pd13 cluster, in addition to the nonmagnetic state previously reported, a state with magnetic moment of 0.46 μB is also found to exist indicating possible magnetism in cluster phase. 相似文献
997.
Novel 5-5-6 tricyclic compounds containing cyclopentenone have been constructed by the dicobalt octacarbonyl catalyzed tandem [2 + 2 + 1] and [2 + 2 + 2] cycloaddition reaction between a diyne and two phenylacethylene under CO pressure. 相似文献
998.
Lee LF Adronov A Schaller RD Fréchet JM Saykally RJ 《Journal of the American Chemical Society》2003,125(2):536-540
Near-field scanning optical microscopy (NSOM) has been used to investigate the photophysical characteristics of first- to fourth-generation (G1 to G4) light-harvesting dendrimer thin films containing coumarin-343 and coumarin-2 as the core and peripheral chromophores, respectively. Thin film photoluminescence (PL) spectra exhibit a significant red shift in the lower generations (G1, G2, and G3) as compared to their respective solution PL spectra, implying the formation of excimers. Spatially resolved PL NSOM images exhibit pronounced nanoscopic domains in G1, which become more homogeneous in higher generations due to site-isolation of the core chromophore. G4 exhibits complete site-isolation for these light-harvesting dendrimer films. 相似文献
999.
Jin Seok Choi Younjoo Lee Eunmyoung KimNakcheol Jeong Hosung YuHogyu Han 《Tetrahedron letters》2003,44(8):1607-1610
The development of the 2-(4-trifluoromethylphenylsulfonyl)ethoxycarbonyl (Tsc) function, a novel base-sensitive amino-protecting group, and its application to the preparation of DNA-binding polyamides are described. Pyrrole-imidazole polyamides were synthesized by an efficient solid-phase method under conditions compatible with Fmoc chemistry using two Tsc-protected amino acids, Tsc-Py-OH 1a and Tsc-Im-OH 1b. 相似文献
1000.