全文获取类型
收费全文 | 7974篇 |
免费 | 235篇 |
国内免费 | 43篇 |
专业分类
化学 | 5596篇 |
晶体学 | 54篇 |
力学 | 198篇 |
数学 | 1041篇 |
物理学 | 1363篇 |
出版年
2022年 | 46篇 |
2021年 | 57篇 |
2020年 | 97篇 |
2019年 | 93篇 |
2018年 | 66篇 |
2017年 | 54篇 |
2016年 | 167篇 |
2015年 | 148篇 |
2014年 | 155篇 |
2013年 | 344篇 |
2012年 | 454篇 |
2011年 | 506篇 |
2010年 | 248篇 |
2009年 | 221篇 |
2008年 | 464篇 |
2007年 | 499篇 |
2006年 | 525篇 |
2005年 | 461篇 |
2004年 | 405篇 |
2003年 | 344篇 |
2002年 | 316篇 |
2001年 | 137篇 |
2000年 | 89篇 |
1999年 | 65篇 |
1998年 | 90篇 |
1997年 | 97篇 |
1996年 | 109篇 |
1995年 | 95篇 |
1994年 | 107篇 |
1993年 | 148篇 |
1992年 | 94篇 |
1991年 | 90篇 |
1990年 | 98篇 |
1989年 | 86篇 |
1988年 | 67篇 |
1987年 | 59篇 |
1986年 | 56篇 |
1985年 | 104篇 |
1984年 | 79篇 |
1983年 | 64篇 |
1982年 | 86篇 |
1981年 | 86篇 |
1980年 | 70篇 |
1979年 | 75篇 |
1978年 | 71篇 |
1977年 | 54篇 |
1976年 | 67篇 |
1975年 | 51篇 |
1974年 | 50篇 |
1973年 | 51篇 |
排序方式: 共有8252条查询结果,搜索用时 15 毫秒
71.
Tong X Benz L Kemper P Metiu H Bowers MT Buratto SK 《Journal of the American Chemical Society》2005,127(39):13516-13518
Experiments in which mass-selected gold clusters were deposited on a surface have found that the catalytic properties depend strongly on cluster size. However, these experiments have not established definitively that the clusters maintain their size after deposition. We report here work in which we deposit low kinetic energy, mass-selected Aun+ (n = 1-8) clusters on a rutile TiO2(1 x 1) surface and use ultrahigh vacuum scanning tunneling microscopy (UHV-STM) to determine their size and shape. 相似文献
72.
73.
[reaction: see text] Pd nanoparticles (approximately 2 nm in size) microencapsulated in polyurea is an efficient and recyclable catalyst for reductive ring-opening hydrogenolysis of epoxides, using either HCOOH/Et(3)N or H(2) as a hydrogen donor. 相似文献
74.
Yu Yang Maya Belghazi Arnaud Lagadec David J. Miller Steven B. Hawthorne 《Journal of chromatography. A》1998,810(1-2):149-159
The intermolecular interactions between organic solutes and sorbent matrices under subcritical water conditions have been investigated at a pressure of 50 bar and temperatures ranging from 50 to 250°C. Both polar and nonpolar organics (chlorophenols, amines, n-alkanes, and polycyclic aromatic hydrocarbons) and five different sorbent matrices (glass beads, alumina, Florisil, silica-bonded C18, and polymeric XAD-4 resins) were used. From the same matrix, the polar solutes always eluted at lower temperatures, while the moderately polar and nonpolar solutes only eluted at higher temperatures. Similar to matrix effects previously observed using supercritical carbon dioxide, the sorbent type greatly influenced the elution efficiency under subcritical water conditions. Lower temperatures are sufficient to elute a particular solute from glass beads, alumina, and Florisil, but higher temperatures (less polar water) are needed to elute the same solute from silica-bonded C18. The highest temperatures were required to elute aromatic organics from XAD-4. These matrix effects demonstrate that, while low temperature water can break inert or dipole interactions between solutes and glass beads, alumina, and Florisil, higher temperature water is required to interrupt the van der Waals attractions between solutes and silica-bonded C18, and even higher temperatures needed to overcome the π-electron interactions between aromatic solutes and XAD-4. 相似文献
75.
Ronald D. Ley Lee A. Applegate R. Steven Padilla Thomas D. Stuart 《Photochemistry and photobiology》1989,50(1):1-5
Several lines of evidence support the hypothesis that ultraviolet radiation (UVR) is involved in the etiology of cutaneous melanoma in humans. However, progress in understanding the mechanisms involved in induction of melanotic tumors by UVR has been hindered by lack of a suitable animal model. During the course of multiple exposures (3 times/wk for 70 wk) of the South American opossum, Monodelphis domestica, to UVR, we first observed the appearance of areas of dermal melanocytic hyperplasia (MH) on the exposed skin. Post-UVR exposure to photoreactivating light (320-500 nm) suppressed the occurrence of MH. We also observed at 100 weeks from first exposure that 10 of 46 surviving animals had developed melanotic tumors which arose, presumably, from areas of MH. Tumors on three of the 10 animals have been classified as malignant melanomas based on metastasis to lymph nodes. We conclude from these results that UVR can act as a complete carcinogen for melanoma induction and, based on the photoreactivation of MH induction, that DNA damage is involved in melanoma formation. 相似文献
76.
Leatherman MD Svejda SA Johnson LK Brookhart M 《Journal of the American Chemical Society》2003,125(10):3068-3081
The synthesis of a series of (alpha-diimine)NiR(2) (R = Et, (n)Pr) complexes via Grignard alkylation of the corresponding (alpha-diimine)NiBr(2) precursors is presented. Protonation of these species by the oxonium acid [H(OEt(2))(2)](+)[BAr'(4)](-) at low temperatures yields cationic Ni(II) beta-agostic alkyl complexes which model relevant intermediates present in nickel-catalyzed olefin polymerization reactions. The highly dynamic nature of these agostic alkyl cations is quantitatively addressed using NMR line broadening techniques. Trapping of these complexes with ethylene provides cationic Ni alkyl ethylene species, which are used to determine rates of ethylene insertion into primary and secondary carbon centers. The Ni agostic alkyl cations are also trapped by CH(3)CN and Me(2)S to yield Ni(R)(L)(+) (L = CH(3)CN, Me(2)S) complexes, and the dynamic behavior of these species in the presence of varied [L] is discussed. The kinetic data obtained from these experiments are used to present an overall picture of the ethylene polymerization mechanism for (alpha-diimine)Ni catalysts, including effects of reaction temperature and ethylene pressure on catalyst activity, polyethylene branching, and polymer architecture. Detailed comparisons of these systems to the previously presented analogous palladium catalysts are made. 相似文献
77.
The chemical potential μ, as it appears in density functional theory, is examined extensively for harmonically interacting spin-½ fermions in three dimensions. For this system the energy and chemical potential are discontinuous functions of the particle number if the most straightforward equation is used to define the energy for a noninteger particle number. 相似文献
78.
Suspended trapping is used to eject electrons in negative-ion Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometric experiments. In contrast to electron ejection by resonant excitation of the trapping motion, suspended trapping involves allowing the electrons to escape along the z-axis (perpendicular to the trap plates) while the trapping potential is briefly removed. The duration of this event is sufficiently short (~10 μs) so that ion losses are negligible; the overall effect is that of a ‘high-pass mass filter’. Suspended trapping is simpler to implement and more generally applicable to various cell geometries than resonant electron ejection. The effectiveness of the suspended trapping technique is not compromised by the anharmonicity of the potential well in ‘elongated’ ICR traps, but depends simply on the time it takes the electrons to escape the cell. Finally, a small, positive offset potential (~+0.25 V) applied to the trap plates during the suspended trapping event increases the efficiency of the ejection. 相似文献
79.
[reaction: see text] Protein farnesyltransferase (PFTase) catalyzes alkylation of cysteine residues by farnesyl diphosphate (FPP). The dissociation constants for the PFTase-peptide analogue complexes for the series of analogues fl-RTRC(X)VIA (X = H, methyl, dodecyl, farnesyl) were measured by fluorescence anisotropy. The results indicate that an ionizable sulfhydryl moiety is important for substrate binding and the farnesyl group in the product facilitates binding. 相似文献
80.
Nicholas D. Adams Stanley J. Schmidt Steven D. Knight Dashyant Dhanak 《Tetrahedron letters》2007,48(23):3983-3986
A novel synthesis of 4H-pyrazolo-[3,4-d]pyrimidin-4-ones is described. This approach utilizes an in situ generated iminochloride as a key precursor for amidine formation, with subsequent base-catalyzed ring closure. This method represents a mild and efficient entry into this ring system which is amenable to diversification of the core template. 相似文献