全文获取类型
收费全文 | 3018篇 |
免费 | 116篇 |
国内免费 | 21篇 |
专业分类
化学 | 2028篇 |
晶体学 | 20篇 |
力学 | 149篇 |
数学 | 527篇 |
物理学 | 431篇 |
出版年
2023年 | 24篇 |
2022年 | 37篇 |
2021年 | 45篇 |
2020年 | 51篇 |
2019年 | 79篇 |
2018年 | 44篇 |
2017年 | 28篇 |
2016年 | 88篇 |
2015年 | 97篇 |
2014年 | 111篇 |
2013年 | 156篇 |
2012年 | 219篇 |
2011年 | 249篇 |
2010年 | 159篇 |
2009年 | 134篇 |
2008年 | 172篇 |
2007年 | 188篇 |
2006年 | 190篇 |
2005年 | 188篇 |
2004年 | 177篇 |
2003年 | 115篇 |
2002年 | 116篇 |
2001年 | 44篇 |
2000年 | 41篇 |
1999年 | 24篇 |
1998年 | 28篇 |
1997年 | 27篇 |
1996年 | 27篇 |
1995年 | 25篇 |
1994年 | 11篇 |
1993年 | 23篇 |
1992年 | 12篇 |
1991年 | 8篇 |
1990年 | 5篇 |
1989年 | 11篇 |
1987年 | 5篇 |
1986年 | 6篇 |
1985年 | 11篇 |
1984年 | 19篇 |
1983年 | 13篇 |
1982年 | 17篇 |
1981年 | 12篇 |
1980年 | 21篇 |
1979年 | 9篇 |
1978年 | 9篇 |
1977年 | 9篇 |
1976年 | 11篇 |
1975年 | 8篇 |
1974年 | 9篇 |
1973年 | 10篇 |
排序方式: 共有3155条查询结果,搜索用时 312 毫秒
961.
Carvalho S Mom T Gilain L Avan P 《The Journal of the Acoustical Society of America》2004,116(3):1639-1648
Distortion product otoacoustic emissions (DPOAEs) are thought to stem from the outer hair cells (OHCs) around the normally narrow place tuned to the primary tone stimuli. They are thus said to be frequency-specific: their local absence should accurately pinpoint local OHC damage. Yet the influence of impaired tuning on DPOAE frequency specificity is poorly documented. Mice with local damage to OHCs were examined. Their DPOAEs were frequency-specific in that audiometric notches were accurately tracked. The same cochleae were further impaired by ischemia or furosemide injection inducing strial dysfunction with flat loss of sensitivity and tuning, while the preexisting pattern of damaged OHCs remained unaltered. Despite the loss of cochlear activity, DPOAEs produced by high-level (> or =70 dB SPL) primaries remained large in about the same interval where they had been initially normal, i.e., that with nondamaged OHCs, albeit with a slight frequency shift, of -1.1 kHz on average. Thus, the ability of DPOAEs to map structurally intact OHCs cannot be a mere consequence of cochlear tuning as it largely persists in its absence. The key element for this correct mapping is likely part of intact OHC structures (e.g., stereocilia bundles) and must have some tuning of its own. 相似文献
962.
In the framework of the independent-hot-spot model, it is shown that the reflectivity resulting from scattering instabilities when a spatially smoothed laser beam interacts with a plasma exhibits large statistical fluctuations near threshold. The importance of the fluctuations is discussed in terms of a confidence interval for the reflectivity, which is more relevant to experimental measurements than the average reflectivity. An analytical model for the fluctuating reflectivity is developed and shown to be in good agreement with numerical simulations. The influence of the transverse size of the interaction region is studied. 相似文献
963.
Cortez S Krebs O Laurent S Senes M Marie X Voisin P Ferreira R Bastard G Gérard JM Amand T 《Physical review letters》2002,89(20):207401
We show that the spin state of the resident electron in an n-doped self-assembled InAs-GaAs quantum dot can be written and read using nonresonant, circularly polarized optical pumping. A simple theoretical model is presented and accounts for the remarkable dynamics producing counterpolarized photoluminescence. 相似文献
964.
Functionalized indole and quinoline derivatives are conveniently prepared from nitrogen-tethered 2-methoxyphenols via phenyliodine(III) diacetate mediated oxidative acetoxylation, followed by a fluoride- or base-induced intramolecular nucleophilic addition reaction. This regioselective Michael-type addition step is further discussed in view of the rearrangement of orthoquinol acetate intermediates into paraquinol acetates that is sometimes observed in situ. Application of this methodology to the synthesis of a functionalized phenanthridine, and its potential for the construction of polyoxygenated lycorine-type alkaloid skeleta are here described. 相似文献
965.
966.
[reaction: see text] Easily available chiral allylsilanes were used as substrate for carboazidation reactions. For the first time, a substantial control of the stereochemistry of the azidation of acyclic nonconjugated radicals was achieved. 相似文献
967.
Pouységu L De Jéso B Lartigue JC Pétraud M Ratier M 《Chemical & pharmaceutical bulletin》2002,50(8):1114-1117
The application of the refocused-decoupled INEPT pulse sequence in 13C-NMR spectroscopy allows, in conjonction with computer simulations, a straightforward editing of the spectra of a raw mixture of glycosidic compounds. 相似文献
968.
L Lamarque P Navarro C Miranda V J Arán C Ochoa F Escartí E García-Espa?a J Latorre S V Luis J F Miravet 《Journal of the American Chemical Society》2001,123(43):10560-10570
The interaction with Cu2+ and dopamine of three polyazacyclophanes containing pyrazole fragments as spacers is described. Formation of mixed complexes Cu2+-macrocycle-dopamine has been studied by potentiometric methods in aqueous solution. The crystal structures of the complexes [Cu2(L1)(H2O)2](ClO4)4*2H2O (4) (L1 = 13,26-dibenzyl-3,6,9,12,13,16,19,22,25,26-decaazatricyclo[22.2.1.1(11,14)]octacosa-1(27),11,14(28),24-tetraene) and [Cu2(H-1L3)](HClO4)(ClO4)2*2H2O (6) (L3 = 1,4,7,8,11,14,17,20,21,24,29,32,33,36-tetradecaazapentacyclo[12.12.12.1(6,9).1(19,22).1(31,34)]hentetraconta-6,9(41),19(40),21,31,34(39)-hexaene) are presented. In the first one (4), each Cu2+ coordination site is made up by the three nitrogens of the polyamine bridge, a sp2 pyrazole nitrogen and one water molecule that occupies the axial position of a square pyramid. The distance between the copper ions is 6.788(2) A. In the crystal structure of 6, the coordination geometry around each Cu2+ is square pyramidal with its base being formed by two secondary nitrogens of the bridge and two nitrogen atoms of two different pyrazolate units which act as exobidentate ligands. The axial positions are occupied by the bridgehead nitrogen atoms; the elongation is more pronounced in one of the two sites [Cu(1)-N(1), 2.29(2) A; Cu(2)-N(6), 2.40(1) A]. The Cu-N distances involving the deprotonated pyrazole moieties are significantly shorter than those of the secondary nitrogens. The Cu(1)...Cu(2) distance is 3.960(3) A. The pyrazole in the noncoordinating bridge does not deprotonate and lies to one side of the macrocyclic cavity. One of the aliphatic nitrogens of this bridge is protonated and hydrogen bonded to a water molecule, which is further connected to the sp2 nitrogen of the pyrazole moiety through a hydrogen bond. The solution studies reveal a ready deprotonation of the pyrazole units induced by coordination to Cu2+. In the case of L2 (L2 = 3,6,9,12,13,16,19,22,25,26-decaazatricyclo[22.2.1.1(11,14)]octacosa-1(27),11,14(28),24-tetraene), deprotonation of both pyrazole subunits is already observed at pH ca. 4 for 2:1 Cu2+:L2 molar ratios. All three free receptors interact with dopamine in aqueous solution. L3 is a receptor particularly interesting with respect to the values of the interaction constants over five logarithmic units at neutral pH, which might suggest an encapsulation of dopamine in the macrocyclic cage. All three receptors form mixed complexes Cu2+-L-dopamine. The affinity for the formation of ternary dopamine complexes is particularly high in the case of the binuclear Cu2+ complexes of the 1-benzyl derivative L1. 相似文献
969.
We are interested in approximating the solution of a first-order quasi-linear equation associated with a forced unilateral obstacle condition. With this view, we make use of the time-splitting method developed classically to compute discontinuous solutions of nonhomogeneous scalar conservation laws. Here, one proves that this fractional step method converges in L1 to the weak entropy solution of the considered obstacle problem. In the case of the Cauchy problem, an L1-error bound in
is established. 相似文献
970.
Laurent Herr 《Mathematische Annalen》2001,320(2):307-337
Sans résumé
Received July 11, 1999 / Accepted September 25, 2000 / Published online February 5, 2001 相似文献