全文获取类型
收费全文 | 2137篇 |
免费 | 91篇 |
国内免费 | 22篇 |
专业分类
化学 | 1383篇 |
晶体学 | 14篇 |
力学 | 78篇 |
数学 | 424篇 |
物理学 | 351篇 |
出版年
2023年 | 19篇 |
2022年 | 33篇 |
2021年 | 46篇 |
2020年 | 53篇 |
2019年 | 58篇 |
2018年 | 57篇 |
2017年 | 40篇 |
2016年 | 77篇 |
2015年 | 77篇 |
2014年 | 93篇 |
2013年 | 143篇 |
2012年 | 169篇 |
2011年 | 235篇 |
2010年 | 114篇 |
2009年 | 100篇 |
2008年 | 152篇 |
2007年 | 126篇 |
2006年 | 131篇 |
2005年 | 105篇 |
2004年 | 92篇 |
2003年 | 76篇 |
2002年 | 53篇 |
2001年 | 27篇 |
2000年 | 17篇 |
1999年 | 10篇 |
1998年 | 13篇 |
1997年 | 14篇 |
1996年 | 16篇 |
1995年 | 7篇 |
1994年 | 6篇 |
1993年 | 8篇 |
1992年 | 8篇 |
1991年 | 3篇 |
1990年 | 2篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1985年 | 8篇 |
1984年 | 2篇 |
1983年 | 3篇 |
1982年 | 3篇 |
1981年 | 2篇 |
1978年 | 7篇 |
1977年 | 2篇 |
1976年 | 4篇 |
1973年 | 3篇 |
1961年 | 3篇 |
1942年 | 6篇 |
1940年 | 3篇 |
1929年 | 1篇 |
1926年 | 1篇 |
排序方式: 共有2250条查询结果,搜索用时 15 毫秒
971.
We show that a separable Banach space with property (M*) has a Szlenk index equal to ω0, and a norm with an optimal modulus of asymptotic uniform smoothness. From this we derive a condition on the Szlenk functions
of the space and its dual which characterizes embeddability into c
0 or an ℓ
p
-sum of finite dimensional spaces. We also prove that two Lipschitz-isomorphic Orlicz sequence spaces contain the same ℓ
p
-spaces.
相似文献
972.
973.
Lampkins AJ Li Y Al Abbas A Abboud KA Ghiviriga I Castellano RK 《Chemistry (Weinheim an der Bergstrasse, Germany)》2008,14(5):1452-1463
Reported are the syntheses of ester-functionalized (6-8) and alkyl-substituted (9) 1-aza-adamantanones; the easy handling of the compounds provides an opportunity to comprehensively study the fundamental changes in structure and reactivity that can accompany the donor-acceptor arrangement in rigid beta-aminoketones. X-ray structural analysis of trione 6 and dione 7 reveals bond length and angle variations consistent with through-bond (hyperconjugative) donor-acceptor interactions. Observed is a shortening of the C-N bond, elongation of the central C-C bond (to approximately 1.6 A), and a significant pyramidalization of the carbonyl carbon within the donor-sigma-acceptor pathway. UV/Vis spectra of 6-9 show a new absorption maximum (lambda(max)=260-275 nm in three solvents), the so-called "sigma-coupled transition"; the molar absorptivity scales with the number of carbonyl groups (for trione 6, epsilon approximately 3000, for dione 7, epsilon approximately 2000) and the band reversibly disappears upon addition of acid. IR and (13)C NMR spectroscopic data show trends consistent with through-bond donation to the carbonyl acceptor groups and commensurate weakening of the carbonyl pi bond. High yielding acid-mediated fragmentations are used to illustrate the effects of the donor-acceptor arrangement on the reactivity of the molecules. Given that donor-sigma-acceptor molecules have recently been found to show self-assembly behavior and macromolecular properties linked to their unusual structure, the current analysis encourages further consideration of the systems in advanced materials applications. 相似文献
974.
MacQuarrie S Thompson MP Blanc A Mosey NJ Lemieux RP Crudden CM 《Journal of the American Chemical Society》2008,130(43):14099-14101
Chiral mesoporous materials have been prepared with purely organic monomers employing an enantiomerically pure biphenyl bis-siloxane as a dopant along with simple 4,4'-bis(triethoxysilyl)biphenyl as the bulk material. Analysis of these materials by CD spectroscopy indicates the formation of new chiral aggregates in the solid state consistent with charge delocalization from adjacent aryl rings in close contact. These aggregates are only observed when the biphenyl and enantiomerically pure biaryl are both present. The red-shift resulting of the CD signal resulting from these interactions is modeled computationally using time-dependent DFT calculations. 相似文献
975.
Samira Benali Sophie Peeterbroeck Fabien Monteverde Michaël Alexandre Philippe Dubois 《European Polymer Journal》2008,44(6):1673-1685
Exfoliated nanocomposites were prepared by dispersion of poly(ε-caprolactone) (PCL) grafted montmorillonite nanohybrids used as masterbatches in chlorinated polyethylene (CPE). The PCL-grafted clay nanohybrids with high inorganic content were synthesized by in situ intercalative polymerization of ε-caprolactone between silicate layers organo-modified by alkylammonium cations bearing two hydroxyl functions. The polymerization was initiated by tin alcoholate species derived from the exchange reaction of tin(II) bis(2-ethylhexanoate) with the hydroxyl groups borne by the ammonium cations that organomodified the clay. These highly filled PCL nanocomposites (25 wt% in inorganics) were dispersed as masterbatches in commercial chlorinated polyethylene by melt blending. CPE-based nanocomposites containing 3-5 wt% of inorganics have been prepared. The formation of exfoliated nanocomposites was assessed both by wide-angle X-ray diffraction and transmission electron microscopy. The thermal and thermo-mechanical properties were studied as a function of the filler content, by differential scanning calorimetry and dynamic mechanical analysis, respectively. The mechanical properties were also assessed by tensile tests. The Young’s modulus of CPE is increased by a decade when a PCL-grafted clay masterbatch is exfoliated to reach 5 wt% of clay in the resulting nanocomposite. The influence of PCL-grafting on the properties of these nanocomposites was investigated by comparison with materials obtained with ungrafted-PCL. 相似文献
976.
Montiel-Palma V Piechaczyk O Picot A Auffrant A Vendier L Le Floch P Sabo-Etienne S 《Inorganic chemistry》2008,47(19):8601-8603
The phosphinosilane compound PPh 2CH 2OSiMe 2H is potentially a bifunctional P approximately Si-H ligand. By treatment with the Ru (II) precursor RuH 2(H 2) 2(PCy 3) 2, the complex Ru(PPh 2CH 2OSiMe 2H) 3 ( 2), resulting from the coordination of three ligands and the displacement of two PCy 3 and two dihydrogen ligands, was formed. The different bonding modes for each of the three bifunctional P approximately Si-H ligands are discussed on the basis of multinuclear NMR, X-ray diffraction, and density functional theory studies. One ligand acts as a monodentate phosphine ligand with a pendant Si-H group, whereas the two others act as bidentate ligands with different Si-H bond activations. Indeed, an intermediate structure between two arrested forms 2a and 2b can be proposed: a dihydrido(disilyl)ruthenium(IV) species (form 2a) resulting from two Si-H oxidative additions or a hydrido(silyl)ruthenium(II) species (form 2b) presenting an agostic Si-H bond and only one oxidative addition. 相似文献
977.
Gautier A Juillerat A Heinis C Corrêa IR Kindermann M Beaufils F Johnsson K 《Chemistry & biology》2008,15(2):128-136
The visualization of complex cellular processes involving multiple proteins requires the use of spectroscopically distinguishable fluorescent reporters. We have previously introduced the SNAP-tag as a general tool for the specific labeling of SNAP-tag fusion proteins in living cells. The SNAP-tag is derived from the human DNA repair protein O6-alkylguanine-DNA alkyltransferase (AGT) and can be covalently labeled in living cells using O6-benzylguanine derivatives bearing a chemical probe. Here we report the generation of an AGT-based tag, named CLIP-tag, which reacts specifically with O2-benzylcytosine derivatives. Because SNAP-tag and CLIP-tag possess orthogonal substrate specificities, SNAP and CLIP fusion proteins can be labeled simultaneously and specifically with different molecular probes in living cells. We furthermore show simultaneous pulse-chase experiments to visualize different generations of two different proteins in one sample. 相似文献
978.
Yin LW Bando Y Golberg D Gloter A Li MS Yuan X Sekiguchi T 《Journal of the American Chemical Society》2005,127(47):16354-16355
Porous boron carbonitride nanotubular fibers with BCN stoichiometry and homogeneous B, C, and N species distribution were fabricated via the CVD method. Spatially resolved cathodoluminescence measurements on individual nanostructures revealed intense ultraviolet emission centered at 319 nm, suggesting the characteristics of a semiconductor with a band gap of 3.89 eV. It is believed that the present nanostructures may have a variety of applications in ultraviolet optical devices, hydrogen storage systems, and field emission apparatus. 相似文献
979.
[Reaction: see text] A total synthesis of (+/-)-mycothiazole and a formal enantioselective approach have been achieved from 2,4-dibromothiazole. A chain extension of a homoallylic alcohol proceeding through an unsaturated sultone intermediate, generated by ring-closing metathesis, was used as a key step for the elaboration of the conjugated (Z)-dienol moiety. 相似文献
980.
Leroy V Devaud M Hocquet T Bacri JC 《The European physical journal. E, Soft matter》2005,17(2):189-198
The dynamics of a two-dimensional N-bubble static cloud is investigated and shown to be well described by an N-degree of freedom harmonic oscillator model, at least at low enough frequencies. Eigenmodes and eigenfrequencies are calculated and compared with experimental results obtained with an assembly of bubbles caught up under a net in a water tank. Accordance is found to be excellent in the frequency range of validity of the model, the limits of which are discussed. An interpretation of the low-frequency branch of Foldy’s dispersion relation in bubbly liquids is suggested in terms of “bubble waves” in a quasi-incompressible medium. 相似文献