全文获取类型
收费全文 | 641篇 |
免费 | 10篇 |
国内免费 | 1篇 |
专业分类
化学 | 278篇 |
晶体学 | 6篇 |
力学 | 40篇 |
数学 | 150篇 |
物理学 | 178篇 |
出版年
2021年 | 6篇 |
2020年 | 4篇 |
2019年 | 6篇 |
2018年 | 3篇 |
2016年 | 3篇 |
2015年 | 11篇 |
2014年 | 8篇 |
2013年 | 25篇 |
2012年 | 22篇 |
2011年 | 29篇 |
2010年 | 9篇 |
2009年 | 15篇 |
2008年 | 28篇 |
2007年 | 32篇 |
2006年 | 33篇 |
2005年 | 34篇 |
2004年 | 22篇 |
2003年 | 15篇 |
2002年 | 9篇 |
2001年 | 17篇 |
2000年 | 11篇 |
1999年 | 13篇 |
1998年 | 6篇 |
1997年 | 6篇 |
1996年 | 10篇 |
1995年 | 12篇 |
1994年 | 25篇 |
1993年 | 22篇 |
1992年 | 18篇 |
1991年 | 10篇 |
1990年 | 5篇 |
1989年 | 8篇 |
1988年 | 10篇 |
1987年 | 7篇 |
1986年 | 7篇 |
1985年 | 12篇 |
1984年 | 8篇 |
1983年 | 11篇 |
1982年 | 9篇 |
1981年 | 7篇 |
1980年 | 6篇 |
1979年 | 10篇 |
1978年 | 6篇 |
1977年 | 8篇 |
1976年 | 7篇 |
1975年 | 11篇 |
1974年 | 6篇 |
1973年 | 8篇 |
1968年 | 5篇 |
1907年 | 4篇 |
排序方式: 共有652条查询结果,搜索用时 31 毫秒
641.
M.G. Payne 《Optics Communications》2007,271(1):269-273
We investigate a coherently prepared cold medium for efficient single-photon inelastic two-wave mixing (ITWM), maximum Fock state entanglement and single photon self-interference. We show the possibility of generating maximally entangled single-photon state, and near 100% conversion efficiency for generating a frequency shifted TWM photon by proper choice of medium length and concentration. In addition, we demonstrate a new type of transparency effect produced by an efficient single photon self-interference, a transparency effect that is very different from the conventional electromagnetically induced transparency (EIT) process. 相似文献
642.
Tracy L. Payne 《Proceedings of the American Mathematical Society》1999,127(3):829-833
It is established that if and are connected locally symmetric spaces of noncompact type where has finite volume, and is a totally geodesic immersion, then the closure of in is an immersed ``algebraic' submanifold. It is also shown that if in addition, the real ranks of and are equal, then the the closure of in is a totally geodesic submanifold of The proof is a straightforward application of Ratner's Theorem combined with the structure theory of symmetric spaces.
643.
(+/-)-Bakkenolide A was prepared in five steps from ethyl 4-benzyloxyacetoacetate by sequential alkylations with tiglyl bromide nd cis-5-bromo-1,3-pentadiene, followed by an intramolecular Diels-Alder reaction as the key step. The known 7-epibakkenolide A and novel 10-epi- and 7,10-diepibakkenolide A stereoisomers were obtained as minor byproducts. 相似文献
644.
Selective catalytic reduction (SCR) with ammonia has been considered as the most promising technology, as its effect deals with the NOX. Novel Fe-doped V2O5/TiO2 catalysts were prepared by sol–gel and impregnation methods. The effects of iron content and reaction temperature on the catalyst SCR reaction activity were explored by a test device, the results of which revealed that catalysts could exhibit the best catalytic activity when the iron mass ratio was 0.05%. It further proved that the VTiFe (0.05%) catalyst performed the best in denitration and its NOX conversion reached 99.5% at 270 °C. The outcome of experimental procedures: Brunauer–Emmett–Teller surface area, X-ray powder diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, temperature-programmed reduction and adsorption (H2-TPR, NH3-TPD) techniques showed that the iron existed in the form of Fe3+ and Fe2+ and the superior catalytic performance was attributed to the highly dispersed active species, lots of surface acid sites and absorbed oxygen. The modified Fe-doped catalysts do not only have terrific SCR activities, but also a rather broad range of active temperature which also enhances the resistance to SO2 and H2O. 相似文献
645.
Scanlon DO Kehoe AB Watson GW Jones MO David WI Payne DJ Egdell RG Edwards PP Walsh A 《Physical review letters》2011,107(24):246402
Lead dioxide has been used for over a century in the lead-acid battery. Many fundamental questions concerning PbO2 remain unanswered, principally: (i) is the bulk material a metal or a semiconductor, and (ii) what is the source of the high levels of conductivity? We calculate the electronic structure and defect physics of PbO2, using a hybrid density functional, and show that it is an n-type semiconductor with a small indirect band gap of ~0.2 eV. The origin of electron carriers in the undoped material is found to be oxygen vacancies, which forms a donor state resonant in the conduction band. A dipole-forbidden band gap combined with a large carrier induced Moss-Burstein shift results in a large effective optical band gap. The model is supported by neutron diffraction, which reveals that the oxygen sublattice is only 98.4% occupied, thus confirming oxygen substoichiometry as the electron source. 相似文献
646.
Yunlong Cui R. Hawrami Pijush Bhattacharya Mark Bryant Arnold Burger S.A. Payne 《Solid State Communications》2011,151(7):541-544
Europium-doped barium and strontium iodide crystals are high light yield scintillator materials with excellent energy resolution. In this communication, BaI2:Eu and SrI2:Eu single crystals with space groups of Pnma () and Pbca () respectively were grown from the vertical Bridgman method. The crystals were investigated with polarized Raman spectroscopy at temperatures varied from 77 to 300 K. The observed Raman-active modes for each crystal were assigned with the help of group theory analysis. It is found that the absence of the site symmetry leads to splitting of a spectral line in Pnma structure into two lines in the Pbca structure. Structural defects including dark spots formed during crystal growth and new species produced from hydration were characterized. 相似文献
647.
Rodriguez-Aristegui S Clapham KM Barrett L Cano C Desage-El Murr M Griffin RJ Hardcastle IR Payne SL Rennison T Richardson C Golding BT 《Organic & biomolecular chemistry》2011,9(17):6066-6074
Amino-substituted biphenyls were obtained by Suzuki cross-coupling of 2,6-dibromoaniline with a phenylboronic acid (substituted with Me, NO(2), OH, OMe or Cl) preferably assisted by microwave irradiation. Conversion of the amino group into a thiol preceded a base-induced intramolecular substitution, also facilitated by microwave heating, to generate the second C-S bond of the target dibenzothiophene. The 1-, 2-, 3- or 4-substituted 6-halodibenzothiophenes obtained were subjected to a palladium-mediated coupling with 2-morpholin-4-yl-8-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-4H-chromen-4-one to give the respective 6-, 7-, 8- or 9-substituted dibenzothiophen-4-ylchromenones. These compounds were evaluated as inhibitors of DNA-dependent protein kinase (DNA-PK) and compared to the parent 8-(dibenzo[b,d]thiophen-4-yl)-2-morpholin-4-yl-4H-chromen-4-one. Notably, derivatives bearing hydroxy or methoxy substituents at C-8 or C-9 retained activity, whereas substitution at C-7 lowered activity. Substitution with chloro at C-6 was not detrimental to activity, but a chloro group at C-7 or C-8 reduced potency. The data indicate permissive elaboration of hydroxyl at C-8 or C-9, enabling the possibility of improved pharmaceutical properties, whilst retaining potency against DNA-PK. 相似文献
648.
Pierre Garcia Philippa R. Payne Eugene Chong Ruth L. Webster Benedict J. Barron Andrew C. Behrle Joseph A.R. Schmidt Laurel L. Schafer 《Tetrahedron》2013,69(27-28):5737-5743
The influence of structurally related N,O-chelating ligands with additional heteroatoms (N, O, P, S) on the reactivity of in situ generated tantalum complexes for the hydroaminoalkylation of amines has been explored. Reactivity was probed by evaluating the catalytic ability of these N,O-chelating systems with N-methylaniline and 4-methoxy-N-methylaniline substrates. Enhanced reactivity is observed with amide proligands bearing an ortho-methoxyphenyl group on the nitrogen. 4-Methoxy-N-methylaniline is found to be more prone to undergo C–H functionalization via hydroaminoalkylation than N-methylaniline. The use of the related substrate 2-methoxy-N-methylaniline is not tolerated, and instead C(sp3)–O bond cleavage was observed. 相似文献
649.
The pH-dependence and reversibility of uranium and thorium binding onto a modified bauxite refinery residue (MBRR) were studied in laboratory uptake/leaching experiments. Natural (238)U and (232)Th isotopes were contacted with MBRR in an 8day loading period (equilibrium pH≈8.5) then leached in pH-dependent experiments, where the pH was decreased from 8 to 3 over several hours following addition of exchange isotopes (232)U and (229)Th. Relative concentrations of the thorium isotope pair ((232)Th and (229)Th) indicate that Th is very strongly bound to MBRR, and although at pH 3, some de-sorption is observed (232)Th (≈3%) and (229)Th (≈2.5%), released thorium is partially re-adsorbed during an overnight equilibration. During the initial equilibration, approximately 50% of the (238)U was adsorbed, and a U adsorption maximum occurs between pH 5 and pH 6, where <0.5% of the U remains in solution. However, at a pH between 5 and 3, some 60% of the bound U releases, hence the pH range of maximum U retention on the MBRR is relatively narrow. When equilibrated overnight, the MBRR releases additional U, suggesting a kinetically controlled de-sorption linked to mineral dissolution. Plots of U isotope exchange between (232)U and (238)U are linear, and suggest that U adsorption is mostly reversible. Data for adsorption in mixed systems of U and Th suggest that Th and U compete for similar binding sites. 相似文献
650.
Walsh A Da Silva JL Wei SH Körber C Klein A Piper LF DeMasi A Smith KE Panaccione G Torelli P Payne DJ Bourlange A Egdell RG 《Physical review letters》2008,100(16):167402
Bulk and surface sensitive x-ray spectroscopic techniques are applied in tandem to show that the valence band edge for In2O3 is found significantly closer to the bottom of the conduction band than expected on the basis of the widely quoted bulk band gap of 3.75 eV. First-principles theory shows that the upper valence bands of In2O3 exhibit a small dispersion and the conduction band minimum is positioned at Gamma. However, direct optical transitions give a minimal dipole intensity until 0.8 eV below the valence band maximum. The results set an upper limit on the fundamental band gap of 2.9 eV. 相似文献