全文获取类型
收费全文 | 400篇 |
免费 | 3篇 |
专业分类
化学 | 241篇 |
力学 | 30篇 |
数学 | 83篇 |
物理学 | 49篇 |
出版年
2022年 | 4篇 |
2021年 | 7篇 |
2020年 | 3篇 |
2019年 | 4篇 |
2017年 | 5篇 |
2016年 | 11篇 |
2015年 | 12篇 |
2014年 | 9篇 |
2013年 | 28篇 |
2012年 | 19篇 |
2011年 | 23篇 |
2010年 | 20篇 |
2009年 | 7篇 |
2008年 | 12篇 |
2007年 | 16篇 |
2006年 | 20篇 |
2005年 | 14篇 |
2004年 | 14篇 |
2003年 | 11篇 |
2002年 | 17篇 |
2001年 | 3篇 |
2000年 | 5篇 |
1999年 | 3篇 |
1998年 | 7篇 |
1997年 | 6篇 |
1995年 | 5篇 |
1994年 | 5篇 |
1993年 | 8篇 |
1991年 | 4篇 |
1990年 | 4篇 |
1989年 | 8篇 |
1988年 | 4篇 |
1987年 | 2篇 |
1986年 | 3篇 |
1985年 | 5篇 |
1984年 | 6篇 |
1983年 | 2篇 |
1982年 | 6篇 |
1981年 | 5篇 |
1980年 | 3篇 |
1979年 | 4篇 |
1978年 | 6篇 |
1977年 | 2篇 |
1976年 | 3篇 |
1975年 | 2篇 |
1974年 | 2篇 |
1973年 | 2篇 |
1942年 | 2篇 |
1940年 | 3篇 |
1891年 | 3篇 |
排序方式: 共有403条查询结果,搜索用时 34 毫秒
71.
72.
Potential Analysis - We prove an estimate on the modulus of continuity at a boundary point of a cylindrical domain for local weak solutions to degenerate parabolic equations of p-laplacian type.... 相似文献
73.
We establish the intrinsic Harnack inequality for non-negative solutions of a class of degenerate, quasilinear, parabolic equations, including equations of the p-Laplacian and porous medium type. It is shown that the classical Harnack estimate, while failing for degenerate parabolic equations, it continues to hold in a space-time geometry intrinsic to the degeneracy. The proof uses only measure-theoretical arguments, it reproduces the classical Moser theory, for non-degenerate equations, and it is novel even in that context. Hölder estimates are derived as a consequence of the Harnack inequality. The results solve a long standing problem in the theory of degenerate parabolic equations. 相似文献
74.
Pd(OAc)2 in combination with P(t-Bu)3 catalyzes the coupling of β-methallyl alcohol with 1-bromo-3,4-(methylenedioxy)benzene (1a), 1-bromo-4-methoxybenzene (1b), or 1-bromo-4-tert-butylbenzene (1c). The reaction affords the corresponding 2-methyl-3-aryl-propanals, which are valuable floral fragrances. With 1a or 1b high reaction rates are obtained at 130 °C using NMP/water mixtures and an inorganic base such as Na2CO3. The chemoselectivity of the reaction is almost complete, so that the process appears practically feasible. In contrast, the coupling of β-methallyl alcohol with 1c proceeds with low reaction rates. 相似文献
75.
Ugo Jacovella Corentin Rossi Claire Romanzin Christian Alcaraz Roland Thissen 《Chemphyschem》2023,24(2):e202200474
Structural rearrangements in ions are essential for understanding the composition and evolution of energetic and chemically active environments. This study explores the interconversion routes for simple polycyclic aromatic hydrocarbons, namely naphthalene and azulene radical cations (C10H8+), by combining mass spectrometry and vacuum ultraviolet tunable synchrotron radiation through the chemical monitoring technique. Products of ion-molecule reactions are used to probe C10H8+ structures that are formed as a function of their internal energies. Isomerisation from azulene radical cation towards naphthalene radical cation in a timescale faster than 80 μs was monitored, whereas no reverse isomerisation was observed in the same time window. When energising C10H8+ with more than 6 eV, the reactivity of C10H8+ unveils the formation of a new isomeric group with a contrasted reactivity compared with naphthalene and azulene cations. We tentatively assigned these structures to phenylvinylacetylene cations. 相似文献
76.
Cariati E Macchi R Roberto D Ugo R Galli S Casati N Macchi P Sironi A Bogani L Caneschi A Gatteschi D 《Journal of the American Chemical Society》2007,129(30):9410-9420
Mixed M(II)/M(III) metal oxalates, as "stripes" connected through strong hydrogen bonding by para-dimethylaminobenzaldeide (DAMBA) and water, form an organic-inorganic 2D network that enables segregation in layers of the cationic organic NLO-phore trans-4-(4-dimethylaminostyryl)-1-methylpyridinium, [DAMS+]. The crystalline hybrid materials obtained have the general formula [DAMS]4[M2M'(C2O4)6].2DAMBA.2H2O (M = Rh, Fe, Cr; M' = Mn, Zn), and their overall three-dimensional packing is non-centrosymmetric and polar, therefore suitable for second harmonic generation (SHG). All the compounds investigated are characterized by an exceptional SHG activity, due both to the large molecular quadratic hyperpolarizability of [DAMS+] and to the efficiency of the crystalline network which organizes [DAMS+] into head-to-tail arranged J-type aggregates. The tunability of the pairs of metal ions allows exploiting also the magnetic functionality of the materials. Examples containing antiferro-, ferro-, and ferri-magnetic interactions (mediated by oxalato bridges) are obtained by coupling proper M(III) ions (Fe, Cr, Rh) with M(II) (Mn, Zn). This shed light on the role of weak next-nearest-neighbor interactions and main nearest-neighbor couplings along "stripes" of mixed M(II)/M(III) metal oxalates of the organic-inorganic 2D network, thus suggesting that these hybrid materials may display isotropic 1D magnetic properties along the mixed M(II)/M(III) metal oxalates "stripes". 相似文献
77.
Claudio Bartocci Ugo Bruzzo Daniel Hernndez Ruiprez Jos M. Muoz Porras 《Mathematische Nachrichten》2002,238(1):23-36
We consider a relative Fourier‐Mukai transform de.ned on elliptic fibrations over an arbitrary base scheme. This is used to construct relative Atiyah sheaves and generalize Atiyah and Tu's results about semistable sheaves over elliptic curves to the case of elliptic fibrations. Moreover we show that this transform preserves relative (semi)stability of sheaves of positive relative degree. 相似文献
78.
The generation of a series of α-methoxymethoxy-substituted arylmethyllithiums was achieved by direct metalation of the corresponding arylmethyl methoxymethyl ethers. While the effect of substituents at the benzylic position is straightforward, substituents located on the aromatic ring promote the set up of a competition between lateral and aromatic metalation, strongly affected by the position and relative ortho directing properties of the new substituent. The proposed methodology allows a simple approach to the generation of a wide array of functionalized organolithium reagents. 相似文献
79.
Ortica F Smimmo P Mazzucato U Favaro G Heynderickx A Moustrou C Samat A 《Photochemistry and photobiology》2006,82(5):1326-1333
The photobehavior of five photochromic dipyrrolyl-perfluoro-cyclopentenes was studied by steady state and time-resolved absorption spectroscopy. The quantum yields of the UV-photoinduced ring-closing reaction (coloration) and the visible-stimulated cycloreversion reaction (bleaching) were measured. Kinetic and thermodynamic parameters of thermal bleaching were also determined. Nanosecond time-resolved experiments showed formation of a transient, which was not a precursor of the reaction photoproduct. This transient was tentatively assigned to a radical cation formed by direct photoionization through a short-lived triplet state. The nature of the transient species was supported by photoinduced electron transfer to electron acceptors. 相似文献
80.
Ivette Rodriguez Oriol Lehmkuhl Ugo Piomelli Jorge Chiva Ricard Borrell Assensi Oliva 《Flow, Turbulence and Combustion》2017,99(3-4):729-763
This paper investigates the effects of surface roughness on the flow past a circular cylinder at subcritical to transcritical Reynolds numbers. Large eddy simulations of the flow for sand grain roughness of size k/D = 0.02 are performed (D is the cylinder diameter). Results show that surface roughness triggers the transition to turbulence in the boundary layer at all Reynolds numbers, thus leading to an early separation caused by the increased momentum deficit, especially at transcritical Reynolds numbers. Even at subcritical Reynolds numbers, boundary layer instabilities are triggered in the roughness sublayer and eventually lead to the transition to turbulence. The early separation at transcritical Reynolds numbers leads to a wake topology similar to that of the subcritical regime, resulting in an increased drag coefficient and lower Strouhal number. Turbulent statistics in the wake are also affected by roughness; the Reynolds stresses are larger due to the increased turbulent kinetic energy production in the boundary layer and separated shear layers close to the cylinder shoulders. 相似文献