全文获取类型
收费全文 | 18432篇 |
免费 | 706篇 |
国内免费 | 514篇 |
专业分类
化学 | 3477篇 |
晶体学 | 125篇 |
力学 | 613篇 |
综合类 | 68篇 |
数学 | 10269篇 |
物理学 | 5100篇 |
出版年
2024年 | 30篇 |
2023年 | 171篇 |
2022年 | 264篇 |
2021年 | 268篇 |
2020年 | 410篇 |
2019年 | 437篇 |
2018年 | 419篇 |
2017年 | 343篇 |
2016年 | 346篇 |
2015年 | 370篇 |
2014年 | 698篇 |
2013年 | 1055篇 |
2012年 | 554篇 |
2011年 | 1513篇 |
2010年 | 1060篇 |
2009年 | 1255篇 |
2008年 | 1398篇 |
2007年 | 1306篇 |
2006年 | 990篇 |
2005年 | 795篇 |
2004年 | 651篇 |
2003年 | 556篇 |
2002年 | 557篇 |
2001年 | 308篇 |
2000年 | 308篇 |
1999年 | 349篇 |
1998年 | 276篇 |
1997年 | 255篇 |
1996年 | 275篇 |
1995年 | 247篇 |
1994年 | 278篇 |
1993年 | 248篇 |
1992年 | 211篇 |
1991年 | 150篇 |
1990年 | 116篇 |
1989年 | 122篇 |
1988年 | 87篇 |
1987年 | 90篇 |
1986年 | 80篇 |
1985年 | 147篇 |
1984年 | 108篇 |
1983年 | 76篇 |
1982年 | 110篇 |
1981年 | 84篇 |
1980年 | 75篇 |
1979年 | 36篇 |
1978年 | 49篇 |
1977年 | 41篇 |
1976年 | 43篇 |
1974年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
Denote by σ the subspace of Hilbert space {(xi)?l2:xi=0 for all but finitely many i}. Examples of cell-like decompositions of σ are constructed that have decomposition spaces that are not homeomorphic to σ. At one extreme is a cell-like decomposition G of σ produced using ghastly finite dimensional examples such that the decomposition space σ?G contains no embedded 2-cell but (σ?G)× is homeomorphic to σ. At the other extreme is a cell-like decomposition G of σ satisfying: (a) the nondegeneracy set NG={g?G:g≠point} consists of countably many arcs (necessarily tame); (b) the nondegeneracy set NG is a closed subset of the decomposition space σ?G; (c) each map f:B2→σ?G of a 2-cell into σ?G can be approximated arbitrarily closely by an embedding; (d) σ?G is not homeomorphic to σ but (σ?G)× is homeomorphic to σ. The fact that both conditions (a) and (b) can be satisfied (and have (d) hold) is directly attributable to σ’s incompleteness as a topological space. 相似文献
982.
Mishkovsky M Frydman L 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2005,173(2):344-350
A new protocol for processing the data arising in ultrafast 2D NMR is discussed and exemplified, based on the interlaced Fourier transformation. This approach is capable of dealing in a single, combined fashion, with the two mirror-imaged interferograms arising in this kind of experiment as a result of the acquisition of a train of magnetic field gradient echoes. By combining all the acquired data points into a common Fourier processing procedure the spectral width along the direct-acquisition domain becomes effectively doubled, giving the opportunity of employing acquisition gradients that are approximately half as strong as hitherto required. This in turn should lead to an overall enhancement in the signal-to-noise ratio of the experiment of ca. 2, as well as to improvements in the achievable digital resolution. These expectations were tested by carrying out a series of homo- and heteronuclear ultrafast 2D NMR acquisitions, and found systematically fulfilled. The robustness and conditions that allow the interlaced numerical procedure to be implemented in routine analytical applications were explored and are briefly discussed. 相似文献
983.
We present the infrared and Raman study of the optical phonon modes of the defective compounds ZnGa2Se4 and ZnGa2S4. Most of the compounds have been found to crystallize in the thiogallate structure (defect chalcopyrite) with space group where all cations and vacancies are ordered. For some Zinc compounds a partially disordered cationic sublattice with various degrees of cation and vacancy statistical distribution, which lead to the higher symmetry (defect stannite), has been reported. For ZnGa2Se4 we have found three modes of A symmetry, showing Raman activity only. In addition, we have observed each five modes of B and E symmetry, showing infrared as well as Raman activity. The number of modes and their symmetry assignment, based on polarized measurements, clearly indicate space group for the investigated crystals of ZnGa2Se4.Regarding ZnGa2S4 we have found three modes exclusively showing Raman activity (2A⊕1B1), and only eight modes showing infrared as well as Raman activity (3B2⊕5E). The assignment of the modes has been derived by analyzing the spectral positions of the vibrational modes in comparison to a number of compounds. From the number and symmetry assignment of the optical phonon modes we confirm that ZnGa2S4 most likely crystallizes in space group . 相似文献
984.
Using first-principles electronic structure theory, we have calculated defect formation energies and defect transition levels in CuInSe2 and CuGaSe2. We show that (i) it is easy to form Cu vacancies in CuInSe2, and (ii) it is also relatively easy to form cation antisite defects (e.g. InCu) for this ternary compound. Consequently, defect pairs such as (2VCu+InCu) have a remarkably low formation enthalpy. As a result, the formation of a series of Cu-poor compounds (CPCs) such as CuIn5Se8 and CuIn3Se5, is explained as a repeat of (2VCu+InCu) pairs in CuInSe2. The very efficient p-type self-doping ability of CuInSe2 is explained by the easy formation of the shallow Cu vacancies. The electrically benign character of the natural defect in CuInSe2 is explained in terms of an electronic passivation of the by . For CuGaSe2, we find that (i) the native acceptor formation energies and transition energy levels are similar to that in CuInSe2, but the donor formation energy is larger in CuGaSe2. (ii) The GaCu donor level in CuGaSe2 is deeper than InCu donor level in CuInSe2, therefore, GaCu behaves as an electron trap in CuGaSe2, even when it is passivated by VCu. We have also calculated the band alignment between the CPCs and CuInSe2, showing that it could have significant effect on the solar cell performance. 相似文献
985.
The chemical preparation, the calorimetric studies and the crystal structure are given for two new organic sulfates NH3(CH2)5NH3SO4 1.5H2O (DAP-S) and NH3(CH2)9NH3SO4·H2O (DAN-S). DAP-S is monoclinic P21/n with unit cell dimensions: a=11.9330(2) Å; b=10.9290(2) Å; c=17.5260(2) Å; β=101.873(1)°; V=2236.77(6) Å3; and Z=8. Its atomic arrangement is described as inorganic layers of units and water molecules separated by organic chains. DAN-S is monoclinic P21/c with unit cell parameters: a=5.768(2) Å; b=25.890(10) Å; c=11.177(5) Å; β=115.70(4)°; V=1504.0(11) Å3 and Z=4. Its structure exhibits infinite chains, parallel to the [100] direction where the organic cations are interconnected. In both structures a network of strong and weak hydrogen bonds connects the different components in the building of the crystal. 相似文献
986.
987.
We classify the irreducible components of the varieties
\[
\V(n,a,b) = \{ (A,B) \in \M_n(\field) \times \M_n(\field) \mid
AB = BA = A^a = B^b = 0 \}.
\] 相似文献
988.
We reformulate a recent result of Arason and
Elman by giving a very simple presentation of the
powers of the fundamental ideal of the Witt ring of a field
of characteristic 2.
Sur les puissances de lidéal fondamental de lanneau de Witt
Nous reformulons un résultat récent de Arason et Elman en donnant une présentation très simple des puissances de lidéal fondamental de lanneau de Witt dun corps de caractéristique 2.相似文献
989.
Jaume Giné 《Journal of Differential Equations》2004,197(1):147-161
We consider the class of polynomial differential equations , where Pn and Qn are homogeneous polynomials of degree n. These systems have a focus at the origin if λ≠0, and have either a center or a focus if λ=0. Inside this class we identify a new subclass of Darbouxian integrable systems having either a focus or a center at the origin. Additionally, under generic conditions such Darbouxian integrable systems can have at most one limit cycle, and when it exists is algebraic. For the case n=2 and 3, we present new classes of Darbouxian integrable systems having a focus. 相似文献
990.