全文获取类型
收费全文 | 15753篇 |
免费 | 725篇 |
国内免费 | 178篇 |
专业分类
化学 | 11389篇 |
晶体学 | 134篇 |
力学 | 708篇 |
综合类 | 5篇 |
数学 | 2007篇 |
物理学 | 2413篇 |
出版年
2024年 | 35篇 |
2023年 | 146篇 |
2022年 | 745篇 |
2021年 | 706篇 |
2020年 | 552篇 |
2019年 | 653篇 |
2018年 | 651篇 |
2017年 | 527篇 |
2016年 | 882篇 |
2015年 | 604篇 |
2014年 | 759篇 |
2013年 | 1559篇 |
2012年 | 1148篇 |
2011年 | 1130篇 |
2010年 | 778篇 |
2009年 | 641篇 |
2008年 | 742篇 |
2007年 | 683篇 |
2006年 | 509篇 |
2005年 | 489篇 |
2004年 | 348篇 |
2003年 | 330篇 |
2002年 | 297篇 |
2001年 | 134篇 |
2000年 | 116篇 |
1999年 | 105篇 |
1998年 | 70篇 |
1997年 | 94篇 |
1996年 | 87篇 |
1995年 | 63篇 |
1994年 | 76篇 |
1993年 | 50篇 |
1992年 | 65篇 |
1991年 | 60篇 |
1990年 | 70篇 |
1989年 | 60篇 |
1988年 | 54篇 |
1987年 | 59篇 |
1986年 | 56篇 |
1985年 | 68篇 |
1984年 | 51篇 |
1983年 | 36篇 |
1982年 | 50篇 |
1981年 | 38篇 |
1980年 | 37篇 |
1979年 | 36篇 |
1978年 | 29篇 |
1977年 | 25篇 |
1976年 | 24篇 |
1975年 | 18篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
61.
In this paper we construct a positive doubly power bounded operator
with a nonpositive inverse on an AL-space. 相似文献
62.
In this paper, it is shown that some of the results of torus actions on Poincaré duality spaces, Borel’s dimension formula
and topological splitting principle to local weights, hold if ‘torus’ is replaced by ‘pro-torus’. 相似文献
63.
Felix Ali Mehmeti 《Mathematical Methods in the Applied Sciences》1994,17(9):697-752
Consider two copies N1, N2 of the interval [0, ∞]. Consider Klein-Gordon equations with (different) constant coefficients on ? × Nj ( = time × space). Assume the coincidence of the values of the solution at the boundary points of the Nj for all times and a transmission condition relating its first (one-sided) space derivatives at these points. Under a symmetry condition, we extend the spatial part of the equation and the transmission conditions to a self-adjoint operator (by Friedrichs extension) and reformulate our problem in terms of an abstract wave equation in a suitable Hilbert space. We derive an expansion of the solution in generalized eigenfunctions of this self-adjoint extension and show, that the L∞-norms (in space) of the solution and its first k space derivatives at the time t decay for t → ∞ at least as const. t¼, if the initial conditions satisfy a compatibility condition of order k derived in this paper. The loss of decay rate in comparison with the full line case (const. t?½, cf. [28]) is caused by the tunnel effect. Further we show that an abstract wave equation in a Hilbert space with a Friedrichs extension as spatial part can always be derived from a stationarity principle for an associated action-type functional. This yields a physical legitimation of our model by the principle of stationary action and moreover a criterion for the physical interpretability of all models created by the linear interaction concept [4, 6, 8, 10], in particular for the coupling of media of different dimension (alternative to [13, 16] for similar models). 相似文献
64.
We report on the lattice-mismatched growth of step-graded InxAl1−xAs buffer layers on GaAs (0 0 1) substrates by molecular beam epitay (MBE). The approach to growing highly lattice-mismatched epilayers is to interpose a buffer layer between the substrate and the active layer. Two samples G30 and G40 with active layer compositions, respectively, x = 0.46 and x = 0.41, are studied by photoluminescence (PL). At low temperature, the PL spectra show a large broadened band whose energy and intensity depend on the active layer composition. The step-graded layer compositions improved the crystalline quality of these structures and increase the active layer PL band intensity. 相似文献
65.
Joel M. Kauffman Ali Khalaj Peter T. Litak John A. Novinski Gurdip S. Bajwa 《Journal of heterocyclic chemistry》1994,31(4):957-965
To improve the solubility, photostability and fluorescence quantum yield (Φ) of 2-arylbenzimidazolium ions the N-1 hydrogen was replaced by an alkyl group before N-3 was quatemized; this substitution did not reduce Φ, proving the absence of steric inhibition to attainment of coplanarity in the excited state. A related symmetrical 2,2′-arylenebis(1,3-dimethyldibenzimidazolium ion) had Φ = 0.86. The related 2,5-bis(1-methyl-2-benzimidazolyl)phenol had Φ = 0.38 at 22° as well as a very large Stokes' shift due to proton-transfer. These findings supported a new insight into the electron distribution of the first excited singlet state of oligophenylenes. The compounds are of interest as laser dyes and scintillation fluors. 相似文献
66.
67.
68.
Abnormal cilia were demonstrated in the lining epithelial cells of three cases of intraspinal endodermal (bronchogenic) cyst. The changes comprised a wide spectrum of ultrastructural abnormalities, including (a). cilia with abnormal axonemal microtubules, (b). swollen cilia, (c). compound cilia with or without excessive ciliary matrix, (d). naked cilia without limiting membrane, and (e). intracytoplasmic cilia and aggregates of microtubules. Of these, compound cilia and swollen cilia were most common. Cilia with dynein arm deficiency were not observed. Ciliary abnormalities found in the present study were very similar to those described in the bronchial epithelium of various diseases. The present findings suggest that the lining epithelium of intraspinal endodermal cyst shares similar ciliogenesis and susceptibility to abnormal ciliary formation as that of the bronchial epithelium. 相似文献
69.
Herein we describe the synthesis and characterization of compounds having the formulae R2SnL2 and R3SnL, where R = Me, n-Bu, Ph and n-Oct and L = 2-[N-(2,4,6-tribromophenylamido]propanoic acid. All the complexes have been characterized by various spectroscopic methods (IR and 1H, 13C, 119Sn NMR), elemental analysis, mass spectrometry and physical data. These compounds were also screened for their biological activity and found some encouraging results. 相似文献
70.
Mohamed JazouliDominique Guianvarc'h Mohamed SoufiaouiPierre Vierling Rachid Benhida 《Tetrahedron letters》2003,44(31):5807-5810
A short route to a series of 2′-deoxy-C-nucleosides featuring substituted nucleobases has been developed. The key step is the formation of the cyclized products following Mukaiyama's type amide coupling and a simple dehydration, starting from readily accessible synthons. The epimerization of the C1′-stereogenic center was avoided under mild and controlled conditions. 相似文献