全文获取类型
收费全文 | 43185篇 |
免费 | 3735篇 |
国内免费 | 5641篇 |
专业分类
化学 | 37471篇 |
晶体学 | 881篇 |
力学 | 329篇 |
综合类 | 238篇 |
数学 | 5957篇 |
物理学 | 7685篇 |
出版年
2024年 | 76篇 |
2023年 | 520篇 |
2022年 | 791篇 |
2021年 | 811篇 |
2020年 | 1108篇 |
2019年 | 2049篇 |
2018年 | 1180篇 |
2017年 | 1908篇 |
2016年 | 1591篇 |
2015年 | 1600篇 |
2014年 | 2062篇 |
2013年 | 3759篇 |
2012年 | 2647篇 |
2011年 | 2861篇 |
2010年 | 2220篇 |
2009年 | 2512篇 |
2008年 | 2758篇 |
2007年 | 2845篇 |
2006年 | 2546篇 |
2005年 | 2132篇 |
2004年 | 2093篇 |
2003年 | 1782篇 |
2002年 | 1536篇 |
2001年 | 1165篇 |
2000年 | 1139篇 |
1999年 | 932篇 |
1998年 | 887篇 |
1997年 | 724篇 |
1996年 | 627篇 |
1995年 | 685篇 |
1994年 | 488篇 |
1993年 | 411篇 |
1992年 | 397篇 |
1991年 | 257篇 |
1990年 | 194篇 |
1989年 | 162篇 |
1988年 | 147篇 |
1987年 | 86篇 |
1986年 | 80篇 |
1985年 | 110篇 |
1984年 | 86篇 |
1983年 | 40篇 |
1982年 | 97篇 |
1981年 | 81篇 |
1980年 | 54篇 |
1979年 | 84篇 |
1978年 | 52篇 |
1977年 | 43篇 |
1976年 | 44篇 |
1973年 | 32篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
Intense multi- and single-line laser operation is reported from semiconductively preionised atomic fluorine lasers employing an optimised capacitor transfer excitation circuitry. The performance and spectral characteristics of these lasers are discussed and compared to the characteristics of previously described atomic fluorine lasers. The efficiency of one of these lasers described, at optimum conditions, is reaching a value as high as 0.14%. To the best of our knowledge, this is the highest efficiency reported up date, for an atomic fluorine laser. 相似文献
32.
一个改进的Hardy-Hilbert不等式 总被引:1,自引:1,他引:0
通过建立权系数的不等式,得到一个改进的Hardy-Hilbert不等式. 相似文献
33.
《Journal of Chemical Sciences》2005,117(5):525-531
The silicon atom may increase its coordination number to values greater than four, to form pentacoordinated compounds. It
has been observed experimentally that, in general, pentacoordinated compounds show greater reactivity than tetracoordinated
compounds. In this work, density functional theory is used to calculate the global softness and the condensed softness of
the silicon atom for SiH
n
F4−n
and SiH
n
F
5−n
1−
. The values obtained show that the global and condensed softness are greater in the pentacoordinated compounds than in the
tetracoordinated compounds, a result that explains the enhanced reactivity. If the results are analysed through a local version
of the hard and soft acids and bases principle, it is possible to suggest that in nucleophilic substitution reactions, soft
nucleophiles preferably react with SiH
n
F
5−n
1−
, and hard nucleophiles with SiH
n
F4−n
. 相似文献
34.
Pál Sohár Gábor Bernáth Géza Stájer Angela E. Szabo 《Magnetic resonance in chemistry : MRC》1989,27(9):872-876
Pentacyclic isoxazolines were obtained by the cycloaddition of benzonitrile oxide to norbornene-azetidinone-fused 3,1-oxazines. The constitutions of two of the isomers obtained, and the configurations and conformations of all products, were determined by means of 1H and 13C NMR spectroscopy and DNOE experiments. 相似文献
35.
Igor Chueshov 《Journal of Differential Equations》2007,233(1):42-86
We study von Karman evolution equations with non-linear dissipation and with partially clamped and partially free boundary conditions. Two distinctive mechanisms of dissipation are considered: (i) internal dissipation generated by non-linear operator, and (ii) boundary dissipation generated by shear forces friction acting on a free part of the boundary. The main emphasis is given to the effects of boundary dissipation. Under suitable hypotheses we prove existence of a compact global attractor and finiteness of its fractal dimension. We also show that any solution is stabilized to an equilibrium and estimate the rate of the convergence which, in turn, depends on the behaviour at the origin of the functions describing the dissipation. 相似文献
36.
The initial stage of cubic silicon carbide (3C-SiC) growth on a Si(0 0 1) surface using dimethylsilane (DMS) as a source gas was observed using scanning tunneling microscopy (STM) and reflection high-energy electron diffraction (RHEED). It was found that the dimer vacancies initially existing on the Si(0 0 1)-(2 × 1) surface were repaired by the Si atoms in DMS molecules, during the formation of the c(4 × 4) surface. From the STM measurement, nucleation of SiC was found to start when the Si surface was covered with the c(4 × 4) structure but before the appearance of SiC spots in the RHEED pattern. The growth mechanism of SiC islands was also discussed based on the results of RHEED, STM and temperature-programmed desorption (TPD). 相似文献
37.
Yi-Shiue Lin 《Applied Surface Science》2006,252(16):5892-5899
Dissociative adsorption of CCl4 on TiO2 at 35 °C has been studied by Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, and electron spin resonance. CCl4 decompose to form CO, CO2, and CO3 on the surface, at such a low temperature, in which CO2 formation is not from CO oxidation on TiO2, but CO3 can be produced by CO and CO2 adsorption. The Cl generated from CCl4 decomposition is left on the surface and bonded to titanium ions. Mineralization of CCl4 on TiO2 involves the lattice oxygens. Thermodynamical driving force and possible reaction routes for CO and CO2 formation in the CCl4 decomposition on TiO2 are discussed. 相似文献
38.
S.A. Grigorian 《Topology and its Applications》2006,153(18):3598-3614
Finite-sheeted covering mappings onto compact connected groups are studied. We show that for a covering mapping from a connected Hausdorff topological space onto a compact (in general, non-abelian) group there exists a topological group structure on the covering space such that the mapping becomes a homomorphism of groups. To prove this fact we construct an inverse system of covering mappings onto Lie groups which approximates the given covering mapping. As an application, it is shown that a covering mapping onto a compact connected abelian group G must be a homeomorphism provided that the character group of G admits division by degree of the mapping. We also get a criterion for triviality of coverings in terms of means and prove that each finite covering of G is equivalent to a polynomial covering. 相似文献
39.
We prove a conjecture by F. Ferrari. Let X be the total space of a nonlinear deformation of a rank two holomorphic vector bundle on a smooth rational curve, such that X has trivial canonical bundle and has sections. Then the normal bundle to such sections is computed in terms of the rank of the Hessian of a suitably defined superpotential at its critical points. 相似文献
40.
Lin and Zhang (J. Roy. Statist. Soc. Ser. B 61 (1999) 381) proposed the generalized additive mixed model (GAMM) as a framework for analysis of correlated data, where normally distributed random effects are used to account for correlation in the data, and proposed to use double penalized quasi-likelihood (DPQL) to estimate the nonparametric functions in the model and marginal likelihood to estimate the smoothing parameters and variance components simultaneously. However, the normal distributional assumption for the random effects may not be realistic in many applications, and it is unclear how violation of this assumption affects ensuing inferences for GAMMs. For a particular class of GAMMs, we propose a conditional estimation procedure built on a conditional likelihood for the response given a sufficient statistic for the random effect, treating the random effect as a nuisance parameter, which thus should be robust to its distribution. In extensive simulation studies, we assess performance of this estimator under a range of conditions and use it as a basis for comparison to DPQL to evaluate the impact of violation of the normality assumption. The procedure is illustrated with application to data from the Multicenter AIDS Cohort Study (MACS). 相似文献