全文获取类型
收费全文 | 9855篇 |
免费 | 222篇 |
国内免费 | 94篇 |
专业分类
化学 | 6098篇 |
晶体学 | 108篇 |
力学 | 236篇 |
数学 | 1983篇 |
物理学 | 1746篇 |
出版年
2020年 | 93篇 |
2019年 | 84篇 |
2018年 | 97篇 |
2017年 | 59篇 |
2016年 | 169篇 |
2015年 | 154篇 |
2014年 | 159篇 |
2013年 | 492篇 |
2012年 | 434篇 |
2011年 | 509篇 |
2010年 | 301篇 |
2009年 | 249篇 |
2008年 | 453篇 |
2007年 | 447篇 |
2006年 | 519篇 |
2005年 | 493篇 |
2004年 | 404篇 |
2003年 | 362篇 |
2002年 | 374篇 |
2001年 | 150篇 |
2000年 | 132篇 |
1999年 | 129篇 |
1998年 | 137篇 |
1997年 | 132篇 |
1996年 | 187篇 |
1995年 | 115篇 |
1994年 | 113篇 |
1993年 | 114篇 |
1992年 | 120篇 |
1991年 | 110篇 |
1990年 | 88篇 |
1989年 | 92篇 |
1988年 | 87篇 |
1987年 | 102篇 |
1986年 | 97篇 |
1985年 | 148篇 |
1984年 | 164篇 |
1983年 | 115篇 |
1982年 | 126篇 |
1981年 | 148篇 |
1980年 | 157篇 |
1979年 | 129篇 |
1978年 | 138篇 |
1977年 | 110篇 |
1976年 | 119篇 |
1975年 | 133篇 |
1974年 | 123篇 |
1973年 | 116篇 |
1972年 | 63篇 |
1971年 | 62篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
51.
The detection of serial changes in magnetic resonance (MR) signal intensity of the heart following acute myocardial infarction may provide a useful method of characterizing tissue healing. Fourteen patients with acute Q-wave infarction underwent T2-weighted, spin-echo cardiac imaging during hospitalization, followed by one or more additional MR studies (total 31) over a 6- to 27-wk period (mean: 3 mo). Visual assessment of the images demonstrated a gradual reduction in signal intensity and localization of the bright signal to the subendocardium of the infarction region over the three-mo study period. A quantitative measurement of signal intensity (infarction/normal myocardium) fell from 1.81 +/- 0.42 on the initial study to 1.34 +/- 0.37 (p less than 0.05) at a mean of 14 wk. Two patients had an increase in signal intensity on the follow-up study and both patients had been readmitted with acute coronary syndromes. In summary, characterization of changes in signal intensity may provide a useful method of assessing myocardial healing following acute myocardial infarction. Further studies are indicated to determine the prognostic significance of these parameters. 相似文献
52.
53.
Alick KT Law Deepak Gupta Shawn Levy Douglas C Wallace Robert J McKeon Charles R Buck 《BMC neuroscience》2004,5(1):1
Background
The adenine nucleotide translocator 1 (Ant1) is an inner mitochondrial membrane protein involved with energy mobilization during oxidative phosphorylation. We recently showed that rodent Ant1 is upregulated by transforming growth factor-beta (TGF-β) in reactive astrocytes following CNS injury. In the present study, we describe the molecular mechanisms by which TGF-β1 regulates Ant1 gene expression in cultured primary rodent astrocytes. 相似文献54.
Jean-Philippe Bouillon Charles Portella Alessandro Degl’Innocenti 《Tetrahedron letters》2004,45(1):87-90
The thionation of bis(acylsilanes) with spacers of variable size with hexamethyldisilathiane under cobalt(II) chloride or trimethylsilyl triflate catalysis affords 2,5-bis(trialkylsilyl)-thiophenes, 2,6-bis(trialkylsilyl)-4H-thiopyrans and 2,7-bis(trialkylsilyl)-4,5-dihydrothiepine generally along with a minor amount of the corresponding oxo analogue. The synthesis of both symmetrical and unsymmetrical bis(trialkylsilyl) derivatives was achieved. 相似文献
55.
The passive control of bluff body flows using porous media is investigated by means of the penalization method. This method is used to create intermediate porous media between solid obstacles and the fluid in order to modify the boundary layer behaviour. The study covers a wide range of two‐dimensional flows from low transitional flow to fully established turbulence by direct numerical simulation of incompressible Navier–Stokes equations. A parametric study is performed to illustrate the effect of the porous layer permeability and thickness on the passive control. The numerical results reveal the ability of porous media to both regularize the flow and to reduce the drag forces up to 30%. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
56.
Alex Charles Cumming 《Fresenius' Journal of Analytical Chemistry》1911,50(2):111-112
Ohne Zusammenfassung 相似文献
57.
A central problem that arises in evolutionary biology is that of displaying partitions of subsets of a finite set X on a tree whose vertices are partially labelled with the elements of X. Such a tree is called an X-tree and, for a collection of partitions of subsets of X, characterisations for the existence and uniqueness of an X-tree that displays have been previously given in terms of chordal graphs. In this paper, we obtain two closely related characterisations also in terms of chordal graphs. The first describes when identifies an X-tree, and the second describes when a compatible subset of is of maximum size. 相似文献
58.
Non-compact generalized variational inequalities for quasi-monotone and hemi-continuous operators with applications 总被引:1,自引:0,他引:1
Some results are obtained for non-compact cases in topological vector spaces for the existence problem of solutions for some
set-valued variational inequalities with quasi-monotone and lower hemi-continuous operators, and with quasi-semi-monotone
and upper hemi-continuous operators. Some applications are given in non-reflexive Banach spaces for these existence problems
of solutions and for perturbation problems for these set-valued variational inequalities with quasi-monotone and quasi-semi-monotone
operators.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
59.
In previous work, a probabilistic approach to controlling difficulties of density in hyperbolic space led to a workable notion of optimal density for packings of bodies. In this paper we extend an ergodic theorem of Nevo to provide an appropriate definition of those packings to be considered optimally dense. Examples are given to illustrate various aspects of the density problem, in particular the shift in emphasis from the analysis of individual packings to spaces of packings. 相似文献
60.
Logic Regression is an adaptive regression methodology mainly developed to explore high-order interactions in genomic data. Logic Regression is intended for situations where most of the covariates in the data to be analyzed are binary. The goal of Logic Regression is to find predictors that are Boolean (logical) combinations of the original predictors. In this article, we give an overview of the methodology and discuss some applications. We also describe the software for Logic Regression, which is available as an R and S-Plus package. 相似文献