全文获取类型
收费全文 | 2170篇 |
免费 | 59篇 |
国内免费 | 22篇 |
专业分类
化学 | 1139篇 |
晶体学 | 9篇 |
力学 | 31篇 |
数学 | 272篇 |
物理学 | 485篇 |
无线电 | 315篇 |
出版年
2020年 | 18篇 |
2019年 | 25篇 |
2018年 | 22篇 |
2017年 | 14篇 |
2016年 | 42篇 |
2015年 | 36篇 |
2014年 | 45篇 |
2013年 | 91篇 |
2012年 | 90篇 |
2011年 | 112篇 |
2010年 | 70篇 |
2009年 | 66篇 |
2008年 | 122篇 |
2007年 | 109篇 |
2006年 | 124篇 |
2005年 | 102篇 |
2004年 | 117篇 |
2003年 | 82篇 |
2002年 | 85篇 |
2001年 | 65篇 |
2000年 | 52篇 |
1999年 | 41篇 |
1998年 | 33篇 |
1997年 | 33篇 |
1996年 | 46篇 |
1995年 | 29篇 |
1994年 | 29篇 |
1993年 | 54篇 |
1992年 | 34篇 |
1991年 | 31篇 |
1990年 | 23篇 |
1989年 | 23篇 |
1988年 | 22篇 |
1987年 | 10篇 |
1986年 | 21篇 |
1985年 | 20篇 |
1984年 | 15篇 |
1983年 | 11篇 |
1982年 | 16篇 |
1981年 | 13篇 |
1980年 | 12篇 |
1979年 | 22篇 |
1978年 | 22篇 |
1977年 | 14篇 |
1976年 | 22篇 |
1975年 | 16篇 |
1974年 | 12篇 |
1973年 | 12篇 |
1972年 | 10篇 |
1971年 | 11篇 |
排序方式: 共有2251条查询结果,搜索用时 16 毫秒
131.
Reconstruction and quantification of the carotid artery bifurcation from 3-D ultrasound images 总被引:4,自引:0,他引:4
Barratt DC Ariff BB Humphries KN Thom SA Hughes AD 《IEEE transactions on medical imaging》2004,23(5):567-583
Three-dimensional (3-D) ultrasound is a relatively new technique, which is well suited to imaging superficial blood vessels, and potentially provides a useful, noninvasive method for generating anatomically realistic 3-D models of the peripheral vasculature. Such models are essential for accurate simulation of blood flow using computational fluid dynamics (CFD), but may also be used to quantify atherosclerotic plaque more comprehensively than routine clinical methods. In this paper, we present a spline-based method for reconstructing the normal and diseased carotid artery bifurcation from images acquired using a freehand 3-D ultrasound system. The vessel wall (intima-media interface) and lumen surfaces are represented by a geometric model defined using smoothing splines. Using this coupled wall-lumen model, we demonstrate how plaque may be analyzed automatically to provide a comprehensive set of quantitative measures of size and shape, including established clinical measures, such as degree of (diameter) stenosis. The geometric accuracy of 3-D ultrasound reconstruction is assessed using pulsatile phantoms of the carotid bifurcation, and we conclude by demonstrating the in vivo application of the algorithms outlined to 3-D ultrasound scans from a series of patient carotid arteries. 相似文献
132.
The canonical quantization of the photon field in covariant gauge is studied in the presence of static boundaries, on which the field satisfies either “bag” or superconductor boundary conditions. The inclusion of the Fadeev-Popov ghost fields is found to be essential for agreement with Coulomb gauge calculations of the Casimir energy. 相似文献
133.
Vokes J.C. Hughes B.T. Wight D.R. Dawsey J.R. Shrubb S.J.W. 《Electronics letters》1979,15(20):627-629
A technology is described for the fabrication of Schottky-barrier f.e.t.s with electrodes on either or both sides of a submicrometre thick single-crystal layer of GaAs. Preliminary d.c. and microwave results are given together with possible advantages of these novel f.e.t. structures. 相似文献
134.
Cron GO Wallace JC Stevens WD Fortin T Pappas BA Wilkins RC Kelcz F Santyr GE 《Magnetic resonance imaging》2005,23(5):619-627
The arterial input function (AIF) is important for quantitative MR imaging perfusion experiments employing Gd contrast agents. This study compared the accuracy of T(2)*-weighted magnitude and phase imaging for noninvasive measurement of the AIF in the rat aorta. Twenty-eight in vivo experiments were performed involving simultaneous arterial blood sampling and MR imaging following Gd injection. In vitro experiments were also performed to confirm the in vivo results. At 1.89 T and TE=3 ms, the relationship between changes in 1/T(2)* in blood (estimated from MR signal magnitude) and Gd concentration ([Gd]) was measured to be approximately 19 s(-1) mM(-1), while that between phase and [Gd] was approximately 0.19 rad mM(-1). Both of these values are consistent with previously published results. The in vivo phase data had approximately half as much scatter with respect to [Gd] than the in vivo magnitude data (r(2)=.34 vs. r(2)=.17, respectively). This is likely due to the fact that the estimated change in 1/T(2)* is more sensitive than the phase to a variety of factors such as partial volume effects and T(1) weighting. Therefore, this study indicates that phase imaging may be a preferred method for measuring the AIF in the rat aorta compared to T(2)*-weighted magnitude imaging. 相似文献
135.
Brewer C Brewer G Butcher RJ Carpenter EE Cuenca L Schmiedekamp AM Viragh C 《Dalton transactions (Cambridge, England : 2003)》2005,(22):3617-3619
The iron(II) and manganese(II) complexes of the N(7) Schiff-base condensate of tris(3-aminopropyl)amine with 1-methyl-2-imidazolecarbaldehyde and the manganese(II) complex of the N(7) Schiff-base condensate of tris(3-aminopropyl)amine with 4-imidazolecarbaldehyde are high-spin mono capped octahedral seven-coordinate complexes with a short, approximately 2.44 è, metal to apical nitrogen bond. 相似文献
136.
Sporolides A and B: structurally unprecedented halogenated macrolides from the marine actinomycete Salinispora tropica 总被引:4,自引:0,他引:4
Buchanan GO Williams PG Feling RH Kauffman CA Jensen PR Fenical W 《Organic letters》2005,7(13):2731-2734
[structure: see text] Analysis of the fermentation broth of a strain of the marine actinomycete Salinispora tropica has led to the isolation of two unprecedented macrolides, sporolides A (1) and B (2). The structures and absolute stereochemistries of both metabolites were elucidated using a combination of NMR spectroscopy and X-ray crystallography. 相似文献
137.
138.
[reaction: see text] The expedient synthesis of a library of kanamycin B analogues is reported. The revealed SAR will guide future designs in developing kanamycin-type aminoglycoside antibiotics against drug-resistant bacteria. 相似文献
139.
140.
Hughes CE Carravetta M Levitt MH 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2004,167(2):306-265
Cogwheel phase cycles are often significantly shorter than traditional nested phase cycles. However, optimal solutions for cogwheel cycles are often difficult to find. This paper presents techniques and conjectures which the authors have found useful for generating cogwheel phase cycles either without the need for computer searches or with significantly smaller searches than would otherwise be necessary. The conjectures presently lack proofs but have been tested successfully for a large number of cases. 相似文献