全文获取类型
收费全文 | 1358篇 |
免费 | 222篇 |
国内免费 | 75篇 |
专业分类
化学 | 530篇 |
晶体学 | 10篇 |
力学 | 39篇 |
综合类 | 11篇 |
数学 | 113篇 |
物理学 | 952篇 |
出版年
2024年 | 2篇 |
2023年 | 17篇 |
2022年 | 23篇 |
2021年 | 44篇 |
2020年 | 37篇 |
2019年 | 33篇 |
2018年 | 33篇 |
2017年 | 57篇 |
2016年 | 97篇 |
2015年 | 70篇 |
2014年 | 116篇 |
2013年 | 105篇 |
2012年 | 80篇 |
2011年 | 91篇 |
2010年 | 92篇 |
2009年 | 90篇 |
2008年 | 99篇 |
2007年 | 81篇 |
2006年 | 59篇 |
2005年 | 56篇 |
2004年 | 49篇 |
2003年 | 45篇 |
2002年 | 38篇 |
2001年 | 28篇 |
2000年 | 23篇 |
1999年 | 28篇 |
1998年 | 28篇 |
1997年 | 21篇 |
1996年 | 19篇 |
1995年 | 10篇 |
1994年 | 10篇 |
1993年 | 8篇 |
1992年 | 8篇 |
1991年 | 6篇 |
1990年 | 9篇 |
1989年 | 3篇 |
1988年 | 5篇 |
1987年 | 4篇 |
1986年 | 4篇 |
1985年 | 7篇 |
1984年 | 8篇 |
1983年 | 1篇 |
1982年 | 4篇 |
1981年 | 1篇 |
1979年 | 2篇 |
1978年 | 1篇 |
1973年 | 1篇 |
1972年 | 1篇 |
1970年 | 1篇 |
排序方式: 共有1655条查询结果,搜索用时 546 毫秒
1.
Jonathan M. Levin Lawrence L. Wald Marc J. Kaufman Marjorie H. Ross Luis C. Maas Perry F. Renshaw 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1998,130(2):292-295
Residual effects of an initial bolus of gadolinium contrast agent have been previously demonstrated in sequential dynamic susceptibility contrast MR experiments. While these residual effects quickly reach a saturation steady state, their etiology is uncertain, and they can lead to spurious estimates of hemodynamic parameters in activation experiments. The possible influence ofT1effects is now investigated with experiments in whichT1weighting is varied as well as with serial regionalT1measurements. Little evidence for significant residualT1effects is found, suggesting instead that susceptibility effects underlie these observations. An initial saturation dose of contrast agent minimizes this effect. 相似文献
2.
Recursive algorithms for the computation of standard deviation and average deviation are derived and their applications in data acquisition are discussed. The relative speeds and accuracies of the two algorithms are compared for synthetic data. The performance of recursive estimation under shot and proportional noise limitations is also described. As an example of the utility of these algorithms, absorbance data with constant confidence intervals are collected regardless of incident and transmitted intensities. The desired precision is specified prior to data acquisition and used to control signal-averaging of the data in real time. 相似文献
3.
光纤倒像器是一种特殊类型的光纤板,可将传递图像直接倒转180°并应用于像增强器。由于光纤倒像器的特殊结构,造成光纤随着距扭转轴心的距离增加,数值孔径和光通量逐渐下降。相对于光纤板,光纤倒像器在透过率和对比度传递特性上均有一定程度的下降。通过改进玻璃系统,在一定程度上降低芯皮玻璃间离子扩散和相互渗透的程度,可提高光纤倒像器的实际数值孔径;通过调节EMA吸收量,既能满足像增强器对光纤倒像器荧光屏透过率的要求,又能保证最大程度地吸收光纤中逸出的杂散光,提高对比度。选择合理的芯皮比,不仅可弥补因扭转拉伸造成的皮层厚度减薄,还可进一步增加皮层厚度,抑制光从光纤中逸出。通过以上改进,可改善光纤倒像器的对比度传递特性。 相似文献
4.
5.
本文提出从高分辨电子显微像中,利用光学、全息和部分相干等方法,提取传递函数,制作相位和振幅滤波器,实现解卷,提高高分辨电子显微像的分辨率. 相似文献
6.
An experimental method is developed to examine the near tip deformation at the mesoscopic scale level. The differential interference contrast (DIC) method is used by application of the Nomarski prism in polarized microscope for measuring the out of surface deformation. The method is very sensitive to small height changes detected by different interference color. Discussed are results for the crack tip deformation field. 相似文献
7.
8.
基于正弦灰度变换的X光图像增强 总被引:11,自引:0,他引:11
介绍了灰度变换法的几种形式 ,提出了一种正弦非线性变换法 ,对三幅数字X光医学图像进行了处理 ,获得了令人满意的对比度增强效果 ,证明该方法有效而实用 相似文献
9.
《Magnetic resonance in chemistry : MRC》2003,41(8):585-588
A novel, radical responsive MRI contrast agent based on a gadolinium chelate conjugated to a liposome through a disulfide linker was synthesized, with the aim of pursuing the in vivo mapping of radicals. The liposome was prepared by incorporating a thiol‐activated phospholipid, which was subsequently reacted with a gadolinium chelate containing a free thiol group. The long reorientational motion of the supramolecular adduct endows the paramagnetic agent with a relaxivity significantly higher than that of the free complex. The disulfide bond represents a radical‐sensitive moiety and a large decrease in contrast efficacy (T1 relaxivity) is shown upon its cleavage. A preliminary assessment of the system was made by means of in vitro gamma‐irradiation and thiol–disulfide bond exchange with dithiothreitol. Both methods showed a clear dose‐dependent decrease in T1‐relaxivity. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
10.
Labib H. Ghaoui 《Journal of separation science》1992,15(7):449-451
Ensemble or signal averaging, and the use of the technique for the solution of some unique problems with minimum sample handling, have previously been reported. The capability of the technique has been evaluated with microbore gas chromatography for the fast handling of a large number of analyses: ca 20–30 replicate injections are easily attainable in a reasonable analysis time. The implementation of the signal averaging technique with other concentration techniques for improving detection limits is also demonstrated. 相似文献