全文获取类型
收费全文 | 5526篇 |
免费 | 1275篇 |
国内免费 | 430篇 |
专业分类
化学 | 2922篇 |
晶体学 | 15篇 |
力学 | 76篇 |
综合类 | 14篇 |
数学 | 62篇 |
物理学 | 4142篇 |
出版年
2024年 | 26篇 |
2023年 | 48篇 |
2022年 | 244篇 |
2021年 | 307篇 |
2020年 | 348篇 |
2019年 | 299篇 |
2018年 | 245篇 |
2017年 | 318篇 |
2016年 | 377篇 |
2015年 | 340篇 |
2014年 | 594篇 |
2013年 | 498篇 |
2012年 | 425篇 |
2011年 | 431篇 |
2010年 | 338篇 |
2009年 | 367篇 |
2008年 | 356篇 |
2007年 | 349篇 |
2006年 | 222篇 |
2005年 | 181篇 |
2004年 | 133篇 |
2003年 | 109篇 |
2002年 | 93篇 |
2001年 | 65篇 |
2000年 | 84篇 |
1999年 | 70篇 |
1998年 | 62篇 |
1997年 | 54篇 |
1996年 | 44篇 |
1995年 | 28篇 |
1994年 | 25篇 |
1993年 | 17篇 |
1992年 | 22篇 |
1991年 | 16篇 |
1990年 | 21篇 |
1989年 | 14篇 |
1988年 | 11篇 |
1987年 | 13篇 |
1986年 | 3篇 |
1985年 | 10篇 |
1984年 | 11篇 |
1983年 | 1篇 |
1982年 | 9篇 |
1980年 | 2篇 |
1975年 | 1篇 |
排序方式: 共有7231条查询结果,搜索用时 312 毫秒
991.
Weis J Ahlström H Hlavcak P Häggman M Ortiz-Nieto F Bergman A 《Magnetic resonance imaging》2009,27(1):87-93
Purpose
To minimize user and vendor dependence of the spectrum processing of prostate spectra, to measure the ratio of choline (Cho) plus creatine (Cr) to citrate (Cit) in the prostate tissue of normal volunteers and cancer patients, and to compare the results with pathologic findings after radical prostatectomy.Materials and methods
Four healthy volunteers and 13 patients with prostate cancer were measured. Measurements were performed using two-dimensional magnetic resonance spectroscopic imaging (MRSI) and endorectal coil. A standard vendor's spectrum processing approach has been modified. An original feature of this methodology was the combination of vendor-optimized and user-independent spectrum preprocessing in the scanner and user-independent quantitation in the environment of an MRUI software package. (Cho+Cr)/Cit ratio was used for the classification of prostate tissue. Results were compared with histopathology after radical prostatectomy.Results
Eight of 13 cancer patients were classified as suspicious or very suspicious for cancer at spectroscopy, three were ambiguous for cancer and two patients were evaluated as false negative. A considerable overlap of metabolite ratios at various Gleason score was found.Conclusion
The proposed spectrum processing has the potential to improve the accuracy and user independency of the (Cho+Cr)/Cit quantitation. This study confirmed the previous results that a considerable overlap of (Cho+Cr)/Cit ratios exists at various Gleason score levels. 相似文献992.
Kiryu S Inoue Y Watanabe M Izawa K Shimada M Tojo A Yoshikawa K Ohtomo K 《Magnetic resonance imaging》2009,27(1):101-107
We investigated the characteristics of gadoxetate disodium (Gd-EOB-DTPA) as a contrast agent for magnetic resonance imaging of the mouse liver. Mice were imaged sequentially under isoflurane anesthesia using a T1-weighted, three-dimensional fast low-angle shot (3D FLASH) sequence after an intravenous injection of Gd-EOB-DTPA or gadobenate dimeglumine (Gd-BOPTA), and the time course of the contrast effect was examined. The time course of the contrast effect of Gd-EOB-DTPA was also assessed after intravenous injection under pentobarbital anesthesia and after subcutaneous injection while awake or under isoflurane or pentobarbital anesthesia. Moreover, different doses of Gd-EOB-DTPA or Gd-BOPTA were injected subcutaneously into conscious mice, and the clarity of the liver border was evaluated visually. Intravenous injection under isoflurane anesthesia caused rapid contrast enhancement in the liver with both Gd-EOB-DTPA and Gd-BOPTA, and the contrast effect was 41% stronger with Gd-EOB-DTPA. Subcutaneous injection of Gd-EOB-DTPA caused delayed but favorable contrast enhancement in the liver. Washout of Gd-EOB-DTPA was faster in mice injected while awake than in those injected under anesthesia. After intravenous injection, washout was faster under pentobarbital anesthesia than under isoflurane anesthesia. The peak liver contrast was 11% and 18% stronger under pentobarbital anesthesia than under isoflurane anesthesia, after intravenous and subcutaneous injections, respectively. Subcutaneous injection of Gd-EOB-DTPA or Gd-BOPTA caused dose-dependent contrast effects in the liver. At a given dose, the contrast effect tended to be stronger and liver demarcation tended to be clearer with Gd-EOB-DTPA than with Gd-BOPTA. In conclusion, intravenous or subcutaneous injection of Gd-EOB-DTPA produces a favorable contrast effects in the mouse liver, indicating its potential in investigating mouse models of liver diseases. The contrast effects vary between conscious mice and anesthetized mice and among anesthetic agents used. 相似文献
993.
Manganese enhanced MRI (MEMRI) is an emerging technique for tracing neuronal pathways in vivo. However, manganese may leak into blood vessels or cerebrospinal fluid (CSF) after local injection and can be circulated to and taken up by brain regions that may not have connections to the targeted pathways. Comparing enhancement time courses after intranasal injection with intravenous infusion of MnCl2 in rats, the early enhancements in the pituitary gland (Pit) and hippocampus indicate the contrasts in those regions in the olfactory tract-tracing experiment were caused by such systemic effects. Since the Pit has easy access to manganese from the blood and its signal is proportional to other brain regions after intravenous infusion, it was used as an internal reference for the systemic effects. Applying intensity normalization by the Pit signal to tract-tracing data from the olfactory bulb led to reduced contrast in the hippocampus. These results demonstrate that nonspecific enhancements in MEMRI tract-tracing studies may have to be taken into account and that normalization by the Pit signal can compensate these effects. 相似文献
994.
Y. Tsuchiya Y. Nakajima T. Tamegai 《Physica C: Superconductivity and its Applications》2009,469(15-20):1084-1087
We have fabricated finite-sized Pb superconducting networks with 10 × 10 square (each 6 × 6 μm2) holes by using the electron beam lithography and vortex arrangements are visualized by using magneto-optical imaging. We find that the vortex penetration at low temperature is controlled by defects in the network. We also find nearly regular arrangements of vortices with defects close to 1/2 and1/3 of the matching field. 相似文献
995.
996.
997.
Characterization of a velocity-tunable 87Rb cold atomic source with a high-speed imaging technology
下载免费PDF全文
![点击此处可从《中国物理 B》网站下载免费的PDF全文](/ch/ext_images/free.gif)
This paper has developed and characterized a method to produce a velocity-tunable 87 Rb cold atomic source for atomic interferometry application.Using a high speed fluorescence imaging technology,it reports that the dynamic process of the atomic source formation is observed and the source performances including the flux and the initial velocity are characterized.A tunable atomic source with the initial velocity of 1.4~2.6 m/s and the atomic source flux of 2 × 10 8 ~ 6 × 10 9 atoms/s has been obtained with the built experimental setup. 相似文献
998.
平面三维显示技术的研究现状 总被引:6,自引:0,他引:6
平面三维显示技术是近年来最新出现的虚拟现实显示技术,其最大的特点是观察者无需使用任何辅助附加设备,直接用肉眼就可看到屏幕上显示的三维图像。为推进三维显示技术的发展,进一步研究了视差立体成像原理,并据此介绍几种平面三维显示方法及其工作原理,包括障栅立体显示、微柱透镜阵列立体显示、偏振片立体显示和基于微柱透镜立体显示原理的多视点系统,阐述并分析了系统的优缺点。以日本三洋公司的四视角立体显示装置、南京大学的多视点三维显示系统和NEC液晶科技的HDDP三维显示系统为例,描述了国内外该项技术近期的研究现状,分析了存在的技术难点,展望了该应用领域的发展前景。 相似文献
999.
应用CS理论实现同步采样压缩成像 总被引:2,自引:0,他引:2
郭军伟 《中国光学与应用光学》2009,2(6):525-530
为了减轻图像数据存储负担,实现图像在网络上的快速传输和实时处理,对一种新的压缩传感(CS)理论进行了研究。介绍了压缩传感理论的主要思想和基于压缩传感理论的光学成像系统,给出了一种新型图像重建算法—和谐正交匹配追踪算法,并进行了相应的模拟实验。实验结果显示,该成像机制可同步完成图像的采样与数据压缩,同时可获得良好的图像重建效果。由于该方法所要传输的信号数据量较小,所以十分有利于远距离的图像传输。 相似文献
1000.
总结了双棱镜干涉实验的调整难点及原因。提出解决的措施并对要点进行研究分析,收到良好效果。提出评价学生调整光路质量的方法。 相似文献