全文获取类型
收费全文 | 16558篇 |
免费 | 2434篇 |
国内免费 | 1134篇 |
专业分类
化学 | 7356篇 |
晶体学 | 168篇 |
力学 | 710篇 |
综合类 | 72篇 |
数学 | 2440篇 |
物理学 | 9380篇 |
出版年
2024年 | 28篇 |
2023年 | 151篇 |
2022年 | 397篇 |
2021年 | 577篇 |
2020年 | 642篇 |
2019年 | 618篇 |
2018年 | 542篇 |
2017年 | 683篇 |
2016年 | 779篇 |
2015年 | 689篇 |
2014年 | 1127篇 |
2013年 | 1643篇 |
2012年 | 1021篇 |
2011年 | 1054篇 |
2010年 | 823篇 |
2009年 | 980篇 |
2008年 | 1006篇 |
2007年 | 1050篇 |
2006年 | 814篇 |
2005年 | 672篇 |
2004年 | 623篇 |
2003年 | 517篇 |
2002年 | 455篇 |
2001年 | 415篇 |
2000年 | 382篇 |
1999年 | 338篇 |
1998年 | 288篇 |
1997年 | 290篇 |
1996年 | 185篇 |
1995年 | 180篇 |
1994年 | 163篇 |
1993年 | 133篇 |
1992年 | 118篇 |
1991年 | 104篇 |
1990年 | 71篇 |
1989年 | 57篇 |
1988年 | 107篇 |
1987年 | 64篇 |
1986年 | 41篇 |
1985年 | 64篇 |
1984年 | 50篇 |
1983年 | 24篇 |
1982年 | 42篇 |
1981年 | 30篇 |
1980年 | 25篇 |
1979年 | 15篇 |
1978年 | 9篇 |
1977年 | 11篇 |
1973年 | 10篇 |
1972年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 581 毫秒
71.
激光水下扫描成像系统的图像修正机理 总被引:2,自引:0,他引:2
同步扫描系统可以克服后向散射,但同时也产生了两种非线性,一是扫描线速度的非线性,一是由于距离对光能衰减造成的非线性,两种非线性都需要修正,本文从原理上讨论水下激光成像时能量衰减的修正原理与修正方法,并给出仿真结果。 相似文献
72.
针对我军现代夜间反空袭作战的制约瓶颈——缺乏夜视装备,提出在预警领域采用被动红外周视扫描系统,在先进的火控领域采用制冷式红外热成像系统.在老式的防空火控系统上利用微光和非制冷红外技术进行改造的方案比较符合我国国情,也是比较现实和可行的。 相似文献
73.
74.
75.
By using the methods of the matrix decomposition and expansion of the hard-edged aperture function into a finite sum of complex Gaussian functions, the recurrence propagation expressions for a flattened Gaussian beam (FGB) through multi-apertured optical imaging systems of B = 0 are derived and illustrated with numerical examples. Comparisons with the straightforward numerical integration of the Collins formula and with the previous work are made. It is shown that the main advantages of our methods and results are the more accuracy and great reduction of computer time. 相似文献
76.
J. H. Lu Y. Y. Yang C. C. Chen C. H. Kuan H. T. Chen S. C. Lee 《Infrared Physics & Technology》2003,44(5-6):399-409
Superlattices have been demonstrated previously by our group in the design of the multicolor infrared photodetector. In general, the period number of the superlattice may be up to several dozens. In this paper, we have investigated the performance of the infrared photodetectors especially with 3, 5 and 15 periods. The detector structure contains a thick blocking barrier embedded between two superlattices with different period numbers but with the same well and barrier widths. This double-superlattice structure shows switchable spectral responses between two spectral regions by the voltage polarities. The photoresponse in each spectral region is also tunable by the magnitude of the applied voltage. The voltage-dependent behavior reveals the photoelectron relaxation and transport mechanism in the superlattice miniband. Superlattice with few periods has high electron group velocity, less relaxation effect and less collection efficiency. Therefore the superlattice with few periods may have better responsivity and narrower photoresponse range than the one with many periods. Based on the experimental results of our devices, it is observed that the superlattice with fewer periods has better detectivity, responsivity, wider range of the operational temperature, and more flexible miniband engineering than the conventional multiple quantum well infrared photodetector. 相似文献
78.
The atomic structures of indium (In) on silicon (Si) (1 0 0)-(2 × 1) surface are investigated by the local density approximation using first-principles pseudopotentials. Total energy optimizations show that the energetically favored structure is the parallel ad-dimer model. The adsorption energy of In on ideal Si(1 0 0)-(1 × 1) surface is significantly higher than that on reconstructed Si(1 0 0)-(2 × 1) surface, suggesting that In adsorption does not break the Si-Si dimer bond of the substrate. When Si surface contains single dimer vacancy defects, In chain will be interrupted, leading to disconnected In nanowires. Displacive adsorption of In on Si(1 0 0) is also considered, and the calculation suggests that interdiffusion of In into Si substrate will not be favorable under equilibrium conditions. 相似文献
79.
C. Baumgarten B. Braun G. Court G. Ciullo P. Ferretti G. Graw W. Haeberli M. Henoch R. Hertenberger N. Koch H. Kolster P. Lenisa A. Nass S.P. Pod'yachev D. Reggiani K. Rith M.C. Simani E. Steffens J. Stewart T. Wise 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2002,18(1):37-49
The use of storage cells has become a standard technique for internal gas targets in conjunction with high energy storage
rings. In case of spin-polarized hydrogen and deuterium gas targets the interaction of the injected atoms with the walls of
the storage cell can lead to depolarization and recombination. Thus the number of wall collisions of the atoms in the target
gas is important for modeling the processes of spin relaxation and recombination. It is shown in this article that the diffusion
process of rarefied gases in long tubes or storage cells can be described with the help of the one-dimensional diffusion equation.
Mathematical methods are presented that allow one to calculate collision age distributions (CAD) and their moments analytically.
These methods provide a better understanding of the different aspects of diffusion than Monte Carlo calculations. Additionally
it is shown that measurements of the atomic density or polarization of a gas sample taken from the center of the tube allow
one to determine the possible range of the corresponding density weighted average values along the tube. The calculations
are applied to the storage cell geometry of the HERMES internal polarized hydrogen and deuterium gas target.
Received 9 July 2001 and Received in final form 18 September 2001 相似文献
80.
Endoscopic Observation of Tissue by Narrowband Illumination 总被引:6,自引:0,他引:6
Kazuhiro Gono Kenji Yamazaki Nobuyuki Doguchi Tetsuo Nonami Takashi Obi Masahiro Yamaguchi Nagaaki Ohyama Hirohisa Machida Yasushi Sano Shigeaki Yoshida Yasuo Hamamoto Takao Endo 《Optical Review》2003,10(4):211-215
We propose a new illumination method for a medical endoscope: narrow band imaging (NBI), in which the spectral bandwidth of the filtered light is narrowed. To confirm how the spectral specifications of the filtered light influence a reproduced image, an experiment was conducted observing the endoscopic images of the back mucosa of a human tongue. In addition, the effect of NBI on endoscopic images was investigated through preliminary clinical tests in colonoscopy and upper gastrointestinal endoscopy. It has been shown that NBI can enhance the capillary pattern and the crypt pattern on the mucosa. These patterns are useful features for diagnosing an early cancer. 相似文献