全文获取类型
收费全文 | 255635篇 |
免费 | 2186篇 |
国内免费 | 791篇 |
专业分类
化学 | 127481篇 |
晶体学 | 3374篇 |
力学 | 8816篇 |
综合类 | 6篇 |
数学 | 25653篇 |
物理学 | 66964篇 |
无线电 | 26318篇 |
出版年
2021年 | 2061篇 |
2020年 | 2411篇 |
2019年 | 2712篇 |
2018年 | 3675篇 |
2017年 | 3503篇 |
2016年 | 4894篇 |
2015年 | 2788篇 |
2014年 | 4464篇 |
2013年 | 10519篇 |
2012年 | 7953篇 |
2011年 | 9833篇 |
2010年 | 6999篇 |
2009年 | 7099篇 |
2008年 | 9779篇 |
2007年 | 10087篇 |
2006年 | 9532篇 |
2005年 | 8705篇 |
2004年 | 7886篇 |
2003年 | 7251篇 |
2002年 | 7096篇 |
2001年 | 7062篇 |
2000年 | 5849篇 |
1999年 | 4452篇 |
1998年 | 4026篇 |
1997年 | 3971篇 |
1996年 | 3743篇 |
1995年 | 3174篇 |
1994年 | 3290篇 |
1993年 | 3222篇 |
1992年 | 3355篇 |
1991年 | 3443篇 |
1990年 | 3311篇 |
1989年 | 3193篇 |
1988年 | 3095篇 |
1987年 | 2822篇 |
1986年 | 2724篇 |
1985年 | 3489篇 |
1984年 | 3655篇 |
1983年 | 2945篇 |
1982年 | 3254篇 |
1981年 | 3086篇 |
1980年 | 2895篇 |
1979年 | 3078篇 |
1978年 | 3170篇 |
1977年 | 3167篇 |
1976年 | 3138篇 |
1975年 | 2971篇 |
1974年 | 2951篇 |
1973年 | 3072篇 |
1972年 | 2076篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
121.
122.
Widely tunable bottom-emitting vertical-cavity SOAs 总被引:1,自引:0,他引:1
Cole G.D. Bjorlin E.S. Wang C.S. MacDonald N.C. Bowers J.E. 《Photonics Technology Letters, IEEE》2005,17(12):2526-2528
We present bottom-emitting tunable vertical-cavity semiconductor optical amplifiers (VCSOAs) with an effective wavelength tuning range of >20 nm. These devices utilize a high reflectivity micromechanically tunable Bragg mirror as the back reflector. Compared with our first generation tunable VCSOAs, the bottom-emitting devices exhibit a two-fold increase in the effective tuning range as well as a five-fold reduction in the required tuning voltage. 相似文献
123.
Troya A. Maharatna K. Krstic M. Grass E. Jagdhold U. Kraemer R. 《IEEE transactions on circuits and systems. I, Regular papers》2008,55(2):672-686
In this paper, we propose low-power designs for the synchronizer and channel estimator units of the Inner Receiver in wireless local area network systems. The objective of the work is the optimization, with respect to power, area, and latency, of both the signal processing algorithms themselves and their implementation. Novel circuit design strategies have been employed to realize optimal hardware and power efficient architectures for the fast Fourier transform, arc tangent computation unit, numerically controlled oscillator, and the decimation filters. The use of multiple clock domains and clock gating reduces the power consumption further. These blocks have been integrated into an experimental digital baseband processor for the IEEE 802.11a standard implemented in the 0.25mum- 5-metal layer BiCMOS technology from Institute for High Performance Microelectronics. 相似文献
124.
Yu. A. Chaplygin E. A. Artamonova A. Yu. Krasyukov T. Yu. Krupkina 《Semiconductors》2008,42(13):1522-1526
Heat removal problems, thermal effects, and self-heating phenomena occurring during operation of planar power SOI MOS transistors are considered. Using device-technological simulating methods, the transistor characteristics and safe operation range were studied. It was shown that limitations of the safe operation range are mostly associated with structure self-heating rather than with the parasitic bipolar transistor. 相似文献
125.
126.
We show that, in dimensions equal or greater than three, classical strings do not join or split. This is part of a no-interaction theorem that is also proved. The quantum theory is considered. 相似文献
127.
D. N. Karimov E. A. Krivandina Z. I. Zhmurova B. P. Sobolev V. A. Bezhanov S. P. Chernov G. M. Shapochkin 《Crystallography Reports》2006,51(6):1009-1015
Crystalline materials that are transparent in the vacuum UV spectral region and currently used have been reviewed. Transmission of crystals of solid solutions with the fluorite structure Ca1?x R xF2+x (R = Sc, Y, La, Yb, Lu) in the UV and vacuum UV spectral regions has been investigated. It is shown that application of different methods of purification of fluorides from some impurities can significantly improve the optical quality of fluoride multicomponent crystals in the short-wavelength spectral region. 相似文献
128.
Self-Adapting Linear Algebra Algorithms and Software 总被引:2,自引:0,他引:2
Demmel J. Dongarra J. Eijkhout V. Fuentes E. Petitet A. Vuduc R. Whaley R.C. Yelick K. 《Proceedings of the IEEE. Institute of Electrical and Electronics Engineers》2005,93(2):293-312
One of the main obstacles to the efficient solution of scientific problems is the problem of tuning software, both to the available architecture and to the user problem at hand. We describe approaches for obtaining tuned high-performance kernels and for automatically choosing suitable algorithms. Specifically, we describe the generation of dense and sparse Basic Linear Algebra Subprograms (BLAS) kernels, and the selection of linear solver algorithms. However, the ideas presented here extend beyond these areas, which can be considered proof of concept. 相似文献
129.
H E Cline W E Lorensen R J Herfkens G A Johnson G H Glover 《Magnetic resonance imaging》1989,7(1):45-54
A three-dimensional examination of blood vessels is provided using MR data from seven cases. The vascular surfaces are constructed with an algorithm that automatically follows the selected artery or vein and generates a projected three-dimensional gradient shaded image. Fast 3DFT pulse sequences were optimized to enhance the time-of-flight contrast of the intravascular region. By increasing the surface threshold value in a three-dimensional head study, the flesh of a patient's face was peeled away to demonstrate the superfacial temporal artery. Gated cardiac images show the great vessels and cardiac chambers. A three-dimensional view of the aorta shows an irregular surface in the vicinity of an adrenal tumor. 3D MR exams provide a non-invasive technique for assessing vascular morphology in a clinical setting. 相似文献
130.
Calculations are performed, using a basis of two-center functions described previously, for excited states of the following three-and four-electron compounds: H2
–, He2
+, He2, Li2
+2. High efficiency is demonstrated for the frozen ionic core approximation and the nonorthogonal orbital method in such calculations. For four-electron systems the practical feasibility of the frozen hybrid core approximation is demonstrated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 53–56, August, 1987. 相似文献