首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   317861篇
  免费   35517篇
  国内免费   26931篇
化学   157651篇
晶体学   3308篇
力学   15646篇
综合类   1420篇
数学   30721篇
物理学   93071篇
无线电   78492篇
  2024年   954篇
  2023年   5367篇
  2022年   7719篇
  2021年   10216篇
  2020年   9949篇
  2019年   9670篇
  2018年   9175篇
  2017年   9446篇
  2016年   12341篇
  2015年   12040篇
  2014年   15307篇
  2013年   19949篇
  2012年   22895篇
  2011年   23244篇
  2010年   17784篇
  2009年   17822篇
  2008年   18849篇
  2007年   17155篇
  2006年   16008篇
  2005年   14166篇
  2004年   10811篇
  2003年   9236篇
  2002年   8861篇
  2001年   7582篇
  2000年   6624篇
  1999年   6511篇
  1998年   5644篇
  1997年   4950篇
  1996年   5019篇
  1995年   4181篇
  1994年   4001篇
  1993年   3403篇
  1992年   3146篇
  1991年   2905篇
  1990年   2476篇
  1989年   2014篇
  1988年   1698篇
  1987年   1550篇
  1986年   1485篇
  1985年   1448篇
  1984年   1297篇
  1983年   1121篇
  1982年   1041篇
  1981年   881篇
  1979年   885篇
  1977年   880篇
  1976年   985篇
  1975年   877篇
  1974年   908篇
  1973年   899篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 55, No. 3, pp. 478–484, September, 1991.  相似文献   
82.
A new type of vortices called Chern-Simons vortices is considered and the existence is established.  相似文献   
83.
A polysilicon emitter RCA transistor (an ultra-thin interfacial oxide layer exists between polysilicon and silicon emitter) is presented which can operate at 77 K for the first time. An ultra-thin (1.5 nm) interfacial oxide layer is grown deliberately between polysilicon and silicon emitter using RCA oxidation and excellent device stability is obtained after rapid thermal annealing (RTA) treatment in nitrogen atmosphere. The RCA transistor exhibits good electrical performance at very low temperature for an emitter area of 3 × 8 μm2. The maximum toggle frequency of a 1:2 static divider is 1.2 GHz and 732 MHz at 300 K and 77 K, respectively  相似文献   
84.
In this paper, the influence of viewing distance on subjective assessment of the impairment in video sequences is investigated. Subjective tests using the double-stimulus impairment scale variant II (DSIS II) method have been conducted at viewing distances of 5H (where H stands for the screen height) and 3H, respectively. Several statistical measures have been used to analyze the influence, including correlations and ANOVA (analysis of variance) tests. The results reveal that there is a very high correlation between the subjective scores, the variances are similar under the two viewing distances, the means of subjective data at these two viewing distances are the same, and there is no interaction between the viewing distance and the other two factors, i.e., the codec system and the source sequence. Throughout the tests, there is no evidence that a closer viewing distance such as 3H will vary the subjective test result statistically significantly.  相似文献   
85.
An infinite graph is 2‐indivisible if the deletion of any finite set of vertices from the graph results in exactly one infinite component. Let G be a 4‐connected, 2‐indivisible, infinite, plane graph. It is known that G contains a spanning 1‐way infinite path. In this paper, we prove a stronger result by showing that, for any vertex x and any edge e on a facial cycle of G, there is a spanning 1‐way infinite path in G from x and through e. Results will be used in two forthcoming papers to establish a conjecture of Nash‐Williams. © 2005 Wiley Periodicals, Inc. J Graph Theory  相似文献   
86.
本文介绍了用单片机实现的小型燃油蒸汽锅炉控制系统 ,对硬件电路组成及其软件设计作了较为详细的介绍。  相似文献   
87.
We present a new iterative alternating-direction-implicit finite-difference time-domain (ADI-FDTD) method. By recognizing the ADI-FDTD method as a special case of a more general iterative approach to solve the Crank-Nicolson (CN) FDTD scheme, the splitting error in ADI-FDTD can be reduced systematically. Numerical examples are used to illustrate the improved accuracy of this method.  相似文献   
88.
Widely tunable bottom-emitting vertical-cavity SOAs   总被引:1,自引:0,他引:1  
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   
89.
In this paper, RF noise in 0.18-mum NMOSFETs concerning the contribution of carrier heating and hot carrier effect is characterized and analyzed in detail via a novel approach that modulates the channel carrier heating and number of hot carriers using body bias. We confirm qualitatively a negligible role of hot carrier effect on the channel noise in deep-submicrometer MOSFETs. For a device under reverse body bias (Vb), even though the increase in hot carrier population is clearly characterized by dc measurements, the device high-frequency noise is found to be irrelevant to the increase in the channel hot carriers. Experimental results show that the high-frequency noise is slightly reduced with the increase in |Vb|, and can be qualitatively explained by secondary effects such as the suppression of nonequilibrium channel noise and substrate induced noise. The reduction of NFmin and Rn with the increase in |Vb| may provide a possible methodology to finely adjust the device high-frequency noise performance for circuit design  相似文献   
90.
This paper studies the combination of practical trellis and convolution codes with Tomlinson-Harashima precoding (THP) for the presubtraction of multiuser interference that is known at the transmitter but not known at the receiver. It is well known that a straightforward application of THP suffers power, modulo, and shaping losses. This paper proposes generalizations of THP that recover some of these losses. At a high signal-to-noise ratio (SNR), the precoding loss is dominated by the shaping loss, which is about 1.53 dB. To recover shaping loss, a trellis-shaping technique is developed that takes into account the knowledge of a noncausal interfering sequence, rather than just the instantaneous interference. At rates of 2 and 3 bits per transmission, trellis shaping is shown to be able to recover almost all of the 1.53-dB shaping loss. At a low SNR, the precoding loss is dominated by power and modulo losses, which can be as large as 3-4 dB. To recover these losses, a technique that incorporates partial interference presubtraction (PIP) within convolutional decoding is developed. At rates of 0.5 and 0.25 bits per transmission, PIP is able to recover 1-1.5 dB of the power loss. For intermediate SNR channels, a combination of the two schemes is shown to recover both power and shaping losses.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号