全文获取类型
收费全文 | 676081篇 |
免费 | 7061篇 |
国内免费 | 2176篇 |
专业分类
化学 | 306694篇 |
晶体学 | 8835篇 |
力学 | 26233篇 |
综合类 | 14篇 |
数学 | 71380篇 |
物理学 | 180935篇 |
无线电 | 91227篇 |
出版年
2021年 | 5052篇 |
2020年 | 5537篇 |
2019年 | 6009篇 |
2018年 | 7477篇 |
2017年 | 7484篇 |
2016年 | 11453篇 |
2015年 | 7181篇 |
2014年 | 11442篇 |
2013年 | 29106篇 |
2012年 | 21927篇 |
2011年 | 26653篇 |
2010年 | 18995篇 |
2009年 | 19879篇 |
2008年 | 26095篇 |
2007年 | 27229篇 |
2006年 | 25690篇 |
2005年 | 23171篇 |
2004年 | 21479篇 |
2003年 | 19256篇 |
2002年 | 19023篇 |
2001年 | 21155篇 |
2000年 | 16824篇 |
1999年 | 13772篇 |
1998年 | 11865篇 |
1997年 | 11556篇 |
1996年 | 11112篇 |
1995年 | 10143篇 |
1994年 | 9823篇 |
1993年 | 9572篇 |
1992年 | 10255篇 |
1991年 | 10290篇 |
1990年 | 9705篇 |
1989年 | 9201篇 |
1988年 | 9080篇 |
1987年 | 8086篇 |
1986年 | 7695篇 |
1985年 | 10198篇 |
1984年 | 10310篇 |
1983年 | 8541篇 |
1982年 | 8847篇 |
1981年 | 8576篇 |
1980年 | 8268篇 |
1979年 | 8530篇 |
1978年 | 8593篇 |
1977年 | 8502篇 |
1976年 | 8426篇 |
1975年 | 8147篇 |
1974年 | 7880篇 |
1973年 | 8070篇 |
1972年 | 5277篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
Near-field corrections to site attenuation 总被引:1,自引:0,他引:1
The theoretical model used for calculating normalized site attenuation for broadband antennas in ANSI C63.4-1992 and for antenna calibration in ANSI C63.5-1988 includes only the radiation terms in the electric field. The omission of the near field terms leads to errors of as much as 2.0 dB at 30 MHz for horizontally polarized antennas separated by 3 m. Corrected values of normalized site attenuation and E Dmax are presented for the 30-300 MHz frequency range 相似文献
22.
Uddenfeldt J. 《Proceedings of the IEEE. Institute of Electrical and Electronics Engineers》1998,86(7):1319-1324
Cellular radio started in the early 1980s by using analog technologies. Research in voice coding, modulation, and channel coding resulted in second-generation cellular radio based on digital technologies, which were introduced in the early 1990s. These were all based on advanced time division multiple access technology, resulting in better capacity and lower cost. Today, these digital technologies-based on the Global System for Mobility, digital Advanced Mobile Phone System, and personal digital cellular-have more than 100 million subscribers worldwide. The next step is to introduce wide-band packet services for wireless Internet up to 2 Mb/s. These so-called third-generation systems (Universal Mobile Telecommunications Services, IMT2000) are planned to be introduced in the early 2000s 相似文献
23.
A general (possibly asymmetric noncausal and/or nonminimum phase) 2D autoregressive moving average random field model driven by an independent and identically distributed 2D nonGaussian sequence is considered. The model is restricted to be invertible, i.e., system zeros are not allowed to lie on the unit bicircle. Three performance criteria are investigated for parameter estimation of the system parameters given only the output measurements (image pixels). The proposed criteria are functions of the higher order cumulant statistics of an inverse filter output. One of these criteria is novel and the others have been considered in past only for moving average inverses and without any analysis of their consistency. In the paper strong consistency of the proposed methods under the assumption that the system order is known is proved. The convergence of the proposed parameter estimators under overparametrization is also analyzed. Experimental results involving synthesized as well as real life textures are presented to illustrate the performance of two of the considered approaches. Experimental results of synthesis of 128x128 textures visually resembling several real life textures in the Brodatz album (and other sources) are presented. 相似文献
24.
This investigation focuses on the correlation between the crystallographic orientation of grains with respect to magnetic
properties in Co-Cr films. Based on a distribution measured for the (0002) crystallographic pole densities, modified formulae
describing the texture were used to calculate the coercivity Hc, the squareness ratio Rs (Mr⊥/TMs) and the anisotropy field
Ha for Co-Cr films. In general, calculated values for the coercive force He agreed well with the measured values. The calculated
Rs⊥. values were much larger than the measured ones, however, indicating that the influence of the demagnetizing field and
the magnetostriction on the orientation of the magnetization cannot be neglected.
On leave from Institute of Computing Technology, Academia Sinica, P.O.Box 2704-6, Beijing, P.R. China. 相似文献
25.
Code-division multiple-access (CDMA) implemented with direct-sequence spread spectrum (DS/SS) signaling is a promising multiplexing technique for cellular telecommunications services. The efficiency of a direct-sequence spread-spectrum code-division multiple-access (DS-CDMA) system depends heavily on the shape of the spectrum of the spread signal. Maximum efficiency is obtained with an ideal brick-wall bandpass spectrum. There are two approaches toward achieving such a spectrum. One is to use a simple spreader that produces a broad spectrum and then follow it with a precise, high order filter to band limit the spectrum. A second approach, which is the approach taken in this paper, is to use a spreader that produces a spectrum close to the ideal spectrum and then employ a simple filter to control the out-of-band power. The proposed spreader/despreader is based on a simple hybrid function and can be easily implemented. An analysis provides a compact expression for the signal-to-noise ratio (SNR) of a RAKE receiver. The expression includes the effects of baseband, intermediate frequency (IF) and RF filtering as well as the effects of the spectral densities of the spreading/despreading functions. The analysis shows that the proposed spreader/despreader yields superior performance over a conventional pseudo noise (PN) spreading/despreading mechanism 相似文献
26.
This paper presents a new self-routing packet network called the plane interconnected parallel network (PIPN). In the proposed design, the traffic arriving at the network is shaped and routed through two banyan network based interconnected planes. The interconnections between the planes distribute the incoming load more homogeneously over the network. The throughput of the network under uniform and heterogeneous traffic requirements is studied analytically and by simulation. The results are compared with the results of the baseline network and another banyan network based parallel interconnection network. It is shown that, for the proposed design, a higher degree of heterogeneity results in better performance 相似文献
27.
This paper presents the design criteria, procedure, and implementation of a soft-switched power-factor-correction (PFC) circuit based on the extended-period quasi-resonant (EPQR) principles. All power electronic devices including switches and diodes in the circuit are fully soft switched. The design method is demonstrated in a prototype circuit. The operating principles are confirmed with computer simulation and experimental results. A comparison of the EP-QR operation and zero-voltage-transition (ZVT) pulse-width modulation (PWM) method 相似文献
28.
Douay M. Xie W.X. Taunay T. Bernage P. Niay P. Cordier P. Poumellec B. Dong L. Bayon J.F. Poignant H. Delevaque E. 《Lightwave Technology, Journal of》1997,15(8):1329-1342
A comprehensive survey of photosensitivity in silica glasses and optical fiber is reviewed. Recent work on understanding the mechanisms contributing to germanium or aluminum doped fiber photosensitivity is discussed within the framework of photoelastic densification models 相似文献
29.
Watanabe H. Komori J. Higashitani K. Sekine M. Koyama H. 《Semiconductor Manufacturing, IEEE Transactions on》1997,10(2):228-232
A novel monitoring method for plasma-charging damage is proposed. This method performs a quick and accurate evaluation using antenna PMOSFET. It was found that not only hot-carrier (HC) lifetime but transistor parameters such as initial gate current and substrate current were changed according to the degree of plasma-charging damage. However, the present work suggests that monitoring the shift of drain current after a few seconds of HC stress is a more accurate method to indicate plasma-charging damage. The monitoring method using the present test structure is demonstrated to be useful for realizing highly reliable devices 相似文献
30.
J Havick 《The Journal of communication》1997,47(2):97-111
This study employed two perspectives to investigate media attention given women congressional candidates. The first perspective is that media attention may be explained by typical and normal media processes, such as focusing on incumbents. The second perspective considers a partisan explanation in which media attention is weighted more to Democratic candidates than Republican candidates. This study employs two established sources, Vanderbilt's Television News Index and Abstract and Information Access's National Newspaper Index , to examine the national media attention of Democratic and Republican women congressional candidates in 1990 and 1992. The investigation determined that media attention is related to whether the candidates are running for the Senate or House, incumbency, and state population. The results also reveal that party is a statistically significant factor associated with media attention. The investigation also determined that ethnic candidates tend to receive slightly less attention from the print media than nonethnic candidates. 相似文献