全文获取类型
收费全文 | 573405篇 |
免费 | 6441篇 |
国内免费 | 2032篇 |
专业分类
化学 | 267156篇 |
晶体学 | 7232篇 |
力学 | 20545篇 |
综合类 | 9篇 |
数学 | 59403篇 |
物理学 | 146015篇 |
无线电 | 81518篇 |
出版年
2020年 | 3817篇 |
2019年 | 3996篇 |
2018年 | 4541篇 |
2017年 | 4320篇 |
2016年 | 7676篇 |
2015年 | 5644篇 |
2014年 | 8265篇 |
2013年 | 24012篇 |
2012年 | 17589篇 |
2011年 | 21887篇 |
2010年 | 14422篇 |
2009年 | 15242篇 |
2008年 | 21569篇 |
2007年 | 22520篇 |
2006年 | 21531篇 |
2005年 | 20166篇 |
2004年 | 18216篇 |
2003年 | 16377篇 |
2002年 | 16065篇 |
2001年 | 17681篇 |
2000年 | 14293篇 |
1999年 | 11753篇 |
1998年 | 10098篇 |
1997年 | 9909篇 |
1996年 | 9622篇 |
1995年 | 9029篇 |
1994年 | 8727篇 |
1993年 | 8583篇 |
1992年 | 9062篇 |
1991年 | 9046篇 |
1990年 | 8451篇 |
1989年 | 8025篇 |
1988年 | 8114篇 |
1987年 | 6946篇 |
1986年 | 6712篇 |
1985年 | 9134篇 |
1984年 | 9330篇 |
1983年 | 7763篇 |
1982年 | 8324篇 |
1981年 | 8102篇 |
1980年 | 7841篇 |
1979年 | 7841篇 |
1978年 | 8069篇 |
1977年 | 7890篇 |
1976年 | 7923篇 |
1975年 | 7487篇 |
1974年 | 7447篇 |
1973年 | 7797篇 |
1972年 | 4796篇 |
1971年 | 3704篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
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
52.
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
53.
Croon J.A. Rosmeulen M. Decoutere S. Sansen W. Maes H.E. 《Solid-State Circuits, IEEE Journal of》2002,37(8):1056-1064
In this paper, a physics-based mismatch model is presented. It is demonstrated on a 0.18-/spl mu/m technology that a simple mismatch model can still be used to characterize deep-submicron technologies. The accuracy of the model is examined and found to be within 20% in the strong inversion region. Bulk bias dependence is modeled in a physical way. To extract the mismatch parameters, a weighted fit is introduced. It is shown that the width and length dependence of the mismatch parameters is given by the Pelgrom model. 相似文献
54.
A PAC-Bayesian margin bound for linear classifiers 总被引:3,自引:0,他引:3
Herbrich R. Graepel T. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》2002,48(12):3140-3150
We present a bound on the generalization error of linear classifiers in terms of a refined margin quantity on the training sample. The result is obtained in a probably approximately correct (PAC)-Bayesian framework and is based on geometrical arguments in the space of linear classifiers. The new bound constitutes an exponential improvement of the so far tightest margin bound, which was developed in the luckiness framework, and scales logarithmically in the inverse margin. Even in the case of less training examples than input dimensions sufficiently large margins lead to nontrivial bound values and-for maximum margins-to a vanishing complexity term. In contrast to previous results, however, the new bound does depend on the dimensionality of feature space. The analysis shows that the classical margin is too coarse a measure for the essential quantity that controls the generalization error: the fraction of hypothesis space consistent with the training sample. The practical relevance of the result lies in the fact that the well-known support vector machine is optimal with respect to the new bound only if the feature vectors in the training sample are all of the same length. As a consequence, we recommend to use support vector machines (SVMs) on normalized feature vectors only. Numerical simulations support this recommendation and demonstrate that the new error bound can be used for the purpose of model selection. 相似文献
55.
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
56.
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
57.
Network fault identification is an important network management function, which is closely related to fault management and has an impact on other network management functions such as configuration management, and performance management. This paper investigates fault surveillance and fault identification mechanisms for a transparent optical network in which data travels optically from the source node to the destination node without going through any optical-to-electrical (O/E) or electrical-to-optical (E/O) conversion. Mechanisms and algorithms are proposed to detect and isolate faults such as fiber cuts, laser, receiver, or router failures. These mechanisms allow nonintrusive device monitoring without requiring any prior knowledge of the actual protocols being used in the data transmission 相似文献
58.
Single-phase voltage source power converters (VSCs) under consideration are AC-DC current-controlled boost-type power converters with bidirectional power-handling capability. Equivalence between two series-connected two-level power converters and a single three-level power converter is considered here. Further considered is the series operation of three-level power converters. Simulation results and experimental verification for both are provided. Economical configurations of three-level power converters leading to multilevel waveforms are presented thereafter 相似文献
59.
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
60.
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