全文获取类型
收费全文 | 18366篇 |
免费 | 1833篇 |
国内免费 | 1053篇 |
专业分类
化学 | 9301篇 |
晶体学 | 197篇 |
力学 | 687篇 |
综合类 | 75篇 |
数学 | 1716篇 |
物理学 | 4321篇 |
无线电 | 4955篇 |
出版年
2024年 | 54篇 |
2023年 | 351篇 |
2022年 | 417篇 |
2021年 | 546篇 |
2020年 | 563篇 |
2019年 | 518篇 |
2018年 | 443篇 |
2017年 | 386篇 |
2016年 | 644篇 |
2015年 | 647篇 |
2014年 | 806篇 |
2013年 | 1124篇 |
2012年 | 1252篇 |
2011年 | 1306篇 |
2010年 | 864篇 |
2009年 | 778篇 |
2008年 | 1104篇 |
2007年 | 990篇 |
2006年 | 966篇 |
2005年 | 816篇 |
2004年 | 640篇 |
2003年 | 609篇 |
2002年 | 498篇 |
2001年 | 409篇 |
2000年 | 440篇 |
1999年 | 415篇 |
1998年 | 344篇 |
1997年 | 290篇 |
1996年 | 353篇 |
1995年 | 286篇 |
1994年 | 261篇 |
1993年 | 235篇 |
1992年 | 239篇 |
1991年 | 186篇 |
1990年 | 168篇 |
1989年 | 127篇 |
1988年 | 123篇 |
1987年 | 94篇 |
1986年 | 91篇 |
1985年 | 125篇 |
1984年 | 82篇 |
1983年 | 54篇 |
1982年 | 63篇 |
1981年 | 78篇 |
1980年 | 54篇 |
1979年 | 55篇 |
1978年 | 46篇 |
1977年 | 37篇 |
1975年 | 38篇 |
1974年 | 58篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
51.
Chia-Ching Lin Gwo-Yun Lee Kin-Lu Wong 《Electronics letters》2003,39(18):1302-1304
A surface-mount dual-loop antenna suitable for dual-frequency WLAN operation is presented. For achieving dual-frequency operation with a compact size, the antenna comprises two loop strips of different sizes printed on a flexible printed circuit board, which is then bent and attached onto a foam base of compact size. The antenna shows an attractive feature of high antenna gain, about 4 and 5 dBi for frequencies across the 2.4 and 5 GHz WLAN bands, respectively. 相似文献
52.
A jet-printed digital-lithographic method, in place of conventional photolithography, was used to fabricate 64 /spl times/ 64 pixel (300 /spl mu/m pitch) matrix addressing thin-film transistor (TFT) arrays. The average hydrogenated amorphous silicon TFT device within an array had a threshold voltage of /spl sim/3.5 V, carrier mobility of 0.7 cm/sup 2//V/spl middot/s, subthreshold slope of 0.76 V/decade, and an on/off ratio of 10/sup 8/. 相似文献
53.
Zukerman M. Wong E.W.M. Rosberg Z. Gyu Myoung Lee Hai Le Vu 《Communications Letters, IEEE》2004,8(2):116-118
We provide teletraffic models for loss probability evaluation of optical burst switching (OBS). We show that the popular Engset formula is not exact for OBS modeling and demonstrate that in certain cases it is not appropriate. A new exact model is provided. The various models are compared using numerical results for various OBS alternatives with and without burst segmentation. 相似文献
54.
A new method of using time-averaged digital speckle pattern interferometry for the quantitative measurement of vibration amplitude was developed. Signal processing techniques especially the Hilbert transformation for quantitative evaluation of the Bessel fringes obtained in time-averaged digital speckle pattern interferometry were explored. The quadrature signal after Hilbert transformation is equivalent to a 90° phase-shifted interferogram for a monotonically increasing or decreasing phase function. An algorithm was developed for Bessel fringe contrast enhancement and phase extraction. The techniques were tested numerically and experimentally. Sub-fringe quantification of the time-averaged vibration fringes is realised with the proposed method. Compared with the commonly used phase shift method which requires a minimum of two images for image processing, this method requires only one fringe pattern for data extraction. 相似文献
55.
Deale O.C. Ng K.T. Kim E.J. Lerman B.B. 《IEEE transactions on bio-medical engineering》1995,42(12):1208-1211
A high common mode voltage (Vcm) relative to earth ground is produced on the myocardium during the delivery of a defibrillator pulse and can generate a differential error signal when potential gradients are recorded with bipolar electrodes and isolation amplifiers. The error signal is proportional to Vcm, and therefore, a reduction in Vcm improves the accuracy of the potential gradient data. Experiments were conducted on 5 dogs to determine whether Vcm can be controlled using a bridge circuit. The bridge circuit consisted of a 5 kΩ power rheostat in parallel with the transthoracic resistance of the dog. The variable contact of the rheostat was connected to earth ground, and by adjusting the rheostat, Vcm on the myocardium could be varied. In each dog, 20 A shocks were delivered through stainless steel transthoracic electrodes. Point contact electrodes sutured to the epicardium were used to measure Vcm. It was determined that Vcm could be reduced to approximately zero at a given electrode on the heart. In addition, for the 5 dogs studied, the maximum measured Vcm on the heart was only 10% of the transthoracic voltage when the bridge circuit was balanced for an interior point in the heart 相似文献
56.
Lee V.V. Kuehne S.C. Nguyen C.T. Beiley M.A. Wong S.S. 《Electron Devices, IEEE Transactions on》1993,40(7):1223-1230
A local interconnection technology utilizing polysilicon strapped with selective-chemical-vapor-deposited (CVD) tungsten has been developed. Both n- and p-channel MOS transistors have been successfully fabricated using this technology. Tungsten deposited on polysilicon is an attractive gate shunt and local interconnection material because of its low resistivity, immunity to dopant segregation and diffusion, and resistance to electromigration. A potential problem of this technology is the excessive diode leakage current associated with strapping shallow source/drain diodes with tungsten. The leakage is attributed to defects induced by the heavy source/drain implant, which can be effectively eliminated with a proper annealing procedure 相似文献
57.
58.
Oblatum 4-VI-1990 &; 5-III-1991 相似文献
59.
Dagan Feng Koon-Pong Wong Chi-Ming Wu Wan-Chi Siu 《IEEE transactions on information technology in biomedicine》1997,1(4):243-254
Positron emission tomography (PET) is an important tool for enabling quantification of human brain function. However, quantitative studies using tracer kinetic modeling require the measurement of the tracer time-activity curve in plasma (PTAC) as the model input function. It is widely believed that the insertion of arterial lines and the subsequent collection and processing of the biomedical signal sampled from the arterial blood are not compatible with the practice of clinical PET, as it is invasive and exposes personnel to the risks associated with the handling of patient blood and radiation dose. Therefore, it is of interest to develop practical noninvasive measurement techniques for tracer kinetic modeling with PET. In this paper, a technique is proposed to extract the input function together with the physiological parameters from the brain dynamic images alone. The identifiability of this method is tested rigorously by using Monte Carlo simulation. The results show that the proposed method is able to quantify all the required parameters by using the information obtained from two or more regions of interest (ROIs) with very different dynamics in the PET dynamic images. There is no significant improvement in parameter estimation for the local cerebral metabolic rate of glucose (LCMRGlc) if there are more than three ROIs. The proposed method can provide very reliable estimation of LCMRGlc, which is our primary interest in this study 相似文献
60.
An ordered dynamic channel assignment with reassignment (ODCAR) scheme is proposed, and its performance is studied in a highway microcellular radio environment. Channels are assigned in an ordered basis in conjunction with a minimax algorithm under cochannel interference constraints, to provide high capacity and to alleviate worst case channel congestion in each microcell. Simulation results show significant performance improvements in terms of channel utilization and probability of call failure, at the expense of an increase in complexity and call switching requirements 相似文献