全文获取类型
收费全文 | 471950篇 |
免费 | 4068篇 |
国内免费 | 1456篇 |
专业分类
化学 | 219385篇 |
晶体学 | 6473篇 |
力学 | 19189篇 |
综合类 | 12篇 |
数学 | 47328篇 |
物理学 | 127974篇 |
无线电 | 57113篇 |
出版年
2020年 | 3925篇 |
2019年 | 4720篇 |
2018年 | 6141篇 |
2017年 | 6040篇 |
2016年 | 8606篇 |
2015年 | 4883篇 |
2014年 | 8262篇 |
2013年 | 19651篇 |
2012年 | 14558篇 |
2011年 | 17588篇 |
2010年 | 13087篇 |
2009年 | 13699篇 |
2008年 | 17303篇 |
2007年 | 17675篇 |
2006年 | 16255篇 |
2005年 | 15077篇 |
2004年 | 13874篇 |
2003年 | 12634篇 |
2002年 | 12457篇 |
2001年 | 13808篇 |
2000年 | 10924篇 |
1999年 | 8765篇 |
1998年 | 7645篇 |
1997年 | 7748篇 |
1996年 | 7153篇 |
1995年 | 6647篇 |
1994年 | 6593篇 |
1993年 | 6563篇 |
1992年 | 6942篇 |
1991年 | 7130篇 |
1990年 | 6872篇 |
1989年 | 6679篇 |
1988年 | 6545篇 |
1987年 | 5901篇 |
1986年 | 5644篇 |
1985年 | 7330篇 |
1984年 | 7503篇 |
1983年 | 6288篇 |
1982年 | 6462篇 |
1981年 | 6303篇 |
1980年 | 5960篇 |
1979年 | 6293篇 |
1978年 | 6438篇 |
1977年 | 6417篇 |
1976年 | 6646篇 |
1975年 | 6022篇 |
1974年 | 6094篇 |
1973年 | 6477篇 |
1972年 | 4438篇 |
1971年 | 3805篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
Lightly p-doped (3×1017 cm-3) GaN grown on GaAs substrates by metal organic molecular beam epitaxy (MOMBE) shows deactivation of the residual acceptors on exposure to a microwave (2.45 GHz) hydrogen plasma at 250°C. Subsequent annealing to 350°C produces further dopant passivation, while higher temperatures (450°C) restore the initial conductivity. These results suggest that hydrogen carrier gases should be avoided during vapour phase growth of III-V nitrides 相似文献
952.
The feasibility of using a thick ground plane with an aperture coupled microstrip patch antenna while maintaining reasonable antenna performance is demonstrated using moment method and reciprocity analyses in the spectral domain. The thick ground plane, which may serve as a heatsink for active MMIC circuitry, or as a mechanical support for thin substrates, is particularly advantageous in millimetre wave phased array applications. It is found that the effect of ground plane thickness is to reduce the level of coupling from the feed line to the patch 相似文献
953.
Chung S.S. Shui-Ming Cheng Lee R.G.-H. Song-Nian Kuo Mong-Song Liang 《Electron Devices, IEEE Transactions on》1997,44(12):2220-2226
This paper reports a simple I-V method for the first time to determine the lateral lightly-doped source/drain (S/D) profiles (n- region) of LDD n-MOSFETs. One interesting result is the direct observation of the reverse-short-channel effect (RSCE). It is observed that S/D n- doping profile is channel length dependent if reverse short-channel effect exists as a result of the interstitial imperfections caused by Oxide Enhanced Diffusion (OED) or S/D implant. Not only the lateral profiles for long-channel devices but also for short-channel devices can be determined. One other practical application of the present method for device drain engineering has been demonstrated with a LATID MOS device drain engineering work. It is convincible that the proposed method is well suited for the characterization and optimization of submicron and deep-submicron MOSFETs in the current ULSI technology 相似文献
954.
Presents a method to solve the inverse problem of electrocardiography using the Laplacian of the body surface potentials. The method presented is studied first using trade-off curves from a concentric spheres model representing a heart-torso system. Then a more conventional study is undertaken where a limited number of current dipoles are placed within the inner sphere and noise is added to the resulting potentials and Laplacians on the surface of the outer sphere. The results indicate that measurements of the outer surface Laplacian can more accurately reconstruct epicardial potentials than measurements of the outer surface potentials. The reconstructions are more accurate in that extrema are placed very close to their correct positions and multiple extrema and high potential gradients are recovered. Identical conclusions are observed in the presence of noise and even when the Laplacians are subject to greater noise than the potentials 相似文献
955.
Transistor equivalent circuits 总被引:1,自引:0,他引:1
Pritchard R.L. 《Proceedings of the IEEE. Institute of Electrical and Electronics Engineers》1998,86(1):150-162
This paper surveys the history of the electric-circuit representation of the transistor over the past fifty years. During the first two decades after the transistor was announced in 1948, primary emphasis was on small-signal equivalent circuits, which could be used for linear-circuit analysis and design. In addition, parameters of many of these equivalent circuits for the bipolar junction transistor, which are described, were related to the physical construction of the device. Approximately two-thirds of the paper is devoted to this period, when the writer personally contributed to this effort. By the beginning of the third decade, transistor circuits had became more complex, and circuit analysis was carried out with the help of digital computers. Interest then shifted away from small-signal equivalent circuits to “models” for computer-aided circuit design (CACD). This transition, including the models used in the widely used CACD program SPICE, is described. MOS transistors are treated only briefly; by the time MOS transistors became commercially viable devices, emphasis then also had shifted to “models” for CACD. In conclusion, the writer notes that there is still hope for us aficionados of small-signal equivalent circuits; new types of transistors are still being characterized in this manner 相似文献
956.
Fukuda H. Ebara M. Kobayashi A. Sugiura N. Yoshikawa M. Saisho H. Kondo F. Yoshino S. Yahagi T. 《IEEE transactions on bio-medical engineering》1998,45(3):396-400
To objectively evaluate the parenchymal echo pattern of cirrhotic liver and chronic hepatitis, the authors applied an image analyzing system (IAS) using a neural network. Autopsy specimens in a water tank (n=13) were used to examine the relationship between the diameter of the regenerative nodule and the coarse score (CS) calculated by IAS. CS was significantly correlated with the diameter of the regenerative nodule (p<0.0001, r=0.966). CS is considered to be useful for evaluating the coarseness of the parenchymal echo pattern 相似文献
957.
Summary X-ray reflectivity may be used to determine the internal structure of thin polymer films. An electron density difference of 10% for polystyrene and polyisoprene is sufficient to distinguish between a random distribution of lamellae, complete orientation parallel to the substrate surface and a surface induced formation of lamellae. The disappearance of the lamellar Bragg-peaks, with heating of the film, shows the transition into the disordered state. 相似文献
958.
We present an isovector Lagrangian, which admits stable, nonsingular soliton solutions in three space dimensions. The spherical solution and its total energy are obtained via a variational procedure. An antisymmetric, second-rank tensor is defined in terms of the isovector field and its derivatives. This tensor satisfies Maxwell's equations. The corresponding current is identically conserved and the total charge is topologically quantized. 相似文献
959.
G. Baldsiefen H. Hübel W. Korten U. J. van Severen J. A. Cizewski N. H. Medina D. R. Napoli C. Rossi Alvarez G. Lo Bianco S. Signorelli 《Zeitschrift für Physik A Hadrons and Nuclei》1996,355(1):337-338
High-spin states in196Pb have been populated using the reaction170Er(30Si,4n). The previously observed shears bands in this nucleus have been extended and some of their transitions have been reordered. They now form regular bands with band crossings. One of the bands splits into two pathways at high spin. 相似文献
960.
Yu. Dryagin N. Scalyga T. Geist 《International Journal of Infrared and Millimeter Waves》1996,17(7):1199-1204
The construction of a notch-filter for 140 GHz with very low passband insertion loss is described. It is based on a single-mode rectangular waveguide and cylindrical cavities matched to it. The typical transmission characteristic is also presented. 相似文献