全文获取类型
收费全文 | 536084篇 |
免费 | 5424篇 |
国内免费 | 1731篇 |
专业分类
化学 | 233289篇 |
晶体学 | 6879篇 |
力学 | 22298篇 |
综合类 | 8篇 |
数学 | 65082篇 |
物理学 | 144345篇 |
无线电 | 71338篇 |
出版年
2020年 | 3021篇 |
2019年 | 3276篇 |
2018年 | 12623篇 |
2017年 | 13440篇 |
2016年 | 10275篇 |
2015年 | 5264篇 |
2014年 | 6593篇 |
2013年 | 18699篇 |
2012年 | 17998篇 |
2011年 | 29239篇 |
2010年 | 18799篇 |
2009年 | 19444篇 |
2008年 | 27041篇 |
2007年 | 31053篇 |
2006年 | 16860篇 |
2005年 | 22703篇 |
2004年 | 17790篇 |
2003年 | 16115篇 |
2002年 | 13766篇 |
2001年 | 14228篇 |
2000年 | 11513篇 |
1999年 | 9114篇 |
1998年 | 7837篇 |
1997年 | 7714篇 |
1996年 | 7456篇 |
1995年 | 6895篇 |
1994年 | 6702篇 |
1993年 | 6699篇 |
1992年 | 6971篇 |
1991年 | 7010篇 |
1990年 | 6500篇 |
1989年 | 6312篇 |
1988年 | 6163篇 |
1987年 | 5356篇 |
1986年 | 5072篇 |
1985年 | 6940篇 |
1984年 | 7039篇 |
1983年 | 5868篇 |
1982年 | 6262篇 |
1981年 | 5992篇 |
1980年 | 5819篇 |
1979年 | 5784篇 |
1978年 | 6036篇 |
1977年 | 5716篇 |
1976年 | 5890篇 |
1975年 | 5445篇 |
1974年 | 5450篇 |
1973年 | 5845篇 |
1972年 | 3621篇 |
1971年 | 2769篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
A novel moment-based m parameter estimator using noisy channel samples is derived. This estimator is simpler than known estimators. Numerical results are presented to demonstrate that, under some practical fading conditions, it outperforms previous estimators. 相似文献
992.
N. Nishiyama C. Caneau S. Tsuda G. Guryanov M. Hu R. Bhat Chung-En Zah 《Photonics Technology Letters, IEEE》2005,17(8):1605-1607
Error-free transmission through 10-km single-mode fiber at 10 Gb/s under -13-dB optical reflections has been demonstrated for the first time using a directly modulated 1.3-/spl mu/m InP-based VCSEL without any optical isolator. The 13-GHz relaxation oscillation frequency and stable polarization suppresses relative intensity noise degradation under optical reflection. Only 1-dB error-free power penalty has been observed with optical reflection set with the worst polarization direction. 相似文献
993.
Y. Rey-Tauriac J. Badoc B. Reynard R.A. Bianchi D. Lachenal A. Bravaix 《Microelectronics Reliability》2005,45(9-11):1349
This paper presents results of reliability investigation of 20 V N-Drift MOS transistor in 0.13 μm CMOS technology. Due to high performances required for CMOS applications, adding high voltage devices becomes a big challenge to guarantee the reliability criteria. In this context, new reliability approaches are needed. Safe Operating Area are defined for switch, Vds limited and Vgs limited applications in order to improve circuit designs. For Vds limited applications, deep doping dose effects in drift area are investigated in correlation to lifetime evaluations based on device parameter shifts under hot carrier stressing. To further determine the amount and locations of hot carriers injections, accurate 2D technological and electrical simulations are performed and permit to select the best compromise between performance and reliability for N-Drift MOS transistor. 相似文献
994.
N. V. Vostokov Yu. N. Drozdov Z. F. Krasil’nik O. A. Kuznetsov A. V. Novikov V. A. Perevoshchikov M. V. Shaleev 《Russian Microelectronics》2005,34(4):203-209
The results are presented of the fabrication of strain-relaxed graded Si1 − x
Gex/Si(001) buffer layers with a maximum Ge fraction of about 0.25 that have a low density of threading dislocations (<106 cm−2) and low surface roughness. The buffer layers are grown by atmospheric-pressure hydride CVD. It is found that chemical mechanical polishing can reduce their surface roughness to a level comparable with that of the original Si(001) substrates. It is shown that the polished buffer layers can serve as substrates for MBE-grown SiGe/Si heterostructures.__________Translated from Mikroelektronika, Vol. 34, No. 4, 2005, pp. 243–250.Original Russian Text Copyright © 2005 by Vostokov, Drozdov, Krasil’nik, Kuznetsov, Novikov, Perevoshchikov, Shaleev. 相似文献
995.
What if every public space and surface were transformed into an interactive experience - capturing our movements, gestures, sounds and actions and using these to produce new sounds, images, text and actions? Bus stops could come alive and talk back, advertisements could look back at you and talk to you and public park robotic devices could play games with you. Will public places eventually become our social barometer as well as director? Will billboards display the populace's discourse? The author look at some special streets in France where the city is literally alive with art and explores what it might mean for us to live in a world of artistic interactivity. 相似文献
996.
We develop a theory of the magnetic-zenith effect (MZE) observed upon the ionosphere modification by powerful radio waves.
The dependences of MZE on main parameters, such as the magnetic-field inclination angle, pump-wave frequency and power, and
angular width of HF beam are obtained. The optimal elevation angle of pump-wave beam is predicted.
__________
Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 48, No. 9, pp. 772–787, September 2005. 相似文献
997.
998.
Data on methods for the production of isatin and indole aldoximes, ketoximes, and amidoximes and their reactions are reviewed. Individual syntheses of new heterocycles from indole and isatin oximes are discussed. The principal results from investigation of the biological activity of derivatives of the oximes are also presented. 相似文献
999.
E.Ann Hallinan Clifford R. DornWilliam M. Moore Gina M. JeromePamela T. Manning Barnett S. Pitzele 《Tetrahedron letters》2003,44(39):7345-7347
Described is a method of introducing trifluoroalkyl groups at C-6 of lysine. This chemistry has the potential to introduce a variety of functionality at C-6 of lysine. 相似文献
1000.
Philip A. Wheeler Michael Brungs Graham R. Atkins 《Journal of Sol-Gel Science and Technology》2003,27(3):321-326
Organically modified silicates (ormosils) have been viewed as possible materials for optical signal processing devices for communications networks. One significant problem obstructing their use in this application is the absorption of infrared light by hydroxy species (present for example in silanol groups (SiOH) and water) around the communications wavelengths (1310 and 1550 nm). We have investigated several ormosil compositions, synthesised via an aqueous sol-gel method, to determine if residual hydroxy groups can be removed more effectively via reducing the connectivity of the material by increasing the number of organic groups (such as Si—CH3). Furthermore, we have investigated the effect of different inert atmospheres on the drying process. It was found that reducing the connectivity does promote the removal of hydroxy absorption, mainly via the condensation of residual silanol groups, but also by removal of residual water, depending on the other precursors in the system. It was also found that the drying atmosphere used (nitrogen or helium) had no effect on the drying process. 相似文献