全文获取类型
收费全文 | 124811篇 |
免费 | 19017篇 |
国内免费 | 12616篇 |
专业分类
化学 | 65632篇 |
晶体学 | 988篇 |
力学 | 6038篇 |
综合类 | 536篇 |
数学 | 11132篇 |
物理学 | 39064篇 |
无线电 | 33054篇 |
出版年
2024年 | 515篇 |
2023年 | 3143篇 |
2022年 | 3854篇 |
2021年 | 4985篇 |
2020年 | 4963篇 |
2019年 | 4392篇 |
2018年 | 3954篇 |
2017年 | 3756篇 |
2016年 | 5468篇 |
2015年 | 5719篇 |
2014年 | 6808篇 |
2013年 | 8823篇 |
2012年 | 10426篇 |
2011年 | 10560篇 |
2010年 | 7357篇 |
2009年 | 7371篇 |
2008年 | 7979篇 |
2007年 | 7027篇 |
2006年 | 6763篇 |
2005年 | 5650篇 |
2004年 | 4000篇 |
2003年 | 3377篇 |
2002年 | 3096篇 |
2001年 | 2661篇 |
2000年 | 2479篇 |
1999年 | 2785篇 |
1998年 | 2390篇 |
1997年 | 2217篇 |
1996年 | 2171篇 |
1995年 | 1835篇 |
1994年 | 1681篇 |
1993年 | 1378篇 |
1992年 | 1256篇 |
1991年 | 1059篇 |
1990年 | 825篇 |
1989年 | 630篇 |
1988年 | 504篇 |
1987年 | 398篇 |
1986年 | 405篇 |
1985年 | 356篇 |
1984年 | 250篇 |
1983年 | 179篇 |
1982年 | 177篇 |
1981年 | 121篇 |
1980年 | 109篇 |
1979年 | 75篇 |
1978年 | 60篇 |
1977年 | 51篇 |
1976年 | 47篇 |
1973年 | 41篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
51.
52.
A possible biological intermediate in the reduction and methylation of selenium oxyanions, dimethyl selenone, was synthesized, and the first experiments involving the amendment of selenium resistant bacterial cultures with this compound are reported. The amount of volatile, reduced selenium-containing species released from these cultures into the headspace is significantly more than that produced in analogous experiments involving sodium selenate amended cultures. Dimethyl selenone is reduced in the presence of dimethyl sulfide and dimethyl disulfide in a complex growth medium, trypticase soy broth with 0.1% nitrate. This reduction occurs whether or not the reduced sulfur compounds are biologically produced. 相似文献
53.
Jackson T.J. Engman E.T. Le Vine D. Schmugge T.J. Lang R. Wood E. Teng W. 《Geoscience and Remote Sensing, IEEE Transactions on》1994,32(1):201-206
MACHYDR0'90 was an experiment conducted in Pennsylvania in 1990 to study the synergistic use of remote sensors in multitemporal hydrologic studies. As part of this mission the pushbroom microwave radiometer was flown and used to produce brightness temperature maps. Verification studies and vegetation algorithms for mixed land cover areas are described 相似文献
54.
Yanbin Luo Jianhui Zhang Alexandrov P. Fursin L. Zhao J.H. Burke T. 《Electron Device Letters, IEEE》2003,24(11):695-697
This letter reports the design and fabrication of 4H-SiC bipolar junction transistors with both high voltage (>1kV) and high dc current gain (/spl beta/=32) at a collector current level of I/sub c/=3.83A (J/sub c/=319 A/cm/sup 2/). An Al-free base ohmic contact has been used which, when compared with BJTs fabricated with Al-based base contact, shows clearly improved blocking voltage. A specific on-resistance of 17 m/spl Omega//spl middot/cm/sup 2/ has been achieved for collector current densities up to 289 A/cm/sup 2/. 相似文献
55.
56.
Le Guillou Y. Gaborieau O. Gamand P. Isberg M. Jakobsson P. Jonsson L. Le Deaut D. Marie H. Mattisson S. Monge L. Olsson T. Prouet S. Tired T. 《Solid-State Circuits, IEEE Journal of》2005,40(2):403-411
This paper describes a highly digitized direct conversion receiver of a single-chip quadruple-band RF transceiver that meets GSM/GPRS and EDGE requirements. The chip uses an advanced 0.25-/spl mu/m BiCMOS technology. The I and Q on-chip fifth-order single-bit continuous-time sigma-delta (/spl Sigma//spl Delta/) ADC has 84-dB dynamic range over a total bandwidth of /spl plusmn/135 kHz for an active area of 0.4 mm/sup 2/. Hence, most of the channel filtering is realized in a CMOS IC where digital processing is achieved at a lower cost. The systematic analysis of dc offset at each stage of the design enables to perform the dc offset cancellation loop in the digital domain as well. The receiver operates at 2.7 V with a current consumption of 75 mA. A first-order substrate coupling analysis enables to optimize the floor plan strategy. As a result, the receiver has an area of 1.8 mm/sup 2/. 相似文献
57.
Sensitivity analysis of lossy coupled transmission lines 总被引:3,自引:0,他引:3
An analysis method, based on the numerical inversion of the Laplace transform, is described for the evaluation of the time domain sensitivity of networks that include lossy coupled transmission lines. The sensitivity can be calculated with respect to network components and parameters of the transmission lines. Sensitivity analysis is useful for waveform shaping and optimization. Examples and comparisons with sensitivity determined by perturbation are presented 相似文献
58.
This paper presents a new method, based on a multi-agent system and on a digital mock-up technology, to assess an efficient path planner for a manikin or a robot for an access and visibility task taking into account ergonomic constraints or joint and mechanical limits. In order to solve this problem, the human operator is integrated in the process optimization to contribute to a global perception of the environment. This operator cooperates, in real time, with several automatic local elementary agents. The result of this work validates solutions through the digital mock-up; it can be applied to simulate maintainability and mountability tasks. 相似文献
59.
A novel soft decision-based median subtraction filter is presented for clutter suppression and infrared (IR) point targets enhancement. The decision is made based on a jump Markov model and its state and parameter estimation using a particle filter. The scheme is compared with other conventional clutter background removal techniques and good results are obtained. 相似文献
60.
Li Changjun; Li Zheng; Evans David J.; Zhang Tie 《IMA Journal of Numerical Analysis》2003,23(4):581-592
Golub et al. (2001, BIT, 41, 7185) gave a generalizedsuccessive over-relaxation method for the augmented systems.In this paper, the connection between the SOR-like method andthe preconditioned conjugate gradient (PCG) method for the augmentedsystems is investigated. It is shown that the PCG method isat least as accurate (fast) as the SOR-like method. Numericalexamples demonstrate that the PCG method is much faster thanthe SOR-like method. 相似文献