全文获取类型
收费全文 | 316949篇 |
免费 | 12932篇 |
国内免费 | 6276篇 |
专业分类
电工技术 | 10968篇 |
技术理论 | 9篇 |
综合类 | 8831篇 |
化学工业 | 51031篇 |
金属工艺 | 18042篇 |
机械仪表 | 14894篇 |
建筑科学 | 12603篇 |
矿业工程 | 5592篇 |
能源动力 | 6258篇 |
轻工业 | 18101篇 |
水利工程 | 5637篇 |
石油天然气 | 16661篇 |
武器工业 | 1061篇 |
无线电 | 30766篇 |
一般工业技术 | 60258篇 |
冶金工业 | 36349篇 |
原子能技术 | 10192篇 |
自动化技术 | 28904篇 |
出版年
2023年 | 2475篇 |
2022年 | 5479篇 |
2021年 | 7514篇 |
2020年 | 5704篇 |
2019年 | 5326篇 |
2018年 | 7565篇 |
2017年 | 8128篇 |
2016年 | 7852篇 |
2015年 | 7044篇 |
2014年 | 9764篇 |
2013年 | 14616篇 |
2012年 | 13282篇 |
2011年 | 14984篇 |
2010年 | 12586篇 |
2009年 | 12811篇 |
2008年 | 12833篇 |
2007年 | 12233篇 |
2006年 | 11051篇 |
2005年 | 9716篇 |
2004年 | 8054篇 |
2003年 | 7893篇 |
2002年 | 7871篇 |
2001年 | 7495篇 |
2000年 | 6780篇 |
1999年 | 6291篇 |
1998年 | 9403篇 |
1997年 | 7130篇 |
1996年 | 5648篇 |
1995年 | 4513篇 |
1994年 | 3921篇 |
1993年 | 3777篇 |
1992年 | 3253篇 |
1991年 | 3126篇 |
1990年 | 3218篇 |
1989年 | 3052篇 |
1988年 | 2960篇 |
1987年 | 2773篇 |
1986年 | 2754篇 |
1985年 | 2760篇 |
1984年 | 2606篇 |
1983年 | 2524篇 |
1981年 | 2573篇 |
1979年 | 2698篇 |
1978年 | 2880篇 |
1977年 | 2860篇 |
1976年 | 3507篇 |
1975年 | 2680篇 |
1974年 | 2706篇 |
1973年 | 2752篇 |
1972年 | 2560篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
32.
Passaro V.M.N. Armenise M.N. Nesheva D. Savatinova I.T. Pun E.Y.B. 《Lightwave Technology, Journal of》2002,20(1):71-77
Proton-exchanged planar waveguides have been fabricated on Z-cut and X-cut lithium niobate crystals by using a new proton source formed by a mixture of benzoic and adipic acids. Waveguide index profiles and optical characteristics have been obtained at different values of the adipic-benzoic acid concentration ratio. The samples have been structurally characterized by Raman and infrared (IR) absorption spectroscopy and double-crystal X-ray diffraction. Good quality samples have been fabricated by using 30 mol% ratio dilution, showing very low scattering levels (<0.1 dB/cm), relatively high electrooptic coefficient (r33=0.88 pm/V), and low relative percentage of interstitial protons (26%). The main factor limiting the waveguide optical properties is the substitutional-interstitial proton ratio, which can be easily controlled to produce good quality waveguides. A demonstration of the repeatability of the exchange process in the acid mixture is also provided 相似文献
33.
Younan N.H. Cox B.I. Taylor C.D. Prather W.D. 《Electromagnetic Compatibility, IEEE Transactions on》1994,36(4):394-398
Efficient numerical solution techniques have been developed and used to examine the electromagnetic fields that can be developed in the working volume of the CW Ellipticus antenna operated at frequencies from 100 kHz to 1 GHz. An exponentially tapered transition section is designed to obtain the desired illumination pattern in the working volume. The input transition section is needed for impedance matching and to drive efficiently the Ellipticus antenna. A parametric study is performed to ascertain the performance of the Ellipticus antenna for frequencies up to 1 GHz 相似文献
34.
35.
36.
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
37.
38.
对菱镁矿在固体燃料燃烧过程中的固硫特性进行了理论研究,所得结论对进一步将菱镁矿作为固体燃料燃烧过程中的固硫剂具有非常重要的意义。 相似文献
39.
Fahmy H.I. Develekos G. Douligeris C. 《Selected Areas in Communications, IEEE Journal on》1997,15(2):226-237
Communication network design is becoming increasingly complex, involving making networks more usable, affordable, and reliable. To help with this, we have proposed an expert network designer (END) for configuring, modeling, simulating, and evaluating large structured computer networks, employing artificial intelligence, knowledge representation, and network simulation tools. We present a neural network/knowledge acquisition machine-learning approach to improve the END's efficiency in solving the network design problem and to extend its scope to acquire new networking technologies, learn new network design techniques, and update the specifications of existing technologies 相似文献
40.
T Mukasa Y Khoroku T Tsukahara MY Momoi I Kimura T Momoi 《Canadian Metallurgical Quarterly》1997,232(1):192-197
P19 EC cells undergoes apoptosis during neuronal differentiation induced by retinoic acid. Two CPP32-like proteases, CPP32 and Mch-3, are expressed in untreated and retinoic acid-treated P19 EC cells. CPP32-like activity is remarkably increased in apoptosis during neuronal differentiation of P19 EC cells. Inhibition of CPP32-like proteases prevents apoptosis, suggesting that activation of CPP32-like proteases play central roles in the apoptosis during neuronal differentiation of P19 EC cells. Wortmannin, PI-3K inhibitor, enhances the CPP32-like activity of the retinoic acid-treated P19 EC cells. PI-3K may be involved in the apoptosis during neuronal differentiation as negative regulator. 相似文献