全文获取类型
收费全文 | 51325篇 |
免费 | 1450篇 |
国内免费 | 1202篇 |
专业分类
电工技术 | 1285篇 |
技术理论 | 3篇 |
综合类 | 675篇 |
化学工业 | 8047篇 |
金属工艺 | 3632篇 |
机械仪表 | 1964篇 |
建筑科学 | 1538篇 |
矿业工程 | 419篇 |
能源动力 | 1750篇 |
轻工业 | 3116篇 |
水利工程 | 570篇 |
石油天然气 | 1851篇 |
武器工业 | 98篇 |
无线电 | 4976篇 |
一般工业技术 | 11708篇 |
冶金工业 | 7576篇 |
原子能技术 | 872篇 |
自动化技术 | 3897篇 |
出版年
2022年 | 577篇 |
2021年 | 854篇 |
2020年 | 642篇 |
2019年 | 680篇 |
2018年 | 1010篇 |
2017年 | 1024篇 |
2016年 | 1069篇 |
2015年 | 1020篇 |
2014年 | 1407篇 |
2013年 | 3015篇 |
2012年 | 1996篇 |
2011年 | 2640篇 |
2010年 | 2074篇 |
2009年 | 2289篇 |
2008年 | 2088篇 |
2007年 | 2164篇 |
2006年 | 1826篇 |
2005年 | 1507篇 |
2004年 | 1419篇 |
2003年 | 1264篇 |
2002年 | 1269篇 |
2001年 | 1280篇 |
2000年 | 1155篇 |
1999年 | 1186篇 |
1998年 | 2245篇 |
1997年 | 1760篇 |
1996年 | 1538篇 |
1995年 | 1070篇 |
1994年 | 839篇 |
1993年 | 838篇 |
1992年 | 654篇 |
1991年 | 602篇 |
1990年 | 546篇 |
1989年 | 510篇 |
1988年 | 386篇 |
1987年 | 406篇 |
1986年 | 371篇 |
1985年 | 383篇 |
1984年 | 314篇 |
1983年 | 301篇 |
1982年 | 305篇 |
1981年 | 296篇 |
1980年 | 349篇 |
1979年 | 342篇 |
1978年 | 294篇 |
1977年 | 415篇 |
1976年 | 629篇 |
1975年 | 308篇 |
1974年 | 292篇 |
1973年 | 311篇 |
排序方式: 共有10000条查询结果,搜索用时 16 毫秒
91.
This paper presents a 20-Gb/s 1:4-demultiplexer for future fiber-optic transmission systems. It uses an 0.4-μm emitter double polysilicon 21-GHz fT Si bipolar foundry process. This is the highest data rate of a 1:4-DEMUX reported so far in any technology. The 1:4-DEMUX features a tree-type architecture with one frequency divider and a channel switch circuit. The circuit design was carefully optimized to achieve high speed and moderate power dissipation. It consumes 1.4 W with a single -4.5-V supply 相似文献
92.
Thermal stratification in a mantled hot water storage tank is analysed numerically for different water inlet velocities. The aim is to obtain higher thermal stratification and supply hot water for usage as long as possible. Twelve different water inlet velocities to the hot water storage tank are considered. The numerical method is validated by comparing its results against experimental and numerical results from the literature. It turned out that the results obtained from the numerical analysis have shown very good agreements with the results from previous works. As a result, the water temperature in the tank increases with the increase of the water inlet velocities to the mantle but this increment is not proportional. After a period of operation of 7.2 h, which corresponds to the average sunshine duration in Turkey, temperature increments of 6.5 and 35 K have been estimated for the hot water inlet velocities of 0.01 and 0.3 m s?1, respectively, at a radial distance of 0.1 m and a height of 1 m inside the storage tank. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
93.
94.
A new unsymmetrical chiral thioindigo dopant 6‐[(R,R)‐2,3‐difluorooct‐1‐yloxy]‐5′‐nitro‐6′‐[(R)‐2‐octyloxy]thioindigo ( 4 ) designed to photoinvert the sign of spontaneous polarization (PS) in a ferroelectric chiral smectic C (SmC*) liquid crystal was prepared using a synthetic approach previously developed in our laboratory. In this new “ambidextrous” design, the (R)‐2‐octyloxy side‐chain is sterically coupled to the thioindigo core and induces a positive PS, whereas the (R,R)‐2,3‐difluorooctyloxy side‐chain is decoupled from the core and induces a larger negative PS. In the trans form, this dopant induces a negative polarization in the SmC host (+)‐4‐(4‐methylhexyloxy)phenyl 4‐decyloxybenzoate ( PhB ). Irradiation of a 1 mol‐% mixture of 4 in PhB at λ = 510 nm caused a sign inversion of PS, from –0.88 to +0.42 nC cm–2 at T – TC = –5 °C, which is consistent with an increase in the polarization power of the coupled 2‐octyloxy/thioindigo unit over that of the 2,3‐difluorooctyloxy unit, due to the increase in transverse dipole moment of the thioindigo core upon trans–cis photoisomerization. The PS sign inversion was confirmed by a surface‐stabilized ferroelectric liquid crystal photoswitching experiment. Spectroscopic measurements on films of the doped liquid crystal mixtures showed that trans–cis photoisomerization is gradually suppressed with increasing dopant mole fraction, possibly as a result of increased dopant aggregation. 相似文献
95.
Miaris G. Kaitas T. Zaharis Z. Babas D. Vafladis E. Samaras T. Sahalos J.N. 《Antennas and Propagation Magazine, IEEE》2003,45(4):35-46
The current work presents the procedure of designing and carrying out measurements of radiation emission from an air-traffic surveillance radar. The system requirements are first given, and then the equipment used is described. The steps to determine the operating characteristics of the radar are specified in detail. The necessary methods for manual power measurements and calculations are next explained. Since the final measurement system was fully automated, its main features (topology, protocol of operation) are also given. Finally, some measurement results taken during the initial phase of system operation are presented and discussed. 相似文献
96.
Linear birefringence studies were performed for the BBO crystal in the temperature range of 120-300 K. The linear birefringence changes induced by the ferroelastic phase transition at about 145 K were analysed and the critical index β was determined. The layered ferroelastic domain structure with domain walls oriented along the c- and a-axis was observed on the b-plate. 相似文献
97.
98.
Z Lukmanji 《Progress in food & nutrition science》1992,16(2):163-179
This paper highlights various issues in relation to the workload of women in developing countries and its impact on health and nutritional status. The determining factors in women's workload and work-time and the methods employed for assessment are described. The drawbacks of the methods used and the resulting inconsistencies in the data are reviewed. How women are subjected to different health stresses owing to their productive and reproductive roles has been examined under three categories of work: economic, domestic and agricultural. The interaction of women's workload and health is complex and multifactorial owing to variations in the environment and socio-economic conditions within developing countries. There is a critical need to re-examine the assumptions existing about women's workload in programmes aimed at reducing work-time and workload of women in developing countries. 相似文献
99.
100.