全文获取类型
收费全文 | 261141篇 |
免费 | 50209篇 |
国内免费 | 39530篇 |
专业分类
化学 | 135575篇 |
晶体学 | 2531篇 |
力学 | 13060篇 |
综合类 | 920篇 |
数学 | 24567篇 |
物理学 | 95814篇 |
无线电 | 78413篇 |
出版年
2024年 | 1361篇 |
2023年 | 5141篇 |
2022年 | 6600篇 |
2021年 | 7975篇 |
2020年 | 8117篇 |
2019年 | 8753篇 |
2018年 | 7786篇 |
2017年 | 8523篇 |
2016年 | 11021篇 |
2015年 | 12319篇 |
2014年 | 13612篇 |
2013年 | 17805篇 |
2012年 | 20467篇 |
2011年 | 21065篇 |
2010年 | 20398篇 |
2009年 | 20165篇 |
2008年 | 14903篇 |
2007年 | 13343篇 |
2006年 | 12520篇 |
2005年 | 11291篇 |
2004年 | 10114篇 |
2003年 | 8259篇 |
2002年 | 7652篇 |
2001年 | 7223篇 |
2000年 | 6173篇 |
1999年 | 6394篇 |
1998年 | 5317篇 |
1997年 | 4835篇 |
1996年 | 5136篇 |
1995年 | 5038篇 |
1994年 | 4833篇 |
1993年 | 4526篇 |
1992年 | 3960篇 |
1991年 | 3451篇 |
1990年 | 2780篇 |
1989年 | 2621篇 |
1988年 | 2403篇 |
1987年 | 1579篇 |
1986年 | 1608篇 |
1985年 | 1239篇 |
1984年 | 1242篇 |
1983年 | 577篇 |
1982年 | 1037篇 |
1981年 | 860篇 |
1980年 | 883篇 |
1979年 | 594篇 |
1978年 | 581篇 |
1977年 | 706篇 |
1976年 | 1101篇 |
1972年 | 570篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
92.
针对国外变频器价格昂贵、调试周期和维修周期长等特点,我公司自主研发了可替代进口机的高性能钻机国产专用变频器。该变频器能够从零开始调速。当钻井卡钻时,能及时断开电动机,在低速提升时,能最大限度地发挥电动机的动力特性,提供220%的转矩输出。变频器具有易安装、调试、维护,性能稳定。应用中据实测数据显示,采用变频改造后,打1口井可节省柴油15t。 相似文献
93.
Spectral shape tunable band-rejection filter using a long-period fiber grating with divided coil heaters 总被引:2,自引:0,他引:2
Jun Kye Bae Sang Hyuck Kim Jun Hee Kim Jinho Bae Sang Bae Lee Je-Myung Jeong 《Photonics Technology Letters, IEEE》2003,15(3):407-409
We have developed a new type of tunable band rejection filter, which provides high spectral-shaping flexibility in a wide tuning range. The filter consists of a long-period fiber grating (LPFG) with divided coil heaters. Each of the divided coil heaters is controlled individually to adjust a temperature distribution along the LPFG and to modify the spectral shape of the LPFG filter. The tunable band rejection filter is demonstrated to function properly when applied as an erbium-doped fiber amplifier gain-flattening filter. 相似文献
94.
In this paper, we propose a routing algorithm called minimum fusion Steiner tree (MFST) for energy efficient data gathering with aggregation (fusion) in wireless sensor networks. Different from existing schemes, MFST not only optimizes over the data transmission cost, but also incorporates the cost for data fusion, which can be significant for emerging sensor networks with vectorial data and/or security requirements. By employing a randomized algorithm that allows fusion points to be chosen according to the nodes' data amounts, MFST achieves an approximation ratio of 5/4log(k + 1), where k denotes the number of source nodes, to the optimal solution for extremely general system setups, provided that fusion cost and data aggregation are nondecreasing against the total input data. Consequently, in contrast to algorithms that only excel in full or nonaggregation scenarios without considering fusion cost, MFST can thrive in a wide range of applications 相似文献
95.
农村通信如何"突围" 总被引:2,自引:0,他引:2
农村通信作为农村地区信息化的基础,对于全面建设小康具有特殊的重要地位。介绍了我国农村通信的发展现状,分析了农村通信发展的困难压其原因,最后对农村压边远、欠发达地区通信的均衡震展提出了一些建议。 相似文献
96.
Supermedia-enhanced Internet-based telerobotics 总被引:4,自引:0,他引:4
Elhajj I. Ning Xi Wai Keung Fung Yun-Hui Liu Hasegawa Y. Fukuda T. 《Proceedings of the IEEE. Institute of Electrical and Electronics Engineers》2003,91(3):396-421
This paper introduces new planning and control methods for supermedia-enhanced real-time telerobotic operations via the Internet. Supermedia is the collection of video, audio, haptic information, temperature, and other sensory feedback. However, when the communication medium used, such as the Internet, introduces random communication time delay, several challenges and difficulties arise. Most importantly, random communication delay causes instability, loss of transparency, and desynchronization in real-time closed-loop telerobotic systems. Due to the complexity and diversity of such systems, the first challenge is to develop a general and efficient modeling and analysis tool. This paper proposes the use of Petri net modeling to capture the concurrency and complexity of Internet-based teleoperation. Combined with the event-based planning and control method, it also provides an efficient analysis and design tool to study the stability, transparency, and synchronization of such systems. In addition, the concepts of event transparency and event synchronization are introduced and analyzed. This modeling and control method has been applied to the design of several supermedia-enhanced Internet-based telerobotic systems, including the bilateral control of mobile robots and mobile manipulators. These systems have been experimentally implemented in three sites test bed consisting of robotic laboratories in the USA, Hong Kong, and Japan. The experimental results have verified the theoretical development and further demonstrated the stability, event transparency, and event synchronization of the systems. 相似文献
97.
98.
The γcmc values of CTAB-SDS decrease from 63.67 mN/m at 10‡C to 36.38 mN/m at 90‡C, slightly lower than those of either CTAB or SDS.
Correspondingly, the CMC of CTAB-SDS decreases almost by half. The increase of surface activity of CTAB-SDS can be attributed
to the relatively weak electrostatic interaction at high temperature, which is supported by the increase of solubility of
CTAB-SDS with rise in temperature. Catalytic effect on oxidation of toluene derivatives with potassium permanganate follows
the order CTAB-SDS > SDS > CTAB. This is not caused by the dissociative effect of CTAB-SDS with low surface activity at low
temperature, as seen from the fact that almost all oxidative products can be retrieved for different toluene derivatives and
surfactants by mimicking the conditions of reaction. In the emulsifications of toluene derivatives at 90‡C, the time that
turbid water layers of surfactant solutions take to become clear is the same as that of the catalytic effect on oxidation
of toluene derivatives. Thus, it can be inferred that surfactants can improve the oxidation yields of toluene derivatives
by increasing the contact between two reacting phases. 相似文献
99.
Ji-Dong Lou Chun-Ling Gao Li Li Zhi-Gang Fang 《Monatshefte für Chemie / Chemical Monthly》2006,41(10):1071-1074
A new procedure for the selective oxidation of alcohols to the corresponding aldehydes and ketones with potassium permanganate
supported on aluminum silicate at room temperature under solvent-free conditions and shaking is reported. 相似文献
100.
托盘能否有效运用对物流的畅通与否关系重大,这已经引起了国内外专家和学者们的重视,并就此制定了托盘电子信息化管理系统,旨在更合理地使用托盘,推进托盘一体化和共用系统的建立。而RFID技术在托盘的电子信息化管理的应用更将大大提高物流的效率与准确性,托盘电子信息化管理将成为今后托盘管理的趋势。 相似文献