首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   742182篇
  免费   9658篇
  国内免费   2006篇
电工技术   13867篇
综合类   1036篇
化学工业   111766篇
金属工艺   27084篇
机械仪表   21116篇
建筑科学   18432篇
矿业工程   2944篇
能源动力   20354篇
轻工业   67070篇
水利工程   6917篇
石油天然气   11572篇
武器工业   44篇
无线电   87188篇
一般工业技术   141457篇
冶金工业   145357篇
原子能技术   14034篇
自动化技术   63608篇
  2021年   6345篇
  2020年   4633篇
  2019年   5943篇
  2018年   10073篇
  2017年   9791篇
  2016年   10285篇
  2015年   7214篇
  2014年   12039篇
  2013年   34141篇
  2012年   19086篇
  2011年   26626篇
  2010年   20911篇
  2009年   23669篇
  2008年   24337篇
  2007年   24031篇
  2006年   21127篇
  2005年   19436篇
  2004年   18753篇
  2003年   18424篇
  2002年   17603篇
  2001年   17743篇
  2000年   16642篇
  1999年   17681篇
  1998年   44969篇
  1997年   31747篇
  1996年   24522篇
  1995年   18545篇
  1994年   16457篇
  1993年   16012篇
  1992年   11669篇
  1991年   10978篇
  1990年   10792篇
  1989年   10486篇
  1988年   9940篇
  1987年   8658篇
  1986年   8647篇
  1985年   9871篇
  1984年   9088篇
  1983年   8248篇
  1982年   7665篇
  1981年   7853篇
  1980年   7392篇
  1979年   7238篇
  1978年   6843篇
  1977年   8346篇
  1976年   10897篇
  1975年   5986篇
  1974年   5706篇
  1973年   5790篇
  1972年   4766篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
We propose and evaluate a medium-access control (MAC) protocol for synchronous cellular packet direct-sequence code-division multiple-access (DS/CDMA). The protocol is designed for handling a variety of multimedia traffic types in an integrated wireless-access network (IWAN). For instance, the protocol is suited for carrying multiple traffic types of different priorities. An analytical model for the equilibrium state has been developed for the proposed protocol. A comparative evaluation of the protocol is done for three different radio frequency bandwidths currently under consideration for personal communication services (PCS), i.e., 1.25 MHz, 5 MHz, and 10 MHz. Our results demonstrate that the proposed protocol is both robust and flexible for the intended IWAN applications. It offers a significant multiplexing gain as the bandwidth increases  相似文献   
82.
In a prospective study we found good results concerning postoperative complications (21%), perioperative lethality (3.2%) and lethality during hospital stay (6.5%) after implantation of dynamic hip screws (DHS) in patients with per- and subtrochanteric fractures of the femur. The ability to walk, as well as housing and social care postoperatively and follow-up for at least six months after dismissal demonstrates the importance of these fractures for the elderly. One of three patients needs professional care, 21% at home, 13% in nursing homes. Six months after operation the lethality is nearly 20%, i.e. three times higher than in hospital. These are the essential data for all techniques of osteosynthesis used in patients with these fractures, which could only be documented in prospective studies.  相似文献   
83.
84.
This paper focuses on the hydrodynamics of third sound on a superfluid 3 He film. We solve the hydrodynamical equations in the limit of thick films with weak interaction with the substrate. The surface tension at the free interface is shown to have a large effect on the third sound velocity and on the attenuation for frequencies larger than 1Hz. In the case of a diffusely scattering substrate a ripplon-like dispersion relation is found for this frequency range.  相似文献   
85.
The Frizzled genes encode receptors for WNTs, secreted glycoproteins implicated in development as well as in carcinogenesis. In this paper, we report molecular cloning of Hfz6, the human homologue of Mfz6. Nucleotide sequence analysis showed that the Hfz6 gene encodes the 706 amino-acid protein with seven transmembrane domains, a cystein-rich domain in the N-terminal extracellular region, two N-linked glycosylation sites, and two cystein residues in the second and third extracellular loops. Hfz6 mRNA 4.4-kb in size was detected in various normal adult and fetal tissues, and a larger amount of Hfz6 mRNA was detected in both fetal lung and fetal kidney. The Hfz6 gene has been mapped to human chromosome 8q22.3-q23.1. In conclusion, we have cloned Hfz6, which encodes a seven-transmembrane receptor with the cystein-rich domain in the N-terminal extracellular region, but without the Ser/Thr-X-Val motif in the C-terminus.  相似文献   
86.
87.
The objective of this study is to determine the economic and operational impact on energy cost of incorporating large photovoltaic (PV) and wind energy conversion systems (WECS) into the electric utility generation mix. In most cases, PV and WECS power outputs are subtracted from the utility load with the expectation that conventional generation would meet the residual load. This approach is valid for small penetration levels and/or for PV and WECS facilities connected near load centers, However, several constraints such as thermal generation characteristics, fuel supply and delivery, spinning reserve requirements, and hydro availability are not adequately represented in this process. To determine the optimal value of large-scale PV and WECS applications, a new methodology that would take into account the aforementioned constraints as well as a more global penetration is developed. Results indicate that while high hydro availability increases PV penetration levels, high ramping rates can also significantly increase penetration levels  相似文献   
88.
89.
The biotransformation of oxazepam by Bifidobacterium bifidum was studied. The major metabolite was purified by chromatographic methods and found to be desmethyldiazepam using NMR, IR and other physicochemical data.  相似文献   
90.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号