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   3040篇
  免费   69篇
  国内免费   19篇
电工技术   184篇
综合类   5篇
化学工业   1018篇
金属工艺   96篇
机械仪表   104篇
建筑科学   96篇
能源动力   140篇
轻工业   194篇
水利工程   6篇
石油天然气   3篇
无线电   149篇
一般工业技术   693篇
冶金工业   90篇
原子能技术   117篇
自动化技术   233篇
  2023年   17篇
  2022年   45篇
  2021年   50篇
  2020年   21篇
  2019年   32篇
  2018年   51篇
  2017年   45篇
  2016年   59篇
  2015年   48篇
  2014年   70篇
  2013年   164篇
  2012年   104篇
  2011年   198篇
  2010年   138篇
  2009年   146篇
  2008年   169篇
  2007年   113篇
  2006年   135篇
  2005年   116篇
  2004年   83篇
  2003年   116篇
  2002年   82篇
  2001年   55篇
  2000年   67篇
  1999年   54篇
  1998年   75篇
  1997年   79篇
  1996年   67篇
  1995年   42篇
  1994年   33篇
  1993年   45篇
  1992年   30篇
  1991年   29篇
  1990年   32篇
  1989年   45篇
  1988年   31篇
  1987年   34篇
  1986年   53篇
  1985年   45篇
  1984年   56篇
  1983年   49篇
  1982年   32篇
  1981年   24篇
  1980年   19篇
  1979年   27篇
  1978年   19篇
  1977年   18篇
  1976年   11篇
  1974年   14篇
  1973年   10篇
排序方式: 共有3128条查询结果,搜索用时 0 毫秒
11.
Optical amplifier techniques have led to the installation of large-capacity submarine systems and further capacity increases seem likely. This paper reviews the FSA submarine system, which flexibly operates at both 2.5 and 10 Gb/s and offers maximum transmission capacity of 60 Gb/s for commercial use. The system configuration as well as its characteristics and upgradability will be introduced, including measurement results on time-division-multiplexing/wavelength-division-multiplexing (TDM–WDM) transmission at bit rates of 10 and 20 Gb/s using non-return-to-zero or soliton pulses. To further increase transmission capacity, TDM–WDM techniques that permit more than 10 Gb/s signal transmission in each data channel should be developed. Thus, pulse formats, which include non-return-to-zero, return-to-zero, or soliton pulses, and dispersion allocation in transmission fibers are significant issues. We introduce and discuss our recent results from high-speed (10 to 40 Gb/s) TDM–WDM signal transmission experiments with regard to the above aspects.  相似文献   
12.
13.
It has been shown in our previous studies that the geographical traffic nonuniformity considerably affects the performance of the low earth orbit satellite communications systems. In this paper, a new scheme for improving the throughput characteristics of these systems at nonuniform traffic distribution is proposed. In this method, some parts of the users under the satellite which is flying over the area with high traffic load are assigned to its neighbor satellites with lower transmitting power levels. It is shown that the method equalizes the traffic loads of the satellites to some degree and, hence, can improve the throughput characteristics of the system.  相似文献   
14.
Despite the recent development of analytical and numerical techniques for problems of scattering from two-dimensional rough surfaces, very few experimental studies were available for verification. The authors present the results of millimeter-wave experiments on scattering from two-dimensional conducting random rough surfaces with Gaussian surface roughness statistics. Machine-fabricated rough surfaces with controlled roughness statistics were examined. Special attention was paid to surfaces with large rms slopes (ranging from 0.35 to 1.00) for which enhanced backscattering is expected to take place. Experimentally, such enhancement was indeed observed in both the copolarized and cross-polarized returns. In addition, it was noticed that at moderate angles of incidence, the scattering profile as a function of observation angle is fairly independent of the incident polarization and operating frequency. This independence justifies the use of the geometric optics approximation embodied in the Kirchhoff formulation for surfaces with large surface radius of curvature. When compared with the experimental data, this analytical technique demonstrates good agreement with the experimental data  相似文献   
15.
The purpose of this study is to investigate the vascular wall with a thermally self-regulating, cylindrical stent made of a low Curie temperature ferromagnetic alloy. Physiologic saline was circulated in the silicone model vessel implanted with the stent. The stent-temperature remained nearly constant for variable saline flows, saline temperatures, and magnetic flux densities. Stent implants of this type in human blood vessels could potentially enable thermotherapy and temperature determination without catheterization.  相似文献   
16.
17.
Cubic BN was synthesized under high-temperature and -pressure conditions from BN powder formed by pressure pyrolysis of borazine below 700°C and 100 MPa. The conversion of BN powder to cubic BN was strongly influenced by the residual hydrogen identified by the BH/BN ratio of IR absorption band. The activation energy for cubic BN synthesis from BN powder-20 mol% AIN was 46 kJ/mol, when the starting BN was synthesized at 250°C. A mixture of BN powder and cubic BN particles was converted to cubic BN in a 100% yield by heat treatment at 1800°C and 6.5 GPa without any catalyst. The presence of cubic BN particles does enhance the conversion to cubic BN from BN powder. The energy required for the transformation of starting BN to cubic BN in the presence of cubic BN seed was 355 kJ/mol.  相似文献   
18.
VSATS in Japan     
In Japan, private domestic communications satellites, JCSAT and SuperBird, carrying Ku-band transponders, were launched in 1989, and the use of the VSATs started. To expedite a wider use of the VSAT systems, regulations were amended in June 1989 to permit simplified licensing procedures for the VSATs which conformed to technical standards. This contribution describes the usage of VSAT systems in Japan along with the licensing procedures and the technical standards.  相似文献   
19.
Characterization of a Si1−xGex layer formed by high-dose germanium implantation and subsequent solid phase epitaxy is reported. Properties of this layer are obtained from electrical measurements on diodes and transistors fabricated in this layer. Results are compared with those of the silicon control devices. It was observed that the germanium implantation created considerable defects that are difficult to eliminate with annealing. These defects result in boron deactivation in the p-type regions of the devices, giving rise to larger resistance. Optimization of the device structure and fabrication process is discussed.  相似文献   
20.
The key words for the future telecommunications are personalization, multi-media services and high accessibility to the network. These trends naturally lead to radio media application to access links. In this sense, this paper proposes an advanced wireless access system with a capacity supporting broadband video and data services.Concept of the system can be defined as a new radiocommunication category with advantages of both fixed and mobile systems.The system is designed to transport ATM-based signals using SHF (or EHF) bands to wireless terminals, interconnected with optical fibers at the network side. The basic system configuration is presented as well as examples of system parameters.For radio aspects the maximum service coverages of the radio base stations are calculated based on availability consideration. Preferred frequency bands for actual operation are also discussed.  相似文献   
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