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   4294篇
  免费   113篇
  国内免费   10篇
电工技术   47篇
综合类   5篇
化学工业   1036篇
金属工艺   79篇
机械仪表   92篇
建筑科学   304篇
矿业工程   6篇
能源动力   103篇
轻工业   405篇
水利工程   27篇
石油天然气   22篇
无线电   238篇
一般工业技术   683篇
冶金工业   801篇
原子能技术   59篇
自动化技术   510篇
  2022年   47篇
  2021年   43篇
  2020年   42篇
  2019年   51篇
  2018年   51篇
  2017年   74篇
  2016年   84篇
  2015年   54篇
  2014年   97篇
  2013年   282篇
  2012年   141篇
  2011年   199篇
  2010年   185篇
  2009年   160篇
  2008年   180篇
  2007年   185篇
  2006年   172篇
  2005年   141篇
  2004年   132篇
  2003年   122篇
  2002年   105篇
  2001年   89篇
  2000年   70篇
  1999年   67篇
  1998年   100篇
  1997年   85篇
  1996年   57篇
  1995年   63篇
  1994年   75篇
  1993年   58篇
  1992年   51篇
  1991年   51篇
  1990年   56篇
  1989年   68篇
  1988年   62篇
  1987年   36篇
  1986年   58篇
  1985年   59篇
  1984年   63篇
  1983年   55篇
  1982年   54篇
  1981年   56篇
  1980年   50篇
  1979年   57篇
  1978年   60篇
  1977年   46篇
  1976年   46篇
  1975年   33篇
  1974年   31篇
  1973年   38篇
排序方式: 共有4417条查询结果,搜索用时 0 毫秒
1.
Area- and time-specific marginal capacity costs of electricity distribution   总被引:2,自引:0,他引:2  
Marginal costs of electricity vary by time and location. In the past, researchers attributed the variations to factors related to electricity generation and transmission. These authors, however, have not analyzed possible variations in marginal distribution capacity costs (MDCC). The objectives of this paper are:
1. (i) to show that large MDCC variations are due to the dispersion in distribution capital expenditures by time and space,
2. (ii) to propose a method for quantifying the area- and time-specific MDCC in the presence of lumpy investments, and
3. (iii) to compare our MDCC estimates to those commonly used in the electric utility industry.
Our proposed method and its results were adopted by the California Public Utilities Commission (CPUC) in 1992 for Pacific Gas and Electric Company (PG&E), the largest privately owned electric utility in the U.S.  相似文献   
2.
3.
4.
5.
This paper presents a numerical study of the Lorentz force and fluid flow induced by a rotating magnetic field in a medium with a nonhomogeneous electric conductivity placed in a cylindrical vessel with insulated walls. The nonhomogeneity is modeled by the presence of a solid and a liquid phase of different electrical conductivity. The solid phase is located orthogonally to the axis of rotation, which corresponds to the case of unidirectional solidification. The simulations were performed for different locations of the solid front and different ratios of the electrical conductivity, σ s /σ l =0.2... 10. Here σ s and σ l are the electric conductivity of the solid and liquid phases, respectively. The results showed that the difference between electrical conductivity of solid and liquid phases has a noticeable effect on the mean-time Lorentz force and the velocity: namely, the presence of the solid phase (σ s >σ l ) leads to an increase of the Lorentz force and fluid flow in the cylinder.  相似文献   
6.
Roger L. Kemp 《Cities》1985,2(3):252-255
Since the early 1970s, local governments in the USA have faced limited tax income combined with double-digit inflation. As a result, every local service from fire prevention to recreation has had to be closely scrutinized. Local administrators and politicians have evolved new and innovative retrenchment management policies to respond to both the demands of citizens and the fewer dollars available. A combination of denial of new programmes and public sector pay austerity, with public education and use of community organizations is providing ways to stretch limited funds. The reality in many communities across the USA is that local government and its services will not be growing in the future.  相似文献   
7.
Develops 3 hypotheses to explain the role of construal processes in conformity behavior. According to the preconformity change of meaning hypothesis, social influence leads to changes in situational construal that precede and mediate conformity behavior; according to the postconformity change of meaning hypothesis, construal change follows and justifies conformity; and according to the construal-consistency hypothesis, preexisting situational construal distinguishes those who conform from those who dissent. Four experimental studies with varying methodology and involving a total of 361 undergraduate students were performed to test these hypotheses. The results of all studies provide strong evidence for postconformity change of meaning effects. Discussion centers on possible domains in which the different construal mechanisms might operate. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
8.
This paper assesses the potential of natural zeolite utilization as a low-cost ion-exchange and sorbent material in pollution control and recovery of metals. The improvement of the purity of natural zeolites coupled with their chemical modification capabilities in order to provide specific properties may provide a cost-effective alternative for the treatment of heavy metal contaminated soil and water. The application of natural zeolites in industrial pollution control is becoming important and the level of technical effort is increasingly expanding. The extent of application is unlikely to be related solely to their low cost, but also to improved properties and performance characteristics, which will advance the practical implementation of natural zeolites' technology.  相似文献   
9.
We present a characterization of some processes affecting the performance of solid state array cameras designed for ground based astronomical imaging in the 8–13μm atmospheric window. Our discussion includes a novel model for electron-hole generationrecombination noise based on the probable pathlength of an electron in a photoconductor. We use the Berkeley mid-IR Array Camera as an example. For this camera, the results show that the total optical system composed of the camera, a 3m telescope, and the atmosphere has an efficiency of about 3%, a 1σ noise equivalent flux density of 25 mJy min?1/2arcsec?2 measured over a Δλ/λ=10% band width, and a noise equivalent expressed as the ambient temperature thermal black body noise of 23%.  相似文献   
10.
Aqueous extracts of the coralline red algaeBossiella orbigniana have been shown to catalyze the enzymatic oxidation of arachidonic acid to a previously unreported ω6 eicosapentaenoic acid product. This unique fatty acid contains a conjugated tetraene with absorption maxima at 293, 306, and 321 nm, and was identified by spectral methods as 5(Z),8(Z),10(E),12(E),14(Z)-eicosapentaenoic acid. The compound was given the trivial name bosseopentaenoic acid. Under experimental conditions, production of bosseopentaenoic acid was linear up to 2 hr and dioxygen was consumed. Bosseopentaenoic acid, along with several other conjugated tetraenes, was also present in the algae endogenously as revealed by a comparison of the ultraviolet (UV) spectra and the high-performance liquid chromatographic (HPLC) pattern of the purified product and the organic extract ofBossiella.  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