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   280181篇
  免费   3088篇
  国内免费   1307篇
电工技术   4830篇
综合类   520篇
化学工业   40934篇
金属工艺   11156篇
机械仪表   8453篇
建筑科学   6474篇
矿业工程   1577篇
能源动力   7032篇
轻工业   22616篇
水利工程   3046篇
石油天然气   5787篇
武器工业   43篇
无线电   33701篇
一般工业技术   56809篇
冶金工业   51024篇
原子能技术   7505篇
自动化技术   23069篇
  2021年   2196篇
  2019年   2079篇
  2018年   3607篇
  2017年   3703篇
  2016年   3937篇
  2015年   2501篇
  2014年   4396篇
  2013年   12058篇
  2012年   7145篇
  2011年   9803篇
  2010年   7605篇
  2009年   8839篇
  2008年   9176篇
  2007年   9219篇
  2006年   7876篇
  2005年   7312篇
  2004年   7256篇
  2003年   7029篇
  2002年   6786篇
  2001年   6871篇
  2000年   6511篇
  1999年   6798篇
  1998年   16341篇
  1997年   11625篇
  1996年   8958篇
  1995年   6879篇
  1994年   6044篇
  1993年   6082篇
  1992年   4425篇
  1991年   4212篇
  1990年   4105篇
  1989年   4084篇
  1988年   3774篇
  1987年   3414篇
  1986年   3417篇
  1985年   3781篇
  1984年   3492篇
  1983年   3236篇
  1982年   3050篇
  1981年   3058篇
  1980年   3047篇
  1979年   2858篇
  1978年   2892篇
  1977年   3165篇
  1976年   4116篇
  1975年   2438篇
  1974年   2393篇
  1973年   2414篇
  1972年   2022篇
  1971年   1792篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
The operation of a torsional oscillator for detection, of flow-driven textural transitions in, 3He-A is described. The detection technique, tracking the shift of the resonant frequency of the torsional oscillator, allows us to observe textural changes and the presence of vortices in zero magnetic field. It relies on. the anisotropic superfluid density of 3He-A and its change due to reorientation of the Î-texture caused by counterflow.  相似文献   
82.
A high-gain ballistic hot-electron device is described. The GaAs-AlGaAs heterostructure device, with a 21-mm-thick pseudomorphic In 0.12Ga0.88As base, had a current gain of 27 at 77 K and 41 at 4.2 K. As characteristically seen in ballistic devices, transfer into the L valley limited the maximum gain. The Γ-L valley separation in the strained In0.12Ga0.88As was estimated to be about 380 meV  相似文献   
83.
84.
85.
A newly developed membrane performance enhancer (MPE) was used to prevent membrane fouling in a membrane bioreactor (MBR) process. It transpired that 1,000 mg/l of MPE reduced polysaccharide levels from 41 mg/I to 21 mg/I on average under the experimental condition. Repeated experiments also confirmed that 50-1,000 mg/l of MPE could reduce membrane fouling significantly and increase the intervals between membrane cleanings. Depending on MPE dosages and experimental conditions, trans-membrane pressure (TMP) increase was suppressed for 20-30 days, while baseline TMP surged within a few days. In addition, MPE allowed MBR operation even at 50,000 mg/l of total solid and reduced permeate COD. However, no evidence of toxicity for sludge was found from respiratory works.  相似文献   
86.
The biochemical composition can be seen as a good indicator of both the biodegradability and the methane potential of a given waste. The work presented here is an attempt to elaborate a typology of wastes and to compare it to the anaerobic degradation characteristics. The first data indicate that there is a link between the ligno-cellulosic content of the waste and the biodegradability. When dealing with application to anaerobic digestion processes, having a tool to predict the ability of the waste to be degraded could be of the greatest interest for preventing failures, estimating biogas production, methane content, or for the management of co-digestion processes.  相似文献   
87.
A novel contact opening mechanism has been developed using a piezoelectric actuator to open the contacts in a low contact opening velocity circuit breaker. The arc control on the contacts is critical for successful current interruption (10/sup 3/-10/sup 4/ A) in low voltage (<250V) devices. Previous work has shown how arc root commutation from the contact region into the arc chamber is affected by arc chamber materials, contact materials and the gap behind the moving contact for contact velocities between 1ms/sup -1/ and 10ms/sup -1/. This work is extended using a commercially available piezoelectric actuator to open the contacts. Contact opening speeds are assessed and the arc root mobility is characterized under this operating regime. A flexible test apparatus and solid-state high-speed arc imaging system are used to gather data on the arc root during the opening of the contacts. New experimental results are presented on the anode and cathode root velocity and arc root motion in an arc chamber with piezoelectrically actuated contact opening. These results can be used to improve the design of high current low voltage circuit breakers suitable for piezoelectric actuation.  相似文献   
88.
The TELEMAC project brings new methodologies from the Information and Science Technologies field to the world of water treatment. TELEMAC offers an advanced remote management system which adapts to most of the anaerobic wastewater treatment plants that do not benefit from a local expert in wastewater treatment. The TELEMAC system takes advantage of new sensors to better monitor the process dynamics and to run automatic controllers that stabilise the treatment plant, meet the depollution requirements and provide a biogas quality suitable for cogeneration. If the automatic system detects a failure which cannot be solved automatically or locally by a technician, then an expert from the TELEMAC Control Centre is contacted via the internet and manages the problem.  相似文献   
89.
90.
A combination of concept (the product portfolio and the product life cycle) and empirical research (the PIMS study) can aid in clarifying the key issues involved in the selection of a firm's core strategies. Strategy control is multistage: periodic re-evaluation of strategy alternatives by use of the marketing audit; an analytical framework for control of chosen strategies by breaking down components and reporting their performance variances; and 'marketing segment' analysis for control of the detailed marketing plan.?  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