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   522987篇
  免费   3908篇
  国内免费   1342篇
电工技术   10165篇
综合类   763篇
化学工业   77158篇
金属工艺   18312篇
机械仪表   15019篇
建筑科学   13021篇
矿业工程   1657篇
能源动力   14601篇
轻工业   47282篇
水利工程   4460篇
石油天然气   6439篇
武器工业   21篇
无线电   67753篇
一般工业技术   99437篇
冶金工业   98797篇
原子能技术   9621篇
自动化技术   43731篇
  2021年   4289篇
  2020年   3369篇
  2019年   4183篇
  2018年   6293篇
  2017年   6248篇
  2016年   6539篇
  2015年   4368篇
  2014年   7642篇
  2013年   23757篇
  2012年   12437篇
  2011年   17161篇
  2010年   13612篇
  2009年   15668篇
  2008年   16419篇
  2007年   16260篇
  2006年   14512篇
  2005年   13191篇
  2004年   12924篇
  2003年   12865篇
  2002年   12290篇
  2001年   12783篇
  2000年   12028篇
  1999年   12793篇
  1998年   31711篇
  1997年   22504篇
  1996年   17444篇
  1995年   13356篇
  1994年   12095篇
  1993年   11784篇
  1992年   8615篇
  1991年   8271篇
  1990年   8070篇
  1989年   7761篇
  1988年   7540篇
  1987年   6351篇
  1986年   6439篇
  1985年   7348篇
  1984年   6629篇
  1983年   6306篇
  1982年   5782篇
  1981年   5864篇
  1980年   5507篇
  1979年   5321篇
  1978年   4951篇
  1977年   6022篇
  1976年   7941篇
  1975年   4283篇
  1974年   4136篇
  1973年   4065篇
  1972年   3480篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
Ten full-scale test fires were conducted in a chamber simulating a three person ship accommodation quarter. The test fires used three different ventilation conditions, two types of bunks and furnishings with either polyurethane foam or chloroprene foam. The chamber was instrumented to follow the development of the fire.The furnishings with polyurethane foam were readily ignited and produced an intense fire within 2 to 4 minutes. The furnishings with chloroprene foam ignited but burned slowly with a small flame or in smoldering combustion.The ventilation conditions had a significant effect on the development and intensity of the fire as the test fires were burning under oxygen-limiting conditions.The solid pan bunks retarded the early development of the fire compared to open bunks for polyurethane furnishings. However, once the mattresses on the three bunks were ignited the intensity of the test fires was similar for both types of bunks.  相似文献   
32.
33.
Gallium was removed selectively from aqueous solutions containing zinc or aluminum using sodium di-(n-octyl) phosphinate as a ligand (NaL). At low pH or low mole ratios, the gallium was removed by complexation with the ligand as GaL(3(S)), while the zinc or the aluminum remained in the solution. Nearly complete separation of gallium was obtained. By increasing the amount of ligand or by increasing the pH, the zinc or aluminum remaining in the solution was then removed as a solid complex: ZnL(2(S)) or AlL(3(S)), respectively. At a pH between 1.5 and 2 and a mole ratio ligand to total metals of 0.75 for zinc solutions and 1.0 for aluminum solutions, more than 98% of the gallium was selectively removed with a high molar selectivity, alpha(Ga/Zn) and alpha(Ga/Al), respectively. Over 95% of gallium was recovered from the solid GaL(3(S)) complex by treatment of the complex with a 3M NaOH solution and diethyl ether. The gallium was concentrated in the aqueous solution to 4 times its initial concentration and the ligand was extracted into the ether phase. After evaporation of the ether, 95% of the ligand was regenerated in its sodium form as a solid.  相似文献   
34.
Underground construction is notorious for high cost and time overruns. This is probably inherent in the nature of the work, which usually involves high risks on the part of both the employer and the contractor, in view of the extent of the uncertainties with regard to such factors as geology, appropriate design and performance. The main civil works constructed for South Africa's Palmiet Pumped Storage Scheme between 1983 and 1987 have demonstrated that with early identification and proper management of the risks and costs, the value of cost and time overruns can be curtailed to a very large extent. This paper describes broadly the process adopted on the project, focusing in particular on the successful management at Palmiet of the financial risks pertaining to the cost of the contractor's overheads, establishment, supervision and plant in a contract that was known to be subject to variations, extensions and delays. This was achieved by appropriate scheduling, provisions for payment and on-site management of these cost items, usually described broadly as the Preliminary and General (P & G) Items. These items accounted for 34% of the value of the contract.  相似文献   
35.
Two approximate yield criteria are proposed for reinforced concrete cylindrical shell element. Three lower bound solutions are developed for cylindrical shell roofs with free longitudinal edges. Two shells with different geometric parameters are analysed. Elastic analysis is also done at working load. Graphs are presented to show the variation of stress resultants at critical sections.  相似文献   
36.
Determination of labile and strongly bound metals in lake water   总被引:1,自引:0,他引:1  
Differential pulse anodic stripping voltammetry is applied to differentiate and determine the labile and strongly bound forms of Zn, Cd, Pb and Cu in lake water without preconcentration of the sample. The sensitivities as established for an oxidation peak current of 0·020 μA are: 0·2 ppb for Zn. 0·4 ppb Cd. 0·7 ppb Pb and 0·5 ppb for Cu. For the analysis of a lake water containing 5·0–24·5 ppb of the four metals, the relative S.D. ranged from 1·6 to 10 per cent. Interferences of cations and anions and the choice of a buffer system have been discussed. The method has been applied to study a number of small lakes in the Sudbury area. Ontario.  相似文献   
37.
38.
Water levels in tidal rivers are subject to the influence of both fluvial and tidal factors. The interaction between these factors is rendered more complex in regions affected by typhoons, which typically result in the combined occurrence of heavy rainfall and high sea levels.
Previous studies have outlined an analytical approach for calculating design water levels based on dividing the fluvial and tidal series into a typhoon and a non-typhoon component. This paper illustrates the steps involved in the analytical approach and describes its application to drainage basins in Hong Kong.
The approach provides a practical means for calculating design water levels, and is recommended for final design purposes. The results of using the full analysis may be approximated by using a sub-set of analytical results, and should prove satisfactory for the purposes of outline design.  相似文献   
39.
40.
The survival, oxygen consumption and spontaneous activity of suckers were determined at 15°C in water containing different concentrations of methoxychlor in ethanol, ethanol, and no added substances. A dynamic bioassay was used to estimate the 96 h LC50, which was found to be 0·0345 ppm. Routine oxygen consumption was determined in a continuous-flow respirometer, and the activity was recorded in separate experiments by measuring the water disturbance using silver wire electrodes.After 24 h fish exposed to untreated, ethanol-treated and 0.01 ppm methoxychlor-treated water had oxygen uptake rates which were relatively constant and near standard levels. At 0.10 ppm methoxychlor (lethal to all fish within 85 h), oxygen consumption rates were 2–3.5 × as high as those of control fish. Activity also greatly increased. The fish lost their balance along with the coordination between opercular and locomotor movements. Suckers that died in 0.04 ppm methoxychlor showed oxygen uptake rates and poisoning symptoms similar to 0.10 ppm treated fish, while those that survived this treatment had rates similar to control fish. The increase in ventilation which accompanies this hyperactivity may result in increased uptake of insecticide and thereby explain the irreversible nature of the response.  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