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   287650篇
  免费   3084篇
  国内免费   813篇
电工技术   5037篇
综合类   163篇
化学工业   45423篇
金属工艺   11897篇
机械仪表   8607篇
建筑科学   6846篇
矿业工程   1835篇
能源动力   6735篇
轻工业   25913篇
水利工程   3195篇
石油天然气   7091篇
武器工业   16篇
无线电   31458篇
一般工业技术   56945篇
冶金工业   51362篇
原子能技术   7538篇
自动化技术   21486篇
  2021年   2098篇
  2018年   3672篇
  2017年   3623篇
  2016年   3905篇
  2015年   2466篇
  2014年   4231篇
  2013年   12019篇
  2012年   6829篇
  2011年   9212篇
  2010年   7565篇
  2009年   8642篇
  2008年   8929篇
  2007年   8813篇
  2006年   7826篇
  2005年   7309篇
  2004年   6800篇
  2003年   6553篇
  2002年   6627篇
  2001年   6497篇
  2000年   6176篇
  1999年   6229篇
  1998年   14749篇
  1997年   11029篇
  1996年   8566篇
  1995年   6523篇
  1994年   5914篇
  1993年   5787篇
  1992年   4479篇
  1991年   4440篇
  1990年   4284篇
  1989年   4301篇
  1988年   4255篇
  1987年   3599篇
  1986年   3595篇
  1985年   4161篇
  1984年   3965篇
  1983年   3620篇
  1982年   3422篇
  1981年   3558篇
  1980年   3426篇
  1979年   3365篇
  1978年   3445篇
  1977年   3923篇
  1976年   5037篇
  1975年   3158篇
  1974年   3007篇
  1973年   3024篇
  1972年   2651篇
  1971年   2473篇
  1970年   2104篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
High strength concretes, in particular at early ages (more than 40 MPa at 3 days and up to 80 MPa at 7 days), are obtained by replacement of certain granular size ranges of sand or gravel by cement-active artificial aggregates in a given concrete composition elaborated with a local supply of aggregates and cement. The strengthening of the paste-aggregate bond by chemical reactions between the artificial aggregates and the cement also involves an improvement of other concrete performances and not only of mechanical strengths. The cracking ability is decreased, the wear strength is higher, and water permeability is low or even nil for certain compositions. The laboratory results are confirmed by tests on the site and show the possibility of achieving high strength concretes without any particular selection of cement and natural aggregates.  相似文献   
52.
Until some thirty years ago tunnelling in southern Africa for civil engineering purposes had been on a relatively small scale and of a sporadic nature. The first major tunnel to be built in the region was the 82 km long Orange-Fish Tunnel. Since 1970 more than 175 tunnels with an aggregate length of just over 400 km have been built. Much of the first phase of the Lesotho Highlands Water Project, which will have more than 100 km of tunnels, is about to be completed. The last thirty years represent the busiest period of tunnelling by civil engineers that southern Africa has ever known. The paper starts by summarising the early history of tunnelling in the region, and goes onto describe recent and current tunnelling projects. It also gives an indication of the possible demand for tunnels in the future.  相似文献   
53.
The levels of lead in city street dirt and in soil from various locations in Glasgow were investigated during spring 1976. Lead concentrations in street dirt ranged from 150–2300 ppm, mean 960 ppm, and were significantly elevated with respect to the observed “natural” level of 78 ppm. Lead derived from anti-knock compounds in petrol and introduced to the environment via automobile exhausts was clearly implicated as the main source of lead pollution in a series of soil lead measurements at the centre and periphery of eight Glasgow parks.Various chemical leaching techniques were employed and compared. Less than 5% of street dirt and soil lead was found to be associated with the organic phase.  相似文献   
54.
A short- and long-term toxicity study with α-hexachlorocyclohexane (α-HCH) was carried out with freshwater organisms of different trophic levels (algae, crustaceae and fishes). In this study mortality, immobilization, growth, reproduction, histopathologic changes and enzyme activities were chosen as criteria.Furthermore accumulation and elimination processes were investigated in separate organisms as well as the uptake of α-HCH by one organism and through a food-chain.Finally a tentative method for deriving an “ecological limit” for χ-HCH in surface water is described.  相似文献   
55.
The degree of biodegradability of three non-ionic surfactants has been assessed using small-scale activated-sludge sewage treatment plants. The effect of temperature on biodegradability was studied by operation at 15°, 11° and 8°C.The two alcohol ethoxylate surfactants tested were well degraded at all three operating temperatures. The alkyl phenol ethoxylate surfactant tested was well degraded at 15°C but at lower temperatures the biodegradability was dependent on concentration. At 5 mg 1−1, greater than 90 per cent removal was achieved but at 20 mg 1−1 the degree of removal fluctuated between 40 and 95 per cent at 11°C and between 20 and 80 per cent at 8°C.The present results have been compared with those obtained using the same surfactants in community trials at a small sewage works employing biological filtration. Although a similar temperature effect was observed with the alkyl phenol ethoxylate, the biodegradabilities obtained in the laboratory were consistently greater than those obtained at the sewage works, perhaps because of the constant conditions of the laboratory test.  相似文献   
56.
Interstitial P levels in Lake Mendota and Lake Wingra were evaluated as a function of season and water column and sediment depth. Interstitial water was obtained by the centrifugation-filtration method. Temporal variations were observed over the entire 15 cm sediment depth interval examined in all four locations evaluated. Interstitial reactive P (IRP) levels in Lake Mendota ranged from 0.014–1.67 mg l−1 at the 5–6 m water column depth and from 1.20–5.75 mg l−1 at the 18–19.5 m depth. IRP levels in Lake Wingra ranged from 0.029–2.15 mg l−1 at 3.5 m and from 0.191–3.96 mg l−1 at 2 m. Variations in interstitial P were attributed to variations in oxidation state of Fe as influenced by oxygen transport and reduction rates.  相似文献   
57.
The paper outlines some of the actions taken by Severn Trent Water to achieve compliance with consent limits since the implementation of the Control of Pollution Act (Part II) in 1985.
Prior to and since 1985, Severn Trent Water has undertaken an extensive programme to analyse the reasons for failure and carry out remedial work. A major feature of this programme, which has been further stimulated by privatization, has been the need to reduce the lead time for carrying out improvements.
The paper discusses the means by which this has been achieved, including the introduction of the 'fast-track'initiative and the 'process matrix'.
Other aspects are outlined including initiatives to develop more robust processes, computerized work scheduling, extensive training programmes and best practice studies.  相似文献   
58.
A matrix method for the analysis of structural systems composed of thin-walled members is presented. The matrix displacement analysis includes the effects of thin-walled non-uniform torsion theory, cross-section asymmetry, eccentric restraint as well as joint types peculiar to thin-walled members. The method is used for a prediction of the elastic behaviour of a set of representative test frames. The test frames were pitched-roof portals constructed from channel sections bent about their major-axis and supported by eccentric restraints simulating purlins and girts.  相似文献   
59.
ABSTRACT

A growing number of workers, particularly in the knowledge and service sectors, can perform their work at multiple locations, and it is decreasingly realistic to assume, as researchers and planners have traditionally done, that employment in cities occurs in fixed locations. This suggests that census data or establishment registries do not fully capture where economic activity takes place. Given the role that ICTs play in enabling daytime workplace mobility, and given that they generate substantial amounts of real-time, geolocated data, we ask whether these Big Data can shed light upon the trajectories of mobile workers at the urban scale.  相似文献   
60.
The human senses play an important role in assessing the quality of food and of the environment. Particularly the chemical senses of taste and smell determine the pleasantness of foods and drinks and may provide a warning mechanism for the presence of more or less toxic contaminants. Even after the recent rapid development of powerful analytical techniques the human nose can easily detect trace amounts of chemicals at levels manytimes lower than the analytical detection limits.Although taste and odour assessment of drinking water has been practized inmany waterworks laboratories by small panels, relatively unreliable results were obtained and such sensory data played up till now only a minor role in the management of the water treatment plant. More sophisticated and reliable methods for sensory water quality evaluation are discussed. Special attention is given to the characteristics of the human senses of taste and smell, to which the methodology of sensory assessment should be better adapted. High numbers of observations on a sample are generally needed, for which purpose the use of large panels has to be realized. In this respect the help of large consumer panels is an alternative for the presently often used small laboratory groups. As an example the preliminary results of a large ongoing experiment of sensory water quality assessment by 2 consumer panels of 100 persons each in the Rotterdam area are discussed. Drinking water quality is judged at the consumer homes every week. The experiment started September 1979 and will be continued for one year.  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