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   368879篇
  免费   4017篇
  国内免费   1213篇
电工技术   5778篇
综合类   2469篇
化学工业   56335篇
金属工艺   16507篇
机械仪表   11552篇
建筑科学   8993篇
矿业工程   2385篇
能源动力   7853篇
轻工业   29374篇
水利工程   4428篇
石油天然气   7439篇
武器工业   16篇
无线电   40498篇
一般工业技术   72541篇
冶金工业   53840篇
原子能技术   7781篇
自动化技术   46320篇
  2021年   2207篇
  2018年   17702篇
  2017年   16615篇
  2016年   13608篇
  2015年   3048篇
  2014年   4446篇
  2013年   12189篇
  2012年   9851篇
  2011年   18290篇
  2010年   15536篇
  2009年   13988篇
  2008年   15439篇
  2007年   16309篇
  2006年   7934篇
  2005年   8455篇
  2004年   7873篇
  2003年   7657篇
  2002年   7135篇
  2001年   6591篇
  2000年   6347篇
  1999年   6280篇
  1998年   14799篇
  1997年   11053篇
  1996年   8614篇
  1995年   6531篇
  1994年   5925篇
  1993年   5796篇
  1992年   4491篇
  1991年   4462篇
  1990年   4284篇
  1989年   4301篇
  1988年   4264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   3446篇
  1977年   3924篇
  1976年   5039篇
  1975年   3160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
21.
Soil, bottom sediment, suspended sediment, and urban street dust and dirt samples from the Menomonee River Watershed, Wisconsin, were dispersed by ultrasound, fractionated and analyzed for the P content of each of three particle-size fractions. The major soil types in the watershed were used as a reference for comparing particle-size distribution and P content in urban street dust and dirt, and in sediments.Phosphorus level was found to be greater in the clay-sized particles than in the sand- or silt-sized particles of urban street dust and dirt samples, but 48% of the P was in the sand-sized fraction because of the predominance of sand-sized particles in these samples. The highest P level in the clay-sized fraction of the bottom sediments occurred at the site below a sanitary treatment plant (STP) outfall with secondary treatment capability. The P level found below a tertiary STP outfall was equal to the level found in agricultural areas. When using P level in sediment for locating areas of possible P input to the river, the clay-sized fraction of bottom sediments was more precise than P levels in unfractionated samples.  相似文献   
22.
Food and knowledge of food is an essential part of life. Knowledge about how to get enough and how to get it right is an essential part of all cultures. Since food is vital, nutrition policy is critical. Nutrition policy is about who should eat what, why, when and how in order to promote better health. This article deals with the politics of nutrition, which is about who eats what, why, when, how, and with what impact on their lives. For food, whether it is scarce or abundant, affects people unequally, hence food has always been a social concern. Regulating its supply has been a source of civilization. The state of nutrition is one of the most potent indicators of the state of society. Hence, food is also one of the oldest objects of politics--indeed, one cannot think of politics without it. This is the first issue I will address: by offering a capsule history of humanity from the perspective of the politics of nutrition. Next, I will address, in a similarly compressed history, the development of the science of nutrition, as nutrition policy has become based on research--particularly on medical science. Finally, I will address nutrition policy as a subpart of the politics of nutrition--how it has been defined and how it has developed, its changing agendas and current concerns.  相似文献   
23.
This essay begins with a reflection on what has been taught in architectural design since the turn of the twentieth century. I shall trace back to the two disciplinary foundations of the French École des Beaux-Arts — parti and poché — in the education of an architect in the eighteenth and nineteenth centuries. I shall then attempt to superimpose parti and poché on a modern disciplinary framework, say that of mathematics, which leads to musings on a series of architectural problems that include pattern versus type, stability versus mobility, orthogonal versus oblique, confinement versus transparency, and aging versus metallic sheen. These paradoxes, I suggest, demand the education of an architect to address both the instrumental pattern of a building configuration and the ambient felt qualities of a room, rather than vision alone.  相似文献   
24.
25.
A review was undertaken of (a) the levels and prevalence of Salmonella spp., Campylobacter spp., Listeria monocytogenes, Escherichia coli 0157. Cryptosporidium parvum and Giardia intestinalis in livestock manure, and (b) factors which affect their survival during storage and following land application. Pathogens are commonly present in livestock manures and can survive in soil for several months or years after spreading or excretion onto land. Temperature was identified as being the most important factor influencing pathogen survival, although pH, UV light and drying were also important. Various practical measures to minimise the risk of pathogen transfer into the food chain were identified, including treatment, extended storage and no/harvest periods following land spreading. Guidelines are being developed on managing farm manures to minimise the risks of microbiological contamination of ready-to-eat crops.  相似文献   
26.
The grain handling and transportation system (GHTS) has been a defining component of the western Canadian economy since the early part of the last century. Only two major railways serve the grain handling industry, so the potential for the exercise of market power in transportation appears to be significant. In fact, spatially oriented regulations applicable to railways exist in Canada to mitigate this, but to date they have not often been used by shippers. While the reasons for this are not completely clear, we suggest that the regulations are now inadequate for the current state of the GHTS. Using a unique data set from the year 2000, we assess the effect on the grain handling industry of modifications to these regulations, and find that significant gains for shippers from these modifications are possible. These gains are computed to be on the order of C$10–15 million per year for grain shippers alone. We would like to thank conference and presentation participants (Transportation Research Board, Canadian Agricultural Economics Society, University of British Columbia, University of Leeds and California State University, Long Beach) as well as the anonymous reviewers who commented on the earlier versions of this paper. We would also like to acknowledge support from the Saskatchewan Department of Highways.  相似文献   
27.
Many lakes and some streams and estuaries are showing signs of excessive fertilization due to the input of aquatic plant nutrients from man-associated sources. The key element often found limiting aquatic plant populations is phosphorus. The attempt to control phosphorus input to natural waters as the overall approach for controlling excessive fertilization is technically sound and economically feasible for many natural waters. However, a much better understanding of the relationship between the phosphorus input to a lake and the excessive growths of aquatic plants within the lake must be developed. This development will require a combined biological and chemical approach toward assessing the role of phosphorus in eutrophication for a specific water body. The biological approach will use tissue content, enzymatic and kinetic uptake analysis of phosphorus limitations as well as bioassays of phosphorus availability in order to determine the limiting nutrient for a body of water. The chemical aaproach will utilize amounts of each of the forms of phosphorus present in the lake and the rates of interchange of phosphorus between these various forms.There will be some waters where control of phosphorus from treatment of domestic waste water input and removal of phosphorus from detergents will not result in significant improvement in water quality. This is because these waters derive their phosphorus from diffuse sources, such as urban and rural stormwater drainage, the atmosphere, and ground waters. In these instances, it may be necessary to initiate in-lake control of phosphorus by the addition of alum or iron salts.  相似文献   
28.
Conclusions The experiment with the use of piles cast in rammed holes in lieu of continuous foundations placed on a bed compacted by tamping produced positive results and will be expanded upon further, since the strength properties of the materials in the foundations and the soils in the foundation beds can be more fully utilized, the reliability of the structure increased, the volume of earthwork reduced, labor and material outlays diminished, and the rate of construction and the level of the industrial nature of the work increased.State Institute for the Projection of the Greenhouse Industry. Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 2, p. 10, March–April, 1977.  相似文献   
29.
30.
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