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   346727篇
  免费   3400篇
  国内免费   776篇
电工技术   5747篇
综合类   266篇
化学工业   52390篇
金属工艺   16358篇
机械仪表   11042篇
建筑科学   7529篇
矿业工程   2954篇
能源动力   7574篇
轻工业   24830篇
水利工程   4726篇
石油天然气   11386篇
武器工业   20篇
无线电   35707篇
一般工业技术   72947篇
冶金工业   61843篇
原子能技术   10594篇
自动化技术   24990篇
  2021年   3058篇
  2019年   2939篇
  2018年   5151篇
  2017年   5307篇
  2016年   5691篇
  2015年   3278篇
  2014年   5620篇
  2013年   14560篇
  2012年   8791篇
  2011年   11699篇
  2010年   9443篇
  2009年   10617篇
  2008年   10809篇
  2007年   10766篇
  2006年   9174篇
  2005年   8529篇
  2004年   8169篇
  2003年   7931篇
  2002年   7572篇
  2001年   7685篇
  2000年   7526篇
  1999年   7539篇
  1998年   17597篇
  1997年   12693篇
  1996年   9752篇
  1995年   7222篇
  1994年   6533篇
  1993年   6464篇
  1992年   5145篇
  1991年   5079篇
  1990年   4965篇
  1989年   4945篇
  1988年   4810篇
  1987年   4292篇
  1986年   4218篇
  1985年   4664篇
  1984年   4374篇
  1983年   4139篇
  1982年   3798篇
  1981年   4084篇
  1980年   3826篇
  1979年   4057篇
  1978年   4197篇
  1977年   4430篇
  1976年   5873篇
  1975年   3748篇
  1974年   3705篇
  1973年   3799篇
  1972年   3303篇
  1971年   2968篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
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   
42.
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   
43.
Data from compartment tests of polyurethane foam seating assemblies are reported. Results show that the behavior of such assemblies during fires is strongly dependent on the type and position of interliners and upholstery fabrics. An incidental comparison of peak rate of heat release obtained in a compartment test with a predictive model equation is made and selected data from the tests are presented.  相似文献   
44.
45.
ABSTRACT

The main purpose of this study is to analyse the phenomenon of pool boiling in water over a horizontally placed copper tube heater of 28?mm diameter. The experiment has been carried out to observe the bubble growth and departure characteristics for the heat flux range up to 40,000?W/m2. The entire process is being recorded by a digital camera at different time intervals. The measured parameters have been used to determine the initial layer thickness, macro layer thickness and critical heat flux and validated with models proposed in the literature.  相似文献   
46.
Conclusions 1. Application of the method of underwater concreting with the use of a container is inexpedient in connection with the presence of a large number of flaws detected in the shafts of cast-in-place piles formed by this method; concreting of holes by the VAT method is preferable.2. The strength of concrete in the shafts of cast-in-place piles increases, reaching a maximum at a depth of 0.4–0.5 the length of the shaft. Variation in strength along the shaft is explained primarily by the percentage of coarse aggregate contained in the concrete.3. The quality of pile concrete should be monitored during all stages of pile installation, and, primarily during the stage of concreting, i.e., when there is a chance of correcting an observed defect.All-Union Institute for the Design and Planning of Health Resorts. Scientific-Research Institute of Foundations and Underground Structures. Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 5, pp. 14–17, September–October, 1976.  相似文献   
47.
Zenaitis MG  Sandhu H  Duff SJ 《Water research》2002,36(8):2053-2061
Batch biological treatment of log yard run-off reduced biochemical oxygen demand (BOD), chemical oxygen demand (COD) and tannin and lignin (TL) concentration by 99%, 80%, and 90%, respectively. Acute (Microtox) toxicity was decreased over treatment, from an initial EC50 of 1.83% to a value of 50.4% after 48 h of treatment. Kinetics of biodegradation were determined using respirometry and fitted using the Monod and Tessier model. For the Monod model the maximum substrate uptake rate, and Ks values determined were 0.0038 mg BOD/mgVSS min, and 1.4 mg/L, respectively. The efficacy of ozone as a pre- and post- biological treatment stage was also assessed. During ozone pretreatment, TL concentration and acute toxicity were rapidly reduced by 70% and 71%, respectively. Pre-ozonation reduced BOD and COD concentration by < 10%, however a larger fraction of residual COD was non biodegradable after ozonation. Biologically treated effluent was subjected to ozonation to determine whether further improvements in effluent quality could be achieved. A reduction in COD and TL concentration was observed during ozonation, however no further improvement in toxicity was observed. Ozonation increased BOD by 38%, due to conversion of COD to BOD.  相似文献   
48.
Concentrations of trace elements in human tissues characteristically vary widely, and the distributions of most concentrations are skewed to the right. Examination of some of the factors which contribute to the marked variability and skewness of the concentrations revealed that: (1) distributions of concentrations are satisfactorily normal (Gaussian) after logarithmic transformation, (2) ash weight is the best frame of reference in which to report results, (3) the distributions of metal concentrations are not further normalized by adjustments which assume that tissue lipid or collagen contains a fixed fraction of the metal found in the parenchyma, and (4) the choice of sample site within the liver is of minimal significance.  相似文献   
49.
Recent changes in the construction of building floors have included the use of light material composite systems and long span floor systems. Although these changes have many advantages, such floor systems can suffer from excessive vibration due to human activities. This problem is exacerbated in office buildings due to the reduction in inherent damping associated with modern fit outs. Excessive floor vibrations are often realised after the completion of construction or following structural modifications and normally arise due to inadequate knowledge of the damping values in the design process. Thus rectification measures are normally required to reduce floor accelerations. This paper proposes a new innovative passive viscoelastic damper to reduce floor vibrations. This damper can be easily tuned to the fundamental frequency of the floor and can be designed to achieve various damping values. The paper discusses the analytical development of the damper with experimental results presented on a prototype to demonstrate its effectiveness.  相似文献   
50.
Analysis of stable lead isotopes and lead concentrations in lake-sediment deposits, not least in varved (annually-laminated) sediments, is a useful method to study lead pollution history. This paper presents details from a study of 31 lakes in Sweden. Using a strong acid digestion of sediment samples and ICP-MS analyses, we have found that Swedish lake sediments have a high natural (pre-pollution) 206[Pb]207[Pb] ratio (mean 1.52+/-0.18, range 1.28-2.01, n=31 lakes). In contrast, atmospheric lead pollution derived from metal smelting processes, coal burning and from alkyl-lead added to petrol has a lower ratio (< 1.2). Consequently, when pollution lead deposition began approximately 3500 years ago, the lead isotope ratio of the sediments started to decline, and in modern sediments it is typically < 1.2. Using the isotope and concentration values and a mixing model, the relative contribution of pollution and natural lead in sediment samples can be calculated. The pollution lead records of the Swedish lake sediments show a consistent picture of the atmospheric lead pollution history. Some noticeable features are the Roman peak (approx. 0 AD), the large and permanent Medieval increase (approx. 1000 AD), peaks at approximately 1200 and 1530 AD, the rapid increase after World War II, the peak in the 1970s, and the large modern decline.  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