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   208027篇
  免费   3233篇
  国内免费   1064篇
电工技术   3962篇
综合类   585篇
化学工业   30401篇
金属工艺   8543篇
机械仪表   6463篇
建筑科学   5362篇
矿业工程   1565篇
能源动力   5395篇
轻工业   17878篇
水利工程   2477篇
石油天然气   4331篇
武器工业   72篇
无线电   24360篇
一般工业技术   41203篇
冶金工业   37646篇
原子能技术   5166篇
自动化技术   16915篇
  2021年   1978篇
  2020年   1458篇
  2019年   1834篇
  2018年   2857篇
  2017年   2960篇
  2016年   3108篇
  2015年   2165篇
  2014年   3520篇
  2013年   9585篇
  2012年   5609篇
  2011年   7620篇
  2010年   6103篇
  2009年   6858篇
  2008年   6909篇
  2007年   6939篇
  2006年   6013篇
  2005年   5617篇
  2004年   5266篇
  2003年   5130篇
  2002年   4739篇
  2001年   4830篇
  2000年   4749篇
  1999年   4884篇
  1998年   11511篇
  1997年   8239篇
  1996年   6501篇
  1995年   4751篇
  1994年   4339篇
  1993年   4132篇
  1992年   3253篇
  1991年   3119篇
  1990年   2897篇
  1989年   2974篇
  1988年   2849篇
  1987年   2408篇
  1986年   2317篇
  1985年   2726篇
  1984年   2521篇
  1983年   2351篇
  1982年   2103篇
  1981年   2215篇
  1980年   2040篇
  1979年   2131篇
  1978年   2126篇
  1977年   2373篇
  1976年   3189篇
  1975年   1834篇
  1974年   1765篇
  1973年   1778篇
  1972年   1448篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Urban sources account for significant quantities of important diffuse pollutants, and urban watercourses are typically badly polluted. As well as toxic metals, hydrocarbons including PAHs, and suspended matter, priority urban pollutants include faecal pathogens and nutrients. Can urban watercourses be restored by sufficient reductions in pollution loads? Case studies in the UK and Sweden provide insights and some grounds for optimism. A major trans-Atlantic review of the performance of best management practices (BMPs) is informing BMP planning. New approaches such as the maximisation of self-purification capacity in the receiving waters may also need to be developed, alongside BMPs at source. Other initiatives in Europe, USA and China, including collaborative projects, are trying to address the intractable issues such as persistent pollutants from transport and urban infrastructure. The challenge is daunting, but there are clear ways forward and future research needs are evident.  相似文献   
992.
Optical channel waveguides in silicon diffused from GeSi alloy   总被引:1,自引:0,他引:1  
A technique for fabricating low-loss and polarisation-independent channel waveguides in silicon is reported. The waveguides are obtained by Ge-indiffusion using either a GeSi alloy or a system of alternating Ge and Si layers. Typical fabrication parameters for single-mode waveguides are given.<>  相似文献   
993.
During the last decade, a substantial amount of progress has been made in the understanding of the relationship between the dietary constituents and the development of colon cancer in man. The information base is sufficiently convincing with respect to an enhancing effect as a function of total fat intake and a protective effect of certain dietary fibers in colon cancer. The populations with high incidence of colon cancer are characterized by consumption of high-dietary fat which may be a risk factor in the absence of factors that are protective, such as use of whole-grain cereals, high fibrous foods and vegetables mainly of cruciferous type. Application of the findings made thus far in colon cancer research for the general public is, therefore, to have a far-reaching impact on the major premature, killing diseases in the western world.  相似文献   
994.
Fermented salami was prepared containing 0, 9, 18 and 27% defatted sesame flour (DSF). Chemical and sensory analyses were performed on the products. Results indicated that DSF could be used at either the 9 or 18% level without detrimental effect upon sensory attributes but salami containing 27% DSF will have less desirable flavor.  相似文献   
995.
A dozen CuO samples prepared under various conditions and from different starting materials were evaluated as cathode materials for a primary Li/CuO cell. The “thin electrode” method was used for rapid evaluation of the samples. Both coulombic efficiency and discharge voltage depend considerably on the method of synthesis. No correlation was found between the specific surface area and the resistivity of the samples on the one hand and the cathode performance on the other. Best results were obtained from CuO prepared by the oxidation of Cu2O under controlled temperature and time of oxidation.  相似文献   
996.
997.
This newsletter provides information on past and upcoming events related to the IEEE Computer Society's Test Technology Technical Council and the test community.  相似文献   
998.
Alewives (Alosa pseudoharengus), the major prey fish for Lake Ontario, contain thiaminase. They are associated with development of a thiamine deficiency in salmonines which greatly increases the potential for developing an early mortality syndrome (EMS). To assess the possible effects of thiamine deficiency on salmonine reproduction we measured egg thiamine concentrations for five species of Lake Ontario salmonines. From this we estimated the proportion of families susceptible to EMS based on whether they were below the ED20, the egg thiamine concentration associated with 20% mortality due to EMS. The ED20s were 1.52, 2.63, and 2.99 nmol/g egg for Chinook salmon (Oncorhynchus tshawytscha), lake trout (Salvelinus namaycush), and coho salmon (Oncorhynchus kisutch), respectively. Based on the proportion of fish having egg thiamine concentrations falling below the ED20, the risk of developing EMS in Lake Ontario was highest for lake trout, followed by coho (O. kisutch), and Chinook salmon, with the least risk for rainbow trout (O. mykiss). For lake trout from western Lake Ontario, mean egg thiamine concentration showed significant annual variability during 1994 to 2003, when the proportion of lake trout at risk of developing EMS based on ED20 ranged between 77 and 100%. Variation in the annual mean egg thiamine concentration for western Lake Ontario lake trout was positively related (p < 0.001, r2 = 0.94) with indices of annual adult alewife biomass. While suggesting the possible involvement of density-dependent changes in alewives, the changes are small relative to egg thiamine concentrations when alewife are not part of the diet and are of insufficient magnitude to allow for natural reproduction by lake trout.  相似文献   
999.
New climate change scenarios for the Netherlands.   总被引:4,自引:0,他引:4  
A new set of climate change scenarios for 2050 for the Netherlands was produced recently. The scenarios span a wide range of possible future climate conditions, and include climate variables that are of interest to a broad user community. The scenario values are constructed by combining output from an ensemble of recent General Climate Model (GCM) simulations, Regional Climate Model (RCM) output, meteorological observations and a touch of expert judgment. For temperature, precipitation, potential evaporation and wind four scenarios are constructed, encompassing ranges of both global mean temperature rise in 2050 and the strength of the response of the dominant atmospheric circulation in the area of interest to global warming. For this particular area, wintertime precipitation is seen to increase between 3.5 and 7% per degree global warming, but mean summertime precipitation shows opposite signs depending on the assumed response of the circulation regime. Annual maximum daily mean wind speed shows small changes compared to the observed (natural) variability of this variable. Sea level rise in the North Sea in 2100 ranges between 35 and 85 cm. Preliminary assessment of the impact of the new scenarios on water management and coastal defence policies indicate that particularly dry summer scenarios and increased intensity of extreme daily precipitation deserves additional attention in the near future.  相似文献   
1000.
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