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   296830篇
  免费   3927篇
  国内免费   1028篇
电工技术   5420篇
综合类   409篇
化学工业   47438篇
金属工艺   12213篇
机械仪表   9035篇
建筑科学   7347篇
矿业工程   1950篇
能源动力   7114篇
轻工业   27138篇
水利工程   3318篇
石油天然气   7252篇
武器工业   29篇
无线电   32257篇
一般工业技术   58573篇
冶金工业   51541篇
原子能技术   7611篇
自动化技术   23140篇
  2021年   2567篇
  2019年   2284篇
  2018年   4134篇
  2017年   4103篇
  2016年   4416篇
  2015年   2863篇
  2014年   4762篇
  2013年   12909篇
  2012年   7480篇
  2011年   9861篇
  2010年   8148篇
  2009年   9205篇
  2008年   9403篇
  2007年   9235篇
  2006年   8162篇
  2005年   7550篇
  2004年   7009篇
  2003年   6721篇
  2002年   6804篇
  2001年   6596篇
  2000年   6271篇
  1999年   6297篇
  1998年   14806篇
  1997年   11083篇
  1996年   8623篇
  1995年   6562篇
  1994年   5955篇
  1993年   5817篇
  1992年   4504篇
  1991年   4460篇
  1990年   4296篇
  1989年   4317篇
  1988年   4266篇
  1987年   3604篇
  1986年   3604篇
  1985年   4186篇
  1984年   3979篇
  1983年   3634篇
  1982年   3439篇
  1981年   3574篇
  1980年   3430篇
  1979年   3379篇
  1978年   3457篇
  1977年   3931篇
  1976年   5045篇
  1975年   3162篇
  1974年   3020篇
  1973年   3030篇
  1972年   2655篇
  1971年   2475篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
More than one hundred samples of European beers have been investigated by the new SNIF-NMR method which is based on 2H NMR at the natural abundance level and enables site-specific natural isotope fractionation factors in ethanol to be determined. The relative (R) and absolute (D/H)i, parameters are shown to be characteristic of the country where the beers are brewed and the observed variations are explained in terms of cereal composition, water resource, and manufacturing processing (fermentation, yeast, temperature cycle). These new parameters find analytical and mechanistic applications in the identification of a beer and in the investigation of a fermentation process.  相似文献   
993.
994.
To monitor the use of synthetic growth promoting hormones in red meat animals requires a sensitive and rapid method of analysis. This paper describes the development of such a method for these agents in bile, based on a high pressure liquid chromatographic purification procedure with detection and estimation by radioimmunoassay.  相似文献   
995.
Two pairs of expanded granular sludge bed (EGSB) bioreactors, R1/R2 and R3/R4, were designed. R1/R2 were used for mesophilic (37 degrees C) treatment of synthetic wastewater over a 100-day trial. A successful start-up was achieved by R1 and R2, with COD removal over 90%. Both reactors were operated under identical parameters; however, increased organic loading induced a reduction in COD removal by R1, while R2 maintained satisfactory performance throughout the experiment. R3/R4 were operated at 15 degrees C throughout a 422-day trial and were used for the stabilisation of volatile fatty acid-based wastewater. Phenol was introduced to R4 at an applied loading rate of 1 kg phenol m(-3)d(-1), which was increased to 2 kg phenol m(-3)d(-1). No phenol was supplied to R3. Efficient COD conversion was recorded in both R3 and R4, thus demonstrating the feasibility of high-rate phenol degradation under psychrophilic conditions. Terminal restriction fragment length polymorphism analysis was applied to the characterisation of microbial community dynamics within each of the reactors. The results indicated a microbiological basis for the deviation, in terms of operational performance, of R1 and R2. TRFLP analyses indicated stable microbial communities in R3 and R4, but detected changes in the abundance of specific ribotypes in response to phenol mineralisation.  相似文献   
996.
997.
The status of invasive dreissenid mussels (Dreissena polymorpha and D. bugensis) and native amphipods (Diporeia spp.) in Lake Ontario was assessed in 2003 and compared with historical data. D. polymorpha (zebra mussels) were rarely observed in 2003, having been displaced by D. bugensis (quagga mussels). D. bugensis expanded its depth range from 38 m depth in 1995 to 174 m in 2003 and this dreissenid reached densities averaging 8,000/m2 at all sites < 90 m. During the same time period, Diporeia populations almost completely disappeared from 0–90 m depth, continuing a declining trend from 1994–1997 reported in previous studies. The average density of Diporeia in the 30–90 m depth interval decreased from 1,380/m2 to 63/m2 between 1997 and 2003. Prior to 2003, areas deeper than 90 m represented a refuge for Diporeia, but even these deep populations decreased, with densities declining from 2,181/m2 in 1999 to 545/m2 in 2003. Two common hypotheses for the decline of Diporeia in the Great Lakes are food limitation and a toxin/pathogen associated with dreissenid pseudofeces. The Diporeia decline in deep waters preceded the expansion of D. bugensis to these depths, and suggests that shallow dreissenid populations remotely influence profundal habitats. This pattern of decline is consistent with mechanisms that act from some distance including nearshore dreissenid grazing and downslope transport of pseudofeces.  相似文献   
998.
Translated from Khimicheskoe i Neftyanoe Mashinostroenie, No. 3, pp. 8–9, March, 1992.  相似文献   
999.
A probabilistic seismic hazard analysis, in conjunction with subsurface geologic investigations, was used to evaluate the liquefaction potential of sand layers at Western Area Power Adminstration's Tracy, California, substation site. The seismic hazard analysis evaluates the peak ground acceleration corresponding to a lower level earthquake with a 50-percent probability of occurrence in 150 years, and two upper level earthquakes with a 10-percent probability of occurrence in 50 and 100 years. Based on the results of the geologic investigations and seismologic studies, it was concluded that the potential of liquefaction at the site for the design earthquakes was low  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