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   26039篇
  免费   458篇
  国内免费   326篇
测绘学   733篇
大气科学   1831篇
地球物理   5029篇
地质学   9031篇
海洋学   2366篇
天文学   6398篇
综合类   60篇
自然地理   1375篇
  2021年   211篇
  2020年   249篇
  2019年   317篇
  2018年   626篇
  2017年   611篇
  2016年   752篇
  2015年   430篇
  2014年   717篇
  2013年   1340篇
  2012年   821篇
  2011年   1080篇
  2010年   984篇
  2009年   1282篇
  2008年   1171篇
  2007年   1184篇
  2006年   1153篇
  2005年   868篇
  2004年   858篇
  2003年   780篇
  2002年   730篇
  2001年   634篇
  2000年   644篇
  1999年   573篇
  1998年   565篇
  1997年   532篇
  1996年   405篇
  1995年   405篇
  1994年   420篇
  1993年   321篇
  1992年   318篇
  1991年   263篇
  1990年   313篇
  1989年   275篇
  1988年   258篇
  1987年   288篇
  1986年   242篇
  1985年   327篇
  1984年   348篇
  1983年   332篇
  1982年   321篇
  1981年   256篇
  1980年   270篇
  1979年   224篇
  1978年   214篇
  1977年   218篇
  1976年   189篇
  1975年   193篇
  1974年   186篇
  1973年   169篇
  1972年   114篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
871.
In order to prepare iron phosphate by waste sludge, we report a method for effective utilization of the sludge obtained from the electrocoagulation treatment of source-separated urine. The sludge was dissolved with hydrochloric acid and pretreated with H2O2 and Na3PO4; finally, NaOH was added to precipitate iron phosphate from the solution. Thermal treatment of the precipitate at 750 °C in air yielded crystalline quartz-like anhydrous FePO4. The precipitate was characterized by a number of thermal techniques such as thermogravimetry/differential thermal analysis, scanning electron microscopy, and X-ray powder diffraction.  相似文献   
872.
873.
874.
875.
876.
The KwaZulu-Natal Bight is a shallow indentation of the eastern seaboard of South Africa, characterised by a narrow (45 km wide) extension of the continental shelf, with a shelf break at about 100 m. It has a complex hydrography: the waters of the bight are derived from the fast-flowing, southward-trending Agulhas Current, which is fed mostly by the tropical and subtropical surface waters of the South-West Indian Ocean subgyre, which are generally oligotrophic in nature, notably depleted in reduced nitrogen and phosphate except at river mouths and during periodic upwelling of deeper nutrient-rich water. Despite this, the bight is believed to be relatively productive, and it is suggested that efficient nutrient recycling by prokaryotes may sustain primary productivity efficiently, even in the absence of new nutrient inputs. Here we have measured bacterial numbers, biomass and heterotrophic productivity during summer and winter in conjunction with phytoplankton standing stock and factors that influence it. Bacterial distribution closely matched phytoplankton distribution in surface waters, and was highest close to the coast. Bacterial standing stocks were similar to those of oligotrophic systems elsewhere (0.5–5.0 × 105 cells ml–1; 1 × 10–8 to 1.25 × 10–7 g C ml–1) and increased in association with the development of phytoplankton blooms offshore and with inputs of allochthonous material by rivers at the coast. Heterotrophic productivity in summer was lowest in the far south and north of the bight (0.5 × 10–10 g C ml–1 h–1) but higher close to the shore, over shallow banks, and in association with increased phytoplankton abundance over the midshelf (1.0–3.5 × 10–9 g C ml–1 h–1). There were marked seasonal differences with lower bacterial standing stocks (5 × 104 to 2 × 105 cells ml–1; 4–5 × 10–9 to 1–2 × 10–8 g C ml–1) and very low bacterial productivity (4 × 10–11 to 1 × 10–10 g C ml–1 h–1) in winter, probably resulting from lowered rates of primary productivity and dissolved organic matter release as well as reduced riverine allochthonous inputs during the winter drought.  相似文献   
877.
878.
879.
The peculiarities of catastrophic earthquakes that occurred in the Northwest Pacific region on January 13, 2007, and January 15, 2007, east of the Kuril Islands and the Tohoku megaearthquake of March 11, 2011, east of Japan are considered and analyzed. It is revealed that these earthquakes, apart from the fact that they occurred in the transition zone from the Pacific to island arcs and the Eurasian continent, have common features and similar characteristics. The seismotectonic slip in the source of the chronologically first event, the 2006 Kuril earthquake, was a gentle thrust, while that of the second event of January 13, 2007, was a downthrow beneath the Kuril deep trench.  相似文献   
880.
Using high-resolution analytical electron microscopy, heterogeneous fragments of cosmogenic substances, such as nickelphosphide Ni3P and the unnamed intermetallide ZnAl2, are detected for the first time for a large meteorite crater, the Zhamanshin circular structure. Due to the impossibility of simultaneous finding these phases in the same meteorite, an assumption is made on the cometary nature of the impactor of the Zhamanshin crater.  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