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   8319篇
  免费   341篇
  国内免费   95篇
测绘学   200篇
大气科学   596篇
地球物理   1948篇
地质学   3007篇
海洋学   777篇
天文学   1216篇
综合类   19篇
自然地理   992篇
  2021年   89篇
  2020年   113篇
  2019年   115篇
  2018年   167篇
  2017年   164篇
  2016年   231篇
  2015年   199篇
  2014年   199篇
  2013年   476篇
  2012年   257篇
  2011年   316篇
  2010年   287篇
  2009年   343篇
  2008年   322篇
  2007年   284篇
  2006年   323篇
  2005年   239篇
  2004年   294篇
  2003年   265篇
  2002年   267篇
  2001年   175篇
  2000年   178篇
  1999年   142篇
  1998年   142篇
  1997年   104篇
  1996年   116篇
  1995年   118篇
  1994年   131篇
  1993年   112篇
  1992年   112篇
  1991年   113篇
  1990年   101篇
  1989年   86篇
  1988年   89篇
  1987年   123篇
  1986年   100篇
  1985年   165篇
  1984年   188篇
  1983年   145篇
  1982年   132篇
  1981年   134篇
  1980年   109篇
  1979年   125篇
  1978年   121篇
  1977年   105篇
  1976年   96篇
  1975年   91篇
  1974年   67篇
  1973年   83篇
  1972年   49篇
排序方式: 共有8755条查询结果,搜索用时 265 毫秒
121.
122.
123.
124.
Marine manganese nodules and crusts, when processed, yield tailings which may be utilized for environmental and economic benefit. The key to the reasonable and effective utilization of these tailings lies in making a systematic appraisal of their composition and properties. This article gives an introduction to the investigation of manganese tailings properties. The tailings have a high iron and/or manganese content, high surface area, high porosity, and fine grain size. Some tailings have a high rare earth element content which is valuable. They may also have high SO3, arsenic, and uranium contents which are harmful. Depending on the process used to produce the tailings, there will likely be some differences in chemical composition, mineral assemblages, surface area and adsorption capability, pore diameter and volume, density and pH. In assigning potentially beneficial applications for these tailings, these differences should be taken into account to optimize utilization.  相似文献   
125.
126.
127.
Reflection from submerged cylinders are studied by means of integral equations. By expressing the solution as a distribution of vortices, the integral equations become non-singular for closed contours. It is shown that the method gives a short and easy proof for the classical result that no reflection occurs for the circular cylinder. The reflection power for the elliptic contour and the flat plate are studied when the bodies are situated deeply below the surface.  相似文献   
128.
The tolerance of the oligochaete Monopylephorus rubroniveus and the polychaete Streblospio benedicti to sediment-associated fluoranthene was characterized under normoxic (>80% dissolved oxygen saturation) and moderately hypoxic ( approximately 50% dissolved oxygen saturation) conditions. Under both conditions, M. rubroniveus was highly tolerant of fluoranthene. Streblospio benedicti was considerably less tolerant of fluoranthene compared with M. rubroniveus. In addition, S. benedicti was less tolerant to fluoranthene under moderately hypoxic conditions, although no differences in sensitivity between the two oxygen conditions were observed based upon median lethal tissue residues. Bioaccumulation factors were higher for S. benedicti exposed to moderate hypoxia, suggesting that behavioral adaptations to compensate for the lower dissolved oxygen increased its bioaccumulation of fluoranthene. The results of the present laboratory study demonstrate that (1). changes in annelid tolerance to fluoranthene under varying oxygen conditions is a species-dependent phenomenon and (2). the differential tolerance of these two annelids to these combined stressors is consistent with their relative abundances in the field.  相似文献   
129.
130.
In mammals, the induction of cytochrome P4501A forms by chlorinated dibenzo-p-dioxins, chlorinated dibenzofurans, and halogenated biphenyls is under control of a soluble protein known as the Ah (aromatic hydrocarbon) receptor. Little is known about the presence and properties of the Ah receptor in other vertebrate and invertebrate species. In these studies, we sought evidence for an Ah receptor in the liver or liver-equivalent of 20 species of marine and freshwater animals, using the photoaffinity ligand 2-azido-3-[125I] iodo-7,8-dibromodibenzo-p-dioxin (N3[125I]Br2DD). Specific labeling of cytosolic proteins by N3[125I]Br2DD was observed in seven species of teleost and elasmobranch fish, in PLHC-I fish hepatoma cells, and in beluga whales. No specifically labeled proteins were found in cytosol from two species of agnathan fish nor in any of nine invertebrate species representing eight classes of four phyla. The presence or absence of specifically labeled polypeptides corresponds with the inducibility of cytochrome P4501A and sensitivity to the toxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin and related planar halogenated aromatic hydrocarbons in many of these groups. Thus, Ah receptor function may have arisen early invertebrate evolution and has been conserved from elasmobranch and teleost fish to mammals.  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