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   41529篇
  免费   591篇
  国内免费   320篇
测绘学   959篇
大气科学   2948篇
地球物理   8018篇
地质学   14280篇
海洋学   3859篇
天文学   9754篇
综合类   74篇
自然地理   2548篇
  2021年   408篇
  2020年   408篇
  2019年   455篇
  2018年   948篇
  2017年   852篇
  2016年   1099篇
  2015年   611篇
  2014年   1006篇
  2013年   2084篇
  2012年   1135篇
  2011年   1627篇
  2010年   1473篇
  2009年   1988篇
  2008年   1654篇
  2007年   1737篇
  2006年   1591篇
  2005年   1312篇
  2004年   1272篇
  2003年   1241篇
  2002年   1195篇
  2001年   1056篇
  2000年   981篇
  1999年   810篇
  1998年   813篇
  1997年   830篇
  1996年   668篇
  1995年   656篇
  1994年   617篇
  1993年   570篇
  1992年   526篇
  1991年   495篇
  1990年   506篇
  1989年   496篇
  1988年   475篇
  1987年   554篇
  1986年   489篇
  1985年   604篇
  1984年   650篇
  1983年   592篇
  1982年   527篇
  1981年   577篇
  1980年   469篇
  1979年   459篇
  1978年   433篇
  1977年   434篇
  1976年   383篇
  1975年   378篇
  1974年   376篇
  1973年   384篇
  1971年   224篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
Izvestiya, Atmospheric and Oceanic Physics - The results of the fractal analysis of a drainage network reconstructed using a digital elevation model and the structural and geomorphological analysis...  相似文献   
972.
The spectra of ten objects discovered by the Tautenburg objective prism survey were obtained at a higher spectral resolution in order to refine the selection criteria. We found four objects to be quasars and three MARKARIAN -type galaxies.  相似文献   
973.
The arcuate pattern of the main Caledonian cleavage and associated fold axial plane traces in North Wales is due partly to NW-SE compression with tectonic transport to the southeast against the concealed crop of the Tan y grisiau Microgranite. Low-angle cleavage close to the microgranite is shown to be a local variant of the regional cleavage formed during the main deformation and not an earlier phase as previously supposed. Transcurrent movements along several major fault systems are also related to compression around the microgranite and the Harlech Dome block.  相似文献   
974.
975.
An examination of day-to-day and monthly mean positions of the electrojet axis in relation to the changes in the apparent solar declination, in the Indian equatorial region shows marked association between the two. For relatively quiet days, significant correlations are observed between the solar declination and each of the parameters, the northernSq focal latitude, the jet axis and the line of maximumSq(H). From the significant mutual association of these parameters, it has been suggested that the equatorial electrojet could be a part of the world-wideSq current system.  相似文献   
976.
A property of swelling on heating to yield firm lightweight granules has first been revealed for lowcarbonaceous shales (LCS) from the Riphean stratotype in the Bashkirian Megaanticlinorium, Southern Urals. These granules possess all technological properties of “keramzite” or claydite (an artificial, expanded clayey material) and can successfully substitute this material. Moreover, the procedure of their manufacture, analogous to the manufacture of “shungizite” (an artificial carbonaceous-clayey material) from Lower Proterozoic clayey rocks of Karelia, is more economical than that for keramzit. Productive beds of the homogeneous LCS make up extended bodies tens of meters thick. At the present time, they can be used as cover rocks during the exploitation of the Bakal siderite deposits. The LCS represent fine-grained quartz-chlorite-sericite aggregates (with variable relationships between the major components), bearing fine-dispersed shungite in amounts of 0.5–1.0%. High-quality raw material of this kind is characterized by a fine-dispersed admixture of a mixed-layer clayey material bearing montmorillonite and vermiculite. Geological, lithological, mineralogical, and chemical prognostic criteria have been developed for the shungizite raw material. It has been concluded that this construction material, new for the Urals, is promising for industrial use  相似文献   
977.
978.
Discrimination diagrams have been developed that source Egyptian basaltic artefacts using whole‐rock major element geochemistry. These include K2O versus SiO2, TiO2 and P2O5 against MgO/Fe2O3t (total Fe as Fe2O3), and a discriminant analysis diagram using SiO2, Fe2O3t, CaO, and MnO. A complementary set of diagrams uses easily obtained trace element data (Nb/Y versus Zr/Nb; Zr [ppm] versus Rb/Sr; TiO2 [wt % volatile free] versus V; and Cr [ppm] versus Zr/Y) to determine the bedrock sources. These diagrams have been applied to seven First Dynasty basalt vessels (Abydos), two Fourth Dynasty basalt paving stones (Khufu's funerary temple, Giza), and two Fifth Dynasty paving stones (Sahure's complex, Abu Sir). They show that the bedrock source for all the artefacts was the Haddadin flow in northern Egypt. Multidimensional scaling and cluster analysis applied to the whole‐rock data (major elements and trace elements together) and previously published mineral fingerprinting studies confirm these results. Comparing mineral versus whole‐rock fingerprinting techniques, a major advantage of the former is the small sample size required (0.001 g compared to ≥ 0.1 g). Analytical costs are similar for both methods assuming that a comparison (bedrock) database can be assembled from the literature. For most archaeological problems, a whole‐rock bedrock database is more likely to exist than a mineral database, and whole‐rock analyses on artefacts will generally be easier to obtain than mineral analyses. Whole‐rock fingerprinting may be more sensitive than mineral‐based fingerprinting. Thus, if sample quantity is not an issue, whole‐rock analysis may have a slight cost, convenience, and technical advantage over mineral‐based methods. Our results also emphasize that the Egyptians cherished their Haddadin basalt flow and used it extensively and exclusively for manufacturing basalt vessels and paving stones for at least 600 years (∼3150 B.C. to 2500 B.C., approximate ages of the vessels and Abu Sir paving stones, respectively). © 2001 John Wiley & Sons, Inc.  相似文献   
979.
Geochemical processes, occurring in a stable transition zone between saltwater and freshwater, were simulated in a 2D, multi-layer flow chamber experiment. Mixing, calcite dissolution, and oxidative degradation of organic matter were identified as the main controlling factors. The results of the chamber experiment were compared to field data and verified by thermodynamic modeling. Similarity in most ion distributions suggests the general applicability of the experimental method. Differences in the redox conditions between field and experiment were reflected by the oxidants involved in the mineralization of organic carbon; while field data show evidence of sulfate reduction, the presence of oxygen in the laboratory experiment resulted in the reoxidation of sulfides.  相似文献   
980.
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