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   30931篇
  免费   462篇
  国内免费   381篇
测绘学   791篇
大气科学   2811篇
地球物理   6356篇
地质学   10770篇
海洋学   2378篇
天文学   6754篇
综合类   70篇
自然地理   1844篇
  2020年   179篇
  2019年   196篇
  2018年   491篇
  2017年   481篇
  2016年   689篇
  2015年   448篇
  2014年   676篇
  2013年   1406篇
  2012年   739篇
  2011年   1026篇
  2010年   873篇
  2009年   1241篇
  2008年   1058篇
  2007年   941篇
  2006年   1043篇
  2005年   875篇
  2004年   848篇
  2003年   869篇
  2002年   867篇
  2001年   746篇
  2000年   788篇
  1999年   660篇
  1998年   629篇
  1997年   666篇
  1996年   575篇
  1995年   541篇
  1994年   482篇
  1993年   427篇
  1992年   420篇
  1991年   416篇
  1990年   422篇
  1989年   398篇
  1988年   380篇
  1987年   466篇
  1986年   435篇
  1985年   464篇
  1984年   558篇
  1983年   560篇
  1982年   501篇
  1981年   490篇
  1980年   447篇
  1979年   433篇
  1978年   447篇
  1977年   394篇
  1976年   355篇
  1975年   355篇
  1974年   405篇
  1973年   389篇
  1972年   245篇
  1971年   224篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
941.
Rapid mass movements are common features of hilly or mountainous terrains, and depending on the type of movements or processes involved these give rise to characteristic deposits. Identification of these deposits in mapping programmes gives a clue as to their mode of origin (rockslide/rockfall, snow avalanche, debris flow, fluidal sediment flow) and this can be used to predict rapid mass movements in the future.

A case story from western Norway is described and the value of mapping such deposits for land-use planning and hazard estimation stressed.  相似文献   

942.
The most important measure is that the freshwater demand of 0.5 m3 per t beet processing is strictly kept to or fallen below, to which a catalogue of measures with ten points is presented. The wastewaters which cannot be used again have to be stored prior to the final purification in order to compensate for quantities and concentrations, aerated storage ponds being successful for small sugar factories as the purification technology. For this, three cases are described as examples: supplementary equipment of two anaerobic ponds with a rod-roll aeration, by which within two months (May to July) the BOD3 is reduced from 1,200 to 11 mg/l. The aeration of a pond of 12.5 ha (370,000 … 500,000 m3) by means of a centrifugal aerator (45 kW) does not lead to completely aerobic conditions: Accumulation up to 350 t BOD5, 200 t organic acids at a reduction of 100 t sulphate, 45 t of which being converted via sulphides by photoautotrophic sulphur bacteria. This combination of anaerobic and aerobic processes is very economical. At aeration with ca. 1 W/m3 in the third case in summer there are achieved wastewater loads which are of a receiving-water quality.  相似文献   
943.
944.
The vibrational properties of metastable Ca-GeO3 perovskite are investigated as a function of temperature using Raman scattering. Mode-assignments are derived from polarized spectra of optically oriented single crystals. Neither symmetry-breaking structural transformations nor soft-mode behavior is revealed in the Raman spectra between room temperature and 650° C. Only a small decrease in the local static octahedral tilt angles can be inferred from the Raman data over this temperature range. A Landau extrapolation of the lowest frequency Ag modes suggests that these modes become critical near 2000° C, a temperature that is above the extrapolated zero pressure melting point.  相似文献   
945.
946.
947.
948.
The reduction of the spectral accelerations to account for inelastic response of structures as given by the 1976 Regulations for the Federal District of Mexico is evaluated relative to the records obtained at the Secretaria de Communicaciones y Transportes (SCT) during the September 19, 1985 Mexico earthquake. It is concluded that the 1976 Regulations do not treat all types of structural systems equally. On a relative basis the 1976 Regulations provide less protection for those structures designed to larger values of ductility factors. This could have contributed, together with other characteristics of the ground motions and construction practices, to the heavy damage of highrise buildings in Mexico City.  相似文献   
949.
The National Defense University's study of climate change to the year 2000 was based largely on the judgments of the members of two expert panels. Although the study has been widely distributed and apparently read by policy makers in the U.S. and abroad, the method of eliciting and analyzing expert judgment has not been critically reviewed. This paper uses the literature on judgment and subjective probability to evaluate the expert judgment methods used in the study.The National Center for Atmospheric Research is sponsored by the National Science Foundation.  相似文献   
950.
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