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   92761篇
  免费   1585篇
  国内免费   1546篇
测绘学   3029篇
大气科学   6878篇
地球物理   18355篇
地质学   35322篇
海洋学   7134篇
天文学   17559篇
综合类   2339篇
自然地理   5276篇
  2022年   396篇
  2021年   685篇
  2020年   777篇
  2019年   822篇
  2018年   6280篇
  2017年   5473篇
  2016年   4619篇
  2015年   1534篇
  2014年   2246篇
  2013年   3780篇
  2012年   3160篇
  2011年   5592篇
  2010年   4456篇
  2009年   5629篇
  2008年   4964篇
  2007年   5200篇
  2006年   2928篇
  2005年   2542篇
  2004年   2643篇
  2003年   2491篇
  2002年   2158篇
  2001年   1845篇
  2000年   1766篇
  1999年   1388篇
  1998年   1488篇
  1997年   1371篇
  1996年   992篇
  1995年   1125篇
  1994年   948篇
  1993年   855篇
  1992年   830篇
  1991年   753篇
  1990年   833篇
  1989年   693篇
  1988年   631篇
  1987年   790篇
  1986年   646篇
  1985年   841篇
  1984年   905篇
  1983年   846篇
  1982年   812篇
  1981年   715篇
  1980年   664篇
  1979年   597篇
  1978年   597篇
  1977年   547篇
  1976年   535篇
  1975年   496篇
  1974年   502篇
  1973年   465篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
92.
The UBV photometric observations of RT Per, from Sanwal and Chaubey (1981), were analyzed by the Wilson and Devinney code (1971). The light curves include reflection effects that for the first time has been suggested by Dugan (1911). RT Per has a semi-detached configuration where the lower-mass component is in contact with its respective Roche surface. The higher-mass component very nearly fills its Roche lobe. It has the characteristic of an Algol type system. The absolute dimensions for the primary and secondary of this system were calculated from its spectral types and by combining the photometric solution with inferred component radial velocities (Lu, 1990).  相似文献   
93.
Dry valleys are a striking geographic landscape in Hengduan Mountains Region and are characterized by low rainfall, desert type of vegetation and fragile environment. Past efforts and resources have been concentrated mainly on rehabilitation of degraded ecosystem and fragile environment, particularly reforestation, while socio-economic development has been largely overlooked. Despite successes in pocket areas, the overall trend of unsustainability and environmental deterioration are continuing. It is important to understand that uplift of the Tibetan Plateau is the root cause of development of dry valleys, and development and formation of dry valleys is a natural process. Human intervention has played a secondary role in development of dry valleys and degradation of dry valleys though human intervention in many cases has speeded up environmental degradation of the dry valleys. It is important to understand that dry valleys are climatic enclaves and an integrated approach that combines rehabilitation of degraded ecosystems and socio-economic development should be adopted if the overall goal of sustainable development of dry valleys is to be achieved. Promotion of niche-based cash crops, rural energy including hydropower, solar energy, biogas and fuelwood plantation is recommended as the priority activities.  相似文献   
94.
95.
96.
Recent findings by the MER rover opportunity confirming the presence of iron minerals that can only be formed in the presence of water emphasize the study of analogous environments to Mars on Earth. The study of chemolithoautotrophic communities living in acidic iron-rich habitats is highly relevant in order to identify Mars analog environment-specific biomarkers. Iron oxidizing bacteria like Leptospirillum ferrooxidans and Acidithiobacillus ferrooxidans have ways of life for which it is feasible to identify a past or present hypothetical niche on Mars. We have developed a strategy for biomarker identification based on: (i) search for biosignatures on acid and metal-rich environments; (ii) development of an immunosensor microarray; and (iii) integration into an instrument for autonomous and remote operation. The instrument that we have built, called Signs Of LIfe Detector (SOLID), is capable of processing a variety of samples for the detection of specific biomarkers. Antibodies against several bacterial strains have been developed and tested in a microarray biosensor on SOLID. Tests with field samples have been successfully performed, allowing the detection of L. ferrooxidans, A. ferrooxidans present in sediment samples.  相似文献   
97.
We consider the particular solutions of the evolutionary system of equations in elements that correspond to planar and spatial circular orbits of the singly averaged Hill problem. We analyze the stability of planar and spatial circular orbits to inclination and eccentricity, respectively. We construct the instability regions of both particular solutions in the plane of parameters of the problem.  相似文献   
98.
The chemical composition of several thermal springs around Changbaishan area has been investigated. Cenozoic basaltic rocks are widely distributed in Northeast China and geothermal characteristics have been described. About one hundred hot springs exist around Changhaishan Volcano at the border between China and D.P.R.Korea with high temperature about 82~C. The pH values of the spring water range from 6.9 to 7.1 and the total flow rate is about 4.8 L/sec. The chemical composition of the thermal springs is sodium carbonate; the high-mineral contents of thermal water are believed to have medicinal properties. Bathhouses are already built along the hot springs to take the advantage of the supposed healing properties. The high quality of those hot springs is believed to be utilitized for mineral water. The chemical equilibrium temperatures were estimated at about 160℃ based on the Na-K-Ca geochemical thermometer.  相似文献   
99.
100.
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