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   111860篇
  免费   1567篇
  国内免费   795篇
测绘学   2496篇
大气科学   7543篇
地球物理   21622篇
地质学   39792篇
海洋学   10288篇
天文学   25631篇
综合类   277篇
自然地理   6573篇
  2022年   679篇
  2021年   1228篇
  2020年   1304篇
  2019年   1475篇
  2018年   3002篇
  2017年   2779篇
  2016年   3311篇
  2015年   1736篇
  2014年   3172篇
  2013年   5793篇
  2012年   3503篇
  2011年   4614篇
  2010年   4198篇
  2009年   5427篇
  2008年   4689篇
  2007年   4807篇
  2006年   4485篇
  2005年   3352篇
  2004年   3263篇
  2003年   3062篇
  2002年   3040篇
  2001年   2665篇
  2000年   2561篇
  1999年   2074篇
  1998年   2131篇
  1997年   2011篇
  1996年   1701篇
  1995年   1691篇
  1994年   1467篇
  1993年   1431篇
  1992年   1318篇
  1991年   1315篇
  1990年   1296篇
  1989年   1195篇
  1988年   1108篇
  1987年   1291篇
  1986年   1135篇
  1985年   1389篇
  1984年   1564篇
  1983年   1478篇
  1982年   1351篇
  1981年   1320篇
  1980年   1128篇
  1979年   1096篇
  1978年   1054篇
  1977年   967篇
  1976年   921篇
  1975年   903篇
  1974年   888篇
  1973年   961篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
22.
23.
We report intermediate resolution H spectroscopy of the black hole candidate Nova Muscae 1991 during quiescence. We classify the companion star as a K3-K4V which contributes 85±6 percent to the total flux from the binary. The photospheric absorption lines are broadened by 106±13 kms−1 with respect to template field stars, leading to a system mass ratio of q =M1/M2 = 7.8−2.0+3.4. Doppler imaging of the H line shows strong emission coming from the secondary star (EW=3.1±0.6Å) which we associate with chromospheric activity. However, the hot-spot is not detected and this may indicate a lower mass transfer rate than in other X-ray transients of comparable orbital periods. The surface brightness distribution of the accretion disk in H follows a relation I∝R−1.1, less steep than typically observed in cataclysmic variables. Updated system parameters are also presented.  相似文献   
24.
This work presents preliminaries spectroscopic results about ionized xenon and xenon-helium mixture using a capillary pulsed discharge under several experimental results.  相似文献   
25.
26.
27.
Extensive examination of large numbers of spatially orientated thin sections of orientated samples from orogens of all ages around the world has demonstrated that porphyroblasts do not rotate relative to geographical coordinates during highly non-coaxial ductile deformation of the matrix subsequent to their growth. This has been demonstrated for all tectonic environments so far investigated. The work also has provided new insights and data on metamorphic, structural and tectonic processes including: (1) the intimate control of deformation partitioning on metamorphic reactions; (2) solutions to the lack of correlation between lineations that indicate the direction of movement within thrusts and shear zones, and relative plate motion; and (3) a possible technique for determining the direction of relative plate motion that caused orogenesis in ancient orogens.  相似文献   
28.
Summary ?In the south-eastern Altenbergkar–Silbereck area in the eastern Tauern window (Lungau, Salzburg) structurally controlled precious-metal (Au–Ag) mineralization is hosted in marbles of the Permo(?)-Mesozoic Silbereck Formation and in the underlying Variscan Central gneiss. During the Alpine otogeny both lithologies were affected by ductile deformation (shearing, D1; folding, D2/D3) and subsequent brittle deformation (tension gashes, D4; normal faulting, D5) related to the uplift and exhumation of the Tauern window. Mineralization is controlled by brittle D4 structures. NE–SW trending steeply dipping tension gashes of the “Tauerngoldgang” type occur within the Central gneiss. Three different marble-hosted ore types following fracture systems as well as foliation and bedding planes can be distinguished: 1) metasomatic replacement ores, 2) ores in tension gashes and 3) ores in talc-bearing structures, often containing high-grade gold and silver mineralization (native gold in association with Ag–Pb–Bi–sulfosalts). Four stages of mineralization can be distinguished which occur in all ore types: arsenopyrite–pyrite–pyrrhotite (first stage), Au–(Ag–Pb–Bi–sulfosalts) (second stage), base-metal sulfides and tetrahedrite–tennantite (third stage) and Ag-rich galena (fourth stage). Preliminary fluid inclusion data indicate temperatures of ore formation well above 300 °C (346 °C mean) for the second stage within the Central gneiss and temperatures between 310 and 230 °C for the second and third stages in the marble. Received October 12, 2001; revised version accepted September 5, 2002 Published online March 10, 2003  相似文献   
29.
A variety of measures of organic matter concentration and quality were made on samples collected from the top few mm of intertidal mudflat sediment over the course of a year, in order to assess the relative importance of biological and sedimentological influences on sedimentary organic matter. Winter and summer were times of relatively fine-grained sediment accumulation, caused by biological deposition or stabilization processes and resulting in higher organic matter concentrations. Stable carbon isotope and Br:C ratios indicated a planktonic source of bulk organic matter. Ratios of organic carbon to specific surface area of the sediments were consistent with an organic monolayer coverage of sediment grains. Correction for changing grain size during the year showed no change in the organic concentration per unit surface area, in spite of organic matter inputs by in situ primary production, buildup of heterotroph biomass and mucus coatings, and biodeposition of organic-rich seston. There were also no indications of changes in bulk organic quality, measured as hydrolyzable carbohydrates and amino acids, in response to these biological processes. It is concluded that biological processes on a seasonal time scale affect the bulk organic matter of these sediments via a modulation of grain size rather than creation or decay of organic matter.  相似文献   
30.
Laboratory measurements of the OI1173989 Å (3s' 3D° → 2p41D, 3s' 3D° → 2p43P) branching ratio have been made with a value of 1.5 × 10t-4 indicated. This value makes the branching transition at 1173 Å an order of magnitude stronger than the branch at 7990 Å (3s' 3D° → 3p 3P). The 1173 Å branching loss is still too weak a loss process for multiply scattered 989 Å photons to resolve the 989 Å intensity problem in the dayglow.  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