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   42091篇
  免费   924篇
  国内免费   656篇
测绘学   1090篇
大气科学   3550篇
地球物理   8718篇
地质学   14011篇
海洋学   3846篇
天文学   9362篇
综合类   139篇
自然地理   2955篇
  2021年   251篇
  2020年   313篇
  2019年   329篇
  2018年   727篇
  2017年   668篇
  2016年   1026篇
  2015年   749篇
  2014年   1026篇
  2013年   2173篇
  2012年   1254篇
  2011年   1686篇
  2010年   1365篇
  2009年   2038篇
  2008年   1827篇
  2007年   1716篇
  2006年   1602篇
  2005年   1460篇
  2004年   1389篇
  2003年   1345篇
  2002年   1264篇
  2001年   1098篇
  2000年   1110篇
  1999年   1023篇
  1998年   928篇
  1997年   921篇
  1996年   793篇
  1995年   724篇
  1994年   629篇
  1993年   573篇
  1992年   569篇
  1991年   544篇
  1990年   537篇
  1989年   471篇
  1988年   451篇
  1987年   507篇
  1986年   471篇
  1985年   585篇
  1984年   652篇
  1983年   621篇
  1982年   582篇
  1981年   535篇
  1980年   495篇
  1979年   453篇
  1978年   447篇
  1977年   410篇
  1976年   381篇
  1975年   379篇
  1974年   390篇
  1973年   382篇
  1972年   226篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
ABSTRACT. Although considerable attention has been paid to the record of temperature change over the last few centuries, the range and rate of change of atmospheric circulation and hydrology remain elusive. Here, eight latitudinally well-distributed (pole-equator-pole), highly resolved (annual to decadal) climate proxy records are presented that demonstrate major changes in these variables over the last 2000 years. A comparison between atmospheric 14C and these changes in climate demonstrates a first-order relationship between a variable Sun and climate. The relationship is seen on a global scale.  相似文献   
142.
143.
An empirical analysis is performed on the most detailed, recent measurements of turbulent oscillatory boundary layer flow. The measurements show that throughout elevations where the flow can be considered horizontally uniform, one deficit model is sufficient for describing the fundamental mode. Some general properties of the non dimensional velocity deficit D1(z) appear with striking consistency. First of all the identity ln ¦D1¦ ≡ Arg D1, which is a theoretical result for smooth laminar flow, seems to hold with great accuracy for a large range of turbulent flow conditions as well. This is of principal theoretical interest because all previous analytical eddy viscosity models as well as numerical mixing length models predict a consistent and fairly large difference between Arg D1 and ln ¦D1¦. If the identity between ln ¦D1¦ and Arg D1 extends all the way to the bed, it means that the bed shear stress leads the free stream velocity by 45 degrees. It is also found that the structure of smooth turbulent oscillatory flows as measured by Kalkanis (1964) corresponds to a sharp maximum in the normalized energy dissipation rate.  相似文献   
144.
Nucleosynthetic yields and production rates of helium and heavy elements are derived using new initial mass functions which take into account the recent revisions in O star counts and the stellar models of Maeder (1981a, b) which incorporate the effects of massloss on evolution. The current production rates are significantly higher than the earlier results due to Chiosi & Caimmi (1979) and Chiosi (1979), and a near-uniform birthrate operating over the history of the galactic disc explains the currently observed abundances. However, the yields are incompatibly high, and to obtain agreement it is necessary to assume that stars above a certain mass do not explode but proceed to total collapse. Further confirmation of this idea comes from the consideration of the specific yields and production rates of oxygen, carbon and iron and the constraints imposed by the observational enrichment history in the disc as discussed by Twarog & Wheeler (1982). Substantial amounts of4He and14C, amongst the primary synthesis species, are contributed by the intermediate mass stars in their wind phases. If substantial numbers of them exploded as Type I SN, their contribution to the yields of12C and56Fe would be far in excess of the requirements of galactic nucleosynthesis. Either efficient massloss precludes such catastrophic ends for these stars, or the current stellar models are sufficiently in error to leave room for substantial revisions in the specific yields. The proposed upward revision of the12C (α,γ)16O rate may produce the necessary changes in stellar yields to provide a solution to this problem. Stars that produce most of the metals in the Galaxy are the same ones that contribute most to the observed supernova rate.  相似文献   
145.
146.
Evidence was found that uptake of lead from seawater in both model and natural systems by the leaves of the seagrass Zostera muelleri does occur for live, dead and scraped leaves at all the lead concentrations tested. Positive uptake of lead was measured using the three analytical techniques of radio-tracer, differential pulse anodic stripping voltammetry and atomic absorption spectrometry; similar uptake profiles were obtained for each technique. Profiles always showed an initial rapid uptake phase followed by a gradual transition to a plateau phase at which an approximate equilibrium between lead attached to seagrass and free ionic lead in seawater was reached. The presence of active lead uptake processes was indicated in experiments which attempted to remove lead from leaves by a chelating agent.  相似文献   
147.
The multiplicity of early-type stars is still not well established. The derived binary fraction is different for individual star forming regions, suggesting a connection with the age and the environment conditions. The few studies that have investigated this connection do not provide conclusive results. To fill in this gap, we started the first detailed adaptive-optic-assisted imaging survey of early-type field stars to derive their multiplicity in a homogeneous way. The sample has been extracted from the Hipparcos Catalog and consists of 341 BA-type stars within ∼300 pc from the Sun. We report the current status of the survey and describe a Monte-Carlo simulation that estimates the completeness of our companion detection.  相似文献   
148.
149.
Abstract— Amoeboid olivine aggregates (AOAs) are irregularly shaped, fine‐grained aggregates of olivine and Ca, Al‐rich minerals and are important primitive components of CR chondrites. The AOAs in CR chondrites contain FeNi metal, and some AOAs contain Mn‐rich forsterite with up to 0.7 MnO and Mn:Fe ratios greater than one. Additionally, AOAs in the CR chondrites do not contain secondary phases (nepheline and fayalitic olivine) that are found in AOAs in some CV chondrites. The AOAs in CR chondrites record a complex petrogenetic history that included nebular gas‐solid condensation, reaction of minerals with the nebular gas, small degrees of melting, and sintering of the assemblage. A condensation origin for the Mn‐rich forsterite is proposed. The Mn‐rich forsterite found in IDPs, unequilibrated ordinary chondrite matrix, and AOAs in CR chondrites may have had a similar origin. A type A calcium, aluminum‐rich inclusion (CAI) with an AOA attached to its Wark‐Lovering rim is also described. This discovery reveals a temporal relationship between AOAs and type A inclusions. Additionally, a thin layer of forsterite is present as part of the Wark‐Lovering rim, revealing the crystallization of olivine at the end stages of Wark‐Lovering rim formation. The Ca, Al‐rich nodules in the AOAs may be petrogenetically related to the Ca, Al‐rich minerals in Wark‐Lovering rims on type A CAIs. AOAs are chondrite components that condensed during the final stage of Wark‐Lovering rim formation but, in general, were temporally, spatially, or kinetically isolated from reacting with the nebula vapor during condensation of the lower temperature minerals that were commonly present as chondrule precursors.  相似文献   
150.
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