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   6491篇
  免费   236篇
  国内免费   81篇
测绘学   235篇
大气科学   514篇
地球物理   1522篇
地质学   2161篇
海洋学   523篇
天文学   1177篇
综合类   28篇
自然地理   648篇
  2022年   30篇
  2021年   76篇
  2020年   87篇
  2019年   122篇
  2018年   179篇
  2017年   161篇
  2016年   231篇
  2015年   173篇
  2014年   196篇
  2013年   407篇
  2012年   258篇
  2011年   342篇
  2010年   292篇
  2009年   381篇
  2008年   336篇
  2007年   291篇
  2006年   268篇
  2005年   259篇
  2004年   261篇
  2003年   205篇
  2002年   214篇
  2001年   115篇
  2000年   144篇
  1999年   102篇
  1998年   114篇
  1997年   81篇
  1996年   85篇
  1995年   83篇
  1994年   90篇
  1993年   73篇
  1992年   88篇
  1991年   68篇
  1990年   55篇
  1989年   50篇
  1988年   57篇
  1987年   54篇
  1986年   56篇
  1985年   67篇
  1984年   64篇
  1983年   76篇
  1982年   57篇
  1981年   65篇
  1980年   55篇
  1979年   65篇
  1978年   51篇
  1977年   34篇
  1976年   28篇
  1975年   26篇
  1974年   26篇
  1973年   30篇
排序方式: 共有6808条查询结果,搜索用时 15 毫秒
1.
2.
EARTHWORKS     
  相似文献   
3.
The quantity of phytoplankton in Newark Bay, New Jersey as indicated by chlorophyll-a content of the water, is low in the winter and early spring, and fluctuates greatly during the spring and summer. Chlorophyll-a concentrations are generally less than 20 μg/l until April. Between April and August, three phytoplankton blooms were indicated by chlorophyll-a concentrations as high as 81.4 μg/l. Net phytoplankton diversity values indicated generally eutrophic conditions; however, there was no significant correlation between diversity and chlorophyll-a concentrations. A role of nannoplankton in blooms is indicated.  相似文献   
4.
Several double salts have been detected in building materials and most of these salts are incongruently soluble compounds. In contrast to single salts, however, no systematic investigations of the crystallization behavior and deleterious effects of incongruently soluble double salts exist. To assess the damage potential of these salts, a systematic investigation of their highly complex behavior is desirable. This paper deals with the crystallization behavior of various solids in the ternary mixed NaNO3–Na2SO4 system including the formation of the double salt darapskite, Na3NO3SO4·H2O. The crystallization sequence during droplet evaporation experiments at room conditions was determined using Raman and polarization microscopy. The basic idea of this research is to use deviations of the crystallization sequence of a salt or a mixed salt solution from the equilibrium pathway as an indicator to detect the degree of supersaturation. The observed crystallization pathway includes the formation of the metastable phases Na2SO4(III), Na2SO4(V) and darapskite. The experimental observations are discussed on the basis of the NaNO3–Na2SO4–H2O phase diagram and the results provide evidence for crystal growth from highly supersaturated solutions in both systems. If the crystals growing under these conditions are confined, these supersaturations result in substantial crystallization pressures.  相似文献   
5.
The Principle of Least Interaction Action, which explains the observed preference in the Solar System for two-satellite resonant configurations, is shown to apply also to the Laplacean satellites of Jupiter and Uranus, in the sense that these triplet resonant structures lie close to configurations for which the time-mean of the action associated with the mutual interaction of the satellites is an overall minimum. Far from the minimum configuration, significant changes take place in the major semi-axes on time-scales ~106-107y. Both systems require times ~108 y to come close to the minimum configuration; to approach resonance to the observed precision of the Laplace relationship (3×10?4 for the Uranian case, 2×10?7 for the Jovian case) requires, for both systems, a time closely comparable with the age of the solar system.  相似文献   
6.
As an alternative to computationally expensive N-body simulations for gravitional clustering, the Zel'dovich approximation (ZA) was studied in 3D, 2D and 1D. Plots of the density contrast were compared against linear theory and the exact solution. The ZA was found to perform very well in the linear regime, better than linear theory, and to give a good approximation well into the non-linear regime. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
7.
8.
9.
Over the past decade, the electric vehicle (EV) industry has experienced unprecedented growth and diversification, resulting in a complex ecosystem. To effectively manage this multifaceted field, we present an EV-centric knowledge graph (EVKG) as a comprehensive, cross-domain, extensible, and open geospatial knowledge management system. The EVKG encapsulates essential EV-related knowledge, including EV adoption, EV supply equipment, and electricity transmission network, to support decision-making related to EV technology development, infrastructure planning, and policy-making by providing timely and accurate information and analysis. To enrich and contextualize the EVKG, we integrate the developed EV-relevant ontology modules from existing well-known knowledge graphs and ontologies. This integration enables interoperability with other knowledge graphs in the Linked Data Open Cloud, enhancing the EVKG's value as a knowledge hub for EV decision-making. Using six competency questions, we demonstrate how the EVKG can be used to answer various types of EV-related questions, providing critical insights into the EV ecosystem. Our EVKG provides an efficient and effective approach for managing the complex and diverse EV industry. By consolidating critical EV-related knowledge into a single, easily accessible resource, the EVKG supports decision-makers in making informed choices about EV technology development, infrastructure planning, and policy-making. As a flexible and extensible platform, the EVKG is capable of accommodating a wide range of data sources, enabling it to evolve alongside the rapidly changing EV landscape.  相似文献   
10.
Abstract

Multi‐temporal ERS‐1 SAR data acquired over a large agricultural region in West Bengal was used to classify kharif crops like rice, jute and sugarcane. Rice crop grown under lowland management practice showed a temporal characteristic. The dynamic range of backscatter was highest for this crop in temporal SAR data. This was used to classify rice using temporal SAR data. Such temporal character was not observed for the other study crops, which may be due to the difference in cultivation practice and crop calendar. Significant increase in backscatter from the ploughed fields was used to derive information on onset and duration of land preparations. Synergistic use of optical remote sensing data and SAR data increased the separability of rice crop from homesteads and permanent vegetation classes.  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