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   14836篇
  免费   2566篇
  国内免费   4377篇
测绘学   1997篇
大气科学   2941篇
地球物理   3257篇
地质学   6155篇
海洋学   2851篇
天文学   1366篇
综合类   1315篇
自然地理   1897篇
  2024年   46篇
  2023年   175篇
  2022年   500篇
  2021年   551篇
  2020年   749篇
  2019年   734篇
  2018年   645篇
  2017年   761篇
  2016年   755篇
  2015年   898篇
  2014年   873篇
  2013年   1044篇
  2012年   982篇
  2011年   956篇
  2010年   786篇
  2009年   905篇
  2008年   1003篇
  2007年   1187篇
  2006年   1174篇
  2005年   1016篇
  2004年   854篇
  2003年   705篇
  2002年   654篇
  2001年   479篇
  2000年   614篇
  1999年   583篇
  1998年   458篇
  1997年   323篇
  1996年   268篇
  1995年   204篇
  1994年   179篇
  1993年   169篇
  1992年   147篇
  1991年   95篇
  1990年   55篇
  1989年   76篇
  1988年   43篇
  1987年   35篇
  1986年   35篇
  1985年   18篇
  1984年   12篇
  1983年   4篇
  1982年   7篇
  1981年   4篇
  1980年   3篇
  1979年   2篇
  1978年   2篇
  1977年   3篇
  1954年   4篇
  1877年   1篇
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
51.
Keck's Long Wavelength Spectrometer (LWS), is the facility instrument used for imaging and spectroscopy in the wavelength range of 3–28 μm at the Keck Observatory. LWS uses an 128 × 128 Si:As blocked impurity band (BIB) array manufactured by the Boeing Corporation. This paper discusses the method used for optimizing the detector's operating parameters at a temperature of 8.5 K and bias voltage of 1.2 V. A process for characterizing detective quantum efficiency of BIB detectors is also presented. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
52.
The paper deals with the methods of formation pressure evaluation for a single well by using the very common ac-cepted parameters, such as drilling exponent , and flowline temperature , etc. which is part of compiling the end well report.  相似文献   
53.
54.
1 IntroductionDiscontinuousDeformationAnalysis(DDA)developedbyShiin 1 980’s[1 ,2 ] maybeusedtocalculatethedeformationanddisplacementsinamulti blocksystemandtoanalyzetherelationbe tweenforcesanddisplacementsinthesystem .Ifthesurveydisplacementofeverypointordirectionisknown,alldisplacementsanddeformationsofblocks,eventheglide ,theclosureandthestretchoftheboundary planeofblocks ,canbecalculated .TheresultisoptimallyfittedaccordingtotheLeastSquarePrinciple.DDAismainlyusedinrockblockssystems…  相似文献   
55.
Linux 防火墙的研究与设计   总被引:4,自引:0,他引:4  
阐述了Linux2.4内核的防火墙Linux的原理,并分析了netfilter的设计与实现,叙述了Linux的动态地址转换,介绍了在Linux下基于netfilter架构如何开发自己的防火墙并给出具体实例。  相似文献   
56.
We propose a zero-point photometric calibration of the data from the Advanced Camera for Surveys (ACS) Wide Field Channel (WFC) on board the Hubble Space Telescope , based on a spectrum of Vega and the most up-to-date in-flight transmission curves of the camera. This calibration is accurate at the level of a few hundredths of a magnitude. The main purpose of this effort is to transform the entire set of evolutionary models into a simple observational photometric system for ACS/WFC data, and to make them available to the astronomical community. We provide the zero-points for the most used ACS/WFC bands, and give basic recipes for calibrating both the observed data and the models. We also present the colour–magnitude diagram from ACS data of five Galactic globular clusters, spanning the metallicity range  −2.2 <[Fe/H] < −0.04  , and we provide fiducial points representing their sequences from several magnitudes below the turn-off to the red giant branch tip. The observed sequences are compared with the models in the newly defined photometric system.  相似文献   
57.
Vertical measurements of NH4+, NO3? and N2O concentrations, NO3? and NH4+ uptake, and NH4+ oxidation rates were measured at 5 sites in western Cook Strait, New Zealand, between 31 March and 3 April 1983. Nitrate increased with depth at all stations reaching a maximum of 10.5 μg-atom NO3?N l?1 at the most strongly stratified station whereas NH4+ was relatively constant with depth at all stations (~0.1 μg-atom NH4+N l?1). The highest rates of NH4+ oxidation generally occurred in the near surface waters and decreased with depth. N2O levels were near saturation with respect to the air above the sea surface and showed no obvious changes during 24 h incubation. NH4+ oxidation by nitrifying bacteria may account for about 30% of the total NH4+ utilization (i.e. bacterial+agal) and, assuming oxidation through to NO3?, may supply about 40% of the algal requirements of NO3? in the study area. These results suggest that bacterial nitrification is of potential importance to the nitrogen dynamics of the western Cook Strait, particularly with respect to the nitrogen demands of the phytoplankton.  相似文献   
58.
Based on the Germany Koldwey Station's 1994-2003 conventional observation hourly data, this paper conducts a statistical analysis on the short-term climate characteristics for an arctic tundra region (Ny-(A)lesund island) where our first arctic expedition station (Huanghe Station) was located. Affected by the North Atlantic warming current, this area has a humid temperate climate, and the air temperature at Ny-(A)lesund rose above 0 ℃ even during deep winter season during our research period. The wind speed in this area was low and appeared most at southeast direction. We find that the temperature at Ny-(A)lesund rose in the faster rate (0.68 ℃/10 a) than those at the whole Arctic area. Compared with the floating ices where our expedition conducted in the Arctic, Ny-(A)lesund was warmer and more humid and had lower wind speed. Comparison of the near surface air temperature derived by NCEP/NCAR reanalysis to the conventional measurements conducted at the Koldwey site in Ny-(A)lesund area shows a good agreement for winter season and a significant difference for summer season.  相似文献   
59.
Galileo系统及其在中国的应用   总被引:9,自引:0,他引:9  
较详细地介绍了Galileo系统的全面体系结构、频率设计、服务内容及其广泛的应用范围,指出了它与GPS系统相比的优点,并对开发和增强Galileo系统在我国多层次、全方位的应用提出了建议。  相似文献   
60.
INTRODUCTIONItwasgenerallyconsideredpreviouslythatthecorrosionofsteelinseamudwasnotserious.However,itwaslaterfoundthatthecorrosionrateofsteelinsomeseamudwashigherthanthatinseawater(King,1980;Sehmide,1982;Guoetal.,1997;Hou,1998).Therearesomereportsthatthecorrosionr…  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