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   21082篇
  免费   2992篇
  国内免费   4468篇
测绘学   3579篇
大气科学   2426篇
地球物理   2927篇
地质学   10763篇
海洋学   2032篇
天文学   1942篇
综合类   1670篇
自然地理   3203篇
  2024年   81篇
  2023年   237篇
  2022年   562篇
  2021年   643篇
  2020年   689篇
  2019年   863篇
  2018年   627篇
  2017年   805篇
  2016年   816篇
  2015年   936篇
  2014年   1145篇
  2013年   1138篇
  2012年   1278篇
  2011年   1344篇
  2010年   1129篇
  2009年   1248篇
  2008年   1294篇
  2007年   1541篇
  2006年   1450篇
  2005年   1336篇
  2004年   1261篇
  2003年   1090篇
  2002年   1062篇
  2001年   848篇
  2000年   828篇
  1999年   745篇
  1998年   687篇
  1997年   524篇
  1996年   459篇
  1995年   405篇
  1994年   358篇
  1993年   282篇
  1992年   212篇
  1991年   147篇
  1990年   127篇
  1989年   111篇
  1988年   73篇
  1987年   67篇
  1986年   33篇
  1985年   21篇
  1984年   10篇
  1982年   2篇
  1981年   4篇
  1980年   2篇
  1979年   5篇
  1977年   4篇
  1976年   3篇
  1975年   1篇
  1972年   1篇
  1954年   6篇
排序方式: 共有10000条查询结果,搜索用时 171 毫秒
21.
Recent Doppler velocity measurements have revealed the existence of two planets orbiting the star HD 12661 on medium-eccentricity orbits. The inner planet has a period of 263.6 d and a mass of 2.3 Jupiter masses, and the outer planet has a period of 1444.5d and a mass of 1.57 Jupiter masses. The stability of this system requires the two planets to be in a state of mean-motion orbit resonances. By numerical method we have studied the orbit migration and stability of the system in its early ages under the action of the proto-stellar disk, and calculated the probabilities of the planets being captured into the mean -motion resonances during their migrations. It is found that at present the two planets are probably situated at the edge of the 11:2 mean-motion resonance and are in chaotic motions. This result may be helpful to clarify the arguments on the present configuration. Besides, it is indicated that very probably, after the formation of the system, the gaseous disk has almost disappeared before the planets migrated to the present configuration.  相似文献   
22.
23.
24.
In this work, the factors controlling the formation and preservation of high-pressure mineral assemblages in the metamorphosed orthopyroxene-bearing metagranitoids of the Sandmata Complex, Aravalli-Delhi Mobile Belt (ADMB), northwestern India have been modelled. The rocks range in composition from farsundite through quartz mangerite to opdalite, and with varying K2O, Ca/(Ca + Na)rock and FeOtot + MgO contents. A two stage metamorphic evolution has been recorded in these rocks.
An early hydration event stabilized biotite with or without epidote at the expense of magmatic orthopyroxene and plagioclase. Subsequent high-pressure granulite facies metamorphism (∼15 kbar, ∼800 °C) of these hydrated rocks produced two rock types with contrasting mineralogy and textures. In the non-migmatitic metagranitoids, spectacular garnet ± K-feldspar ± quartz corona was formed around reacting biotite, plagioclase, quartz and/or pyroxene. In contrast, biotite ± epidote melting produced migmatites, containing porphyroblastic garnet incongruent solids and leucosomes.
Applying NCKFMASHTO T–M (H2O) and P–T pseudosection modelling techniques, it is demonstrated that the differential response of these magmatic rocks to high-pressure metamorphism is primarily controlled by the scale of initial hydration. Rocks, which were pervasively hydrated, produced garnetiferous migmatites, while for limited hydration, the same metamorphism formed sub-solidus garnet-bearing coronae. Based on the sequence of mineral assemblage evolution and the mineral compositional zoning features in the two metagranitoids, a clockwise metamorphic P–T path is constrained for the high-pressure metamorphic event. The finding has major implications in formulating geodynamic model of crustal amalgamation in the ADMB.  相似文献   
25.
针对信息系统安全的考虑,介绍在PowerBuilder环境中对用户权限的具体控制.  相似文献   
26.
We present a quantitative estimate of the relativistic corrections to the thermal SZ power spectrum produced by the energetic electrons in massive clusters. The corrections are well within 10% for current experiments with working frequencies below v < 100 GHz, but become non-negligible at high frequencies v > 350 GHz. Moreover, the corrections appear to be slightly smaller at higher l or smaller angular scales. We conclude that there is no need to include the relativistic corrections in the theoretical study of the SZ power spectrum especially at low frequencies unless the SZ power spectrum is used for precision cosmology.  相似文献   
27.
28.
利用Logisitic曲线模型和扩展支撑模型对居民可支配收入预测和单项消费预测,以河北省居民的收入及其消费支出为实例,研究了城镇居民和农民的可支配收入分配和单项边际消费倾向,合理建立了预测模型,并理论计算了1999~2003年的通信消费方面的边际消费倾向、预测了2004~2008年通信消费情况。最后,根据预测结果对通信网络建设提出了一些有价值的建议。  相似文献   
29.
30.
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