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   21067篇
  免费   3844篇
  国内免费   4890篇
测绘学   1050篇
大气科学   4186篇
地球物理   5409篇
地质学   10760篇
海洋学   2446篇
天文学   953篇
综合类   2422篇
自然地理   2575篇
  2024年   99篇
  2023年   400篇
  2022年   916篇
  2021年   1088篇
  2020年   907篇
  2019年   992篇
  2018年   1198篇
  2017年   1074篇
  2016年   1276篇
  2015年   943篇
  2014年   1275篇
  2013年   1226篇
  2012年   1125篇
  2011年   1205篇
  2010年   1139篇
  2009年   1128篇
  2008年   1003篇
  2007年   980篇
  2006年   766篇
  2005年   822篇
  2004年   582篇
  2003年   619篇
  2002年   616篇
  2001年   571篇
  2000年   670篇
  1999年   1003篇
  1998年   810篇
  1997年   884篇
  1996年   817篇
  1995年   686篇
  1994年   564篇
  1993年   514篇
  1992年   395篇
  1991年   291篇
  1990年   232篇
  1989年   182篇
  1988年   174篇
  1987年   126篇
  1986年   107篇
  1985年   76篇
  1984年   53篇
  1983年   46篇
  1982年   38篇
  1981年   28篇
  1980年   41篇
  1979年   28篇
  1978年   15篇
  1976年   11篇
  1975年   14篇
  1958年   18篇
排序方式: 共有10000条查询结果,搜索用时 21 毫秒
71.
To decipher the origin of oxygen-deficient shelfal deposits is significant for tracing the distribution of marine source rocks and interpreting the evolution of depositional environment. The origin of the Middle Permian Chihsia Formation in South China remains a puzzle for long with its evident oxygen-deficient features but diverse benthos. This paper shows a typical Chihsian depositional rhythm composed of the massive and the laminated limestones with ecological and geochemical features. Massive bioclastic limestone from the rhythm was aerobic in paleoxygenation condition indicated by both the ecological and geochemical features. However, a contradictory oxygenation was inferred for the “laminated” counterpart from the rhythm, with the ecological signal being aerobic and the geochemical one being anoxic. The difference in ecological and geochemical indications was interpreted as the instability of paleoxygenation condition in shelf environments, caused by an enhanced paleoproductivity. Rhythmic occurrence of the oxygen-deficient condition might have been stemmed from paleo-Tethyan paleocurrents flowing across South China. __________ Translated from Earth Science—Journal of China University of Geosciences, 2007, 32(6): 789–796 [译自: 地球科学—中国地质大学学报]  相似文献   
72.
基于主体功能区约束的大气污染物总量控制目标分配研究   总被引:1,自引:0,他引:1  
考虑经济发展水平、污染物排放现状、污染物治理水平、空气质量,特别是国家主体功能区环境目标约束等因素,构建大气污染物排放总量分配的指标体系,用改进的等比例分配方法对2015年国家SO2,NOx总量控制目标进行区域分配。分配结果表明:SO2和NOx削减量大的省份主要集中在华北平原及其周围地区,这些地区污染物排放量大、空气质量较差;削减比例较大的地区主要集中在西部地区以及北京、天津2个直辖市,这些地区单位GDP能耗高、工业污染物去除率低、空气质量差;削减量相对较小的地区主要集中在西南和南部一些省份;削减比例较小的地区主要集中在中南部和南部几个省份,这些地区污染物排放量相对较少,空气质量好于其他省份。  相似文献   
73.
74.
Huang  Yubing  Zhang  Tiantao  Lu  Wei  Wei  Huayong  Liu  Yue  Xiao  Yuchi  Zeng  Zhaonan 《Geotechnical and Geological Engineering》2021,39(7):5213-5227
Geotechnical and Geological Engineering - In order to solve the difficulties of creep deformation of surrounding rock in super large section tunnel, taking Letuan tunnel of Binlai expressway, a...  相似文献   
75.
Using a semi-analytical method, we have obtained a first-order solution of the equations of motion of near-Earth satellites under the combined perturbing effect due to Earth's figure and air drag.  相似文献   
76.
We have obtained a solution of the Dirac equation near the horizon of a Kerr-Newman black hole and compared it with the solution of the Klein-Gordon equation. We first generalized the work of LIU Liao and XU Dian-yan for a quasi-extreme Kerr black hole to a quasi-extreme Kerr-Newman black hole, then further generalized to a general Kerr-Newman black hole, and thereby verified that the Dirac particles emitted by a general Kerr-Newman black hole do have a black-body energy spectrum.  相似文献   
77.
Field observations of suspended particulate matter (SPM) in the Bohai Bay, China have not been widely reported. The aim of this paper is to describe the horizontal and vertical distribution of mass and volume concentrations of SPM, respectively, based on observed data at 312 stations in the northern Bohai Bay during summer of 2006. A numerical model ECOMSED coupled with a sediment transport module was also established to further discuss the mechanism of the thermocline effect on the vertical distribution of SPM. The mass concentrations of SPM exhibited high inshore values and low offshore values in the horizontal distribution; while in the vertical direction, characteristics of the volume concentration of SPM can be divided into two types: one with a sharp peak at depth of 10–15 m and another without. The peak value at the depth of the thermocline was resulted from concentrated phytoplankton. A numerical experiment further displayed that the thermocline can also prevent particles from being resuspended upward.  相似文献   
78.
For the future development of Chinese Giant Solar Telescope (CGST) in Western China, a new sky brightness monitor (SBM) has been produced for the site survey for CGST. To critically examine the performance and sensitivity of SBM, we used it in the observation of the annular solar eclipse in Dali City, Yunnan, on 15 January 2010. The observation met good weather conditions with an almost clear sky during the eclipse. The SBM measurement translates into the solar illuminance changes at a level of 2.4×10?4 I?s?1 during the eclipse. The time of the minimal sky brightness in the field of view (FOV) is found consistent with the time of maximum eclipse. Two local sky regions in the FOV are chosen to make a time series of the calibrated skylight profiles. The evolution of the sky brightness thus calibrated also shows good consistency with the eclipse, particularly between the second and the third contacts. The minimal sky brightness in each local sky region took place within half a minute from the corresponding predicted contact time. Such small time delays were mainly caused by occasional cirri. The minimal sky brightness measured during the eclipse is a few millionths of I ?? with standard deviation of 0.11 millionths of I ??. The observation supports that the single-scattering process (optically thin conditions) is the main contributor to the atmospheric scattering. We have demonstrated that many important aerosol optical parameters can be deduced from our data. We conclude that the new SBM is a sensitive sky photometer that can be used for our CGST and coronagraph site surveys.  相似文献   
79.
This work analyzes the annual fluctuation of the observation data of the Mingantu Solar radio Telescope(MST) in S, C and X bands. It is found that the data vary with local air temperature as the logarithmic attenuation of equipment increases with temperature and frequency. A simplified and effective calibration method is proposed, which is used to calibrate the MST data in 2018–2020, while the correction coefficients are calculated from data in 2018–2019. For S, C and X bands, the root mean square errors of one polarization are 2.7, 5.7 and 20 sfu, and the relative errors are 4%, 6% and 8% respectively. The calibration of MUSER and SBRS spectra is also performed. The relative errors of MUSER at 1700 MHz,SBRS at 2800 MHz, 3050 MHz and 3350 MHz are 8%, 8%, 11% and 10% respectively. We found that several factors may affect the calibration accuracy, especially at X-band. The method is expected to work for other radio telescopes with similar design.  相似文献   
80.
据统计,1992~2013年苡茨河曾多次发生泥石流地质灾害,并造成村民生命财产巨大损失。2013年7月24日再次爆发泥石流,破坏农田71280m2(108亩)。因此在对苡茨河泥石流形成条件、活动特征分析基础上,分析其泥石流的形成机制及其造成巨大危害的原因,预测其发展趋势,在已有治理措施的基础上,提出两种防治对策进行对比,得出一个较为合理的防治方案。  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