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   51196篇
  免费   678篇
  国内免费   399篇
测绘学   1478篇
大气科学   3646篇
地球物理   10341篇
地质学   17463篇
海洋学   4556篇
天文学   12079篇
综合类   161篇
自然地理   2549篇
  2022年   310篇
  2021年   491篇
  2020年   519篇
  2019年   631篇
  2018年   1387篇
  2017年   1232篇
  2016年   1525篇
  2015年   814篇
  2014年   1442篇
  2013年   2491篇
  2012年   1591篇
  2011年   2055篇
  2010年   1888篇
  2009年   2526篇
  2008年   2238篇
  2007年   2233篇
  2006年   2041篇
  2005年   1531篇
  2004年   1547篇
  2003年   1408篇
  2002年   1434篇
  2001年   1272篇
  2000年   1212篇
  1999年   1032篇
  1998年   1034篇
  1997年   1012篇
  1996年   808篇
  1995年   771篇
  1994年   716篇
  1993年   623篇
  1992年   536篇
  1991年   567篇
  1990年   551篇
  1989年   550篇
  1988年   506篇
  1987年   586篇
  1986年   523篇
  1985年   632篇
  1984年   736篇
  1983年   663篇
  1982年   633篇
  1981年   578篇
  1980年   519篇
  1979年   503篇
  1978年   514篇
  1977年   451篇
  1976年   392篇
  1975年   404篇
  1974年   387篇
  1973年   417篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
The activity concentration and the gamma-absorbed dose rates of the terrestrial naturally occurring radionuclides (232Th, 226Ra and 40K) were determined in soil samples collected from ten different locations of Sirsa district of Haryana, using HPGe detector based on high-resolution gamma spectrometry system. The range of activity concentrations of 226Ra, 232Th and 40K in the soil samples from the studied areas varies from 19.18 Bq kg−1 (Moriwala) to 40.31 Bq kg−1 (Rori), 59.43 Bq kg−1 (Pipli) to 89.54 Bq kg−1 (Fatehpur) and 223.22 Bq kg−1 (Moriwala) to 313.32 Bq kg−1 (SamatKhera) with overall mean values of 27.94, 72.75 and 286.73 Bq kg−1 respectively. The absorbed dose rate calculated from activity concentration of 226Ra, 232Th and 40K ranges between 8.84 and 18.58, 37.02 and 55.78, and 9.24 and 12.97 nGy h−1, respectively. The total absorbed dose in the study area ranges from 60.40 to 82.15 nGy h−1 with an average value of 70.12 nGy h−1. The calculated values of external hazard index (H ex) for the soil samples of the study area range from 0.36 to 0.49 with an average value of 0.42.  相似文献   
962.
963.
Previous work in the Gulf of Lions (western Mediterranean Sea) has suggested that significant amounts of sediment escape through the western part of this tectonically passive margin, despite it being far removed from the primary sediment source (the Rhone River, ∼160 km to the NE). The primary mechanism behind this export is hypothesized to be the interaction of a regional, southwestward sediment-transport path with a canyon deeply incising the southwestern part of the shelf, Cap de Creus Canyon.  相似文献   
964.
A palaeomagnetic study has been carried out in the Tethyan Himalaya (TH; the northern margin of Greater India). Twenty-six palaeomagnetic sites have been sampled in Triassic low-grade metasediments of western Dolpo. Two remanent components have been identified. A pyrrhotite component, characterized by unblocking temperatures of 270–335 °C, yields an in situ mean direction of D=191.7°, I=−30.9° (k=29.5, α95=5.7°, N=23 sites). The component fails the fold test at the 99% confidence level (kin situ/kbed=6.9) and is therefore of postfolding origin. For reason of the low metamorphic grade, this pyrrhotite magnetization is believed to be of thermo-chemical origin. Geochronological data and inclination matching indicate an acquisition age around 35 Ma. The second remanence component has higher unblocking temperatures (>400 °C and up to 500–580 °C range) and resides in magnetite. A positive fold test and comparison with expected Triassic palaeomagnetic directions suggest a primary origin.The postfolding character of the pyrrhotite component, and its interpreted age of remanence acquisition, implies that the main Himalayan folding is older than 35 Ma in the western Dolpo area. This study also suggests that the second metamorphic event (Neo-Himalayan) was more significant in the Dolpo area than the first (Eo-Himalayan) one.A clockwise rotation of 10–15° is inferred from the pyrrhotite component, which is compatible with oroclinal bending and/or rotational underthrusting models. This rotation is also supported by the magnetite component, indicating that no rotation of the Tethyan Himalaya relative to India took place before 35 Ma.  相似文献   
965.
The atmospheric disturbances caused by the first rocket launch from the Vostochnyi Cosmodrome on April 28, 2016, were registered 10–24 min after the launch using the signals of the GPS/GLONASS global navigation satellite systems. The analysis of the spatial distribution of the disturbances allowed the conclusion that the launch vehicle moved northwest from the cosmodrome, which corresponds to a trajectory of the satellite movement to the orbit with an inclination of 98º.  相似文献   
966.
Astronomy Reports - An algorithm has been developed for finding the global minimum of a multidimensional error function by fitting model spectral maps into observed ones. Principal component...  相似文献   
967.
Doklady Earth Sciences - This paper discusses sources of significant errors that occur in the course of determination of the primary environmental standard for natural waters: the maximum...  相似文献   
968.
969.
Properly designed precast concrete cladding could potentially provide lateral stiffness, ductility, and energy dissipation for an overall building structure, especially during earthquakes. This paper describes a set of advanced connections that take advantage of the interaction between facade panels and structure (mainly due to horizontal interstorey drift) to dissipate energy, thereby reducing the response of the main structure. The results of an experimental program to characterize the hysteretic behaviour of advanced connections are presented. Design equations for the advanced connections are then calibrated against the test results, and the corresponding design charts are presented. It is anticipated that this research will lead to innovative ways of viewing the entire cladding system of a building.  相似文献   
970.
Gromov  A. O.  Nikiforov  I. I. 《Astronomy Letters》2021,47(6):357-376
Astronomy Letters - Techniques for solving the problem of constructing a Stäckel model by generalizing the potential from the equatorial plane to the entire space are considered. The initial...  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