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   271707篇
  免费   8185篇
  国内免费   7099篇
测绘学   8188篇
大气科学   22051篇
地球物理   57616篇
地质学   97438篇
海洋学   23542篇
天文学   55723篇
综合类   2590篇
自然地理   19843篇
  2022年   2040篇
  2021年   3081篇
  2020年   3382篇
  2019年   3674篇
  2018年   4874篇
  2017年   4500篇
  2016年   6885篇
  2015年   5021篇
  2014年   7913篇
  2013年   15184篇
  2012年   7566篇
  2011年   9081篇
  2010年   8160篇
  2009年   10631篇
  2008年   9315篇
  2007年   8663篇
  2006年   10166篇
  2005年   8264篇
  2004年   8027篇
  2003年   7487篇
  2002年   7051篇
  2001年   6381篇
  2000年   6353篇
  1999年   5793篇
  1998年   5719篇
  1997年   5423篇
  1996年   4667篇
  1995年   4655篇
  1994年   4346篇
  1993年   4048篇
  1992年   3782篇
  1991年   3703篇
  1990年   3768篇
  1989年   3450篇
  1988年   3257篇
  1987年   3778篇
  1986年   3299篇
  1985年   4163篇
  1984年   4659篇
  1983年   4335篇
  1982年   4247篇
  1981年   3853篇
  1980年   3600篇
  1979年   3446篇
  1978年   3446篇
  1977年   3222篇
  1976年   2966篇
  1975年   2900篇
  1974年   2862篇
  1973年   3059篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Wuyep  E. O.  Oluyemi  G. F.  Yates  K.  Akisanya  A. R. 《Natural Resources Research》2020,29(2):1239-1258
Natural Resources Research - Sand failure and production occurs when the formation stress exceeds the strength of the formation, which is derived majorly from the natural material that cements the...  相似文献   
102.
Natural Resources Research - Borehole-controlled one-dimensional geo-electric data were integrated with water samples obtained from boreholes drilled near the vertical electrical sounding at 12...  相似文献   
103.
Abstract— Active capture is a new process for the incorporation of large quantities of heavy noble gases into growing surfaces. Adsorption in the conventional sense involves surface bonding by polarization (Van der Waals forces). What is referred to as “anomalous adsorption” of heavy noble gases involves chemical bonds and can occur when other (more chemically active) species are not available to preempt sites with unfilled bonds. Anomalous adsorption has been observed under conditions of fracture, vacuum deposition and ionizing radiation. Active capture depends upon anomalous adsorption to retain noble gases on a surface long enough to be captured in a growing surface film as it is deposited. The fundamental principle may be the impingement onto the growing film with sufficient energy to liberate surface electrons (work function energy of a few electronvolts) so that they are retained by anomalous adsorption long enough to be entrapped in the growing surface. Trapping efficiencies of ?1% have been observed for Kr and Xe in laboratory experiments, implying a fundamentally new mechanism for the incorporation of heavy noble gases onto surfaces. It may play a role in explaining the large concentrations of planetary noble gases contained in phase‐Q.  相似文献   
104.
The vertical deformation rates (VDRs) and horizontal deformation rates (HDRs) of Shanghai VLBI station in China and Kashima and Kashima34 VLBI stations in Japan were re-analysed using the baseline length change rates from Shanghai to 13 global VLBI stations, and from Kashima to 27 stations and from Kashima34 to 12 stations, based on the NASA VLBI global solution glb1123 (Ma, 1999). The velocity vectors of the global VLBI stations were referred to the ITRF97 reference frame, and the Eulerian vectors of different models of plate motion were used for comparative solutions. The VDR of Shanghai station is estimated to be −1.91±0.56 mm/yr, and those of Kashima and Kashima34 stations, −3.72±0.74mm/yr and −8.81±0.84mm/yr, respectively. The difference between the last two was verified by further analysis. Similar estimates were also made for the Kokee, Kauai and MK_VLBA VLBI stations in mid-Pacific.  相似文献   
105.
We present our observations of the galaxy UGS 5600 with a long-slit spectrograph (UAGS) and a multipupil field spectrograph (MPFS) attached to the 6-m Special Astrophysical Observatory telescope. Radial-velocity fields of the stellar and gaseous components were constructed for the central region and inner ring of the galaxy. We proved the existence of two nearly orthogonal kinematic subsystems and conclude that UGC 5600 is a galaxy with an inner polar ring. In the circumnuclear region, we detected noncircular stellar motions and suspected the existence of a minibar. The emission lines are shown to originate in H II regions. We estimated the metallicity from the intensity ratio of the [N II]λ6583 and Hα lines to be nearly solar, which rules out the possibility that the polar ring was produced by the accretion of gas from a dwarf companion.  相似文献   
106.
Abstract— The possibility of volcanism on Mercury has been a topic of discussion since Mariner 10 returned images of half the planet's surface showing widespread plains material. These plains could be volcanic or lobate crater ejecta. An assessment of the mechanics of the ascent and eruption of magma shows that it is possible to have widespread volcanism, no volcanism on the surface whatsoever, or some range in between. It is difficult to distinguish between a lava flow and lobate crater ejecta based on morphology and morphometry. No definite volcanic features have been identified on Mercury. However, known lunar volcanic features cannot be identified in images with similar resolutions and viewing geometries as the Mariner 10 dataset. Examination of high‐resolution, low Sun angle Mariner 10 images reveals several features which are interpreted to be flow fronts; it is unclear if these are volcanic flows or ejecta flows. This analysis implies that a clear assessment of volcanism on Mercury must wait for better data. MESSENGER (MErcury: Surface, Space ENvironment, GEochemistry, Ranging) will take images with viewing geometries and resolutions appropriate for the identification of such features.  相似文献   
107.
C.B Olkin  L.H Wasserman  O.G Franz 《Icarus》2003,164(1):254-259
The mass ratio of Charon to Pluto is a basic parameter describing the binary system and is necessary for determining the individual masses and densities of these two bodies. Previous measurements of the mass ratio have been made, but the solutions differ significantly (Null et al., 1993; Young et al., 1994; Null and Owen, 1996; Foust et al., 1997; Tholen and Buie, 1997). We present the first observations of Pluto and Charon with a well-calibrated astrometric instrument—the fine guidance sensors on the Hubble Space Telescope. We observed the motion of Pluto and Charon about the system barycenter over 4.4 days (69% of an orbital period) and determined the mass ratio to be 0.122±0.008 which implies a density of 1.8 to 2.1 g cm−3 for Pluto and 1.6 to 1.8 g cm−3 for Charon. The resulting rock-mass fractions for Pluto and Charon are higher than expected for bodies formed in the outer solar nebula, possibly indicating significant postaccretion loss of volatiles.  相似文献   
108.
109.
110.
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