全文获取类型
收费全文 | 52384篇 |
免费 | 17869篇 |
国内免费 | 42155篇 |
专业分类
测绘学 | 2781篇 |
大气科学 | 21975篇 |
地球物理 | 15944篇 |
地质学 | 41167篇 |
海洋学 | 17781篇 |
天文学 | 6158篇 |
综合类 | 3986篇 |
自然地理 | 2616篇 |
出版年
2023年 | 242篇 |
2022年 | 545篇 |
2021年 | 958篇 |
2020年 | 2172篇 |
2019年 | 5680篇 |
2018年 | 6398篇 |
2017年 | 6040篇 |
2016年 | 6100篇 |
2015年 | 4959篇 |
2014年 | 4801篇 |
2013年 | 5898篇 |
2012年 | 5225篇 |
2011年 | 5081篇 |
2010年 | 5093篇 |
2009年 | 4621篇 |
2008年 | 3728篇 |
2007年 | 3819篇 |
2006年 | 3339篇 |
2005年 | 2851篇 |
2004年 | 3190篇 |
2003年 | 2943篇 |
2002年 | 2678篇 |
2001年 | 2396篇 |
2000年 | 1939篇 |
1999年 | 1871篇 |
1998年 | 1985篇 |
1997年 | 2047篇 |
1996年 | 1596篇 |
1995年 | 1546篇 |
1994年 | 1397篇 |
1993年 | 1307篇 |
1992年 | 1162篇 |
1991年 | 899篇 |
1990年 | 884篇 |
1989年 | 756篇 |
1988年 | 642篇 |
1987年 | 570篇 |
1986年 | 481篇 |
1985年 | 444篇 |
1984年 | 471篇 |
1983年 | 363篇 |
1982年 | 394篇 |
1981年 | 318篇 |
1980年 | 269篇 |
1979年 | 289篇 |
1978年 | 204篇 |
1977年 | 183篇 |
1976年 | 164篇 |
1973年 | 188篇 |
1971年 | 153篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
A synthesis of Holocene pollen records from the Tibetan Plateau shows the history of vegetation and climatic changes during the Holocene. Palynological evidences from 24 cores/sections have been compiled and show that the vegetation shifted from subalpine/alpine conifer forest to subalpine/alpine evergreen sclerophyllous forest in the southeastern part of the plateau; from alpine steppe to alpine desert in the central, western and northern part; and from alpine meadow to alpine steppe in the eastern and southern plateau regions during the Holocene. These records show that increases in precipitation began about 9 ka from the southeast, and a wide ranging level of increased humidity developed over the entire of the plateau around 8-7 ka, followed by aridity from 6 ka and a continuous drying over the plateau after 4-3 ka. The changes in Holocene climates of the plateau can be interpreted qualitatively as a response to orbital forcing and its secondary effects on the Indian Monsoon which expanded northwards 相似文献
32.
33.
34.
同时采用4个台站的国际超导重力仪长期连续重力观测资料和国际地球自转服务中心提供的同步地球自转参数,研究了极移引起的地球重力场变化特征。利用自回归模型估计了各序列的功率谱密度和积谱密度,结果表明极移导致的重力效应的主要能量集中在Chandler摆动和周年项附近,叠积后实际重力观测与极移重力信号理论值之间的差异分别为0.4%和3.9%,说明超导重力仪可有效监测极移导致的重力变化。 相似文献
35.
地震前后垂直形变场动态演化的量化指标 总被引:3,自引:2,他引:1
提出一种表达垂直形变场动态演化过程的量化指标——区域应变率、应变集中度。在此基础上,对南北地震带各水准监测区近30年的垂直形变资料进行了实际计算.并结合具体震例进行了对比研究。结果表明:量化指标在一定程度上能够反映地震前后形变场的动态演化过程和地壳运动状态,对地震的中长期预报有一定的积极意义。 相似文献
36.
J. Zhang D. B. Williams J. I. Goldstein R. S. Clarke 《Meteoritics & planetary science》1990,25(3):167-175
Abstract— Characterization of the microstructural features of the metal of the Santa Catharina meteorite was performed using a variety of electron optical techniques. Sample USNM#6293 is chemically homogeneous on the micron scale and has a Ni content of 28.2 wt.%. Its microstructure is similar to that of the Twin City ataxite and contains clear taenite II, i.e., fcc taenite with domains of tetrataenite, < 10 nm in size. Sample USNM#3043 is a more typical Santa Catharina specimen with dark and light regions as observed with the light optical microscope. The dark regions are inhomogeneous and contain 45–50 wt.% Ni and 7–12 wt.% O. The light regions are homogeneous and contain 35 wt.% Ni and no detectable oxygen. The microstructure is that of cloudy zone, i.e., islands of tetrataenite, ~20 nm in size, in a honeycomb matrix. The honeycomb phase contains Ni rich oxide in the dark regions and contains metal, fcc taenite, in the light regions. The original metal structure of USNM#3043 is cloudy zone which formed during cooling into the low temperature miscibility gap of the Fe-Ni phase diagram. The dark regions were developed from the metal by selective corrosion of the honeycomb structure, transforming it into Ni containing oxides, possibly non-stoichiometric Fe2NiO4 while retaining the tetrataenite islands. Using the results of this study, many of the existing discrepancies concerning the microstructure of Santa Catharina can be explained. 相似文献
37.
In the frame of 2D-static problems one approaches the problem of elastic-NRT (not-resisting tension) semi-plane loaded on its limit line. This problem is intended to model the stress situation induced in the soil by a foundation structure. The solution, in terms of activated stress field, is searched for in the class of stress fields satisfying equilibrium and admissibility conditions, by applying an energy approach. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
38.
阿吾拉勒山体西段发育了厚度巨大的下二叠统火山岩,这些火山岩均属碱性玄武岩系列,以钾质类型为主,钠质类型次之,岩浆演化呈现出跨越B型趋势。这种火山活动是在大陆地壳内部产生的,代表了陆内裂谷演化的早期阶段。在早二叠世末期,由于受到新源运动的影响,这种火山活动被终止了。 相似文献
39.
40.
Gavin L. England Birger Rasmussen Neal J. McNaughton Ian R. Fletcher David I. Groves & Bryan Krapez 《地学学报》2001,13(5):360-367
SHRIMP dating of xenotime overgrowths on detrital zircon grains can constrain maximum durations since diagenesis and therefore provide minimum dates of sediment deposition. Thus, xenotime dating has significant economic application to Precambrian sediment-hosted ore deposits, such as Witwatersrand Au–U, for which there are no precise depositional ages. The growth history of xenotime in the Witwatersrand Supergroup is texturally complex, with several phases evident. The oldest authigenic xenotime 207 Pb/206 Pb age obtained in sandstone underlying the Vaal Reef is 2764 ± 5 Myr (1 σ), and most likely represents a mixture of diagenetic and hydrothermal growth. Nevertheless, this represents the oldest authigenic mineral age yet recorded in the sequence and provides a minimum age of deposition. Other xenotime data record a spread of ages that correspond to numerous post-diagenetic thermotectonic events (including a Ventersdorp event at ≈ 2720 Ma) up to the ≈2020 Ma Vredefort event. 相似文献