全文获取类型
收费全文 | 6956篇 |
免费 | 1566篇 |
国内免费 | 1612篇 |
专业分类
测绘学 | 414篇 |
大气科学 | 1515篇 |
地球物理 | 2051篇 |
地质学 | 3299篇 |
海洋学 | 812篇 |
天文学 | 544篇 |
综合类 | 702篇 |
自然地理 | 797篇 |
出版年
2024年 | 23篇 |
2023年 | 127篇 |
2022年 | 341篇 |
2021年 | 384篇 |
2020年 | 307篇 |
2019年 | 338篇 |
2018年 | 599篇 |
2017年 | 382篇 |
2016年 | 377篇 |
2015年 | 315篇 |
2014年 | 371篇 |
2013年 | 369篇 |
2012年 | 332篇 |
2011年 | 409篇 |
2010年 | 391篇 |
2009年 | 358篇 |
2008年 | 326篇 |
2007年 | 324篇 |
2006年 | 246篇 |
2005年 | 207篇 |
2004年 | 270篇 |
2003年 | 203篇 |
2002年 | 220篇 |
2001年 | 200篇 |
2000年 | 220篇 |
1999年 | 335篇 |
1998年 | 316篇 |
1997年 | 268篇 |
1996年 | 274篇 |
1995年 | 203篇 |
1994年 | 184篇 |
1993年 | 147篇 |
1992年 | 134篇 |
1991年 | 112篇 |
1990年 | 103篇 |
1989年 | 80篇 |
1988年 | 69篇 |
1987年 | 56篇 |
1986年 | 46篇 |
1985年 | 27篇 |
1984年 | 19篇 |
1983年 | 10篇 |
1982年 | 15篇 |
1981年 | 14篇 |
1980年 | 11篇 |
1977年 | 5篇 |
1976年 | 7篇 |
1937年 | 6篇 |
1933年 | 9篇 |
1924年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
本文提出OH超脉泽源的一个具体模型,在该模型中OH超脉泽源由非饱和的球型子源组成.在此估算出超脉泽星系致密射电核的尺度和连续谱亮温度,以及OH源距星系核的最小非饱和距离.计算表明,OH超脉泽源所包含的子源数目可能相当大.R_H~log ρ_(25μm)/ρ_(60μm)图、log L_(OH)/L_(IR)~log ρ_(25μm)/ρ_(60μm)图和其它结果显示,有一部分OH超脉泽源很可能是弱饱和的,本文对此作了解释. 相似文献
993.
本文通过在厦门地区大直径桩孔钻遇“孤石”的施工实践,总结了一套利用钢粒取心钻进“孤石”的技术方法,有效地解决了钻进“孤石”效率低,易偏斜,事故多的技术难题。 相似文献
994.
Liu Xue-fu
K. C. Leung
Tan Hui-song
《Chinese Astronomy and Astrophysics》1988,12(4):298-303New UBV photometric observations of IU Aur were made at McDonald Observatory in December 1984 and January 1985, showing that the eclipse depth had kept on increasing and the light minimum time had shown periodic changes. Orbital elements were obtained using the Wilson-Devinney method. The inclination is now close to 90°. The continuous variation of inclination over the last few decades suggests the presence of a third body. Absolute parameters of IU Aur were calculated with the help of Mammano's spectral data. 相似文献
995.
996.
Recent glacial retreat in High Asia in China and its impact on water resource in Northwest China 总被引:48,自引:0,他引:48
YAO Tandong WANG Youqing LIU Shiying PU Jianchen SHEN Yongping & LU Anxin .Institute of Tibetan Plateau Research Chinese Academy of Sciences Beijing China .Cold Arid Regions Environmental Engineering Research Institute Chinese Academy of Sciences Lanzhou China 《中国科学D辑(英文版)》2004,47(12):1065-1075
There are about 46298 glaciers in the High Asiain China, the total glacial area is about 59406 km , andtotal glacial volume about 5590 km3. These glaciersmainly concentrate around the Himalayas Mountains,Nyainqentanglha Mountains, Kunlun Mountains,Karakoram Mountains and Tianshan Mountains. Theglaciers in the Tibetan Plateau are the major compo-nent of the glaciers in the High Asia in China. Theseglaciers extend north to the arid and desert regions,and become the main water resource in … 相似文献
997.
Jianping?Zheng W.?L.?GriffinEmail author Suzanne?Y.?O’Reilly Fengxiang?Lu Chunmei?Yu Ming?Zhang Huimin?Li 《Contributions to Mineralogy and Petrology》2004,148(1):79-103
Mafic xenoliths from the Paleozoic Fuxian kimberlites in the North China craton include garnet granulite, and minor pyroxene amphibolite, metagabbro, anorthosite and pyroxenite. The formation conditions of the amphibolites are estimated at 745–820 °C and 7.6–8.8 Kb (25–30 km); the granulites probably are derived from greater depths in the lower crust. LAM-ICPMS U–Pb dating of zircons from four granulites reveals multiple age populations, recording episodes of magmatic intrusion and metamorphic recrystallisation. Concordant ages and upper intercept ages, interpreted as minimum estimates for the time of magmatic crystallisation, range from 2,620 to 2,430 Ma in three granulites, two amphibolites and two metagabbros. Lower intercept ages, represented by near-concordant zircons, are interpreted as reflecting metamorphic recrystallisation, and range from 1,927 to 1,852 Ma. One granulite contains two metamorphic zircon populations, dated at 1,927±55 Ma and 600–700 Ma. Separated minerals from one granulite and one amphibolite yield Sm–Nd isochron ages of 1,619±48 Ma (143Nd/144Nd)i=0.51078), and 1,716±120 Ma (143Nd/144Nd)i=0.51006), respectively. These ages are interpreted as recording cooling following metamorphic resetting; model ages for both samples are in the range 2.40–2.66 Ga. LAM-MC-ICPMS analyses of zircon show a range in 176Hf/177Hf from 0.28116 to 0.28214, corresponding to a range of Hf from –34 to +12. The relationships between 207Pb/206Pb age and Hf show that: (1) the granulites, amphibolites and metagabbro were derived from a depleted mantle source at 2.6–2.75 Ga; (2) zircons in most samples underwent recrystallisation and Pb loss for 100–200 Ma after magmatic crystallisation, consistent with a residence in the lower crust; (3) metamorphic zircons in several samples represent new zircon growth, incorporating Hf liberated from breakdown of silicates with high Lu/Hf; (4) in other samples metamorphic and magmatic zircons have identical 176Hf/177Hf, and the younger ages reflect complete resetting of U–Pb systems in older zircons. The Fuxian mafic xenoliths are interpreted as the products of basaltic underplating, derived from a depleted mantle source in Neoarchean time, an important period of continental growth in the North China craton. Paleoproterozoic metamorphic ages indicate an important tectonic thermal event in the lower crust at 1.8–1.9 Ga, corresponding to the timing of collision between the Eastern and Western Blocks that led to the final assembly of the North China craton. The growth of metamorphic zircon at 600–700 Ma may record an asthenospheric upwelling in Neoproterozoic time, related to uplift and a regional disconformity in the North China craton. 相似文献
998.
Carvalho S Gaspar MB Moura A Vale C Antunes P Gil O da Fonseca LC Falcão M 《Marine pollution bulletin》2006,52(11):1414-1424
This study reports the longitudinal distribution of sediment properties, including inorganic and organic contaminants, and the structure of the benthic community in Obidos lagoon, a coastal system permanently connected to the sea and with negligible freshwater sources. Sediments from the upper to central lagoon consist of fine particles (91%) and from the lower lagoon of sands (94%). Chemical composition is strongly correlated to the percentage of fine particles. Contamination is relatively low in those sediments suggesting the effect of diffuse sources. The increase in organic matter content from down- to upstream areas was associated with the dominance of opportunistic species, while sensitive and indifferent species to organic enrichment were mainly associated to the clean sandy downstream area. The marine biotic index (AMBI) was suitable for the discrimination of the biological and environmental gradients in the Obidos lagoon and was highly related with the gradient of organic matter content in this system. 相似文献
999.
1000.
关于下一代地震区划图编制原则与关键技术的初步探讨 总被引:22,自引:10,他引:12
本文在回顾我国四代地震区划编图原则的基础上,结合对未来我国社会经济发展状况和国家地震安全政策变化的分析,同时对国际上地震区划研究进展与编图工作的现状了解,探讨了下一代地震区划编图的基本原则,提出了与之相关的基本科学技术问题,对第五代地震区划图的编制提出了初步的建议。 相似文献