全文获取类型
收费全文 | 2482篇 |
免费 | 420篇 |
国内免费 | 550篇 |
专业分类
测绘学 | 283篇 |
大气科学 | 398篇 |
地球物理 | 580篇 |
地质学 | 1060篇 |
海洋学 | 372篇 |
天文学 | 365篇 |
综合类 | 187篇 |
自然地理 | 207篇 |
出版年
2024年 | 9篇 |
2023年 | 40篇 |
2022年 | 87篇 |
2021年 | 82篇 |
2020年 | 88篇 |
2019年 | 115篇 |
2018年 | 89篇 |
2017年 | 90篇 |
2016年 | 114篇 |
2015年 | 111篇 |
2014年 | 121篇 |
2013年 | 140篇 |
2012年 | 124篇 |
2011年 | 161篇 |
2010年 | 133篇 |
2009年 | 177篇 |
2008年 | 160篇 |
2007年 | 177篇 |
2006年 | 135篇 |
2005年 | 164篇 |
2004年 | 157篇 |
2003年 | 106篇 |
2002年 | 102篇 |
2001年 | 116篇 |
2000年 | 121篇 |
1999年 | 124篇 |
1998年 | 80篇 |
1997年 | 44篇 |
1996年 | 30篇 |
1995年 | 52篇 |
1994年 | 47篇 |
1993年 | 33篇 |
1992年 | 32篇 |
1991年 | 13篇 |
1990年 | 15篇 |
1989年 | 6篇 |
1988年 | 5篇 |
1987年 | 2篇 |
1986年 | 6篇 |
1985年 | 4篇 |
1984年 | 5篇 |
1983年 | 5篇 |
1982年 | 5篇 |
1981年 | 4篇 |
1980年 | 2篇 |
1976年 | 2篇 |
1905年 | 3篇 |
1880年 | 2篇 |
1877年 | 3篇 |
1875年 | 1篇 |
排序方式: 共有3452条查询结果,搜索用时 15 毫秒
791.
R. A. L. Wray 《Australian Journal of Earth Sciences》2013,60(4):623-632
Speleothems of silica are far rarer than those of calcite but occur in a range of types including stalactites, stalagmites and flowstones. This study has found a wider range and far greater number of silica speleothems on the quartz sandstones of the Sydney region than the small number of previous accounts had suggested. Speleothems on the Sydney region sandstones are composed of multiple layers of amorphous opal‐A and cryptocrystalline chalcedony. Silica slowly dissolved from detrital and diagenetic quartz and kaolinite clays of the host arenites is redeposited as opal‐A at the sandstone surface when groundwater evaporates. This amorphous silica converts over time by Ostwald‐type paragenesis to the cryptocrystalline form, but the expected intermediate opal‐CT phase has not been detected. The crystallisation of chalcedony at earth‐surface temperatures is generally believed to take an extremely long time and its presence makes these speleothems very significant, especially as it is reported in only a small number of silica speleothems elsewhere. Furthermore, a similar paragenetic silica‐‘ripening’ mechanism may also be involved in the low‐temperature earth‐surface formation of other crystalline silica deposits such as silcrete duricrusts and pedogenic quartz. Additional closely coupled laboratory and field investigations into the processes that control silica paragenesis under earth‐surface conditions are sorely needed. 相似文献
792.
793.
794.
795.
Hirotaka Ito Motoki Kino Nozomu Kawakatu Naoki Isobe Shoichi Yamada 《Astrophysics and Space Science》2007,311(1-3):335-339
We investigate the total kinetic powers (L
j) and ages (t
age) of powerful jets of four FR II radio sources (Cygnus A, 3C 223, 3C 284, and 3C 219) by the detail comparison of the dynamical
model of expanding cocoons with observed ones. It is found that these sources have quite large kinetic powers with the ratio
of L
j to the Eddington luminosity (L
Edd) resides in 0.02<L
j/L
Edd<10. Reflecting the large kinetic powers, we also find that the total energy stored in the cocoon (E
c) exceed the energy derived from the minimum energy condition (E
min ): 2<E
c/E
min <160. This implies that a large amount of kinetic power is carried by invisible components such as thermal leptons (electron
and positron) and/or protons. 相似文献
796.
Ke-Fu Yu Jian-Xin Zhao Gang-Jian Wei Xin-Rong Cheng Pin-Xian Wang 《Global and Planetary Change》2005,47(2-4):301
South China Sea (SCS) is a major moisture source region, providing summer monsoon rainfall throughout Mainland China, which accounts for more than 80% total precipitation in the region. We report seasonal to monthly resolution Sr/Ca and δ18O data for five Holocene and one modern Porites corals, each covering a growth history of 9–13 years. The results reveal a general decreasing trend in sea surface temperature (SST) in the SCS from 6800 to 1500 years ago, despite shorter climatic cycles. Compared with the mean Sr/Ca–SST in the 1990s (24.8 °C), 10-year mean Sr/Ca–SSTs were 0.9–0.5 °C higher between 6.8 and 5.0 thousand years before present (ky BP), dropped to the present level by 2.5 ky BP, and reached a low of 22.6 °C (2.2 °C lower) by 1.5 ky BP. The summer Sr/Ca–SST maxima, which are more reliable due to faster summer-time growth rates and higher sampling resolution, follow the same trend, i.e. being 1–2 °C higher between 6.8 and 5.0 ky BP, dropping to the present level by 2.5 ky BP, and reaching a low of 28.7 °C (0.7 °C lower) by 1.5 ky BP. Such a decline in SST is accompanied by a similar decrease in the amount of monsoon moisture transported out of South China Sea, resulting in a general decrease in the seawater δ18O values, reflected by offsets of mean δ18O relative to that in the 1990s. This observation is consistent with general weakening of the East Asian summer monsoon since early Holocene, in response to a continuous decline in solar radiation, which was also found in pollen, lake-level and loess/paleosol records throughout Mainland China. The climatic conditions 2.5 and 1.5 ky ago were also recorded in Chinese history. In contrast with the general cooling trend of the monsoon climate in East Asia, SST increased dramatically in recent time, with that in the 1990s being 2.2 °C warmer than that 1.5 ky ago. This clearly indicates that the increase in the concentration of anthropogenic greenhouse gases played a dominant role in recent global warming, which reversed the natural climatic trend in East Asian monsoon regime. 相似文献
797.
798.
Huib Jan Van Langevelde Ylva PihlstrÖm Anthony Beasley 《Astrophysics and Space Science》2005,295(1-2):249-255
Centaurus A, the nearest AGN shows molecular absorption in the millimeter and radio regime. By observing the absorption with VLBI, we try to constrain the distribution of the gas, in particular whether it resides in the circumnuclear region. Analysis of VLBA observations in four OH and two H2CO transitions is presented here, as well as molecular excitation models parameterized with distance from the AGN. We conclude that the gas is most likely associated with the tilted molecular ring structure observed before in molecular emission and IR continuum. The formaldehyde absorption shows small-scale absorption which requires a different distribution than the hydroxyl. 相似文献
799.
800.
Three kinds of tundra plant samples including Dicranum angnstum(a type of boreal bryophyte) , PuccineUia phryganodes (a type of fringy p/ant),Salix polaris (a type of vascular plant) and surface soil were samples in 200 at Ny-Alesund of the Arctic.The levels of eight heavy metal elements (Hg, Pb, Cd, Cu, Zn, Ni, Fe and Mn) and three metal-like dements (As, Se, Sr) in the plant and soil samples of the areas within previous coal mining activities are significantly higher than those of other areas.The relative accumulation of these elements in these tundra plant samples is consistent with the one in the soft samples, especially in the areas affected by previous coal-mining activities.Thus, the pollution is apparently from local coal mining activity.Dicranum angustum has the highest concentrations among those elements, and it can be a good bio-indicator for heavy metal pollution in Ny(A)lesund.Though Ny(A)lesund is less polluted by heavy metal than nearby Northern European human living areas, but much more than the tundras of the Alaska, Greenland and the Antarctic. 相似文献