全文获取类型
收费全文 | 2433篇 |
免费 | 279篇 |
国内免费 | 354篇 |
专业分类
测绘学 | 156篇 |
大气科学 | 317篇 |
地球物理 | 619篇 |
地质学 | 1128篇 |
海洋学 | 315篇 |
天文学 | 255篇 |
综合类 | 88篇 |
自然地理 | 188篇 |
出版年
2024年 | 18篇 |
2023年 | 49篇 |
2022年 | 121篇 |
2021年 | 144篇 |
2020年 | 131篇 |
2019年 | 129篇 |
2018年 | 143篇 |
2017年 | 141篇 |
2016年 | 151篇 |
2015年 | 128篇 |
2014年 | 156篇 |
2013年 | 154篇 |
2012年 | 161篇 |
2011年 | 185篇 |
2010年 | 111篇 |
2009年 | 178篇 |
2008年 | 119篇 |
2007年 | 103篇 |
2006年 | 95篇 |
2005年 | 103篇 |
2004年 | 72篇 |
2003年 | 51篇 |
2002年 | 75篇 |
2001年 | 41篇 |
2000年 | 38篇 |
1999年 | 47篇 |
1998年 | 18篇 |
1997年 | 31篇 |
1996年 | 17篇 |
1995年 | 13篇 |
1994年 | 16篇 |
1993年 | 6篇 |
1992年 | 13篇 |
1991年 | 15篇 |
1990年 | 13篇 |
1989年 | 4篇 |
1988年 | 8篇 |
1987年 | 9篇 |
1986年 | 4篇 |
1985年 | 5篇 |
1984年 | 5篇 |
1983年 | 5篇 |
1982年 | 6篇 |
1981年 | 6篇 |
1980年 | 3篇 |
1979年 | 4篇 |
1976年 | 4篇 |
1973年 | 2篇 |
1971年 | 3篇 |
1970年 | 2篇 |
排序方式: 共有3066条查询结果,搜索用时 15 毫秒
21.
闽江口第四纪地层中的孢粉、有孔虫、硅藻组合及其古地理意义 总被引:7,自引:1,他引:7
本文通过对闽江口琅岐岛钻孔的孢粉,有孔虫及硅藻的综合分析,发现第四纪地层中有五个孢粉组合带,五个硅藻组合带与一个微体化石层。结合~(14)C测年资料与沉积物特征认为该地区第四纪地层可以划分为晚更新世晚期与全新世(早期、中期、晚期)。气候演化依次为:冷干→温暖略干→温暖湿润→炎热潮湿→温暖略干。沉积相发展顺序为:冲积相、冲洪积相→河流相、湖沼相→湖沼相、河口相→海湾相→河口相。 相似文献
22.
A population of killifish (Fundulus heteroclitus) inhabiting a creosote-polluted inlet of the Elizabeth River demonstrates tolerance to the acute toxic effects exerted by contaminated sediments on reference site killifish. Previous data have suggested that upregulated antioxidant defenses contribute to short-term tolerance in killifish exposed to Elizabeth River sediments. This study investigated population differences in antioxidant defenses from wild caught Elizabeth River and reference population killifish in different seasons, and after being held in the laboratory. Parameters measured in the killifish were total glutathione concentrations (GSH(T)), activities of glutathione reductase (GR), glutathione peroxidase (GPx), and lipid peroxidation (LPO), all in adult hepatic tissues. The Elizabeth River population exhibited greater GSH(T), higher GPx activities, and increased LPO as compared to the reference population. Sex specific population differences were also observed in GSH(T) and GPx. Both populations displayed decreased GSH(T) and increased GR from early to late summer, as well as after being held in the laboratory. This study indicates that there are many factors that may contribute to differences in levels of antioxidant defenses in addition to exposure to contaminants, including reproductive status and environmental conditions. 相似文献
23.
Low multipath antennas for GNSS-based attitude determination systems applied to high-altitude platforms 总被引:1,自引:1,他引:0
Luigi Boccia Pasquale Pace Giandomenico Amendola Giuseppe Di Massa 《GPS Solutions》2008,12(3):163-171
High-altitude platforms (HAPs) are a flexible and attractive technology for providing innovative wireless services. These
aerial platforms can be successfully employed for mobile or broadband communications and for disaster monitoring or response.
However, one of the open issues is whether HAP stations can provide reliable services without temporal outages owing to stratospheric
winds that can cause positional and attitude instabilities thus affecting the communication system operation. To counteract
this issue, one possible solution is to use reconfigurable antennas whose pointing direction can be adjusted depending on
the platform spatial orientation. However, this would require real-time three-axial attitude data. As a possible solution,
this paper will review the potential of GNSS-based attitude determination systems with reference to HAP stations. In particular,
it will be shown how the use of a particular class of low multipath and lightweight antennas can provide a high degree of
accuracy without altering the avionic ballast. 相似文献
24.
Jerome Métral Laurent Charlet Sara Bureau Sukumar Basu Mallik Sudipta Chakraborty Kazi M Ahmed MW Rahman Zhongqi Cheng Alexander van Geen 《Geochemical transactions》2008,9(1):1
Background
The origin of the spatial variability of dissolved As concentrations in shallow aquifers of the Bengal Basin remains poorly understood. To address this, we compare here transects of simultaneously-collected groundwater and aquifer solids perpendicular to the banks of the Hooghly River in Chakdaha, India, and the Old Brahmaputra River in Araihazar, Bangladesh. 相似文献25.
New basis for the constitutive modelling of aggregated soils 总被引:1,自引:1,他引:0
Natural and compacted soils are usually characterized by aggregation of particles. The mechanical behaviour of these materials
depends on soil structure. The oedometric compression tests performed on aggregated samples presented here showed that these
materials exhibit a yield limit depending not only on stress history and stress state but also on soil structure. Evidence
is provided using the neutron tomography technique. These results revealed that soil structure modification occurs together
with plastic deformations. The experimental results are used to propose a new state parameter to quantify the soil structure.
Based on pore-scale experimental observations, an evolution law for this parameter is proposed as a function of associated
plastic strains. Considering both soil fabric and inter-particle bonding effects, a new yield limit depending on stress state,
stress history and soil structure is introduced for the aggregated soils. Accordingly, a new constitutive framework consistent
with strain hardening plasticity is proposed to consider soil structure effects in the modelling of aggregated soils. 相似文献
26.
Maria Luce Frezzotti Gianfranco Di Vincenzo Claudio Ghezzo Ernst A. J. Burke 《Contributions to Mineralogy and Petrology》1994,117(2):111-123
Fine-grained peraluminous synkinematic leuco-monzogranites (SKG), of Cambro-Ordovician age, occur as veins and sills (up to 20–30 m thick) in the Deep Freeze Range, within the medium to high-grade metamorphics of the Wilson Terrane. Secondary fibrolite + graphite intergrowths occur in feldspars and subordinately in quartz. Four main solid and fluid inclusion populations are observed: primary mixed CO2+H2O inclusions + Al2SiO5 ± brines in garnet (type 1); early CO2-rich inclusions (± brines) in quartz (type 2); early CO2+CH4 (up to 4 mol%)±H2O inclusions + graphite + fibrolite in quartz (type 3); late CH4+CO2+N2 inclusions and H2O inclusions in quartz (type 4). Densities of type 1 inclusions are consistent with the crystallization conditions of SKG (750°C and 3 kbar). The other types are post-magmatic: densities of type 2 and 3 inclusions suggest isobaric cooling at high temperature (700–550°C). Type 4 inclusions were trapped below 500°C. The SKG crystallized from a magma that was at some stage vapour-saturated; fluids were CO2-rich, possibly with immiscible brines. CO2-rich fluids (±brines) characterize the transition from magmatic to post-magmatic stages; progressive isobaric cooling (T<670°C) led to a continuous decrease off
O
2 can entering in the graphite stability field; at the same time, the feldspars reacted with CO2-rich fluids to give secondary fibrolite + graphite. Decrease ofT andf
O
2 can explain the progressive variation in the fluid composition from CO2-rich to CH4 and water dominated in a closed system (in situ evolution). The presence of N2 the late stages indicates interaction with external metamorphic fluids.Contribution within the network Hydrothermal/metamorphic water-rock interactions in crystalline rocks: a multidisciplinary approach on paleofluid analysis. CEC program: Human Capital and Mobility 相似文献
27.
We present 50 and 100µm photometry and size information for several main sequence stars surrounded by dust shells. The observations from NASA's Kuiper Airborne Observatory include the Vega-like stars, Beta Pic, Fomalhaut, as well as four stars suggested by Walker and Wolstencroft to belong possibly to the same class. The results of our observations are best interpreted as upper limits to the far-infrared sizes of the dust clouds around all of the stars except Fomalhaut and Beta Pic. We have also fit simple, optically thin models to the Beta Pic data to explore the range of shell parameters consistent with our limits and with previous observations. 相似文献
28.
L. Bassani R. C. Butler A. J. Dean G. Di Cocco A. Della Ventura F. Perotti G. Villa 《Astrophysics and Space Science》1982,82(2):311-316
During a balloon flight in September 1979 of the MISO low-energy -ray telescope, the BL Lac-object MkN 501 was studied in the hard X-ray range above 30 keV and in the low energy -ray range up to 19 MeV. No statistically significant X- and -ray fluxes were detected. The implications of the upper limits obtained are discussed in the light of the relativistic jet theories recently proposed. 相似文献
29.
30.
Ice and snow have often helped physicists understand the world. On the contrary it has taken them a very long time to understand the flow of the glaciers. Naturalists only began to take an interest in glaciers at the beginning of the 19th century during the last phase of glacier advances. When the glacier flow from the upslope direction became obvious, it was then necessary to understand how it flowed. It was only in 1840, the year of the Antarctica ice sheet discovery by Dumont d'Urville, that two books laid the basis for the future field of glaciology: one by Agassiz on the ice age and glaciers, the other one by canon Rendu on glacier theory. During the 19th century, ice flow theories, adopted by most of the leading scientists, were based on melting/refreezing processes. Even though the word ‘fluid’ was first used in 1773 to describe ice, more the 130 years would have to go by before the laws of fluid mechanics were applied to ice. Even now, the parameter of Glen's law, which is used by glaciologists to model ice deformation, can take a very wide range of values, so that no unique ice flow law has yet been defined. To cite this article: F. Rémy, L. Testut, C. R. Geoscience 338 (2006). 相似文献