全文获取类型
收费全文 | 988篇 |
免费 | 42篇 |
国内免费 | 5篇 |
专业分类
测绘学 | 18篇 |
大气科学 | 69篇 |
地球物理 | 237篇 |
地质学 | 378篇 |
海洋学 | 57篇 |
天文学 | 197篇 |
综合类 | 2篇 |
自然地理 | 77篇 |
出版年
2023年 | 8篇 |
2022年 | 3篇 |
2021年 | 16篇 |
2020年 | 17篇 |
2019年 | 17篇 |
2018年 | 32篇 |
2017年 | 20篇 |
2016年 | 32篇 |
2015年 | 29篇 |
2014年 | 39篇 |
2013年 | 57篇 |
2012年 | 48篇 |
2011年 | 53篇 |
2010年 | 49篇 |
2009年 | 69篇 |
2008年 | 59篇 |
2007年 | 35篇 |
2006年 | 44篇 |
2005年 | 52篇 |
2004年 | 45篇 |
2003年 | 35篇 |
2002年 | 42篇 |
2001年 | 23篇 |
2000年 | 13篇 |
1999年 | 16篇 |
1998年 | 19篇 |
1997年 | 16篇 |
1996年 | 10篇 |
1995年 | 12篇 |
1994年 | 11篇 |
1993年 | 12篇 |
1992年 | 12篇 |
1991年 | 7篇 |
1990年 | 5篇 |
1989年 | 5篇 |
1988年 | 4篇 |
1987年 | 3篇 |
1986年 | 9篇 |
1985年 | 3篇 |
1984年 | 8篇 |
1983年 | 5篇 |
1982年 | 3篇 |
1981年 | 3篇 |
1980年 | 7篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1975年 | 3篇 |
1973年 | 3篇 |
1972年 | 4篇 |
1970年 | 4篇 |
排序方式: 共有1035条查询结果,搜索用时 46 毫秒
121.
122.
123.
A solution to the orbital motion of an Earth satellite at the critical inclination and with near-zero eccentricity is developed by the von Zeipel method to the first order in the eccentricity, and to the first order in the higher gravitational harmonics, using elements which do not degenerate at zero eccentricity. 相似文献
124.
125.
A. Niezurawska M. Szymczak A. M. S. Richards R. J. Cohen 《Astrophysics and Space Science》2005,295(1-2):37-42
Two star-forming regions Cepheus A and W75N, were searched for the 4765-MHz OH maser emission using the multi-element radio linked interferometer network (MERLIN). The excited OH emission has an arc-like structure of 40 mas in Cep A and a linear structure of size 45 mas in W75N. We also found the 1720-MHz line in Cep A and Hutawarakorn [MNRAS 330 (2002) 349] reported the 1720-MHz emission in W75N. The 1720- and 4765-MHz OH spots coincided in space within 60 mas and in velocity within 0.3 km s–1 in both targets implying that both maser transitions arise from the same region. According to the modelling by Gray [MNRAS 252 (1991) 30] the 1720/4765-MHz co-propagation requires a low density, warm environment. The masers lie at the edges of H II regions where such conditions are expected. 相似文献
126.
William D. Langer Leonard C. Rosen Jeffrey M. Cohen A. G. W. Cameron 《Astrophysics and Space Science》1969,5(3):259-271
An equation of state for cold matter above white dwarf densities is evaluated. The gas is considered to be a mixture of degenerate neutrons, protons and electrons combined with nuclei of one type (that is only oneA andZ value). We derive the equilibrium equations for the mixture and calculate the number densities as well as theA andZ of the nucleus. Finally we calculate an equation of state, which smoothly goes over to that of a neutron, proton electron gas mixture at a density of 5×1013 g/cm3. 相似文献
127.
David?L.?DettmanEmail author Manuel?R.?Palacios-Fest Hudson?H.?Nkotagu Andrew?S.?Cohen 《Journal of Paleolimnology》2005,34(1):93-105
Evaporation dominates the removal of water from Lake Tanganyika, and therefore the oxygen isotope composition of lake water has become very positive in comparison to the waters entering the lake. The surface water in Lake Tanganyika has remained relatively unchanged over the last 30 years with a seasonal range of +3.2 to +3.5 VSMOW. Water from small rivers entering the lake seems to have a 18O value between –3.5 and –4.0, based on scattered measurements. The two largest catchments emptying into the lake deliver water that has a 18O value between these two extremes. This large contrast is the basis of a model presented here that attempts to reconstruct the history of runoff intensity based on the 18O of carbonate shells from Lake Tanganyika cores. In order to use biogenic carbonates to monitor changes in the 18O of mixing-zone water, however, the oxygen isotope fractionation between water and shell carbonate must be well understood. The relatively invariant environmental conditions of the lake allow us to constrain the fractionation of both oxygen and carbon isotope ratios. Although molluskan aragonitic shell 18O values are in agreement with published mineral-water fractionations, ostracode calcite is 1.2 more positive than that of inorganic calcite precipitated under similar conditions. Ostracode shell 18O data from two cores from central Lake Tanganyika suggest that runoff decreased in the first half of this millennium and has increased in the last century. This conclusion is poorly constrained, however, and much more work needs to be done on stable isotope variation in both the waters and carbonates of Lake Tanganyika. We also compared the 13C of shells against predicted values based solely on the 13C of lake water dissolved inorganic carbon (DIC). The ostracode Mecynocypria opaca is the only ostracode or mollusk that falls within the predicted range. This suggests that M. opaca has potential for reconstructing the carbon isotope ratio of DIC in Lake Tanganyika, and may be a useful tool in the study of the history of the lakes productivity and carbon cycle. 相似文献
128.
Geochemical signals of bulk sedimentary organic matter from three cores from Lake Tanganyika provided information about both internal processes and terrestrial inputs to the lake. Indications of land use change were detected in the geochemical records of the watersheds, and the timing of these changes was consistent with historical records of population demographics. While C:N ratios suggested that the distance from shore might be important in influencing the relative amount of allochthonous vs. autochthonous material, all cores were dominated by autochthonous organic matter. In general, nitrogen isotopes were more positive at disturbed sites, indicating inputs of enriched soil nitrate that was subsequently taken up by phytoplankton. In contrast, carbon isotopes did not reflect land use patterns, and a post-1950s decline in carbon isotope ratios found in all cores may indicate a lake-wide decrease in productivity. These interpretations were consistent with pollen and climate records. 相似文献
129.
Jeffrey M. Cohen William D. Langer Leonard C. Rosen A. G. W. Cameron 《Astrophysics and Space Science》1970,6(2):228-239
Using an equation of state for cold degenerate matter which takes nuclear forces and nuclear clustering into account, neutron star models are constructed. Stable models were obtained in the mass range above 0.065M
and density range 1014.08 to 1015.4 gm/cm3. All of these models were found to be bound. The outer crystalline layer of the star was found to have a thickness of 200 m or more depending on the mass of the model. 相似文献
130.
Olivier?FéméniasEmail author Nicolas?Coussaert Julien?Berger Jean-Claude?C.?Mercier Daniel?Demaiffe 《Contributions to Mineralogy and Petrology》2004,148(1):13-28
Mantle xenoliths from Puy Beaunit (French Massif Central) are compositionally varied, ranging from relatively fertile spinel lherzolites to refractory spinel dunites. Fertile peridotites have registered a modal (amphibole-bearing lherzolites) and cryptic metasomatic event that took place before the last Permian (257 Ma) melting episode. Depletion processes have been constrained by chemical modelling: the depletion is related to different degrees of partial melting, but two major melt extraction episodes are needed to explain the range of major element composition. The second event was responsible for the local large-scale dunitification of former residues. The first melting event (F25%) and metasomatic enrichment are attributed to an ancient fluid and/or liquid infiltration that could be related to a pre-Variscan regional subduction (located to the north of the Beaunit area). Texture acquisition and major deformation of the mantle xenoliths were sub-contemporaneous of the subduction and would result from lithospheric delamination. The second melting event (F17%) produced high-Mg basalts with calc-alkaline trace element signature that gave rise to the Permian underplating episode recognised in western Europe. 相似文献