全文获取类型
收费全文 | 78081篇 |
免费 | 1318篇 |
国内免费 | 868篇 |
专业分类
测绘学 | 2075篇 |
大气科学 | 6061篇 |
地球物理 | 15144篇 |
地质学 | 28053篇 |
海洋学 | 6779篇 |
天文学 | 17780篇 |
综合类 | 246篇 |
自然地理 | 4129篇 |
出版年
2021年 | 725篇 |
2020年 | 803篇 |
2019年 | 865篇 |
2018年 | 1940篇 |
2017年 | 1854篇 |
2016年 | 2383篇 |
2015年 | 1313篇 |
2014年 | 2230篇 |
2013年 | 3995篇 |
2012年 | 2403篇 |
2011年 | 3105篇 |
2010年 | 2737篇 |
2009年 | 3606篇 |
2008年 | 3088篇 |
2007年 | 3046篇 |
2006年 | 2950篇 |
2005年 | 2248篇 |
2004年 | 2283篇 |
2003年 | 2195篇 |
2002年 | 2115篇 |
2001年 | 1865篇 |
2000年 | 1827篇 |
1999年 | 1473篇 |
1998年 | 1509篇 |
1997年 | 1457篇 |
1996年 | 1294篇 |
1995年 | 1216篇 |
1994年 | 1079篇 |
1993年 | 960篇 |
1992年 | 940篇 |
1991年 | 932篇 |
1990年 | 984篇 |
1989年 | 832篇 |
1988年 | 843篇 |
1987年 | 931篇 |
1986年 | 855篇 |
1985年 | 1022篇 |
1984年 | 1149篇 |
1983年 | 1075篇 |
1982年 | 1001篇 |
1981年 | 918篇 |
1980年 | 868篇 |
1979年 | 857篇 |
1978年 | 855篇 |
1977年 | 726篇 |
1976年 | 669篇 |
1975年 | 680篇 |
1974年 | 675篇 |
1973年 | 705篇 |
1971年 | 462篇 |
排序方式: 共有10000条查询结果,搜索用时 234 毫秒
991.
An investigation has been made of available data on the saturation state of seawater with respect to calcium carbonate and its possible significance for scale formation on Ocean Thermal Energy Conversion (OTEC) heat exchangers. Pertinent oceanographic data is lacking at or near potential OTEC sites for the calculation of the degree of saturation of seawater with respect to calcium carbonate. Consequently, only “extrapolated” saturation values can be used. These indicate that near surface seawater is probably supersaturated, with respect to the calcium carbonate phases calcite and aragonite, at all potential OTEC sites. The deep seawater that would be brought to the surface at the potential Atlantic Ocean sites is also likely to be supersaturated with respect to calcium carbonate. The deep seawater at the potential Pacific Ocean sites may be slightly undersaturated.The fact that OTEC heat exchangers will be operating in seawater, which is supersaturated with respect to calcium carbonate, means that if nucleation of calcite or aragonite occurs on the heat exchanger surfaces, significant growth rates of calcium carbonate scale may be expected. The potential for calcium carbonate nucleation is highest at cathodic metal surface locations, which are produced as the result of aluminum corrosion in seawater. Consequently, corrosion and scale formation may be closely related. What the possible effects of biofouling may be on this process are not known. 相似文献
992.
S.A. Mavrakos 《Ocean Engineering》1997,24(4):381-399
The paper deals with the linearized exciting wave forces and hydrodynamic coefficients of a toroidal body floating in water of finite depth. For the solution of the diffraction and the radiation problems the flow field around the body is subdivided into ring-shaped fluid regions, in each of which axisymmetric eigenfunction expansions for the velocity potential is made. By implementing Galerkin's method the various potential solutions are matched and numerical results concerning the exciting wave forces and the hydrodynamic coefficients in all modes of motion are obtained. 相似文献
993.
Long-term changes in ray populations in the North Sea 总被引:2,自引:3,他引:2
994.
995.
The concentrations of a number of trace metals (Co, Ni, Cu, Zn, Ag, Cd and Hg) have been determined in sediments from cores collected in the Southampton Water region. Measurements of total hydrocarbons were also made on several of the cores. Markedly elevated concentrations of copper, up to 362 μg/g dry wt, were found in sediments which were collected close to the discharge points of oil refinery outfalls and which also showed concentrations of total hydrocarbons considerably greater than the background levels for the estuary. The results of selective chemical extractions showed that much of the copper was present in the non-lattice fraction of the sediment. Although the concentrations of Co, Ni, Zn and Cd were significantly correlated with those of copper in the core showing the highest concentrations of this element, these metals were not enhanced in comparison with the levels found in several other parts of the estuary. Variations in concentrations of metals in sediments throughout the estuary were not correlated with differences in organic carbon content. 相似文献
996.
997.
Spectral analysis is used to determine the wave force characteristics on structures exposed to random waves. Considerable work has been carried out to determine the magnitude of random wave forces acting on a single cylinder, but little information is available in the case of a group of cylinders in random waves. Such situations arise when structures comprise multiple tubular members which are in close proximity, and wave forces cannot be calculated precisely by analytical methods due to complicated flow conditions past the group. Experimental studies are also required for proper understanding and analysis. An experimental scheme was carried out to study the wave force characteristics on a single cylinder and on a group of cylinders in response to two different wave spectra with the same significant wave height, and the results are compared. 相似文献
998.
This paper considers the relationship between multi-year air temperature and pressure fluctuations over the ocean, based on an approximate solution of the problem on the determination of large-scale seawater temperature anomalies from the conditions predominating at the sea surface. The dependence derived is numerically analysed using observations made in the North Atlantic. It is shown that the variability of annual mean air temperature anomalies is largely controlled by the air pressure field which has taken place during the preceding long-term period. The dependence derived may be applied to generate long-term forecasts of the ocean's hydrometeorological regime.Translated by Vladimir A. Puchkin. 相似文献
999.
1000.
Within the framework of the reduced-gravity model of the ocean taking into account the effect of friction in the Rayleigh form, we study the two-dimensional problem of nonlinear motions of a subsurface front of finite width. We consider the conservation laws and the character of motion of the center-of-mass of the cross section of the front and their variations caused by the losses of energy. For fields with special structure, the problem is reduced to the solution of a system of nonlinear ordinary differential equations. It is shown that the initially geostrophic frontal current decays with time according to a power law. The deviations of the initial state of the front from the state of geostrophic balance result in the generation of superinertial oscillations of the hydrodynamic fields. 相似文献