全文获取类型
收费全文 | 40573篇 |
免费 | 1626篇 |
国内免费 | 1834篇 |
专业分类
测绘学 | 1286篇 |
大气科学 | 3711篇 |
地球物理 | 8126篇 |
地质学 | 15762篇 |
海洋学 | 3840篇 |
天文学 | 7962篇 |
综合类 | 549篇 |
自然地理 | 2797篇 |
出版年
2022年 | 446篇 |
2021年 | 572篇 |
2020年 | 591篇 |
2019年 | 643篇 |
2018年 | 1230篇 |
2017年 | 1123篇 |
2016年 | 1357篇 |
2015年 | 871篇 |
2014年 | 1337篇 |
2013年 | 2194篇 |
2012年 | 1405篇 |
2011年 | 1878篇 |
2010年 | 1608篇 |
2009年 | 2057篇 |
2008年 | 1782篇 |
2007年 | 1678篇 |
2006年 | 1625篇 |
2005年 | 1286篇 |
2004年 | 1226篇 |
2003年 | 1143篇 |
2002年 | 1139篇 |
2001年 | 1052篇 |
2000年 | 1027篇 |
1999年 | 875篇 |
1998年 | 795篇 |
1997年 | 861篇 |
1996年 | 693篇 |
1995年 | 685篇 |
1994年 | 636篇 |
1993年 | 537篇 |
1992年 | 512篇 |
1991年 | 473篇 |
1990年 | 470篇 |
1989年 | 454篇 |
1988年 | 420篇 |
1987年 | 482篇 |
1986年 | 402篇 |
1985年 | 491篇 |
1984年 | 485篇 |
1983年 | 517篇 |
1982年 | 484篇 |
1981年 | 427篇 |
1980年 | 456篇 |
1979年 | 374篇 |
1978年 | 328篇 |
1977年 | 334篇 |
1976年 | 310篇 |
1975年 | 294篇 |
1974年 | 293篇 |
1973年 | 288篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
491.
A simple and effective method of heating divers in extreme cold water has been developed. The local heating concept utilizes a uniformly distributed granular mixture of magnesium and iron particles packed in small sachets. Upon activation by sea water, the two metal mixture behaves as a multitude of short-circuited electrolytic cells, producing thermal energy rather than electrical energy. The 45 mm-square sachets may be placed where heating is required on the diver's body, thus giving rise to the term “local heating”, obviously, the heating system requires no pumping device, or distribution network. Heating rate and duration of output of the sachets are controlled by particle size, and mixture ratio of the constituent Mg and Fe particles.This paper described the development, testing and performance of the heating sachets. Results of live tests in different dive situations are also presented and discussed. 相似文献
492.
Emma L. Jackson Martin J. Attrill Malcolm B. Jones 《Estuarine, Coastal and Shelf Science》2006,68(3-4):421
Recent research and management plans for seagrass habitats have called for landscape level approaches. The present study examines the spatial utilisation of subtidal seagrass beds by fish and decapods around the coast of Jersey (49°N 02° W). A hierarchical scale of landscape configuration and the plant characteristics of eight seagrass beds were measured and the contributions of these variables as predictors of the properties of the fish and decapod assemblages were evaluated using multiple linear regression models. The results indicated that total diversity had a negative relationship with transect heterogeneity and total species number had a weak negative association with increasing fragmentation. Both total diversity and total species number showed a positive relationship with depth. In fact, in all models of species number and densities, values were higher in deeper seagrass beds. Total decapod density increased with aggregation of seagrass patches within a landscape. In addition to landscape configuration, smaller-scale structural changes in both canopy height and epiphyte load appeared to influence densities of decapod crustaceans. At night, fewer patterns could be explained by the independent variables in the model. 相似文献
493.
494.
495.
Dissolved and particulate manganese in seawater samples derived from the English Channel has been analyzed using graphite furnace atomic absorption (GFAAS), and inductively coupled plasma atomic emission (ICP—AES) spectroscopies. Because of the high contents of carbonate minerals found in the suspended matter samples, the English Channel constitutes an ideal field area for the study of the Mn2+/Ca2+/CaCO3 system. Owing to the chemical speciation of particulate manganese and the combined use of the X-ray diffraction and electron spin resonance spectroscopy we have shown the importance of the carbonate phase in the stabilization of manganese (II). This has been confirmed by p-pH measurements in the field. All these studies have also indicated that: (1) manganese is associated with calcite in the form of a solid solution, MnxCa1−xCO3; and (2) significant increases in the concentrations of particulate manganese, especially in offshore waters, occur in summer. This seasonal phenomenon has been attributed to the proliferation of coccolithophorids, which are known to be covered with calcified skeletons at high specific surface areas. To appraise the implication of the coccolithophorid-blooms phenomenon on the Mn2+/Ca2+/CaCO3 system, we have used the manganese distribution coefficient, Di, between the liquid phase and CaCO3 particles. Overall we have shown that: (1) Di in summer (i.e. when coccoliths considered as very fine-grained calcite are abundant) is much higher than that obtained in winter; and (2) in the vicinity of the French coast, Di does not vary significantly even in summer. This is because of the high content of chalk-derived particles found in the near-shore waters. 相似文献
496.
The intertidal erosion rate of cohesive sediment: a case study from Long Island Sound 总被引:2,自引:0,他引:2
Over the past decades, many attempts have been made to generate useful bottom erosion models for the study of cohesive sediment movement. This study addresses some of the key questions involved in determining the functional relationship between erosion rate and bottom shear stress. Current, wave, and turbidity data were collected from a bottom mounted instrument array in a moderately energetic estuarine environment. The bottom shear stress was calculated from a wave–current interaction model. The erosion rate was derived from the observed sediment concentration using a vertical mixing model. Examination of the relationship between erosion rate and bottom stress showed that the erosion rate varied at intertidal frequency. When averaged over the tidal fluctuation, the erosion rate remained approximately constant at low stress, but increased sharply when the shear stress rose above a critical value. This suggests two-stage erosion. The bed has a layered structure, in which a thin layer of loose, high water content material overlies a more consolidated bed. The top layer of high water content material (fluff) was easily disturbed and re-suspended by tidal currents, but the consolidated bottom layer was eroded only under conditions of high shear stress. 相似文献
497.
The transfer of liquid bulk through flexible transfer lines can be accomplished despite relatively large ship motions. For this reason, open sea berth terminals have replaced conventional harbors in many places. The thorough understanding of moored ships dynamics when subjected to waves, wind and currents, combined with the use of flexible dolphins, flexible fenders, asymmetric constant-tension winches and nylon tails permit us to design open sea berth terminals for relatively exposed areas. The design of an integrated system of marine sensors to continuously monitor all the critical elements-ship motion and mooring and fender forces-will enhance the operational procedure whilst insuring safe operations. A berth code can then be established for insuring the maximum efficiency of the terminal on the basis of the measurement of these critical elements. 相似文献
498.
M. Tiwari R. Ramesh B. L. K. Somayajulu A. J. T. Jull G. S. Burr 《Geo-Marine Letters》2006,26(1):23-30
Rapid shifts in past climate recorded in polar ice sheets have elicited various explanations relating to either thermohaline
circulation changes by ice-rafting or natural greenhouse gas concentrations modulated by climatic conditions in the tropics.
To compare the tropical paleoclimate record with the polar record, one must choose sediment cores from highly productive ocean
regions. Necessarily, such regions reflect the wind records in the tropics, because high productivity is associated with upwelling
driven by winds. Comparing tropical precipitation records with high-latitude records is, however, a more difficult task because
sediments recording paleoprecipitation usually have low sedimentation rates, and offer coarser resolution relative to polar
ice cores. Here, we present δ
18O data of three planktonic species of Foraminifera (a proxy for precipitation) from such a sediment core, spanning the past
35 ka for the equatorial Indian Ocean, which falls under the southwest monsoon (SWM) realm. Results show that minimum SWM
precipitation occurred at the Last Glacial Maximum, with a subsequent increase at Termination IA. During the Holocene, SWM
precipitation intensified uniformly up to the core top (∼2.2 ka b.p.), as revealed by generally decreasing δ
18O values. Variations in precipitation are consistent with climate changes recorded in polar ice sheets. Although the different resolutions of the two records preclude
a rigorous comparison, abrupt cooling/warming events appear to be accompanied by sudden reduction/enhancement in (SWM) rainfall.
Thus, mechanisms with time scales much shorter than a millennium, such as natural greenhouse warming (e.g., CH4 concentration), controlled by emissions from the tropics, could have played a major role in high-latitude climate change. 相似文献
499.
500.
董胜 《中国海洋大学学报(自然科学版)》1997,(2)
针对海上矿石运输,介绍了一种完成定量任务并能使投资费用最少为目标的最优船队组建方法,同时对易变参数进行了灵敏度分析,使决策更加科学化 相似文献