全文获取类型
收费全文 | 7850篇 |
免费 | 1692篇 |
国内免费 | 2555篇 |
专业分类
测绘学 | 351篇 |
大气科学 | 362篇 |
地球物理 | 3044篇 |
地质学 | 6193篇 |
海洋学 | 1127篇 |
天文学 | 72篇 |
综合类 | 390篇 |
自然地理 | 558篇 |
出版年
2024年 | 39篇 |
2023年 | 141篇 |
2022年 | 234篇 |
2021年 | 334篇 |
2020年 | 375篇 |
2019年 | 467篇 |
2018年 | 371篇 |
2017年 | 383篇 |
2016年 | 407篇 |
2015年 | 452篇 |
2014年 | 539篇 |
2013年 | 513篇 |
2012年 | 498篇 |
2011年 | 574篇 |
2010年 | 466篇 |
2009年 | 552篇 |
2008年 | 556篇 |
2007年 | 626篇 |
2006年 | 662篇 |
2005年 | 468篇 |
2004年 | 471篇 |
2003年 | 388篇 |
2002年 | 335篇 |
2001年 | 314篇 |
2000年 | 281篇 |
1999年 | 276篇 |
1998年 | 220篇 |
1997年 | 223篇 |
1996年 | 176篇 |
1995年 | 156篇 |
1994年 | 153篇 |
1993年 | 98篇 |
1992年 | 99篇 |
1991年 | 59篇 |
1990年 | 41篇 |
1989年 | 37篇 |
1988年 | 40篇 |
1987年 | 28篇 |
1986年 | 11篇 |
1985年 | 6篇 |
1984年 | 6篇 |
1983年 | 2篇 |
1982年 | 4篇 |
1981年 | 3篇 |
1980年 | 1篇 |
1978年 | 1篇 |
1977年 | 2篇 |
1954年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 795 毫秒
61.
A finite element model is set up and experimental tests are performed to help understand the behavior of a concrete canoe and subsequently optimize its design. First, the performance criteria that must be satisfied to participate at the annual ASCE/Master Builders competition are described. Then, the finite element model and the different loading cases that were studied are presented. Results from these loading cases are discussed and used to optimize the material properties as well as the thickness of the hull and the dimensions of the gunwale and reinforcing ribs. Static and dynamic experimental tests were also conducted to validate the results of the finite element analyses. The results indicate that the main stresses and strains are caused by the static load cases. The additional stresses caused during races are small. 相似文献
62.
The characteristics of the ratios between cadmium (Cd) and phosphorus (P) in settling particles collected from the Okinawa
Trough in the East China Sea were examined using a sediment trap, moored at a depth of 811 m for one year. The Cd/P ratios
varied within a narrow range throughout the year, in spite of the large seasonal change in the total dry mass, Cd, and P fluxes.
The average Cd/P ratio of settling particles was 0.062 (nmol/μmol), which was obviously lower than that of surface seawater around the study site (0.16). This lower ratio in the Okinawa
Trough particles collected using the 811 m-moored trap certainly reflected the mixture of biologically produced organic matter
around the study site and other components that were mainly transported as lateral flux from the shelf edge and slope area
of the East China Sea. 相似文献
63.
64.
Serena M. Moseman Lisa A. Levin Carolyn Currin Charlotte Forder 《Estuarine, Coastal and Shelf Science》2004,60(4):755-770
Modes of colonization, the successional trajectory, and trophic recovery of a macrofaunal community were analyzed over 19 months in the Friendship marsh, a 20-acre restored wetland in Tijuana Estuary, California. Traditional techniques for quantifying macrofaunal communities were combined with emerging stable isotopic approaches for evaluation of trophic recovery, making comparisons with a nearby natural Spartina foliosa habitat. Life history-based predictions successfully identified major colonization modes, although most taxa employed a variety of tactics for colonizing the restored marsh. The presence of S. foliosa did not seem to affect macrofaunal colonization or succession at the scale of this study. However, soil organic matter content in the restored marsh was positively correlated with insect densities, and high initial salinities may have limited the success of early colonists. Total macrofaunal densities recovered to natural marsh levels after 14 months and diversity, measured as species richness and the Shannon index (H′), was comparable to the natural marsh by 19 months. Some compositional disparities between the natural and created communities persisted after 19 months, including lower percentages of surface-feeding polychaetes (Polydora spp.) and higher percentages of dipteran insects and turbellarians in the Friendship marsh. As surficial structural similarity of infaunal communities between the Friendship and natural habitat was achieved, isotopic analyses revealed a simultaneous trajectory towards recovery of trophic structure. Enriched δ13C signatures of benthic microalgae and infauna, observed in the restored marsh shortly after establishment compared to natural Spartina habitat, recovered after 19 months. However, the depletion in δ15N signatures of macrofauna in the Friendship marsh indicated consumption of microalgae, particularly nitrogen-fixing cyanobacteria, while macroalgae and Spartina made a larger contribution to macrofaunal diets in the natural habitat. Future successional studies must continue to develop and employ novel combinations of techniques for evaluating structural and functional recovery of disturbed and created habitats. 相似文献
65.
66.
67.
This paper describes the simulation of the flow of a viscous incompressible Newtonian liquid with a free surface. The Navier–Stokes equations are formulated using a streamline upwind Petrov–Galerkin scheme, and solved on a Q-tree-based finite element mesh that adapts to the moving free surface of the liquid. Special attention is given to fitting the mesh correctly to the free surface and solid wall boundaries. Fully non-linear free surface boundary conditions are implemented. Test cases include sloshing free surface motions in a rectangular tank and progressive waves over submerged cylinders. 相似文献
68.
A numerical model to compute wave field is developed. It is based on the Berkhoff diffraction-refraction equation, in which an energy dissipation term is added, to take into account the breaking and the bottom friction phenomena. The energy dissipation function, by breaking and by bottom friction, is introduced in the Berkhoff equation to obtain a new equation of propagation.The resolution is done with the hybrid finite element method, where lagrangians elements are used. 相似文献
69.
Seismic noise induced by the seismic source during continuous profiling reduces the signal-to-noise ratio and hence the data quality. This noise is largely dependent on the shot interval. In this paper, the noise amplitude of refraction seismic records from a special experiment is analysed as a function of the shot interval. An empirical exponential relationship between mean peak noise amplitude and shot interval is deduced. By increasing the shot interval, the induced noise can be minimized on all successive records. This results in an improvement of the data, predominantly the signal-to-noise ratio. Because the seismic signal and the shot-induced noise have nearly the same spectra, the chance of improving the signal-to-noise ratio by stacking is significantly reduced. 相似文献
70.