全文获取类型
收费全文 | 57121篇 |
免费 | 14352篇 |
国内免费 | 21219篇 |
专业分类
测绘学 | 10305篇 |
大气科学 | 8221篇 |
地球物理 | 10445篇 |
地质学 | 38156篇 |
海洋学 | 11154篇 |
天文学 | 775篇 |
综合类 | 5037篇 |
自然地理 | 8599篇 |
出版年
2024年 | 501篇 |
2023年 | 1315篇 |
2022年 | 3513篇 |
2021年 | 4228篇 |
2020年 | 3448篇 |
2019年 | 4153篇 |
2018年 | 3688篇 |
2017年 | 3497篇 |
2016年 | 3410篇 |
2015年 | 4106篇 |
2014年 | 4017篇 |
2013年 | 4842篇 |
2012年 | 5315篇 |
2011年 | 5359篇 |
2010年 | 5334篇 |
2009年 | 4931篇 |
2008年 | 5039篇 |
2007年 | 4692篇 |
2006年 | 4455篇 |
2005年 | 3831篇 |
2004年 | 2933篇 |
2003年 | 2009篇 |
2002年 | 2017篇 |
2001年 | 1836篇 |
2000年 | 1549篇 |
1999年 | 785篇 |
1998年 | 337篇 |
1997年 | 237篇 |
1996年 | 187篇 |
1995年 | 149篇 |
1994年 | 117篇 |
1993年 | 121篇 |
1992年 | 111篇 |
1991年 | 66篇 |
1990年 | 81篇 |
1989年 | 50篇 |
1988年 | 28篇 |
1987年 | 42篇 |
1986年 | 45篇 |
1985年 | 31篇 |
1984年 | 38篇 |
1983年 | 30篇 |
1982年 | 23篇 |
1981年 | 12篇 |
1979年 | 27篇 |
1978年 | 11篇 |
1976年 | 9篇 |
1957年 | 19篇 |
1954年 | 30篇 |
1933年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 281 毫秒
151.
旅游生态效率是反映旅游经济与旅游环境双向效应(bi-effect)的重要判断指标。本文基于非期望产出视角,采用超效率SBM模型,评价了我国东部沿海18个主要滨海旅游城市的旅游生态效率,并分析了其时空演变特征。研究结果表明:(1)中国东部沿海18市滨海旅游生态效率整体呈中等水平,平均值处于0.727~0.894,阶段特征明显,呈现前期平稳发展,中期下降,后期回升的发展趋势;(2)各城市滨海旅游生态效率水平差异显著,分为下行风险型,持续成长型,稳定型和波动型4种发展类型。平均效率值最高为天津,最低为三亚;(3)分解效率均值中纯技术效率较高,规模效率次之,技术进步是影响综合效率的主要因素;(4)投入冗余和非期望产出冗余已经成为阻碍滨海旅游生态效率进步的关键因素。 相似文献
152.
Photoinduced toxicity single and binary mixtures of four polycyclic aromatic hydrocarbons to the marine diatom Skeletonema costatum 总被引:1,自引:0,他引:1
Diatom Skeletonema costatum Cleve is one of the main predominant phytoplankton species in the Changjiang Estuary in China. In order to provide some basic information for future assessment of the potential risk on phytoplankton communities in this estuary caused by polycyclic aromatic hydrocarbons(PAHs), this alga was selected as a representative to investigate the photoinduced toxicity of PAHs, in single and mixture. Four PAHs including three-ring phenanthrene and anthracene, four-ring fluoranthene and pyrene were tested in the laboratory. The single toxicity of each PAH on this microalga was compared with and without the simulated solar UV radiation. The results showed that this microalga was sensitive to PAH's photoinduced toxicity. Ratios of the 72 h median effect concentration obtained for fluorescent and UV light tests were about 8.4 for phenanthrene, 13.0 for anthracene, 6.5 for fluoranthene, and 5.7 for pyrene, indicating that UV light enhanced the PAH toxicity to this alga significantly. Under the fluorescent radiation (lacking UV), the dose-response curves based on chemical concentrations revealed that the order of toxic strength was fluoranthene greater than pyrene greater than anthracene greater than phenanthrene; while under the UV radiation (476 μW/cm2 for UVA, 6.5 μW/cm2 for UVB) it became fluoranthene approximately equaling anthracene greater than pyrene greater than phenanthrene, indicating that the UV light also changed its relative toxicity to this alga. The photoinduced toxicity of PAHs to the marine diatom S. costatum might be a synergistic effect of photosensitization reactions (e.g., generation of single-state oxygen) and photomodification (photooxidation and/or photolysis).The combined effects of six binary mixtures on the marine diatom S. costatum were investigated using the additive-index method. Four binary-mixtures (phenanthrene plus anthracene; phenanthrene plus pyrene; anthracene plus fluoranthene; anthracene plus pyrene) were found to be synergistic joint action mode, while two binary-mixtures (phenanthrene plus fluoranthene; fluoranthene plus pyrene) displayed antagonist, revealing a complex pattern of possible interactions. The combined action mode of PAHs might be related to various factors such as the mixture compounds, mixture ratios and test conditions, etc. 相似文献
153.
154.
本文根据胶州湾东北部岸滩目前污染严重、脏乱不堪的现状及城市总体规划的要求.提出了胶州湾东北部岸滩的改造意见.着重从海洋动力、生态、水量平衡的角度论证了改造工程的必要性和可行性。 相似文献
155.
A method combining boundary and finite element approaches as well as modal analysis to solve the acoustical scattering from elastic obstacle in infinite fluid is described. An elastic cylinder with finite length immersed in water is chosen as an example for numerical demonstration. The computed frequency response and direction pattern of the backscattering target strength of the cylinder are conformable to those measured. 相似文献
156.
Up to now, available method of numerical forecasting and suitable wind field model for the Bohai Sea storm surge have been few. In this paper, through the analysis of the weather situation is presented a mathematical model for the wind fields involved mainly the deformation field of a high pressure matched with a low pressure, the temperate cyclone, the cold wave and the northword typhoon. Meanwhile, numerical computations combined with the nonlinear storm surge models are made by using "ADI" method. The computed results are generally coincident with the practical observations. It has showed a success in the simulated wind field and the feasibility of using "ADI" method to forecast the Bohai Sea storm surge. 相似文献
157.
The soil permeability of many natural marine sediments decreases with depth because of consolidation under overburden pressure. This is accompanied by a decrease in porosity and void ratio that also affect the permeability. Conventional theories for wave-induced soil response have assumed a homogeneous porous seabed. This paper presents a new approach for the wave-induced response in a soil matrix, with variable permeability as a function of burial depth. The soil matrix considered is unsaturated and anisotropic, and is subject to a three-dimensional wave system. The pore pressure and effective stresses induced by such a system are obtained from a set of equations incorporating a variable permeability. Verification is available through reduction to the simple case of uniform permeability. The results indicate that the effect of variable soil permeability on pore pressure and vertical effective stress may be significant, especially in a gravelled seabed and for unsaturated sandy soils. 相似文献
158.
Relationship between biogeochemical features of biogenic elements and flocculation in the Changjiang Estuary 总被引:1,自引:3,他引:1
RelationshipbetweenbiogeochemicalfeaturesofbiogenicelementsandflocculationintheChangjiangEstuary¥LinYi'an;TangRenyou;LiYan;Do... 相似文献
159.
大连湾牡蛎的染色体数目 总被引:1,自引:0,他引:1
以大连湾牡蛎的早期胚胎为材料。经染色体数目分析,首次提出大连湾牡蛎的染色体数目为2n=20。为牡蛎遗传育种工作提供依据。 相似文献
160.
Using 203HgCl2 as a tracer, uptake and combination of inorganic mercury in Tilapia mossambica (petres) and its transfer between different tissues of the fish were studied. The results demonstrate that gill is the main tissue uptaking mercury directly from seawater and that the mercury in gill, skin, digestive tube and fin would be transferred to the other tissues. The result of gel chromatography with Sephadex G-75 shows that most of mercury in gill filaments, liver, and digestive tube are bound in high molecules (MW-70000) and not in metallothionein, but in kidney and bile mercury binding proteins are similar to metallothionein and are accounted for up to 60% and 46% respectively. 相似文献