全文获取类型
收费全文 | 6793篇 |
免费 | 1167篇 |
国内免费 | 1431篇 |
专业分类
测绘学 | 727篇 |
大气科学 | 1262篇 |
地球物理 | 1294篇 |
地质学 | 3257篇 |
海洋学 | 1039篇 |
天文学 | 254篇 |
综合类 | 656篇 |
自然地理 | 902篇 |
出版年
2024年 | 59篇 |
2023年 | 208篇 |
2022年 | 230篇 |
2021年 | 227篇 |
2020年 | 212篇 |
2019年 | 413篇 |
2018年 | 263篇 |
2017年 | 165篇 |
2016年 | 156篇 |
2015年 | 200篇 |
2014年 | 320篇 |
2013年 | 247篇 |
2012年 | 273篇 |
2011年 | 244篇 |
2010年 | 295篇 |
2009年 | 303篇 |
2008年 | 281篇 |
2007年 | 280篇 |
2006年 | 314篇 |
2005年 | 290篇 |
2004年 | 319篇 |
2003年 | 288篇 |
2002年 | 273篇 |
2001年 | 306篇 |
2000年 | 423篇 |
1999年 | 377篇 |
1998年 | 194篇 |
1997年 | 235篇 |
1996年 | 270篇 |
1995年 | 235篇 |
1994年 | 223篇 |
1993年 | 160篇 |
1992年 | 161篇 |
1991年 | 136篇 |
1990年 | 146篇 |
1989年 | 97篇 |
1988年 | 66篇 |
1987年 | 56篇 |
1986年 | 37篇 |
1985年 | 41篇 |
1984年 | 60篇 |
1983年 | 42篇 |
1982年 | 48篇 |
1981年 | 41篇 |
1980年 | 17篇 |
1979年 | 12篇 |
1978年 | 11篇 |
1964年 | 30篇 |
1958年 | 11篇 |
1957年 | 14篇 |
排序方式: 共有9391条查询结果,搜索用时 15 毫秒
71.
渤海湾海河口区是北京、天津二大城市工业及生活污水排放入海处,是一比较典型的海域。1972年以来曾对渤海湾进行过综合性的环境污染调查和研究,但由于受调查范围和项目的限制,对河口区特别是大沽口、北塘口区有机污染物的迁移转化研究甚少,尚不足以就有机污染对河口生态环境的潜在影响作出科学判断。目前,有机污染已成为海河口区特别是其附近大城市的主要污染问题之一。本文以化学耗氧量(COD)、生物耗氧量(BOD5)、溶解氧(DO)等项目作为耗氧有机物污染程度评价的主要指标(还有氮、磷、本集另有专文讨论),对海河口区有机污染物的迁移、转化及生态效应进行了调查研究,为有机污染评价提供数据并作初步探讨。 相似文献
72.
73.
Fuzzy Optimum Model of Semi-Structural Decision for Lectotype Optimization of Offshore Platforms 总被引:8,自引:0,他引:8
In the process of concept design of offshore platforms, it is necessary to select the best from feasible alternatives through comparison and filter. The criterion set, used to evaluate and select the satisfying alternative, consists of many qualitative and quantitative factors. Therefore, the selection is a problem of multicriteria and semi- structural decision- making. Different from traditional methods in semi- structural decision- making, a new framework and methodology is presented in this paper for evaluation of offshore platform alternatives. First, the criterion set is established for the evaluation of alternatives. Next, the approach is studied to construct the relative membership degree matrix, in which both qualitative and quantitative factors are consistent with the uniform calculating standard. And then a new weight-assessing method is developed for calculation of the weights based on the relative membership degree matrix. Finally, a multi-hierarchy fuzzy optimum model is adopted to select t 相似文献
74.
The effects of different lateral confinement stress on the fatigue behavior of and cumulative damage to plain concrete are investigated experimentally. Eighty 100 mm x 100 mm x 100 mm specimens of ordinary strength concrete are tested under constant- or variable-amplitude fatigue loading and lateral confinement pressure in two orthogonal directions. A fatigue equation is developed by modifying the classical Aas-Jakobsen S-N equation for taking into account the effect of the confined stress on fatigue strength of plain concrete. The results of variable-amplitude fatigue tests indicate that the linear damage theory proposed by Palmgren and Miner is unreasonable in the biaxial stress state. A nonlinear cumulative damage model that could model the effects of the magnitude and sequence of variable-amplitude fatigue loading and lateral confinement pressure is proposed on the basis of the evolution laws of the residual strains in the longitudinal direction during fatigue tests. The residual fatigue life predic 相似文献
75.
21世纪是海洋世纪。中国加入世界贸易组织,加大了向海外开放的力度。世界《21世纪议程》将海洋作为实施人类社会可持续发展战略的重点领域,在全球掀起了一场开发海洋的“蓝色革命”。“九五”之初,国务院批准了《联合国海洋法公约》在我国的实施,制定了《中国海洋21世纪议程》,确立了海洋经济发展战略。当前沿海各省市区制定“十五”规划,都重视海洋产业规划和海洋经济区的建设。例如,江苏和山东分别提出建设“海上苏东”和“海上山东”的设想。福建和海南分别提出建设“海上强省”和“海上大省”的战略。21世纪20年代,中… 相似文献
76.
The effect of the cone spacing of a conieal structure on the ice force is studied by model experiments. The ice force reduction coefficient presented in this paper expresses the relationship between the ice force and the arrangement of cones. The experiments prove that the mode of the ice failure before the boundary of upward-downward bending cone (UDBC) is crushing. A conclusion can also be drawn from the experiments that the ice force on the boundary of UDBC is by far less than that on a vertical pile with the same diameter. Moreover, the ice force frequencies on upright and inve-rted cones are obtained under the field condition of the platform JZ20-2, respectively. The results show that the alterna-tion of the ice force on UDBC can hardly induce resonance of platform JZ20-2. 相似文献
77.
罗非鱼、南美白对虾对重金属富集的研究 总被引:11,自引:0,他引:11
对广东省养殖的罗非鱼Oreochromis niloticus及南美白对虾Litopeneaus vannamei体内Pb、Cd、Hg、As 4种重金属富集及分布状况进行研究。比较了不同规格罗非鱼及南美白对虾体内重金属差异。分析重金属在罗非鱼及南美白对虾不同部位的分布状况。结果表明,罗非鱼及南美白对虾体内Pb、Cd、Hg、As均有不同程度的检出,但含量均低于国家限定标准(NY5125-2002);其中Pb的含量最高,罗非鱼和南美白对虾体内Pb含量最高值分别为限量的39.32%和42.18%;重金属含量因其规格大小及部位而异,趋势为大规格样品重金属含量相对较高,重金属含量在罗非鱼及南美白对虾体内的增长情况与其体重具有正相关性;罗非鱼内脏重金属含量明显高于肌肉组织,虾头为虾重金属富集的主要部位。 相似文献
78.
79.
Following Bagnold's approach, a relationship between sediment transport and energy dissipation is developed. The major assumption made in the study is that the near bed velocity plays a dominant role in the process of sediment transport. A general relationship between energy dissipation and sediment transport is first proposed. Then the equations for total sediment transport are derived by introducing the appropriate expression of energy dissipation rate under different conditions, such as open channel flows, combination of wave and current, as well as longshore sediment transport. Within the flows investigated, the derived relationships are fairly consistent with the available data over a wide range of conditions. 相似文献
80.