全文获取类型
收费全文 | 8289篇 |
免费 | 1280篇 |
国内免费 | 1213篇 |
专业分类
测绘学 | 1430篇 |
大气科学 | 1118篇 |
地球物理 | 1662篇 |
地质学 | 3956篇 |
海洋学 | 1113篇 |
天文学 | 80篇 |
综合类 | 487篇 |
自然地理 | 936篇 |
出版年
2024年 | 18篇 |
2023年 | 83篇 |
2022年 | 222篇 |
2021年 | 297篇 |
2020年 | 259篇 |
2019年 | 353篇 |
2018年 | 237篇 |
2017年 | 299篇 |
2016年 | 292篇 |
2015年 | 308篇 |
2014年 | 495篇 |
2013年 | 495篇 |
2012年 | 524篇 |
2011年 | 549篇 |
2010年 | 520篇 |
2009年 | 512篇 |
2008年 | 512篇 |
2007年 | 632篇 |
2006年 | 593篇 |
2005年 | 492篇 |
2004年 | 466篇 |
2003年 | 439篇 |
2002年 | 309篇 |
2001年 | 295篇 |
2000年 | 277篇 |
1999年 | 260篇 |
1998年 | 237篇 |
1997年 | 156篇 |
1996年 | 148篇 |
1995年 | 104篇 |
1994年 | 85篇 |
1993年 | 51篇 |
1992年 | 60篇 |
1991年 | 26篇 |
1990年 | 30篇 |
1989年 | 27篇 |
1988年 | 29篇 |
1987年 | 18篇 |
1986年 | 7篇 |
1985年 | 10篇 |
1984年 | 18篇 |
1983年 | 11篇 |
1982年 | 14篇 |
1981年 | 7篇 |
1980年 | 3篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
62.
In this paper, we focus on the status and trends of the current Korean fisheries management regime. Specifically, this paper briefly introduces the Korean conventional fisheries management regime (KCFMR) and discusses its problems and limitations. In describing policy evolution, this paper finds reasons why the Korean government has chosen a TAC system, an output control approach, besides input control approaches in force for almost a century. This paper also describes the evolution of the Korean TAC system, which is carrying out a pivotal role in Korean fisheries development, and analyzes problems of the Korean TAC system. Finally, this paper gives a perspective on expanding the Korean TAC system toward Output Control Systems (OCSs) such as Individual Quotas (IQs), Individual Vessel Quotas (IVQs), Individual Transferable Traps (ITTs), Community Quotas (CQs), and Individual Transferable Quotas (ITQs). 相似文献
63.
据开发应用水产饲料膨化机实践,初步论述了膨化机生产能力与动力匹配,螺杆设计参数长径比,物料在腔内滞留时间,膨化腔设计及其温度调控,并提出改进设计的见解. 相似文献
64.
65.
The tolerance of the oligochaete Monopylephorus rubroniveus and the polychaete Streblospio benedicti to sediment-associated fluoranthene was characterized under normoxic (>80% dissolved oxygen saturation) and moderately hypoxic ( approximately 50% dissolved oxygen saturation) conditions. Under both conditions, M. rubroniveus was highly tolerant of fluoranthene. Streblospio benedicti was considerably less tolerant of fluoranthene compared with M. rubroniveus. In addition, S. benedicti was less tolerant to fluoranthene under moderately hypoxic conditions, although no differences in sensitivity between the two oxygen conditions were observed based upon median lethal tissue residues. Bioaccumulation factors were higher for S. benedicti exposed to moderate hypoxia, suggesting that behavioral adaptations to compensate for the lower dissolved oxygen increased its bioaccumulation of fluoranthene. The results of the present laboratory study demonstrate that (1). changes in annelid tolerance to fluoranthene under varying oxygen conditions is a species-dependent phenomenon and (2). the differential tolerance of these two annelids to these combined stressors is consistent with their relative abundances in the field. 相似文献
66.
Nicolas Mazzella Josiane Molinet Agung Dhamar Syakti Jean-Claude Bertrand Pierre Doumenq 《Marine Chemistry》2007,103(3-4):304-317
This in situ study reports on the relevance of intact phospholipids as new biomarkers and highlights the limits of the classical fatty acid analysis after the hydrolysis of the polar lipid fraction. The analysis of polar lipid fractions revealed significant contributions of marine macrophytes with either non-phospholipid compounds (sulfoquinovosyl diacylglycerol) or with both non-bacterial phospholipid classes (phosphatidylcholine) and non-bacterial phospholipid molecular species (some molecular species of phosphatidylglycerol). The analysis of intact phospholipids showed also the predominance of gram-negative bacteria in sediments whereas the analysis of fatty acids alone cannot confirm such information. Lastly, as regard to the effects of petroleum hydrocarbons on the sedimentary bacterial communities, we observed a higher degree of saturation of the fatty acyl chains of the phospholipids extracted from sediment exposed to a massive and chronic crude oil contamination. Such a result was interpreted as an adaptive response resulting in higher membrane rigidity of the bacterial communities. 相似文献
67.
Adaptive optimal control of an autonomous underwater vehicle in the dive plane using dorsal fins 总被引:1,自引:0,他引:1
In this paper, adaptive control of low speed bio-robotic autonomous underwater vehicles (BAUVs) in the dive plane using dorsal fins is considered. It is assumed that the model parameters are completely unknown and only the depth of the vehicle is measured for feedback. Two dorsal fins are mounted in the horizontal plane on either side of the BAUV. The normal force produced by the fins, when cambered, is used for the maneuvering. The BAUV model considered here is non-minimum phase. An indirect adaptive control system is designed for the depth control using the dorsal fins. The control system consists of a gradient based identifier for online parameter estimation, an observer for state estimation, and an optimal controller. Simulation results are presented which show that the adaptive control system accomplishes precise depth control of the BAUV using dorsal fins in spite of large uncertainties in the system parameters. 相似文献
68.
针对具有外部持续扰动的线性系统,研究前馈-反馈最优控制律的设计问题。给出了最优控制律的存在唯一性条件。并提出了最优控制律的设计算法。利用滤波器解决了前馈控制的物理不可实现问题。仿真结果表明,此算法易于实现,与传统的反馈最优控制相比对抑制外部扰动具有较强的鲁棒性。 相似文献
69.
70.
Particularly Sensitive Sea Areas (PSSAs) are a tool designed to protect vulnerable marine areas from vessel-source pollution through measures approved by the governing body, the International Maritime Organisation. Recent developments triggered by the proposal to designate, amongst others, a large part of the Western European Atlantic a PSSA have brought the instrument to a decisive stage. Although the proposal appears to be lawful, its implications for the concept, which this paper seeks to assess, are questionable. The significance of the instrument is at risk if it is overused. As a consequence, new protective mechanisms should be contemplated. 相似文献