全文获取类型
收费全文 | 5178篇 |
免费 | 976篇 |
国内免费 | 726篇 |
专业分类
测绘学 | 887篇 |
大气科学 | 616篇 |
地球物理 | 1551篇 |
地质学 | 1335篇 |
海洋学 | 746篇 |
天文学 | 396篇 |
综合类 | 557篇 |
自然地理 | 792篇 |
出版年
2024年 | 5篇 |
2023年 | 44篇 |
2022年 | 114篇 |
2021年 | 148篇 |
2020年 | 180篇 |
2019年 | 212篇 |
2018年 | 166篇 |
2017年 | 212篇 |
2016年 | 215篇 |
2015年 | 233篇 |
2014年 | 262篇 |
2013年 | 304篇 |
2012年 | 330篇 |
2011年 | 339篇 |
2010年 | 270篇 |
2009年 | 345篇 |
2008年 | 295篇 |
2007年 | 382篇 |
2006年 | 381篇 |
2005年 | 280篇 |
2004年 | 263篇 |
2003年 | 250篇 |
2002年 | 202篇 |
2001年 | 193篇 |
2000年 | 166篇 |
1999年 | 147篇 |
1998年 | 180篇 |
1997年 | 105篇 |
1996年 | 95篇 |
1995年 | 104篇 |
1994年 | 103篇 |
1993年 | 87篇 |
1992年 | 63篇 |
1991年 | 41篇 |
1990年 | 27篇 |
1989年 | 45篇 |
1988年 | 23篇 |
1987年 | 17篇 |
1986年 | 12篇 |
1985年 | 14篇 |
1984年 | 5篇 |
1983年 | 1篇 |
1982年 | 6篇 |
1981年 | 3篇 |
1980年 | 4篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1954年 | 3篇 |
排序方式: 共有6880条查询结果,搜索用时 515 毫秒
101.
102.
以往的地下水环境健康研究忽视了单个地下水功能对地下水环境健康的决定作用,因此当不同功能的健康程度差别较大时,地下水环境健康的模糊评价结果被平均化,而得到不合理的结论。提出分功能模糊评价方法,并用其评价郑州、开封、新乡和濮阳四城市地下水环境健康状况。结果表明:分功能模糊评价方法不仅可以得到研究区地下水环境的整体健康状况,而且能得到地下水环境不同的功能或属性的健康状况,能够反映研究区地下水的保护、开发和管理的重点。此外,虽然采用数学方法选择评价因子能够避免人类的主观臆断,提高工作效率,但它可能会忽略重要因子或引入扰动因子,所以需要人工检查数学方法筛选出的评价因子。客观赋权方法无法根据各因子的实际意义给出各因子的合理权重,因此需要人工检查和调整各因子权重的分配,从而避免得出错误的评价结果。 相似文献
103.
104.
105.
106.
Study on Probability Distributions of Multi-Timescale Aerosol Optical
Depth Using AERONET Data 下载免费PDF全文
The probability distribution analysis is per-formed for multi-timescale aerosol optical depth (AOD) using AErosol RObotic NETwork (AERONET) level 2.0 data.The maximum likelihood estimation is employed to determine the best-fit probability density function (PDF),and the statement that the fitting Weibull distribution will be light-tailed is proved true for these AOD samples.The best-fit PDF results for multi-site data show that the PDF of AOD samples with longer timescale in most sites tends to be stably represented by lognormal distribution,while Weibull distribution is a better fit for AOD samples with short timescales.The reason for this difference is ana-lyzed through tail characteristics of the two distributions,and an indicator for the selection between Weibull and lognormal distributions is suggested and validated.The result of this research is helpful for determining the most accurate AOD statistics for a given site and a given time-scale and for validating the retrieved AOD through its PDF. 相似文献
107.
108.
109.
Because a conventional isolation system with constant isolation frequency is usually a long‐period dynamic system, its seismic response is likely to be amplified in earthquakes with strong long‐period wave components, such as near‐fault ground motions. Seismic isolators with variable mechanical properties may provide a promising solution to alleviate this problem. To this end, in this work sliding isolators with variable curvature (SIVC) were studied experimentally. An SIVC isolator is similar to a friction pendulum system (FPS) isolator, except that its sliding surface has variable curvature rather being spherical. As a result, the SIVC's isolation stiffness that is proportional to the curvature becomes a function of the isolator displacement. By appropriately designing the geometry of the sliding surface, the SIVC is able to possess favorable hysteretic behavior. In order to prove the applicability of the SIVC concept, several prototype SIVC isolators, whose sliding surfaces are defined by a sixth‐order polynomial function, were fabricated and tested in this study. A cyclic element test on the prototype SIVC isolators and a shaking table test on an SIVC isolated steel frame were all conducted. The results of both tests have verified that the prototype SIVC isolators do indeed have the hysteretic property of variable stiffness as prescribed by the derived formulas in this study. Moreover, it is also demonstrated that the proposed SIVC is able to effectively reduce the isolator drift in a near‐fault earthquake with strong long‐period components, as compared with that of an FPS system with the same friction coefficient. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
110.