全文获取类型
收费全文 | 7574篇 |
免费 | 1505篇 |
国内免费 | 2712篇 |
专业分类
测绘学 | 244篇 |
大气科学 | 2015篇 |
地球物理 | 1365篇 |
地质学 | 3842篇 |
海洋学 | 1679篇 |
天文学 | 1114篇 |
综合类 | 389篇 |
自然地理 | 1143篇 |
出版年
2024年 | 41篇 |
2023年 | 115篇 |
2022年 | 248篇 |
2021年 | 261篇 |
2020年 | 306篇 |
2019年 | 406篇 |
2018年 | 316篇 |
2017年 | 284篇 |
2016年 | 324篇 |
2015年 | 352篇 |
2014年 | 509篇 |
2013年 | 483篇 |
2012年 | 507篇 |
2011年 | 553篇 |
2010年 | 480篇 |
2009年 | 591篇 |
2008年 | 554篇 |
2007年 | 706篇 |
2006年 | 682篇 |
2005年 | 588篇 |
2004年 | 557篇 |
2003年 | 481篇 |
2002年 | 376篇 |
2001年 | 294篇 |
2000年 | 297篇 |
1999年 | 288篇 |
1998年 | 257篇 |
1997年 | 187篇 |
1996年 | 161篇 |
1995年 | 153篇 |
1994年 | 130篇 |
1993年 | 74篇 |
1992年 | 62篇 |
1991年 | 50篇 |
1990年 | 19篇 |
1989年 | 17篇 |
1988年 | 15篇 |
1987年 | 10篇 |
1986年 | 9篇 |
1985年 | 7篇 |
1984年 | 6篇 |
1983年 | 5篇 |
1982年 | 3篇 |
1981年 | 6篇 |
1977年 | 2篇 |
1976年 | 7篇 |
1973年 | 4篇 |
1972年 | 1篇 |
1971年 | 2篇 |
1954年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
821.
822.
P. C. Gregory † 《Monthly notices of the Royal Astronomical Society》2007,381(4):1607-1616
A Bayesian multiplanet Kepler periodogram has been developed for the analysis of precision radial velocity data. The periodogram employs a parallel tempering Markov chain Monte Carlo algorithm. The HD 11964 data have been re-analysed using 1, 2, 3 and 4 planet models. Assuming that all the models are equally probable a priori, the three planet model is found to be ≥600 times more probable than the next most probable model which is a two planet model. The most probable model exhibits three periods of 38.02+0.06 −0.05 , 360+4 −4 and 1924+44 −43 d , and eccentricities of 0.22+0.11 −0.22 , 0.63+0.34 −0.17 and 0.05+0.03 −0.05 , respectively. Assuming the three signals (each one consistent with a Keplerian orbit) are caused by planets, the corresponding limits on planetary mass ( M sin i ) and semimajor axis are respectively. The small difference (1.3σ) between the 360-d period and one year suggests that it might be worth investigating the barycentric correction for the HD 11964 data. 相似文献
823.
Peter Faber Alice C. Quillen 《Monthly notices of the Royal Astronomical Society》2007,382(4):1823-1828
Infrared spectra from the Spitzer Space Telescope ( SSC ) of many debris discs are well fit with a single blackbody temperature which suggest clearings within the disc. We assume that clearings are caused by orbital instability in multiple planet systems with similar configurations to our own. These planets remove dust-generating planetesimal belts as well as dust generated by the outer disc that is scattered or drifts into the clearing. From numerical integrations, we estimate a minimum planet spacing required for orbital instability (and so planetesimal and dust removal) as a function of system age and planet mass. We estimate that a 108 yr old debris disc with a dust disc edge at a radius of 50 au hosted by an A star must contain approximately five Neptune mass planets between the clearing radius and the iceline in order to remove all primordial objects within it. We infer that known debris disc systems contain at least a fifth of a Jupiter mass in massive planets. The number of planets and spacing required is insensitive to the assumed planet mass. However, an order of magnitude higher total mass in planets could reside in these systems if the planets are more massive. 相似文献
824.
825.
826.
This paper investigates the possibility of interpreting progressive shear failure in hard soils and soft rocks as the result of shear propagation of a pre‐existing natural defect. This is done through the application of the principles of fracture mechanics, a slip‐weakening model (SWM) being used to simulate the non‐linear zone at the tips of the discontinuity. A numerical implementation of the SWM in a computation method based on the boundary element technique of the displacement discontinuity method (DDM) is presented. The crack and the non‐linear zone at the advancing tip are represented through a set of elements, where the displacement discontinuity (DD) in the tangential direction is determined on the basis of a friction law. A residual friction angle is assumed on the crack elements. Shear resistance decreases on elements in the non‐linear zone from a peak value at the tip, which is characteristic of intact material, to the residual value. The simulation of a uniaxial compressive test in plane strain conditions is carried out to exemplify the numerical methodology. The results emphasize the role played by the critical DD on the mechanical behaviour of the specimen. A validation of the model is shown through the back analysis of some experimental observations. The results of this back analysis show that a non‐linear fracture mechanics approach seems very promising to simulate experimental results, in particular with regards to the shear band evolution pattern. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
827.
Jackie L. Cooper Geoffrey V. Bicknell Ralph S. Sutherland Joss Bland-Hawthorn 《Astrophysics and Space Science》2007,311(1-3):99-103
In order to better understand the formation of a starburst-driven wind, we have performed a series of three-dimensional hydrodynamical
simulations in an inhomogeneous interstellar medium. We present the results of these simulations, which provide new insights
into the formation of the optical filaments and the origin of the soft X-ray emission. 相似文献
828.
Artificial neural networks are used to predict the micro‐properties of particle flow code in three dimensions (PFC3D) models needed to reproduce macro‐properties of cylindrical rock samples in uniaxial compression tests. Data for the training and verification of the networks were obtained by running a large number of PFC3D models and observing the resulting macro‐properties. Four artificial networks based on two different architectures were used. The networks used different numbers of input parameters to predict the micro‐properties. Multi‐layer perceptron networks using Young's modulus, Poisson's ratio, uniaxial compressive strength, model particle resolution and the maximum‐to‐minimum particle ratio showed excellent performance in both training and verification. Adding one more variable—namely, minimum particle radius—showed degrading performance. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
829.
《海洋技术学报》2024,(1)
为提高基础利用率增加海上风电设施的可行性,对楔形单桩基础竖向承载力特性进行研究分析。采用PLAXIS 3D 有限元软件建立楔形单桩基础模型,从桩侧摩阻力、桩侧法应力及土体位移对比分析楔形单桩基础与等截面单桩竖向承载特性差异,并探讨内摩擦角、楔角及楔高对承载力的影响。研究表明:楔形单桩基础竖向承载力高于等截面单桩基础,且承载力随着楔角、楔高的增大而增大,提高率最大达24.786%。倾斜侧壁的引入改变了桩侧摩阻力的传递规律;倾斜侧壁挤密桩周土体,桩侧摩阻力与法向应力增大,从而有效提高单桩基础的竖向承载力。研究成果可为今后海上风电单桩基础截面型式的设计提供参考。 相似文献
830.