全文获取类型
收费全文 | 13655篇 |
免费 | 2698篇 |
国内免费 | 4312篇 |
专业分类
测绘学 | 1709篇 |
大气科学 | 2479篇 |
地球物理 | 2639篇 |
地质学 | 7092篇 |
海洋学 | 3303篇 |
天文学 | 491篇 |
综合类 | 1122篇 |
自然地理 | 1830篇 |
出版年
2024年 | 138篇 |
2023年 | 355篇 |
2022年 | 751篇 |
2021年 | 815篇 |
2020年 | 748篇 |
2019年 | 763篇 |
2018年 | 764篇 |
2017年 | 742篇 |
2016年 | 785篇 |
2015年 | 767篇 |
2014年 | 839篇 |
2013年 | 1004篇 |
2012年 | 969篇 |
2011年 | 1046篇 |
2010年 | 1003篇 |
2009年 | 934篇 |
2008年 | 906篇 |
2007年 | 888篇 |
2006年 | 841篇 |
2005年 | 733篇 |
2004年 | 584篇 |
2003年 | 431篇 |
2002年 | 447篇 |
2001年 | 491篇 |
2000年 | 404篇 |
1999年 | 349篇 |
1998年 | 276篇 |
1997年 | 242篇 |
1996年 | 230篇 |
1995年 | 194篇 |
1994年 | 181篇 |
1993年 | 154篇 |
1992年 | 157篇 |
1991年 | 127篇 |
1990年 | 92篇 |
1989年 | 86篇 |
1988年 | 69篇 |
1987年 | 62篇 |
1986年 | 48篇 |
1985年 | 39篇 |
1984年 | 34篇 |
1983年 | 29篇 |
1982年 | 40篇 |
1981年 | 19篇 |
1980年 | 11篇 |
1979年 | 16篇 |
1976年 | 10篇 |
1975年 | 7篇 |
1973年 | 6篇 |
1954年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
对于改进的Encke方法,选择适当的参考轨道是一个关键.然而,对于人造地球卫星长弧轨道计算,目前所给出的几种参考轨道均需要逐段校正,这将给定轨问题带来附加的复杂性.本文将仔细探讨如何选择参考轨道和减少校正次数. 相似文献
2.
Lin Liu 《Chinese Astronomy and Astrophysics》1997,21(4):488-500
With the aim of analyzing the errors in the radial position of satellites, we give in this paper the expressions for the radial perturbation which include the complete zeroth- and first-order terms in the orbital eccentricity. A simpler and faster method of calculating the perturbation is given. 相似文献
3.
1882年李希霍芬在太原西山建立石炭系的太原系含煤建造以来,石炭系与二叠系的分界意见,归纳起来有三:①以Pseudoschwagerina(s.1)带之底为界,下面为石炭系,上面为二叠系;②以晋东南的灰白色“佳祥砂岩”与太原西山黄绿色“骆驼脖子砂岩”对比,砂岩底面为石炭系与二叠系的分界线,下面是石炭系山西组,上面是二叠系石盒子组,山西组的地层时代定为晚石炭世;③以晋东南灰白色第三砂岩(陵川砂岩)与太原西山“北岔沟砂岩”对比,砂岩底面为石炭系与二叠系的分界线,下面是石炭系太原组,上面是二叠系山西组,山西组的地层时代为早二叠世。笔者认为石炭系与二叠系的划分,以第③方案较适宜。 相似文献
4.
5.
Numerical methods are usually used for the computation of ephemerides with perturbations for the precise orbital determination of an artificial satellite. But their numerical stability will be encountered in a long arc. In this case the use the improved Encke special perturbation methods has been suggested. The results of this paper show that Encke's method does indeed have a certain effectiveness, but cannot yet completely resolve the numerical stability, and the more efficient method is to use the energy integral or its variational relation to control the growth of the along-track error in general numerical calculations so that the aim of stabilization can be achieved. 相似文献
6.
7.
Hong Lin Zhang Douglas H. Sampson 《Monthly notices of the Royal Astronomical Society》2001,322(3):433-437
Calculations are made of the resonance contribution to electron-impact excitation of H-like 13 C and Li-like 23 Na, 25 Mg, 27 Al and 29 Si to the upper hyperfine levels that produce millimetre (mm) lines of likely astrophysical interest. The resonance contribution is found to be very important for these Li-like ions, considerably more important than for Li-like 57 Fe considered previously. However, resonances are found to be rather unimportant for H-like 13 C. The effect of radiative decay on the resonance contribution is found to be insignificant in all of the present calculations. 相似文献
8.
BIAN Jian-min LIN Nian-feng TANG Jie College of Environment Resources Jilin University Changchun Jilin China 《东北亚地学研究》2005,(Z1)
Desertification is an environmental issue in the world. The salt-alkalization desertification land area formed by both primary and secondary salt-alkalization has extended in a large scale, which has become a significant eco-environmental problem. Based on the characteristics of eco-environment and the situation of desertification in western Songnen plain, this paper reports the analyzes of its formation in background and cause. An early warning system on the salt-alkalization desertification is established and the GIS technology is used to abstract the information of desertification evaluation index. Supported by the integrated technology of the GIS and ANN, the orientation and quantitative result of desertification are gained, which is helpful to the eco-environment protection and resource development in western Songnen Plain. 相似文献
9.
10.
The paper presents a simple approximate analytical solution of the remote stresses that cause the collapse of a borehole or other circular cylindrical cavity in an infinite elastic space. Regions of parallel equidistant splitting cracks are assumed to form on the sides of the cavity. Their boundary is assumed to be an ellipse of a growing horizontal axis, the other axis remaining equal to the borehole diameter. The slabs of rock between the splitting cracks are assumed to buckle as slender columns, and their post-critical stress is considered as the residual stress in the cracked rock. The buckling of these slab columns is assumed to be resisted not only by their elastic bending stiffness but also shear stresses produced on rough crack faces by relative shear displacements. The energy release from the infinite medium caused by the growth of the elliptical cracking region is evaluated according to Eschelby's theorem. This release is set equal to the energy dissipated by the formation of all the splitting cracks, which is calculated under the assumption of constant fracture energy. This yields the collapse stress as a function of the elastic moduli, fracture energy, ratio of the remote principal stresses, crack shear resistance characteristic and borehole diameter. The collapse stress as a function of crack spacing is found to have a minimum, and the correct crack spacing is determined from this minimum. For small enough diameters, the crack spacing increases as the (4/5)-power of the borehole diameter, while for large enough diameters a constant spacing is approached. In contrast to plastic solutions, the breakout stress exhibits a size effect, such that for small enough diameters the breakout stress decreases as the (? 2/5)-power of the borehole diameter, while for large enough diameters a constant limiting value is approached. Finally, some numerical estimates are given and the validity of various simplifying assumptions made is discussed. 相似文献