全文获取类型
收费全文 | 28269篇 |
免费 | 5132篇 |
国内免费 | 6829篇 |
专业分类
测绘学 | 1851篇 |
大气科学 | 5702篇 |
地球物理 | 6884篇 |
地质学 | 14177篇 |
海洋学 | 3823篇 |
天文学 | 1251篇 |
综合类 | 3052篇 |
自然地理 | 3490篇 |
出版年
2024年 | 161篇 |
2023年 | 545篇 |
2022年 | 1256篇 |
2021年 | 1460篇 |
2020年 | 1271篇 |
2019年 | 1366篇 |
2018年 | 1584篇 |
2017年 | 1407篇 |
2016年 | 1669篇 |
2015年 | 1347篇 |
2014年 | 1697篇 |
2013年 | 1720篇 |
2012年 | 1625篇 |
2011年 | 1726篇 |
2010年 | 1592篇 |
2009年 | 1560篇 |
2008年 | 1400篇 |
2007年 | 1360篇 |
2006年 | 1100篇 |
2005年 | 1134篇 |
2004年 | 839篇 |
2003年 | 808篇 |
2002年 | 814篇 |
2001年 | 798篇 |
2000年 | 867篇 |
1999年 | 1237篇 |
1998年 | 1011篇 |
1997年 | 1072篇 |
1996年 | 1000篇 |
1995年 | 849篇 |
1994年 | 721篇 |
1993年 | 647篇 |
1992年 | 525篇 |
1991年 | 406篇 |
1990年 | 313篇 |
1989年 | 259篇 |
1988年 | 234篇 |
1987年 | 172篇 |
1986年 | 132篇 |
1985年 | 100篇 |
1984年 | 75篇 |
1983年 | 62篇 |
1982年 | 65篇 |
1981年 | 40篇 |
1980年 | 47篇 |
1979年 | 35篇 |
1978年 | 17篇 |
1976年 | 19篇 |
1975年 | 17篇 |
1958年 | 20篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
12.
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
13.
在文献」1「4 基础上,根据近年文选址实践中提出的问题,先进一步讨论了大气相干直径和时间的的物理和意义,然后强调了自由大气湍流的天文成像效应及选址中进行研究和测量的重要性。最后对差分像运动方法测量视宁度的定标和系统误差等问题作了分析。 相似文献
14.
15.
16.
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
17.
THECONSTRUCTIONANDITSDEVELOPMENTOFTHEOVERSEASTRANSPORTSYSTEMINNORTHEASTCHINAGaoShali(高莎丽)(DepartmentofGeography,NortheastNorm... 相似文献
18.
APPLICATION OF GEOGRAPHICAL PARAMETER DATABASE TO ESTABLISHMENT OF UNIT POPULATION DATABASE 总被引:1,自引:1,他引:0
Now GIS is turning into a good tool in handling geographical, economical, and population data, so we can obtain more and more information from these data. On the other hand, in some cases, for a calamity, such as hurricane, earthquake, flood, drought etc., or a decision-making, such as setting up a broadcasting transmitter, building a chemical plant etc., we have to evaluate the total population in the region influenced by a calamity or a project. In this paper, a method is put forward to evaluate the population in such special region. Through exploring the correlation of geographical parameters and the distribution of people in the same region by means of quantitative analysis and qualitative analysis, unit population database (1km× 1km) is established. In this way, estimating the number of people in a special region is capable by adding up the population in every grid involved in this region boundary. The geographical parameters are obtained from topographic database and DEM database on the scale of 相似文献
19.
A THREE-DIMENSIONAL ELASTIC NESTED-GRID MESO-(β-γ)SCALE ATMOSPHERIC MODEL——PARTI:MODEL DESCRIPTION
下载免费PDF全文
![点击此处可从《Acta Meteorologica Sinica》网站下载免费的PDF全文](/ch/ext_images/free.gif)
A three-dimensional elastic nonhydrostatic mesoscale(β-γ)model with nested-grid is presented.It uses a set of fullequations in terrain-following coordinates as its basic dynamic frame,which is solved with a time-splitting algorithmfor acoustic and gravity waves.The model physical parameterization includes a K-theory subgrid eddy mixing for cloudand free atmosphere,a bulk planetary boundary layer parameterization,and three types of sofisticated cloudmicrophysics schemes with double-parameters for hail-bearing clouds,warm clouds and snowing clouds respectively.The model is designed to be used flexibly for simulations of a variety of meso-and small-scale atmospheric processes,and can be improved as a regional and local operational NWP system in future. 相似文献
20.
本文对徐州地震台JB—72水平摆倾斜仪1988年11月至1989年5月间所记录到的24次同震阶变进行了深入分析,认为:a.造成同震阶变的原因是多方面的,而反应了地球应变场准静态调整的同震阶变才是有价值的价变异常;b.多观测手段的地震台站,同震阶变应该在不止一台仪器上得到响应。 相似文献