全文获取类型
收费全文 | 82242篇 |
免费 | 1000篇 |
国内免费 | 495篇 |
专业分类
测绘学 | 2008篇 |
大气科学 | 5033篇 |
地球物理 | 15477篇 |
地质学 | 32330篇 |
海洋学 | 6846篇 |
天文学 | 18009篇 |
综合类 | 314篇 |
自然地理 | 3720篇 |
出版年
2022年 | 599篇 |
2021年 | 923篇 |
2020年 | 1036篇 |
2019年 | 1162篇 |
2018年 | 4208篇 |
2017年 | 3820篇 |
2016年 | 3351篇 |
2015年 | 1176篇 |
2014年 | 2186篇 |
2013年 | 3476篇 |
2012年 | 3086篇 |
2011年 | 4837篇 |
2010年 | 4262篇 |
2009年 | 4940篇 |
2008年 | 4246篇 |
2007年 | 4782篇 |
2006年 | 2914篇 |
2005年 | 2015篇 |
2004年 | 1966篇 |
2003年 | 2050篇 |
2002年 | 1806篇 |
2001年 | 1530篇 |
2000年 | 1353篇 |
1999年 | 1060篇 |
1998年 | 1082篇 |
1997年 | 1121篇 |
1996年 | 878篇 |
1995年 | 874篇 |
1994年 | 789篇 |
1993年 | 698篇 |
1992年 | 652篇 |
1991年 | 687篇 |
1990年 | 731篇 |
1989年 | 634篇 |
1988年 | 601篇 |
1987年 | 640篇 |
1986年 | 579篇 |
1985年 | 776篇 |
1984年 | 813篇 |
1983年 | 802篇 |
1982年 | 741篇 |
1981年 | 692篇 |
1980年 | 672篇 |
1979年 | 590篇 |
1978年 | 597篇 |
1977年 | 574篇 |
1976年 | 507篇 |
1975年 | 514篇 |
1974年 | 495篇 |
1973年 | 554篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
A numerical scheme is developed in order to simulate fluid flow in three dimensional (3‐D) microstructures. The governing equations for steady incompressible flow are solved using the semi‐implicit method for pressure‐linked equations (SIMPLE) finite difference scheme within a non‐staggered grid system that represents the 3‐D microstructure. This system allows solving the governing equations using only one computational cell. The numerical scheme is verified through simulating fluid flow in idealized 3‐D microstructures with known closed form solutions for permeability. The numerical factors affecting the solution in terms of convergence and accuracy are also discussed. These factors include the resolution of the analysed microstructure and the truncation criterion. Fluid flow in 2‐D X‐ray computed tomography (CT) images of real porous media microstructure is also simulated using this numerical model. These real microstructures include field cores of asphalt mixes, laboratory linear kneading compactor (LKC) specimens, and laboratory Superpave gyratory compactor (SGC) specimens. The numerical results for the permeability of the real microstructures are compared with the results from closed form solutions. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
22.
23.
V. G. Gorbatskii 《Astrophysics》1990,32(2):149-154
Leningrad State University. Translated from Astrofizika, Vol. 32, No. 2, pp. 267–275, March–April, 1990. 相似文献
24.
25.
J. V. WALTHER 《Journal of Metamorphic Geology》1996,14(3):351-360
ABSTRACT With increasing temperature during prograde metamorphism reactions will occur first at the lithological contacts of mixed pelite and calcsilicate terranes. At these interfaces, a fluid of lower chemical potential of H2O and CO2 than that required to produce a fluid in either layer can be produced whether reaction is caused by fluid infiltration or is initially fluid absent. If the interface region does not allow fluid transport then as temperature increases, a fluid pressure greater than lithostatic can develop. At some degree of over-pressure relative to rock pressure, the fluid hydraulically fractures the rock and a gradient in fluid composition away from the contact can be produced. These phenomena occur at the compositional interfaces whenever univariant reactions in the differing layers cross on a temperature vs. mole fraction of CO2 diagram with slopes of opposite sign. The first occurrence of these reaction products at lithological contacts delineates an isograd that defines temperature as well as the mole fraction of CO2 at constant pressure in systems open to fluid transport. These isograds can be contrasted with fluid-producing isograds in closed systems. As an illustration of possible effects, the reactions quartz + clinozoisite + muscovite = anorthite + K-feldspar + H2O and phlogopite + quartz + calcite = tremolite + K-feldspar + H2O + CO2 at 4 kbar are analysed and equations for fluid production and transport are developed. 相似文献
26.
27.
28.
We report intermediate resolution H spectroscopy of the black hole candidate Nova Muscae 1991 during quiescence. We classify the companion star as a K3-K4V which contributes 85±6 percent to the total flux from the binary. The photospheric absorption lines are broadened by 106±13 kms−1 with respect to template field stars, leading to a system mass ratio of q =M1/M2 = 7.8−2.0+3.4. Doppler imaging of the H line shows strong emission coming from the secondary star (EW=3.1±0.6Å) which we associate with chromospheric activity. However, the hot-spot is not detected and this may indicate a lower mass transfer rate than in other X-ray transients of comparable orbital periods. The surface brightness distribution of the accretion disk in H follows a relation I∝R−1.1, less steep than typically observed in cataclysmic variables. Updated system parameters are also presented. 相似文献
29.
The dynamics of the vortex lattice in the inner crust of a neutron star is considered. A general equation of motion is obtained
and solved under the assumption that there are regions of pinned and of free vortices. By comparing these solutions with observational
data for the Vela pulsar, the relative moments of inertia of regions of relaxation with the corresponding characteristic times
are calculated for two model stars with different equations of state. It is shown that the theory can be reconciled with observations
of the relaxation of pulsar angular velocity only for model stars with extremely stiff equations of state.
Translated from Astrofizika, Vol. 40, No. 1, pp. 67–76, January–March, 1997. 相似文献
30.
Harker David E. Woodward Charles E. McMURTRY Craig W. Goetz J. A. Pipher J. L. Forrest W. J. 《Earth, Moon, and Planets》1997,78(1-3):259-264
We present 1- to 5-μm broadband and CVF images of comet Hale-Bopp taken 1997 February 10.5 UT, 50 days before perihelion.
All the images exhibit a nonspherical coma with a bright “ridge” in the direction of the dust tail approximately 10″ from
the coma. Synthetic aperture spectrophotometry implies that the optically important grains are of a radius ≤0.4 μm; smallest
radius for any comet seen to date. The variation of the integrated surface brightness with radial distance from the coma (ρ)
in all the images closely follows the “steady state” ρ−1 model for comet dust ablation (Gehrz and Ney, 1992). The near-infrared colors taken along the dust tail are not constant
implying the dust grain properties vary with coma distance.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献