全文获取类型
收费全文 | 52147篇 |
免费 | 969篇 |
国内免费 | 425篇 |
专业分类
测绘学 | 1237篇 |
大气科学 | 3546篇 |
地球物理 | 10725篇 |
地质学 | 18099篇 |
海洋学 | 4560篇 |
天文学 | 12080篇 |
综合类 | 137篇 |
自然地理 | 3157篇 |
出版年
2021年 | 514篇 |
2020年 | 598篇 |
2019年 | 602篇 |
2018年 | 1245篇 |
2017年 | 1141篇 |
2016年 | 1393篇 |
2015年 | 837篇 |
2014年 | 1380篇 |
2013年 | 2649篇 |
2012年 | 1556篇 |
2011年 | 2117篇 |
2010年 | 1892篇 |
2009年 | 2617篇 |
2008年 | 2174篇 |
2007年 | 2191篇 |
2006年 | 2014篇 |
2005年 | 1533篇 |
2004年 | 1539篇 |
2003年 | 1463篇 |
2002年 | 1350篇 |
2001年 | 1204篇 |
2000年 | 1172篇 |
1999年 | 990篇 |
1998年 | 1000篇 |
1997年 | 946篇 |
1996年 | 830篇 |
1995年 | 833篇 |
1994年 | 747篇 |
1993年 | 639篇 |
1992年 | 609篇 |
1991年 | 575篇 |
1990年 | 669篇 |
1989年 | 566篇 |
1988年 | 536篇 |
1987年 | 677篇 |
1986年 | 569篇 |
1985年 | 729篇 |
1984年 | 837篇 |
1983年 | 788篇 |
1982年 | 684篇 |
1981年 | 691篇 |
1980年 | 565篇 |
1979年 | 546篇 |
1978年 | 549篇 |
1977年 | 502篇 |
1976年 | 487篇 |
1975年 | 488篇 |
1974年 | 456篇 |
1973年 | 497篇 |
1971年 | 290篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
A new approach to constraining seawater δ34S and sulphate concentration using francolite‐bound sulphate reveals an abrupt increase in δ34S to +50‰ around the Early–Middle Cambrian boundary. Such high δ34S values are best explained by increased rates of pyrite burial due to ocean anoxia coupled with an increased sensitivity of the ocean sulphate reservoir to perturbations due to low sulphate concentrations of 500–700 μgL?1. We argue that the spread of anoxic waters at this time was partly the result of greenhouse warming related to the eruption of the Kalkarindji Large Igneous Province of northern Australia and that it triggered the collapse of early metazoan reef ecosystems during the latest Early Cambrian. Mass extinctions of the last 260 Myr have all coincided with enhanced volcanic activity, while several are also associated with positive shifts in seawater δ34S. Extending this correlation back in time further implicates volcanically induced climate change as a major determining factor in biosphere evolution. Terra Nova, 18, 257–263, 2006 相似文献
82.
An approximate Riemann solver for the equations of relativistic magnetohydrodynamics (RMHD) is derived. The Harten–Lax–van Leer contact wave (HLLC) solver, originally developed by Toro, Spruce and Spears, generalizes the algorithm described in a previous paper to the case where magnetic fields are present. The solution to the Riemann problem is approximated by two constant states bounded by two fast shocks and separated by a tangential wave. The scheme is Jacobian-free, in the sense that it avoids the expensive characteristic decomposition of the RMHD equations and it improves over the HLL scheme by restoring the missing contact wave.
Multidimensional integration proceeds via the single step, corner transport upwind (CTU) method of Colella, combined with the constrained transport (CT) algorithm to preserve divergence-free magnetic fields. The resulting numerical scheme is simple to implement, efficient and suitable for a general equation of state. The robustness of the new algorithm is validated against one- and two-dimensional numerical test problems. 相似文献
Multidimensional integration proceeds via the single step, corner transport upwind (CTU) method of Colella, combined with the constrained transport (CT) algorithm to preserve divergence-free magnetic fields. The resulting numerical scheme is simple to implement, efficient and suitable for a general equation of state. The robustness of the new algorithm is validated against one- and two-dimensional numerical test problems. 相似文献
83.
A. G. Nikoghossian 《Astrophysics》2006,49(2):228-239
The problem of finding statistical averages which characterize the radiation field in a semi-infinite atmosphere has been
discussed by the author in a series of papers. In this paper some of the results obtained there are generalized to media of
finite optical thickness. The average number of scattering events undergone by a photon during its random walk in an atmosphere
and the average time required for this are determined. Three different types of averages are considered, depending on the
photon characteristics over which the averages are taken. Special attention is devoted to the asymptotic behavior of these
quantities for media with large optical depths taking the influence of continuum absorption into account.
__________
Translated from Astrofizika, Vol. 49, No. 2, pp. 263–275 (May 2006). 相似文献
84.
85.
Radiation exposure and Mission Strategies
for Interplanetary Manned Missions (REMSIM) 总被引:1,自引:0,他引:1
C. Cougnet N. B. Crosby S. Eckersley C. Foullon V. Guarnieri S. Guatelli D. Heynderickx A. Holmes-Siedle C. Lobascio S. Masiello P. Nieminen G. Parisi P. Parodi M. A. Perino M. G. Pia R. Rampini P. Spillantini V. Tamburini E. Tracino 《Earth, Moon, and Planets》2004,94(3-4):279-285
Cosmic radiation is an important problem for human interplanetary missions. The “Radiation Exposure and Mission Strategies for Interplanetary Manned Missions–REMSIM” study is summarised here. They are related to current strategies and countermeasures to ensure the protection of astronauts from radiation during interplanetary missions, with specific reference to: radiation environment and its variability; radiation effects on the crew; transfer trajectories and associated fluences; vehicle and surface habitat concepts; passive and active shielding concepts; space weather monitoring and warning systems. 相似文献
86.
A. G. Nikoghossian 《Astrophysics》2004,47(3):412-421
The approach proposed in the previous parts of this series of papers is used to solve the radiative transfer problem in scattering and absorbing multicomponent atmospheres. Linear recurrence relations are obtained for both the reflectance and transmittance of these kinds of atmospheres, as well as for the emerging intensities when the atmosphere contains energy sources. Spectral line formation in a one-dimensional inhomogeneous atmosphere is examined as an illustration of the possibility of generalizing our approach to the matrix case. It is shown that, in this case as well, the question reduces to solving an initial value problem for linear differential equations. Some numerical calculations are presented. 相似文献
87.
PatMoore 《成都信息工程学院学报》2002,17(4):289-299
尽管语法在交际教学法鼎盛时期一度遭到冷遇,但现在又重新被英语语言教学所接纳,作为一种新的语法教学方式,发现原则综合了传统归纳发和演绎法的长处,概述了发现原则的理论依据,指出发现原则旨在鼓励学习者积极参与而非被动接受,符合当前认知主义对二语学习的认识。 相似文献
88.
89.
Irina N Belskaya Vasilij G Shevchenko Yurij N Krugly Yurij S Efimov Alberto Cellino 《Icarus》2003,166(2):276-284
The first results of the observational program devoted to simultaneous investigation of asteroid polarimetric and photometric opposition phenomena are presented. UBVRI polarimetric and V-band photometric observations of the S-type Asteroid 20 Massalia and the E-type Asteroids 214 Aschera and 620 Drakonia were carried out in 1996-1999 down to phase angles of 0.08°, 0.7°, and 1.2°, correspondingly. The S-type Asteroid 20 Massalia is characterized by the pronounced brightness opposition surge with an amplitude larger than that observed for the E-type asteroids. A sharp peak of negative polarization at small phase angles was not observed for this asteroid. The value of polarization degree at phase angle α<1° is less than 0.5% for both S and E types. The negative polarization branches of S and especially E-asteroids have an asymmetrical shape. The phase angle at which the polarization minimum occurs is close to the angle at which non-linear increase begins in the asteroid magnitude phase curves. A relation of the observed effects to the mechanism of coherent backscattering is discussed. 相似文献
90.