全文获取类型
收费全文 | 1242篇 |
免费 | 122篇 |
国内免费 | 368篇 |
专业分类
测绘学 | 11篇 |
大气科学 | 4篇 |
地球物理 | 149篇 |
地质学 | 801篇 |
海洋学 | 53篇 |
天文学 | 643篇 |
综合类 | 23篇 |
自然地理 | 48篇 |
出版年
2024年 | 2篇 |
2023年 | 14篇 |
2022年 | 24篇 |
2021年 | 18篇 |
2020年 | 27篇 |
2019年 | 43篇 |
2018年 | 35篇 |
2017年 | 19篇 |
2016年 | 37篇 |
2015年 | 39篇 |
2014年 | 67篇 |
2013年 | 70篇 |
2012年 | 61篇 |
2011年 | 87篇 |
2010年 | 51篇 |
2009年 | 104篇 |
2008年 | 103篇 |
2007年 | 122篇 |
2006年 | 120篇 |
2005年 | 94篇 |
2004年 | 74篇 |
2003年 | 76篇 |
2002年 | 68篇 |
2001年 | 44篇 |
2000年 | 44篇 |
1999年 | 34篇 |
1998年 | 68篇 |
1997年 | 16篇 |
1996年 | 18篇 |
1995年 | 14篇 |
1994年 | 22篇 |
1993年 | 18篇 |
1992年 | 12篇 |
1991年 | 8篇 |
1990年 | 16篇 |
1989年 | 8篇 |
1988年 | 11篇 |
1987年 | 8篇 |
1986年 | 8篇 |
1985年 | 5篇 |
1984年 | 4篇 |
1983年 | 4篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1978年 | 6篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1973年 | 1篇 |
排序方式: 共有1732条查询结果,搜索用时 718 毫秒
101.
L. G. Kiseleva J. Colin B. Dauphole & P. Eggleton 《Monthly notices of the Royal Astronomical Society》1998,301(3):759-766
In this paper we present numerical results on the decay of small stellar systems under different initial conditions (multiplicity 3 ≤ N ≤ 10, and various mass spectra, initial velocities and initial configurations). The numerical treatment uses the CHAIN1 code (Mikkola &38; Aarseth). Particular attention is paid to the distribution of high-velocity escapers: we define these as stars with velocity above 30 km s−1 . These numerical experiments confirm that small N -body systems are dynamically unstable and produce cascades of escapers in the process of their decay. It is shown that the fraction of stars that escape from small dense stellar systems with an escape velocity greater than 30 km s−1 is ∼1 per cent for all systems treated here. This relatively small fraction must be considered in relation to the rate of star formation in the Galaxy in small groups: this could explain some moderately high-velocity stars observed in the Galactic disc and possibly some young stars with relatively high metallicity in the thick disc. 相似文献
102.
Melvyn B. Davies Richard Blackwell Vernon C. Bailey & Steinn Sigurdsson 《Monthly notices of the Royal Astronomical Society》1998,301(3):745-753
We consider the destructive effects of encounters between binaries and red giant stars in the Galactic Centre. Such encounters may explain the observed depletion of luminous red giants within the central 0.2 pc of the galaxy. We consider encounters involving 2- and 8-M⊙ red giants, and thus span the range of stellar masses contributing to the most luminous red giants observed in the Galactic Centre. To explore the phase space of encounters thoroughly, we simulate 18 × 103 encounters using a modified four-body code in which the red giant core and components of the binary are treated as point masses, and where the envelope configuration is assumed to remain static throughout the encounter. We then rerun a small number of encounters with a smoothed particle hydrodynamics (SPH) code to confirm the reliability of conclusions drawn from the four-body runs. We see two possible pathways to red giant destruction. A large fraction of encounters lead to the formation of common-envelope systems, where two compact objects (drawn from the red giant core and the components of the original binary) form a binary within a common gaseous envelope, whilst the third body is ejected. The destruction of the red giant will then follow when the envelope is ejected as the binary hardens. In a smaller number of encounters, the intruding binary passes through the star and ejects the red giant core from the envelope. The red giant envelope will then disperse on short time-scales. We compute the time-scales for both of these processes to occur in the Galactic Centre for a variety of binary populations. 相似文献
103.
The short-term evolution of spheroidal galaxies has been explored by S. Chandrasekhar et al. and G. S. Sunder et al. In this paper, we study their long-term evolution with Laskar's method of frequency analysis. The main new results are as follows: (1) There exists a unique equilibrium, which is spherically symmetric. This equilibrium has a critical linear stability. (2) Generally speaking, the semi-axes exhibit quasiperiodic or nearly quasi-periodic (in a time scale longer than a Hubble time) oscillations around the radius of the above-mentioned equilibrium, so the equilibrium is practically stable. (3) There are cases in which one of the semi-axes tends fast to zero while the other to some finite value. The limit state is generally planar rather than linear, i.e. it is the symmetric semi-axis that tends to zero. This implies that some disk galaxies may have originated from spheroidal pregalaxy material. 相似文献
104.
Transient electromagnetic (TEM), self-potential (SP) and geoelectrical mapping measurements were carried out at the Chernorud-Mukhor site in the Priolkhonje area on the western shore of the Lake Baikal. All measurements were made along several profiles across the main strike of the regional Primorsky fault. TEM measurements were carried out in a time range from a few tens of microseconds to several tens of milliseconds. The most important result of the 1D modelling of TEM soundings is the discovery of nearly horizontal boundaries that divide high resistive overlying and well conducting underlying rocks. The resistivity of the former is in the range from 100 Ωm to 1000 Ωm, while the resistivity of the latter varies from less than 1 Ωm to several tens of Ωm. This good conductive zone could also be verified by geoelectrical mapping using Schlumberger array (AB/2=100 m). Due to high conductivity of the underlying rocks only the upper boundary of the conductive layer could be determined by TEM soundings. A regional SP anomaly with amplitude of about −450 mV has also been observed above the low resistivity zone indicating the electron nature of its conductance. Geologically, the conductive zone is represented by a graphite-bearing layer within the region of archean rocks. Since that layer extends over a large area, it may be used as a key in studying structures and tectonics of the Priolkhonje area. A 1D TEM geoelectric section shows a wide, gently sloping syncline as a probable base structure of the Chernorud-Mukhor site. Neotectonic faults divide the syncline into vertically displaced blocks that form a wide complicated graben with a total amplitude of about 250 m. 相似文献
105.
地震前兆复杂性成因机理研究的讨论(一):地震前兆复杂性的表现形式 总被引:10,自引:2,他引:8
通过国内外成功与失败的震例分析,介绍了地震前兆共性的一面,如在台网具有一定监测能力的地区,如果发生6级以上地震,震前总能或多或少地观测到异常现象。地震越大震前异常越多,分布越广,持续时间越长,异常幅度也较大,同时章着重对前兆复杂性的表现形式进行了较系统的归纳,整理,主要从以下几方面进行了分析,异常与地震关系的不确定性,主要表现在除了“有异常有地震”和“无异常无地震”的理论情况外,还存在“有异常无 相似文献
106.
地震前兆复杂性成因机理研究的讨论(二):地震前兆复杂性成因机理 总被引:8,自引:2,他引:6
针对前兆复杂性的种种表现,作从震源、构造应力场、断裂力学、前兆监测等方面探讨了前兆复杂性产生的可能原因。认为前兆复杂性是客观存在的,与共性一起构成前兆的二重性,因此在研究前兆时,既要研究共性,也要研究复杂性,才能更好地掌握或认识前兆。同时从复杂性中再寻求普遍性,进一步完善孕震理论和开发新的分析预报方法;造成前兆复杂性的根本原因是孕震的物理力学过程的复杂性,其中包括地质构造、孕震环境、动力学过程、 相似文献
107.
General absorbing boundary conditions based on Biot's two-phase mixture theory and paraxial approximation is presented for the dynamic analysis of fluid-saturated porous media with isotropic, transverse isotropic, and anisotropic properties. For the last two cases, the equivalent Lame's constants, under conditions of uniqueness, are introduced to facilitate the analytical solutions. The numerical results show that the proposed absorbing boundary can greatly suppress spuriously reflected waves and efficiently model the far field of the system with sufficient accuracy. 相似文献
108.
遗传规划在岩石力学中的应用 总被引:1,自引:0,他引:1
本文探讨了遗传规划这一新的优化方法在岩石力学参数确定中的应用。由于影响岩石力学性能的各因素之间关系不明确,而遗传规划特别适用于各影响因素之间因果关系不明确的复杂非线性问题。它为预测岩石力学性能提供了一条新的技术途径。通过例子,说明该方法的实用性。 相似文献
109.
Adrián Brunini 《Monthly notices of the Royal Astronomical Society》1998,293(4):405-410
In this paper we consider the dynamical evolution and orbital stability of objects in the asteroid belt. A simple physical model, including full gravitational perturbations from both giant planets, is used to compute the dynamical evolution of 1000 test particles simulating the primitive asteroids. The criterion of planet crossing (or close approach in the case of resonant objects) is used to reject particles from the simulation. 44 per cent of the particles survived for the whole time-span covered by the numerical integration (∼109 yr).
The 4:1, 3:1 and to a lesser extent the 2:1 Kirkwood gaps are formed in ∼107 yr of evolution, representing direct numerical evidence about their gravitational origin.
We found that the rms eccentricity and inclination of the sample experience a fast increase during the first 106 yr. The final rms eccentricity is 0.11, ∼60 per cent smaller than the present rms eccentricity (0.17). Nevertheless, the gravitational action of the giant planets suffices to prevent the formation of large objects, allowing catastrophic collisions and the subsequent depletion of material from this zone of the Solar system. The excited eccentricity by Jupiter and Saturn may favour mutual encounters and the further increase of the relative velocities up to their present values. 相似文献
The 4:1, 3:1 and to a lesser extent the 2:1 Kirkwood gaps are formed in ∼10
We found that the rms eccentricity and inclination of the sample experience a fast increase during the first 10
110.
We present a hybrid technique of N -body simulation to deal with collisionless stellar systems having an inhomogeneous global structure. We combine a treecode and a self-consistent field code such that each of the codes models a different component of the system being investigated. The treecode is suited to treatment of dynamically cold or clumpy components, which may undergo significant evolution within a dynamically hot system. The hot system is appropriately evolved by the self-consistent field code. This combined code is particularly suited to a number of problems in galactic dynamics. Applications of the code to these problems are briefly discussed. 相似文献