全文获取类型
收费全文 | 1352篇 |
免费 | 107篇 |
国内免费 | 385篇 |
专业分类
测绘学 | 11篇 |
大气科学 | 11篇 |
地球物理 | 195篇 |
地质学 | 827篇 |
海洋学 | 54篇 |
天文学 | 645篇 |
综合类 | 28篇 |
自然地理 | 73篇 |
出版年
2024年 | 4篇 |
2023年 | 13篇 |
2022年 | 21篇 |
2021年 | 15篇 |
2020年 | 24篇 |
2019年 | 46篇 |
2018年 | 35篇 |
2017年 | 19篇 |
2016年 | 43篇 |
2015年 | 43篇 |
2014年 | 67篇 |
2013年 | 69篇 |
2012年 | 62篇 |
2011年 | 94篇 |
2010年 | 56篇 |
2009年 | 114篇 |
2008年 | 110篇 |
2007年 | 130篇 |
2006年 | 127篇 |
2005年 | 98篇 |
2004年 | 80篇 |
2003年 | 86篇 |
2002年 | 80篇 |
2001年 | 46篇 |
2000年 | 48篇 |
1999年 | 36篇 |
1998年 | 70篇 |
1997年 | 18篇 |
1996年 | 24篇 |
1995年 | 17篇 |
1994年 | 22篇 |
1993年 | 22篇 |
1992年 | 14篇 |
1991年 | 10篇 |
1990年 | 17篇 |
1989年 | 8篇 |
1988年 | 12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1844条查询结果,搜索用时 93 毫秒
111.
112.
We obtain the viscous stirring and dynamical friction rates of planetesimals with a Rayleigh distribution of eccentricities and inclinations, using three-body orbital integration and the procedure described by Ohtsuki (1999, Icarus137, 152), who evaluated these rates for ring particles. We find that these rates based on orbital integrations agree quite well with the analytic results of Stewart and Ida (2000, Icarus 143, 28) in high-velocity cases. In low-velocity cases where Kepler shear dominates the relative velocity, however, the three-body calculations show significant deviation from the formulas of Stewart and Ida, who did not investigate the rates for low velocities in detail but just presented a simple interpolation formula between their high-velocity formula and the numerical results for circular orbits. We calculate evolution of root mean square eccentricities and inclinations using the above stirring rates based on orbital integrations, and find excellent agreement with N-body simulations for both one- and two-component systems, even in the low-velocity cases. We derive semi-analytic formulas for the stirring and dynamical friction rates based on our numerical results, and confirm that they reproduce the results of N-body simulations with sufficient accuracy. Using these formulas, we calculate equilibrium velocities of planetesimals with given size distributions. At a stage before the onset of runaway growth of large bodies, the velocity distribution calculated by our new formulas are found to agree quite well with those obtained by using the formulas of Stewart and Ida or Wetherill and Stewart (1993, Icarus106, 190). However, at later stages, we find that the inclinations of small collisional fragments calculated by our new formulas can be much smaller than those calculated by the previously obtained formulas, so that they are more easily accreted by larger bodies in our case. The results essentially support the previous results such as runaway growth of protoplanets, but they could enhance their growth rate by 10-30% after early runaway growth, where those fragments with low random velocities can significantly contribute to rapid growth of runaway bodies. 相似文献
113.
I. Charvátová 《Annales Geophysicae》2000,18(4):399-405
A solar activity cycle of about 2400 years has until now been of uncertain origin. Recent results indicate it is caused by solar inertial motion. First we describe the 178.7-year basic cycle of solar motion. The longer cycle, over an 8000 year interval, is found to average 2402.2 years. This corresponds to the Jupiter/Heliocentre/Barycentre alignments (9.8855 × 243). Within each cycle an exceptional segment of 370 years has been found characterized by a looping pattern by a trefoil or quasitrefoil geometry. Solar activity, evidenced by 14C tree-ring proxies, shows the same pattern. Solar motion is computable in advance, so this provides a basis for future predictive assessments. The next 370-year segment will occur between AD 2240 and 2610. 相似文献
114.
The scaled boundary finite‐element method has been developed for the dynamic analysis of unbounded domains. In this method only the boundary is discretized resulting in a reduction of the spatial dimension by one. Like the finite‐element method no fundamental solution is required. This paper extends the scaled boundary finite‐element method to simulate the transient response of non‐homogeneous unbounded domains with the elasticity modulus and mass density varying as power functions of spatial coordinates. To reduce the number of degrees of freedom and the computational cost, the technique of reduced set of base functions is applied. The scaled boundary finite‐element equation for an unbounded domain is reformulated in generalized coordinates. The resulting acceleration unit‐impulse response matrix is obtained and assembled with the equation of motion of standard finite elements. Numerical examples of non‐homogeneous isotropic and transversely isotropic unbounded domains demonstrate the accuracy of the scaled boundary finite‐element method. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
115.
Time delays associated with processes leading to a failure or stress relaxation in materials and earthquakes are studied in
terms of continuum damage mechanics. Damage mechanics is a quasi-empirical approach that describes inelastic irreversible
phenomena in the deformation of solids. When a rock sample is loaded, there is generally a time delay before the rock fails.
This period is characterized by the occurrence and coalescence of microcracks which radiate acoustic signals of broad amplitudes.
These acoustic emission events have been shown to exhibit power-law scaling as they increase in intensity prior to a rupture.
In case of seismogenic processes in the Earth's brittle crust, all earthquakes are followed by an aftershock sequence. A universal
feature of aftershocks is that their rate decays in time according to the modified Omori's law, a power-law decay. In this
paper a model of continuum damage mechanics in which damage (microcracking) starts to develop when the applied stress exceeds
a prescribed yield stress (a material parameter) is introduced to explain both laboratory experiments and systematic temporal
variations in seismicity. 相似文献
116.
117.
非饱和土土水特征曲线(SWCC)测试与预测 总被引:3,自引:0,他引:3
非饱和土土水特征曲线(SWCC)表示了土中含水量与吸力之间的关系。文章介绍了6种常用方法,各有其适用范围。体积压力板仪可量测最大基质吸力值为1500kPa的干燥曲线和浸湿曲线;超过1500kPa时,可用盐溶液法进行量测;Tem-ple仪可量测基质吸力达100kPa的干燥曲线;滤纸法可用于测量土体的基质吸力与总吸力;Dew-point电位计可用于量测土样总吸力变化,尤其适合渗透吸力的量测;TDR探头适合于量测小于300kPa的基质吸力。用GDS非饱和土三轴仪可以进行SWCC测试,测试范围主要取决于陶土板的进气值。用准确的数学模型对测得的含水量、吸力数据进行拟合,对于预测非饱和土力学性质、渗透系数、抗剪强度及分析边坡稳定性有重要意义。由于准确测试SWCC难度较大,并且测试影响因素较多,所以根据土体孔隙大小分布和颗粒大小分布情况预测SWCC,也是一种较好的方法。 相似文献
118.
Most main sequence stars are binaries or higher multiplicity Systems and it appears that at birth most stars have circumstellar
disks. It is commonly accepted that planetary systems arise from the material of these disks; consequently, binary and multiple
systems may have a main role in planet formation. In this paper, we study the stage of planetary formation during which the
particulate material is still dispersed as centimetre-to-metre sized primordial aggregates. We investigate the response of
the particles, in a protoplanetary disk with radius RD = 100 AU around a solar-like star, to the gravitational field of bound perturbing companions in a moderately wide (300–1600
AU) orbit. For this purpose, we have carried out a series of simulations of coplanar hierarchical configurations using a direct
integration code that models gravitational and viscous forces. The massive protoplanetary disk is around one of the components
of the binary. The evolution in time of the dust sub-disk depends mainly on the nature (prograde or retrograde) of the relative
revolution of the stellar companion, and on the temperature and mass of the circumstellar disk. Our results show that for
binary companions near the limit of tidal truncation of the disk, the perturbation leads to an enhanced accretion rate onto
the primary, decreasing the lifetime of the particles in the protoplanetary disk with respect to the case of a single star.
As a consequence of an enhanced accretion rate the mass of the disk decreases faster, which leads to a longer resultant lifetime
for particles in the disk. On the other hand, binary companions may induce tidal arms in the dust phase of protoplanetary
disks. Spiral perturbations with m = 1 may increase in a factor 10 or more the dust surface density in the neighbourhood of
the arm, facilitating the growth of the particles. Moreover, in a massive disk (0.01M⊙) the survival time of particles is
significantly shorter than in a less massive nebula (0.001M⊙) and the temperature of the disk severely influences the spiral-in
time of particles. The rapid evolution of the dust component found in post T Tauri stars can be explained as a result of their
binary nature. Binarity may also influence the evolution of circumpulsar disks.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
119.
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
120.
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