全文获取类型
收费全文 | 59601篇 |
免费 | 1095篇 |
国内免费 | 533篇 |
专业分类
测绘学 | 1419篇 |
大气科学 | 4074篇 |
地球物理 | 11666篇 |
地质学 | 21300篇 |
海洋学 | 5431篇 |
天文学 | 13739篇 |
综合类 | 134篇 |
自然地理 | 3466篇 |
出版年
2022年 | 339篇 |
2021年 | 586篇 |
2020年 | 689篇 |
2019年 | 806篇 |
2018年 | 1509篇 |
2017年 | 1467篇 |
2016年 | 1669篇 |
2015年 | 924篇 |
2014年 | 1590篇 |
2013年 | 2973篇 |
2012年 | 1836篇 |
2011年 | 2466篇 |
2010年 | 2208篇 |
2009年 | 2814篇 |
2008年 | 2540篇 |
2007年 | 2585篇 |
2006年 | 2466篇 |
2005年 | 1772篇 |
2004年 | 1763篇 |
2003年 | 1730篇 |
2002年 | 1579篇 |
2001年 | 1391篇 |
2000年 | 1324篇 |
1999年 | 1181篇 |
1998年 | 1178篇 |
1997年 | 1151篇 |
1996年 | 925篇 |
1995年 | 909篇 |
1994年 | 852篇 |
1993年 | 721篇 |
1992年 | 670篇 |
1991年 | 637篇 |
1990年 | 702篇 |
1989年 | 622篇 |
1988年 | 599篇 |
1987年 | 637篇 |
1986年 | 615篇 |
1985年 | 784篇 |
1984年 | 831篇 |
1983年 | 827篇 |
1982年 | 761篇 |
1981年 | 658篇 |
1980年 | 657篇 |
1979年 | 589篇 |
1978年 | 560篇 |
1977年 | 567篇 |
1976年 | 495篇 |
1975年 | 513篇 |
1974年 | 487篇 |
1973年 | 510篇 |
排序方式: 共有10000条查询结果,搜索用时 428 毫秒
181.
A tachometer for measuring the displacements of the center of gravity of spectral line profiles is developed. The implementation of the device, basically a Fabry-Pérot etalon, is given. The spectral shifts are determined, using the frequency of a laser as reference. 相似文献
182.
183.
This paper is aimed towards investigating the filtration law of an incompressible viscous Newtonian fluid through a rigid non-inertial porous medium (e.g. a porous medium placed in a centrifuge basket). The filtration law is obtained by upscaling the flow equations at the pore scale. The upscaling technique is the homogenization method of multiple scale expansions which rigorously gives the macroscopic behaviour and the effective properties without any prerequisite on the form of the macroscopic equations. The derived filtration law is similar to Darcy's law, but the tensor of permeability presents the following remarkable properties: it depends upon the angular velocity of the porous matrix, it verifies Hall–Onsager's relationship and it is a non-symmetric tensor. We thus deduce that, under rotation, an isotropic porous medium leads to a non-isotropic effective permeability. In this paper, we present the results of numerical simulations of the flow through rotating porous media. This allows us to highlight the deviations of the flow due to Coriolis effects at both the microscopic scale (i.e. the pore scale), and the macroscopic scale (i.e. the sample scale). The above results confirm that for an isotropic medium, phenomenological laws already proposed in the literature fails at reproducing three-dimensional Coriolis effects in all types of pores geometry. We show that Coriolis effects may lead to significant variations of the permeability measured during centrifuge tests when the inverse Ekman number Ek−1 is 𝒪(1). These variations are estimated to be less than 5% if Ek−1<0.2, which is the case of classical geotechnical centrifuge tests. We finally conclude by showing that available experimental data from tests carried out in centrifuges are not sufficient to determining the effective tensor of permeability of rotating porous media. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
184.
Photoionization by stars cannot be the only cause for the diffuseionized gas component in the distant halos of galaxies.Quasistationary localized magnetic reconnection can efficientlycontribute to the ionization of the extraplanar halo gas. The gas isheated and re-ionized in numerous current filaments fast enough tocompensate recombination. The necessary magnetic energy is provided bythe disk activity. 相似文献
185.
Astronomy Letters - The acceleration of anomalous cosmic rays (ACRs) at the heliospheric termination shock and their influence on the shock structure and location are analyzed in terms of a... 相似文献
186.
187.
This paper considers the problem of estimatingm, the number of components in a finite mixture of distributions from a parametric family. A step-up procedure using the bootstrap method is proposed. Some properties of the procedure are illustrated with simulation studies. An example of the method, applied to orientation of beach clasts, is given. 相似文献
188.
189.
C. Tadhunter K. Wills R. Morganti T. Oosterloo R. Dickson 《Monthly notices of the Royal Astronomical Society》2001,327(1):227-232
We present new spectroscopic observations of the southern radio galaxy . Despite the flat-spectrum character of the radio emission from this source, our optical spectra show no sign of the broad permitted lines and non-stellar continuum characteristic of quasar nuclei and broad-line radio galaxies. However, the high-ionization forbidden lines, including [O iii ] λλ 5007, 4959, are unusually broad for a narrow-line radio galaxy , and are blueshifted by 600 km s−1 relative to the low-ionization lines such as [O ii ] λλ 3726,3729. The [O ii ] lines are also considerably narrower than the [O iii ] lines, and have a redshift consistent with that of the recently detected H i 21-cm absorption-line system. Whereas the kinematics of the [O iii ] emission lines are consistent with outflow in an inner narrow-line region, the properties of the [O ii ] emission lines suggest that they are emitted by a more extended and quiescent gaseous component. We argue that, given the radio properties of the source, our line of sight is likely to be lying close to the direction of bulk outflow of the radio jets. In this case it is probable that the quasar nucleus is entirely obscured at optical wavelengths by the material responsible for the H i absorption-line system. The unusually broad [O iii ] emission lines suggest that the radio source is intrinsically compact. Overall, our data are consistent the idea that is a radio source in an early stage of evolution. 相似文献
190.
R. Wieler Th. Graf P. Signer S. Vogt G. F. Herzog C. Tuniz D. Fink L. K. Fifield J. Klein R. Middleton A. J. T. Jull P. Pellas J. Masarik G. Dreibus 《Meteoritics & planetary science》1996,31(2):265-272
Abstract— We determined He, Ne, Ar, 10Be, 26Al, 36Cl, and 14C concentrations, as well as cosmic-ray track densities and halogen concentrations in different specimens of the H6 chondrite Torino, in order to constrain its exposure history to cosmic radiation. The Torino meteoroid had a radius of ~20 cm and travelled in interplanetary space for 2.5–10 Ma. Earlier, Torino was part of a larger body. The smallest possible precursor had a radius of 55 cm and a journey through space longer than ~65 Ma. If the first-stage exposure took place in a body with a radius of >3 m or in the parent asteroid, then it lasted nearly 300 Ma. The example of Torino shows that it is easy to underestimate first-stage exposure ages when constructing two-stage histories. 相似文献