全文获取类型
收费全文 | 71553篇 |
免费 | 747篇 |
国内免费 | 417篇 |
专业分类
测绘学 | 1860篇 |
大气科学 | 4793篇 |
地球物理 | 13347篇 |
地质学 | 28088篇 |
海洋学 | 5766篇 |
天文学 | 15478篇 |
综合类 | 273篇 |
自然地理 | 3112篇 |
出版年
2022年 | 327篇 |
2021年 | 532篇 |
2020年 | 581篇 |
2019年 | 613篇 |
2018年 | 3914篇 |
2017年 | 3613篇 |
2016年 | 2851篇 |
2015年 | 899篇 |
2014年 | 1482篇 |
2013年 | 2633篇 |
2012年 | 2537篇 |
2011年 | 4385篇 |
2010年 | 3959篇 |
2009年 | 4732篇 |
2008年 | 3892篇 |
2007年 | 4389篇 |
2006年 | 2102篇 |
2005年 | 1899篇 |
2004年 | 1808篇 |
2003年 | 1776篇 |
2002年 | 1567篇 |
2001年 | 1208篇 |
2000年 | 1196篇 |
1999年 | 960篇 |
1998年 | 1000篇 |
1997年 | 955篇 |
1996年 | 783篇 |
1995年 | 801篇 |
1994年 | 724篇 |
1993年 | 620篇 |
1992年 | 558篇 |
1991年 | 585篇 |
1990年 | 663篇 |
1989年 | 555篇 |
1988年 | 519篇 |
1987年 | 666篇 |
1986年 | 552篇 |
1985年 | 706篇 |
1984年 | 768篇 |
1983年 | 747篇 |
1982年 | 645篇 |
1981年 | 680篇 |
1980年 | 598篇 |
1979年 | 550篇 |
1978年 | 546篇 |
1977年 | 496篇 |
1976年 | 472篇 |
1975年 | 469篇 |
1974年 | 457篇 |
1973年 | 481篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Due to the geological time scales required for observation of catchment evolution, surrogates or analogues of field data are necessary to understand long‐term processes. To investigate long‐term catchment behaviour, two experimental model catchments that developed without rigid boundaries under controlled conditions are examined and a qualitative and quantitative analysis of their evolution is presented. Qualitatively, the experimental catchments have the visual appearance of field scale data. Observation demonstrates that changes in catchment shape and network form are conservative. Quantitative analysis suggests that the catchments reach an equilibrium form while a reduction in the channel network occurs. While the catchments are laboratory scale models, the results provide insights into field scale behaviour. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
102.
G. V. Abrahamyan 《Astrophysics》2003,46(3):304-318
New methods are applied to samples of classical cepheids in the galaxy, the Large Magellanic Cloud, and the Small Magellanic Cloud to determine the interstellar extinction law for the classical cepheids, R
B:R
V:R
I:R
J:R
H:R
K= 4.190:3.190:1.884:0.851:0.501:0.303, the color excesses for classical cepheids in the galaxy, E(B-V)=-0.382-0.168logP+0.766(V-I), and the color excesses for classical cepheids in the LMC and SMC, E(B-V)=-0.374-0.166logP+0.766(V-I). The dependence of the intrinsic color (B-V)0 on the metallicity of classical cepheids is discussed. The intrinsic color (V-I)0 is found to be absolutely independent of the metallicity of classical cepheids. A high precision formula is obtained for calculating the intrinsic colors of classical cepheids in the galaxy: (<B>-<V>)0=0.365(±0.011)+0.328(±0.012)logP. 相似文献
103.
In this paper general solutions are found for domain walls in Lyra geometry in the plane symmetric spacetime metric given
by Taub. Expressions for the energy density and pressure of domain walls are derived in both cases of uniform and time varying
displacement field β. It is also shown that the results obtained by Rahaman et al [IJMPD, 10, 735 (2001)] are particular case
of our solutions. Finally, the geodesic equations and acceleration of the test particle are discussed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
104.
C.S. Botzler J. Snigula R. Bender N. Drory G. Feulner G.J. Hill U. Hopp C. Maraston C. Mendes de Oliveira 《Astrophysics and Space Science》2003,284(2):393-396
The Munich Near-IR Cluster Survey (MUNICS) is a wide-area, medium-deep, photometric survey selected in the K' band. The project's
main scientific aims are the identification of galaxy clusters up to redshifts of unity and the selection of a large sample
of field early-type galaxies up to z < 1.5 for evolutionary studies. We created a Large Scale Structure catalog, using a new structure finding technique specialized
for photometric datasets, that we developed on the basis of a friends-of-friends algorithm. We tested the plausibility of
the resulting galaxy group and cluster catalog with the help of Color-Magnitude Diagrams (CMD), as well as a likelihood- and
Voronoi-approach.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
105.
We explore the possibility of searching for groups of radio sources from the FIRST catalog on angular scales 1′–5′. We developed an efficient method of searching for such groups that takes into account the need for combining the components of extended sources represented in the catalog by separate objects. We found 31 groups of radio sources with angular sizes <5′ that contain no fewer than five sources with flux densities ≥3 mJy. This number is at least triple the expected number of such groups for a random Poisson distribution of radio sources in the sky. The prospects for using groups of radio sources to detect and study distant systems of galaxies are discussed. 相似文献
106.
We consider the effects of projection, internal absorption, and gas-or stellar-velocity dispersion on the measured rotation curves of galaxies with edge-on disks. Axisymmetric disk models clearly show that the rotational velocity in the inner galaxy is highly underestimated. As a result, an extended portion that imitates nearly rigid rotation appears. At galactocentric distances where the absorption is low (i.e., it does not exceed 0.3–0.5m kpc?1), the line profiles can have two peaks, and a rotation curve with minimum distortions can be obtained by estimating the position of the peak that corresponds to a higher rotational velocity. However, the high-velocity peak disappears in high-absorption regions and the actual shape of the rotation curve cannot be reproduced from line-of-sight velocity estimates. In general, the optical rotation curves for edge-on galaxies are of little use in reconstructing the mass distribution in the inner regions, particularly for galaxies with a steep velocity gradient in the central region. In this case, estimating the rotation velocities for outer (transparent) disk regions yields correct results. 相似文献
107.
108.
109.
We present two spectra of the supernova SN1988A in M58 (NGC4579) over the wavelength range 4000–9700 Å, as recorded by the Faint Object Spectrograph on the 4.2 m William Herschel Telescope at the Observatorio del Roque de los Muchachos. We conclude that SN1988A was a type II supernova. 相似文献
110.
G. A. GRAHAM P. G. GRANT R. J. CHATER A. J. WESTPHAL A. T. KEARSLEY C. SNEAD G. DOMÍNGUEZ A. L. BUTTERWORTH D. S. McPHAIL G. BENCH J. P. BRADLEY 《Meteoritics & planetary science》2004,39(9):1461-1473
Abstract— In 2006, the Stardust spacecraft will return to Earth with cometary and perhaps interstellar dust particles embedded in silica aerogel collectors for analysis in terrestrial laboratories. These particles will be the first sample return from a solid planetary body since the Apollo missions. In preparation for the return, analogue particles were implanted into a keystone of silica aerogel that had been extracted from bulk silica aerogel using the optical technique described in Westphal et al. (2004). These particles were subsequently analyzed using analytical techniques associated with the use of a nuclear microprobe. The particles have been analyzed using: a) scanning transmission ion microscopy (STIM) that enables quantitative density imaging; b) proton elastic scattering analysis (PESA) and proton backscattering (PBS) for the detection of light elements including hydrogen; and c) proton‐induced X‐ray emission (PIXE) for elements with Z > 11. These analytical techniques have enabled us to quantify the composition of the encapsulated particles. A significant observation from the study is the variable column density of the silica aerogel. We also observed organic contamination within the silica aerogel. The implanted particles were then subjected to focused ion beam (FIB) milling using a 30 keV gallium ion beam to ablate silica aerogel in site‐specific areas to expose embedded particles. An ion polished flat surface of one of the particles was also prepared using the FIB. Here, we show that ion beam techniques have great potential in assisting with the analysis and exposure of Stardust particles. 相似文献