全文获取类型
收费全文 | 86143篇 |
免费 | 980篇 |
国内免费 | 548篇 |
专业分类
测绘学 | 2143篇 |
大气科学 | 5431篇 |
地球物理 | 16169篇 |
地质学 | 33260篇 |
海洋学 | 7402篇 |
天文学 | 19028篇 |
综合类 | 290篇 |
自然地理 | 3948篇 |
出版年
2022年 | 573篇 |
2021年 | 924篇 |
2020年 | 978篇 |
2019年 | 1112篇 |
2018年 | 4258篇 |
2017年 | 3813篇 |
2016年 | 3461篇 |
2015年 | 1225篇 |
2014年 | 2167篇 |
2013年 | 3756篇 |
2012年 | 3105篇 |
2011年 | 4931篇 |
2010年 | 4419篇 |
2009年 | 5196篇 |
2008年 | 4375篇 |
2007年 | 4979篇 |
2006年 | 3057篇 |
2005年 | 2276篇 |
2004年 | 2166篇 |
2003年 | 2176篇 |
2002年 | 2007篇 |
2001年 | 1715篇 |
2000年 | 1548篇 |
1999年 | 1150篇 |
1998年 | 1179篇 |
1997年 | 1220篇 |
1996年 | 954篇 |
1995年 | 959篇 |
1994年 | 905篇 |
1993年 | 808篇 |
1992年 | 775篇 |
1991年 | 760篇 |
1990年 | 798篇 |
1989年 | 740篇 |
1988年 | 704篇 |
1987年 | 780篇 |
1986年 | 657篇 |
1985年 | 871篇 |
1984年 | 926篇 |
1983年 | 848篇 |
1982年 | 768篇 |
1981年 | 793篇 |
1980年 | 712篇 |
1979年 | 609篇 |
1978年 | 651篇 |
1977年 | 609篇 |
1976年 | 545篇 |
1975年 | 533篇 |
1974年 | 528篇 |
1973年 | 563篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
111.
Moroz L. V. Baratta G. Distefano E. Strazzulla G. Starukhina L. V. Dotto E. Barucci M. A. 《Earth, Moon, and Planets》2003,92(1-4):279-289
Trans-Neptunian Objects (TNOs) and Centaurs show remarkable colour variationsin the visual and near-infrared spectral regions. Surface alteration processes such asspace weathering (e.g., bombardment with ions) and impact resurfacingmay play an important role in the colour diversity of such bodies. Ion irradiation ofhydrocarbon ices and their mixtures with water ice transforms neutral (grey) surfacecolours of ices to red and further to grey. Along with the ices, TNOs and Centaursprobably contain complex carbonaceous compounds, in particular, complexhydrocarbons. Unlike ices, such refractory organic materials have originally lowvisual albedos and red colours in the visible and near-infrared ranges. Here wepresent the first results of ion irradiation experiments on asphaltite. Asphaltite isa natural complex hydrocarbon material. The reflectance spectra of asphaltite inthe 0.4–0.8 μm range have been recorded before irradiation and after eachirradiation step. We demonstrate that irradiation of this red dark material with30 keV H+ and 15 keV N+ ions gradually transforms its colour from redto grey as a result of carbonization. A moderate increase in the visual albedo hasbeen observed. These results may imply that the surfaces of primitive red objectsoptically dominated by complex refractory organics may show a similar spaceweathering trend. Our laboratory results were compared with published coloursof TNOs and Centaurs. A broad variety of spectral colours observed for TNOs andCentaurs may be reproduced by various spectra of irradiated organics correspondingto different ion fluences. However, such objects probably also contain ices and silicatecomponents which show different space weathering trends. This fact, together with alack of information about albedos, may explain difficulties to reveal correlations between surface colours within TNO and Centaur populations and their other properties, such as absolute magnitudes and orbital parameters. 相似文献
112.
Larisa A. Yakovina Yakiv V. Pavlenko Carlos Abia 《Astrophysics and Space Science》2003,288(3):279-286
Lithium abundances in the atmospheres of the super Li-rich C-giants WZ Cas and WX Cyg are derived by the spectral synthesis
technique using the Li I resonance line at λ670.8 nm and three subordinate lines at λλ 812.6, 610.4 and 497.2 nm. The differences
between the Li abundances derived from the λ670.8 nm line and the λλ 497.2, 812.6 nm lines do not exceed ±0.5 dex. The lithium
line at λ610.4 nm provides typically lower abundances than the resonance line (by ≈ 1 dex). The mean LTE and NLTE Li abundances
from three Li I lines (excluding λ610.4 nm) are 4.7, 4.9 for WZ Cas, and 4.6, 4.8 for WX Cyg, respectively.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
113.
Summary ?A single-crystal X-ray investigation was performed on crystals of P21/c natural pigeonite with varying Ca and Fe* ( = Fe2+ + Mn2+) contents, in order to verify the effect of microtextural disorder on structure refinements and to constrain the crystal
chemistry of pigeonite. Antiphase domains and exsolution lamellae affect differently the refinement results. In a crystal
free of exsolution the structure obtained after refinement with all reflections is an average of that of the antiphase domains
and of their boundaries, whereas in an exsolved crystal it represents only the structure of the prevailing pigeonite lamellae.
The refinement using only h + k odd reflections seems to give the structure of the Ca-free pigeonite characteristic of the antiphase domains rather than
that of Ca-rich domain walls. The ratio of the scale factors in refinements with all reflections and with only h + k odd reflections allows the ratios of the exsolved augite and pigeonite phases to be estimated.
The crystal chemistry of the investigated samples follows the trends outlined by data on Ca-free and Fe-free synthetic samples.
In particular, it is shown that Ca and Fe* substitution for Mg induce similar changes in the average structure, i.e. both induce an expansion in the M1 polyhedron and
decrease the difference between the M2–O3 distances.
Received October 18, 2001; revised version accepted February 15, 2002 相似文献
114.
We report three new or updated techniques for probing the parameters of active galaxies based on the masses of their central
black holes MBH). First, we derived a near-IR analog of the bulge luminosity versus MBH relationship. The low scatter makes it a promising new tool to study the black hole demographics. Next, we present relations
between MBH and the10 μm and 2-10 keV nuclear luminosity. They may help to study the MBH evolution over wide redshift ranges. Finally, we measured MBH in quasars from z ∼ 3.4 to z ∼ 0.3 to search directly for MBH growth. Surprisingly, we found no evidence for growth implying that the majority of quasar host galaxies have undergone their
last major merger at z ≥ 3.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
115.
116.
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
117.
118.
119.
The Speckle camera of the 3.5 m Telescopio Nazionale Galileo (TNG) has been used to measure apparent sizes and shapes of a number of main belt asteroids. The average size measurements are in a generally good agreement with the results of indirect IRAS-based radiometric techniques. The measured shapes are compared with predictions based on previously derived spin axis directions and lightcurve photometry of some of the observed objects. Also in this case the agreement is reasonable and the speckle observations allow us to discriminate in some cases between the two pole solutions usually found for each object. No clear evidence of binaries was found. The results show that the TNG speckle camera can be a powerful tool to resolve relatively large main belt asteroids and to calibrate the results of the IRAS survey. 相似文献
120.
Variable surface albedo features on Mars are likely caused by the entrainment and deposition of dust by the wind. Most discrete markings are associated with topographic forms or with regional slopes that serve to alter the effective wind shear stress on the surface. Some of the largest variable features, here termed mesoscale linear streaks, are up to 400 km in length and repeatedly occur in one of the smoothest regions of Mars: Amazonis Planitia. Their orientations and apparent season of variability as observed by Viking and Mars Orbiter cameras indicate linear streak formation by enhanced surface wind stresses during regional or local dust storms and during the initial stages of global dust storms. They provide an example of the ability of large-scale winds, without significant local enhancement, to initiate dust motion on Mars. The sizes and spacing of the linear streaks may be controlled by boundary layer rolls. The repetitive formation of these streaks, over a span of more than 11 Mars years, gives one measure of the stability of Mars’ eolian processes. 相似文献