全文获取类型
收费全文 | 19078篇 |
免费 | 3458篇 |
国内免费 | 2968篇 |
专业分类
测绘学 | 804篇 |
大气科学 | 711篇 |
地球物理 | 1344篇 |
地质学 | 9758篇 |
海洋学 | 1965篇 |
天文学 | 5510篇 |
综合类 | 1179篇 |
自然地理 | 4233篇 |
出版年
2024年 | 64篇 |
2023年 | 168篇 |
2022年 | 583篇 |
2021年 | 674篇 |
2020年 | 617篇 |
2019年 | 758篇 |
2018年 | 599篇 |
2017年 | 580篇 |
2016年 | 689篇 |
2015年 | 712篇 |
2014年 | 1085篇 |
2013年 | 1202篇 |
2012年 | 1189篇 |
2011年 | 1277篇 |
2010年 | 1253篇 |
2009年 | 1597篇 |
2008年 | 1451篇 |
2007年 | 1385篇 |
2006年 | 1341篇 |
2005年 | 1194篇 |
2004年 | 1059篇 |
2003年 | 919篇 |
2002年 | 805篇 |
2001年 | 785篇 |
2000年 | 744篇 |
1999年 | 684篇 |
1998年 | 484篇 |
1997年 | 307篇 |
1996年 | 258篇 |
1995年 | 174篇 |
1994年 | 171篇 |
1993年 | 144篇 |
1992年 | 93篇 |
1991年 | 80篇 |
1990年 | 69篇 |
1989年 | 40篇 |
1988年 | 51篇 |
1987年 | 25篇 |
1986年 | 29篇 |
1985年 | 36篇 |
1984年 | 29篇 |
1983年 | 19篇 |
1982年 | 21篇 |
1981年 | 8篇 |
1980年 | 12篇 |
1979年 | 4篇 |
1978年 | 5篇 |
1977年 | 18篇 |
1971年 | 3篇 |
1954年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
61.
DEVELOPMENTSTRATEGIESOFWATERANDLANDRESOURCESINTHEHEXIREGION,CHINA肖洪浪,高前兆,李福兴DEVELOPMENTSTRATEGIESOFWATERANDLANDRESOURCESINTHE... 相似文献
62.
Brian Warner Patrick A. Woudt 《Monthly notices of the Royal Astronomical Society》2006,367(4):1562-1576
We present new observations of the rapid oscillations in the dwarf nova VW Hyi, made late in outburst. These dwarf nova oscillations (DNOs) increase in period until they reach 33 s, when a transition to a strong 1st harmonic and weak fundamental takes place. After further period increase, the 2nd harmonic appears; often all three components are present simultaneously. This 1:2:3 frequency suite is similar to what has been seen in some neutron star and black hole X-ray binaries, but has not previously been seen in a cataclysmic variable. When studied in detail, the fundamental and 2nd harmonic vary similarly in phase, but the 1st harmonic behaves independently, though keeping close to twice the frequency of the fundamental. The fundamental period of the DNOs, as directly observed or inferred from the harmonics, increases to ∼100 s before the oscillation disappears as the star reaches quiescence. Its maximum period is close to that of the 'longer-period' DNOs observed in VW Hyi. The quasi-periodic oscillations (QPOs), which have fundamental periods 400–1000 s, behave in the same way, showing 1st and 2nd harmonics at approximately the same times as the DNOs. We explore some possible models. One in which the existence of the 1st harmonic is due to the transition from viewing a single accretion region to viewing two regions, and the rate of accretion on to the primary is modulated at the frequency of the 1st harmonic, as in the 'beat frequency model', can generate the suite of DNO frequencies observed. But the behaviour of the QPOs is not yet understood. 相似文献
63.
64.
65.
The lensing properties of the Plummer model with a central point mass and external shear are derived, including the image multiplicities, critical curves and caustics. This provides a simple model for a flattened galaxy with a central supermassive black hole. For the Plummer model with black hole, the maximum number of images is four, provided the black hole mass is less than an upper bound which is calculated analytically. This introduces a method to constrain black hole masses by counting images, thus applicable at cosmological distance. With shear, the maximum number of images is six and we illustrate the occurrence of an astroid caustic and two metamorphoses. 相似文献
66.
Distances to nine dark globules are determined by a method using optical ( VRI ) and near-infrared (near-IR) ( JHK ) photometry of stars projected towards the field containing the globules. In this method, we compute intrinsic colour indices of stars projected towards the direction of the globule by dereddening the observed colour indices using various trial values of extinction A V and a standard extinction law. These computed intrinsic colour indices for each star are then compared with the intrinsic colour indices of normal main-sequence stars and a spectral type is assigned to the star for which the computed colour indices best match with the standard intrinsic colour indices. Distances ( d ) to the stars are determined using the A V and absolute magnitude ( MV ) corresponding to the spectral types thus obtained. A distance versus extinction plot is made and the distance at which A V undergoes a sharp rise is taken to be the distance to the globule. All the clouds studied in this work are in the distance range 160–400 pc. The estimated distances to dark globules LDN 544, LDN 549, LDN 567, LDN 543, LDN 1113, LDN 1031, LDN 1225, LDN 1252 and LDN 1257 are 180 ± 35, 200 ± 40, 180 ± 35, 160 ± 30, 350 ± 70, 200 ± 40, 400 ± 80, 250 ± 50 and 250 ± 50 pc, respectively. Using the distances determined, we have estimated the masses of the globules and the far-IR luminosity of the IRAS sources associated with them. The mass of the clouds studied are in the range 10–200 M⊙ . 相似文献
67.
68.
69.
K. Iwasawa A.C. Fabian S.W. Allen S. Ettori 《Monthly notices of the Royal Astronomical Society》2001,328(1):L5-L10
We report on the metal distribution in the intracluster medium around the radio galaxy 4C+55.16 observed with the Chandra X-ray Observatory . The radial metallicity profile shows a dramatic change at 10 arcsec (∼50 kpc) in radius from half solar to twice solar at inner radii. Also found was a plume-like feature located at ∼3 arcsec to the south-west of the centre of the galaxy, which is mostly accounted for by a strong enhancement of iron L emission. The X-ray spectrum of the plume is characterized by the metal abundance pattern of Type Ia supernovae (SNeIa), i.e. large ratios of Fe to α elements, with the iron metallicity being unusually high at solar (90 per cent error). How the plume has been formed is not entirely clear. The inhomogeneous iron distribution suggested in this cluster provides important clues to understanding the metal enrichment process of the cluster medium. 相似文献
70.
M. A. Iatì C. Cecchi-Pestellini D. A. Williams F. Borghese P. Denti R. Saija S. Aiello 《Monthly notices of the Royal Astronomical Society》2001,322(4):749-756
Recent observations of stellar composition suggest that elements in the Sun are significantly more abundant than in other stars. The reduction in the available element budget implies a drastic revision in current models of interstellar dust. Theoretical models are therefore exploring fluffy, porous physical structure for the grain material. Since a detailed exact treatment of extinction cross-sections is mandatory for a correct understanding of the nature of interstellar dust, we present a technique based on the multipole expansions of the electromagnetic field, which has proven to be general, flexible and powerful in treating scattering of light by porous, composite, arbitrarily shaped particles. The results of this study speak in favour of core–mantle structures characterized by the presence of porosities. 相似文献