全文获取类型
收费全文 | 88256篇 |
免费 | 1098篇 |
国内免费 | 675篇 |
专业分类
测绘学 | 2592篇 |
大气科学 | 6210篇 |
地球物理 | 16916篇 |
地质学 | 33462篇 |
海洋学 | 6901篇 |
天文学 | 19636篇 |
综合类 | 350篇 |
自然地理 | 3962篇 |
出版年
2021年 | 634篇 |
2020年 | 677篇 |
2019年 | 728篇 |
2018年 | 4070篇 |
2017年 | 3777篇 |
2016年 | 3253篇 |
2015年 | 1250篇 |
2014年 | 1986篇 |
2013年 | 3608篇 |
2012年 | 2990篇 |
2011年 | 4884篇 |
2010年 | 4341篇 |
2009年 | 5217篇 |
2008年 | 4338篇 |
2007年 | 4835篇 |
2006年 | 2684篇 |
2005年 | 2431篇 |
2004年 | 2267篇 |
2003年 | 2229篇 |
2002年 | 2045篇 |
2001年 | 1669篇 |
2000年 | 1620篇 |
1999年 | 1384篇 |
1998年 | 1358篇 |
1997年 | 1343篇 |
1996年 | 1172篇 |
1995年 | 1143篇 |
1994年 | 1053篇 |
1993年 | 893篇 |
1992年 | 828篇 |
1991年 | 863篇 |
1990年 | 932篇 |
1989年 | 837篇 |
1988年 | 783篇 |
1987年 | 949篇 |
1986年 | 802篇 |
1985年 | 997篇 |
1984年 | 1104篇 |
1983年 | 1051篇 |
1982年 | 970篇 |
1981年 | 908篇 |
1980年 | 829篇 |
1979年 | 751篇 |
1978年 | 744篇 |
1977年 | 692篇 |
1976年 | 658篇 |
1975年 | 643篇 |
1974年 | 646篇 |
1973年 | 693篇 |
1972年 | 435篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
171.
Impulsively generated waves in solar coronal loops are numerically simulated in the frame-work of cold magnetohydrodynamics. Coronal inhomogeneities are approximated by gas density slabs embedded in a uniform magnetic field. The simulations show that an initially excited pulse results in the propagation of wave packets which correspond to both trapped and leaky waves. Whereas the leaky waves propagate outside the slab, the trapped waves occur as a result of a total reflection from the slab walls. Time signatures of these waves are made by a detection of the trapped waves at a fixed spatial location. For waves excited within the slab, time signatures exhibit periodic, quasi-periodic and decay phases. The time signatures for waves excited outside the slab, or for a multi-series of variously shaped impulses generated at different places and times, can possess extended quasi-periodic phases. The case of parallel slabs, when the presence of a second slab influences the character of wave propagation in the first slab, exhibits complex time signatures as a result of solitary waves interaction. 相似文献
172.
Z. Balog † A. J. Delgado ‡ A. Moitinho G. Frész † G. Kaszás † J. Vinkó E. J. Alfaro 《Monthly notices of the Royal Astronomical Society》2001,323(4):872-886
CCD photometry in Johnson UBV and Strömgren uvby systems and medium-resolution spectroscopy of the galactic open cluster NGC 7128 are presented. Spectral types of the brightest 12 stars in the cluster field were determined based on equivalent widths of the H α and the He i 6678-Å line. The spectroscopic observations also revealed two obvious and one probable Be-type stars showing H α emission. The analysis of the photometric diagrams gave a colour excess of E ( B − V )=1.03±0.06 mag, a distance modulus DM =13.0±0.2 mag and an age above 10 Myr. Time-resolved photometric observations obtained on one night resulted in the detection of short time-scale light variations of seven new and three already known variable stars in the cluster field. 相似文献
173.
Summary
Silica-undersaturated phlogopite schists from the Cackleberry Metamorphics, Arunta Inlier, central Australia, preserve relatively
low-temperature sapphirine-bearing parageneses that developed during low-pressure upper amphibolite facies metamorphism. Peak
metamorphic phlogopite–cordierite–sapphirine assemblages are interpreted to have formed during the same event recorded in
nearby metapelites, at c.3 kbar and 650–700 °C. Initial cooling of the terrain resulted in the breakdown of sapphirine to corundum–chlorite–phlogopite
and corundum–spinel–chlorite assemblages. Further retrogression at greenschist facies conditions resulted in the replacement
of sapphirine by diaspore–chlorite intergrowths. The reaction textures are consistent with a near-isobaric heating-cooling
path at low-pressure, and provide evidence for the stability of sapphirine at c.700 °C at low pressures in rocks of an appropriate Mg- and Fe3+-rich bulk composition.
Received August 15, 2001 accepted December 27, 2001 相似文献
174.
A. Greco G. Zimbardo P. Veltri A.L. Taktakishvili L.M. Zelenyi 《Astrophysics and Space Science》2001,277(1-2):35-38
We study the ion dynamics in a magnetic field reversal with a constant electric field and with a model of three dimensional
magnetic turbulence. By computing the mean square displacements in the plane of the current sheet we find superdiffusive and
superballistic transport regimes. Since velocity increases with the length of the free path, we have accelerated Lévyflights.
The possibility to generate power law velocity distribution functions is pointed out, as well as the long memory effects and
non local properties of ion transport.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
175.
176.
G. Vedrenne 《Astrophysics and Space Science》2001,276(1):337-346
In spite of the recent successes of SIGMA, RXTE andCompton GRO, and the launch of INTEGRAL in 2001, a lot ofquestions will stay opened for the next decades in gamma-ray astronomy.In this context we have to think about future instrument concepts whichwill allow a new step in the understanding of high-energy phenomena atwork in many exciting objects: binary systems with compact objects,active galactic nuclei, supernovae and novae, gamma ray bursters...A short overview of these new types of instruments will be given. 相似文献
177.
K. Nagamine † M. Fukugita R. Cen J.P. Ostriker 《Monthly notices of the Royal Astronomical Society》2001,327(1):L10-L14
The luminosity function of galaxies is derived from a cosmological hydrodynamic simulation of a Λ cold dark matter universe with the aid of a stellar population synthesis model. At , the resulting B -band luminosity function has a flat faint-end slope of with the characteristic luminosity and the normalization in fair agreement with observations, while the dark matter halo mass function is steep with a slope of . The colour distribution of galaxies also agrees well with local observations. We also discuss the evolution of the luminosity function, and the colour distribution of galaxies from to 5. A large evolution of the characteristic mass in the stellar mass function as a result of number evolution is compensated by luminosity evolution; the characteristic luminosity increases only by 0.8 mag from to 2, and then declines towards higher redshift, while the B -band luminosity density continues to increase from to 5 (but only slowly at . 相似文献
178.
We report on SWS and LWS observations of the circumstellar disks of young stars of a few solar masses. The ISO spectra of
these objects present a diversity of emission features of carbon-rich and oxygen-rich grains. The similarity of the forsterite
spectra observed for Comet Hale-Bopp and the Haebe star HD100546 is particularly striking and provides a new argument that
huge comet swarms are formed in the disks surrounding young stars. While the data suggest that the formation of crystalline
silicates in the dust disks essentially occurs when a Haebe star has already reached the main sequence, no clear correlation
with stellar age only is apparent.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
179.
G. S. Sahakian 《Astrophysics》1997,40(1):77-82
The problem of the equation of state of cosmic matter is discussed and the constants of integration in the Friedmann solutions
are determined.
Translated from Astrofizika, Vol. 40, No. 1, pp. 117–124, January–March, 1997. 相似文献
180.
Micro- and non-crystalline opals, chalcedony and flint show diffuse small angle neutron scattering (SANS). Precious opals give rise to two additional intensity maxima at very small scattering angles which are due to Bragg reflections from the closest packed non-crystalline silica spheres. A small angle texture diagram reveals that the closest packing is faulty. Synthetic non-crystalline opals yield much less intense small angle scattering due to lower contrast between silica spheres and interstitial cement or particles; in this case intensity maxima were not observed. The outer part of the scattering curves of opal-CT and microcrystalline quartz deviates from Porod's law. The specific surface of natural non-crystalline opals ranges from 0.006 to 0.018 nm–1. In microcrystalline opals, the specific surface is about 10 times larger than in non-crystalline opals. 相似文献