全文获取类型
收费全文 | 118693篇 |
免费 | 1918篇 |
国内免费 | 789篇 |
专业分类
测绘学 | 2774篇 |
大气科学 | 8209篇 |
地球物理 | 22909篇 |
地质学 | 42049篇 |
海洋学 | 10686篇 |
天文学 | 27312篇 |
综合类 | 308篇 |
自然地理 | 7153篇 |
出版年
2022年 | 697篇 |
2021年 | 1192篇 |
2020年 | 1324篇 |
2019年 | 1451篇 |
2018年 | 3039篇 |
2017年 | 2838篇 |
2016年 | 3390篇 |
2015年 | 1797篇 |
2014年 | 3349篇 |
2013年 | 6114篇 |
2012年 | 3551篇 |
2011年 | 4659篇 |
2010年 | 4278篇 |
2009年 | 5450篇 |
2008年 | 4815篇 |
2007年 | 4938篇 |
2006年 | 4589篇 |
2005年 | 3491篇 |
2004年 | 3409篇 |
2003年 | 3195篇 |
2002年 | 3094篇 |
2001年 | 2746篇 |
2000年 | 2686篇 |
1999年 | 2278篇 |
1998年 | 2283篇 |
1997年 | 2155篇 |
1996年 | 1844篇 |
1995年 | 1820篇 |
1994年 | 1613篇 |
1993年 | 1532篇 |
1992年 | 1418篇 |
1991年 | 1453篇 |
1990年 | 1498篇 |
1989年 | 1318篇 |
1988年 | 1246篇 |
1987年 | 1441篇 |
1986年 | 1253篇 |
1985年 | 1588篇 |
1984年 | 1819篇 |
1983年 | 1734篇 |
1982年 | 1628篇 |
1981年 | 1525篇 |
1980年 | 1338篇 |
1979年 | 1298篇 |
1978年 | 1262篇 |
1977年 | 1130篇 |
1976年 | 1093篇 |
1975年 | 1026篇 |
1974年 | 1075篇 |
1973年 | 1111篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
651.
In recent years, interface waves such as the Scholte wave have become important tools in the study of the geoacoustic properties of near-bottom seafloor sediments. Traditionally, these waves have been generated by explosive or pneumatic sources deployed at or near the seafloor and monitored by ocean-bottom seismographs or geophone arrays. While these sources generate the requisite interface waves, they also produce higher frequency compressional waves in the water and sediment that tend to contaminate the surface wave and make inversion of the data difficult in the near field. In this paper, a new source consisting of a freely falling projectile instrumented with an accelerometer is described. When the projectile impacts the bottom, the exact time history of the vertical force applied to the sediment is known and therefore may be convolved with the transfer function of a sediment geoacoustic model to produce accurate synthetic seismograms. Moreover, the vertical force applied to the seafloor is very efficient in generating surface wave motion while producing very little compressional wave energy so that the near-field signals are much more easily analyzed. An example of the use of the new source is presented including inversion of the received signals to obtain shear-wave velocity and attenuation as a function of depth in the near bottom sediments at a shallow-water site 相似文献
652.
653.
654.
O. Sadot K. Levy A. Yosef-Hai D. Cartoon Y. Elbaz Y. Srebro G. Ben-Dor D. Shvarts 《Astrophysics and Space Science》2005,298(1-2):305-312
The hydrodynamic instability, which develops on the contact surface between two fluids, has great importance in astrophysical
phenomena such as the inhomogeneous density distribution following a supernova event. In this event acceleration waves pass
across a material interface and initiate and enhance unstable conditions in which small perturbations grow dramatically.
In the present study, an experimental technique aimed at investigating the above-mentioned hydrodynamic instability is presented.
The experimental investigation is based on a shock-tube apparatus by which a shock wave is generated and initiates the instability
that develops on the contact surface between two gases. The flexibility of the system enables one to vary the initial shape
of the contact surface, the shock-wave Mach number, and the density ratio across the contact surface.
Three selected sets of shock-tube experiments are presented in order to demonstrate the system capabilities: (1) large-initial
amplitudes with low-Mach-number incident shock waves; (2) small-initial amplitudes with moderate-Mach-number incident shock
waves; and (3) shock bubble interaction.
In the large-amplitude experiments a reduction of the initial velocity with respect to the linear growth prediction was measured.
The results were compared to those predicted by a vorticity-deposition model and to previous experiments with moderate- and
high-Mach number incident shock waves that were conducted by others. In this case, a reduction of the initial velocity was
noted. However, at late times the growth rate had a 1/t behavior as in the small-amplitude low-Mach number case. In the small-amplitude moderate-Mach number shock experiments a
reduction from the impulsive theory was noted at the late stages.
The passage of a shock wave through a spherical bubble results in the formation of a vortex ring. Simple dimensional analysis
shows that the circulation depends linearly on the speed of sound of the surrounding material and on the initial bubble radius. 相似文献
655.
656.
The fundamental plane (FP) scaling relations and their evolution are a powerful tool for studying the global properties of
early-type galaxies and their evolutionary history. The form of the FP, as derived by surveys in the local Universe at wavelengths
ranging from the U to the K band, cannot be explained by metallicity variations alone among early-type galaxies; systematic variations in age, dark matter
content, or homology breaking are required. A large-scale study of early-type galaxies at 0.1 < z < 0.6demonstrates that the SB intercept of the FP, the rest frame (U-V) colour, and the absorption line strengths all evolve passively, thereby implying a high mean formation redshift for the stellar
content. The slope of the FP evolves with redshift, which is broadly consistent with systematic age effects occurring along
the early-type galaxy sequence. The implication that the least luminous early-type galaxies formed later than the luminous
galaxies is discussed in the context of the evolution of thecolour–magnitude relation, the Butcher–Oemler effect and hierarchical
galaxy formation models.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
657.
A.J. Walsh F. Bertoldi M.G. Burton T. Nikola 《Monthly notices of the Royal Astronomical Society》2001,326(1):36-56
N -band (10.5 μm) and/or Q -band (20.0 μm) images taken with MANIAC on the ESO/MPI 2.2-m telescope are presented for 31 methanol maser sites and 19 ultracompact (UC) H ii regions. Most of the maser sites and UC H ii regions are coincident with mid-infrared (MIR) sources to within the positional uncertainties of ∼ 3 arcsec, consistent with the maser emission being powered by the MIR source. The IRAS source positions, however, do not always coincide with the MIR sources.
Based on an average infrared spectral energy distribution, we deduce that the MIR objects are luminous enough that they should also produce a strong ionizing radiation. Some sources are consistent with stars of later spectral type, but not all can be. A number of maser sites show no detectable radio continuum emission associated with MIR emission, despite a powering source luminous enough potentially to produce an UC H ii region. Since no signs of an UC H ii region are detected here, these maser sites might be produced during a very early stage of stellar evolution.
We present objects that show evidence of outflow activity stemming from a maser site, exhibiting CO and/or CS line profiles indicative of outflows coincident with the MIR source. These cases are promising examples of maser sites signposting the earliest stages of high-mass star formation. 相似文献
Based on an average infrared spectral energy distribution, we deduce that the MIR objects are luminous enough that they should also produce a strong ionizing radiation. Some sources are consistent with stars of later spectral type, but not all can be. A number of maser sites show no detectable radio continuum emission associated with MIR emission, despite a powering source luminous enough potentially to produce an UC H ii region. Since no signs of an UC H ii region are detected here, these maser sites might be produced during a very early stage of stellar evolution.
We present objects that show evidence of outflow activity stemming from a maser site, exhibiting CO and/or CS line profiles indicative of outflows coincident with the MIR source. These cases are promising examples of maser sites signposting the earliest stages of high-mass star formation. 相似文献
658.
C. S. Crawford A. C. Fabian P. Gandhi R. J. Wilman R. M. Johnstone 《Monthly notices of the Royal Astronomical Society》2001,324(2):427-442
We present observations of a sample of optically faint, hard X-ray sources of the kind likely to be responsible for much of the hard X-ray background. We confirm that such sources are easily detected in the near-infrared, and find that they have a featureless continuum suggesting that the active nucleus is heavily obscured. The infrared colours of the majority of the targets observed are consistent with absorbed elliptical host galaxies at z =1–2. It is likely that we are observing some of the brighter members of the important new class of X-ray type II quasars. 相似文献
659.
We have undertaken a study of coronal features observed at meter-decameter wavelengths using the Clark Lake radioheliograph.
Among the coronal structures we have studied are the radio manifestations of coronal streamers on the solar disk and above
the solar limb. We have analyzed the radio data quantitatively, using ray-tracing models for comparison with the maps. Our
study provides information about the streamers' three-dimensional shapes, scales, and density profiles, for comparison with
related observations using white-light coronagraphs. 相似文献
660.