全文获取类型
收费全文 | 58787篇 |
免费 | 873篇 |
国内免费 | 1366篇 |
专业分类
测绘学 | 2616篇 |
大气科学 | 4790篇 |
地球物理 | 11698篇 |
地质学 | 23675篇 |
海洋学 | 3639篇 |
天文学 | 9545篇 |
综合类 | 2341篇 |
自然地理 | 2722篇 |
出版年
2021年 | 357篇 |
2020年 | 345篇 |
2019年 | 366篇 |
2018年 | 5414篇 |
2017年 | 4682篇 |
2016年 | 3675篇 |
2015年 | 955篇 |
2014年 | 1255篇 |
2013年 | 1963篇 |
2012年 | 2151篇 |
2011年 | 3927篇 |
2010年 | 3143篇 |
2009年 | 3709篇 |
2008年 | 3112篇 |
2007年 | 3471篇 |
2006年 | 1334篇 |
2005年 | 1271篇 |
2004年 | 1333篇 |
2003年 | 1307篇 |
2002年 | 1158篇 |
2001年 | 892篇 |
2000年 | 860篇 |
1999年 | 746篇 |
1998年 | 718篇 |
1997年 | 682篇 |
1996年 | 635篇 |
1995年 | 583篇 |
1994年 | 561篇 |
1993年 | 465篇 |
1992年 | 414篇 |
1991年 | 442篇 |
1990年 | 443篇 |
1989年 | 422篇 |
1988年 | 384篇 |
1987年 | 466篇 |
1986年 | 393篇 |
1985年 | 475篇 |
1984年 | 516篇 |
1983年 | 509篇 |
1982年 | 479篇 |
1981年 | 412篇 |
1980年 | 414篇 |
1979年 | 349篇 |
1978年 | 344篇 |
1977年 | 330篇 |
1976年 | 290篇 |
1975年 | 294篇 |
1974年 | 316篇 |
1973年 | 343篇 |
1972年 | 218篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
122.
123.
Wimbush M. Chiswell S.M. Lukas R. Donohue K.A. 《Oceanic Engineering, IEEE Journal of》1990,15(4):380-383
A four-year record from an inverted echo sounder deployed near Palmyra Island at 6°N in the central Pacific Ocean is compared with a simultaneous record of subsurface pressure from this island lagoon. A factor m , converting round-trip acoustic travel time to surface dynamic height relative to a deep pressure level, was estimated from the ratio of the spectra of the two records in the energetic synoptic oscillation band. Year-to-year variation in m was not statistically significant. For the overall record, m was found to be -70±8 dynamic m/s, where the error bounds represent a 90% confidence interval. This is consistent with first-baroclinic-mode excitation 相似文献
124.
125.
126.
C. Tadhunter K. Wills R. Morganti T. Oosterloo R. Dickson 《Monthly notices of the Royal Astronomical Society》2001,327(1):227-232
We present new spectroscopic observations of the southern radio galaxy . Despite the flat-spectrum character of the radio emission from this source, our optical spectra show no sign of the broad permitted lines and non-stellar continuum characteristic of quasar nuclei and broad-line radio galaxies. However, the high-ionization forbidden lines, including [O iii ] λλ 5007, 4959, are unusually broad for a narrow-line radio galaxy , and are blueshifted by 600 km s−1 relative to the low-ionization lines such as [O ii ] λλ 3726,3729. The [O ii ] lines are also considerably narrower than the [O iii ] lines, and have a redshift consistent with that of the recently detected H i 21-cm absorption-line system. Whereas the kinematics of the [O iii ] emission lines are consistent with outflow in an inner narrow-line region, the properties of the [O ii ] emission lines suggest that they are emitted by a more extended and quiescent gaseous component. We argue that, given the radio properties of the source, our line of sight is likely to be lying close to the direction of bulk outflow of the radio jets. In this case it is probable that the quasar nucleus is entirely obscured at optical wavelengths by the material responsible for the H i absorption-line system. The unusually broad [O iii ] emission lines suggest that the radio source is intrinsically compact. Overall, our data are consistent the idea that is a radio source in an early stage of evolution. 相似文献
127.
A.G.G.M. Tielens L.B.F.M. Waters F.J. Molster K. Justtanont 《Astrophysics and Space Science》1997,255(1-2):415-426
This paper reviews spectra obtained with the SWS on board of ISO of dust shells around O-rich objects. These spectra reveal
the presence of many new emission features between 10 and 45 μm. These bands are generally much narrower than the well-known
10 and 20 μm silicates features. The strength of these features relative to the underlying broad continuum varies from source
to source (≅ 5-50%). The 10 μm region shows evidence for the presence of Al2O3 grains. At longer wavelength, the spectra are
dominated by features due to crystalline olivine and pyroxene. The exact peak position of these features shows that the emitting
grains consist of the Mg-rich end-members of these minerals with an Fe-content of < 10%. The underlying continuum is attributed
to amorphous silicate grains.
These observations of aluminum-rich and magnesium-rich compounds compare well with the thermodynamic condensation sequence
of minerals expected for O-rich outflows. The observations also imply that freeze out (ie., kinetics) of this condensation
sequence at different temperatures is an important characteristic of dust formation in these objects. It is suggested that
the absence of Fe-rich silicates is a natural consequence of the low temperature at which gaseous Fe reacts with Mg-rich silicates
in these outflows, resulting in amorphous grains with little characterizing spectral detail.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
128.
R. Wieler Th. Graf P. Signer S. Vogt G. F. Herzog C. Tuniz D. Fink L. K. Fifield J. Klein R. Middleton A. J. T. Jull P. Pellas J. Masarik G. Dreibus 《Meteoritics & planetary science》1996,31(2):265-272
Abstract— We determined He, Ne, Ar, 10Be, 26Al, 36Cl, and 14C concentrations, as well as cosmic-ray track densities and halogen concentrations in different specimens of the H6 chondrite Torino, in order to constrain its exposure history to cosmic radiation. The Torino meteoroid had a radius of ~20 cm and travelled in interplanetary space for 2.5–10 Ma. Earlier, Torino was part of a larger body. The smallest possible precursor had a radius of 55 cm and a journey through space longer than ~65 Ma. If the first-stage exposure took place in a body with a radius of >3 m or in the parent asteroid, then it lasted nearly 300 Ma. The example of Torino shows that it is easy to underestimate first-stage exposure ages when constructing two-stage histories. 相似文献
129.
White mica from the Liassic black shales and slates in Central Switzerland was analysed by transmission electron microscopy (TEM) and electron microprobe to determine its textural and compositional evolution during very low-grade prograde metamorphism. Samples were studied from the diagenetic zone, anchizone and epizone (T ≈100°–450 °C). Phyllosilicate minerals analysed include illite/smectite (I/S), phengite, muscovite, brammallite, paragonite, margarite and glauconite. Textural evolution primarily is towards larger, more defect-free grains with compositions that approach those of their respective end-members. The smectite-to-illite transformation reduced the amounts of the exchange components SiK?1Al?1, MgSiAl?2, and Fe3+Al?1. These trends continue to a lesser degree in the anchizone and epizone. Correlations between the proportion of smectite in I/S and the composition of I/S indicate that smectite layers may contain a high layer charge. Illite in I/S bears a compositional resemblance to macrocrystalline phengite in some samples, but is different in others. Paragonite first appears in the upper diagenetic zone or lower anchizone as an interlayer-deficient brammallite, and it may be mixed with muscovite on the nanometre scale. Owing to the small calculated structure factor for paragonite-muscovite superstructures, conventional X-ray powder diffraction cannot distinguish between mixed-layer structures and a homogeneous compositionally intermediate solid solutions. However, indirect TEM evidence shows that irregularly shaped domains of Na- and K-rich mica exist below 10 nm. Subsequent coarsening of domains at higher grades produced discrete paragonite grains at the margins of muscovite crystals or in laths parallel to the basal plane of the host muscovite. Margarite appears in the epizone and follows a textural evolution similar to paragonite in that mixtures of margarite, paragonite, and muscovite may initially occur on the nanometre scale. However, no evidence of interlayer-poor margarite has been found. 相似文献
130.
The Square Kilometer Array (SKA) will enable studies of star formation in nearby galaxies with a level of detail never before possible outside of the Milky Way. Because the earliest stages of stellar evolution are often inaccessible at optical and near-infrared wavelengths, high spatial resolution radio observations are necessary to explore extragalactic star formation. The SKA will have the sensitivity to detect individual ultracompact HII regions out to the distance of nearly 50 Mpc, allowing us to study their spatial distributions, morphologies, and populations statistics in a wide range of environments. Radio observations of Wolf-Rayet stars outside of the Milky Way will also be possible for the first time, greatly expanding the range of conditions in which their mass loss rates can be determined from free-free emission. On a vastly larger scale, natal of super star clusters will be accessible to the SKA out to redshifts of nearly z 0.1. The unprecedented sensitivity of radio observations with the SKA will also place tight constraints on the star formation rates as low as 1M yr−1 in galaxies out to a redshift of z 1 by directly measuring the thermal radio flux density without assumptions about a galaxy’s magnetic field strength, cosmic ray production rate, or extinction. 相似文献