全文获取类型
收费全文 | 32705篇 |
免费 | 6129篇 |
国内免费 | 7958篇 |
专业分类
测绘学 | 623篇 |
大气科学 | 1494篇 |
地球物理 | 5202篇 |
地质学 | 24085篇 |
海洋学 | 3622篇 |
天文学 | 5156篇 |
综合类 | 1519篇 |
自然地理 | 5091篇 |
出版年
2024年 | 101篇 |
2023年 | 489篇 |
2022年 | 1087篇 |
2021年 | 1269篇 |
2020年 | 1273篇 |
2019年 | 1508篇 |
2018年 | 1356篇 |
2017年 | 1471篇 |
2016年 | 1550篇 |
2015年 | 1588篇 |
2014年 | 2053篇 |
2013年 | 2173篇 |
2012年 | 2158篇 |
2011年 | 2339篇 |
2010年 | 2110篇 |
2009年 | 2610篇 |
2008年 | 2486篇 |
2007年 | 2575篇 |
2006年 | 2351篇 |
2005年 | 1988篇 |
2004年 | 1774篇 |
2003年 | 1608篇 |
2002年 | 1270篇 |
2001年 | 1091篇 |
2000年 | 1118篇 |
1999年 | 961篇 |
1998年 | 731篇 |
1997年 | 580篇 |
1996年 | 518篇 |
1995年 | 499篇 |
1994年 | 450篇 |
1993年 | 371篇 |
1992年 | 288篇 |
1991年 | 229篇 |
1990年 | 197篇 |
1989年 | 155篇 |
1988年 | 112篇 |
1987年 | 59篇 |
1986年 | 47篇 |
1985年 | 40篇 |
1984年 | 39篇 |
1983年 | 24篇 |
1982年 | 16篇 |
1981年 | 19篇 |
1980年 | 15篇 |
1979年 | 10篇 |
1978年 | 6篇 |
1977年 | 16篇 |
1954年 | 8篇 |
1875年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
The absolute magnitudeM
v
of the hydrogen deficient binary υ Sgr has been estimated as -4.8 ± 1.0 from the distribution of the interstellar reddening,
polarization and interstellar lines of the surrounding stars. From the ANS observations obtained at the time of the secondary
eclipse, it appears that the hotter secondary is surrounded by a disc with colours of a B8-B9 star. The λ 1550 CIv absorption
line arising in the stellar wind does not show any change in strength during the secondary minimum. The upper limit to the
mass-loss rate from the high temperature wind is estimated as ≤ 5 × 10-7 M⊙ yr-1 from the 2 cm and 6 cm radio observations.
Based on observations obtained with the Astronomical Netherlands Satellite and VLA. The National Radio Astronomy Observatory’s
Very Large Array at Socorro, New Mexico is operated by Associated Universities Inc. under contract with the National Science
Foundation. 相似文献
112.
113.
114.
We present a new Very Large Array (VLA) image of Saturn, made from data taken in October 1998 at a wavelength of λ3.6 cm. The moderate ring opening angle (B≈15°) allows us to explore direct transmission of microwave photons through the A and C rings. We find a strong asymmetry of photons transmitted through the A ring, but not in the C ring, a new diagnostic of wake structure in the ring particles. We also find a weak asymmetry between east and west for the far side of the ansae. To facilitate quantitative comparison between dynamic models of the A ring and radio observations, we extend our Monte Carlo radiative transfer code (described in Dunn et al., 2002, Icarus 160, 132-160) to include idealized wakes. We show the idealized model can reproduce the properties of dynamic simulations in directly transmitted light. We examine the model behavior in directly transmitted and scattered light over a range of physical and geometric wake parameters. Finally, we present a wake model with a plausible set of physical parameters that quantitatively reproduces the observed intensity and asymmetry of the A ring both across the planet and in the ansae. 相似文献
115.
Geophysical data from Gerlache Strait, Croker Passage, Bismarck Strait and the adjacent continental shelf reveal streamlined subglacial bedforms that were produced at the bed of the Antarctic Peninsula Ice Sheet (APIS) during the last glaciation. The spatial arrangement and orientation of these bedforms record the former drainage pattern and flow dynamics of an APIS outlet up‐flow, and feeding into, a palaeo‐ice stream in the Western Bransfield Basin. Evidence suggests that together, they represent a single ice‐flow system that drained the APIS during the last glaciation. The ice‐sheet outlet flowed north/northeastwards through Gerlache Strait and Croker Passage and converged with a second, more easterly ice‐flow tributary on the middle shelf to form the main palaeo‐ice stream. The dominance of drumlins with low elongation ratios suggests that ice‐sheet outlet draining through Gerlache Strait was comparatively slower than the main palaeo‐ice stream in the Western Bransfield Basin, although the low elongation ratios may also partly reflect the lack of sediment. Progressive elongation of drumlins further down‐flow indicates that the ice sheet accelerated through Croker Passage and the western tributary trough, and fed into the main zone of streaming flow in the Western Bransfield Basin. Topography would have exerted a strong control on the development of the palaeo‐ice stream system but subglacial geology may also have been significant given the transition from crystalline bedrock to sedimentary strata on the inner–mid‐shelf. In the broader context, the APIS was drained by a number of major fast‐flowing outlets through cross‐shelf troughs to the outer continental shelf during the last glaciation. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
116.
WANG Li-ping LI Chuang TANG Bing-yong 《东北亚地学研究》2005,8(1):78-93
The Donghetang Formation (Upper Devonian) in central Tarim Basin has been thought an important oil and gas reservoir since the abundant oil and gas resources were found in the wells W16, W20, W34, and other fields. However, the sedimentary environment of the Donghetang Formation has been disputed because it suffered from both tidal and fluvial actions and there were not rich fossils in the sandstone. After the authors analyzed sedimentary features by means of drill cores, well logging data, paleosols, and with SEM obseruations, three kinds of sedimentary environments were distinguished: alluvial fan, tide-dominated estuary, and shelf. Particularly, the sedimentary features of tide-dominated estuary were studied in detail. Besides, the authors discussed sedimentary characteristics of the Donghetang Formation which was divided into two fourth-order sequences and five system tracts. At the same time, according to the forming process of five system tracts, the whole vertical evolution and lateral transition of tide-dominated estuary were illustrated clearly. Finally, the reservoir quality was evaluated based on porosity and permeability. 相似文献
117.
118.
119.
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
120.