全文获取类型
收费全文 | 34321篇 |
免费 | 2863篇 |
国内免费 | 4606篇 |
专业分类
测绘学 | 2508篇 |
大气科学 | 3137篇 |
地球物理 | 3731篇 |
地质学 | 7779篇 |
海洋学 | 3095篇 |
天文学 | 17750篇 |
综合类 | 1419篇 |
自然地理 | 2371篇 |
出版年
2024年 | 99篇 |
2023年 | 239篇 |
2022年 | 623篇 |
2021年 | 616篇 |
2020年 | 820篇 |
2019年 | 1003篇 |
2018年 | 691篇 |
2017年 | 802篇 |
2016年 | 794篇 |
2015年 | 1020篇 |
2014年 | 996篇 |
2013年 | 1179篇 |
2012年 | 1177篇 |
2011年 | 1149篇 |
2010年 | 1018篇 |
2009年 | 2451篇 |
2008年 | 2485篇 |
2007年 | 2909篇 |
2006年 | 2929篇 |
2005年 | 2612篇 |
2004年 | 2553篇 |
2003年 | 2195篇 |
2002年 | 1928篇 |
2001年 | 1602篇 |
2000年 | 1527篇 |
1999年 | 1415篇 |
1998年 | 1478篇 |
1997年 | 546篇 |
1996年 | 364篇 |
1995年 | 478篇 |
1994年 | 465篇 |
1993年 | 295篇 |
1992年 | 220篇 |
1991年 | 180篇 |
1990年 | 139篇 |
1989年 | 208篇 |
1988年 | 135篇 |
1987年 | 124篇 |
1986年 | 100篇 |
1985年 | 59篇 |
1984年 | 41篇 |
1983年 | 33篇 |
1982年 | 18篇 |
1981年 | 13篇 |
1980年 | 8篇 |
1979年 | 5篇 |
1977年 | 8篇 |
1954年 | 5篇 |
1897年 | 7篇 |
1877年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 745 毫秒
151.
The large-scale structure associated with the 2′N HNCO peak in Sgr B2 [Minh, Y.C., Haikala, L., Hjalmarson, Å., Irvine, W.M., 1998. ApJ 498, 261 (Paper I)] has been investigated. A ring-like morphology of the HNCO emission has been found; this structure may be colliding with the Principal Cloud of Sgr B2. This “HNCO Ring” appears to be centered at (l,b) = (0.7°,−0.07°), with a radius of 5 pc and a total mass of 1.0 × 105 to 1.6 × 106 M. The expansion velocity of the Ring is estimated to be 30–40 km s−1, which gives an expansion time scale of 1.5 × 105 year. The morphology suggests that collision between the Ring and the Principal Cloud may be triggering the massive star formation in the Sgr B2 cloud sequentially, with the latest star formation taking place at the 2′N position. The chemistry related to HNCO is not certain yet, but if it forms mainly via reaction with the evaporated OCN− from icy grain mantles, the observed enhancement of the HNCO abundance can be understood as resulting from shocks associated with the collision between the Principal Cloud and the expanding HNCO Ring. 相似文献
152.
This research paper analyses the grain-size characteristics of the Quaternary deposits at Xingshan near Siping, Jilin province in China by employing graphic measures to study the grain size distribution and its mode of transport and deposition. The Quaternary deposits at Xingshan lie unconformable on Cretaceous rocks made of siltstone, mudstone and sandstone. The average grain size is between 8.06 to 8.55Φ (0.002 6 ~0.003 7 mm). The Quaternary deposits at Xingshan mainly compose of very fine silt to clay. The compositions of the grade are clay 63% and silt 37%. The clay size components are weathered debris transported and deposited by flowing water from the SE highlands or hills to the low lying NW Xingshan plains whereas the silty components accumulated by aoelian process. The Quaternary deposits at Xingshan accumulated in the middle and late Pleistocene interglacial periods from (459.12~39.03) ka to (88.92~7.56) ka. The standard deviation ranged from 0.96 to 1.36Φ, indicating that the sediments are moderately to poorly sorted, Coefficient of skewness ranged from 0.16~0.31 with an average skewness of 0.218, (Positively skewed towards fine). Kurtosis values (0.84~1.05) from the grain size distribution and visual inspection of the frequency curves indicate platykurtic to mesokurtic curves and unimodal to bimodal grain-size distribution. The type of deposit formation is sand dune and the source is at a distal from its provenance. 相似文献
153.
154.
155.
156.
157.
158.
159.
The study of the chemical stability of vitreous material in aqueous media is well‐established. There has to date been little consideration of the implications of variations in the chemical durability of tephra in Quaternary tephrochronology. Chemical alteration can take the form of cationic leaching from the matrix, or complete destruction of the silica network, either of which could constrain the ability to chemically identify distal tephra. Here we apply established models of vitreous durability to the published chemical analyses of a large number of Icelandic tephras in order to predict their relative durabilities under equivalent conditions. This suggests that some important tephras have relatively poor chemical stability, and that rhyolitic tephras are, in general, more stable than basaltic. We conclude that tephras should be expected to show predictable differential chemical stability in the post‐depositional environment. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
160.
The results of a photometric monitoring of the quasar 4C 38.41, performed at the optical R and B bands in 2002 February–March, are presented. With a 60/90 cm Schmidt telescope at the Xinglong station of the National Astronomical Observatories of China, we observed the source exhibiting amplitude variations of up to 0.78 mag in both bands during the whole campaign. Intraday and even intranight variations are detected as well. A typical variability time-scale of about 36 d is derived from our 2-month observations at the optical bands, which is identical to that found at a radio wavelength of 92 cm, suggesting a common origin for the variations in 4C 38.41 from optical to radio bands. 相似文献