全文获取类型
收费全文 | 535450篇 |
免费 | 62613篇 |
国内免费 | 81326篇 |
专业分类
测绘学 | 49772篇 |
大气科学 | 63789篇 |
地球物理 | 113049篇 |
地质学 | 243348篇 |
海洋学 | 69490篇 |
天文学 | 56812篇 |
综合类 | 25719篇 |
自然地理 | 57410篇 |
出版年
2023年 | 5085篇 |
2022年 | 14870篇 |
2021年 | 19212篇 |
2020年 | 16624篇 |
2019年 | 19488篇 |
2018年 | 22633篇 |
2017年 | 21499篇 |
2016年 | 23694篇 |
2015年 | 21214篇 |
2014年 | 25646篇 |
2013年 | 34344篇 |
2012年 | 30872篇 |
2011年 | 34367篇 |
2010年 | 33092篇 |
2009年 | 34744篇 |
2008年 | 33597篇 |
2007年 | 32217篇 |
2006年 | 30263篇 |
2005年 | 24899篇 |
2004年 | 20425篇 |
2003年 | 16764篇 |
2002年 | 16113篇 |
2001年 | 14684篇 |
2000年 | 14705篇 |
1999年 | 11496篇 |
1998年 | 8853篇 |
1997年 | 7889篇 |
1996年 | 7728篇 |
1995年 | 7147篇 |
1994年 | 6394篇 |
1993年 | 4610篇 |
1992年 | 4216篇 |
1991年 | 3675篇 |
1990年 | 3581篇 |
1989年 | 3186篇 |
1988年 | 2900篇 |
1987年 | 3218篇 |
1986年 | 2997篇 |
1985年 | 3376篇 |
1984年 | 3672篇 |
1983年 | 3284篇 |
1982年 | 2979篇 |
1981年 | 2746篇 |
1980年 | 2471篇 |
1979年 | 2344篇 |
1978年 | 2216篇 |
1977年 | 1975篇 |
1976年 | 1853篇 |
1975年 | 1758篇 |
1973年 | 1778篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
Mercury (Hg) was investigated in bone tissues of skua ( Catharacta maccormick) and penguin (Pygoscelis adeliae) collected in the maritime Antarctic using atomic fluorescence spectrometry (AFS) and synchrotron radiation X-ray fluorescence (SR-XRF) method. The total levels of mercury in bone tissues of penguin and skua are much lower than those in other organs (e. g. , kidney, liver). The toxic effects of mercury in bone tissues of seabirds in polar region are not known. We have used SR- XRF method to map the distribution of trace levels of mercury in bones. The levels of mercury are found to be enriched somewhere near the periosteal surface and/or endosteal surface. The distribution of mercury shows strongly correlation with that of some essential elements and probably poses negative effect on the bone metabolism inferring from the relationship of mercury with the other elements. These studies represent a first step toward understanding the toxic effects of mercury on bone of polar animals by suggesting the possible microscopic investigation. 相似文献
142.
Mallory S. E. Roberts Crystal L. Brogan Bryan M. Gaensler Jason W. T. Hessels C.-Y. Ng Roger W. Romani 《Astrophysics and Space Science》2005,297(1-4):93-100
A remarkable number of pulsar wind nebulae (PWN) are coincident with EGRET γ-ray sources. X-ray and radio imaging studies of unidentified EGRET sources have resulted in the discovery of at least six new pulsar wind nebulae (PWN). Stationary PWN (SPWN) appear to be
associated with steady EGRET sources with hard spectra, typical for γ-ray pulsars. Their toroidal morphologies can help determine the geometry of the
pulsar which is useful for constraining models of pulsed γ-ray emission. Rapidly moving PWN (RPWN) with more cometary morphologies
seem to be associated with variable EGRET sources in regions where the ambient medium is dense compared to what is typical for the ISM. 相似文献
143.
太阳发电机理论主要解释太阳磁场的起源和演化,是太阳物理中最基本、最重要的问题之一.对它的研究不仅具有重要的科学价值,而且对空间天气的预测和空间技术的发展有一定的影响.随着日震学的发展,该理论也得到了很大改进。从相关观测入手,综述了发电机理论需解释的观测事实,以及为发电机理论提供约束的太阳内部动力学结构;介绍了基本理论和近来的主流模型,并指出尚待解决的问题和进一步的努力方向。 相似文献
144.
Xue-Fei Chen Zhan-Wen HanNational Astronomical Observatories Yunnan Observatory Chinese Academy of Sciences. Kunming Graduate School of the Chinese Academy of Sciences Beijing 《中国天文和天体物理学报》2005,5(1):65-76
We have studied the influence of different choices of core-envelope transition point on the final merger of contact binaries with two main-sequence components. A binary of 1.00 + 0.90M⊙ with an initial orbital period of 0.35d is examined. The mass fraction of the primary mixed with the matter of the secondary, qmix, determined by the chosen core-envelope transition point, ranges from 0.04 to 1.00 in our analysis. If as qmix< 0.8, none of the 相似文献
145.
146.
147.
148.
Abstract The Yaxcopoil‐1 (Yax‐1) drill hole comprises Cretaceous limestones and calcarenites, the K/P boundary cocktail unit (including impact breccia), and a Danian marly clay layer overlain by calcareous marls. The biostratigraphy, paleobathymetry, and environmental turnover across the K/P interval were inferred after analyzing the planktic and benthic foraminiferal assemblages. The Cretaceous samples only contain a few poorly preserved planktic foraminifera of a middle Campanian to Maastrichtian age, while low‐diversity benthic foraminiferal assemblages suggest a sufficient nutrient supply to the sea floor and a shallow neritic, occasionally stressed environment. The impact breccia and the redeposited suevite are overlain by a 46 cm‐thick dolomitic calcareous sandstone unit that contains scarce, reworked planktic foraminiferal specimens. This unit probably represents the uppermost part of the initial infill of the crater. The uppermost centimeters of this unit are bioturbated, and its top represents a hiatus that spans at least the G. cretacea, Pv. eugubina, and part of the P. pseudobulloides biozones. This unit is overlain by a 3–4 cm‐thick marly clay layer that represents a condensed layer. Benthic foraminiferal assemblages suggest a low food supply to the sea floor and environmental instability during the deposition of the marly clay layer. The increase in diversity of the assemblages indicates that the environmental conditions improved and stabilized from the G. compressa biozone toward the A. uncinata (P2) biozone. The Danian planktic and benthic foraminiferal assemblages indicate a deeper, probably bathyal environment. 相似文献
149.
150.
A spectacular change in the lower corona on the south-west limb has been found in solar images taken by the Yohkoh soft X-ray telescope. The event is characterized by a large topological change in magnetic field and a large intensity decrease observed after the X1. 1/1B flare on 9 November, 1991. A coronal mass ejection (CME) was observed by the Mark III K-coronameter (MK3) at the HAO/Mauna Loa Observatory. Both the MK3 (white-light) and soft X-ray observations showed that one leg of this CME was located above the flare site. An interplanetary shock associated with this event was observed by Pioneer Venus Orbiter, and, possibly, by IMP-8.Also Cooperative Institute for Research in the Environmental Sciences (CIRES), University of Colorado, Boulder, CO 80309, U.S.A. 相似文献