全文获取类型
收费全文 | 49437篇 |
免费 | 7359篇 |
国内免费 | 9447篇 |
专业分类
测绘学 | 8564篇 |
大气科学 | 6456篇 |
地球物理 | 8422篇 |
地质学 | 22589篇 |
海洋学 | 5232篇 |
天文学 | 4670篇 |
综合类 | 3606篇 |
自然地理 | 6704篇 |
出版年
2024年 | 315篇 |
2023年 | 805篇 |
2022年 | 1710篇 |
2021年 | 2047篇 |
2020年 | 1968篇 |
2019年 | 2300篇 |
2018年 | 1638篇 |
2017年 | 2123篇 |
2016年 | 2109篇 |
2015年 | 2283篇 |
2014年 | 2779篇 |
2013年 | 2939篇 |
2012年 | 2976篇 |
2011年 | 3020篇 |
2010年 | 2556篇 |
2009年 | 3229篇 |
2008年 | 3127篇 |
2007年 | 3342篇 |
2006年 | 3232篇 |
2005年 | 2942篇 |
2004年 | 2621篇 |
2003年 | 2376篇 |
2002年 | 2046篇 |
2001年 | 1797篇 |
2000年 | 1737篇 |
1999年 | 1526篇 |
1998年 | 1307篇 |
1997年 | 934篇 |
1996年 | 806篇 |
1995年 | 713篇 |
1994年 | 657篇 |
1993年 | 570篇 |
1992年 | 393篇 |
1991年 | 343篇 |
1990年 | 236篇 |
1989年 | 202篇 |
1988年 | 156篇 |
1987年 | 96篇 |
1986年 | 54篇 |
1985年 | 42篇 |
1984年 | 27篇 |
1983年 | 16篇 |
1982年 | 17篇 |
1981年 | 14篇 |
1980年 | 14篇 |
1979年 | 9篇 |
1978年 | 16篇 |
1977年 | 17篇 |
1971年 | 7篇 |
1954年 | 12篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
82.
应用显微光度术、显微傅立叶红外光谱(Micro-FT.IR)和飞行时间二次离子质谱(TOF-SIMS)等原位微分析技术并结合均一温度测量对胜利油气区下第三系沙河街组沙三段中有机包裹体进行了研究。区分出两类有机包裹体,即原生有机包裹体和次生有机包裹体。结果表明两类有机包裹体特征不同,二者物质组成、有机质成分及化学结构、热演化程度等差别也较大。结合地质分析表明原生有机包裹体是沙三段烃源岩生成烃类运移产物,具“自生自储”特点,而次生有机包裹体是沙四段烃源岩生成的烃类二次运移的产物。沙三段是沙河街组油气运移和聚集的主要层位,因而是寻找油气资源的主要目标层。研究表明,有机包裹体是研究油气生成、运移、聚集和演化等成藏系统最有效的手段之一,在油气勘探中有重要的应用意义。 相似文献
83.
N.C. Hambly A.C. Davenhall M.J. Irwin H.T. MacGillivray 《Monthly notices of the Royal Astronomical Society》2001,326(4):1315-1327
In this, the third in a series of three papers concerning the SuperCOSMOS Sky Survey, we describe the astrometric properties of the data base. We describe the algorithms employed in the derivation of the astrometric parameters of the data, and demonstrate their accuracies by comparison with external data sets using the first release of data, the South Galactic Cap survey. We show that the celestial coordinates, which are tied to the International Celestial Reference Frame via the Tycho–2 reference catalogue, are accurate to better than ±0.2 arcsec at J , R ∼19,18 , rising to ±0.3 arcsec at J , R ∼22,21 , with positional-dependent systematic effects from bright to faint magnitudes at the ∼0.1-arcsec level. The proper motion measurements are shown to be accurate to typically ±10 mas yr−1 at J , R ∼19,18 , rising to ±50 mas yr−1 at J , R ∼22,21 , and are tied to zero using the extragalactic reference frame. We show that the zero-point errors in the proper motions are ≤1 mas yr−1 for R >17 , and are no larger than ∼10 mas yr−1 for R <17 mas yr−1 . 相似文献
84.
Boli basin, between Yishu fracture belt and Dunmi fracture belt, is the biggest Mesozoic coal basin in the east of Heilongjiang Province. Now it is a fault - fold remnant basin. The basin' s shape is generally consistent with the whole distribution of the cover folds, an arc protruding southwards. The basement of the basin can be divided into three fault blocks or structural units. The formation and evoluation of the basin in Mesozoic was determined by the basement fault blocks' displacement features rusulted from by the movement of the edge faults and the main basement faults. 相似文献
85.
The dust shell around the evolved star HD 179821 has been detected in scattered light in near-IR imaging polarimetry observations. Here, we subtract the contribution of the unpolarized stellar light to obtain an intrinsic linear polarization of between 30 and 40 per cent in the shell that seems to increase with radial offset from the star. The J - and K -band data are modelled using a scattering code to determine the shell parameters and dust properties. We find that the observations are well described by a spherically-symmetric distribution of dust with an r −2 density law, indicating that when mass-loss was occurring, the mass-loss rate was constant. The models predict that the detached nature of a spherically-symmetric, optically-thin dust shell, with a distinct inner boundary, will only be apparent in polarized flux. This is in accordance with the observations of this and other optically-thin circumstellar shells, such as IRAS 17436+5003. By fitting the shell brightness we derive an optical depth to the star that is consistent with V -band observations and that, assuming a distance of 6 kpc, gives an inner-shell radius of , a dust number density of at r in and a dust mass of . We have explored axisymmetric shell models but conclude that any deviations from spherical symmetry in the shell must be slight, with an equator-to-pole density contrast of less than 2:1. We have not been able to fit simultaneously the high linear polarizations and the small colour excess of the shell and we attribute this to the unusual scattering properties of the dust. We suggest that the dust grains around HD 179821 either are highly elongated or consist of aggregates of smaller particles. 相似文献
86.
C. Jordan S.A. Sim A.D. McMurry M. Aruvel 《Monthly notices of the Royal Astronomical Society》2001,326(1):303-312
Observations of ε Eri (K2 V) have been made with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope . The spectra obtained show a number of emission lines which can be used to determine, or place limits on, the electron density and pressure. Values of the electron pressure are required in order to make quantitative models of the transition region and inner corona from absolute line fluxes, and to constrain semi-empirical models of the chromosphere. Using line flux ratios in Si iii and O iv a mean electron pressure of P e = N e T e =4.8×1015 cm−3 K is derived. This value is compatible with the lower and upper limits to P e found from flux ratios in C iii , O v and Fe xii . Some inconsistencies which may be because of small uncertainties in the atomic data used are discussed. 相似文献
87.
88.
M. A. P. Torres J. Casares I. G. Martínez-Pais P. A. Charles 《Monthly notices of the Royal Astronomical Society》2002,334(1):233-240
We present a library of 47 open-cluster integrated spectra, mostly obtained at CASLEO (San Juan, Argentina) in the range 3600 < λ < 7400 Å , which are made available at CDS. The data are combined with previous spectra to obtain 10 high signal-to-noise ratio basic templates in the young and intermediate-age domains, which are also provided in the library. These Galactic disc templates represent the increased time resolution spectral evolution of a stellar population unit around the Solar metallicity level. The improved signal-to-noise ratio of the present templates with respect to previous template lists, together with their increased time resolution, allowed us to improve the fundamental parameters of some open clusters. The present spectral library will be useful for several astrophysical applications, particularly for population syntheses of star-forming giant galaxies. 相似文献
89.
In recent years,scholars at home and abroad have method,and that vanous disasters would be analyzed assynthetically studied natural dlsaste‘theoretically and an Integral.SHI Peilun(1991)putforwad a scientificmethodologlcally,as well as its cases analys。s(CND,term—regional disaster system,which Indicates that1987; MA et al,1990; MARBLE,1990; NE et al,the situation of a disaster(calamity loss)results from1999; PATAK et al,1982; SHI,1991;VAN et al,h… 相似文献
90.