全文获取类型
收费全文 | 92784篇 |
免费 | 1566篇 |
国内免费 | 791篇 |
专业分类
测绘学 | 2354篇 |
大气科学 | 7168篇 |
地球物理 | 18368篇 |
地质学 | 33996篇 |
海洋学 | 7679篇 |
天文学 | 19748篇 |
综合类 | 288篇 |
自然地理 | 5540篇 |
出版年
2020年 | 524篇 |
2019年 | 561篇 |
2018年 | 3931篇 |
2017年 | 3751篇 |
2016年 | 3044篇 |
2015年 | 1216篇 |
2014年 | 1610篇 |
2013年 | 3515篇 |
2012年 | 2706篇 |
2011年 | 5088篇 |
2010年 | 4671篇 |
2009年 | 5672篇 |
2008年 | 4747篇 |
2007年 | 5118篇 |
2006年 | 2691篇 |
2005年 | 2821篇 |
2004年 | 2778篇 |
2003年 | 2658篇 |
2002年 | 2346篇 |
2001年 | 1935篇 |
2000年 | 1927篇 |
1999年 | 1623篇 |
1998年 | 1607篇 |
1997年 | 1584篇 |
1996年 | 1378篇 |
1995年 | 1304篇 |
1994年 | 1161篇 |
1993年 | 1051篇 |
1992年 | 991篇 |
1991年 | 862篇 |
1990年 | 1063篇 |
1989年 | 906篇 |
1988年 | 795篇 |
1987年 | 998篇 |
1986年 | 862篇 |
1985年 | 1076篇 |
1984年 | 1231篇 |
1983年 | 1175篇 |
1982年 | 1072篇 |
1981年 | 1042篇 |
1980年 | 892篇 |
1979年 | 859篇 |
1978年 | 890篇 |
1977年 | 828篇 |
1976年 | 789篇 |
1975年 | 724篇 |
1974年 | 727篇 |
1973年 | 740篇 |
1972年 | 460篇 |
1971年 | 402篇 |
排序方式: 共有10000条查询结果,搜索用时 718 毫秒
451.
452.
453.
Nonstationary hydrodynamic models of a viscous accretion disk around a central compact object were constructed. Two different numerical methods (TVD and SPH) are used to study the dynamics of dissipatively unstable acoustic perturbations at the nonlinear stage in terms of the standard α-disk model. The standard disk accretion in the Shakura-Sunyaev model is unstable against acoustic waves for various parameters of the system. If the α parameter, which specifies the level of turbulent viscosity, exceeds α?0.03, then a complex nonstationary system of small-scale weak shock waves is formed. The growth rate of the perturbations is higher in the central disk region. For α?0.2, the relative shock amplitude can exceed 50% of the equilibrium disk parameters. The reflection of waves from the disk boundaries and their nonlinear interaction are important factors that can produce unsteady accretion. The luminosity of such a disk undergoes quasi-periodic oscillations at a level of several percent (?5%) of the equilibrium level. 相似文献
454.
The effect of particle-particle interaction on the adiabatic index γ for an electron-positron plasma is considered. An improved method for numerically calculating the Hartree-Fock exchange integral is presented and its relativistic asymptotics is determined. An approximation formula is derived for the correlation part of the interaction in the low-density limit. This formula includes degeneracy and the positron component. 相似文献
455.
A high-velocity ionized outflow and XUV photosphere in the narrow emission line quasar PG1211+143 总被引:1,自引:0,他引:1
K. A. Pounds J. N. Reeves A. R. King K. L. Page P. T. O'Brien M. J. L. Turner 《Monthly notices of the Royal Astronomical Society》2003,345(3):705-713
We report on the analysis of a ∼60-ks XMM–Newton observation of the bright, narrow emission line quasar PG1211+143. Absorption lines are seen in both European Photon Imaging Camera and Reflection Grating Spectrometer spectra corresponding to H- and He-like ions of Fe, S, Mg, Ne, O, N and C. The observed line energies indicate an ionized outflow velocity of ∼24 000 km s−1 . The highest energy lines require a column density of N H ∼ 5 × 1023 cm−2 , at an ionization parameter of log ξ∼ 3.4 . If the origin of this high-velocity outflow lies in matter being driven from the inner disc, then the flow is likely to be optically thick within a radius of ∼130 Schwarzschild radii, providing a natural explanation for the big blue bump (and strong soft X-ray) emission in PG1211+143. 相似文献
456.
Introducing a concept of equivalent mass depth of flow, this study describes the phenomenon of non‐point source pollutant (metal) transport for pavement (or overland) flow in analogy with wave propagation in wide open channels. Hysteretic and normal mass rating curves are developed for runoff rate and mass of 12 dissolved and particulate‐bound metal elements (pollutants) using the rainfall‐runoff and water quality data of the 15 × 20 m2 instrumented pavement in Cincinnati, USA. Normal mass rating curves developed for easy computation of pollutant load are found to be of a form similar to Manning's, which is valid for open channel flows. Based on the hysteresis analysis, wave types for dissolution and mixing of particulate‐bound metals are identified. The analysis finds that the second‐order partial‐differential equation normally used for metal transport does not have the efficacy to describe fully the strong non‐linear phenomena such as is described for various metal elements by dynamic waves. In addition, the proportionality concept of the popular SCS‐CN concept is extended for determining the potential maximum metal mass Mp of all the 12 elements transported by a rain storm and related to the antecedent dry period (ADP). For the primary metal zinc element, Mp is found to increase with the ADP and vice versa. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
457.
Lance B. Morrissey Philippe Janvier Simon J. Braddy John P. Bennett Susan B. Marriott Peter R. Tarrant 《Geology Today》2006,22(2):66-67
Armoured jawless fish, or 'ostracoderms', lived 450–360 million years ago, and display unusual morphologies, unlike any modern fish group. Since they left no living descendants, their mode of swimming has, until recently, remained speculative, although this is a crucial question as the first true pectoral fins evolved within the 'ostracoderms'. The discovery of the oldest-known fish trails, from the Early Devonian (400 million year old) 'Lower Old Red Sandstone' of south-east Wales offers new insights into the swimming behaviour of these early fish, notably the osteostracan 'ostracoderms' (or cephalaspids), whose horseshoe-shaped head and paddle-shaped pectoral fins have remained a functional riddle. 相似文献
458.
A. D. Biggs D. Rusin I. W. A. Browne A. G. de Bruyn N. J. Jackson L. V. E. Koopmans J. P. McKean S. T. Myers R. D. Blandford K.-H. Chae C. D. Fassnacht M. A. Norbury T. J. Pearson P. M. Phillips A. C. S. Readhead P. N. Wilkinson 《Monthly notices of the Royal Astronomical Society》2003,338(4):1084-1088
459.
460.