全文获取类型
收费全文 | 94877篇 |
免费 | 1885篇 |
国内免费 | 749篇 |
专业分类
测绘学 | 2394篇 |
大气科学 | 7196篇 |
地球物理 | 19410篇 |
地质学 | 31870篇 |
海洋学 | 8513篇 |
天文学 | 21377篇 |
综合类 | 195篇 |
自然地理 | 6556篇 |
出版年
2021年 | 682篇 |
2020年 | 811篇 |
2019年 | 868篇 |
2018年 | 1696篇 |
2017年 | 1610篇 |
2016年 | 2198篇 |
2015年 | 1445篇 |
2014年 | 2174篇 |
2013年 | 4741篇 |
2012年 | 2335篇 |
2011年 | 3426篇 |
2010年 | 3045篇 |
2009年 | 4266篇 |
2008年 | 3777篇 |
2007年 | 3612篇 |
2006年 | 3502篇 |
2005年 | 2988篇 |
2004年 | 3034篇 |
2003年 | 2860篇 |
2002年 | 2754篇 |
2001年 | 2436篇 |
2000年 | 2363篇 |
1999年 | 2008篇 |
1998年 | 2017篇 |
1997年 | 1998篇 |
1996年 | 1736篇 |
1995年 | 1646篇 |
1994年 | 1495篇 |
1993年 | 1362篇 |
1992年 | 1298篇 |
1991年 | 1152篇 |
1990年 | 1352篇 |
1989年 | 1194篇 |
1988年 | 1100篇 |
1987年 | 1294篇 |
1986年 | 1180篇 |
1985年 | 1436篇 |
1984年 | 1624篇 |
1983年 | 1522篇 |
1982年 | 1403篇 |
1981年 | 1397篇 |
1980年 | 1171篇 |
1979年 | 1152篇 |
1978年 | 1165篇 |
1977年 | 1100篇 |
1976年 | 1009篇 |
1975年 | 963篇 |
1974年 | 962篇 |
1973年 | 974篇 |
1972年 | 600篇 |
排序方式: 共有10000条查询结果,搜索用时 764 毫秒
31.
32.
J. Raitala 《Earth, Moon, and Planets》1996,74(3):191-214
The intrablock deformation of Meshkenet Tessera on Venus is mostly due to responses of the uppermost surface bedrock to tensional stresses. It is found that complex deformation structures within the highland blocks resemble those of formed in chocolate tablet boudinaging which has taken place after original parallel faulting and bar-like crustal block formation. The high-angle tessera structures with varying cross-cutting relations define styles and locations of multiphase deformation most evidently related to local relaxation of tessera topography. Series of progressive or superposed fracturing events with alternating fault directions took place at high angles during this relaxational deformation. Compressional ridges often surround these tesserae. 相似文献
33.
E. C. Hopewell M. J. Barlow J. E. Drew Y. C. Unruh Q. A. Parker M. J. Pierce P. A. Crowther C. Knigge S. Phillipps A. A. Zijlstra 《Monthly notices of the Royal Astronomical Society》2005,363(3):857-866
We report the discovery of five massive Wolf–Rayet (WR) stars resulting from a programme of follow-up spectroscopy of candidate emission-line stars in the Anglo-Australian Observatory United Kingdom Schmidt Telescope (AAO/UKST) Southern Galactic Plane Hα survey. The 6195–6775 Å spectra of the stars are presented and discussed. A WC9 class is assigned to all five stars through comparison of their spectra with those of known late-type WC stars, bringing the known total number of Galactic WC9 stars to 44. Whilst three of the five WC9 stars exhibit near-infrared (NIR) excesses characteristic of hot dust emission (as seen in the great majority of known WC9 stars), we find that two of the stars show no discernible evidence of such excesses. This increases the number of known WC9 stars without NIR excesses to seven. Reddenings and distances for all five stars are estimated. 相似文献
34.
35.
36.
Abstract— We have analyzed several types of data associated with the well‐documented fall of the Neuschwanstein meteorites on April 6, 2002 (a total of three meteorites have been recovered). This includes ground‐based photographic and radiometer data as well as infrasound and seismic data from this very significant bolide event (Spurný et al. 2002, 2003). We have also used these data to model the entry of Neuschwanstein, including the expected dynamics, energetics, panchromatic luminosity, and associated fragmentation effects. In addition, we have calculated the differential efficiency of acoustical waves for Neuschwanstein and used these values to compare against the efficiency calculated using available ground‐based infrasound data. This new numerical technique has allowed the source height to be determined independent of ray tracing solutions. We have also carried out theoretical ray tracing for a moving point source (not strictly a cylindrical line emission) and for an infinite speed line source. In addition, we have determined the ray turning heights as a function of the source height for both initially upward and downward propagating rays, independent of the explicit ray tracing (detailed propagation path) programs. These results all agree on the origins of the acoustic emission and explicit source heights for Neuschwanstein for the strongest infrasonic signals. Calculated source energies using more than four different independent approaches agree that Neuschwanstein was certainly <500 kg in initial mass, given the initial velocity of 20.95 km/s, resulting in an initial source energy ≤0.0157‐0.0276 kt TNT equivalent (4.185 times 1012 J). Local source energies at the calculated infrasonic/seismic source altitudes are up to two orders of magnitude smaller than this initial source energy. 相似文献
37.
New determination of the Earth orientation parameters (EOP), based on optical astrometry observations since the beginning of the century, is now under preparation by the Working group established by Commission 19 of the IAU. The Hipparcos catalog is to define the celestial reference frame in which the new series of EOP are to be described, The novelties of the prepared solution are the higher resolution (5 days) and more parameters estimated from the solution (celestial pole offsets, rheological parameters of the Earth, certain instrumental constants). The mathematical model of the solution is described, and the results based on the observations made with 46 instruments at 29 observatories and a preliminary Hipparcos catalog are presented. 相似文献
38.
39.
40.
Non‐equilibrium concepts lead to a unified explanation of the formation of chondrules and chondrites
Milton BLANDER Arthur D. PELTON In‐Ho JUNG Richard WEBER 《Meteoritics & planetary science》2004,39(12):1897-1910
Abstract— Calculations of the formation of seven types of chondrules in Semarkona from a gas of solar composition were performed with the FACT computer program to predict the chemistries of oxides (including silicates), developed by the authors and their colleagues. The constrained equilibrium theory was used in the calculations with two nucleation constraints suggested by nucleation theory. The first constraint was the blocking of Fe and other metal gaseous atoms from condensing to form solids or liquids because of very high surface free energies and high surface tensions of the solid and liquid metals, respectively. The second constraint was the blocking of the condensation of solids and the formation of metastable liquid oxides (including silicates) well below their liquidus temperatures. Our laboratory experiments suggested subcooling of type IIA chondrule compositions of 400 degrees or more below the liquidus temperature. The blocking of iron leads to a supersaturation of Fe atoms, so that the partial pressure of Fe (pFe) is larger than the partial pressure at equilibrium (pFe(eq)). The supersaturation ratio S = pFe/pFe(eq) becomes larger than 1 and increases rapidly with a decrease in temperature. This drives the reaction Fe + H2O ? H2 + FeO to the right. With S = 100, the activity of FeO in the liquid droplet is 100 times as large as the value at equilibrium. The FeO activities are a function of temperature and provide relative average temperatures of the crystallization of chondrules. Our calculations for the LL3.0 chondrite Semarkona and our study of some non‐equilibrium effects lead to accurate representations of the compositions of chondrules of types IA, IAB, IB, IIA, IIAB, IIB, and CC. Our concepts readily explain both the variety of FeO concentrations in the different chondrule types and the entire process of chondrule formation. Our theory is unified and could possibly explain the formation of chondrules in all chondritic meteorites as well as provide a simple explanation for the complex chemistries of chondrites, and especially for type 3 chondrites. 相似文献