全文获取类型
收费全文 | 3847篇 |
免费 | 633篇 |
国内免费 | 736篇 |
专业分类
测绘学 | 226篇 |
大气科学 | 304篇 |
地球物理 | 827篇 |
地质学 | 2107篇 |
海洋学 | 366篇 |
天文学 | 929篇 |
综合类 | 123篇 |
自然地理 | 334篇 |
出版年
2024年 | 21篇 |
2023年 | 50篇 |
2022年 | 95篇 |
2021年 | 147篇 |
2020年 | 138篇 |
2019年 | 142篇 |
2018年 | 127篇 |
2017年 | 159篇 |
2016年 | 165篇 |
2015年 | 154篇 |
2014年 | 206篇 |
2013年 | 208篇 |
2012年 | 197篇 |
2011年 | 228篇 |
2010年 | 195篇 |
2009年 | 260篇 |
2008年 | 234篇 |
2007年 | 242篇 |
2006年 | 224篇 |
2005年 | 244篇 |
2004年 | 218篇 |
2003年 | 211篇 |
2002年 | 189篇 |
2001年 | 174篇 |
2000年 | 170篇 |
1999年 | 147篇 |
1998年 | 161篇 |
1997年 | 91篇 |
1996年 | 65篇 |
1995年 | 69篇 |
1994年 | 59篇 |
1993年 | 40篇 |
1992年 | 36篇 |
1991年 | 23篇 |
1990年 | 30篇 |
1989年 | 26篇 |
1988年 | 22篇 |
1987年 | 6篇 |
1986年 | 13篇 |
1985年 | 8篇 |
1984年 | 7篇 |
1982年 | 5篇 |
1981年 | 1篇 |
1980年 | 4篇 |
1979年 | 1篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1971年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有5216条查询结果,搜索用时 109 毫秒
1.
Effects of the geometry of two‐dimensional fractures on their hydraulic aperture and on the validity of the local cubic law 下载免费PDF全文
Flow through rough fractures is investigated numerically in order to assess the validity of the local cubic law for different fracture geometries. Two‐dimensional channels with sinusoidal walls having different geometrical properties defined by the aperture, the amplitude, and the wavelength of the walls' corrugations, the corrugations asymmetry, and the phase shift between the two walls are considered to represent different fracture geometries. First, it is analytically shown that the hydraulic aperture clearly deviates from the mean aperture when the walls' roughness, the phase shift, and/or the asymmetry between the fracture walls are relatively high. The continuity and the Navier–Stokes equations are then solved by means of the finite element method and the numerical solutions compared to the theoretical predictions of the local cubic law. Reynolds numbers ranging from 0.066 to 66.66 are investigated so as to focus more particularly on the effect of flow inertial effects on the validity of the local cubic law. For low Reynolds number, typically less than 15, the local cubic law properly describes the fracture flow, especially when the fracture walls have small corrugation amplitudes. For Reynolds numbers higher than 15, the local cubic law is valid under the conditions that the fracture presents a low aspect ratio, small corrugation amplitudes, and a moderate phase lag between its walls. 相似文献
2.
3.
4.
Harker David E. Woodward Charles E. McMURTRY Craig W. Goetz J. A. Pipher J. L. Forrest W. J. 《Earth, Moon, and Planets》1997,78(1-3):259-264
We present 1- to 5-μm broadband and CVF images of comet Hale-Bopp taken 1997 February 10.5 UT, 50 days before perihelion.
All the images exhibit a nonspherical coma with a bright “ridge” in the direction of the dust tail approximately 10″ from
the coma. Synthetic aperture spectrophotometry implies that the optically important grains are of a radius ≤0.4 μm; smallest
radius for any comet seen to date. The variation of the integrated surface brightness with radial distance from the coma (ρ)
in all the images closely follows the “steady state” ρ−1 model for comet dust ablation (Gehrz and Ney, 1992). The near-infrared colors taken along the dust tail are not constant
implying the dust grain properties vary with coma distance.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
5.
J. M. De Buizer 《Monthly notices of the Royal Astronomical Society》2003,341(1):277-298
The results of a survey searching for outflows using near-infrared imaging are presented. Targets were chosen from a compiled list of massive young stellar objects associated with methanol masers in linear distributions. Presently, it is a widely held belief that these methanol masers are found in (and delineate) circumstellar accretion discs around massive stars. If this scenario is correct, one way to test the disc hypothesis is to search for outflows perpendicular to the methanol maser distributions. The main objective of the survey was to obtain wide-field near-infrared images of the sites of linearly distributed methanol masers using a narrow-band 2.12-μm filter. This filter is centred on the H2 v = 1–0 S(1) line; a shock diagnostic that has been shown to successfully trace CO outflows from young stellar objects. 28 sources in total were imaged of which 18 sources display H2 emission. Of these, only two sources showed emission found to be dominantly perpendicular to the methanol maser distribution. Surprisingly, the H2 emission in these fields is not distributed randomly, but instead the majority of sources are found to have H2 emission dominantly parallel to their distribution of methanol masers. These results seriously question the hypothesis that methanol masers exist in circumstellar discs. The possibility that linearly distributed methanol masers are instead directly associated with outflows is discussed. 相似文献
6.
7.
Flavio Scappini Cesare Cecchi-Pestellini Harvey Smith William Klemperer Alexander Dalgarno 《Monthly notices of the Royal Astronomical Society》2003,341(2):657-661
We consider sulphur depletion in dense molecular clouds, and suggest hydrated sulphuric acid, H2 SO4 · n H2 O, as a component of interstellar dust in icy mantles. We discuss the formation of hydrated sulphuric acid in collapsing clouds and its instability in heated regions in terms of the existing hot core models and observations. We also show that some features of the infrared spectrum of hydrated sulphuric acid have correspondence in the observed spectra of young stellar objects. 相似文献
8.
9.
F. Marcolini G. Bigazzi F. P. Bonadonna E. Centamore R. Cioni G. Zanchetta 《第四纪科学杂志》2003,18(6):545-556
The stratigraphical context of two Middle Pleistocene fossiliferous palaeosols from Central Italy (Abruzzo and Tuscany) have been studied. Small mammals and molluscs occur in both palaeosols, which are covered by tephra layers that were analysed using an interdisciplinary approach. Application of fission‐track dating to apatites separated from the Case Picconetto tephra (Pescara, Abruzzo), yielded an age of 0.48 ± 0.04 Ma, indistinguishable from those previously determined for the Campani Quarry (Lower Valdarno, Tuscany) (0.46 ± 0.05 Ma and 0.48 ± 0.05 Ma). Geochemical and petrographic investigations indicate that these tephra originated from different volcanoes, the Alban Hills Volcanic Complex and the Vico Volcano (Latium) respectively. Small mammal and mollusc assemblages indicate different palaeoclimatic and palaeoenvironmental conditions for the Case Picconetto and Campani Quarry palaeosols. Warm and humid conditions can be inferred for the Campani Quarry site, whereas open and cold conditions can be inferred for Case Picconetto. On the basis of faunal data, fission‐track dates and attribution of tephra to specific volcanic eruptions, we suggest a correlation of these faunas with marine oxygen isotope stage 14 (Case Picconetto) and with marine oxygen isotope stage 11 (Campani Quarry), respectively. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
10.