全文获取类型
收费全文 | 1671篇 |
免费 | 66篇 |
国内免费 | 38篇 |
专业分类
测绘学 | 45篇 |
大气科学 | 135篇 |
地球物理 | 316篇 |
地质学 | 638篇 |
海洋学 | 104篇 |
天文学 | 404篇 |
综合类 | 11篇 |
自然地理 | 122篇 |
出版年
2024年 | 5篇 |
2023年 | 5篇 |
2022年 | 9篇 |
2021年 | 44篇 |
2020年 | 36篇 |
2019年 | 39篇 |
2018年 | 52篇 |
2017年 | 45篇 |
2016年 | 64篇 |
2015年 | 55篇 |
2014年 | 53篇 |
2013年 | 84篇 |
2012年 | 63篇 |
2011年 | 83篇 |
2010年 | 86篇 |
2009年 | 121篇 |
2008年 | 87篇 |
2007年 | 105篇 |
2006年 | 93篇 |
2005年 | 58篇 |
2004年 | 88篇 |
2003年 | 55篇 |
2002年 | 61篇 |
2001年 | 50篇 |
2000年 | 40篇 |
1999年 | 37篇 |
1998年 | 38篇 |
1997年 | 15篇 |
1996年 | 14篇 |
1995年 | 21篇 |
1994年 | 15篇 |
1992年 | 15篇 |
1991年 | 6篇 |
1989年 | 10篇 |
1988年 | 4篇 |
1987年 | 13篇 |
1986年 | 6篇 |
1985年 | 5篇 |
1984年 | 7篇 |
1983年 | 6篇 |
1982年 | 8篇 |
1981年 | 8篇 |
1980年 | 6篇 |
1978年 | 4篇 |
1977年 | 6篇 |
1976年 | 4篇 |
1975年 | 6篇 |
1974年 | 5篇 |
1973年 | 5篇 |
1969年 | 5篇 |
排序方式: 共有1775条查询结果,搜索用时 0 毫秒
21.
Simon P. Goodwin 《Astrophysics and Space Science》2009,324(2-4):259-263
The star-formation efficiency (SFE) of a star cluster is thought to be the critical factor in determining if the cluster can survive for a significant (>50 Myr) time. There is an often quoted critical SFE of ~30% for a cluster to survive gas expulsion. I reiterate that the SFE is not the critical factor; rather, it is the dynamical state of the stars (as measured by their virial ratio) immediately before gas expulsion. If the stars in a star cluster are born in an (even slightly) cold dynamical state, then the survivability of a cluster can be greatly increased. 相似文献
22.
23.
24.
Road Reduction Filtering for GPS-GIS Navigation 总被引:2,自引:0,他引:2
George Taylor Geoffrey Blewitt Doerte Steup Simon Corbett & Adrijana Car 《Transactions in GIS》2001,5(3):193-207
A novel method of map matching using the Global Positioning System (GPS) has been developed for civilian use, which uses digital mapping data to infer the systematic position errors of less than 100m which result largely from 'selective availability' (S/A) imposed by the US military. Selective availability was switched off on the 2nd of May 2000, and is to be replaced with 'regional denial capabilities in lieu of global degradation'. The system tracks a vehicle on all possible roads (road centre-lines) in a computed error region, then uses a method of rapidly detecting inappropriate road centre-lines from the set of all those possible. This is called the Road Reduction Filter (RRF) algorithm. Point positioning is computed using C/A code pseudorange measurements direct from a GPS receiver. The least squares estimation is performed in the software developed for the experiment described in this paper. Virtual differential GPS (VDGPS) corrections are computed and used from a vehicle's previous positions, thus providing an autonomous alternative to DGPS for in-car navigation and fleet management. Height aiding is used to augment the solution and reduce the number of satellites required for a position solution. Ordnance Survey (OS) digital map data was used for the experiment, i.e. OSCAR 1 m resolution road centre-line geometry and Land Form PANORAMA 1:50,000, 50 m-grid digital terrain model (DTM). Testing of the algorithm is reported and results are analysed. Vehicle positions provided by RRF are compared with the 'true' position determined using high precision (cm) GPS carrier phase techniques. It is shown that height aiding using a DTM and the RRF significantly improve the accuracy of position provided by inexpensive single frequency GPS receivers. 相似文献
25.
Matthieu?E.?GalvezEmail author Olivier?Beyssac Karim?Benzerara Nicolas?Menguy Sylvain?Bernard Simon?C.?Cox 《Contributions to Mineralogy and Petrology》2012,164(5):895-914
Understanding the mobility of chemical elements during fluid–rock interactions is critical to assess the geochemical evolution of a rock undergoing burial and metamorphism and, more generally, to constrain the geochemical budget of the subduction factory. In particular, determining the behavior and mobility of Ti in aqueous fluids constitutes a great challenge that is still under scrutiny. Here, we study plant fossils preserved in blueschist metasedimentary rocks from the Marybank Formation (New Zealand). Using scanning and transmission electron microscopies (SEM and TEM), we show that the carbonaceous material (CM) composing the fossils contains abundant nano-inclusions of Ti- and Fe-oxides. These nanocrystals are mainly anatase, rutile, and Fe–Ti oxides. The mineral composition observed within the fossils is significantly different from that detected in the surrounding rock matrix. We propose that Ti and Fe might have been mobilized by the alteration of a detrital Ti–Fe-rich protolith during an early diagenetic event under acidic and reducing conditions. Aqueous fluids rich in organic ligands released by the degradation of organic matter may have been involved. Moreover, using mass balance and petrological observations, we show that the contrasted mineralogy between the rock matrix and the fossil CM might be the consequence of the chemical isolation of fossil CM during the prograde path of the rock. Such an isolation results from the early formation of quartz and Fe-rich phyllosilicate layers enclosing the fossil as characterized by SEM and TEM investigations. Overall, this study shows that investigating minerals associated with CM down to the nanometer scale in metamorphic rocks can provide a precious record of early prograde geochemical conditions. 相似文献
26.
27.
Vincent M. Woolf C. Simon Jeffery Donald L. Pollacco 《Monthly notices of the Royal Astronomical Society》2002,329(3):497-501
We present an analysis of high-speed spectroscopy of the pulsating subdwarf B star PG 1605+072. Periodic radial motions are detected at frequencies similar to those reported for photometric variations in the star, with amplitudes of up to 6 km s−1 . Differences between relative strengths for given frequency peaks for our velocity data and previously measured photometry are probably a result of shifting of power between modes over time. Small differences in the detected frequencies may also indicate mode-shifting. We report the detection of line-shape variations using the moments of the cross-correlation function profiles. It may be possible to use the moments to identify the pulsation modes of the star. 相似文献
28.
C. Simon Jeffery & Don L. Pollacco 《Monthly notices of the Royal Astronomical Society》1998,298(1):179-184
Intermediate dispersion spectroscopy of a sample of 40 hot subdwarf B stars between 5500 and 9000Å has been obtained. The sample includes a large fraction of targets which have been studied photometrically. Seven targets show strong lines arising from the infrared Ca ii triplet, the unequivocal signature of a cool companion. The positive Ca ii identifications include known photometric binaries and new targets; all are associated with a photometric red excess. Assuming a canonical value for the subdwarf luminosity, all of the detected companions are overluminous compared with the main sequence. The detection procedure indicates an improved and more secure method for the measurement of the binary frequency of hot subdwarfs. 相似文献
29.
The host galaxies of active galactic nuclei 总被引:2,自引:0,他引:2
Guinevere Kauffmann Timothy M. Heckman Christy Tremonti Jarle Brinchmann Stéphane Charlot Simon D. M. White Susan E. Ridgway Jon Brinkmann Masataka Fukugita Patrick B. Hall eljko Ivezi Gordon T. Richards Donald P. Schneider 《Monthly notices of the Royal Astronomical Society》2003,346(4):1055-1077
We examine the properties of the host galaxies of 22 623 narrow-line active galactic nuclei (AGN) with 0.02 < z < 0.3 selected from a complete sample of 122 808 galaxies from the Sloan Digital Sky Survey. We focus on the luminosity of the [O iii ]λ5007 emission line as a tracer of the strength of activity in the nucleus. We study how AGN host properties compare with those of normal galaxies and how they depend on L [O iii ]. We find that AGN of all luminosities reside almost exclusively in massive galaxies and have distributions of sizes, stellar surface mass densities and concentrations that are similar to those of ordinary early-type galaxies in our sample. The host galaxies of low-luminosity AGN have stellar populations similar to normal early types. The hosts of high-luminosity AGN have much younger mean stellar ages. The young stars are not preferentially located near the nucleus of the galaxy, but are spread out over scales of at least several kiloparsecs. A significant fraction of high-luminosity AGN have strong Hδ absorption-line equivalent widths, indicating that they experienced a burst of star formation in the recent past. We have also examined the stellar populations of the host galaxies of a sample of broad-line AGN. We conclude that there is no significant difference in stellar content between type 2 Seyfert hosts and quasars (QSOs) with the same [O iii ] luminosity and redshift. This establishes that a young stellar population is a general property of AGN with high [O iii ] luminosities. 相似文献
30.
After briefly recalling basic facts about the metallicity gradients inspiral galaxies, we summarize two recent N-body gas-dynamical modelsthat implement a non-instantaneous gas recycling and point our their potential for an improved treatment of the chemical evolution ingalactic disks. 相似文献