全文获取类型
收费全文 | 5569篇 |
免费 | 248篇 |
国内免费 | 30篇 |
专业分类
测绘学 | 304篇 |
大气科学 | 345篇 |
地球物理 | 1402篇 |
地质学 | 1921篇 |
海洋学 | 404篇 |
天文学 | 1118篇 |
综合类 | 39篇 |
自然地理 | 314篇 |
出版年
2023年 | 21篇 |
2022年 | 46篇 |
2021年 | 93篇 |
2020年 | 115篇 |
2019年 | 95篇 |
2018年 | 243篇 |
2017年 | 218篇 |
2016年 | 313篇 |
2015年 | 239篇 |
2014年 | 293篇 |
2013年 | 365篇 |
2012年 | 279篇 |
2011年 | 354篇 |
2010年 | 269篇 |
2009年 | 341篇 |
2008年 | 291篇 |
2007年 | 202篇 |
2006年 | 211篇 |
2005年 | 196篇 |
2004年 | 179篇 |
2003年 | 145篇 |
2002年 | 140篇 |
2001年 | 106篇 |
2000年 | 91篇 |
1999年 | 84篇 |
1998年 | 104篇 |
1997年 | 87篇 |
1996年 | 52篇 |
1995年 | 43篇 |
1994年 | 50篇 |
1993年 | 28篇 |
1992年 | 45篇 |
1991年 | 30篇 |
1990年 | 43篇 |
1989年 | 27篇 |
1988年 | 28篇 |
1987年 | 26篇 |
1986年 | 15篇 |
1985年 | 32篇 |
1984年 | 28篇 |
1983年 | 41篇 |
1982年 | 21篇 |
1981年 | 21篇 |
1980年 | 19篇 |
1979年 | 17篇 |
1978年 | 19篇 |
1977年 | 14篇 |
1975年 | 15篇 |
1973年 | 11篇 |
1972年 | 14篇 |
排序方式: 共有5847条查询结果,搜索用时 15 毫秒
21.
A solution of Einstein-Maxwell equations representing a distribution of equal and opposite charges moving in circles in opposite directions around thez-axis is obtained. In a particular case the solution reduces to the special case of Levi-Civita metric for a line-mass placed along thez-axis. The asymptotic behaviour of the solution is also discussed. 相似文献
22.
Geometric analysis of nested Riedel structures was used to identify and quantify strain localization processes within faulted Navajo sandstone. The analysis shows systematic deviation from the basic Riedel geometry complying with the Mohr–Coulomb criterion. Using cross-cutting relations amongst deformation bands within the Riedel structures, and comparing the orientations of the deformation bands to theoretical strain calculations, we identify two coupling deformation mechanisms involved in the early stages of shear-zone evolution, namely, granular flow and discrete faulting. Both mechanisms localize during strain accumulation, and the granular flow facilitates considerable change in the initial geometry of the Riedel structures. The analysis demonstrates a systematic sequence, by which new Riedel structures form after a constant amount of shear strain takes place in the sandstone. Analysis further indicates that granular flow is the major deformation mechanism during early stages of shear-zone evolution and discrete faulting is the dominant mechanism during later deformation stages. 相似文献
23.
A. Cassatella F. A. Córdova M. Friedjung J. Kenyon L. Piro R. Viotti 《Astrophysics and Space Science》1987,131(1-2):763-763
We describe the first X-ray monitoring of a symbiotic star during phases of enhanced activity. AG Dra is a Pop II object with a composite spectrum, characterized by a cool K-type component, prominent high ionization emission lines and a strong UV continuum which is attributed to a hot dwarf companion. Periodic variability of the UV radiation during minimum could be attributed to the orbital motion of the system. In April 1980 HEAO-2 detected an intense, soft X-ray flux from AG Dra, stronger than in other symbiotic stars. After one major outburst of November 1980, which continued until 1983, two more outbursts occurred in February 1985 and January 1986, and coordinated X-ray (EXOSAT) and ultraviolet (IUE) observations were organized to study the behaviour of AG Dra during different activity phases. EXOSAT observations made during decline after the 1985 outburst, revealed a weak X-ray flux in the Thin Lexan filter of the Low Energy dedtector. Observations made during minimum, in June and November 1985, at phases 0.22 and 0.50 of the UV light curve, disclosed the presence of an intense X-ray flux, which was not occulted in November. AG Dra was again observed with EXOSAT in February 1986 when the stellar luminosity was still at maximum. No X-ray flux was detected, in spite of the prominent, high ionization UV spectrum observed with IUE.A detailed discussion of the X-ray and ultraviolet results on AG Dra in the light of possible models is in progress.Paper presented at the IAU Colloquium No. 93 on Cataclysmic Variables. Recent Multi-Frequency Observations and Theoretical Developments, held at Dr. Remeis-Sternwarte Bamberg, F.R.G., 16–19 June, 1986. 相似文献
24.
Ram S 《Population geography : a journal of the Association of Population Geographers of India》1987,9(1-2):37-44
The period of highest migration from India to England was 1955-1975. In 1981, the Bradford metropolitan district had about 13,000 Indians. 46% were Punjabis and 43.3% were from Gujarat. Using a 10% sample of Indian households in the Bradford district in 1984 and secondary information, this study examines the impact of the following reasons on decision to migrate: 1) push factors in the area of origin, 2) the 1947 partition of India, 3) strong economic attraction of the destination, and 4) "cultural ethos" and "status competition" among the migrant communities in the areas of emigration. Findings show that 1) Punjab and Gujarat do not have high poverty levels nor very high population densities; 2) the partition of India did not influence migration decisions; 3) the economic well-being of returning migrants and the high conversion rate of English currency did attract migrants to England (39% of surveyed migrants emigrated to England for purely economic reasons); and 4) 60% of the Punjabis and over 50% of the Gujaratis state that local status competition had a significant influence on their decisions to migrate. Thus, the economic attraction of England and the status competition among local families and the community were the dominant factors in migration decision making. 相似文献
25.
The 5-dimensional Jordan-Brans-Dicke cosmologies in vacuum are found for the Bianchi type I metric, their relation with general relativity cosmologies is studied. Two solutions are possible, both produce effective pressure and energy density in the 4-dimensional G.R.-universes. One is a power-law relation, with two cases, the first one is forp
eff=eff and the other forp
eff=eff(– 1 < < 1) has a behaviour as the open flat universe. The second solution is an exponential only valid forp
eff=–eff. In all cases the three-space expansion reaches infinity ast and the fifth dimension can be made to decrease approaching zero. The scalar field can increase or decrease with time. 相似文献
26.
Ram S 《Population geography : a journal of the Association of Population Geographers of India》1986,8(1-2):17-26
The author analyzes the spatial distribution of Indians living in Great Britain in 1981. The impact of factors such as demand for labor, location of manufacturing industries, and the attraction of urban areas is examined. 相似文献
27.
On the coronograph spectrophotographic records taken on 31.372 UT, August 1979, some faint emission features were found which can be ascribed to Siii and Niii. These emissions were obviously a transient phenomena which were detected only 10 hr after the supposed fall of Comet 1979 XI in the Sun's photosphere. It cannot be excluded that the appearance of Si and Ni lines was triggered by the evaporation of dust particles with a high abundance of heavier elements in the solar corona. This assumption is also supported by intensity distribution of the Fex coronal line around the Sun's limb. The maximum coincide with the position angle of the projected path of the comet. 相似文献
28.
Hannes Alfvén 《Astrophysics and Space Science》1983,97(1):79-94
The paper is based on Holberg's analysis of the Voyager photographs in both reflected and transparent light, combined with occultation data of stars seen through the rings. Besides rapidly varying phenomena (spokes, braided ring, etc.), which according to Mendis are due to gravito-electromagnetic effects, the ring consists of abulk structure, a fine structure, and also ahyperfine structure, showing more than 10000 ringlets. The large number of ringlets can be explained by the Baxter-Thompson ‘negative diffusion’. This gives the ringlets a stability which makes it possible to interprete them as ‘fossils’, which originated at cosmogonic times. It is shown that thebulk structure can be explained by the combined ‘cosmogonic shadows’ of Mimas, the co-orbiting satellites, and the Shepherd satellites. This structure originated at the transition from the plasma phase to the planetesimal phase (which probably took place 4–5×109 y ago). Further, Holberg has discovered that the shadows are not simple void region but exhibit a certain characteristic ‘signature’. This is not yet understood theoretically. Parts of thefine structure are explained by Holberg as resonances with the satellites. Parts are here interpreted as cosmogonic shadow effects. However, there are a number of ringlets which can neither be explained by cosmogonic nor by resonance effects. The most important conclusion is that an analysis of the ring data is likely to lead to areconstruction of the plasma-planetesimal transition with an accuracy of a few percent. 相似文献
29.
High-velocity impacts of interplanetary meteoroids on Saturn's rings are discussed. It is shown that the neutral gas emitted by impact vaporization may be responsible, to a large part, for the observed neutral ring atmosphere. Both the predicted neutral gas injection rate and the gas temperature (or kinetic energy) are compatible with the measurements (see Broadfoot, A. L., B. R. Sandel, D. E. Shemansky, J. B. Holberg, G. R. Smith, D. F. Strobel, J. C. McConnell, S. Kumar, D. M. Hunten, S. K. Atreya, T. M. Dohnahne, H. W. Moos, J. L. Bertaux, J. E. Blamont, R. B. Pomphrey, and S. Linik, Science212, 206–211, 1981). Heavy ejecta particles produce a particulate ring “halo”. The physical properties of this halo are calculated, and it appears to be identical with the tenous particle population discussed by Baum and Kreidl (1982). Erosion of Saturn's ring particles, the resulting mass balance, and regolith formation are estimated. This provides some constraints on surface properties and optical albedo. 相似文献
30.
Verónica Oliveros Gilbert Féraud Luis Aguirre Luis Ramírez Michel Fornari Carlos Palacios Miguel Parada 《Mineralium Deposita》2008,43(3):281-293
The 40Ar/39Ar geochronological method was applied to date magmatic and hydrothermal alteration events in the Mantos Blancos mining district
in the Coastal Cordillera of northern Chile, allowing the distinction of two separate mineralization events. The Late Jurassic
Mantos Blancos orebody, hosted in Jurassic volcanic rocks, is a magmatic-hydrothermal breccia-style Cu deposit. Two superimposed
mineralization events have been recently proposed. The first event is accompanied by a phyllic hydrothermal alteration affecting
a rhyolitic dome. The second mineralization event is related to the intrusion of bimodal stocks and sills inside the deposit.
Because of the superposition of several magmatic and hydrothermal events, the obtained 40Ar/39Ar age data are complex; however, with a careful interpretation of the age spectra, it is possible to detect complex histories
of successive emplacement, alteration, mineralization, and thermal resetting. The extrusion of Jurassic basic to intermediate
volcanic rocks of the La Negra Formation is dated at 156.3 ± 1.4 Ma (2σ) using plagioclase from an andesitic lava flow. The first mineralization event and associated phyllic alteration affecting
the rhyolitic dome occurred around 155–156 Ma. A younger bimodal intrusive event, supposed to be equivalent to the bimodal
stock and sill system inside the deposit, is probably responsible for the second mineralization event dated at ca. 142 Ma.
Other low-temperature alteration events have been dated on sericitized plagioclase at ca. 145–146, 125, and 101 Ma. This is
the first time that two distinct mineralization events have been documented from radiometric data for a copper deposit in
the metallogenic belt of the Coastal Cordillera of northern Chile.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献