全文获取类型
收费全文 | 123398篇 |
免费 | 1980篇 |
国内免费 | 1597篇 |
专业分类
测绘学 | 3650篇 |
大气科学 | 8709篇 |
地球物理 | 25143篇 |
地质学 | 45929篇 |
海洋学 | 10039篇 |
天文学 | 24356篇 |
综合类 | 2369篇 |
自然地理 | 6780篇 |
出版年
2022年 | 636篇 |
2021年 | 1083篇 |
2020年 | 1270篇 |
2019年 | 1340篇 |
2018年 | 7045篇 |
2017年 | 6212篇 |
2016年 | 5466篇 |
2015年 | 1894篇 |
2014年 | 2959篇 |
2013年 | 5334篇 |
2012年 | 4001篇 |
2011年 | 6526篇 |
2010年 | 5525篇 |
2009年 | 6838篇 |
2008年 | 5988篇 |
2007年 | 6335篇 |
2006年 | 4111篇 |
2005年 | 3088篇 |
2004年 | 3322篇 |
2003年 | 3119篇 |
2002年 | 2843篇 |
2001年 | 2482篇 |
2000年 | 2341篇 |
1999年 | 1866篇 |
1998年 | 1883篇 |
1997年 | 1884篇 |
1996年 | 1571篇 |
1995年 | 1508篇 |
1994年 | 1388篇 |
1993年 | 1250篇 |
1992年 | 1211篇 |
1991年 | 1068篇 |
1990年 | 1304篇 |
1989年 | 1108篇 |
1988年 | 990篇 |
1987年 | 1158篇 |
1986年 | 989篇 |
1985年 | 1287篇 |
1984年 | 1453篇 |
1983年 | 1380篇 |
1982年 | 1258篇 |
1981年 | 1212篇 |
1980年 | 1074篇 |
1979年 | 1010篇 |
1978年 | 1083篇 |
1977年 | 974篇 |
1976年 | 917篇 |
1975年 | 864篇 |
1974年 | 853篇 |
1973年 | 912篇 |
排序方式: 共有10000条查询结果,搜索用时 617 毫秒
231.
232.
The observed properties of the long-period comet system, and its periodic disturbance by galactic forces manifesting as terrestrial impact episodes, may be indicative of a comet capture/escape cycle as the Solar System orbits the Galaxy. A mean number density of comets in molecular clouds of ~10?1±1 AU?3 is implied. This is sufficient to deplete metals from the gaseous component of the interstellar medium, as observed, but leads to the problem of how stars are formed nevertheless with solar metal abundances. Formation of comets prior to stars in dense systems of near-zero energy may be indicated, and isotope signatures in cometary particles may be diagnostic of conditions in young spiral arm material. 相似文献
233.
J. Malý 《洁净——土壤、空气、水》1985,13(2):137-147
As the introduction, the advantages and drawbacks of a large number of phosphorus determination methods and digestion techniques for particulate matter, as e. g. sewage sludge, used in practice are described. The results of some investigations concerning the techniques of digestion as well as the further preparation of the extracts for the phosphorus determination proper are graphically and in tabular form represented and interpreted. The investigated photo-metric methods may be disturbed by iron and silicic-acid components. Advice about, and possibilities for avoiding such disturbances are given. In the case of the simultaneous application of the Kjeldahl digestion for the determination of nitrogen as well as of phosphorus there should be paid attention to possible disturbances by the selenium- or mercury-containing catalyst additives in the further processing of samples due to the formation of deposits. 相似文献
234.
Andrea PATZER Jochen SCHLÜTER Ludolf SCHULTZ Dolores H. HILL William V. BOYNTON 《Meteoritics & planetary science》2005,40(6):869-879
Abstract— Mineralogical and chemical studies of Dar al Gani 983 show that this meteorite is a eucrite. Its texture is that of an impact breccia. It contains cumulate pyroxene and feldspar megacrysts, a variety of recrystallized melt clasts, clasts of subophitic basalt, and mesostasis. These components are embedded in a matrix of fragmental pyroxene and plagioclase. In addition, the entire rock is penetrated by glassy melt veins and patches, and displays features of strong shock. The mineralogical and chemical evidence obtained for DaG 983 indicates that this meteorite experienced a complex evolutionary history. The presence of cumulate silicate crystals implies substantial, large scale cratering events on the HED asteroid. As a result of these impacts, rocks from different intrusive bodies to extrusive surface layers were laterally and vertically transported to form a thoroughly mixed megaregolith. DaG 983 represents a sample of this megabreccia. 相似文献
235.
236.
237.
238.
F. J. Ormeling Sr. 《GeoJournal》1986,13(4):413-416
Conclusion The above account is just a selection of Perthes' multifarious activities. As such it is fragmentary and incomplete. Many other publications and collaborators could have been mentioned. Even in its incompleteness the report may justify the statement that the success of Justus Perthes Geographische Verlagsanstalt, was apart from the managing qualities of its governors, undoubtedly due to the long series of scholarly competent collaborators whom they succeeded in engaging. As illustrated above most of theme were among the avant garde of the profession. Apparently they were at ease with Justus Perthes where they were granted enough freedom of action to realise their ambitious projects under the vigilant eye of the management. In fact the history of the firm is a continuous story of their achievements. Their combined effort initiated modern scientific atlas-making, laid down the foundation of methodic school cartography and accelerated the growth of thematic cartography. The Justus Perthes maps and atlasses stand for intellectual honestly and have an authoritative appearance of truth and exactness. For a long time to come the name of the firm will be considered as a classical example of the promotion of geography and cartography by private enterprise. 相似文献
239.
N.J.P. Owens 《Estuarine, Coastal and Shelf Science》1985,20(4):505-510
In this study, the 14N:15N ratio of suspended particulate material collected from the Tamar river estuary, south-west England, is described. Three populations of particles, distinguishable by their 15N content, were observed. This investigation has shown that populations of estuarine particles are generated by biological transformations in situ and that the 15N content of estuarine particles does not merely reflect hydrodynamic mixing of the freshwater and seawater source particulate material. 相似文献
240.
P. Lorén-Aguilar J. Guerrero J. Isern J. A. Lobo E. García-Berro 《Monthly notices of the Royal Astronomical Society》2005,356(2):627-636
We compute the emission of gravitational radiation from the merging of a close white dwarf binary system. This is done for a wide range of masses and compositions of the white dwarfs, ranging from mergers involving two He white dwarfs, through mergers in which two CO white dwarfs coalesce, to mergers in which a massive ONe white dwarf is involved. In doing so we follow the evolution of the binary system using a smoothed particle hydrodynamics code. Even though the coalescence process of the white dwarfs involves considerable masses, moving at relatively high velocities with a high degree of asymmetry we find that the signature of the merger is not very strong. In fact, the most prominent feature of the coalescence is that in a relatively small time-scale (of the order of the period of the last stable orbit, typically a few minutes) the sources stop emitting gravitational waves. We also discuss the possible implications of our calculations for the detection of the coalescence within the framework of future space-borne interferometers like LISA. 相似文献