全文获取类型
收费全文 | 103388篇 |
免费 | 1313篇 |
国内免费 | 697篇 |
专业分类
测绘学 | 2259篇 |
大气科学 | 6553篇 |
地球物理 | 20019篇 |
地质学 | 36999篇 |
海洋学 | 9337篇 |
天文学 | 24492篇 |
综合类 | 294篇 |
自然地理 | 5445篇 |
出版年
2022年 | 757篇 |
2021年 | 1282篇 |
2020年 | 1361篇 |
2019年 | 1505篇 |
2018年 | 3129篇 |
2017年 | 2894篇 |
2016年 | 3360篇 |
2015年 | 1666篇 |
2014年 | 3238篇 |
2013年 | 5393篇 |
2012年 | 3547篇 |
2011年 | 4523篇 |
2010年 | 4078篇 |
2009年 | 5179篇 |
2008年 | 4537篇 |
2007年 | 4700篇 |
2006年 | 4284篇 |
2005年 | 3027篇 |
2004年 | 2914篇 |
2003年 | 2725篇 |
2002年 | 2683篇 |
2001年 | 2401篇 |
2000年 | 2289篇 |
1999年 | 1798篇 |
1998年 | 1851篇 |
1997年 | 1747篇 |
1996年 | 1469篇 |
1995年 | 1466篇 |
1994年 | 1253篇 |
1993年 | 1187篇 |
1992年 | 1125篇 |
1991年 | 1129篇 |
1990年 | 1136篇 |
1989年 | 995篇 |
1988年 | 908篇 |
1987年 | 1071篇 |
1986年 | 910篇 |
1985年 | 1148篇 |
1984年 | 1290篇 |
1983年 | 1237篇 |
1982年 | 1140篇 |
1981年 | 1062篇 |
1980年 | 994篇 |
1979年 | 880篇 |
1978年 | 898篇 |
1977年 | 763篇 |
1976年 | 741篇 |
1975年 | 741篇 |
1974年 | 718篇 |
1973年 | 791篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
971.
The physicomathematical features of the plasma explosion region dynamics at altitudes higher than 100 km at the early stage of its development, which culminates in the transfer of a considerable portion of the initial plasma energy into the environment, are discussed in the first part of the work. The radiation gas dynamic, inertial, and shock stages of the disturbed region development are calculated. 相似文献
972.
Selected-area measurements of lead-isotope ratios in zircons have been made at high mass resolution using a modified A.E.I. IM20 ion-microprobe. At a working resolution of 3200 it is shown that the important interfering molecular peaks generated from the zircon matrix may be separated from atomic lead peaks. Corrections for overlapping peaks, which are necessary in measurements of lead isotopes at low mass resolution, are then only required for 208Pb.Measurements on N.B.S. standards show no isotope discrimination within counting statistics. Determination of 207Pb/206Pb on 29 single zircon grains from a tonalite gneiss and a granite from Lac Seul, northwestern Ontario, give very variable ratios with mean values close to those of bulk separates. Peaks in the distribution of 207/206 ratios obtained from the tonalite gneiss may be interpreted as evidence for two stages of lead loss. High 207/206 ratios are shown to be significant for at least two zircons and probably record a minimum age of formation of 3.3 ± 0.1 b.y. for the gneiss. 相似文献
973.
MERLIN and VLA observations of the galaxy M82 have detected a jet-like feature emanating from one of the compact sources, 44.01+59.6. The proximity of this source to the dynamical centre of M82 led us to suspect that it could be a weak active nucleus rather than an SNR. We imaged this source using the EVN at 15 mas and although it shows a compact shell-like structure which could be consistent with an SNR, we note that the EVN image shows a bright region within the source which is elongated along the jet direction. There are distinct similarities between this source and the Sgr A complex at the centre of our own Galaxy which contains the compact AGN, Sgr A*. 相似文献
974.
The effect of highly saline oil-field water on soils and potable groundwater is considered. Three phases of the hydrochemical evolution of groundwater quality affected by transborder pollution and field development that has deteriorated potable groundwater in the two topmost aquifers are identified. 相似文献
975.
976.
We present simple analytic models which predict the peak X-ray emission measure and temperature attained in flares in which
the chromospheric evaporation process takes place either in a single ‘monolithic’ loop or in a loop consisting of several
filaments that are created successively as the energy release process proceeds in time. As possible mechanisms driving chromospheric
evaporation we consider both classical heat conduction from the loop top and non-thermal electron beams. The model predictions
are tested for a set of 18 well studied RHESSI microflares. The results suggest beam driven evaporation in filamented loops
as being capable of accounting for the observed emission measures and temperatures though there are issues with the very high
beam densities needed. On the other hand, estimates of the emission measures achieved by conductive evaporation which are
derived by using the Rosner – Tucker – Vaiana (RTV) scaling law are much larger than the observed ones. Possible reasons for
this discrepancy are discussed. 相似文献
977.
978.
Raman K. Prinja & Paul A. Crowther 《Monthly notices of the Royal Astronomical Society》1998,300(3):828-836
Archival HST FOS and GHRS data sets have been used to collect ultraviolet evidence for large- and small-scale stellar wind structure in extragalactic Local Group OB stars (i.e. SMC, LMC including R136, M31, M33 and NGC 6822). By comparison with previous studies of Galactic OB stars, wind activity is principally diagnosed in individual spectrograms via the presence of 'narrow absorption components' and saturated 'black' absorption troughs in the resonance line doublets. Their characteristics broadly suggest that these stars share the same physical mechanisms for perturbing the winds as those that act in Galactic stars. Both of these spectral indicators are also used to provide reliable measures of wind terminal velocities. These velocities are directly compared with previously published Galactic values, without reliance on model profile fitting. Relative to Galactic OB stars, the most discrepant terminal velocities (and wind line profiles) result from main-sequence early O-type stars in the SMC. 相似文献
979.
980.
John A. Knox 《Mathematical Geology》2000,32(2):203-215
The Fahrenheit-to-Celsius temperature-conversion equation is a basic component of many introductory earth science courses. Despite its simplicity, it presents a challenge to students and instructors alike because residents of the United States are unfamiliar with the Celsius scale. By solving for the point at which these two temperature scales are equal, it is possible to use the equations for temperature conversion as a springboard to more advanced topics. It is demonstrated that temperature-conversion equations and chaotic equations can be solved using identical numerical and graphical techniques. As a result, the fundamental concepts of chaos theory and numerical methods can be introduced to students in the context of the simplest equations in the earth sciences. These solution methods are applied to the quantitative theory of the extratropical cyclone as an example of the utility and broad scope of this educational approach. 相似文献