全文获取类型
收费全文 | 94426篇 |
免费 | 2339篇 |
国内免费 | 1480篇 |
专业分类
测绘学 | 2621篇 |
大气科学 | 7459篇 |
地球物理 | 19701篇 |
地质学 | 32459篇 |
海洋学 | 8281篇 |
天文学 | 20754篇 |
综合类 | 377篇 |
自然地理 | 6593篇 |
出版年
2021年 | 783篇 |
2020年 | 891篇 |
2019年 | 943篇 |
2018年 | 1761篇 |
2017年 | 1704篇 |
2016年 | 2258篇 |
2015年 | 1618篇 |
2014年 | 2308篇 |
2013年 | 4775篇 |
2012年 | 2661篇 |
2011年 | 3737篇 |
2010年 | 3244篇 |
2009年 | 4444篇 |
2008年 | 3934篇 |
2007年 | 3716篇 |
2006年 | 3736篇 |
2005年 | 3004篇 |
2004年 | 3038篇 |
2003年 | 2795篇 |
2002年 | 2657篇 |
2001年 | 2359篇 |
2000年 | 2297篇 |
1999年 | 1960篇 |
1998年 | 1993篇 |
1997年 | 1923篇 |
1996年 | 1650篇 |
1995年 | 1590篇 |
1994年 | 1420篇 |
1993年 | 1310篇 |
1992年 | 1251篇 |
1991年 | 1091篇 |
1990年 | 1333篇 |
1989年 | 1131篇 |
1988年 | 1024篇 |
1987年 | 1244篇 |
1986年 | 1073篇 |
1985年 | 1322篇 |
1984年 | 1570篇 |
1983年 | 1459篇 |
1982年 | 1347篇 |
1981年 | 1278篇 |
1980年 | 1145篇 |
1979年 | 1091篇 |
1978年 | 1121篇 |
1977年 | 1050篇 |
1976年 | 975篇 |
1975年 | 927篇 |
1974年 | 946篇 |
1973年 | 958篇 |
1972年 | 581篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
72.
Stress wave attenuation across fractured rock masses is a great concern of underground structure safety. When the wave amplitude is large, fractures experience nonlinear deformation during the wave propagation. This paper presents a study on normal transmission of P‐wave across parallel fractures with nonlinear deformational behaviour (static Barton–Bandis model). The results show that the magnitude of transmission coefficient is a function of incident wave amplitude, nondimensional fracture spacing and number of fractures. Two important indices of nondimensional fracture spacing are identified, and they divide the area of nondimensional fracture spacing into three parts (individual fracture area, transition area and small spacing area). In the different areas, the magnitude of transmission coefficient has different trends with nondimensional fracture spacing and number of fractures. In addition, the study reveals that under some circumstances, the magnitude of transmission coefficient increases with increasing number of fractures, and is larger than 1. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
73.
Carlos J.A.P. Martins 《Astrophysics and Space Science》2003,283(4):439-444
I briefly present the Organizing Committee's and my own motivation for organizing this workshop, and I suggest a few key questions
for which we will try to find possible answers in the coming days.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
74.
75.
76.
77.
78.
In this work, the factors controlling the formation and preservation of high-pressure mineral assemblages in the metamorphosed orthopyroxene-bearing metagranitoids of the Sandmata Complex, Aravalli-Delhi Mobile Belt (ADMB), northwestern India have been modelled. The rocks range in composition from farsundite through quartz mangerite to opdalite, and with varying K2 O, Ca/(Ca + Na)rock and FeOtot + MgO contents. A two stage metamorphic evolution has been recorded in these rocks.
An early hydration event stabilized biotite with or without epidote at the expense of magmatic orthopyroxene and plagioclase. Subsequent high-pressure granulite facies metamorphism (∼15 kbar, ∼800 °C) of these hydrated rocks produced two rock types with contrasting mineralogy and textures. In the non-migmatitic metagranitoids, spectacular garnet ± K-feldspar ± quartz corona was formed around reacting biotite, plagioclase, quartz and/or pyroxene. In contrast, biotite ± epidote melting produced migmatites, containing porphyroblastic garnet incongruent solids and leucosomes.
Applying NCKFMASHTO T–M (H2 O) and P–T pseudosection modelling techniques, it is demonstrated that the differential response of these magmatic rocks to high-pressure metamorphism is primarily controlled by the scale of initial hydration. Rocks, which were pervasively hydrated, produced garnetiferous migmatites, while for limited hydration, the same metamorphism formed sub-solidus garnet-bearing coronae. Based on the sequence of mineral assemblage evolution and the mineral compositional zoning features in the two metagranitoids, a clockwise metamorphic P–T path is constrained for the high-pressure metamorphic event. The finding has major implications in formulating geodynamic model of crustal amalgamation in the ADMB. 相似文献
An early hydration event stabilized biotite with or without epidote at the expense of magmatic orthopyroxene and plagioclase. Subsequent high-pressure granulite facies metamorphism (∼15 kbar, ∼800 °C) of these hydrated rocks produced two rock types with contrasting mineralogy and textures. In the non-migmatitic metagranitoids, spectacular garnet ± K-feldspar ± quartz corona was formed around reacting biotite, plagioclase, quartz and/or pyroxene. In contrast, biotite ± epidote melting produced migmatites, containing porphyroblastic garnet incongruent solids and leucosomes.
Applying NCKFMASHTO T–M (H
79.
We have investigated the central regions of the galaxies in the NGC 3169/NGC 3166/NGC 3156 group with the multipupil fiber spectrograph of the 6-m telescope; the first (central) galaxy in the group is a spiral (Sa) one and the other two galaxies are lenticular ones. The group is known to have an extended HI cloud with a size of more than 100 kpc that is associated in its position, orientation, and rotation with the central galaxy NGC 3169. The mean age of the stellar populations in the centers of all three galaxies has been found to be approximately the same, ~1 Gyr. Since the galaxies are early-type ones and since NGC 3166 and NGC 3156 show no global star formation, we are dealing here with a synchronous star formation burst in the centers of all three galaxies. 相似文献
80.
D. Montes M. C. Gálvez M. J. Fernández-Figueroa I. Crespo-Chacón 《Astrophysics and Space Science》2006,304(1-4):367-369
High resolution echelle spectroscopic observations taken with the FEROS spectrograph at the 2.2 m telescope ESO confirm the binary nature of the flare M3.5V star LU Vel (GJ 375, RE J0958-462) previously reported by Christian and Mathioudakis (2002). Emission of similar intensity from both components is detected in the Balmer, Na i D1&D2, He i D3, Ca ii H&K, and Ca ii IRT lines. We have determined precise radial velocities by cross correlation with radial velocity standard stars, which have allowed us to obtain for the first time the orbital solution of the system. The binary consists of two near-equal M3.5V components with an orbital period shorter than 2 days. We have analyzed the behaviour of the chromospheric activity indicators (variability and possible flares). In addition, we have determined its rotational velocity and kinematics. 相似文献