全文获取类型
收费全文 | 7550篇 |
免费 | 311篇 |
国内免费 | 81篇 |
专业分类
测绘学 | 166篇 |
大气科学 | 607篇 |
地球物理 | 1745篇 |
地质学 | 2937篇 |
海洋学 | 604篇 |
天文学 | 1159篇 |
综合类 | 21篇 |
自然地理 | 703篇 |
出版年
2022年 | 35篇 |
2021年 | 99篇 |
2020年 | 107篇 |
2019年 | 152篇 |
2018年 | 221篇 |
2017年 | 195篇 |
2016年 | 229篇 |
2015年 | 204篇 |
2014年 | 263篇 |
2013年 | 401篇 |
2012年 | 299篇 |
2011年 | 371篇 |
2010年 | 314篇 |
2009年 | 382篇 |
2008年 | 332篇 |
2007年 | 301篇 |
2006年 | 295篇 |
2005年 | 264篇 |
2004年 | 284篇 |
2003年 | 273篇 |
2002年 | 253篇 |
2001年 | 139篇 |
2000年 | 160篇 |
1999年 | 123篇 |
1998年 | 135篇 |
1997年 | 118篇 |
1996年 | 97篇 |
1995年 | 93篇 |
1994年 | 87篇 |
1993年 | 100篇 |
1992年 | 83篇 |
1991年 | 61篇 |
1990年 | 69篇 |
1989年 | 77篇 |
1988年 | 78篇 |
1987年 | 90篇 |
1986年 | 81篇 |
1985年 | 87篇 |
1984年 | 106篇 |
1983年 | 88篇 |
1982年 | 86篇 |
1981年 | 73篇 |
1980年 | 87篇 |
1979年 | 69篇 |
1978年 | 65篇 |
1977年 | 59篇 |
1976年 | 49篇 |
1975年 | 51篇 |
1974年 | 46篇 |
1973年 | 47篇 |
排序方式: 共有7942条查询结果,搜索用时 15 毫秒
991.
A thermodynamic model for haplogranitic melts in the system Na2O–CaO–K2O–Al2O3–SiO2–H2O (NCKASH) is extended by the addition of FeO and MgO, with the data for the additional end‐members of the liquid incorporated in the Holland & Powell (1998) internally consistent thermodynamic dataset. The resulting dataset, with the software thermocalc , is then used to calculate melting relationships for metapelitic rock compositions. The main forms for this are P–T and T–X pseudosections calculated for particular rock compositions and composition ranges. The relationships in these full‐system pseudosections are controlled by the low‐variance equilibria in subsystems of NCKFMASH. In particular, the solidus relationships are controlled by the solidus relationships in NKASH, and the ferromagnesian mineral relationships are controlled by those in KFMASH. However, calculations in NCKFMASH allow the relationships between the common metapelitic minerals and silicate melt to be determined. In particular, the production of silicate melt and melt loss from such rocks allow observations to be made about the processes involved in producing granulite facies rocks, particularly relating to open‐system behaviour of rocks under high‐grade conditions. 相似文献
992.
993.
Jo Thomas 《Geology Today》2004,20(4):149-152
Famously, geology is on our doorstep, and an understanding of the geology of everyday things helps us communicate to those for whom it is a closed subject. The stone bridges of the River Stour from Stourhead to Christchurch in southern England are a topic worthy of study and rewarding as an example of the variety of the geology of this part of England. 相似文献
994.
Laurent Koechlin Denis Serre Gerald K. Skinner Peter Von Ballmoos Thomas Crouzil 《Experimental Astronomy》2005,20(1-3):307-315
The light deviation caused by the gravitational potential in the vicinity of the sun could be used as a means of focussing radiation that cannot be focussed easily otherwise. The gravitational lens formed by the sun is not stigmatic, but does have the advantage of being achromatic and acts identically on all types of mass-less radiations. For a source at infinity, its geometrical characteristics present a “caustic” line starting at 550 astronomical units (UA) downstream from the sun. In a plane perpendicular to that caustic line, images of distant objects are formed.The perturbations by the solar corona plasma will significantly blur electromagnetic radiation for wavelengths longer than those of the IR domain. At shorter wavelengths, for example the γ domain, the focussing process could lead to 108 amplification factors. In order to reach the regions where images are formed, long distance space missions are necessary. Once launched, missions of this type would be dedicated to a single field. Some possible targets are considered, such as Sagitarius A observed in X and γ rays.In this paper we study the point spread function (PSF) of the sun as a gravitational lens. Taking into account perturbations by the planets, the non sphericity of the sun and coronal plasma index, we derive limits within which such observations could be possible. 相似文献
995.
W. M. Davis E. R. Cespedes L. T. Lee J. F. Powell R. A. Goodson 《Environmental Geology》1997,29(3-4):228-237
The Site Characterization and Analysis Penetrometer System (SCAPS) was used to investigate subsurface petroleum contamination
at Aberdeen Proving Ground, Maryland. The SCAPS is a rapid site-screening tool developed by the Tri-Services (U.S. Army, Navy,
and Air Force) to reduce the cost of installation restoration activities. The system, consisting of a geophysical cone penetrometer
fitted with a laser-induced fluorescence petroleum sensor, is capable of simultaneous, real-time delineation of subsurface
stratigraphy and petroleum contamination. Onsite processing of the SCAPS data allows three-dimensional visualization of both
subsurface stratigraphy and petroleum contaminant distribution. The site investigation described herein successfully identified
three distinct areas of subsurface petroleum contamination caused by leaking underground storage tanks containing No. 2 healing
oil. The SCAPS petroleum sensor response was verified by obtaining 20 soil samples and performing onsite and conventional
laboratory analyses for petroleum contaminants. Verification sample results indicate a strong correlation between the SCAPS
fluorescence data and conventional measures of petroleum contamination. This investigation illustrates the capabilities of
the SCAPS for rapid site characterization.
Received: 27 November 1995 · Accepted: 11 December 1995 相似文献
996.
Marcin Jankiewicz Roman V. Buniy Thomas W. Kephart Thomas J. Weiler 《Astroparticle Physics》2004,21(6):651-666
It has been proposed that propagation of cosmic-rays at extreme-energy may be sensitive to Lorentz-violating metric fluctuations (“foam”). We investigate the changes in interaction thresholds for cosmic-rays and gamma-rays interacting on the CMB and IR backgrounds, for a class of stochastic models of space–time foam. The strength of the foam is characterized by the factor (E/MP)a, where a is a phenomenological suppression parameter. We find that there exists a critical value of a (dependent on the particular reaction: acrit3 for cosmic-rays, 1 for gamma-rays), below which the threshold energy can only be lowered, and above which the threshold energy may be raised, but at most by a factor of two. Thus, it does not appear possible in this class of models to extend cosmic-ray spectra significantly beyond their classical absorption energies. However, the lower thresholds resulting from foam may have signatures in the cosmic-ray spectrum. In the context of this foam model, we find that cosmic-ray energies cannot exceed the fundamental Planck scale, and so set a lower bound of 108 TeV for the scale of gravity. We also find that suppression of p→pπ0 and γ→e−e+ “decays” favors values aacrit. Finally, we comment on the apparent non-conservation of particle energy–momentum, and speculate on its re-emergence as dark energy in the foamy vacuum. 相似文献
997.
998.
999.
Since the discovery of quasars, it has been recognized that these objects must have either an extraordinary intrinsic brightness or a nonlinear redshift. The most widely accepted current belief incorporates a linear (Hubble) redshift- distance relation and time evolution within a Big Bang model. We reconsider the possibility of a nonlinear (de Sitter) redshift-distance relation and find quasar intrinsic brightness to be not at all extraordinary. Given a de Sitter law, intrinsic brightness is found to be independent of redshift over five orders of magnitude. 相似文献
1000.