全文获取类型
收费全文 | 6347篇 |
免费 | 293篇 |
国内免费 | 47篇 |
专业分类
测绘学 | 173篇 |
大气科学 | 502篇 |
地球物理 | 2223篇 |
地质学 | 2206篇 |
海洋学 | 374篇 |
天文学 | 851篇 |
综合类 | 39篇 |
自然地理 | 319篇 |
出版年
2022年 | 43篇 |
2021年 | 114篇 |
2020年 | 109篇 |
2019年 | 85篇 |
2018年 | 259篇 |
2017年 | 229篇 |
2016年 | 348篇 |
2015年 | 247篇 |
2014年 | 308篇 |
2013年 | 414篇 |
2012年 | 320篇 |
2011年 | 347篇 |
2010年 | 276篇 |
2009年 | 275篇 |
2008年 | 241篇 |
2007年 | 210篇 |
2006年 | 186篇 |
2005年 | 154篇 |
2004年 | 156篇 |
2003年 | 120篇 |
2002年 | 119篇 |
2001年 | 106篇 |
2000年 | 96篇 |
1999年 | 101篇 |
1998年 | 98篇 |
1997年 | 96篇 |
1996年 | 74篇 |
1995年 | 69篇 |
1994年 | 73篇 |
1993年 | 61篇 |
1992年 | 61篇 |
1991年 | 60篇 |
1990年 | 71篇 |
1989年 | 52篇 |
1987年 | 58篇 |
1986年 | 48篇 |
1985年 | 46篇 |
1984年 | 56篇 |
1983年 | 50篇 |
1982年 | 64篇 |
1981年 | 49篇 |
1980年 | 54篇 |
1979年 | 47篇 |
1978年 | 43篇 |
1977年 | 48篇 |
1976年 | 49篇 |
1975年 | 47篇 |
1974年 | 43篇 |
1973年 | 49篇 |
1971年 | 47篇 |
排序方式: 共有6687条查询结果,搜索用时 781 毫秒
841.
From the assumption of symmetry of ratio of Fermion masses and masses of bosons ratio results the hypothetical mass of quark-lepton bosonm
wes and hypothetical mass of electron neutrinom
ve. 相似文献
842.
From the observed present parameters of the Universe and the model properties of an expansive non-decelerative universe it results that the value of Boltzmann's constant (coefficient)k does not change only before the end of radiation era, but also in the matter era; with the increase of gauge factora, it decreases as (a
–1)1/4. 相似文献
843.
R. Thompson T. D. J. Cameron C. Schwarz K. A. Jensen V. Van Maenhaut Lemberge L. P. Sha 《第四纪科学杂志》1992,7(4):319-334
Cores from boreholes penetrating late Quaternary, glacial, interglacial and postglacial sediments and the underlying late Cenozoic delta complex of the southern North Sea have been examined for their magnetic properties. A magnetic polarity stratigraphy has been established as an aid to biostratigraphic dating of the sediments; the Kaena-Gauss and Gauss—Matuyama transitions and the base and top of the Olduvai subchron have been identified. The strength and stability of laboratory-induced isothermal remanent magnetisation display clear magneto-petrological variations, which match lithostratigraphic changes in the cores. Principal component analysis has picked out a basin-wide and palaeoenvironmental consistency in the magnetic data. Large, multi-domain magnetite grains predominate in the post-deltaic and fluvio-deltaic sediments, whereas smaller greigite or titanomagnetite grains are concentrated in the intertidal and marine deltaic facies. Since heavy mineral analysis indicates that most of the deltaic detritus derived from common source areas, the differences in magnetic mineralogy have probably been caused by the sediment transport processes operating within the delta complex. 相似文献
844.
There are many aspects of observational evidence that cometary nuclei have irregular or nonspherical shape. The triaxial figure of the Halley's Comet nucleus is a well known fact. Therefore, the nucleus shape plays a significant role in consideration of the formation and evolution of comets and several attempts have been made to explain their nonsphericity. These studies were mainly based on the random-walk schemes for the aggregation processes. Although some results indeed lead to irregularities and deviation from sphericity, the spherical or irregular shape seem to be prevailing results. On the other hand the triaxial figure can be formed by the tidal and rotational forces. Thus, the assumption that the shape of the cometary nucleus due to some of these effects is in principle acceptable. In here assumed scenario already evolved cometary nucleus is situated as a satellite in the gravitation field of a planetary-like body. Since the rigidity of the nucleus is low, it may be easily transferred in the state of a synchronous satellite and in its shape could be imprinted the dynamical effects from this epoch. Here presented results indicate, that such a possibility should be seriously considered. The theory of this process is applied to the nucleus of comet Halley. It is shown, that the nucleus might be synchronously orbiting around a planetary-like hypothetical body with a period of 0.7 days. The minimal bulk tensile strength of the cometary material of about 102 N m–2 is estimated. 相似文献
845.
The comparisons of the Earth gravity field models by the order of their harmonic coefficients, performed with the basic lumped coefficients (Planet. Space Sci.29, 653, 1981, Paper I) are here extended to cover all harmonic coefficients (both odd and even degree). The lumped coefficients (the “e-terms” and “longitudinal” terms), corresponding to 18 Earth models, are compared mutually (Figs. 2–15). The large differences, observed for various models and orders, are of particular interest: they are gathered into Table 1. The result of Paper I was a little pessimistic. The same is true here: various inhomogeneities, sometimes very large, in the accuracy of the harmonic coefficients must exist—even for low orders. Most of our comments and objections, however, relate to the older Earth models, which have only a historical value now. Our comparisons are only relative ones; an actual test of the accuracy of the models (their calibration) is possible via data with independent status (Kloko?ník, 1982, 1983). 相似文献
846.
R. Vandelannoote W. Blommaert A. Sadurski L. Van T Dack R. Gijbels R. Van Grieken B. Bosch M. Leleu J. Rochon C. Sarcia J.F. Sureau 《Journal of Geochemical Exploration》1984,20(1):33-46
Sensitive multi-element analysis techniques were applied to water samples collected in the vicinity of a small Zn-Pb-Cu sulfide mineralization in the region of Menez-Albot (Finistère, France).The variation of the trace-element content along a local stream shows the presence of the mineralization, mainly through a simultaneous positive anomaly in solution for a group of about 10 elements (e.g. Ni, Cu, Zn, As, Sb) which are connected with this type of mineralization. The anomaly decreases steeply due to the influx of swamp water rich in Fe, Mn and organic matter. The precipitation barrier is reflected in the stream sediment composition. Contamination from fertilizers was observed in some samples. 相似文献
847.
Edward P. J. Van den Heuvel 《Journal of Astrophysics and Astronomy》1984,5(3):209-233
The peculiar combination of a relatively short pulse period and a relatively weak surface dipole magnetic field strength of
binary radio pulsars finds a consistent explanation in terms of (i) decay of the surface dipole component of neutron-star
magnetic fields on a timescale of (2–5) × 106 yr, in combination with (ii) spin-up of the rotation of the neutron star during a subsequent mass-transfer phase.
The four known binary radio pulsars appear to fall into two different categories. Two of them, PSR 0655 + 64 and PSR 1913
+ 16, have short orbital periods (<25 h) and high mass functions, indicating companion masses 0.7M⊙ (∼1 (± 0.3) M⊙ and 1.4 M⊙, respectively). The other two, PSR 0820 + 02 and PSR 1953 + 29, have long orbital periods (117d),
nearly circular orbits, and low, almost identical mass functions of about 3×10-3 M⊙, suggesting companion masses of about 0.3M⊙. It is pointed out that these two classes of systems are expected to be formed by the later evolution of binaries consisting
of a neutron star and a normal companion star, in which the companion was (considerably) more massive than the neutron star,
or less massive than the neutron star, respectively. In the first case the companion of the neutron star in the final system
will be a massive white dwarf, in a circular orbit, or a neutron star in an eccentric orbit. In the second case the final
companion to the neutron star will be a low-mass (∼ 0.3 M⊙) helium white dwarf in a wide and nearly circular orbit.
In systems of the second type the neutron star was most probably formed by the accretion-induced collapse of a white dwarf.
This explains in a natural way why PSR 1953 + 29 has a millisecond rotation period and PSR 0820 + 02 has not.
Among the binary models proposed for the formation of the 1.5-millisecond pulsar, the only ones that appear to be viable are
those in which the companion disappeared by coalescence with the neutron star. In such models the companion may have been
a red dwarf of mass 0.03M⊙, a neutron star, or a massive (>0.7M⊙) white dwarf. Only in the last-mentioned case is a position of the pulsar close to the galactic plane a natural consequence.
In the first-mentioned case the progenitor system most probably was a cataclysmic-variable binary in which the white dwarf
collapsed by accretion. 相似文献
848.
849.
A.?Pulido-BoschEmail author J.?M.?Calaforra P.?Pulido-Leboeuf S.?Torres-García 《Environmental Geology》2004,46(5):583-590
The Messinian gypsum of Sorbas is intensively karstified, with hundreds of dolines and numerous, highly unusual exo- and endokarstic forms. Amongst these the tumuli are especially spectacular, as are the large caverns, adorned with numerous speleothems, some of which have not been described anywhere else in the World. The extraction of the gypsum has impacted practically all the elements of the environment (landscape, water, soil, subsoil, flora, fauna, etc.) and, in particular, all of the karstic forms. In addition to endangering the caves and the natural treasures that they contain, it affects the surface waters, markedly modifying the hydrographic network. Groundwaters, which now occur in the bottom of a quarry, are also affected. 相似文献
850.
Using the geologic setting of talc deposits as an indicator of amphibole asbestos content 总被引:1,自引:0,他引:1
Bradley?S.?Van GosenEmail author Heather?A.?Lowers Stephen?J.?Sutley Carol?A.?Gent 《Environmental Geology》2004,45(7):920-939
This study examined commercial talc deposits in the U.S. and their amphibole-asbestos content. The study found that the talc-forming environment directly influenced the amphibole and amphibole-asbestos content of the talc deposit. Large talc districts in the U.S. have mined hydrothermal talcs that replaced dolostone. Hydrothermal talcs, created by siliceous fluids heated by magmas at depth, consistently lack amphiboles as accessory minerals. In contrast, mineable talc deposits that formed by contact or regional metamorphism consistently contain amphiboles, locally as asbestiform varieties. Examples of contact metamorphic deposits occur in Death Valley, California; these talc-tremolite deposits contain accessory amphibole-asbestos. Talc bodies formed by regional metamorphism always contain amphiboles, which display a variety of compositions and habits, including asbestiform. Some industrial mineral deposits are under scrutiny as potential sources of accessory asbestos minerals. Recognizing consistent relations between the talc-forming environment and amphibole-asbestos content may be used in prioritizing remediation or monitoring of abandoned and active talc mines. 相似文献