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901.
Integrability, one of the classic issues in galactic dynamics and in general in celestial mechanics, is here revisited in a Riemannian geometric framework, where Newtonian motions are seen as geodesics of suitable -mechanical- manifolds. The existence of constants of motion that entail integrability is associated with the existence of Killing tensor fields on the mechanical manifolds. Such tensor fields correspond to hidden symmetries of non-Noetherian kind. Explicit expressions for Killing tensor fields are given for the N = 2 Toda model, and for a modified Hénon-Heiles model, recovering the already known analytic expressions of the second conserved quantity besides energy for each model respectively. 相似文献
902.
903.
904.
In models with TeV-scale gravity, ultrahigh energy cosmic rays can generate microscopic black holes in the collision with atmospheric and terrestrial nuclei. It has been proposed that stringent bounds on TeV-scale gravity can be obtained from the absence of neutrino cosmic ray showers mediated by black holes. However, uncertainties in the cross section of black hole formation and, most importantly, large uncertainties in the neutrino flux affects these bounds. As long as the cosmic neutrino flux remains unknown, the non-observation of neutrino induced showers implies less stringent limits than present collider limits. 相似文献
905.
Catherine Heymans Michael L. Brown Marco Barden John A. R. Caldwell Knud Jahnke Chien Y. Peng Hans-Walter Rix y Taylor Steven V. W. Beckwith Eric F. Bell rea Borch Boris Häußler Shardha Jogee Daniel H. McIntosh Klaus Meisenheimer Sebastian F. Sánchez Rachel Somerville Lutz Wisotzki Christian Wolf 《Monthly notices of the Royal Astronomical Society》2005,361(1):160-176
906.
Marco Belan Sergio de Ponte Silvano Massaglia Daniela Tordella 《Astrophysics and Space Science》2004,293(1-2):225-232
The experiment described here is focussed to the observation of underexpanded, hypersonic turbulent jets. The experiment is relevant to a few aspects concerning the dynamics of astrophysical phenomena such as the Herbig-Haro jets and to the interaction between the large-scale vortices and the system of shocks that determine the spreading and entrainment properties of highly compressible free-flows. A number of orifice jets with a ratio between the stagnation pressure and the ambient pressure of the order 103-104 have been studied by changing the stagnation/ambient density ratio. This has been realized using dissimilar gases in the jet and in the ambient medium: by using He, Ar and air the stagnation/ambient density ratio can be changed by one order of magnitude while keeping fixed the pressure ratio. It has been possible to visualize the near and mid-term evolution of the jets and measure the axial and transversal density distributions. A comparison relevant to the shock waves configuration in between the nozzle exit and the first Mach's disk is shown for an air in air laboratory jet and its numerical simulation. 相似文献
907.
The United Kingdom government has decided to be part of the European Space Agency’s Aurora programme, but so far it has declared an intention only to participate in aspects of the programme which do not involve human space flight. Personally, we believe this to be a mistake, mainly because of the inherent limitations of robots, especially in unforeseen circumstances. However the arguments we make are different to this and are focussed mainly upon the benefits to earth based science, medicine, technology and education which would accrue from a manned space flight programme. 相似文献
908.
909.
Mauro Gemmi Marco Merlini Alessandro Pavese Nadia Curetti 《Physics and Chemistry of Minerals》2008,35(7):367-379
Phengite samples (2M
1 and 3T politypes) and a synthetic end-member muscovite specimen were studied by in situ high-temperature synchrotron radiation X-ray
diffraction. The measured volume thermal expansion of 2M
1 phengite (<α
V> ≈ 36.6 × 10−6 K−1) was systematically greater than <α
V> of the 3T polytype (≈33.3 × 10−6 K−1). A positive linear correlation between the average thermal expansion on (001) plane and the mean tetrahedral rotation angle
at ambient condition is proposed on the ground of new measurements and literature data. Dehydroxylation processes were observed
in 2M
1, starting at 1,000 K in 3T at 800 and 945 K in synthetic muscovite. Rietveld refinements allowed a determination of structural variations upon heating
of phengite samples and their dehydroxylate phases. The phengite structure expands by regularizing the tetrahedral sheet and
by reducing the bond length differences between the outer and inner coordination shell of the interlayer site. The dehydroxylate
phase derived from 2M
1 is characterized by fivefold polyhedra in the low temperature form as a consequence of two OH groups reacting to form H2O + O (residual). The dehydroxylate exhibits an increase of the cation–cation distances along the M–Or–M bonds with respect
to low-temperature phengite structures. For the 3T phase, we were unable to achieve completion of dehydroxylation. The refined structural model of the dehydroxylate phase shows
two hydroxyl sites, but at a short distance from one another. This result suggests that the dehydroxylation reaction did not
proceed to completion.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
910.
The mitigation of radio frequency interference (RFI) has a fundamental role in global navigation satellite system (GNSS) applications,
especially when a high level of availability is required. Several electromagnetic sources, in fact, might degrade the performance
of the global positioning system (GPS) and Galileo receivers, and their effects can be either in-band (i.e., secondary harmonics
generated by transmitters of other communication systems due to non-linearity distortions) or out-of-band (i.e., strong signals
that occupy frequency bandwidths very close to GNSS bands). We investigated the effects of real out-of-band signals on GNSS
receivers and analyzed the impact on the overall receiver chain in order to evaluate the impact of the interference source.
In particular, the analysis focuses on the spectrum at the front-end output, on the automatic gain control (AGC) behavior,
as well as on the digital processing stages (signal acquisition and tracking) at the analog digital converter (ADC) output.
This study refers to several experiments and data collections performed in interfered areas of downtown Torino (Italy). The
obtained results underline how digital/analog TV transmissions represent a potential interference source for GNSS applications
and might be critical for the safety of life services. 相似文献