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891.
892.
893.
Within the framework of a linear theory, the instability of an electron beam with a finite radius in a cold magnetised boundless plasma is considered. It is shown that a finite beam dimension influences the generation of quasi-potential waves in two aspects: the perpendicular wave number is quantised so that the frequencies of the waves are subjected to strong selection; a new kind of instability appears due to wave energy losses by emission into surrounding space. Growth rate dependence of wave numbers and frequencies is investigated for typical parameters of experiments with electron beams in space. 相似文献
894.
V. A. Marinov S. V. Meledina O. S. Dzyuba O. S. Urman O. V. Yazikova V. A. Luchinina A. G. Zamirailova A. N. Fomin 《Stratigraphy and Geological Correlation》2006,14(4):418-432
Paleontological study of Upper Jurassic and Lower Cretaceous sediments recovered by boreholes in the Agan-Vakh and Nadym-Vengapur interfluves clarified environments of their deposition. As is shown, influx of siliciclastic material to central areas of the West Siberian sea basin varied through time. Taxonomic composition and ecological structure of nektonic and benthic fossil assemblages are analyzed and considered in terms of environmental factors such as hydrodynamics, aeration, temperature, and salinity of seawater. 相似文献
895.
N. S. Bortnikov 《Geology of Ore Deposits》2006,48(1):1-22
Data on fluid inclusions and stable isotope compositions (O, H, C, and S) in minerals have been summarized for large-and middle-scale mesothermal gold deposits (Nezhdaninsk, Berezovsk, Kochkar’, Svetlinsk, Darasun, and Maisk), cassiterite-silicate-sulfide deposits of Sikhote Alin (Solnechnoe, Arsen’evsk, and Vysokogorsk), vein silver-base metal deposits in the Southern Verkhoyansk region (Prognoz and Kupol’noe), and epithermal copper-bismuth-silver-base metal deposits of the Karamazar district in Tajikistan (Kanimansur, Tary Ekan, and Zambarak). It is shown that ores precipitated from fluids with salinity varying from brines (up to 60 wt % NaCl equiv) to dilute fluids (1–3 wt % NaCl equiv). As a rule, fluids of different compositions entered the hydrothermal-magmatic system. A fluid mixture of H2O-CO2-NaCl±CH4±N2 predominated in the orogenic (mesothermal) gold-bearing hydrothermal systems, with deposition of the final-stage gold-bearing sulfosalts from aqueous-salt fluid. Brines played a significant role in the formation of cassiterite-silicate-sulfide and vein silver-base metal deposits. The brines often coexisted with a low-density vapor-rich fluid at the ore deposition site. The obtained data suggest a predominant magmatic component in the hydrothermal-magmatic systems, with a significant contribution of meteoric waters. 相似文献
896.
The composition, structure, and geochemical properties of the thickest, relatively deep-water Lower Miocene sediments developed in western Ciscaucasia are considered. Of particular interest are materials from the Kuban superdeep borehole SG-12000 that recovered the uppermost layers of the Maikop Group at 3148–3961 m in the central Indol-Kuban Trough west of Krasnodar. However, the borehole did not penetrate the whole Lower Miocene section of the Maikop Group. Therefore, characteristics of the Maikop Group are supplemented with new materials from several other boreholes drilled in the eastern Kuban region. Thus, the typical (reference) Lower Miocene section of central and western Ciscaucasia has been sufficiently well described. 相似文献
897.
898.
This study focused on planktic foraminifera in plankton tows and surface sediments from the western Indian sector of Southern Ocean in order to evaluate the potential foraminiferal secondary calcification and/or dissolution in the sediment. It is found that the symbiotic foraminiferal species are abundant in the subtropical region, whereas non-symbiotic species dominate in the sub-Antarctic and polar frontal regions. The distribution of the symbiotic and non-symbiotic foraminiferal species is controlled by temperature, salinity, light, nutrients and phytoplankton biomass. There is also a lateral southern extent in abundance of planktic foraminifera from surface sediments to plankton tows. The shell weights of the planktic foraminifera N. pachyderma, G. bulloides and G. ruber within the surface sediments are on an average heavier by 27%, 34% and 40% respectively than shells of the same size within the plankton tows, indicative of secondary calcification. The planktic foraminiferal isotopes show the presence of heavier isotopes in the surface sediment foraminifera as compared to plankton tows, thus confirming secondary calcification. Secondary calcification in G. ruber occurs in the euphotic zone, whereas in case of N. pachyderma and G. bulloides it is at deeper depths. We also observed a decrease in the shell spines in surface sediment foraminifera as compared to plankton tows, indicative of the morphological changes that foraminifera underwent during gametogenesis. 相似文献
899.
900.
M. V. Belyaeva A. S. Drofa V. N. Ivanov 《Izvestiya Atmospheric and Oceanic Physics》2013,49(2):154-161
Comparative estimates of the efficiency of hygroscopic reagents for the seeding of convective clouds to obtain additional precipitation are performed based on numerical simulation. It is shown that the use of polydisperse salt powders has many benefits over other known hygroscopic reagents. The effect of seeding with salt powders manifests itself at mass concentrations of hygroscopic particles an order of magnitude less than when pyrotechnic flares are used. With salt-powder seeding, it is possible to obtain precipitation from warm convective clouds of moderate thickness from which precipitation does not typically fall. In this case the effect of “reseeding” of clouds, which can be observed in seeding with hygroscopic reagents having narrow size distributions of particles, does not reveal itself. 相似文献