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211.
Hugo Levato Stella Malaroda Beatriz García Nidia Morrell Gladys Solivella Monica Grosso 《Astrophysics and Space Science》1991,183(1):147-153
Spatially-homogenous and anisotropic Bianchi type-II and III cosmological models are obtained in Barber's second self-creation theory of gravitation both in vacuum and in the presence of perfect fluid with pressure equal to energy density. Some properties of the models are discussed. 相似文献
212.
B.?Caba?asEmail author M.?T.?Baeza P.?Martín S.?Salgado F.?Villanueva E.?Monedero K.?Wirtz 《Journal of Atmospheric Chemistry》2005,51(3):317-335
Products of the gas-phase reaction of the NO3 radical with thiophene have been investigated using different experimental systems. On the one hand, experiments have been conducted in our laboratory using two different methods, a Teflon static reactor coupled to a gas chromatograph combined with mass-spectrometry (GC-MS) and a discharge flow tube with direct MS spectroscopic detection. A qualitative analysis in these cases indicates that possible products for the reaction of thiophene+NO3 at room temperature include: sulphur dioxide, acetic and formic acids, a short-chain aldehyde, 2-nitrothiophene and 3-nitrothiophene. On the other hand, quantitative experiments have been performed in the European Photoreactor (EUPHORE) in Valencia, Spain. In this case, the major products were: HNO3 (≈80%), nitrothiophenes (≈30%), SO2 (≈20%), propanal (3%) and a fraction of particles (≈10%). The results obtained indicate that at least 70% of the reaction of NO3 with thiophene proceeds by an H-abstraction process at room temperature. The mechanism of the reaction studied is proposed on the basis of experimental results. 相似文献
213.
214.
The record of Almoloya Lake in the Upper Lerma basin starts with the deposition of the late Pleistocene Upper Toluca Pumice layer. The data from this interval indicate a period of climatic instability that lasted until 8500 cal yr B.P., when temperature conditions stabilized, although moisture fluctuations continued until 8000 cal yr B.P. Between 8500 and 5000 cal yr B.P. a temperate climate is indicated by dominance of Pinus. From 5000 to 3000 cal yr B.P. Quercus forest expanded, suggesting a warm temperate climate: a first indication of drier environmental conditions is an increase in grassland between 4200 and 3500 cal yr B.P. During the Late Holocene (3300 to 500 cal yr B.P.) the increase of Pinus and grassland indicates temperate dry conditions, with a considerable increase of Pinus between 1100 and 950 cal yr B.P. At the end of this period, humidity increased. The main tendency during the Holocene was a change from humid to dry conditions. During the Early Holocene, Almoloya Lake was larger and deeper; the changing humidity regime resulted in a fragmented marshland, with the presence of aquatic and subaquatic vegetation types. 相似文献
215.
The graphite deposit at Borrowdale (UK): A catastrophic mineralizing event associated with Ordovician magmatism 总被引:4,自引:0,他引:4
L. Ortega F.J. Luque J.F. Barrenechea J.-M. Huizenga S.M. Clarke 《Geochimica et cosmochimica acta》2010,74(8):2429-2449
The volcanic-hosted graphite deposit at Borrowdale in Cumbria, UK, was formed through precipitation from C-O-H fluids. The δ13C data indicate that carbon was incorporated into the mineralizing fluids by assimilation of carbonaceous metapelites of the Skiddaw Group by andesite magmas of the Borrowdale Volcanic Group. The graphite mineralization occurred as the fluids migrated upwards through normal conjugate fractures forming the main subvertical pipe-like bodies. The mineralizing fluids evolved from CO2-CH4-H2O mixtures (XCO2 = 0.6-0.8) to CH4-H2O mixtures. Coevally with graphite deposition, the andesite and dioritic wall rocks adjacent to the veins were intensely hydrothermally altered to a propylitic assemblage. The initial graphite precipitation was probably triggered by the earliest hydration reactions in the volcanic host rocks. During the main mineralization stage, graphite precipitated along the pipe-like bodies due to CO2 → C + O2. This agrees with the isotopic data which indicate that the first graphite morphologies crystallizing from the fluid (cryptocrystalline aggregates) are isotopically lighter than those crystallizing later (flakes). Late chlorite-graphite veins were formed from CH4-enriched fluids following the reaction CH4 + O2 → C + 2H2O, producing the successive precipitation of isotopically lighter graphite morphologies. Thus, as mineralization proceeded, water-generating reactions were involved in graphite precipitation, further favouring the propylitic alteration. The structural features of the pipe-like mineralized bodies as well as the isotopic homogeneity of graphite suggest that the mineralization occurred in a very short period of time. 相似文献