首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   7100篇
  免费   225篇
  国内免费   73篇
测绘学   239篇
大气科学   688篇
地球物理   1484篇
地质学   2495篇
海洋学   688篇
天文学   1301篇
综合类   15篇
自然地理   488篇
  2023年   21篇
  2022年   49篇
  2021年   84篇
  2020年   105篇
  2019年   102篇
  2018年   205篇
  2017年   188篇
  2016年   259篇
  2015年   164篇
  2014年   224篇
  2013年   340篇
  2012年   306篇
  2011年   411篇
  2010年   326篇
  2009年   447篇
  2008年   376篇
  2007年   317篇
  2006年   334篇
  2005年   308篇
  2004年   407篇
  2003年   320篇
  2002年   246篇
  2001年   178篇
  2000年   179篇
  1999年   143篇
  1998年   171篇
  1997年   100篇
  1996年   78篇
  1995年   68篇
  1994年   59篇
  1993年   63篇
  1992年   62篇
  1991年   56篇
  1990年   49篇
  1989年   30篇
  1988年   22篇
  1987年   33篇
  1986年   26篇
  1985年   32篇
  1984年   45篇
  1983年   38篇
  1982年   23篇
  1981年   30篇
  1978年   29篇
  1977年   26篇
  1976年   28篇
  1975年   34篇
  1974年   19篇
  1973年   27篇
  1971年   19篇
排序方式: 共有7398条查询结果,搜索用时 312 毫秒
991.
At the celebration of 400 years of telescopic observations, it is appropriate to consider future ground-based facilities enabled by continuing technological developments, and driven by astronomical questions which are amongst the most fundamental in science and are of enormous interest to the general public.  相似文献   
992.
The DynaMICCS mission is designed to probe and understand the dynamics of crucial regions of the Sun that determine solar variability, including the previously unexplored inner core, the radiative/convective zone interface layers, the photosphere/chromosphere layers and the low corona. The mission delivers data and knowledge that no other known mission provides for understanding space weather and space climate and for advancing stellar physics (internal dynamics) and fundamental physics (neutrino properties, atomic physics, gravitational moments...). The science objectives are achieved using Doppler and magnetic measurements of the solar surface, helioseismic and coronographic measurements, solar irradiance at different wavelengths and in-situ measurements of plasma/energetic particles/magnetic fields. The DynaMICCS payload uses an original concept studied by Thalès Alenia Space in the framework of the CNES call for formation flying missions: an external occultation of the solar light is obtained by putting an occulter spacecraft 150 m (or more) in front of a second spacecraft. The occulter spacecraft, a LEO platform of the mini sat class, e.g. PROTEUS, type carries the helioseismic and irradiance instruments and the formation flying technologies. The latter spacecraft of the same type carries a visible and infrared coronagraph for a unique observation of the solar corona and instrumentation for the study of the solar wind and imagers. This mission must guarantee long (one 11-year solar cycle) and continuous observations (duty cycle > 94%) of signals that can be very weak (the gravity mode detection supposes the measurement of velocity smaller than 1 mm/s). This assumes no interruption in observation and very stable thermal conditions. The preferred orbit therefore is the L1 orbit, which fits these requirements very well and is also an attractive environment for the spacecraft due to its low radiation and low perturbation (solar pressure) environment. This mission is secured by instrumental R and D activities during the present and coming years. Some prototypes of different instruments are already built (GOLFNG, SDM) and the performances will be checked before launch on the ground or in space through planned missions of CNES and PROBA ESA missions (PICARD, LYRA, maybe ASPIICS).  相似文献   
993.
The dynamics of the circular restricted three-body Earth-Moon-particle problem predicts the existence of the retrograde periodic orbits around the Lagrangian equilibrium point L1. Such orbits belong to the so-called family G (Broucke, Periodic orbits in the restricted three-body problem with Earth-Moon masses, JPL Technical Report 32–1168, 1968) and starting from them it is possible to define a set of trajectories that form round trip links between the Earth and the Moon. These links occur even with more complex dynamical systems as the complete Sun-Earth-Moon-particle problem. One of the most remarkable properties of these trajectories, observed for the four-body problem, is a meaningful inclination gain when they penetrate into the lunar sphere of influence and accomplish a swing-by with the Moon. This way, when one of these trajectories returns to the proximities of the Earth, it will be in a different orbital plane from its initial Earth orbit. In this work, we present studies that show the possibility of using this property mainly to accomplish transfer maneuvers between two Earth orbits with different altitudes and inclinations, with low cost, taking into account the dynamics of the four-body problem and of the swing-by as well. The results show that it is possible to design a set of nominal transfer trajectories that require ΔV Total less than conventional methods like Hohmann, bi-elliptic and bi-parabolic transfer with plane change.  相似文献   
994.
995.
Complexation of (trace) elements in fluids plays a critical role in determining element mobility in subduction zones, but to date, the atomic-scale processes controlling elemental solubilities are poorly understood. As a first step towards computer simulation of element complexation in subduction zone fluids, a thermodynamic cycle was developed to investigate the hydration environment and energetics of lanthanide complexes using density functional theory. The first solvation shell is explicitly defined and the remaining part of the aqueous fluid is modelled using a polarisable continuum model, which allows extrapolation to a broad pressure and temperature range.We illustrate our method by comparing solvation of lanthanide series elements in H2O in the presence of fluoride or chloride for conditions relevant to subduction zones. The energetics of lanthanide- and lanthanide-fluoride/chloride hydration complexes were determined computationally. Calculated hydration free energies for trivalent lanthanides with explicit eight- and nine-fold coordinated first hydration shells show good agreement with literature data at room pressure and temperature. The hydration free energy is more negative for smaller complexes (heavy lanthanides) relative to larger complexes (light lanthanides), with the difference between La and Lu in water amounting to 361 kJ mol−1. The hydration free energy of all lanthanide ions becomes less negative with increasing pressure (p) and temperature (T) up to 2.5 GPa and 1000 K (typical conditions in the upper part of subducting slabs). The free energy difference between light- and heavy-lanthanides remains essentially unchanged at elevated (p, T) conditions. There are minor geometrical differences in local hydration environment between light lanthanide-chloride (La-Nd) and heavy lanthanide-chloride (Pm-Lu) hydrated complexes, without a distinguishable energy difference. Complexation with fluoride is energetically more favourable than with chloride by 206 ± 4 kJ mol−1 across the entire lanthanide series. The association of fluoride-water and chloride-water fragments with lanthanide-water complexes is energetically more favourable for aqueous lanthanide complexes surrounded by fewer first hydration shell water molecules.The methods developed in this study, in conjunction with simulation of the energetics of trace element incorporation into minerals, open the possibility to use molecular modelling to constrain elemental behaviour in subduction zones.  相似文献   
996.
The present study investigates hydrocarbon oxidation processes at Isis and Amon mud volcanoes (MV’s), in the eastern Nile deep-sea fan. In the water column, molecular and carbon isotopic signatures of light hydrocarbons indicate that gases rapidly dissolve in seawater and are partially oxidized.In the upper sediments, anaerobic oxidation of the light hydrocarbons takes place, as clearly shown by their molecular and isotopic composition. These processes lead to the presence of a distinct Sulfate-Hydrocarbon Interface at 120-145 cm and 20-50 cm below the seafloor, for Isis and Amon MV’s, respectively. In contrast to processes occurring in the water column, a clear preferential oxidation of methane, propane and n-butane over ethane and i-butane is observed in the anoxic sediments. Furthermore, for the first time, fractionation factors have been determined for the anaerobic oxidation of propane and butane, being respectively −4.80‰ and −0.7‰ for δ13C, and −43.3‰ for δ2H of propane.  相似文献   
997.
998.
Conventional (one-dimensional) chemostratigraphy of marine carbonates assesses the chemical archive of individual stratigraphic sections and their correlation in space and time. Whereas this approach has shown to be of value when linking isobathymetric domains, usually characterised by similar facies, more caution is needed when correlations are extended across different physiographic settings and hence different facies belts. Here, the spatial geochemical record of Pennsylvanian platform-margin-to-basin transects across a bathymetric range of about 800 m is documented and discussed in a process-oriented context. Particularly, the presence of layered palaeo-water masses and their potential control on slope facies distribution and geochemical properties requires attention. Whereas Carboniferous thermo- and/or chemo-clines most likely affected depth-related slope facies zonation, it was facies change and hence, variances in porosity–permeability properties, that controlled differential early burial diagenetic alteration. Specifically, the lower-slope related breccia facies is characterised by higher volumes of early burial carbonate cements. This implies that these sediments entered the shallow-burial domain with a considerable open pore space and gave way to an increased rock:fluid ratio. Whereas the δ13C record is invariant with bathymetry, the more diagenesis-sensitive δ18O proxy, records pronounced shifts observed across major facies boundaries. From this it is concluded that although the primary controlling factor of slope facies distribution with depth is probably palaeoceanographic in nature, it is differential rock:fluid ratios that control the first-order, spatial shifts in δ18O composition. These findings show that one-dimensional chemostratigraphy will severely underestimate the complexity of three-dimensional (bathymetric) data sets across platform margins. This is of relevance for the interpretation of the geochemical archive of fossil carbonate platforms in general.  相似文献   
999.
In the interior of the Iberian Peninsula, the main geomorphic features, mountain ranges and basins, seems to be arranged in several directions whose origin can be related to the N–S plate convergence which occurred along the Cantabro–Pyrenean border during the Eocene–Lower Miocene time span. The Iberian Variscan basement accommodated part of this plate convergence in three E–W trending crustal folds as well as in the reactivation of two left-lateral NNE–SSW strike-slip belts. The rest of the convergence was assumed through the inversion of the Iberian Mesozoic Rift to form the Iberian Chain. This inversion gave rise to a process of oblique crustal shortening involving the development of two right lateral NW–SE shear zones. Crustal folds, strike-slip corridors and one inverted rift compose a tectonic mechanism of pure shear in which the shortening is solved vertically by the development of mountain ranges and related sedimentary basins. This model can be expanded to NW Africa, up to the Atlasic System, where N–S plate convergence seems also to be accommodated in several basement uplifts, Anti-Atlas and Meseta, and through the inversion of two Mesozoic rifts, High and Middle Atlas. In this tectonic situation, the microcontinent Iberia used to be firmly attached to Africa during most part of the Tertiary, in such a way that N–S compressive stresses could be transmitted from the collision of the Pyrenean boundary. This tectonic scenario implies that most part of the Tertiary Eurasia–Africa convergence was not accommodated along the Iberia–Africa interface, but in the Pyrenean plateboundary. A broad zone of distributed deformation resulted from the transmission of compressive stresses from the collision at the Pyrenean border. This distributed, intraplate deformation, can be easily related to the topographic pattern of the Africa–Eurasia interface at the longitude of the Iberian Peninsula.Shortening in the Rif–Betics external zones – and their related topographic features – must be conversely related to more “local” driven mechanisms, the westward displacement of the “exotic” Alboran domain, other than N–S convergence. The remaining NNW–SSE to NW–SE, latest Miocene up to Present convergence is also being accommodated in this zone straddling Iberia and Morocco, at the same time as a new ill-defined plate boundary that is being developed between Europe and Africa.  相似文献   
1000.
The detection of microorganisms with potential for biodeterioration and biodegradation in petroleum fields is of great relevance, since these organisms may be related to a decrease in petroleum quality in the reservoirs or damage in the production facilities. In this sense, petroleum formation water and oil samples were collected from the Campos Basin, Brazil, with the aim of isolating microorganisms and evaluating their ability to degrade distinct classes of hydrocarbon biomarkers (9,10-dihydrophenanthrene, phytane, nonadecanoic acid and 5α-cholestane). Twenty eight bacterial isolates were recovered and identified by sequencing their 16S rRNA genes. Biodegradation assays revealed that bacterial metabolism of hydrocarbons occurred through reactions based on oxidation, carbon–carbon bond cleavage and generation of new bonds or by the physical incorporation of hydrocarbons into microbial cell walls. Based on the biodegradation results, selective PCR-based systems were developed for direct detection in petroleum samples of bacterial groups of interest, namely Bacillus spp., Micrococcus spp., Achromobacter xylosoxidans, Dietzia spp. and Bacillus pumilus. Primer sets targeting 16S rRNA genes were designed and their specificity was confirmed in silico (i.e. computational analysis) and in PCR reactions using DNA from reference strains as positive and negative controls. Total DNA from oil was purified and the amplification tests revealed the presence of the target bacteria in the samples, unraveling a significant potential for petroleum deterioration in the reservoirs sampled, once proper conditions are present for hydrocarbon degradation. The application of molecular methods for rapid detection of specific microorganisms in environmental samples would be valuable as a supporting tool for the evaluation of oil quality in production reservoirs.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号