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   40882篇
  免费   212篇
  国内免费   175篇
测绘学   814篇
大气科学   1977篇
地球物理   7736篇
地质学   15956篇
海洋学   3753篇
天文学   9636篇
综合类   115篇
自然地理   1282篇
  2022年   512篇
  2021年   742篇
  2020年   804篇
  2019年   895篇
  2018年   1802篇
  2017年   1633篇
  2016年   1773篇
  2015年   691篇
  2014年   1551篇
  2013年   2219篇
  2012年   1737篇
  2011年   2035篇
  2010年   1935篇
  2009年   2169篇
  2008年   1913篇
  2007年   2113篇
  2006年   1880篇
  2005年   1016篇
  2004年   934篇
  2003年   907篇
  2002年   812篇
  2001年   825篇
  2000年   697篇
  1999年   491篇
  1998年   510篇
  1997年   552篇
  1996年   410篇
  1995年   401篇
  1994年   362篇
  1993年   337篇
  1992年   318篇
  1991年   329篇
  1990年   361篇
  1989年   302篇
  1988年   290篇
  1987年   299篇
  1986年   242篇
  1985年   337篇
  1984年   345篇
  1983年   334篇
  1982年   313篇
  1981年   278篇
  1980年   291篇
  1979年   230篇
  1978年   256篇
  1977年   224篇
  1976年   197篇
  1975年   206篇
  1974年   189篇
  1973年   229篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
Comparison of solar-neutrino signals in SNO [Phys. Rev. Lett. 87 (2001) 071301] and Super-Kamiokande (SK) [Phys. Rev. Lett. 86 (2001) 5651] detectors results in discovery of νe→νμ,τ oscillations at level 3.1–3.3σ [Phys. Rev. Lett. 87 (2001) 071301]. This comparison involves the assumption of neutrino spectrum and a choice for the thresholds of detection in both experiments. In this paper we obtain an exact formula for the comparison of the signals which is valid for arbitrary spectra and thresholds. We find that the no-oscillation hypothesis (astrophysical solutions) is excluded at 3.3σ. If the energy-dependent component of the survival probability for electron neutrinos is small as compared with the average value, i.e. in the case of small distortion of the observed spectrum, the oscillation hypothesis can also be tested to similar accuracy. The oscillation to sterile neutrino only, is excluded at 3.3σ level, and oscillation to active neutrinos is confirmed at 2.8σ.  相似文献   
952.
邹维列  王钊  杨志强 《岩土力学》2004,25(11):1775-1778
尽管击实试验是目前最流行的室内土体压实试验方法,但实际上与现场任何一种压实方法都无相似之处,且不适用于无粘性土(粒状土)的压实试验。无粘性土的振动击实试验方法也存在很多问题。现代重型压实设备的使用,路基粒状填土的压实密度达到了目前室内规范试验方法不可能达到的水平。介绍了美国采用供高性能沥青路面混合料设计与质量控制之用的旋转压实机对粒状土进行的压实试验。其结果表明,旋转压实试验能有效模拟路基粒状填土的现场压实特征;可采用压实压力200 kPa、旋转角为1.25°、旋转次数为90、旋转速度为20 rpm的试验结果来控制粒状填土路基的现场压实质量。  相似文献   
953.
The distribution of temperature and salinity in the upper 500 m of the northwestern Bay of Bengal, adjoining the east coast of India, during the retreat of southwest monsoon (September) of 1983 is presented. This study reveals coastal upwelling (limited to the upper 40 m) induced by the local winds. Waters of higher surface salinity near the coast characterize the upwelling. The freshwater influx near the head of the Bay diluted the surface salinity to as low as 26.0 × 10−3. The surface circulation was weak and led to a net transport of 2.0 × 106m3.s−1 directed towards northeast.  相似文献   
954.
Albian/Cenomanian benthic foraminiferal faunas recovered by the DSDP in the western South Atlantic Ocean (Leg 36) are described and analyzed from the palaeogeographic and palaeo-environmental points of view. In doing this the author compares Leg 36 assemblages in the western South Atlantic Ocean with coeval benthic foraminiferal faunas recovered in the eastern South Atlantic Ocean (Leg 40) and in the eastern Indian Ocean (Legs 26 and 27). The specific composition of these assemblages, except for Leg 27, is virtually the same. Consequently, they are considered to indicate the same depositional water depth at all relevant sites studied, whether located in the Angola Basin, the northern flank of the Walvis Ridge, the eastern margin of the Falkland Plateau or on the Naturaliste Plateau. All the assemblages indicate shallow environments around 100 m and not exceeding 300–400 m in the deepest parts, corresponding to the inner shelf and the inner part of the outer shelf. By contrast the foraminiferal associations of Leg 27 (especially Site 259) indicate a greater depth, of the order of 200–600 m (but not exceeding 1000 m) corresponding to upper slope of Sliter & Baker (1972) and Sliter (1972). These bathymetrical conclusions are in remarkable accord with those of Sliter (1976), based on planktic Foramini fera of Leg 36.Late Cretaceous (Campanian-Maastrichtian) material with benthic Foraminifera was limited to two positive samples; however, these faunas indicate much the same palaeo-environment as do the planktic ones analyzed by Sliter (1976).  相似文献   
955.
Radio frequency electromagnetic radiation (EMR) registered hundreds of kilometres away from an earthquake epicentre is detected hours before earthquakes. Yet, accurate earthquakes prediction by their self-induced EMR still remains in its infancy due in part to the lack of understanding of EMR's origin. Here we present a viable model of this origin, according to which EMR is emitted by an oscillating dipole created by ions moving collectively as a surface wave on both sides of the crack; when the crack halts, the EMR pulse amplitude decays by interaction with bulk phonons. The model is shown to be able to provide crack dimensions and velocities, to explain some general similarities of different fracturing processes and indicate the existence of a general failure mechanism. Results raise the hope of developing an EMR based genuine earthquake prediction system.  相似文献   
956.
The Shah-Kuh granitic pluton of eastern Central Iran was emplaced 165 Ma ago, in an active continental margin setting. It is made of two main units: a granodioritic unit (SiO2=63–71 wt%) to the north–west and a syenogranitic unit (SiO2=73–77 wt%) to the south–east. The former unit displays seriate medium-grained textures and contains locally abundant mafic enclaves. The latter unit is medium- to coarse-grained and porphyritic, with 0.5–3 cm long K-feldspar megacrysts. Fine-grained granitic bodies are present in both units. The rocks are metaluminous to slightly peraluminous (I-type) and peraluminous (S-type) and belong to the ilmenite-series granites. Fractional crystallization appears to have been a very effective differentiation process in both units, and the fractionated mineral assemblages are determined by mass balance calculations. Isotopic data (Sri=0.7065 and εNdt=−2.5) are consistent with a young upper crustal protolith. Tin mineralization in sheeted quartz-tourmaline (-cassiterite) veins is spatially associated with the granodioritic unit. The veins formed by hydraulic fracturing when the granodioritic to monzogranitic magma became water-saturated and exsolved a fluid phase during crystallization. The reduced nature of this magma is responsible for the incompatible behaviour of Sn, likely to favour Sn concentration in the residual melt and then in the exsolved fluid. Another fluid phase was exsolved by the syenogranitic magma and was responsible for local greisenized granites, characterized by high Y and HREE-contents and non-fractionated REE distribution patterns. Field and mineralogical data show that the (B, Sn) vein-forming fluid was different from the (F, Li) greisen-forming fluid.  相似文献   
957.
Pavagada taluk of Tumkur district in Karnataka is one of the most backward taluks receiving less than 500 mm annual rainfall. The maximum area of the taluk is under monocropping, reasons for the same were not documented well. The present study was carried out using remote sensing data along with field survey and laboratory analysis for assessing the potentials and limitations of soil. Using the basic information on soil, climate and topography based on the matching exercise between the growth and production requirements of the crop, suitability of soils for groundnut, paddy and finger millet was assessed as per FAO land evaluation. The soil suitability maps were prepared using Arc GIS software. About 48 per cent of the total area was moderate to marginally suitable and 13 per cent of the area was not suitable for both groundnut and finger millet. Lowland areas covering 12 per cent of the area was highly suitable, 15 per cent was moderate to marginally suitable and 20 per cent was not suitable for paddy cultivation.  相似文献   
958.
959.
A comparative geochemical study of rare-metal granitoids with various fluorine-bearing minerals (fluorite, topaz, and cryolite) was carried out. It was shown that these rocks correspond to both plumasitic and agpaitic geochemical types. The fluorite-, topaz-, and cryolite-bearing granites of these geochemical types are distinctly different in geochemical parameters and the character of magmatic evolution. These differences are related to the composition of initial magmas and their sources. Rare-metal granitoids with fluorine-bearing minerals compose small massifs, stocks, and dike swarms. Their formation is independent of the composition and age of the country rocks or geologic structures where they occur. Original Russian Text ? V.S. Antipin, E.A. Savina, M.A. Mitichkin, 2006, published in Geokhimiya, 2006, No. 10, pp. 1040–1052.  相似文献   
960.
Using a large sample of 78 well-observed double quasars, we have investigated several consequences of the relativistic beaming model. In this model the ratio of the strengths of the central component and outer lobes of a double source depends on whether the jet axis lies close to or away from the line of sight, If this is the actual situation, the fraction of emission from the core,f c, may be used as a statistical measure of the orientation of the source and should be correlated with other source parameters which also depend on the inclination of the jet axis to the line of sight. We findf c to be anticorrelated with the overall projected linear size of the extended emission but to exhibit a positive correlation with both the observed degree of misalignment from a collinear double structure, and the ratio of separations of the outer hotspots from the central component. As might be expected from these relationships, we also find sources of smaller projected linear sizes to appear more misaligned and the degree of misalignment to be correlated with the ratio of separations of the outer hotspots. All these correlations are consistent with the predictions of the relativistic beaming model.  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