全文获取类型
收费全文 | 2360篇 |
免费 | 483篇 |
国内免费 | 842篇 |
专业分类
测绘学 | 39篇 |
大气科学 | 106篇 |
地球物理 | 715篇 |
地质学 | 1956篇 |
海洋学 | 446篇 |
天文学 | 15篇 |
综合类 | 126篇 |
自然地理 | 282篇 |
出版年
2024年 | 2篇 |
2023年 | 33篇 |
2022年 | 66篇 |
2021年 | 68篇 |
2020年 | 80篇 |
2019年 | 79篇 |
2018年 | 89篇 |
2017年 | 101篇 |
2016年 | 89篇 |
2015年 | 84篇 |
2014年 | 138篇 |
2013年 | 155篇 |
2012年 | 167篇 |
2011年 | 120篇 |
2010年 | 118篇 |
2009年 | 182篇 |
2008年 | 198篇 |
2007年 | 216篇 |
2006年 | 177篇 |
2005年 | 165篇 |
2004年 | 164篇 |
2003年 | 142篇 |
2002年 | 138篇 |
2001年 | 105篇 |
2000年 | 101篇 |
1999年 | 104篇 |
1998年 | 93篇 |
1997年 | 80篇 |
1996年 | 59篇 |
1995年 | 63篇 |
1994年 | 61篇 |
1993年 | 53篇 |
1992年 | 34篇 |
1991年 | 33篇 |
1990年 | 32篇 |
1989年 | 22篇 |
1988年 | 19篇 |
1987年 | 14篇 |
1986年 | 7篇 |
1985年 | 4篇 |
1984年 | 8篇 |
1983年 | 6篇 |
1982年 | 5篇 |
1981年 | 8篇 |
1978年 | 1篇 |
1973年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有3685条查询结果,搜索用时 93 毫秒
1.
史生才 《紫金山天文台台刊》2003,(1)
为了研制亚毫米波射电天文用超导SIS(超导 -绝缘体 -超导 )接收机 ,我们重点开展了如下研究 ,1 )Nb超导SIS结在其能隙频率附近的量子混频特性 ,及其结合高能隙超导薄膜 (NbTiN)和高电导率金属薄膜 (Al)分布结阵在 780 - 950GHz频率区间的量子混频特性 ;2 )亚毫米波超导混频器嵌入阻抗的数值和实验表征 ;3)高电流密度小面积Nb超导SIS结的制备和特性表征 ;4)一个 60 0 - 72 0GHz超导SIS混频器的研制和特性表征。本文详细介绍了相关的数值分析和实验测量结果。 相似文献
2.
Tidal effects on temperature front in the Yellow Sea 总被引:5,自引:0,他引:5
Temperature front (TF) is one of the important features in the Yellow Sea, which forms in spring, thrives in summer, and fades
in autumn as thermocline declines. TF intensity ⋎S
T
⋎ is defined to describe the distribution of TF. Based on the MASNUM wave-tide-circulation coupled model, temperature distribution
in the Yellow Sea was simulated with and without tidal effects. Along 36°N, distribution of TF from the simulated results
are compared with the observations, and a quantitative analysis is introduced to evaluate the tidal effects on the forming
and maintaining processes of the TF. Tidal mixing and the circulation structure adapting to it are the main causes of the
TF.
Supported by the National Basic Research Program of China (No. G1999043809) and the National Science Foundation of China (No.
49736190). 相似文献
3.
Cold water in the deep Pacific can be drawn up to the surface (or west warm water drifts eastwards ) because strong tide increases the mixing of seawater both in vertical and horizontal. In this way greenhouse effect is decreased or increased by means of absorbing (or releasing) CO2. Therefore, La Nina cold event (or El Nino warm event) may occur,which is caused by wanning - up or cooling - down air above the ocean. Volcanic action at sea bottom is also controlled by strong tide. 相似文献
4.
Summary. A residual map of the total magnetic field (above 25 000 nT base) is presented for a portion of the central crystalline shield area of Nigeria and overlapping small portions of the Chad basin and the Benue rift (8°30'−12° 00'lat, and 7°−10°30' long). The map (based on a dataset digitized from recently released aeromagnetic sheets of Nigeria) leads to four results. (1) A magnetic boundary, evident on the map, separates the Younger Granite complexes into two groups. The groups are petrologically different, and the boundary may be a fault line with uplift to the south. (2) South of the boundary the map is dominated by a system of sub-parallel anomalies striking NE–SW, possibly representing major tectonic trends, and a set of fractures through which the Younger Granite complexes were intruded. The trend of the system parallels the Benue rift and lineaments in the oceanic crust off West Africa. (3) Negative magnetic anomalies lie over most of the known ring complexes, and over some suspected buried ring complexes and other intrusions. (4) 2½-and 3-D modelling shows that the larger complexes extend to 12 km depth, and the smaller ones to 6 km. They have nearly vertical sides, and magnetization contrasts range from 0.3 to 0.5 A m−1 . 相似文献
5.
Nicolas Rascle Fabrice Ardhuin Pierre Queffeulou Denis Croiz-Fillon 《Ocean Modelling》2008,25(3-4):154-171
Ocean surface mixing and drift are influenced by the mixed layer depth, buoyancy fluxes and currents below the mixed layer. Drift and mixing are also functions of the surface Stokes drift Uss, volume Stokes transport TS, a wave breaking height scale Hswg, and the flux of energy from waves to ocean turbulence Φoc. Here we describe a global database of these parameters, estimated from a well-validated numerical wave model, that uses traditional forms of the wave generation and dissipation parameterizations, and covers the years 2003–2007. Compared to previous studies, the present work has the advantage of being consistent with the known physical processes that regulate the wave field and the air–sea fluxes, and also consistent with a very large number of in situ and satellite observations of wave parameters. Consequently, some of our estimates differ significantly from previous estimates. In particular, we find that the mean global integral of Φoc is 68 TW, and the yearly mean value of TS is typically 10–30% of the Ekman transport, except in well-defined regions where it can reach 60%. We also have refined our previous estimates of Uss by using a better treatment of the high frequency part of the wave spectrum. In the open ocean, Uss 0.013U10, where U10 is the wind speed at 10 m height. 相似文献
6.
1 IntroductionDiscontinuousDeformationAnalysis(DDA)developedbyShiin 1 980’s[1 ,2 ] maybeusedtocalculatethedeformationanddisplacementsinamulti blocksystemandtoanalyzetherelationbe tweenforcesanddisplacementsinthesystem .Ifthesurveydisplacementofeverypointordirectionisknown,alldisplacementsanddeformationsofblocks,eventheglide ,theclosureandthestretchoftheboundary planeofblocks ,canbecalculated .TheresultisoptimallyfittedaccordingtotheLeastSquarePrinciple.DDAismainlyusedinrockblockssystems… 相似文献
7.
O. Sadot K. Levy A. Yosef-Hai D. Cartoon Y. Elbaz Y. Srebro G. Ben-Dor D. Shvarts 《Astrophysics and Space Science》2005,298(1-2):305-312
The hydrodynamic instability, which develops on the contact surface between two fluids, has great importance in astrophysical
phenomena such as the inhomogeneous density distribution following a supernova event. In this event acceleration waves pass
across a material interface and initiate and enhance unstable conditions in which small perturbations grow dramatically.
In the present study, an experimental technique aimed at investigating the above-mentioned hydrodynamic instability is presented.
The experimental investigation is based on a shock-tube apparatus by which a shock wave is generated and initiates the instability
that develops on the contact surface between two gases. The flexibility of the system enables one to vary the initial shape
of the contact surface, the shock-wave Mach number, and the density ratio across the contact surface.
Three selected sets of shock-tube experiments are presented in order to demonstrate the system capabilities: (1) large-initial
amplitudes with low-Mach-number incident shock waves; (2) small-initial amplitudes with moderate-Mach-number incident shock
waves; and (3) shock bubble interaction.
In the large-amplitude experiments a reduction of the initial velocity with respect to the linear growth prediction was measured.
The results were compared to those predicted by a vorticity-deposition model and to previous experiments with moderate- and
high-Mach number incident shock waves that were conducted by others. In this case, a reduction of the initial velocity was
noted. However, at late times the growth rate had a 1/t behavior as in the small-amplitude low-Mach number case. In the small-amplitude moderate-Mach number shock experiments a
reduction from the impulsive theory was noted at the late stages.
The passage of a shock wave through a spherical bubble results in the formation of a vortex ring. Simple dimensional analysis
shows that the circulation depends linearly on the speed of sound of the surrounding material and on the initial bubble radius. 相似文献
8.
Jorge Luis De Souza 《Pure and Applied Geophysics》1991,136(2-3):245-264
The Rayleigh wave phase and group velocities in the period range of 24–39 sec, obtained from two earthquakes which occurred in northeastern brazil and which were recorded by the Brazilian seismological station RDJ (Rio de Janeiro), have been used to study crustal and upper mantle structures of the Brazilian coastal region. Three crustal and upper mantle models have been tried out to explain crustal and upper mantle structures of the region. The upper crust has not been resolved, due basically to the narrow period range of the phase and group velocities data. The phase velocity inversions have exhibited good resolutions for both lower crust and upper mantle, with shear wave velocities characteristic of these regions. The group velocity data inversions for these models have showed good results only for the lower crust. The shear wave velocities of the lower crust (3.86 and 3.89 km/sec), obtained with phase velocity inversions, are similar to that (=3.89 km/sec) found byHwang (1985) to the eastern South American region, while group velocity inversions have presented shear velocity (=3.75 km/sec) similar to that (=3.78 km/sec) found byLazcano (1972) to the Brazilian shield. It was not possible to define sharply the crust-mantle transition, but an analysis of the phase and group velocity inversions results has indicated that the total thickness of the crust should be between 30 and 39 km. The crustal and upper mantle model, obtained with phase velocity inversion, can be used as a preliminary model for the Brazilian coast. 相似文献
9.
Axel Bjrnsson 《Tectonophysics》2008,447(1-4):136
Two different models of the structure of the Icelandic crust have been presented. One is the thin-crust model with a 10–15 km thick crust beneath the axial rift zones, with an intermediate layer of partially molten basalt at the base of the crust and on the top of an up-domed asthenosphere. The thick-crust model assumes a 40 km thick and relatively cold crust beneath central Iceland. The most important and crucial parameter to distinguish between these different models is the temperature distribution with depth. Three methods are used to estimate the temperature distribution with depth. First, the surface temperature gradient measured in shallow wells drilled outside geothermal areas. Second, the thickness of the seismogenic zone which is associated with a 750 °C isothermal surface. Third, the depth to a layer with high electrical conductivity which is associated with partially molten basalt with temperature around 1100 °C at the base of the crust. Combination of these data shows that the temperature gradient can be assumed to be nearly linear from the surface down to the base of the crust. These results are strongly in favour of the thin-crust model. The scattered deep seismic reflectors interpreted as Moho in the thick-crust model could be caused by phase transitions or reflections from melt pockets in the mantle. 相似文献
10.
The role of heterogeneous strain in the development and preservation of a polymetamorphic record in high-P granulites, western Canadian Shield 总被引:1,自引:0,他引:1
K. H. MAHAN P. GONCALVES R. FLOWERS M. L. WILLIAMS D. HOFFMAN-SETKA 《Journal of Metamorphic Geology》2008,26(6):669-694
Mafic rocks in the Chipman domain of the Athabasca granulite terrane, western Canadian Shield, provide the first well‐documented record of two distinct high‐P granulite facies events in the same domain in this region. Textural relations and the results of petrological modelling (NCFMASHT system) of mafic granulites are interpreted in terms of a three‐stage tectonometamorphic history. Stage 1 involved development of the assemblage Grt + Cpx + Qtz ± Pl (M1) from a primary Opx‐bearing igneous precursor at conditions of 1.3 GPa, 850–900 °C. Field and microstructural observations suggest that M1 developed synchronously with an early S1 gneissic fabric. Stage 2 is characterized by heterogeneous deformation (D2) and synkinematic partial retrogression of the peak assemblage to an amphibole‐bearing assemblage (M2). Stage 3 involved a third phase of deformation and a return to granulite facies conditions marked by the prograde breakdown of amphibole (Amph2) to produce matrix garnet (Grt3a) and the coronitic assemblage Cpx3b + Opx3b + Ilm3b + Pl3b (M3b) at 1.0 GPa, 800–900 °C. M1 and M3b are correlated with 2.55 and 1.9 Ga metamorphic generations of zircon, respectively, which were dated in a separate study. Heterogeneous strain played a crucial role in both the development and preservation of these rare examples of multiple granulite facies events within single samples. Without this fortuitous set of circumstances, the apparent reaction history could have incorrectly led to an interpretation involving a single‐cycle high‐grade event. The detailed P–T–t–D history constructed for these rocks provides the best evidence to date that much of the east Lake Athabasca region experienced long‐term lower crustal residence from 2.55 to 1.9 Ga, and thus the region represents a rare window into the reactivation and ultimate stabilization processes of cratonic lithosphere. 相似文献